I ns t i t ut eofManagement & Techni calSt udi es
CAD, CAM ANDROBOTI CS `500
DI PLOMAI NMECHANI CAL ENGI NEERI NG
www. i mt s i ns t i t ut e. com
IMTS (ISO 9001-2008 Internationally Certified)
CAD, CAM AND ROBOTICS
CAD, CAM AND ROBOTICS
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS CONTENTS: UNIT I:
01-18
Introduction: – CAD/CAM defined – the produced cycle and CAD/CAM – automation and
CAD/CAM. Computer Technology:
– Introduction –Control
Processing Unit- Type of memory – input/output – data representation – computer programming language – operating the computer system.
UNIT II:
19-31
Computer fundamentals of CAD: – Introduction- the design process- the application of computer for design-creating the manufacturing data base - benefits of CAD. Hardware of CAD: – the design workstation – the graphics terminal – operation input devices – plotter and other o/p devices – secondary storage.
UNIT-III
32-42
Computer aided manufacturing: Introduction - CAM - Hierarchy - Elements of CAM systems - NC in CAM - Rationale for CAD/CAM. Product Development and Design: Introduction - Life cycle of a product - Product Cycle - Simultaneous Engineering - Design for manufacture and assemblyDFMA - Rules for the design of parts - Total quality approach.
UNIT IV
43-59
Introduction-definition-basic configuration of robotics and its working-robot components0manipulator, end effectors, drive system, controller, sensors-mechanical arm-degrees of freedom-links and joints-construction of links, types of jointsclassification of robots-Cartesian, cylindrical, spherical, horizontal articulated (SCARA), vertical articulated-structural characteristics pf robots-mechanical rigidity-effects of structure on-control-work envelope and work volume-robot works volume and comparison-wrist rotations-mechanical transmission, pulley, belts, gears, harmonic drive-conversion between linear and rotary motion and its devices.
IMTSINSTITUTE.COM
UNIT V
60-91
Robot controller-level of controller-open loop and closed loop controller-servo systems-microprocessor based control system-robot path control-point to pointcontinuous path control-sensor based path control-controller programming-actuators-dc servo motors-stepper motors-hydraulic and pneumatic drives-feedback devicespotentiometers-optical encoders-dc tachometers.
UNIT VI Robot
92-115 application
in
manufacturing-material
handling-press
loading
and
unloading-die casting-M/c tool loading and unloading-spot welding, arc welding-spray painting-assembly finishing-adopting robots to work station- requisite and non-requisite robot characteristics-stages in selecting robot for individual application-precaution for robot-economic analysis-social and labour issues-future of robotics
UNIT QUESTIONS
116-118
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
1
UNIT I Introduction
CAD/CAM defined CAD/CAM is a term which means computer-aided design and computer-aided manufacturing. It is the technology concerned with the use of digital computers to perform certain functions in design and production. This technology is moving in the direction of greater integration of design and manufacturing. CAD can be defined as the use of computer system to assist in the creation, modification, analysis, or optimization of a design. The CAD hardware typically includes the computer, one or more graphics display terminal, keyboards, and other peripheral equipment. The CAD software consists of the computer programs and application programs. CAM can be defined as the use of computer systems to plan, manage, and control the operations of a manufacturing plant. The application of computer-aided manufacturing:
Computer monitoring and control: These are the direct applications in which computer is connected directly to the manufacturing process for the purpose of monitoring or controlling the process.
Manufacturing support applications: These are the indirect applications in which the computer is used in support of the production operation in the plant.
The difference between monitoring and control
In computer monitoring the flow of data between the process and the computer is in one direction only.
Computer
Process data
Process
FIGURE 1.1 Computer monitoring
In control, the computer interface allows for a two-way communication flow of data.
Process data
Computer
Control signals
IMTSINSTITUTE.COM
Process
CAD,CAM AND ROBOTICS
2
FIGURE 1.2 Computer control Computer-aided manufacturing also include indirect applications in which the computer serves a support role in the manufacturing operation of the planet. In these applications, the computer is not linked directly to the manufacturing process. The computer is used “off-line”.
Process data
Computer
Control signals
Process
FIGURE 1.3 CAM for manufacturing support The dashed lines are used to indicate that the communication and control link is an off-line connection, with human beings often required to communicate the interface.
Some examples of CAM for manufacturing support
Numerical control part programming by computers.
Computer-automated process planning.
Computer-generated work standards.
Production scheduling.
Material requirements planning.
Shop floor control.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
3
The product cycle and CAD/CAM The various steps in the product life cycle are presented in FIGURE 1.4.
Product concept
Design engineering
Customers and markets
Quality control
Drafting
Order new equipment and tooling
Process planning
Production
Production scheduling
FIGURE 1.4 Product cycle (design and manufacturing) The cycle is driven by customers and markets which demand the product. Depending on the particular customer group, there will be differences in the way the product cycle is activated. In some cases, the design functions are performed by the customer and the product is manufactured by a different firm. In other cases, design and manufacturing is accomplished by the same firm. Whatever the case, the product cycle begins with a concept, an idea for a product. This concept is cultivate, refined, analyzed, improved, and translated into a plan for the product through the design engineering process. The plan is documented by drafting a set of engineering drawings showing how the product is made and how the product should perform. The next activities involve the manufacture of the product. Scheduling provides a plan that commits the company to the manufacture of certain quantities of the product by certain dates. Once all these plans are formulated, the product goes into production, followed by quality testing, and delivery to the customer. Computer-aided design and automated drafting are utilized in the conceptualization, design, and documentation of the product. Computer are used
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
4
in process planning, and scheduling,
in production to monitor and control the manufacturing operation,
Quality control: – to perform inspection and performance test on the product and its components.
As given in FIGURE 1.5 CAD/CAM is overlaid on virtually all of the activities and functions of the product cycle.
Computeraided design
Product concept
Customers and markets
Quality control
Computer-automated drafting and documentation
Design engineering
Drafting
Order new equipment and tooling
Process planning
Production
Production scheduling
Computeraided process planning
FIGURE 1.5 Product cycle revised with CAD/CAM overlaid.
Computer-aided quality control
Computer controlled robots, machines, etc.
IMTSINSTITUTE.COM
Computerized scheduling, material requirements planning, shop floor control
CAD,CAM AND ROBOTICS
5
In design and production operations of a modern manufacturing firm, the computer has become a useful tool. Automation and CAD/CAM Automation was defined as the technology concerned with the application of complex mechanical, electrical, and computer-based systems in operation and control of production. Production activity can divide into four categories:
Categories
1. Continuous-flow process
2. Mass production of discrete products 3. Batch production
4. Job shop production
Description
Continuous dedicated production of large amounts of bulk product. Dedicated production of large quantities of one product. Production of medium lot sizes of the same product or components. The lots may be produced once or repeated periodically. Production of low quantities, often one of a kind, of specialized products. The products are often customized and technologically complex.
The relationships among the four types in terms of product variety and production quantities can be conceptualized as shown in Figure1.6. There is some overlapping of the categories as the figure indicates.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
Continues -flow process Production quantity
6
Mass production Batch Production
Product variety
Job production
Figure1.6 Four production types related to quantity and production variation Automation achievements for the four types of production. 1. Continuous-flow process:
Flow process form beginning to end.
Sensor technology available to measure important process variables.
Use of sophisticated control and optimization strategies. Flow computer-automated plants.
2. Mass production of discrete products:
Automated transfer machines.
Dial indexing machines.
Partially and fully automated assembly lines.
Industrial robots for spot welding, parts handling, machine loading, spray painting, etc.
Automated materials handling systems.
Computer production monitoring.
3. Batch production:
Numerical control (NC), direct numerical control (DNC), computer numerical control (CNC).
Adaptive control machine.
Robots for arc welding, parts handling, etc,
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
7
Computer-integrated manufacturing systems.
3. Job shop production:
Numerical control, computer numerical control.
The automated production systems implemented today make use of computers. However the cost of computers has decreased and their capabilities have increased, the economics feasibility of using computers in manufacturing and design has developed. The economics of high productions quantities have tended to Stimulate some of the most productive achievements in automation.
The difference between automation and CAD/CAM Let us consider the mathematical model of the product life cycle. This is a model of the amounts of time expended in designing, planning, and producing a typical product. Let T1 be the time required to produce 1 unit of product. This would be the sum of all the individual process times for each component in the product plus the time to assemble, inspect, and package a single product. Let T2 be the time associated with planning and setting up for each batch of production. T 2 would include the ordering of raw material by the purchasing department, time required in production planning to schedule the batch, setup times for each operation, and so forth. If the batch size is very large, the batch time can be spread among many units of production. If the batch is very small, the batch time would become relatively important. Finally, let T3 be the time required for designing the product and for all the other activities that are accomplished once for each different product. These include process planning, cost estimating and pricing, building of special tools and fixtures, and various other functions which must be done to get the product ready for production. Tow additional parameters are needed to complete the model. Let B equal the of batches produced throughout the product’s life cycle. And let Q be the number of units produced in each batch. We assume, for simplicity, that the batch size, Q, will always be the same for each batch. Accordingly, the total number of units produced during the life cycle is BQ. The aggregate time spent on the product thought its life cycle can be defined as TTLC = BQT1 + BT2 + T3 Where TTLC is the total time during the product life cycle. The total can be allocated evenly among the total number of units produced, BQ, to determine the average time spent on each unit of product during its life cycle. Calling this average time TLC, we have TLC = T1 + T2 / Q + T3 / BQ
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
8
The goal of the both CAD/CAM and automation is to reduce the various time elements in the product life cycle.
Computer Technology
Introduction
Computers are now in common use in both scientific and commercial field. Present computers are compact in size, reliable, available at low cost with high computing power which makes them to use in many applications. A computer system consists of hardware and software. The hardware includes Central Processing Unit (CPU) , Mass storage devices, input and output device, where as the software includes operating system (OS), modeling software, application software for design analysis and synthesis.
CPU
Input/Output
Control Unit
Arithmetic and Logic Unit
Memory
FIGURE 2.1. Basic hardware components of a computer There are three basic hardware components of a computer 1. Central Processing Unit. 2. Memory Unit. 3. Input/Output section. The central processing unit consists of two sub sections namely Control Unit, Arithmetic and Logic Unit. The control unit controls and co-ordinates the functions of all the other sections of the computer. It controls the information between CPU and Input/Output devices, and also between other sections of computer. It also commands the other sections of computer to perform their functions.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
9
ALU carries out arithmetic and logic manipulation of data such as add, subtract, divide and compare the data according to perform written. The software consists of the programs and instructions stored in memory and in external storage units.
Central Processing Unit (CPU)
The central processing unit (CPU) regulates the operation of all system components and performs the arithmetic and logical operations on the data. The CPU consists of two operating units: 1. Control Unit 2. Arithmetic-Logic (ALU) The control unit coordinates the various operations specified by the program instructions. These operations include receiving data which enter the computer and deciding how and when the data should be processed. The control unit directs the operation of the arithmetic-logic unit. It sends data to the ALU and tells the ALU what functions to perform on the data and where to store the results. The arithmetic and logic unit performs operations such as addition, subtraction, and comparisons. These operations are carried out on data in binary form. The logic section can also be used to alter the sequence in which instructions are executed when certain conditions are indicated and to perform other functions. Both the control unit and the arithmetic-logic unit perform their functions by utilizing register. Computer memory is small memory devices that can receive, hold, and transfer data. Each register consists of binary cells to hold bits of data. The arrangement of register in the computer’s CPU
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
10 Address connections
Program counter Input data connection
Memory address register
Output
Instruction register
data connection
Accumulator
Status register
Arithmetic-logic unit
FIGURE 2.2 Typical arrangement of register in the computer’s CPU The functions of register units
Program counter: The program counter holds the location or address of the next instruction. An
instruction word contains two parts: an operator and operand. The operator defines the type of arithmetic or logic operation to be carried out. The operand usually specifies the data on which the operation is to be performed. The program counter is incremented to go on the next instruction word.
Memory address register: This unit is used to hold the address if data held in memory. A computer may have
more then single memory addresses register.
Instruction register: The instruction register is used to hold the instruction for decoding.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
11
Accumulator: An accumulator is a temporary storage register used
during an arithmetic or logic operation.
Status register: Status register are used to indicate the internal condition of the CPU. A status
register is a 1-bit register (called a flags).
Arithmetic-logic unit (ALU): The ALU provides the circuitry required to perform the various calculations and
manipulations of data. A typical configuration of the arithmetic-logic unit is illustrated in FIGURE 2.3. The has two input for data, data outputs, and status outputs used to set the status register or flags.
Functions inputs
Data input A Data output C Data input B
ALU
Status output FIGURE 2.3
The two inputs, A and B, enter the ALU and the logical or mathematical operation is performed as defined by the function input. The ALU places the result of the operation on A and B in the output, C, for transfer to the accumulator.
Types of memory
The memory section consists of binary storage units which are organized into bytes. Computer words can typically be 4, 8, 12, 16, 32, or 64 bits long. Each word has an address in the memory. The
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
12
CPU calls words from memory by referring to the word address. The time required to find the correct address and fetch the contents of that memory location is called access time. The memory section stores all the instruction and data of a program. Thus the CPU must transfer these instructions and data to and from the memory throughout the execution of the program. Types of memory 1. Main Memory (Primary storage). 2. Auxiliary memory (secondary storage). Main Memory (Primary storage) The main memory or primary storage is terms which designate storage areas that are physically a part of the computer and connected directly to the CPU. Primary storage can be divided into three
Main data storage, such as magnetic core or solid-state memory. Fast access rate, low storage capacity, and very high cost.
Control storage, which commonly contains the micro programs that that assist the CPU circuitry in performing its functions.
Local storage, the high-speed working register used in the arithmetic and logical operations.
Auxiliary memory (secondary storage) Programs and data files are not generally kept in primary storage but are stored on large-capacity auxiliary devices and loaded into main memory as required. Main storage is very expensive, and has a rather limited capacity, so the entire file is stored on an auxiliary device and individual records are accessed as needed by the program. There are two types of secondary storage: 1. Sequential access storage: A sequential access storage unit is distinguished by the fact that to read one particular record in the file, all records preceding it must also be read. 2. Direct access storage: With this storage method, individual records can be located and read immediately without reading any other records. Sequential access storage is suitable for applications that do not require a high level of file activity. Direct access storage is best suited to files where a high level of activity is involved. List of hardware devices used for computer storage technology
Magnetic tape storage is a prime example of sequential access storage technology.
Magnetic drum storage is a random access storage device with high access rates.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS ďƒ˜
13
Magnetic disk storage is a direct access storage device.
Input /Output
The purpose of the input/output section of the computer is to provide communication with peripheral devices used with the computer system.
There are inverse function 1. Programs and data are read into the computer. The I/O section must interpret the incoming signals and hold them temporarily until they are placed in main memory or into the CPU. 2.
The result of the calculations and data processing operation must be transmitted to the appropriate peripheral equipment.
Common peripheral devices used for computer input/output 1. Card Readers: A card reader transfers data form the punched card (data–recording medium) to the computer system. 2. Card punches: A card punch records the output from the computer onto punched cards. Its speed range form 100 to 300 cards per minute. 3. Magnetic tape unit: Magnetic tape unit can be used for program and data storage, and they can be interfaced to the computer as both input and output units. The tape is moved past a read/write head, usually at constant speed. 4. Keyboard input devices: Input devices employ a typewriter like keyboard. It inputs data and programs directly to the computer 5. The keypunch: The keypunch is an electromechanical keyboard device which converts operator keystrokes into machine-readable holes on cards. The cards are then submitted through a card reader to the computer. 6. Punched tape punches: Data from a computer system can be outputted onto punched paper tape. Data from main storage are converted into the appropriate code and punched on the tape as it is fed through the punching unit. 7. Alphanumeric displays: An alphanumeric display consists of a typewriter like keyboard and a display screen, usually a cathode ray tube (CRT). 8. Teleprinters: A teleprinter consists of an electromechanical or electronic typewriter keyboard and a hard-copy printing device.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
14
9. Magnetic ink character recognition (MICR): MICR readers are electronic devices that operate by interpreting the sensed waveforms of the individual magnetic ink characters.
10. Optical character recognition (OCR): In this a mechanical drum is used to rotate documents past an optical scanning station. A light source and lens system can distinguish the patterns of the character. 11. Optical bar-code reader (OBR): An optical bar code reader senses; the configuration of shaded bars of different widths and correlates them to previously defined characters. It can read 50 to 400 characters per second. 12. Line printers: The volume of output is sufficient to justify a high-speed line printer. These units print entire lines at one cycle at rate that may exceed 1000 line per minute. The recent highspeed printers combine laser and xerography technologies to achieve print speeds of about 10,000 lines per minute.
1.8 Data Representation
The symbols used by the computer are based on electrical signals that can take one of two states. The smallest unit of data is the bit. It has two possible values 1 or 0. The conversion process between the higher-level characters and the basic units of data used by the computer (the bit). The binary number system The bit, or binary digit, is the basic unit of data which can be interpreted by the digital computer. It uses only two digits 0 and 1. The meaning of successive digits in the binary system is based on the 0
1
2
number 2 raised to successive powers. The first digit is 2 , the second 2 , the third 2 and so forth. Data is represented in a computer system by either the presence or absences of electronic signals in its circuitry. This is called binary or two-state representation of data. Binary and Decimal Number System Equivalence
Binary
Decimal
0000
0
0001
1
0010
2
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
15
0011
3
0100
4
0101
5
0110
6
0111
7
1000
8
1001 9 The conversion of binary to decimal systems 1*2
2+
1+
0*2
0
1 * 2 = 4 + 0+ 1 = 5
A minimum of four digits are required in the binary system to represent any single-digit number in the decimal system. By using more then four binary digits, higher valued decimal numbers or other highlevel data can be represented. An alternative way to represent decimal numbers larger than nine involves separate coding of each digit, using four binary digits for each decimal digit. This coding system is known as binary-coded decimal (BCD). Common binary coding schemes Binary-Code Decimal (BCD) system uses a total of 7 bits. The first 6 represent the data itself. The last bit position is used as a parity check. The first 4 bits are called numeric bits. The fifth and sixth bit is called zone bits. The zone bits are both zero when numeric characters are represented. Combination of the zone and numeric bits can be used to code the alphabetic and special characters. EBCDIC The maximum number of character a computer code can represent is 2 raised to the power 6
equal to the number of bits. Thus BCD allows for 2 = 64 distinct characters. EBCDIC (Extended BinaryCoded Decimal Interchange Code) uses an 8-bit code plus a parity bit so that it can define 256 distinct characters. These include upper and lowercase alphabetic, the numerals, many special characters and control characters for I/O devices. ASCII American Standard Code for Information Interchange was developed for telecommunications to simplify machine-to-machine and system-to-system communications. It is a 7-bit code, which provides 128 bits patterns for character representation.
Computer Programming Language
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
16
Data and instructions are communicated to the computer in the form binary words. In executing a program, the computer interprets the configuration of bits as an instruction. The sequences of these binary-coded instructions define the set of calculations and data manipulation by which computer executes the programs. Three levels of computer programming languages: 1. Machine language 2. Assembly language 3. Procedure-oriented (high level) languages Machine and assembly languages The language used by the computer is called machine language. It is written in binary, with each instruction containing an operation code and an operand. The operand might be a memory address, a device address, or data. In machine language programming, storage locations are designated for the program and data, and these are used throughout the program to refer to specific data or program steps. Machine language instructions are different for each computer. Programming in machine language is tedious, complicated, and time consuming. To alleviate the difficulties in writing programs in binary, symbolic languages have been developed which substitute English like language mnemonics instructions. Mnemonics are easier to remember than binary, so they help speed up programming process. A language consisting of mnemonics instructions is called an assembly language. Assembly languages are considered to be low-level language. It is converted into machine languages before the computer can execute them. The conversion is carried out by a program called an assembler. High level language High level language consists of English-like statements and traditional mathematical symbols. Each high-level statement is equivalent to many instructions in machine language. The advantage of high-level language is that it is not necessary for the programmer to be familiar with machine language. High- level languages must also be converted to machine code. This is accomplished by a special program called a compiler. The compiler takes the high-level program, and converts it into a lower-level code, such as the machine language. If there are any statements errors messages are printed in a special program listing by the compiler. There are many different high-level languages FORTRAN
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
17
FORTRAN stands for FORmula TRANslation. It was developed in the mid-1950s for scientists, engineers, and mathematicians.
COBOL COBOL stands for COmmon Business-Oriented Language. It was developed around 1959. It has become a major computer language for business data processing applications. BASIC BASIC stands for Beginner’s All-purpose Instruction Code. It was developed in the 1960s at Dartmouth College to be an east-to-learn language. It was developed as an interactive language. APL APL stands for A Programming Language. It was designed for interactive problem solving. A significant feature is that it permits users to define complex algorithms efficiently. The primary data structure in APL is in the form of arrays and an extensive set of array operators is provided. RPG RPG stands for Report Program Generator. It is a language designed for writing programs that produce printed reports as output. It is widely used in business environment, where it updates data files, performs analysis of data and generates documents and reports. PL/I PL/I are a general purpose programming language. It is a flexible language which meets the needs of a wide variety of programmers. PASCAL PASCAL is high-level language developed in the early 1970s. The objectives in developing the language were to facilitate the teaching of computer programming as a discipline of knowledge and to accomplish programming implementations which are reliable and efficient on modern computers.
Operating the Computer System
Technological improvements have been made computer system much easier to operate and much more efficient. For example, in early system, the CPU was forced to be idle a slow input/output device transferred data to and form main memory. In modern computer systems, input/output and data processing operations can occur simultaneously to make the operation of the computer system more efficient. Techniques which facilitate the operation of the computers system by the user.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
18
1. I/O control system and operating systems 2. Virtual storage 3. Time sharing 4. Distributed processing
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
19
UNIT II Computer Fundamentals of CAD Introduction The CAD SYSTME DEFINED Computer –aided design involves any type of design activity which makes use of the computer to develop, analyzer, or modify an engineering design. Fundamental reasons for implementing a CAD 1. To increase the productivity of the designer. 2. To improve the quality of design. 3. To improve communications. 4. To create a data base for manufacturing.
The Design Process The process of designing consists of six steps or phases: 1. Recognition of need 2. Definition of problem 3. Synthesis 4. Analysis and optimization 5. Evaluation 6. Presentation
Recognition of need involves the realization by someone that a problem exists for which
some corrective action should be taken in a current machine.
Definition of the problem involves thorough specifications of the item to be designed.
Synthesis and analysis are closely related and highly iterative in the design process. The
process the repeated until the design has been optimized within the constraints imposed on the designer.
Evaluation design has been accomplished on drawing boards, with the design
documented in the form of detailed engineering drawing. Fig.2.1 shows the basic steps in the design process.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
20
Fig.2.1 Basic Steps in the Design Process
The Application of Computers for Design The various design-related tasks which are performed by a modern compute-aided design system and can be grouped into four functional areas:
1. Geometric modeling 2. Engineering analysis 3. Design review and evaluation 4. Automated drafting Design process, illustrated in Fig. 2.2.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
21
Fig. 2.2 Application of Computers to the Design Process
Geometric modeling Geometric modeling is concerned with the computer compatible mathematical description of the geometry of an object. The basic methods for representing the object in geometric modeling are wireframe.
Engineering analysis The analysis may involve stress-strain calculations, heat transfer computations, or the use of differential equations to describe the dynamic behavior of the system being designed. Two examples are i. Analysis of mass properties ii. Finite-element analysis Design review and evaluation A procedure called layering is often helpful in design review. This procedure can be preformed in stage to check each successive step in the processing of the part. Another related procedure for design review is interference checking. One of the most interesting evaluation features available on some computer aided design systems is kinematics Automated drafting It involves the creation of hard-copy engineering drawing directly from the CAD data base.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
22
Parts classification and coding It involves the grouping of similar part designs into classes, and relating the similarities by means of a coding scheme.
Creating the Manufacturing Database The important reason for using a CAD system is that it used to develop the database need for manufacture the product. The conventional manufacturing is used in industry, engineering drawings were prepared by design draftsmen and then used by develop the process plan. The activities involved in designing the product were separated from the activities associated with process of planning. The manufacturing database is an integrated CAD/CAM database. It includes all the data on the product generated during design (geometry data, material specification, parts lists and bill of materials, etc.) as well as additional data require for manufacturing, much of which is based on the product design (Fig. 2.3). CAD
CAM
Tool and fixture design
Geometric Modeling
Interactiv e graphics
Engineerin g analysis Data base Design review and evaluation
Automated drafting
Numerical control Programmin g
Production
Computer –aided process planning Production planning and scheduling
Fig 2.3 Desirable Relationship of Cad/Cam Database to Cad And Cam
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
23
Benefits of Computer-Aided Design There are many benefits of computer-aided design, only some of which can be easily measured. Some benefits are intangible, reflected in improved work quality, more pertinent and usable information and improved control, all of which are difficult to quantify. Other benefits are tangible, but the saving from them shows far downstream in the production process. The benefits of CAD: 1. Productivity improvement in design 2. Design Analysis 3. Fewer design errors 4. Greater accuracy in design calculations 5. Standardization of design, drafting, and documentation procedures 6. Drawings are more understandable 7. Improve procedures for engineering 8. Benefits in manufacturing
Productivity improvement in design: The productivity improvement in CAD as compared to traditional design process in dependent on:
Complexity of the engineering drawing
Level of detail required in the drawing
Degree of repetitiveness in the designed parts
Extensiveness of library of commonly used entities
Design analysis: The design analysis helps to consolidate the design process into a more logical work pattern. Rather than having a back-and-forth exchange between design and analysis groups, the same person can perform the analysis while remaining at the CAD workstation. This helps to improve the concentration of designers, since they are interacting with their designs in real-time sense. This helps designs can be created which are closer to optimum and time saving. Its finds way from the designer’s drawing board to design analyst’s queue and back again. Fewer design errors: The Cad system provides an intrinsic capability for avoiding design, drafting, and documentation errors. In manual compilation data entry, transposition, and extension errors are occurred and it eliminated virtually. Errors are further avoided by perform time-consuming repetitive duties such as multiple symbols placement, and sorts by area and by like item, at high speeds with consistent and accurate result. Greater accuracy in design calculations: The accuracy delivered by interactive CAD systems is three-dimensional curved space designs provide by manual calculation methods that is no real comparison. Computer-based accuracy pays off in many ways,
Parts are labeled by the same recognizable nomenclature and number throughout all drawings.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
24
A change entered on single item can appear throughout the entire documentation package.
Standardization of design, drafting, and documentation procedures: The single database and operating system is common to all workstation in the CAD system. Consequently, the system provides natural standards for design/drafting procedures. With interactive CAD, drawings are
“standardized” are drawn .
no confusion in proper procedure because the entire format is “built into” the system program.
Drawings are more understandable: In general, visualization of drawing related directly to projection used. Orthographic view is less comprehensible than the isometric. Finally, animation of images on CRT screen allows for even greater visualization capability. The various relationships are illustrated in Fig. 2.4.
Orthographic
Oblique
isometric
perspective
Fig. 2.4 Improvement in Visualization of Images of Various Drawing Types and Complete Graphics Features.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
25
Improve procedures for engineering: Control and implementation of engineering changes is significantly improved with CAD. Original drawings and reports are in database of CAD system. This makes them more accessible than document kept drawing Vault and quickly checks the new information. Since data storage is extremely compact, historical information from previous drawings can be easily retained in the system’s database, for easy comparison with current design and drafting needs. Benefits in manufacturing: The following benefits are derived largely from the CAD/CAM database, whose initial frame work is established during CAD.
Numerical control part programming
Tool and fixture design for manufacturing
Computer aided process planning
Assembly lists (generated by CAD) for production
Computer-aided inspection
Robotics planning
Group technology
Shorter manufacturing lead times through better scheduling
Hardware in Computer-Aided Design
Introduction The hardware is restricted to CAD system that utilizes interactive computer graphics. Typically, a stand-alone CAD system would include the following hard ware components: 1. One or more design workstations. These would consists of:
A graphics terminal
Operator input devices
2. One or more plotters and other output devices 3. Central processing unit (CPU) 4. Secondary storage These hardware components would be arranged in as given in Fig.2.5. The various hardware components and the alternatives and options can be obtained in each category.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
26
Secondary storage
Graphics terminal
CPU
Input devices
Output plotters, etc
Fig.2.5 Configuration of Hardware Components in a Stand-Alone Cad System
The Design Workstation It represents the significant in determining how convenient and efficient it is for a designer to use the CAD system and its function s are:
It must interface with the central processing unit.
It must generate a steady graphic image for user.
It must provide the digital descriptions for the graphic image.
It must translate computer commands into operating functions
It must facilitate communication between the user and system.
The use of interactive graphics has been found to be the best approach to accomplish these functions. Atypical interactive graphics workstation would consist of the following hardware components: Hardware Components
A graphics terminal
Operator input devices
The Graphics Terminal
The current technology in interactive computer graphics terminals are: 1. Image generation in computer graphics 2. Graphics terminals for computer-aided design
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
27
3. Color and animation in computer graphics
Image generation in computer graphics: Nearly all computers graphics terminals available today use the cathode ray tube (CRT) as the display device (Fig 2.6). A heated cathode emits a high-speed electron beam onto a phosphor-coated glass screen. The electrons energize the phosphor coating, causing it to glow at the points where the beam makes contact. By focusing the electron beam, changing its intensity, and controlling its point of contact against the phosphor coating through the use of a deflector system, the beam can be made to generate a picture on CRT screen.
Fig 2.6 Cathode Ray Tube
The two basic techniques used in current computer graphics terminals for generating the image on the CRT screen are: ďƒ˜
Stroke writing
ďƒ˜
Raster scan
Stroke writing: Other names for the stroke-writing techniques include line drawing, random position, vector writing, and directed beam. It moves from one point on the screen to the next, where each point is defined by its x and y coordinates. The process is portrayed in Fig. 2.7.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
28
Fig 2.7 Stroke Writing For Generating Images in Computer Graphics
Raster scan: The other name for raster scan technique includes digital TV and scan graphics (Fig. 2.8). Here the viewing screen is divided into larger number of discrete phosphor picture elements called pixels. The number of separate pixels are typically range from 256×256 (a total over 65,000) to 1024×1024 ( a total over 1,000,000).Color screen provides for them pixels to have different brightness.
Fig 2.8 Raster Scan Approach For Generating Images Graphics
Graphics terminal for computer-aided design It includes the type of phosphor coating on the screen, whether color is required, the pixel density, and the mount of computer memory available to generate the picture. The three types are:
Directed-beam refresh
Direct-view storage tube(DVST)
Raster scan(digital TV)
Color and animation in computer graphics The capabilities for multicolored images and animated pictures in computer graphics are largely dependent on the hardware considerations and the relative capabilities for the three types of commercial graphics terminals for color and animation. Advantages:
Finite-element analysis results displayed in color shows the improved clarity of information provided by color in displaying the results in of a finite elements in analysis.
Wire –frame model of jet engine with different sections of the assembly displayed in various colors.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
29
Color display of industrial plant model for industrial building, showing packing lot, landscaping and etc.
Solid modeling assembled for water cooled power frame and display various color.
Operator Input Devices
Operator input devices are provided at the graphics to facilitate convenient communication between the user and system. Workstations generally have several types of input devices to allow the operator to select the various preprogrammed input functions. These functions permit the operator to create or to modify an image on CRT screen or to enter alphanumeric data into system and it results complete part on the CRT screen as well as a complete description of the part in the CAD database. These devices are can be divided into three general categories: 1. Cursor control devices 2. Digitizers 3. Alphanumeric and other keyboard terminals The cursor control devices and digitizer are both used for graphics interaction with the system. Keyboard terminals are used as input devices for commands and numerical data. There are two basic types of graphical interaction accomplish by means of cursor control and digitizing:
Creating and positioning new items on the CRT screen
Pointing at or otherwise identifying locations on the screen, usually associated with existing images.
Cursor Control The cursor normally takes the form of a bright spot on the CRT screen that indicates where lettering or drawing will occur. The computer is capable of reading the current position of the cursor. Varity of cursor control devices
Thumbwheels
Direction keys on a keyboard terminal
Joysticks
Trackball
Light pen
Digitizers The digitizer is an operator input devices which consists of a large, smooth board and an electronic tracking device which can be moved over the surface to follow existing lines. It is
common
technique in CAD system for taking x, y coordinates from a paper drawing. Keyboard terminal Several forms of keyboard terminals are available as CAD input devices. The most familiar type is the alphanumeric terminal. The alphanumeric terminal may be CRT or hard-copy terminal.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
30
The alphanumeric terminal is used to enter commands, functions, and supplemental data to the CAD system. This information is displayed for verification on the CRT or typed on paper. The system also communicates back to the user in a similar manner.
Plotters and Other Output Devices
There is various output devices used in conjunction with a computer-aided design system. These output devices include:
Pen plotters
Hard-copy units
Electrostatic plotters
Computer-output-to-microfilm (COM) units
Pen plotters The accuracy and quality of the hard-copy plot produced by a pen plotter is considerably greater then the apparent accuracy and quality of the corresponding image on the CRT screen. Two types of pen plotters 1. Drum plotters 2. Flat-bed plotters Hard-Copy Unit A hard-copy unit is a machine that can make copies from the same image data displayed on the CRT screen. The image on the screen can be duplicated in a matter of seconds. The copies can be used as records of intermediate steps in the design process or when rough hard copies of the screen are need quickly. The hard copies are not suitable for final copies.
Electrostatic plotters The electrostatic plotter is almost as fast and accurate image on the CRT screen. The limitation is that the data must be in the raster format. If the data are not in raster format, some type of conversion is required to change them into required format. Advantage is which is shared with the drum-type pen plotter is that the length of the paper is virtually unlimited. Computer-output-to-microfilm (COM)units COM units reproduce the drawings on microfilm rather then as full-size engineering drawings. It is expensive equipment. One advantage is storage capacity and speed. Disadvantage of the COM process are that the user cannot write notes on the microfilm as is possible with a paper copy.
Secondary Storage Secondary storage devices are magnetic disk and magnetic tape. The purpose in using secondary storage is to reduce the cost of main computer memory. The secondary storage con be used for engineering drawing files, CAD software which can be transferred to main memory as needed, and
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
31
temporary files for CPU output which will be downloaded to individual graphics terminals, plotters, or other output devices. Disk has the advantage of fairly rapid retrieval, owing to their random access configuration. Because of this feature, the CPU can load and swap programs and files between primary and secondary memory as needed. Magnetic tape would be used for storing programs and files which are less frequently used by the system. Storage on magnetic tape is less expensive then on disks. It would be suitable for disk backup, permanent archival files, and data transfer to output devices or other computers.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
32
UNIT-III INTRODUCTION: CAM means Computer aided manufacturing. It can be defined as the use of computer systems to plan, manage and control the operations of a manufacturing plant through either direct or indirect computer interface with the plant production resources.
COMPUTER AIDED MANUFACTURING (CAM): By introducing the computer the following factions are obtained in the manufacturing industries with less manual effort. 1. Planning 2. Managing and 3. Controlling the operations The computer interface with the plant resources either directly. Therefore, the applications of CAM can be broadly classified into two groups, 1. Computer monitoring and control 2. Manufacturing support applications
1. COMPUTER MONITORING AND CONTROL The computer is directly interfaced with manufacturing tool, for monitoring and control the functions in the manufacturing. The distinction between monitoring and control is shown in fig 1.1
Computer
Process Fig.1.1.a. Computer Monitoring
Process Data
Control Signals
Computer
Process Fig.1.1.b. Computer Control Fig.1.1. Computer Monitoring and Control
2. Manufacturing Support Application There are indirections, which deal essentially with the preparations for actual manufacturing and post manufacturing operations. In this, the computer is used “off-line� to provide. *
Plans
*
Schedules
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS *
Forecasts
*
Information
33 *
Instruction and
By which the resources are utilized more affectivity and economically, the symbolic representation for the CAM as shown in fig.1.2.
Manufacturing operations
Computer
Fig.1.2 CAM for Manufacturing Support
The interrelation of CAM activities as shown in Fig.1.3. Factory Shop Floor
Process Plant
Automatic Measurement & Testing
Quality Control
Materials Handling & Automated Assembly
Robotics Process & Plant Engineering M/c Tool Control
Production control
Material Requirement & Planning
Fig.1.3. Interrelation of CAM activities
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
34
HIERARCHY
The computer interface in the manufacturing unit, carried out the following functions, effectively,
Cost estimating
Computer Aided Process Planning
Machinability data system
Computer assisted NC part programming
Development of work standard
Production and inventory planning
Manufacturing control systems.
1. Cost Estimating: By using the computer system, the total expenditure for the new product, is calculated. For this, the material cost, overheads etc are considered.
2. Computer Aided Process Planning: Preparing a list of operation sequence required to process a particular component, root sheet preparation are done with the aid of computer.
3. Machinability Data: It is the information like speed ,feed etc for machining the component.
4. Computer Assisted NC Part Programming: Preparation of NC part program required for machining the component in NC machine using the computer.
5. Development of Work Standards: It is determination of time standard for a particular production operation by using computer.
6. Production and Inventory Planning: The computer determines an appropriate schedule for meeting production requirements.
7. Manufacturing Control Functions: The manufacturing control functions like process control, quality control, shop floor control and process monitoring are done by using computer.
Elements of CAM system 1. Numerical control part program by computers. Control program are prepared for automated machine tools. 2. Computer –automated process planning. The computer prepares a listing of the operation sequence required to process a particular product or component. 3. Computer-generated work standards. The computer determines the time standard for a particular production operation.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
35
4. Production scheduling. The computer determines an appropriate schedule for meeting production requirements. 5. Material requirement planning. The computer is used to determine when to order raw materials and purchased components and how many should be ordered to achieve the production scheduling. 6. Shop floor control. In this CAM application, data are collected from the factory to determine progress of the various production shop orders.
NC in CAM The control systems and machine tools in numerically controlled machine tools have varying complexities and capabilities. In the initial stages, the NC machine tools had NC systems added to the machine but only to control the position of the work piece relative to the cutting tool. The operator had to select the cutting tools, speed and feeds etc. in the next stages, the capabilities of the machine tools improved and in addition to maintaining cutter/workpiece relationship, the material removal was also controlled by the numerical control system. The mechanical design of the machine tool was also improved with the development of recirculation ball screw and better slide ways. These machines are referred to as NC machines. The instructions to the NC machines are fed through an external; medium i.e paper tape or magnetic tape. The information read from the tape is stored into the memory of the control system called ‘buffer storage’ and is processed by the machine step by step. So when the machine is working on one instruction block, the next block is read from the tape and stored in the memory of the machine control system. Since the part cannot be produced without a tape being run through the machine, these machines are also called tape controlled machines. The tape has to be run repeatedly depending on the number of components to be produced. Also if there is even a minor change in the design of the component, the tape has to be discarded and new tape with changed programme has to be produced.
Rational for CAD/CAM
The rational for CAD/CAM arises for the following reasons
To improve the production rate.
To upgrade the firm to manufacture the variety of jobs
It must able to give assembly list.
It must benefit shorter lead time
It must benefit group technology
It must benefit better scheduling
It must improve productivity in manufacturing.
1.7 Product development and design-Introduction
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS Product development and
36 design is an essential process made before manufacturing a
component. Effective product development and design is made to improve customer satisfaction and profit of the company. The product design may be made for a new product or for old product which need some improvement. Product design is achieved after getting feedbacks from customer and sales people. The following are the factors to considered for any product design. 1. Material of the product 2. Dimensions and tolerances 3. Appearance of the product 4. Standard of performance 5. Quality of the product.
Life Cycle of a Product A new product progresses through a sequence of stages from introduction to growth, maturity, and decline. This sequence is known as the product life cycle and is associated with changes in the marketing situation, thus impacting the marketing strategy and the marketing mix. The product revenue and profits can be plotted as a function of the life-cycle stages as shown in the graph below:
Introduction Stage In the introduction stage, the firm seeks to build product awareness and develop a market for the product. The impact on the marketing mix is as follows: 
Product branding and quality level is established and intellectual property protection such as patents and trademarks are obtained.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
37
Pricing may be low penetration pricing to build market share rapidly, or high skim pricing to recover development costs.
Distribution is selective until consumers show acceptance of the product.
Promotion is aimed at innovators and early adopters. Marketing communications seeks to build product awareness and to educate potential consumers about the product.
Growth Stage In the growth stage, the firm seeks to build brand preference and increase market share.
Product quality is maintained and additional features and support services may be added.
Pricing is maintained as the firm enjoys increasing demand with little competition.
Distribution channels are added as demand increases and customers accept the product.
Promotion is aimed at a broader audience.
Maturity Stage At maturity, the strong growth in sales diminishes. Competition may appear with similar products. The primary objective at this point is to defend market share while maximizing profit.
Product features may be enhanced to differentiate the product from that of competitors.
Pricing may be lower because of the new competition.
Distribution becomes more intensive and incentives may be offered to encourage preference over competing products.
Promotion emphasizes product differentiation.
Decline Stage As sales decline, the firm has several options:
Maintain the product, possibly rejuvenating it by adding new features and finding new uses.
Harvest the product - reduce costs and continue to offer it, possibly to a loyal niche segment.
Discontinue the product, liquidating remaining inventory or selling it to another firm that is willing to continue the product.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
38
The marketing mix decisions in the decline phase will depend on the selected strategy. For example, the product may be changed if it is being rejuvenated, or left unchanged if it is being harvested or liquidated. The price may be maintained if the product is harvested, or reduced drastically if liquidated. Product cycle Product cycle is to examine the various activities and functions that must be accomplished in the design and manufacture of a product. Fig shows the various steps in the product cycle. The cycle is driven by customer and markets which demand the product. It is realistic to think of these as a large collection of diverse industrials and consumer markets rather than one monolithic market. Depending on the particular customer group,there will be differences in the way the product cycle is activated . in some cases, the design functions are performed by the customer and the product is manufactured by a different firm. In other cases, design and manufacturing is accomplished by the same firm. Whatever the case, the product cycle begins with a concept, an idea for a product. This concept is cultivated,refined,analysed,improved and translated into a plan for the product through the design engineering process. The plan is documented by drafting a set of engineering drawings showing how the product is made and providing a set of specifications indicating how the product should perform.
Product concept
customers and Markets
Design engineering
Order new equipment and tooling
Quality control
Production
IMTSINSTITUTE.COM
Drafting
Process planning
Production scheduling
CAD,CAM AND ROBOTICS
39
Simultaneous Engineering: It is a newly emphasized approach that considers design and development of a product concurrently with other related functions of manufacture and support. This approach considers all the factors at the outset e.g., quality, cost, maintenance and repair. It means product design engineering occurs simultaneously with process design engineering. So it yields best-results for both the product developers and consumers.
This approach is getting closer attention because of stringent demands for product quality and shorter times to market. In CIM environment, all relevant information required product design and manufacturing issues can be made available and can be considered simultaneously. Based on current practices, internal partners can work as a unified team from conception to completion of product as ultimate level of concurrent engineering. For implementing the approach of concurrent
Design For Manufacturing And Assembly (DFMA)
The concepts of CAD/CAM are now quite mature and the modern industrial units have developed the capability in its widest scope, as such it becomes imperative that the concept of Design for Manufacturing (DFM) has been drawing more and more attention. To be competitive in today’s marketplace requires a single engineering effort to integrate the process of design to the process of manufacturing. DFM is basically the integration of product-design activities and process-planning and manufacturing activities, i.e., one common integrated activity through Computer Integrated Process Planning (CAPP). The industry has recognized the fact that it cannot meet the requirements of quality at fairly priced product with isolated design and manufacturing activities.
In Non-CIM environment, the interaction between design and production functions has always been weak. There is not doubt that weak integration gives both the design and manufacturing departments sufficient flexibility and independence in achieving their own goals and objectives independently, but fail to achieve the common objective of quality-product at cost-effective price.
The matured concept of CAD/CAM and CIM-environment has forced the management and engineers to bring changes in their attitude primarily because in long run cost-effective improvement in the product quality are possible through improving the design. It is, therefore, obvious that in CIMenvironment, the perfection in product-design cannot be considered without taking into account the manufacturing viewpoint.
Since computers can easily accommodate last minute changes made either in design or subsequently in process planning and production schedule, thus, perfecting the design becomes an easier process. So, the designer must pay more attention to the ideas and drawings from
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
40
manufacturability point of view i.e., the so designed product can be produced with the available manufacturing resources and workforce provided detailed information on the capabilities of existing manufacturing – resources is available.
The enormous power of CAD-CAM work station assists in integration of manufacturing resources and design-process to ensure best matching to the needs of customer. DFM emphasizes an easy integration of design and production functions in CAD/CAM environment. DFM simply reinforces the need that within the functional requirements of product, designers shall consider the manufacturability of their design.
Conclusively, it is intended that DFM integrates product-design, process-planning and manufacturing-resources with the objectives of: Identifying the products that are functionally sound and are inherently easy to manufacture. Focusing on component-design for ease of manufacture. Integrate product design with process design to achieve optimum results.
Looking to the complexities involved in the conceptual framework of DFM, it is still convenient to divide the concept of DFM into two components.
DFM guidelines are systematic and codified statement of good-design-practices empirically derived from years of design and manufacturing experience. Derived guidelines are to be strictly followed.
Derive a product with minimum number of parts to achieve higher reliability Develop modular design Minimize part variation Design the parts to achieve the following Design the parts of multi-functional and for multi-use Design the parts for easy fabrication and easy manufacturing (with the available resources). Avoid separate fasteners Minimize assembly direction i.e., design for top-down assembly Maximize compliance to design for easy assembly Design for easy handling and packaging.
In the broadest sense, the term DFM includes not only production but also the assembly of components. In plants, where Assembly is the major activity, there also DFM starts from production of components and goes well beyond it, to include assembling and other downstream functions. Such functions are designated as Design For Manufacture and Assembly (DFMA). This concept was developed by Boothroyd and Dewhurst. DFMA also aims to achieve high quality product; easy to use and affordable.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
41
Rules for the design of parts
1. Design for function: The product designed should perform the various functions expected by the customers. The factors affecting the function of a product are its strength and durability. 2. Design for reliability The product design should work consistently without more troubles. It should function smoothly and efficiently for a long time. 3. Design for maintainability The product design should be easily and economically maintained for a long duration to satisfy the customers. The maintenance of the product should not be complicated.
4. Design for safety The product designed should ensure the safety of the customer. It should give minimum hazard to the customer and the environment. 5. Design for selling The product design should be attractive in appearance. The shape, size, colour of the product should enhance the aesthetic sense among the customer.
1.13 Total quality approach Quality is excellence which leads one firm’s product to dominate another and to guarantee its survival by establishing a new standard of quality. In this sense quality is a mark of excellence, persistence and maintained over long periods of time. Such excellence is, of course a function of habits, culture and values and may thus vary from person to person and from time to time. The concept and vocabulary of quality are difficult to express. Different people interpret quality differently. Few can define quality is measurable terms that can be operationalized. When asked what differentiates their product or service the banker will answer”service”, the health care worker will answer “quality health care”, the hotel or restarunt employee will answer “customer satisfaction”, and the manufacturer will simply answer “quality product”. There is an old saying in management which says” if you cannot measure it, you cannot manage it “, and so it is with quality. If the strategic management system and the competitive advantage are to be based on quality, every member of the organization should be clear about its concept, definition and measurement as it implies to his job. Harvard Professor David Garvin, in his book Managing Quality summarized five principle approaches to defining quality 1. Transcendent 2. Product based
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
42
3. User based 4. Manufacturing based 5. Value based
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
43
UNIT-IV ROBOTICS INTRODUCTION Robotics, in different forms, has always been on people’s mind, since the time we first built things. You may seen machines that artisans made that tried to mimic a human’s motions and behaviors. Two examples of such machines are the statues in Venice that hit the clock on the hour and toys with repeating movements. Holly-wood and movies have taken this desire one step further by portraying robots and humanoids as even superior to humans. Robotics are very powerful elements of today’s industry. They are capable of performing many different tasks and operations precisely and do not require common safety and comfort elements humans need. However, it takes much effort and many resources to make a robot functions properly. Most companies that made robot in the mid-1980s no longer exist, and only companies that made industrial robots remain in the market (Such as Adept Robotics, Staubli Robotics, and Fanuc Robotics, North America, Inc.,). Early predictions about the possible number of robots in industry never materialized, because high expectations could not be satisfied with the present robots. As a result, although there are many thousands of robots in industry, they have not overwhelmingly replaced workers. They are used where they are useful. As with humans, robots can do certain things, but not other things. As long as they are designed properly for the intended purpose, they are very useful and will continue to be used. The subject of robotics covers many different areas. Robots alone are hardly ever useful. They are used together with other devices, peripherals, and other manufacturing machines. They are generally integrated into a system, which as a whole is designed to perform at task or do an operation. In this book, we will refer to some of these other devices and systems that are used with robots.
DEFINITION Robotics is the are, knowledge base, and the know-how of designing, applying, using robots in human endeavors. Robotics systems consist of not just robots, nut all other devices and systems that are used together with the robots to perform the necessary tasks. Robots may be used in manufacturing environments, in underwater and space exploration, for aiding the disabled, or even for fun. In any capacity, robots and be useful, but need to be programmed and controlled. Robotics is an interdisciplinary subject that benefits from mechanical engineering, electrical and electronic engineering, computer science, biology, and many other disciplines.
BASIC CONFIGURATION OF ROBOTICS AND ITS WORKINGS Four common robot configurations Industrial robots are available in a wide variety of sizes, shapes and physical configurations. The vast majority of today’s commercially available robots posses one of four basic configuration. 1. Polar configuration 2. Cylindrical configuration 3. Cartesian coordinate configuration
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
44
4. Jointed-arm configuration. Polar Configuration It uses a telescoping arm that can be raised or lowered about horizontal pivot. The pivot is mounted on a rotating base. These various joints provide the robot with the capability to move its arm within a spherical space, and hence the name “Spherical coordinate� robot is sometime applied to this type. A number of commercial robots possess the polar configuration. These include the familiar unimate 2000 series. Another which is much smaller than the unimate is the MAKER 110, made by United States Robots.
Cylindrical Configuration The cylindrical configuration uses a vertical column and a slide that can be moved up or down along the column. The robot arm is attached to the slide so that it can be moved radially with respect to the column. By rotating the column, the robot is capable of achieving a work space that approximates a cylinder.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
45
Cartesian Configuration The Cartesian coordinate robot uses three perpendicular slides to construct the x, y and z axes. Other names are sometimes applied to this configuration, including xyz robot and rectilinear robot. By moving the three slides relative to one another, the robot is capable of operating within a rectangular work envelope. An example of this configuration is the IBM RS-1 robot (currently called the model 7565). The RS-1, because of its appearance and construction, is occasionally referred to as a “box” configuration. “Gantry” robot is another name used for Cartesian robots that are generally large and posses the appearance of a gantry- type crane.
Jointed-arm Configuration Its configuration is similar to that the human arm. It consists of two straight components, corresponding to the human forearm and upper arm, mounted on a vertical pedestal. These components are connected by two rotary joints corresponding to the shoulder and elbow. A wrist is attached to the end of the forearm, thus providing several additional joints. A special version of the jointed arm robot is the SCARA, whose shoulder and elbow joints rotate about vertical axes. SCARA stands for Selective Compliance Assembly Robot Arm, and this configuration provides substantial rigidity for the robot in the vertical direction, but compliance in the horizontal plane. This makes it ideal for many assembly tasks.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
46
ROBOT COMPONENTS A robot, as a system, consists of the following elements, which are integrated together to form a whole : Manipulator or Rover This is the main body of the robot and consists of the links, the joints, and other structural elements of the robot. End Effector This is the part that is connected to the last joint (hand) of a manipulator, which generally handles objects, makes connection to other machines, or performs the required tasks. Robot manufacturers generally do not design or sell end effectors. In most cases, all they supply is s simple gripper. Generally, the hand of a robot has provisions for connecting specialty end effectors that are specifically designed for a purpose. This is the job of a company’s engineers or outside consultants to design and install the end effector on the robot and to make it work for the given situation. A welding torch, a paint spray gun, a glue-laying device, and a parts handler are but a few of the possibilities. In most cases, the action of the end effector is either controlled by the robot’s controller, or the controller communicates with the end effector’s controlling device (such as PLC). Sensors Sensors are used to collect information about the internal state of the robot or to communicate with the outside environment. As in humans, the robot controller needs to know where each link of the
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
47
robot is in order to know the robot’s configuration. Even in absolute darkness, you still know where your arms and legs are! This is because feedback sensors in your central nervous system embedded in your muscle tendons sand information to your brain. The brain uses this information to determine the length of your muscle, and thus the state of your arms, legs, etc. The same is true for robots; sensors integrated into the robot send information about each joint of link to the controller, which determines the configuration of the robot. Robots are often equipped with external sensory devices such as a vision system, touch and tactile sensors, speech synthesizers, etc., which enables robot to communicate with the outside world.
Controller The controller is rather similar to your cerebellum, and although it does not have the power of your brain, it still controls your motions. The controller receives its data from the computer, controls the motions of the actuators, and coordinates the motions with the sensory feedback information. Suppose that in order for the robot to pick up a part from a bin, it is necessary that its first joint be at 35°. If the joint is not already at this magnitude, the controller will send a signal to the actuator (a current to an electric motor, air to a pneumatic cylinder, or a signal to a hydraulic servo valve), causing it to move. It will then measure the change in the joint angle through the feedback sensor attached to the joint (a potentiometer, an encoder, etc.). When the joint reaches the desired value, the signal is stopped. In more sophisticated robots, the velocity and the force exerted by the robot are also controlled by the controller. DRIVE SYSTEMS The robot’s capacity to move its body, arm, and wrist is provided by the drive system used to power the robot. The drive system determines the speed of the arm movements. The strength of the robot, and its dynamic performance. To some extent, the drive system determines the kinds of applications that the robot can accomplish. In this and the following sections, we will discuss some of these technical features. Types of drive systems Commercially available industrial robots are powered by one of three types of drive systems. These three systems are: 1. Hydraulic drive 2. Electric drive 3. Pneumatic drive Hydraulic drive and electric drive are the two main types of drives used on more sophisticated robots. Hydraulic drive if generally associated with larger robots, such as the Unimated 2000 series. The usual advantages of the hydraulic drive system are that it provides the robot with greatest speed and strength. The disadvantes of the hydraulic drive system are that it typically adds to the floor space required by the robot, and that a hydraulic system is inclined to leak oil which is a nuisance. Hydraulic drive systems can be designed to actuate either rotational joints or linear joints. Rotary vane actuators can be utilized to provide rotary motion, and hydraulic pistons can be used to accomplish linear motion. Electric drive systems do not generally provide as much speed or power as hydraulic systems. However, the accuracy and repeatability of electric drive robots are usually better. Consequently, electric robots tend to be smaller, requiring less floor space. And their applications tend toward more precise work such as assembly. The MAKER 110 is an example of an electric drive robot that is consistent with these tendencies. Electric drive robots are actuated by dc stepping motors or dc servomotors. These motors are ideally suited to the actuation of rotational joints through appropriate drive train and gear systems. Electric
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
48
motors can also be used to actuate linear joints (E.g., telescoping arms) by means of pulley systems or other translational mechanisms. The economics of the two types of drive systems are also a factor in the decision to utilize hydraulic drive on large robots and electric drive on smaller robots. It turns out that the cost of an electric motor is much more proportional to its size, whereas the cost of a hydraulic drive system is somewhat less dependent on size. These relationships are displayed conceptually in below figure. As the illustration suggests, there is a hypothetical break-even point, below which it is advantageous to use electric drive and above which it is noted that there is a trend in the design of industrial robots toward all electric drives, and away from hydraulic robots because of the disadvantages discussed above.
Electric drive
Cost
Hydraulic drive
BreakEven
Size of equipment
Pneumatic drive is generally reserved for smaller robots that possess fewer degrees of freedom (two-to four-joint motions). These robots are often limited to simple “pick-and-place” operations with fast cycles. Pneumatic power can be readily adapted to the actuation of piston devices to provide translational movement of sliding joint. It can also be used to operate rotary actuators for rotational joints. DEGREES OF FREEDOM As you may remember from your engineering mechanics courses, in order to locate a point in space, one needs to specify three coordinates, such as the x, y, and z coordinates along the three Cartesian axes. Three coordinates are necessary and sufficient to define the location of the point. Although the three coordinates may be expressed in terms of different coordinates systems, they are always necessary. How ever, it is not possible to have two or four coordinates, since two is inadequate to locate a point in space, and four is impossible in three dimensions. Similarly, if you consider a threedimensional device with three degrees of freedom, within the workspace of the device, you should be able to place any point at any desired location. For example, a gantry (x, y, z) crane can place a ball at any location within its work space as specified by the operator. Similarly, to locate a rigid body (a three-dimensional object rather than a point) in space, one needs to specify the location of a selected point on it, and thus it requires three pieces of information to be located as desired. However, although the location of the object is specified, there are infinitely many possible ways to ordinate the object is specified point. To fully specify the object in space, in addition to the location of a selected point on it, one needs to specify the orientation of the object. This means that there is need for a total of six pieces of information to fully specify the location and orientation of s rigid body. By the same token, there needs to be six degrees of freedom available to fully place object in space and also orientate it as desired. If there are fewer than six degrees of freedom, the robot’s capabilities are limited. To demonstrate this, consider a robot with degrees of freedom, where it can only move along the x, y, and z axes. In this case, no orientation can be specified; the entire robot can do is to pick up the part and to move it in space, parallel to the reference axes. The orientation always remains the same. Now consider another robot with five degrees of freedom, capable of rotating about the three axes, but only moving along the x and y axes. Although you may specify any orientation desired, the positioning of the part is only possible along the x and y axes but not z axis. A system with seven degrees of freedom does not have a unique solution. This means that if a robot has seven degrees of freedom, there are an infinite number of ways it can position a part and
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
49
orientate it at the desired location. For the controller to know what to do there must be some additional decision making routine that allows it to pick only one of the infinite ways. As an example, one may use an optimization routine to pick the fastest of the shortest path to the desired destination. Then the computer has to check all solutions to find the shortest or fastest response and perform it. Due to this additional requirement, which can take much computing power and time, no seven degree of freedom robot is used in industry. A similar issue arises when a manipulator robot is mounted on a moving base such as mobile platform or a conveyor belt. The robot then has an additional degree of freedom, which, based on the preceding discussion, is impossible to control. The robot can be at a desired location and orientation fro infinitely many distinct positions on the conveyor belt of the mobile platform. However, in this case, although there are too many degrees of freedom, generally, are additional degrees of freedom are not solved for. In other words, when a robot relative to the belt or is otherwise mobile, the location of the base of the robot relative to the belt or other reference frame is known. Since this location does not need to be defined by the controller, the remaining number of degrees of freedom is still 6 and thus unique. So long as the location of mobile platform is known (or picked), there is no need to find it by solving a set of equation of robot motions, and thus, the system can be solved. Can you determine now many degrees of freedom the human arm has? This should exclude the hand (palm and the finger), but should include the wrist. Before you go on, please try to see if you can determine it. You will notice that the human arm has three joint clusters in it, the shoulder, the elbow and the wrist. The shoulder has three degrees of freedom, since the upper arm (humerus) can rotate in the sagittal plane (parallel from shoulder to shoulder), and about the humerus. (Verify this by rotating your arm about the three different axes). The elbow has only one degree of freedom; it can only flex and extend about the elbow joint. The wrist also has three degrees of freedom. It can abduct and adduct, flex and extend, and since the radius bone can role over the ulna bone, it can rotate longitudinally (pronate and supinate). Thus, the human arm has a total of seven degrees of freedom, even if the ranges of some movements are small. Since a seven degree of freedom system does not have a unique solution, how do you think we can use our arms? There are cases where a joint may have the ability to move, but its movement is not fully controlled. For example, consider a linear joint actuated by a pneumatic cylinder, where the arm is fully extended of fully retracted, but no controlled position can be achieved between the two extremes. In this case, the convention is to assign only a ½ degree of freedom to the joint. This means that the joint can only be at specified locations within its limits of movement. Another possibility for a ½ degree of freedom is to assign only particular values to be joint. For example, suppose that a joint is made to be only at 0, 30, 60 and 90 degrees. Then, as before, the joint is limited to only a few possibilities, and thus has a limited degree of freedom. There are many robots in industry those posses’ fewer than six degrees of freedom. In fact, robots with 3.5, 4 and 5 degrees of freedom and very common. So long as there is no need for the additional degrees of freedom, these robots perform very well. As an example, suppose that you desire to insert electronic components into a circuit board. The circuit board in always laid flat on a known work surface; thus, its height (z value) relative to the base of the robot is known. Therefore, there is only need for two degrees of freedom along the x and y axes to specify any location on the board of insertion. Additionally, suppose that the components would be inserted in any direction on the board, but the board is always flat. In that case, there will be need for one degree of freedom to rotate at the vertical axis (z) in order to orientate the component above the surface. Since there is also need for a ½ degree of freedom to fully extend the en effector to insert the part or to fully retract it to lift the robot before moving, all that is needed is 3.5 degrees of freedom; two to move over the board, one to rotate the component, and ½ to insert or retract. Insertion robots are very common and are used extensively in electronics industry. Their advantages is that are simple to program, are less expensive, and are smaller and faster. Their disadvantages are that although they may be programmed to insert components on any size board in any direction, they cannot perform other jobs. They are limited to what 3.5 degrees of freedom can achieve, but they can perform a variety of functions within this design limit.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
50
LINKS AND JOINTS The manipulator consists of a series of rigid members, called links, connected by joints. Motion of a particular joint cause’s subsequent links attached to it to move. The motion of the joint is accomplished by an actuator mechanism. The actuator can be connected directly to the next link or through some mechanical transmission (in order to produce a torque or speed advantage or “gain”). The manipulator ends with a link on which a tool can be mounted. The interface between the last link and the tool or end effector is called the tool mounting plate or tool flange. The manipulator itself may be thought of as being composed of three divisions:
The major linkages The minor linkages (wrist components) The end effector (gripper or tool)
The major linkages are the set of joint-link pairs that grossly position the manipulator in space. Usually, they consist of the first three sets (counting from the base of the robot). The minor linkages are those joints and links associated with the fine positioning of the end effector. They provide the ability to orient the tool mounting plate and subsequently the end effector once the major linkages get it close to the desired position. The end effector, which is mounted on the tool plate, consists of the particular mechanism needed at the end of the robotic arm to perform a particular task. The end effector may be a tool that does a function such as welding or drilling, or it may be some type of gripper of the robot’s task is to pick up parts and transfer them to another location. A gripper may be a simple pneumatically controlled device which opens and closes or a more complex servo-controlled unit capable or exerting specified forces, or measuring the part within grasp (i.e., gagin).
TYPES OF JOINTS The joints used in the design of industrial robots typically involve a relative motion of the adjoining links that is either liner or rotational. Linear joints involve a sliding or translational motion of the connecting links. This motion can be achieved in a number of ways (e.g., by a piston, a telescoping mechanism, and relative motion along a linear track or rail). Our concern here is not with the mechanical details of the joint, but rather with the relative motion of the adjacent links. We shall refer to the liners joints as a type L joint (L for linear). The term prismatic joint is sometimes used in the literature in place of linear joint. There are at least three types of rotating joint that can be distinguished in robot manipulator. The three types are illustrated below. We shall refer to the first as a type R joint (R for rotational). In the type R joint the axis of rotation is perpendicular to the axes of the two connecting links. The second type of rotating joint involves a twisting motion between the input and output links. The axis of rotation of the twisting joint is parallel to the axes of both links. We shall call this type T joint (T for Twisting). The third type of rotating and the output link in which the input link is parallel to the axis of rotation and the output link is perpendicular to the axis of rotation. In essence, the output link revolves about the input link, as if it were in orbit. This joint will be designed as a type V joint (V for revolving)
Type
Name
Illustration Output link
L
Linear
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
51
Input link Output link R
Rotational Input link Output link
T
Twisting Input link
Output link V
Revolving
Input link
CLASSFICATION OF ROBOTS What exactly is a robot? As mentioned in the preceding section, it is not a C3-P0 like humanoid that has multiple appendages and is capable of decisions and actions based solely on moment to moment processing of information acquired through its external sensors. Webster defines a robot as An automatic apparatus or device that performs functions ordinarily ascribed to humans or operates with what appears to be almost human intelligence. Classification by Coordinates system Cartesian coordinate In this, the simplest of configuration, the links of the manipulator are constrained to move in a linear manner. Axes of a robotic device that behave in this way are referred to as “prismatic�. Let us now consider the two types of Cartesian devices. 1. Cantilevered Cartesian
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
52
The arm is connected to a trunk, which is turn is attached to a base. It is seen that the members of the robot manipulator are constrained to move in directions parallel to the Cartesian x, y, and z axes. Devices like these tend to have a limited extension from the support frame, are less rigid, but have a less restricted workspace than other robots. In addition, they have good repeatability and accuracy (even better that the SCARA types) and are easier to program because of the “more natural” coordinate system. Certain types of motion may be more difficult of achieve with this configuration, due to the significant amount of computation required (e.g straight line in a direction not parallel to any axis). In this respect, Control Automation did manufacturing a robot that was capable of unrestricted straight-line paths. How ever, since 1985, the company has stopped marketing such a manipulator. 2. Gantry-style Cartesian Normally used when extremely heavy loads must be precisely moved, such robots are often mounted on the ceiling. They are generally rigid but many provide less access to the workspace. In the last few years, a number of smaller devices in this class have emerged. In this instance, a framed structure is used to support the root, thereby making it unnecessary to mount the device on the ceiling. It is important to understand that the classifications above take into account only the major axes of the manipulator. However, a robot is not limited to only three degrees of freedom. Normally, a wrist is affixed to the end of the forearm. This appendages if itself, capable of several additional motions. Moreover, the entire base of the robot can be mounted on a device that permits motion a plane (E.g., an x-y table or a track located in either the ceiling or floor). From this discussion it should be clear to the reader that robots with as many as eight (and as few as two) axes can be constructed. Having classified robots according to the geometry of their major axes, we noe look at another way of organizing robot types. Cylindrical coordinate When a horizontal arm (or “boom”) is mounted on a vertical column and this column is then mounted on a rotating base, the configuration is referred to as a cylinder coordinate robot. As can be seen the arm has the ability to move in and out (in the r direction), the carriage can move up and down on the column (in the z direction), and the arm and carriage assembly can rotate as a unit on the base ( in the θ direction). Usually, a full 360° rotation in θ in not permitted, due to restrictions imposed by hydraulic, electrical, or pneumatic connections or lines. Also, there is a minimum, as well as maximum extension (ir., R), due to mechanical requirements. Consequently, the overall volume or work envelope is a portion of a cylinder. Commercial robots having this configuration are (or have been) manufactured by companies such as Prab, Versatran, Auto place, General Numeric, Seiko, and Cincinnati Milacron.
Spherical coordinate When a robotic manipulator bears a resemblance to a tank turret, it is classified as a spherical coordinate device. The reader should observe that the arm can move in and out (in the r direction) and is characterized as being a telescoping boom, can pivot in a vertical plane (in the Ф direction), and can rotate in a horizontal plane about the base ( in the θ direction). Because of mechanical and/or actuator connection limitations, the work envelope of such a robot is a portion of a sphere. Commercially available spherical coordinate robots are being by Prap, Unimation, and United State Robots. Horizontal articulated (SCARA) Selective compliance assembly robot arm (SCARA) robots have two revolute joints that are parallel and allow the robot to move in a horizontal plane, plus and additional prismatic joint that moves vertically below figure SCARA robots are very common in assembly operations. Their specific characteristic is that they are more complain in the x-y plane, but are very stiff along the z- axis and thus have selective compliance. This is an important issue in assembly and will be discussed later.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
53
WORK ENVELOPE AND WORK VOLUME Work volume is the term that refers to the space within which the robot can manipulate its wrist and. The convention of using the wrist end to define the robot’s work volume is adopted to avoid the complication of different sizes of end effectors that might be attached to the robot’s wrist. The end effector is an addition to the basic robot and should not be counted as part of the robot’s working space. A long end effector mounted on the wrist would add significantly to the extension of the robot compared to a smaller and effector. Also, the end effector attached to the work might not be capable or reaching certain points within the robot’s normal work volume because of the particular combination of joint limits of the arm. The work volume is determined by the following physical characteristics of the robot: The robot’s physical configuration The size of the body, arm, and wrist components The limits of the robot’s joint movements. A polar coordinate robot has work volume that is a partial sphere, a cylinder coordinate robot has a cylinder work envelope, a Cartesian coordinate robot has a rectangular shaped work space, and a jointed-arm robot approximates a work volume that is spherical. The size of each work volume shape is influenced by the dimensions of the arm components and by the limits of its joint movements. Using the cylindrical configuration an example, limits on the rotation of the column about the base would determine what portion of a complete cylinder the robot could reach with its wrist end. WRIST ROTATION Wrist movement is designed to enable the robot to orient the end effector properly with respect to the task to be performed. For example, the hand must be oriented at the appropriate angle with respect to the work pieces in order to grasp it. To solve this orientation problem, the wrist is normally provided with up to 3 degree of freedom (the following typical configuration): 1. Wrist roll – Also called wrist swivel, this involves rotation of the wrist mechanism about the arm axis. 2. Wrists pitch – Given that the wrist roll is in its center position, the pitch would involve the up or down rotation of the wrist. Wrist pitch is also sometimes called wrist bend. 3. Wrist yaw – Again, given that the wrist swivel is in the center position of its range, wrist yaw would involve the right or left rotation of the axes. The reasons for specifying that the wrist roll be in its center position in the definition of pitch and yaw is because rotation of the wrist about the arm axis will alter the orientation of the pitch and yaw movements.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
54
MECHANICAL TRANSMISSION The components used to implement a practical joint usually consist of an actuator (E.g motor) coupled to physical joint by a mechanical transmission. This transmission is used to take the actuator motion and direct it to the joint in order to provide such characteristics as a changes of rotational direction, a change of axis, torque multiplication (or reduction), and speed reduction (or multiplication). The transmission may be used to convert rotary motion to linear motion or to provide a “match” between the actuator and load in order that the maximum energy is transferred to the load. Some devices produce linear relationships between input and output motion and thus are well suited for use in closed-loop control. However, some of the more traditional mechanical schemes have highly nonlinear input/output relationships, which can create severe control problems. We have used the word “transmission” to define all the components from the actuator to the physical joint. These components may consist of shaft coupler, multiple sets of gears, and lead screws, to name but a few. Although it is possible, and very desirable for reasons that will become clear throughout this chapter, to implement a joint by direct drive (i.e., actuator and joint are one component with no transmission), it may be impractical for cost or size reasons. One commercially available robot using a direct-drive approach is manufactured by ADEPT. ROTARY TO ROTARY CONVERSION Pulley and belt systems The below figure can be used to model an ideal belt and pulley or chain – and sprocket system. The analysis of this system is similar to that of the ideal gear train. Note that is this case, the relative motions of both shafts are in the same direction timing belts and chains perform essentially the same function as gear trains. However, they allow the transmission of the rotational motion and torque over longer distances without the need for multiple gears. As opposed to a belt and pulley, this method will not allow slippage between the belt and sprocket due to the teeth, however, it will exhibit a spring constant if the output is locked and a torque applied to input. T2, θ2
Belt T1, θ1 Radius of the large circle = r2 Radius of the small circle = r1
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
55
Gears To understand the process of rotary-to-rotary motion conversion, we will focus of the properties of an “ideal” gear train (the concept developed are easily applied to the belt and pulley, or sprocket and chain). By the “ideal gear train” we mean a transmission composed of gears that are perfectly round, rotate on their true centers, and are also inertia less. The surface between the gears is also frictionless, thereby crating no losses. Either shaft may be designated as the input or output. The top gear has a radius of r 1 and N1 teeth, while the bottom gear has a radius of r2 and N2 teeth. Both gears have a rigid shaft attached to then with the torque and displacement of each shaft designated as T and θ, respectively. By examine the same physical relationships between the two gears, equations relating the speeds, displacements, and torques on the two shafts may be obtained. Since the spacing between the teeth on each gear much be the same so that they mesh properly, the number of teeth on each gear is proportional to the radius of the gear. Thus N1 / r 1
=
N2 / r 2
It should also be apparent that the number of teeth on a gear must be a whole number. However, the gear ratio N1 / N2 may be fractional. For ideal gear train, there are no losses, and therefore the work done by the torque acting through an angular displacement on one shaft is equal to the torque acting through the corresponding angular displacement of the other shaft. Therefore T1 θ1 = T2 θ2 The arc length of the distance traveled by on e gear must equal that of the other. That is, the distance traveled along the surface of each gear is the same. Converting the arc length to angular displacement yields r1 θ1 = r2 θ2 Finally noting that since the two gear radii does not vary with time, if the above equations are differentiated with respect to time, their relationship still holds but with respect to the angular velocity or the angular acceleration. Using concept, we may write N1 / N2 = r2 / r2 = T1 / T2 = θ1 / θ2 The above equation can be used to investigate various properties of the “ideal gear train. Harmonic drives A cup type harmonic drive and its components. The three components of the harmonic drive are: 1. Circular spline: a rigid, thick wall ring with internal spline teeth. It is either a fixed or a rotating drive element. 2. Wave generator: an elliptical ball bearing assembly which includes a shaft coupling. It is rotating input drive element. 3. Flexspline: a non rigid cylindrical thin walled cup which has two fewer spline teeth and a smaller pitch diameter than that of the circular spline. It is a fixed or rotating output drive element. Harmonic drive gearing is a patented principle base dons non rigid body mechanics. It employs three concentric components to produce high mechanical advantage and speed reduction. The use of non rigid body mechanics allows a continuous elliptical deflection wave to be induced in a non rigid external gear, thereby providing a continuous rolling mesh with a rigid internal gear. Since the teeth on the no rigid flexspline and the rigid circular spline, are in continuous engagement and since the flexspline has two fewer teeth than the circular spline, one revolution of the
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
56
input causes relative motion between the flexspline and the circular spline equal to two teeth. Thus, with the circular rotationally fixed, the flexspline will rotate in the opposite direction to the input at a reduction ration equal to the number of teeth on the flexspline and the circular spline equal to two teeth. The below figure shows the harmonic drive components may be set up to perform six basic transmission functions, by varying the input, output and fixed element. 1. Speed reducer: Circular spline held stationary. Wave generator is input. Flexspline is output. Ratio as tabulated. Input and output rotate in opposite directions. Out
In
2. Speed Increaser: Circular spline held stationary. Wave generator is Output. Flexspline is input. Ratio as tabulated. Input and output rotate in opposite directions. In
3. Speed reducer: Flexspline held stationary. Wave generator is input. Circular spline is output. Ratio as tabulated plus 1. Input and output rotate in same directions.
Out
Out
in
4. Speed Increaser: Flexspline held stationary. Wave generator is output. Circular spline is input. Ratio as tabulated plus 1. Input and output rotate in same directions.
in
out
5. Speed reducer / Differential: Wave Generator held stationary. Flexspline is input. Circular spline is output.
Out in
or in
Ratio = (Tabulated Ratio + 1) / Tabulated Ratio
6. Speed reducer / Differential: Wave Generator held stationary. Flexspline is Output. Circular spline is input.
Out Out
or Out
Ratio = Tabulated Ratio / (Tabulated Ratio + 1)
The relative rotation may be understood by examine the motion of a single flexspline tooth over one-half an input revolution. Full engagement occurs in the adjacent circular spline tooth space when the
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
57
major axes are rotated at 180°. This motion repeats as the major axis rotates another 180° back to zero, thereby producing the two tooth advancement per input revolution. CONVERSION BETWEEN LINEAR AND ROTARY MOTION AND ITS DEVICES Rotary to linear motion conversion is connected with taking the rotational motion and torque from an actuator and producing a linear motion and force on the output. Components whose ideal model produces a linear relationship between the input shaft’s rotation and the output’s linear displacement are the lead screw, rack and pinion, and Roh’lix. Nonlinear relationships are available by slider-crank assemblies and cams. Rack and pinion systems The pinion is the small gear attached to the actuator, and the rack is a linear member with gear teeth on one side. The transfer relationship of such a mechanical system. x = 2Πrθ As defined the above the linear distance traveled is proportional to the input shaft rotation, with the constant of proportional equal to the circumference of the pinion. That is, linear distance traveled is equal to the distance traveled by the pinion. The reflected inertia, as seen by the input shaft, can be shown to be 2 J = Mr x Payload
r Rack Pinion θ Rack and pinion driving load.
Belt and pulley driving a linear load Another method of generating linear motion is shown in below figure. Note that both pulleys have the same radius. The pulley connected to the input is called the drive pulley and second pulley is called an idler pulley. The distance traveled by the load is equal to the distance traveled by the drive pulley. The equation x = 2Πrθ also describes the transfer relationship of the belt and pulley system while equation J = 2 Mr is the reflected inertia as seen by the input shaft. In practical implementation of the lead screw, rack and pinion, and linear drive via belt and pulley, there are physical limitations on the linear motion. For example, if the linear motion exceeds its designated range, it typically hits and “end stop”. This mechanical contact forces the rotary shaft to lock. In the event that a torque continuous to be applied to the shaft in this locked position, it is possible for extremes forces to be generated, causing the devices to fail (i.e., the threads of screw or nut strip, or gear teeth break). The practical solution on this problem is to place electrical limit switches some distance before physical
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
58
limit is reached. The signal from these switches is used to stop the actuator before a damaging force can be applied at the extreme of motions.
x
M
Drive Pulley
Ideal Pulley
θ Rotary to linear conversion via belt and pulley
Slider cranks The slider crank mechanism is an extremely cost – effective means of converting rotary to linear motion. The below figure a representation of a crank driving a linear stage (or load). In this implementation, the crank portion is the wheel that rotates about its centre and has a rod of fixed length mounted to a point on its circumference. The other end of the connecting rod or link is attached to a linear stage which is constrained to move in only one dimension on a relatively frictionless surface. At both its locations on the disk and linear stage, the connecting rod is free to rotate. Thus the angle formed with the horizontal will change as a function of the disk’s position. As the disk travels from 0° to 180° in the counter clockwise direction, the linear stage moves a distance equal to 2r. If the disk continues to travel from 180° back to 0° (still in the counter clockwise direction), the load will move in the opposite direction over exactly the same linear distance. Note that there are two distances angular positions of the input shaft that correspond to a single linear position of the load. The angular positions that correspond to full extension and retraction of the linear stage are called dead center positions or dead points and occur when the crank and connecting rod are collinear. If this system if used to convert linear to rotary motion (as is typically done is a gasoline reciprocating engine), a flywheel with sufficient inertia must be connected to the rod and crank are in line at a dead point, the direction of the rotation may go either way. Thus the inertia contribution of the flywheel is necessary to bias the mechanism for proper operation. If the input is rotated continuously at some velocity, the motion of the linear stage is reciprocating. As can be observed by a careful examination of below figure, the torque seen by the input and the relationship between the input shaft’s position and linear stage’s position in nature. Crank at 180°
Crank at 0°
2r
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
59
l
Reference point M
θ 0°
180°
Slider crank driving linear storage
Coupler A coupler is a device used to connect shafts so that one can drive the other. Typically, it is used to connect an actuator or motor to the rest of the joint mechanism. In the ideal case, all that is required of the coupling is to transmit the torque, velocity, and position from the actuator to the rest of the system. However, since the world is imperfect, the job of coupler is much more difficult. As nothing ever lines up perfectly (e.g., shafts may be parallel but not collinear, or they may be skewed (so as intersect at a point), rigidly coupling the shafts would not be possible. Thus there is a need for a device that can compensate for these misalignments.
Aligned (axes collinear)
Parallel Misalignment
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
60
UNIT-V
MICROPORCESSOR CONTROL OF ELECTRIC MOTOR A robot is supposed to be a manipulator that is controlled by computers or microprocessors. So, of course, it is important to be able to control the motions of the electric motors with microprocessors. We have already discussed in some detail how a stepper motor or a brushes less DC motor may be controlled by a microprocessor. In the sections that follow, a few other common techniques for controlling electric motors will be mentioned. A microprocessor is a digital device. It only deals with digital inputs and digital outputs. Any voltage that is lower than about 0.8 volt is considered to be low (off, 0). Any voltage greater than 2.4 volts is considered to be high (on, 1). The microprocessor can only read the 0’s and 1’s. It can also only output 0’s and 1’s. All analog or continuous input signals or information must be digitized for use by a microprocessor. All desired analog or continuous output signals or information must be converted from digital to analog as well. Analog to digital converters (ADC) and digital to analog (DAC) are used for this purpose. However, a key element in both DAC and ADC’s is their resolution. Digital devices handle numbers (and all other information) by bits, which can only be 0 or 1. A one-bit piece of information can only have two states: 0, or 1. To add to this capability, one may use two bits. Then, there are four possibilities, 00, 01, 10, and 11. As the number of bits increases, the variations increase with 2”. Thus a four-bit set will have 16 distinct possibilities, and an eight-bit set would have 28, or 256, possibilities. Every four bit is called a word, and every eight bits is a byte. Suppose that you want to read in a variable voltage between 0-5 volts into your microprocessor and be able to use it for running number. Of course, the processor can only read high or low, not a continuous number. If you use a one-bit input port, it can only recognize whether the voltage is high or low, hardly a continuous process. Now suppose you use two bits to read the voltage. Their or four distinct possibilities for two bits. Thus, the voltage can be divided into four different values perhaps 0, 1.67, 3.34, and 5, which corresponds to 00, 01, 10 and 11-bit mapping. Then we will be able to distinguish four different levels of voltage with the processor. If this resolution were not enough, one would have to increase number of bits. With a four-bit input, the 5-volt voltage can be divided into 16 portions, corresponding to 0000, 0001, 0010, 0011… mapping. Then the smallest value of the voltage we could read is 0.33 volt. As you see, the larger the number of bits, the better the resolution becomes. However, what this mean is that in order to read one input voltage, four input ports of the processor would have to be dedicated to it. Additionally, you would have to use an ADV to convert the analog voltage signal to a digital form and feed the information from ADC into the processor. Conversely, suppose that you want to control a servomotor with a microprocessor. To have a variable voltage to control the speed of the motor, the digital information must be converted to analog form through a DAC. The resolution of this information is also limited to the number of bits used. For better resolution, more bits are necessary. Now you can imagine how many input and output ports would be necessary to have an accurate six-axis robot with multiple servomotors, inputs, and sensors.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
61
To control the motions of a robot or to move from one point to another, the robot controller would have to calculate the magnitude of the change in each joint based on the kinematics equations governing the motions of the robot. If it is desired to also have velocity control, the speed at which each joint must move is also calculated. This information determines how much and how fast each joint must move which, alternatively, determines how much and how fast the servomotors of each joint must rotate (based on the gear ratio of the joint, as well as other information). This information is converted to a set of voltages and voltage profiles that govern the servomotors. This means that in order to have a particular servomotor rotate at a particular speed, it will require a particular voltage. The processor calculates this voltage; the voltage is fed to the servomotor, and the feedback signals going back to the controller are checked. The voltage is adjusted accordingly for desired velocity until the joint is moved a desired amount. This process continuous until the robot’s movements are finished. Consequently, it is necessary to have control over the voltage that go to each servomotor and to be able to read the feedback signals from each joint.
PLAYBACK ROBOTS Play back robots use a more sophisticated control unit in which a series of positions or motions are “taught” to the robot, recorded into memory, and then repeated by the robot under its own control. The term “playback” is descriptive to this general mode of operation. The procedure of teaching and recording into memory is referred to as programming the robot. Playback robots usually have some form of servocontrol (e.g., closed loop feedback system) to ensure that the positions achieved by the robot are the positions that have been taught. Playback robots can be classified into two categories: point-to-point (PTP) robots and continuous-path (CP) robots
POINT-TO-POINT CONTORL Point-to-point is capable of performing motion cycles that consist of a series of desired point locations and related actions. The robot is taught each point, and these points are recorded into the robot’s control unit. During playback, the robot is controlled to move from one point to another in the proper sequence. Pointto-point robots do not control the path taken by the robot to get from one point to the next. If the programmer wants to exercise a limited amount on control over the path followed, programming a series of points along the desired path must do this. Control of the sequence of positions is quite adequate for many kinds of applications, including loading and unloading machines and spot welding.
CONTINUOUS PATH CONTROL Continuous-path robots are capable of performing motion cycles in which the path followed by the robot is controlled. This is usually accomplished by making the robot move through a series of closely spaced points, which describe the desired path. The control unit rather than the programme defines the individual points. Straight-line motion is a common form of continuous-path control for industrial robots. The programmer specifies the starting point and the end point of the path, and the control unit calculates the sequence of individual points that permit the robot to follow a straight-line trajectory. Some robots have the capability to follow a smooth, curved path that has been defined by a programmer who manually moves the arm through the desired motion cycle. To achieve continuous-path control to more than a limited extent requires that the controller unit be capable of storing a large number of individual point locations that defines the compound curved path. Today this usually involves the use of a digital computer (a microprocessor is typically used as the central processing unit for the computer) as the robot controller. CP control is required for certain types of industrial applications such as spray coating and arc welding.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
62
ACTUATOR Actuators are the muscles of robots. If you imagine that the links and the joints are the skeleton of the robot, the actuators act as muscles, which move or rotate the links to change the configuration of robots. The actuator must have enough power to accelerate and decelerate the links and to carry the loads, yet to be light, economical, accurate, responsive, reliable, and easy to maintain. There are many types of actuators available, and undoubtedly, there will be more varieties available in the future. The following types are noteworthy: 1. Electric motors a. Servomotors b. Stepper motors c. Direct-drive electric motors 2. Hydraulic actuators 3. Pneumatic actuators 4. Shape memory metal actuators 5. Magnetostrictive actuators Electric motors-especially servomotors are the most commonly used robotic actuators. Hydraulic systems were very popular for large robots in the past and are still around in many places, but are not used in new robots as often any more. Pneumatic cylinders are used in robots that have ½ degree of freedom, on-off type joints, as well as for insertion purposes. Direct drive electric motors, the shape memory metal typeactuators, and other like them are mostly in research and development stage and may become more useful in the near future.
CHARACTERISTICS OF ACTUATING SYSTEMS The characteristics presented next may be used to compare different actuating terms. In addition to these, depending on the special circumstances in which they will be used, other characteristics may play a role in the design of robots. Examples include underwater systems, where waterproof operation of a system is very important, or space systems, where the liftoff weight and reliability are of absolute importance.
DC SERVOMOTORS An important issue in all electric motors is the back electromotive force, or back-emf. As you remember, a wire carrying a current within a magnetic field will experience a force, which causes it to move. Similarly, if a wire (conductor) moves within a magnetic field such that it will cross the field lines, a current will be induced into the conductor. This is the basic principle of electric power generation. However, it also means that when the wires of the windings in a motor are rotating in the magnetic field of the magnets, a current will be induced in them in the opposite direction of the input current. This current is called backemf, and it tends to reduce the effective current of the motor. The faster the motor rotates, the larger the back-emf is. Back-emf current is usually expressed as a function of rotor speed: Vemf = nKE
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
63
Here KE is typically given in volts per 1,000 rpm. As the motor approached its nominal no-load speed, the back-emf is large enough that the motor speed will stabilize at the nominal no-load speed with its corresponding effective current. However, at this nominal speed, the output torque of the motor is essentially zero. The motors velocity is governed by Vin = IR + Vemf = IR + nKE Where R is the windings’ resistance. If a load is applied to the motor, it will slow down, resulting in smaller back-emf, larger effective current, and consequently positive net torque. The larger the load, the slower the motor will rotate in order to develop the larger torque. If the load becomes increasingly larger, there comes a time when the motor stalls, there is no beck-emf, the effective current is at its maximum, and torque is at its minimum. Unfortunately in each case, when the back-emf is smaller, although the output torque is larger, since the net current is larger, so is the generated heat. Under stall or near stall conditions, the generated heat may be large enough to damage the motor. To be able to develop larger torques without showing down the motor, one would have to increase the current (to the rotor, the stator, or both if soft iron cores are used). In such a case, although the motor rotates at the same speed and the back-emf is still the same, the larger current will increase the net effective current and, thus, the torque. By varying the current (or corresponding voltage) the speed torque balance can be maintained as desired. This system is called a servomotor. The output torque T developed by the motor can be expressed as a function of the torque contant KT typically in oz.in/amo, as in T = IKT. The opposing torques are friction torque, T f, viscous damping torque (as a function speed) nKD, and the opposing load TL. Thus the motor’s torque is governed T = TL + Tf + nKD = IKT by the above equations govern the motions of the motor. A servomotor in a DC, AC, brushes less, or even stepper, motor with feedback that can be controlled to move at a desired speed (and consequently, torque), for a desired angle of rotation. To do this, a feedback device sands signals to the controller circuit of the servomotor reporting its angular position and velocity. If as a result of higher loads, the velocity is lower than desired set value, the current is increased until the speed is equal to the desired value. If the speed signal shows that the velocity is larger than desired, the current is reduced accordingly [5]. If position feedback is used as well, the position is used to shut off the motor as the rotor approaches the desired angular position. We will discuss sensors later. For now, let it suffice to say that many different types pf sensors may be used for this purpose, including encoders, resolvers, potentiometers and tachometers. If a position sensor such as a potentiometer or encoder is used, its signal can be differentiated to produce a velocity signal. The below figure show a schematic of a simple control block diagram for servomotor.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
Desired +
64
Error
Position And _ Velocity
Vout
Motor dynamics
Outputpositon
Load
Differentiate Velocity
Feedback
STEPPER MOTOR Stepper motors are versatile, long-lasting, simple motors that can be used in many applications. In most applications, the stepper motors are used without feedback. This is because unless a step is missed, a stepper motor steps s known angle each time it is moved. Thus, its angular position is always known and no feedback is necessary. Stepper motors come in many different forms and principle of operation. Each type has certain characteristics unique to it, yielding it appropriate choice for certain applications. Most stepper motors can be used in different modes by wiring them differently. Unlike regular DC or AC motors if you connect a stepper motor to power, it will not rotate. Stepper rotates only when the magnetic field is rotated through its different windings. In fact, the maximum torque is developed when the do not turn. Even when not powered, steppers have a residual torque called detent torque. It requires external torque to turn the stepper motor, even when not powered. As a result all stepper motors need a microprocessor or driver/controller (indexer) circuit for rotation. You may either create your own driver or you may purchase a device called an indexer that will drive the stepper motor for you. As with servomotor, which need feedback circuitry, stepper motors need drive circuitry. So, in each application, the designer must decide which type of motor is more appropriate. For industrial robotic actuation, stepper motors are hardly using, except in small table-top robots, or, in one case, an industrial robot with stepper motors and feedback. However, stepper motors are used extensively in non-industrial robots and robotic devices, as well as in other devices that are used in conjunction with robots from tooling machines to peripheral devices and from automatic manufacturing to control devices. Structure of stepper motors Generally stepper motors have permanent magnet rotors, while their stators house multiple windings. Based on the discussion, since the heat generate in the coil can easily dissipate through the motor’s body, stepper motors are less susceptible to heat damage, and since there are no brushes or commutators, they have long life. In different types of stepper motors, the permanent magnet rotors are different. In each case, through, the rotor follows a moving magnetic field generated by the coils. As a result, somewhat similar to both AC motors and brush less DC motors, a rotor follows a moving flux under the control of a controller or driver. In the next sections, we will study how stepper motors operate.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
65
Principle of operation Imagine a stepper motor with two coils in its stator and a permanent magnet as its rotor. When each of the coils of the stator is energized, the permanently magnetized rotor will rotate to align itself with the stator magnetic field. (a). The rotor will stay at this position unless the field rotates. As the power of the present coil is distributed and is directed to the next coil, the rotor will rotate again to align itself with the field in the new position (b). Each rotation is equal to the step angle, which may vary from 180ﾂｰ to as little as a fraction of a degree. Next, the first coil will once again turned n, nut in the opposite polarity, while the second is turned off. This will cause the rotor to rotate another step in the same direction. The process continues as one coil is turned off and an other is turned on. A sequence of four steps will bring the rotor back to exactly the same state it was at the beginning of the sequences. Now imagine that at the conclusion of the first step, instead of turning off one coil and turning on the second coil, that both would be turned on. In that case, the rotor would only rotate 45ﾂｰ to align itself to the path of least reluctance. Later, if the first coil is turned off while the second remains on, the rotor will rotate another 45ﾂｰ. This is called half-step operation and includes sequences of eight movements. Of course, with the opposition on-off sequence, the rotor will rotate in the opposite direction. Most industrial steppers run between 1.8 to 7.5 degrees at full stepping. Obviously, to reduce the size of the steps, the number of poles may be increased. However, there is physical limit to how many poles may be used. To further increase the number of steps per revolution, different numbers of teeth can be built into the stator and rotor creating an effect similar to a caliper. For instances, 50 teeth on the rotor and 40 teeth on the stator will result in a 1.8 窶電egree step angle with 200 steps per revolution, as will be discussed later.
POTENTIOMETERS
A potentiometer converts position information into a variable voltage through a resistor. As the sweeper on the resistor moves due to a change in position, the proportion of the resistance before or after the point of contact with the sweeper compared with the total resistance varies. Since in this capacity the potentiometer acts as a voltage divider, the output will be proportional to the resistance as Vout = Vcc (R1/R) Potentiometer can be rotary or linear and thus can measure linear or rotary motions. Rotary potentiometer can also be multiple-turn, enabling the user to measure many revolutions of motion. Potentiometer are either wire wound of thin film deposit (also called conductive plastic), which is a deposit of thin film of resistive material on a surface. The major benefit of thin film potentiometers is that their output is continuous and thus less noisy. As a result, it is possible to electronically differentiate the output of this type of resistors find velocity. However, since the output of a wire wound potentiometer is stepwise, it cannot be differentiated. Potentiometers are generally used as internal feedback sensors in order to report the position of joints and links. Potentiometers are used alone or together with other sensors such as encoders. In this case, the encoder reports the current position of joints and links, whereas the potentiometer reports the startup positions. As a result, the combination of the sensors allows for a minimal input requirement, but maximum accuracy.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
66
R Vcc R1
Vout
A potentiometer as a position sensor
OPTICAL ENCODERS Encoders are simple device that can output a digital signal for each small portion of a movement. To do this, the encoder wheel or strip is divided into small sections a light source, such as an LED, on one side provides a beam of light to the other side of the encoder wheel or strip, where it is seen by an other lightsensitive sensor, such as a phototransistor. If the wheel’s angular position is such that the light can go through, the sensor on the opposite side will be turned on ad will have a high signal. If the angular position of the wheel is such that the light is stopped, the sensor will be off, and its output will be low. As the wheel rotates, it can continuously send signals are counted, the approximate total angular displacement of the wheel can be measured at any time. There are two types of encoders: 1. Incremental encoders 2. Absolute encoders In these incremental encoders the areas of clear and opaque sections are all equal and repeating. Since all arcs are the same size, each arc indicates the certain angle of revolution, which is all the same. If the wheel is divided into only two portions, each portion 180°, its revolution will also be 180°, and within this arc, the system is incapable of reporting any more accurate information about the displacement or position. If the number of division increases, the accuracy increases as well. Thus, the resolution of an optical encoder is related to the number of arcs of clear and opaque areas. Typically incremental encoders can have 512 to 1024 arcs in them, reporting angular displacement with a resolution of 7.0 to 0.35 degree. Optical encoders are either opaque wheels with removed material for clear areas or clear material such as glass with printed opaque areas. Many encoder wheels are also etched, such that they either reflect the light. In that case, the light source and the pick up sensor are both on the same side of the wheel
High Channel A Low
Channel B
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
67
Output of leading edges for one channel only
Output of leading and trailing edges for one channel only.
Output of leading and trailing edges for two channels/
(each portion of the angular position of an absolute encoder has a unique signature. Through this signature, the angular position of the encoder can be performed.) Tachometers A tachometer is a generator hat converts mechanical energy. Its output in an analog voltage proportional to the input angular, it may be sad along with potentiometer to estimate velocities.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
68
ROBOT MOTION ANALYSIS AND KINEMATICS In order for a robot to perform useful work it is necessary for the arm to consist of a number of joint. The typical commercial industrial robot has five of six jaunts. It is also necessary to control the path which the end of the arm follows, as opposed to merely controlling the final positions of the joints. In this chapter we will discuss the robot motion control. Joints are labeled Jn where n begins with 1 at the base of the manipulator, and links are labeled Ln, again with 1 begin the link closest to the base. The below figure illustrate the labeling system for two different robot arm, each possessing 2 degrees of freedom. By the joint notation scheme described in below. We will also use the symbol L n to indicate the length of the link in some of our equation derivations early in the chapter. This mutation system uses the symbol a n to denote length of a manipulator link.
L1
J2
J1 L2
Fig:- Two linear joints (LL)
Forward transformation of 2-Degree of freedom arm
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
69
We can determine the position of the end of the arm in world space by defining a vector for link 1 and another for link 2. r1 = [L1 cos θ1, L1 sin θ1] r2 = [L2 cos (θ1 + θ2), L2 sin (θ1 + θ2)] Vector addition of (1) and (2) yields the coordinates x and y of the end of the arm (point P w) in world space x = L1 cos θ1 + L1 cos (θ1+ θ2) y = L1 sin θ1 + L1 sin (θ1+ θ2) Reverse transformation of 2-Degree of freedom arm In many cases it is more important to be able to derive the joint angles given the end-of-arm position in the world space. The typical situation is where the robots controller’s must compute the joint angles required to move its end-of-arm to a point in space defined by the point’s coordinates. For the twolinks manipulator we have developed, there are two possible configurations for reaching the point (x, y), as shown in the fig. below. Some strategy must be developed to select the appropriate configuration. One approach is that employed in control system of the Ultimate PUMA robot. In the Puma’s control language, VAL, there is a set of commands called ABOVE and BELOW that determines whether the elbow is to make an angle θ2 that is greater or less than zero, as illustrated in fig. below. For our examples, let us assume the θ2 is positive as shown in fig. below. Using the trigonometric identities, cos (A+B) =cos A cos B – sin A sin B sin (A+B) = sin A cos B – sin B cos A We can rewrite the equations as x = L1 cos θ1 + L2 cos θ1 cos θ2 – L2 sin θ1 sin θ2 y = L1 sin θ1 +L2 sin θ1 cos θ2 + L2 cos θ1 sin θ2
Above
θ2 (neg)
(x, y)
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
70
θ2 (pos) Below
Squaring both sides and adding the two equations yields 2
2
2
2
cos θ2 = (x + y – L1 - L2 ) / (2 L1L2 ) Defining α and β as in Fig. below we get tan α = L2 sin θ2 / L2 cos θ2 + L1 tan β = y/x Using the trigonometric identity tan (A-B) = (tan A - tan B) / (1 + tanAtanB) we get tan θ1 = [y(L2 cos θ2 + L1) – x L2 sin θ2] / [x(L2 cos θ2 + L1) – y L2 sin θ2] Knowing the lengths L1 and L2 we are now able to calculate the required joint angles to place the arm at a position ( x, y ) in world space.
y
(x, y)
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
71
θ2
α θ1
β
Adding Orientation: A 3- Degree of Freedom Arm in Two Dimensions The arm we have been modeling is very simple; a two-jointed robot arm has little practical value except for very simple tasks. Let us add to the manipulator a modest capability for orienting as well as positioning a part or tool. Accordingly, we will incorporate a third degree of freedom into the previous configuration to develop the RR:R manipulator shown in Fig. below. This third degree of freedom will represent a wrist joint. The world space coordinates for the wrist end world be We can use the results that we have already obtained for the 2-degree of freedom manipulator to do the reverse transformation for the 3-degree of freedom arm. When defining the position of the end of the arm we use x, y, and Ψ. the angle Ψ is the orientation angle for the wrist. Given these three values, we can solve for the joint angles (θ1 + θ2 + θ3) using Having determined the position of joint 3, the problem of determining θ 1 and θ2 reduces to the cases of the 2-degree of freedom manipulator previously analyzed.
Homogeneous transformation and robot kinematics The approach used in the previous section becomes quite cumbersome when a manipulator with many joints must be analyzed. Another, more general method for solving the kinematics equations of a robot arm makes use of homogeneous transformations. We describe this technique is this section assuming the reader has at least some familiarity with the mathematics of vectors and matrices. Let us begin by defining the mutation to be used.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
72
A point vector, v=ai + bj + ck can be represented in three-dimensional space by the column matrix x y z w
where a=x/w, b=y/w, c=z/w, and w is a scaling factor. For example, any of the following matrices can be used to represent the vector v = 25i + 10j + 20k 25 10 20 1
or
50 20 40 2
or
12.5 5.0 10.0 0.5
Vectors of the above form can be used to define the end-of-arm position for a robot manipulator. (if w=0, then the vector represents direction only.) A vector can be translated of rotated in space by means of a transformation. The transformation is accomplished by a 4 x 4 matrix H. For instance the vector v is transformed into the vector u by the following matrix operation: u = Hv (1) the transformation to accomplish a translation of a vector in space by a distance a in the x direction, b in the y direction, and c in the z direction is given by
H = Trans(a, b, c) =
1 0 0 a 0 1 0 b 0 0 1 c 0 0 0 1
(2)
Example: For the vector v = 25i + 10j + 20k, perform a translation by a distance of 8 in the x direction, and o in the z direction. The translation transformation would be
H = Trans(a, b, c) =
1 0 0 8 0 1 0 5 0 0 1 0 0 0 0 1
The translated vector would be
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS 1 0 0 8 0 1 0 5 Hv = 0 0 1 0 0 0 0 1
73 25 10
20
33 15 =
20
1
1
Rotations of a vector about each of the three axes by an angle θ can be accomplished by rotation transformation. About the x axis, the rotation transformation is
Rot(x, θ) =
1 0 0 0
0 cosθ sinθ 0
0 -sinθ cosθ 0
0 0 0 1
cosθ 0 -sinθ 0
0 1 0 0
sinθ 0 cos θ 0
0 0 0 1
cosθ sinθ 0 0
-sinθ cosθ 0 0
0 0 0 0
0 0 0 1
About the y axis
Rot(x, θ) =
About the z axis
Rot(x, θ) =
In concept it is impossible to develop the rotation transformation about an vector K , where K is not one of the major axes x, y, or z of the coordinate system. This rotation transformation is defined as Rot(K, θ).w e present the concept here but leave the derivation of the transformation. Solving the kinematics equations It is desirable to be able to determine the joint positions required by the manipulator to reach a desired endpoint. Before explaining the procedure for doing this, let us introduce an analysis technique known as the transform graph. The below figure illustrates the transform graph for the manipulator and described in the previous subsection. The transform graph shows the two ways of getting from the base frame 0 to the end-of-arm frame. One way is along the path represented by T3 and the other is along the path A1A2A3. we always travel along the direction of the arrows in a transform graph. If we wish to travel against be arrows, we must use the inverse of the transform. That is, in our example, we could travel along the path -1 A1 T3 to get to the end of the arm. If we again at one point of the graph and end at a second point there are always two paths that we can follow. The transform products represented by these two paths are equivalent. That is -1 1 A1 T3 = T3
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
74
Fig:- transform graph for manipulator And so forth. We can make use of these equivalents to solve for the joint positions given the end-of-arm position in the form of T. Finally d3 = Pz / cosθ2 The solution was obtained in a relatively straightforward manner because of the simple configuration of our 3-degree of freedom arm. In cases where the manipulator is more complex kinematically it may be necessary to use more of the equivalent obtained the transform graph. Manipulator path control In controlling the manipulator we are not only interested in the end points reached by the robot joints, but also it the path followed by the arm in traveling from one point to another in the workspace. Motion types There are three common types of motion that a robot manipulator can make a traveling from point to point. These are slew motion, joint-interpolated motion, and straight line motion. Slew motions represent the simplest type of motion. As the robot is commanded to travel from point A to point B, each axis of the manipulator travels as quickly as possible from its respective initial position to its required final position. Therefore, all axes begin moving at the same time, but each axis ends its motion in an elapsed time that is proportional to the product of its distance moved and its top speed (allowing for acceleration and deceleration). Slew motion generally results in unnecessary wear on the joints and often leads to unanticipated results in terms of the path taken by the manipulator. ROBOT DYNAMICS Accurate control of the manipulator requires precise control of each joint. The control of the joint depends on knowledge of the force that will be acting on the joint and the inertias reflected at the joint (the masses of the joints and links of the manipulator). While these forces and masses are relatively easy to determine for a single joint, it becomes more difficult to determine them as the complexity of the manipulator increases. We will explore these issues using the two-axis manipulator developed earlier. Our purpose is to introduce this problem area to the reader than to analyze its complexities in detail.
Static analysis
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
75
Let us begin by considering the torques required by the joints to produce a force F at the tip of the robot arm as shown below. By balancing the forces on each link we get -F
T2 -F2 F2 T2
T1 F1 F1 – F2 = 0
(1)
F2 – F = 0
(2)
F1 = F2 = F
(3)
And
That is,
The torque is the vector cross-products of the forces and the link vectors r as developed earlier. So that T1 = T2 + r1 x F (4) T2 = r2 x F
(5)
If F = (Fx, Fy) then, T1 = [ L1 cos θ1 + L2 cos (θ1+ θ2)], [L1 sin θ1 + L2 sin (θ1+ θ2)] x (Fx, Fy)
(6)
Since the vector cross-product (a,b) x (c,d) ia ad-bc we get T1 = [ L1 cos θ1 + L2 cos (θ1+ θ2)] x Fx - [L1 sin θ1 + L2 sin (θ1+ θ2)] x Fy
(7)
Therefore, given a force vector F = (Fx, Fy), the torques required by each joint to generate that force at the tip can be computed using the preceding analysis.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
76
In order to calculate the force generated by the end of the arm given joint torques we must solve the equation for Fx and Fy. The solution is obtained by the equation (7) from (6). We can then substitute for Fx, T1, T2, θ1, θ2. After simplification using trigonometry indentities as outlined earlier we get Fx = [T1L2 cos(θ1+ θ2) - T2(L1 cosθ1 + L2 cos(θ1+ θ2)] / L1L2 sinθ2
(8)
Fy = [T1L2 sin(θ1+ θ2) - T2(L1 sinθ1 + L2 sin(θ1+ θ2)] / L1L2 sinθ2
(9)
The technique previously described for kinematics analysis of torque can be extended to more joints. The ability to determine the joint torques and the components of forces applied at the wrist or tools are useful controlling the arm when it must contact the environment.
Robot arm dynamics The topic of robot dynamics is concerned with the analysis of the torques and forces due to acceleration and deceleration. Torques experienced by the joint due to acceleration of the links, as well as forces experienced by the links due to torques applied by the joints, are included within the scope of dynamics analysis. Solving for the acceleration is dependent of the inertia of the arm. However, the inertia is dependent on the configuration of the arm, and this is continuously changing as the joints are moved. An additional factor that influences the inertia is the mass of the payload and its position with respect to joints. This also charges as the joints are moved. The below figures shoes the two link arm in the maximum and minimum inertia configuration. The torque required to drive the robot arm are not only determined by the static and dynamic forces described above: each joint must also react to the torque of other joints in the manipulator, and the effects of these relatively must be included in the analysis. Also, if the arm moves at a relatively high speed, the centrifugal effects may be significant enough to consider.
J2 J2
J1
J1
(a) (b) Arm inertias. (a). Minimum inertia about J1 (b). Maximum inertia about J1 END EFFECTORS
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
77
End effectors are a device that attaches to the wrist of the robot arm and enables the general –purpose robot to perform a specific task. It is sometimes referred to as robot’s “hand”. Most production machines require special purpose fixtures and tools designed for a particular operation, and a robot is no exception. The end effectors are part of that special-purpose tooling for a robot. Usually, end effectors must be custom engineered for the particular task which is to be performed. This can be accomplished either by designing and fabricating the device from scratch, or by purchasing a commercially available device and adapting it to the application. The company installing the robot can either do the engineering work itself or it can contact for the services of a firm that does this kind of work. Most robot manufacturers have special engineering groups whose function is to design end effectors and to provide consultation services to their customers. Also, there are a growing number of robot systems firms which perform some firms which perform some or all of the engineering work to install robot systems. Their services would typically include end effectors design. TYPES OF END EFFECTORS
There is a wide assortment of end effectors required to perform the variety of different work functions. The various types can be divided into two major categories: 1. Gripper 2. Tools
GRIPPERS Grippers are end effectors used to grasp and hold objects. The objects are generally work parts that are to be moved by the robot. These part-handling applications include machine loading and unloading, picking parts from a conveyor, and arranging parts onto a pallet. In addition to work parts, other objects handled by robot grippers include cartons, bottles, raw materials, and tools. We tend to think of grippers as mechanical grasping devices but there are alternative ways of holding objects involving the use of magnets, suction cups, or other means. In this chapter we will divided grippers according to whether they are mechanical grasping devices or some other physical principle is used to retain the object. Different types of mechanical gripper used in robotics, and explores the various grippers that use a means of retention other than mechanical. Gripper can be classified as single grippers or double grippers although this classification applies best to mechanical grippers. The single gripper is distinguished by the fact that only one grasping device is mounted on the robot’s wrist. A double gripper has two gripping devices attached to the wrist and is it used to handle two separate objects. The two gripping devices can be actuated independently. The double gripper is especially useful in machine loading and unloading applications. To illustrate, suppose that a particular job calls for a raw work part to be loaded from a conveyor onto a machine and the finished part to be unloaded onto another conveyor. With a single gripper, the robot would have to unload the finished part before picking up the raw part. This would consume valuable time in the production cycle because the machine would have to remain open during these handling motions. With a double gripper, the robot can pick the part from the incoming conveyor with one of the gripping device and have it ready to exchange for the finished part. When the machine cycle is completed, the robot can reach in for the finished part with the available grasping device, and insert the raw part into the machine with the other grasping device. The amount of time that the machine is open is minimized. The term multiple grippers is applied in the case where two or more grasping mechanisms are fastened to the wrist. Double grippers are a subset of multiple grippers. The occasions when more than two grippers would be required are somewhat rare. There is also a cost and reliability penalty which accompanies an increasing number of gripper devices on one robot arm. Another way of classifying grippers depends on whether the part is grasped on its exterior surface or its internal surface, for example, a ring shaped part. The first type is called an external gripper and the second type is referred to as an internal gripper.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
78
In the context of this chapter, tools are end effectors designed to perform work on the part rater than to merely grasp it. Be definition, the tool-type end effectors is attached to the robot’s wrist. One of the most common applications of industrial robots is spot welding, in which the welding electrodes constitute the end effectors of the robot, other examples of robot applications in which tools are used as end effectors include spray painting and arc welding. Below we discuss the various end effectors in this category. It was mentioned above that gripper are sometimes used to hold tools rather than work part. The reason for using a gripper instead of attaching the tool directly to the robot’s wrist is typically because the job requires several tools to be manipulated by the robot during the work cycle. An example of this kind of application would be deburring operation in which several different sizes and geometries of deburring tool must be held in order to reach all surface of the work part. The gripper serves as a quick change device to provide the capability for a rapid changeover from one tool to the next. In addition to end effectors, other types of fixturing and tooling are required in many industrial robot applications. These include holding fixture, welding fixtures and other forms of holding devices to position the work part or tooling during the work cycle. MECHANICAL GRIPPERS Basic definitions and operation A mechanical gripper is end effectors that use mechanical fingers actuated by a mechanism to grasp an object. The fingers, sometimes called jaws, are the appendages of the gripper that actually make contact with the object. The fingers are either attached to the mechanism or are an integral part of the mechanism. If the fingers are of the attachable type, then can be detached and replaced. The use of replaceable fingers allows for wear and interchangeability. Different sets of fingers for use with the same gripper mechanism can be designed to accommodate different part models. An example of this interchangeability feature is illustrated in below figure, in which the gripper is designed are sufficient to hold the work part or other object. Grippers with three or more fingers are less common. The function of the gripper mechanism is to translate some from of power input into the grasping action of the fingers against the part. The power input is supplied from the robot and can be pneumatic, electric, mechanical, or hydraulic. The mechanism must be able to open and close the fingers and to exert sufficient force against the part when closed to hold it securely. There are two ways of consisting the part in the gripper. The first is by physical constriction of the part within the fingers. In this is by gripper fingers enclose the part to some extent, thereby constraining the motion of the part. This is usually accomplished by designing the contacting surfaces of the fingers to be in the approximate shape of the part geometry. This method of constraining the part. The second way of holding the part is by friction between the fingers and the work part. With this approach, the fingers must apply a force that is sufficient for friction to retain the part against gravity, acceleration, and any other force that might arise during the holding portion of the work cycle. The fingers, or the pads attached to the fingers which make contact with the part, are generally fabricated out of a material that is relatively soft. This tends to increase the coefficient of friction between the part and the contacting finger surface. It also serves to protect the part surface from scratching or other damage. The friction method of holding the part results in a less complicated and therefore less expensive gripper design, and it tends to be readily adapted to a greater variety of work parts. However, there is problem with the friction method that is available magnitude is applied against the part in a direction parallel to the friction surfaces of the fingers. The part might slip of the gripper. To resist this slippage, the gripper must be designed to exert a force that depends on the weight of the part, the coefficient of friction between the part surface and the finger surface, the acceleration (or deceleration) of the part, and the orientation between the direction of fingers. The following force equation can be used to determine the required magnitude of the gripper force as a function of these factors.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
79 Detectable finger
Gripper
μnfFg = w Where
μ = coefficient of friction of the finger contact surface against the part surface nf = number of contacting force Fg = gripper force w = weight of the part or object being gripped
VACUUM GRIPPERS Vacuum grippers, also called suction cups, can be used as gripper devices for handling certain types of objects. The usual requirements on the objects to be handled are that they be flat, smooth, and clean conditions necessary to form a satisfactory vacuum between the object and the suction cup. An example of a vacuum cup used to lift flat glass picture in below. The suction cups used in this type of robot gripper are typically made of elastic material such as rubber or soft plastic. As exception would be when the object to be handled is composed of a soft material. In this, case suction cup would be made of hard substance. The shape of the vacuum cup, as shown in the figure, is usually round. Some means of removing the air between the cup and the part surface to create the vacuum is required. The vacuum pump and the venture are two common devices used for this purpose. The vacuum pump is pistonoperated or vane-driven devices powered by an electric motor. It is capable of creating a relatively high vacuum. The venture is a simple device as pictured below and can drive by means of “shop air pressure.” Its initial cost is less than that of a vacuum pump and it is relatively reliable because of its simplicity. However, the overall reliability of the vacuum system is dependent on the source of air pressure.
Nozzle
Exhaust
Air in
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
80 Vacuum
Fig:- venture device used in operate a suction cup. The lift capacity of the suction cup depends on the effective area of the cup and the negative air pressure between the cup and the object. The relationship can be summarized in the following equation F = PA Where F = the force or lift capacity, lb 2 P = the negative pressure, lb/in 2 A = the total effective are of the suction cup(s) used to create the vacuum, in
MAGNETIC GRIPPERS Magnetic grippers can be a very feasible means of handling ferrous materials. The stainless steel plate would not be an appropriate application for a magnetic gripper because 18-8 stainless steel is not attached by a magnet. Other steels, however, including certain types of stainless steel, would be suitable candidates for this means of handling, especially when the materials are handled in sheet or plate form. In general, magnetic grippers offer the following advantages in robotic handling applications: Pickup times are very fast. Variations in part size can be tolerated. The gripper does not have to be designed for one particular work part. They have the ability to handle metal parts with holes (not possible with vacuum grippers). They require only one surface for gripping. Disadvantages with magnetic gripper include the residual magnetism remaining in the work piece which may cause a problem in subsequent handling, and the possible side slippage and other errors which limit the precision of this mean of handling. Another potential disadvantage of a magnetic gripper is the problem of picking up only one sheet from a stack. The magnetic attraction tends to penetrate beyond the top sheet in the stack resulting in the possibility that more than single sheet will be lifted by the magnet. This problem can be confronted in several ways. First, magnetic grippers can be designed to limit the effective penetration to the desired depth, which would correspond to the thickness of the top sheet. Second, the stacking device used to hold the sheets can be designed to separate the sheets for pickup by the robot. One such type of stacking device is called “fanner�, and it makes use of a magnetic field to induce a charge in the ferrous sheets in the stack. Each sheet toward the top of the stack is given a magnetic charge, causing them to possess the same polarity and repel each other. The sheet most affected is the one at top of the stack. The sheet most affected is the one at the top of the stack. It tends to rise above the remainder of the stack, thus facilitating pickup by the robot gripper. Magnetic gripper can be divided into two categories, those using electromagnets, and those using permanents magnets. Electromagnetic grippers are easier to control, but require a source of dc power and an appropriate controller unit. As with any other robotic gripping device, the part must be released at the end of the handling cycle. This is easier to accomplish with an electromagnet than with a permanent magnet. When the part is to be released, the controller unit reverses the polarity at a record power level before switching off the electromagnet. This procedure acts to cancel the residual magnetism in the work piece and ensures a positive release of the part. Permanent magnets have the advantage of not requiring an external power source to operate the magnet. However, there is a loss of control that accompanies this apparent advantage. For example, when the part is to be released at the end of the handling cycle, some means of separating the part from
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
81
the magnet must be provided. The device which accomplishes this is called a stripper or stripping device. Its function is to mechanically detach the part from the magnet. Permanent magnets are often considered for handling tasks in hazardous environments requiring explosion proof apparatus. The fact that no electrical circuit is needed to operate the magnet reduces the magnet of sparks which might cause ignition in such an environment. TOOLS In many applications, the robot is required to manipulate a tool rather than a work part. In a limited number of these applications, the end effector is a gripper that is designed to grasp and handle the tool. The reason for using a gripper in these applications is that there may be more than one tool to be used by the robot in the work cycle. The use of a gripper permits the tools to be exchanged during the cycle, and thus facilities this multi tool handling function. In most of the robot applications in which a tool is manipulated, the tool is attached directly to the robot wrist. In these cases the tool is the end effector. Some example of tools used as end effectors in robot applications include: Spot-welding tools Arc-welding torch Spray-painting nozzle Rotating spindles for operations such as: Drilling Routing Wire brushing Grinding Liquid cement applications for assembly Heating torches Water jet cutting tool In each case, the robot must control the actuation of the tool. For example, the robot must coordinate the actuation of the spot-welding operation as part of its work cycle. This controlled much in the same manner as the opening and closing of a mechanical gripper. TRANSDUCERS A transducer is a device that converts one type of physical variable (e.g., force, pressure, temperature, velocity, flow rate, etc.,) into another form. A common conversion is to electrical voltage, and the reason for making the conversion is that the converted signal is more convenient to use and evaluate. A sensor is a transducer that is used to make a measurement of a physical variable of interest. Some of the common sensors and transducers include strain gauges (used to measure force and pressure), thermocouples (temperatures), speedometers (velocity), and pitot tubes (flow rates). Any sensors or transducer requires calibration in order to be useful as a measuring device. Calibration is the procedure by which the relationship between the measurement variable and the converted output signal is established. Transducers and sensors can be classified into two basic types depending on the form of the converted signal. The two types are: 1. Analog transducers 2. Digital transducers
Analog transducers provide a continuous analog signal such as electrical voltage or current. This signal can then be interpreted as the value of the physical variable that is being measured. Digital transducers produce a digital output signal, either in the form of set of parallel status bits or a series of
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
82
pulses that can be counted. In either form, the digital signal represents the value of the measured variable. Digital transducers are becoming more popular because of the ease with which they can be read as separate measuring instruments. In addition, they offer the advantage in automation and process control that they are generally more compatible with the digital computer than analog based sensors. In order to be useful as measuring devices, in robotics and in other applications, must possess certain features. Few if any sensors have all of these desirable features, and a compromise must be made among them to select the best sensor for a given application.
SENSORS The sensors used in robotics include a wide range of devices which can be divided into the following general categories: 1. Tactile sensors 2. Proximity and range sensors
TACTILE SENSORS Tactile sensors are devices which indicate contact between themselves and some other solid object. Tactile sensing devices can be divided into two classes: touch sensors and force sensors. Touch sensors provide a binary output signal which indicates whether or not contact has been made with the object. Force sensors (also sometimes (also sometimes called stress sensors) indicate not only that contact has been made with the object but also the magnitude of the contact force between the two objects. Touch sensors Touch sensors are used to indicate that contact has been made between two objects without regard to the magnitude of the contacting force. Included within this category are simple devices such as limit switches, micro switches, and the like. The simpler devices are frequently used in the design of interlock systems in robotics. For example, they can be used to indicate the presence or absence of parts in a fixture or at the pickup point along a conveyor. Another use for touch sensing device would be as part of an inspection probe which is manipulated by the robot to measure dimensions on a work part. A robot with 6 degrees of freedom would be capable of accessing surfaces on the part that would be difficult for a three-axis coordinate measuring machine, the inspection system normally considered for such an inspection task. Unfortunately, the robot’s accuracy would be a limiting factor in contact inspection work. Force sensors The capacity to measure forces permits the robot to perform a number of tasks. These include the capability to grasp parts of different sizes in material handling, machine loading, and assembly work, applying the appropriate level of force for the given part. In assembly applications, force sensing could be used to determine if screw has become cross-threaded or if parts are jammed. Force sensing in robotics can be accomplished in several ways. A commonly used technique is a “force-sensing wrist�. This consists of a special load-cell mounted between the gripper and the wrist. Another technique is to measure the torque being exerted by each joint. This is usually accomplished by sensing motor current for each of the joint motors. Finally, a third technique is to form an array of forcesensing elements so that the shape and other information about the contact surface can be determined.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
83
Proximity and range sensors Proximity sensors are devices that indicate when one object is close to another object. How close the object must be in order to activate the sensor is dependent on the particular device. The distance can be anywhere between several millimeter and several feet. Some of the sensors can also be used to measure the distance between the object and sensor, and these devices are called range sensors. Proximity and range sensors would typically be located on the wrist or end effector since these are the moving parts of the robot. One practical use of a proximity sensor in robotics would be to detect the presence or absence of a work part or other object. Another important application is for sensing human beings in the robot work cell. Range sensors would be useful for determining the location of an object (e.g., the work part) in relation to the robot. A variety of technologies are available for designing proximity and range sensors. These technologies include optical devices, acoustic, electrical field techniques (e.g., eddy current and, magnetic fields), and others. We will survey only a few of the possibilities if the followings. Optical proximity sensors can be designed using either visible or invisible (infrared) light sources. Infrared sensors may be active or passive. The active sensors send out an infrared beam and respond to the reflection of the beam against a target. The infrared-reflectance sensor using an incandescent light source is a common device that is commercially available. The active infrared sensor can be used to indicate not only whether or not a part is present, but also the position of the part. By timing the interval from when the signal is sent also and the echo is received, a measurement of the distance between the object and the sensor can be made. This feature is especially useful for locomotion and guidance systems. Passive infrared sensors are simply devices which detect the presence of infrared radiation in the environment. They are often utilized the presence in security systems to detect the presence of bodies giving off heat within the range of the sensors. These sensors systems are effective at covering large areas in building interiors. Another optical approach for proximity sensing involves the use of a collimated light beam and a linear array of light sensors. By reflecting the light beam off the surface of the object, the location of the object can be determined from the position of its reflected beam on the sensor array. Acoustic devices can be used as proximity sensors. Ultrasonic frequencies (above 20,000 Hz) are often used in these devices because the sound is beyond the range of human hearing. One type pf acoustical proximity sensor end of the chamber. The emitter set up a pattern of standing waves in the cavity which is altered by the presence of an object near the open end. A microphone located in the wall of the chamber is used to sense the change in the sound pattern. This kind of device can also be used as a range sensor. Force sensor The force sensing resistor (FSR) manufactured by interlink electronics, is a polymer thick-film device that exhibits a decreasing resistance with increasing force applied normal to its surface. For a changing for of 10 to 10,000gr, its resistance changes from about 5000 K 立 to about 1 K 立 [4].
VISION SYSTEMS Vision systems are perhaps the most sophisticated sensors used in robotics. Due to their importance and complexity. However, you must realize that vision systems are, in fact, sensors and that they relate the function of a robot to its environment as all other sensors do. IMAGE PROCESSING AND ANALYSIS
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
84
The discussion in the preceding section described now images are obtained, digitized, and stored in a computer. For use of the stored image in industrial applications, the computer must be programmed to operate on the digitally stored image. This is a substantial task considering the large amount of data that must be analyzed. Consider an industrial vision system having a pixel density of 350 pixels per line and 280 lines (a total of 98,000 picture elements), and a 60bit register for each picture element to present various gray levels; this would require a total of 98,000 x 6 = 588,000 bits of data for each 1/30 seconds. This is formidable amount of data to be processed in a short period of time and has led to various techniques to reduce the magnitude of the image processing problem. These techniques include: 1. 2. 3. 4.
Image data reduction Segmentation Feature extraction object recognition
Image Data Reduction In image reduction, the objective is to reduce the volume of data. As a preliminary step in the data analysis, the following two schemes have found common usage for data reduction: 1. Digital conversion 2. Windowing The function of both schemes is to eliminate the bottleneck that can occur from the large volume of data in image processing. Digital conversion reduces the number of gray levels used by the machine vision system. For 8 example, an 8-bit register for each pixel would have 2 = 256 gray levels. Depending on the requirements of the application, digital conversion can be used to reduce the number of gray levels by using fewer nits to represent the pixel light intensity. Four bits would reduce the number of gray levels to 16. This kind of conversion would significantly reduce the magnitude of the image-processing problem.
Segmentation Segmentation is a general term which applies to various methods of data reduction. In segmentation, the objective is to group areas of an image having similar characteristics or features into distinct entities representing parts of the image. For example, boundaries (edges) or regions (areas) represent two natural segments of an image. There are man ways to segment an image. Three important techniques that we will discuss are: 1. Thresholding 2. Region growing 3. Edge detection In its simplest form, thresholding is a binary conversion technique in which each pixel is control into a binary value, either black or white. This is accomplished by utilizing a frequency histogram of the image and establishing what intensity (gray level) is to be border between black and white. Region growing is a collection of segmentation techniques in which pixels are grouped in regions called grid elements based on attributes similarities. Defined regions can then be examined as to whether they are independent or can be merged to other regions by means of an analysis of the difference in their average properties and spatial connectiveness. Edge detection considers the intensity changes that occur in the pixels at the boundary or edges of a part. Give that a region of similar attributes has been found but the boundary shape is unknown, the boundary can be determined by a simple edge image. For the binary image, the procedure is to scan the
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
85
image until a pixel within the region is encountered. For a pixel within the region, turn left and step, otherwise, turn right and step.
Feature extraction In machine vision applications, it is often necessary to distinguish one object from another. This is usually accomplished by means of features that uniquely characterize the object. Some features of objects that can be used in machine vision include are, diameter, and perimeter. A feature, in the context of vision systems, is s single parameter that permits ease of comparison and identification. A list some of the features commonly used in vision applications. The techniques available to extract feature values for two-dimensional cases and can be roughly categorized as those that deal with boundary features and those that deal with area features. The various features can be used to identify the object or part and determine the part location and/or orientation. Object recognition The next step in image data processing is to identify the object the image represents. This identification problem is accomplished using the extracted feature information described in the previous subsection. The recognition algorithm must be powerful enough to uniquely the object. Object recognitions used in industry today may be classified into two major categories: 1. Template-matching techniques 2. Structural techniques
ROBOTIC APPLICATION Current day robot applications include a wide variety of production operations. For purposes of organizations in this book, we will classify the operations into the following categories. There are many robot applications in which the robot is required to move a work part of other material from one location to another. The most basic of the applications is where the robot picks the part up form one position and transfers it to another position. In other applications, the robot is used to load and/or unload a production machine of some type. In this book we divide material-handling applications into two specific categories. 1. Material transfer applications. 2. Machine loading/unloading applications. There are other robot applications which involve part handling. These include assembly operations and holding parts during inspection. Before discussing material transfer and machine loading/unloading applications. Let us review some of the considerations that should be examined when robots are used for material handling.
ROBOTS IN MATERIAL HANDLING In planning an application in which the robot will be used to transfer parts, load a machine, pr other similar operation, there are several considerations that must be reviewed. Most of these considerations have been discussed in previous and we itemize them below as a reference checklist. 1. Part positioning and orientation:In most parts-handling applications the parts must be presented to the robot in a known position and orientation. Robots used in these applications do not generally possess highly sophisticated sensors
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
86
(E.g., Machine vision) that would enable them to seek out a part and identify its orientation before picking it up. 2.
Gripper design:Special and effectors must be designed for the robot to grasp and hold the work part during the handling operation. 3. Minimum distances moved:The material handling application should be planned so as to minimize the distances that the part must be moved. This can be accomplished by proper design of the work cell layout (E.g., Keeping in the equipment in the cell close together), by proper gripper design (E.g., Using a double gripper in a machine loading/unloading operation), and by careful study of the robot motion cycle 4. Robot work volume:The cell layout must be designed with proper consideration given to the robot’s capability to reach the required extreme locations in the cell and still allow room to maneuver the gripper. 5. Robot weight capacity:There is an obvious limitation on the material handling operation that the load capacity of the robot must not be exceeded. A robot with sufficient weight carrying capacity must be specified for the application. 6.
Accuracy and repeatability:Some applications require the materials to be handled with very high precision. Other applications are less demanding in this respect. The robot must be specified accordingly. 7. Robot configuration, degree of freedom, and control:Many parts transfer the operations are simple enough that they can be accomplished by a robot with two to four joints of motion. Machine loading applications often require more degrees of freedom. Robot control requirements are unsophisticated for most material handling operations. Palletizing operations and picking parts from a moving conveyor are example where the control requirements are more demanding. 8. Machine utilization problems:It is important for the application to effectively utilize all pieces of equipment in the cell. In a machine loading/unloading operation, it is common for the robot to be idle while the machine is working, and the machine to be idle while the robot is working. In cases where a long machine cycle is involved, the robot is idle a high proportion of the time. To increase the utilization of the robot, consideration should be given to the possibility for the robot to service more than s single machine. One of the problems arising in the multi machine cell is machine interference, discussed in ROBOTS IN MACHINE LOADING AND UNLOADING These applications are material handling operations in which the robot is used to service a production machine by transferring parts to and/or from the machine. There are three cases that fit into this application category. Machine load/unload:The robot loads a raw work part into the process and unloads a finished part. A machining operation is an example of this case. Machine loading:The robot must load the raw work part or materials into the machine but the part is ejected from the machine by some other means. In a press working operation, the robot may be programmed to load sheet metal blanks into the press, but the finished parts are allowed to drop out of the press by gravity.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
87
Machine unloading:The machine produces finished parts from raw materials that are loaded directly into the machine without robot assistance. The robot unloads the part from the machine. Examples in this category include die casting and plastic modeling application. The application is best typified by a robot-centered work cell which consists of the production machine, the robot, and some form of parts delivery system. To increase the productivity of the cell and the utilization of the robot, the cell may include more than a single production machine. This is desirable when the automatic machine cycle relatively long, hence causing the robot to be idle a high proportion of the time. Some cells are designed so that each machine performs the same identical operation. Other cells are designed as flexible automated systems in which different parts follow a different sequence of operations at different machines in the cell. In either case, the robot is used to perform the parts handling function for the cell. Robots have been successfully applied to accomplish the loading and/or unloading function in the following production operations. Die castings Plastic molding Forging and related operations Machining operations Stamping press operations We will discuss these applications in the following subsections. For each application, a brief description of the manufacturing process will be given. ROBOTS IN DIE CASTINGS Die castings is a manufacturing process in which molten metal is forced into the cavity of a mold under high pressure. The mold is called a die (hence the name, die casting). The process is used to cast metal parts with sufficient accuracy so that subsequent finishing operations are usually not required. Common metals used for die-casted parts include alloys of zinc, tin, lead, aluminum, magnesium and copper. The die consists of two halves that are opened and closed by a die casting machine. During operation the die is closed and molten metal is injected into the cavity by a pump. To ensure that the cavity is filled, enough molten metal is forced into the die that it overflows the cavity and creates “flash� in the space between the die halves. When the metal has solidified, the die is opened and the cast part is ejected, usually by pins which push the part away from the mold cavity. When the part is removed from the machine, it is often quenched (to cool the part) in a water bath. The flash that is created during the casting process must be removed subsequently by a trimming operation which cuts around the periphery of the part. Thus, the typical die-casting production cycle consists of casting, removing the part from the machine, quenching and trimming. The production rates in the die-casting process range from about 100 up to 700 openings of the die per hour, depending on type of machine, the metal being cast and the design of the part. For small parts, the die can be designed with more than one cavity, thus multiplying the number of parts made for each casting cycle. The die-casting machines have traditionally been tended by human operators. The work tends to be not, repetitive, dirty and generally unpleasant for humans. Perhaps because of these conditions, die casting was one of the very first processes to which the robots were applied. The first use of a robot in die casting was around 1961. Engelberger cites one 2 instance in which a Unimate robot has been used in a die casting application for over 90,000 hours The die-casting process represents a relatively straight forward application for industrial robots. The alterations required of the die-casting machine are minimal and the interlocking of the robot cycle with the machine cycle can be accomplished by simple limit switches. Few problems are encountered in
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
88
either the programming of the robot or the design of the gripper to remove the part from the machine when the die is opened. The process requires only that the robot unload the die casting machine, since the metal is in the molten state before the part is formed. On some die-casting machines (called coldchamber die-casting machines), the molten metal must be ladled from the melting container into the injection system. This part of the process is more difficult for robots to accomplish. ROBOTS IN MACHINE TOOL LOADING AND UNLOADING Machining is the metal working process in which the shape of the part is changed by removing the excess material with a cutting tool. It is considered to be a secondary process in which the final form and dimensions are given shape of the part. There are a number of different categories of machining operations. The principal types include turning, drilling, milling, shaping, planning, and grinding. Commercially, machining is a important metal working process and is widely used in many different products, ranging from those that are made in low quantities to those produced in vary high numbers. In mid-volume and high-volume production, the operation is very repetitive with the same machining sequence being repeated on part after part. The machine tools that perform machining operation have achieved a relatively high level of automation after many years of development. In particular, the use of computer control (E.g., computer numerical control and direct numerical control) permits this type of equipment to be interfaced with relative ease to similarly controlled equipment such as robots. Robots have been successfully utilized to perform the loading and unloading functions in machining operations. The robot is typically used to load a raw work part (a casting, forging or other basic form) into the machine tool and to unload the finished part at the completion of the machining cycle. The following robot features generally contribute to the success of the machine too load/unloading 2 application . Dual gripper:The use of the dual gripper permits the robot to handle the raw work part and the finished part at the same time. This permits the production cycle time to be reduced. Up to six joint motions:A large number of degrees of freedom of the arm and wrist are required to manipulate and position the part in the machine tool. Good repeatability:A relatively high level of precision is required to properly position the part into the chuck or other work holding fixture in the machine tool. Palletizing and depalletizing capability:In midvolume production, the raw parts are sometimes most conveniently presented to the work cell and delivered away from the work cell on pallets. The robot’s controller and programming capabilities must be sufficient to accommodate this requirement. Programming features:There are several desirable programming features that facilitate the use of robots in machining applications. In machine cells used for batch production of different parts, there is the need of perform some sort of changeover of the setup between batches. Part of this changeover procedure involves replacing the robot program for the previous batch with the program for the next batch. The robot should able to accept disk, tape, or other storage medium for ease in changing programs. Another programming feature needed for machining is the capability to handle irregular elements, such as tool changes of pallet changes, in the program.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
89
ROBOTS IN SPOT WELDING As the term suggest, spot welding is a process in which two sheet metal parts are fused together at localized points by passing a large electric current through the parts where the weld is to be made. The fusion is accomplished at relatively low voltage levels by using two copper (or copper alloy) electrode squeeze the parts together at the contact points and apply the current to the weld area. The electric current results in sufficient heat in the contact area to fuse the two metal parts, hence producing the weld. The two electrodes have the general shape of a pincer. With the two halves of the pincer open, the electrodes are positioned at the point where the parts are to be fused. Prior clamping or fixturing of the parts is usually required to hold the pieces together for the process. The two electrodes are squeeze together against the mating parts, and the current is applied to cause heating and welding of the contacting surfaces. Then the electrodes are opened and allowed to cool for the next weld. A water circulation system is often is used to accelerate the cooling of the electrodes. The actual welding portion of the sequence typically requires less than a second. Therefore, the rates of production of spot welding are largely dependent on the time required for positioning of the welding electrode and parts relative to each other. Another factor that affects production rate is wear of the electrodes. Because of the heat involve in the process, the tips of the electrodes gradually lose their shape and build up a carbon deposit which affects their electric resistance. Both of those effects reduce the quality of the welds made. Therefore, the electrode tips must periodically be dressed to remove the deposits and restore the desired shape. Spot welding has traditionally been performed manually by either of two methods. The first method uses a spot welding machine in which the parts are inserted between the pair of electrodes that are maintained in a fixed position. This method is normally used for relatively small parts that can be easily handled. The second method involves manipulating a portable spot welding gun into position relative to the parts. This would be used for large work such as automobile bodies. The word “portable� is perhaps an exaggeration. The welding consists of pair of electrodes and a frame to open and close electrodes. In addition, large electrical cables are used to deliver the current to the electrodes from a control panel located near the workstation. The welding gun with cables attached is quite heavy and can easily exceed 100lb in weight. To assist the operator in manipulating the gun, the apparatus a suspended from an overhead hoist system. Even with this assistance, the spot-welding gun represented a heavy mass and is difficult to manipulate by a human worker at the high rates of production desired on a car body assembly line. There are often problems with the consistency of the welded products made on such a manual line as a consequence of this difficult.
ROBOTS IN ARC WELDING Arc welding is a continuous welding process as opposed to spot welding which might be called a discontinues process. Continuous arc welding if used to make long welded joints in which an airtight seal is often required between the two pieces of metal being joined. The process uses an electrode in the form of a rod or wire of metal to supply the high electric current needed for establishing the arc. Currents of typically 100 to 300A at voltages of 10 to 30V. The arc between the welding rod and the metal parts to be joined produces temperatures that are sufficiently high to form a pool of molten metal to fuse the two pieces together. The electrode can also be used to contribute to the molten pool, depending on the type of welding process. Arc welding is usually performed by a skilled human worker who is often assisted by a person called a fitter. The purpose of the fitter is to organize the work and to fixture the parts for the welder. The working conditions of the welder are typically unpleasant and hazardous. The arc from the welding process emits ultraviolet radiation which is injurious to human vision. As a result welders are required to wear eye protection in the form of a welding helmet with a dark window. This dark window filters out the
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
90
dangerous radiation, but it is so dark that the welder is virtually blind while wearing the helmet except when arc is struck. Other aspects of the process are also hazardous. The high temperature created in arc welding and the resulting molten metals are inherently dangerous. The high electrical current used to create the arc is also unsafe. Sparks and smoke are generated during the process and these are a potential threat to the operator. There is a variety of arc welding processes, and the reader is referred to the available manufacturing process texts for more details than we can include here. For robot applications, two types of arc welding seem the most practical: Gas Metal Arc Welding (GMAW) and Gas Tungsten Arc Welding (GTAW). GMA welding (also called MIG welding for metal inert gas welding) involves the use of a welding wire made of the same or similar metals as the parts being joined. The welding wire serves as the electrode in the arc welding process. The wire is continuously fed from a coil and contributes to the molten metal pool used in the fusion process. GMA welding is typically used for welding steel. In GTA welding (also called TIG welding for tungsten inert gas welding), a tungsten rod is used as the electrode to establish the arc. The melting point of tungsten is relatively high, and therefore the electrode does not melt during the fusion process. If filler metal must be added to the weld, it must be added separately from the electrode. The GTA process is typically used for welding aluminum, copper, and stainless steel. In both GMA and GTA welding, inert gases such as helium or argon are used to surround the immediate vicinity of the welding arc to protect the fused surfaces from oxidation.
MDI AND COMPUTER CONTROLLER Some robot applications have requirements for which a digital computer is the most appropriate method of work cell control. We are referring to the use of a stand-alone computer (generally a mini computer or micro computer) rather than the computer which is used as the robot control unit. In cases where a computer is the work cell controller, it would be used either in series with a programmable controller or as a substitute for the PC. The computer might perform other function in the plant, and so it would be implemented to control the robot cell in a time-sharing mode of operation. Also, the computer would probably form a component in a hierarchical computer network in the factory, connected down to the programmable controller(s) and/or robot controller(s) in the cell, and connected up to the next hierarchical level in the plant. Programmable controllers are specialized devices that are designed to be interfaced with industrial processes. They are provided with input/output ports that can be directly wired to the plant equipment. This is an advantage over the digital computer, and special arrangements must be made to interface the computer to the industrial equipment in the cell. However, the PC has certain limitations in data processing and programming language which give the computer an advantage in applications requiring these capabilities. Since example of the kinds of robot application features that might tend to favor the use of computers for work cell control would include the following: Cases in which there are several cells whose operations must be coordinated and significant amounts of data must be communicated between the cells. Cells in which the error detection and recovery problem constitutes, a significant portion of the coding that must be programmed into the work cell operation. Where several different products are made on the same robot-automated production line, the operations at the different stations have to be coordinated and sequenced properly. Computer would be well suited to the data processing chores required in this type of applications, the various sizes and styles of the component parts must be sorted and matched to the particular model being assembled at each respectively work station along the line. Situations in which a high level or production scheduling and inventory control are required in the operation of the cell. Again, this type of data processing function might require the use of a computer in addition to or as a substitute for a programmable controller.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
91
Situations in which a high level of production scheduling and inventory control are required in the operation of the cell. Again, this type of data processing function might require the use of a computer in addition to or as substitute for a programmable controller. The different between digital computer and programmable controller are principally difference in applications rather than differences in basic technology. The PC can, in fact, be considered to be a specialized form and digital computer with dedicated features for input/output control of industrial equipment. The technologies of the two types of control devices are quite similar. TEACH PENDANT The MAKER robot system is produced by United States robots. Incorporated, a subsidiary of square D company. The appends is based on the MAKER Robot system operation manual, copyright 1984 by United States robots. Portions of the manual are reprinted with permission of United States Robots, Inc. The MAKER robot system is some what different to program than the other systems presented in this topic. The MAKER is designed to be operated with a minimum of training of the operator. To accomplish this MAKER is programmed using only the teach pendant. The teach pendant displays menus which allow the operator to select the necessary program commands. By responding to the menu options available, the operator is able to define points or to develop programs.
The below table describes the function of each of the major elements of the teach pendant. Item Display Function 1-5 Numeric Prev Lev Status Halt Vert align JT W/T Relax Speed Grip 1,2 Motion switches
Function Error message, status, operator prompts Buttons with associated displays for menu choices To enter numeric data To move to the previous menu level Displays status of overrides Stops arm motion and put the system in “halt� mode. Aligns the tool Z axis with the would Z axis. Selects joint or world/tool motions in manual mode Relaxes the selected joint. Used to set monitor speed of arm Used to operate the gripper controls Used to move the joints or the arm in world or tool mode depending on JT W/T
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
92
UNIT-VI
ROBOT APPLICATION IN MANUFACTURING Current day robot applications include a wide variety of production operations. For purposes of organizations in this book, we will classify the operations into the following categories. There are many robot applications in which the robot is required to move a work part of other material from one location to another. The most basic of the applications is where the robot picks the part up form one position and transfers it to another position. In other applications, the robot is used to load and/or unload a production machine of some type. In this book we divide material-handling applications into two specific categories. 3. Material transfer applications. 4. Machine loading/unloading applications. There are other robot applications which involve part handling. These include assembly operations and holding parts during inspection. Before discussing material transfer and machine loading/unloading applications. Let us review some of the considerations that should be examined when robots are used for material handling. GENERAL CONSIDERATION IN ROBOT MATERIAL HANDLING In planning an application in which the robot will be used to transfer parts, load a machine, pr other similar operation, there are several considerations that must be reviewed. Most of these considerations have been discussed in previous and we itemize them below as a reference checklist. 9. Part positioning and orientation:In most parts-handling applications the parts must be presented to the robot in a known position and orientation. Robots used in these applications do not generally possess highly sophisticated sensors (E.g., Machine vision) that would enable them to seek out a part and identify its orientation before picking it up. 10. Gripper design:Special and effectors must be designed for the robot to grasp and hold the work part during the handling operation. 11. Minimum distances moved:The material handling application should be planned so as to minimize the distances that the part must be moved. This can be accomplished by proper design of the work cell layout (E.g., Keeping in the equipment in the cell close together), by proper gripper design (E.g., Using a double gripper in a machine loading/unloading operation), and by careful study of the robot motion cycle 12. Robot work volume:The cell layout must be designed with proper consideration given to the robot’s capability to reach the required extreme locations in the cell and still allow room to maneuver the gripper. 13. Robot weight capacity:-
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
93
There is an obvious limitation on the material handling operation that the load capacity of the robot must not be exceeded. A robot with sufficient weight carrying capacity must be specified for the application. 14. Accuracy and repeatability:Some applications require the materials to be handled with very high precision. Other applications are less demanding in this respect. The robot must be specified accordingly. 15. Robot configuration, degree of freedom, and control:Many parts transfer the operations are simple enough that they can be accomplished by a robot with two to four joints of motion. Machine loading applications often require more degrees of freedom. Robot control requirements are unsophisticated for most material handling operations. Palletizing operations and picking parts from a moving conveyor are example where the control requirements are more demanding. 16. Machine utilization problems:It is important for the application to effectively utilize all pieces of equipment in the cell. In a machine loading/unloading operation, it is common for the robot to be idle while the machine is working, and the machine to be idle while the robot is working. In cases where a long machine cycle is involved, the robot is idle a high proportion of the time. To increase the utilization of the robot, consideration should be given to the possibility for the robot to service more than s single machine. One of the problems arising in the multi machine cell is machine interference, discussed in MACHINE LOADING AND UNLOADING These applications are material handling operations in which the robot is used to service a production machine by transferring parts to and/or from the machine. There are three cases that fit into this application category. Machine load/unload:The robot loads a raw work part into the process and unloads a finished part. A machining operation is an example of this case. Machine loading:The robot must load the raw work part or materials into the machine but the part is ejected from the machine by some other means. In a press working operation, the robot may be programmed to load sheet metal blanks into the press, but the finished parts are allowed to drop out of the press by gravity. Machine unloading:The machine produces finished parts from raw materials that are loaded directly into the machine without robot assistance. The robot unloads the part from the machine. Examples in this category include die casting and plastic modeling application. The application is best typified by a robot-centered work cell which consists of the production machine, the robot, and some form of parts delivery system. To increase the productivity of the cell and the utilization of the robot, the cell may include more than a single production machine. This is desirable when the automatic machine cycle relatively long, hence causing the robot to be idle a high proportion of the time. Some cells are designed so that each machine performs the same identical operation. Other cells are designed as flexible automated systems in which different parts follow a different sequence of operations at different machines in the cell. In either case, the robot is used to perform the parts handling function for the cell. Robots have been successfully applied to accomplish the loading and/or unloading function in the following production operations.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
94
Die castings Plastic molding Forging and related operations Machining operations Stamping press operations We will discuss these applications in the following subsections. For each application, a brief description of the manufacturing process will be given. DIE CASTINGS Die castings is a manufacturing process in which molten metal is forced into the cavity of a mold under high pressure. The mold is called a die (hence the name, die casting). The process is used to cast metal parts with sufficient accuracy so that subsequent finishing operations are usually not required. Common metals used for die-casted parts include alloys of zinc, tin, lead, aluminum, magnesium and copper. The die consists of two halves that are opened and closed by a die casting machine. During operation the die is closed and molten metal is injected into the cavity by a pump. To ensure that the cavity is filled, enough molten metal is forced into the die that it overflows the cavity and creates “flash� in the space between the die halves. When the metal has solidified, the die is opened and the cast part is ejected, usually by pins which push the part away from the mold cavity. When the part is removed from the machine, it is often quenched (to cool the part) in a water bath. The flash that is created during the casting process must be removed subsequently by a trimming operation which cuts around the periphery of the part. Thus, the typical die-casting production cycle consists of casting, removing the part from the machine, quenching and trimming. The production rates in the die-casting process range from about 100 up to 700 openings of the die per hour, depending on type of machine, the metal being cast and the design of the part. For small parts, the die can be designed with more than one cavity, thus multiplying the number of parts made for each casting cycle. The die-casting machines have traditionally been tended by human operators. The work tends to be not, repetitive, dirty and generally unpleasant for humans. Perhaps because of these conditions, die casting was one of the very first processes to which the robots were applied. The first use of a robot in die casting was around 1961. Engelberger cites one 2 instance in which a Unimate robot has been used in a die casting application for over 90,000 hours The die-casting process represents a relatively straight forward application for industrial robots. The alterations required of the die-casting machine are minimal and the interlocking of the robot cycle with the machine cycle can be accomplished by simple limit switches. Few problems are encountered in either the programming of the robot or the design of the gripper to remove the part from the machine when the die is opened. The process requires only that the robot unload the die casting machine, since the metal is in the molten state before the part is formed. On some die-casting machines (called coldchamber die-casting machines), the molten metal must be ladled from the melting container into the injection system. This part of the process is more difficult for robots to accomplish. MACHINE TOOL LOADING AND UNLOADING Machining is the metal working process in which the shape of the part is changed by removing the excess material with a cutting tool. It is considered to be a secondary process in which the final form and dimensions are given shape of the part. There are a number of different categories of machining operations. The principal types include turning, drilling, milling, shaping, planning, and grinding. Commercially, machining is a important metal working process and is widely used in many different products, ranging from those that are made in low quantities to those produced in vary high numbers. In mid-volume and high-volume production, the operation is very repetitive with the same machining sequence being repeated on part after part.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
95
The machine tools that perform machining operation have achieved a relatively high level of automation after many years of development. In particular, the use of computer control (E.g., computer numerical control and direct numerical control) permits this type of equipment to be interfaced with relative ease to similarly controlled equipment such as robots. Robots have been successfully utilized to perform the loading and unloading functions in machining operations. The robot is typically used to load a raw work part (a casting, forging or other basic form) into the machine tool and to unload the finished part at the completion of the machining cycle. The following robot features generally contribute to the success of the machine too load/unloading 2 application . Dual gripper:The use of the dual gripper permits the robot to handle the raw work part and the finished part at the same time. This permits the production cycle time to be reduced. Up to six joint motions:A large number of degrees of freedom of the arm and wrist are required to manipulate and position the part in the machine tool. Good repeatability:A relatively high level of precision is required to properly position the part into the chuck or other work holding fixture in the machine tool. Palletizing and depalletizing capability:In midvolume production, the raw parts are sometimes most conveniently presented to the work cell and delivered away from the work cell on pallets. The robot’s controller and programming capabilities must be sufficient to accommodate this requirement. Programming features:There are several desirable programming features that facilitate the use of robots in machining applications. In machine cells used for batch production of different parts, there is the need of perform some sort of changeover of the setup between batches. Part of this changeover procedure involves replacing the robot program for the previous batch with the program for the next batch. The robot should able to accept disk, tape, or other storage medium for ease in changing programs. Another programming feature needed for machining is the capability to handle irregular elements, such as tool changes of pallet changes, in the program. SPOT WELDING As the term suggest, spot welding is a process in which two sheet metal parts are fused together at localized points by passing a large electric current through the parts where the weld is to be made. The fusion is accomplished at relatively low voltage levels by using two copper (or copper alloy) electrode squeeze the parts together at the contact points and apply the current to the weld area. The electric current results in sufficient heat in the contact area to fuse the two metal parts, hence producing the weld. The two electrodes have the general shape of a pincer. With the two halves of the pincer open, the electrodes are positioned at the point where the parts are to be fused. Prior clamping or fixturing of the parts is usually required to hold the pieces together for the process. The two electrodes are squeeze together against the mating parts, and the current is applied to cause heating and welding of the contacting surfaces. Then the electrodes are opened and allowed to cool for the next weld. A water circulation system is often is used to accelerate the cooling of the electrodes. The actual welding portion of the sequence typically requires less than a second. Therefore, the rates of production of spot welding are largely dependent on the time required for positioning of the welding electrode and parts relative to each other. Another factor that affects production rate is wear of the electrodes. Because of the heat involve in the process, the tips of the electrodes gradually lose their shape and build up a carbon deposit which affects their electric resistance. Both of those effects reduce the quality of the welds made.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
96
Therefore, the electrode tips must periodically be dressed to remove the deposits and restore the desired shape. Spot welding has traditionally been performed manually by either of two methods. The first method uses a spot welding machine in which the parts are inserted between the pair of electrodes that are maintained in a fixed position. This method is normally used for relatively small parts that can be easily handled. The second method involves manipulating a portable spot welding gun into position relative to the parts. This would be used for large work such as automobile bodies. The word “portable� is perhaps an exaggeration. The welding consists of pair of electrodes and a frame to open and close electrodes. In addition, large electrical cables are used to deliver the current to the electrodes from a control panel located near the workstation. The welding gun with cables attached is quite heavy and can easily exceed 100lb in weight. To assist the operator in manipulating the gun, the apparatus a suspended from an overhead hoist system. Even with this assistance, the spot-welding gun represented a heavy mass and is difficult to manipulate by a human worker at the high rates of production desired on a car body assembly line. There are often problems with the consistency of the welded products made on such a manual line as a consequence of this difficult. ARC WELDING Arc welding is a continuous welding process as opposed to spot welding which might be called a discontinues process. Continuous arc welding if used to make long welded joints in which an airtight seal is often required between the two pieces of metal being joined. The process uses an electrode in the form of a rod or wire of metal to supply the high electric current needed for establishing the arc. Currents of typically 100 to 300A at voltages of 10 to 30V. The arc between the welding rod and the metal parts to be joined produces temperatures that are sufficiently high to form a pool of molten metal to fuse the two pieces together. The electrode can also be used to contribute to the molten pool, depending on the type of welding process. Arc welding is usually performed by a skilled human worker who is often assisted by a person called a fitter. The purpose of the fitter is to organize the work and to fixture the parts for the welder. The working conditions of the welder are typically unpleasant and hazardous. The arc from the welding process emits ultraviolet radiation which is injurious to human vision. As a result welders are required to wear eye protection in the form of a welding helmet with a dark window. This dark window filters out the dangerous radiation, but it is so dark that the welder is virtually blind while wearing the helmet except when arc is struck. Other aspects of the process are also hazardous. The high temperature created in arc welding and the resulting molten metals are inherently dangerous. The high electrical current used to create the arc is also unsafe. Sparks and smoke are generated during the process and these are a potential threat to the operator. There is a variety of arc welding processes, and the reader is referred to the available manufacturing process texts for more details than we can include here. For robot applications, two types of arc welding seem the most practical: Gas Metal Arc Welding (GMAW) and Gas Tungsten Arc Welding (GTAW). GMA welding (also called MIG welding for metal inert gas welding) involves the use of a welding wire made of the same or similar metals as the parts being joined. The welding wire serves as the electrode in the arc welding process. The wire is continuously fed from a coil and contributes to the molten metal pool used in the fusion process. GMA welding is typically used for welding steel. In GTA welding (also called TIG welding for tungsten inert gas welding), a tungsten rod is used as the electrode to establish the arc. The melting point of tungsten is relatively high, and therefore the electrode does not melt during the fusion process. If filler metal must be added to the weld, it must be added separately from the electrode. The GTA process is typically used for welding aluminum, copper, and stainless steel. In both GMA and GTA welding, inert gases such as helium or argon are used to surround the immediate vicinity of the welding arc to protect the fused surfaces from oxidation. Problems for robots in arc welding
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
97
Because of the hazards for human workers in continuous arc welding, is logical to consider industrial robots for the process. However there are significant technical and economic problems encountered in applying robots to arc welding. Continuous arc welding is commonly used in the fabrication industries where products consisting of many components are made in low quantities. It is difficult to justify automation of any form in these circumstances. A related problem is that arc welding is often performed in confined areas that are difficult to access, such as the insides of tanks, pressure vessels, and ship hulls. Humans can position themselves into these areas more readily than robots. There are two approaches to compensate for these variations and irregularities in robot welding applications:1. Correct the upstream production operations so that the variations are reduced to the point where they do not create a problem in the robot welding process. 2. Provide the robot with sensors to monitor the variations in the welding process and the control logic compensate for part variations and weld gap irregularities.
The typical robot Arc-welding application:Because of the technical and economic problems discussed above, the typical arc-welding application for a robot is one in which the quantities of production are medium of high. In these cases, a robot cell can be justified which consists of a welding robot and a part holder or part manipulator. The part holder is used to fixture the components and position them for welding. A part manipulator provides an additional capability in the form of 1 or 2 degrees of freedom to position and orient the components relative to the robot. The robot is equipped with a welding rod or wire feed system, and the required power source to provide the electric current for the operation. The work cell controller is used to coordinate the robot motion, the welding current, the wire feed, the part manipulator, and any other activities in the cell. SPRAY PAINTING Most products manufactured from metallic materials require some form of painted finish before delivery to the customer. The technology for applying these finishes varies in complexity from simple manual methods to highly sophisticated automatic techniques. We divided the common industrial coating methods into two categories: 1. Immersion and flow painting methods 2. Spray painting methods
Robots in spray painting Because of these hazards to humans, the use of industrial robots has developed as an alternative means of performing spray coating operations. Spray coating operations to which robots have been applied include painting of car bodies, engines, and other components in the automotive industry, spraying of paint and sound absorbing coatings on appliances, application of porcelain coatings in bathroom fixtures, and spray staining of wood products. Some of the applications have consisted of a stand-alone robot spraying a stationary work part that has been positioned in a paint booth by a human worker. However, these applications are generally less successfully because they rely heavily on the human worker and the utilization of the robot is relatively low. In most robot spray-coating applications, the robots are usually part of a system that includes a conveyor for presenting the parts to the robot, and a spray booth for shielding the spraying operation from the factory environment. When a conveyor robot system is used, the operation of the robot and the conveyor must be closely synchronized. In the case of an intermittent conveyor system, interlocks are used to coordinate the start and finish of the robot program with the movement of the conveyor. With a continuously moving conveyor, some form of baseline tracking system is required in order to synchronize the robot’s motions with the movement of the conveyor.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
98
Another feature of many robot spray-coating applications is that the system must be designed to process a variety of part styles, each with its own unique configuration. This is usually accomplished by providing work cell with the parts identification system. Once the part has been properly identified, the robot can apply the correct spray cycle for that part. In general, the requirements of the robot for spray-coating applications are the following: 1. Continuous path control:In order to emulate the smooth movement of a human spray paint operator, the robot must possess man degrees of freedom in its manipulator and it must have continuous path capability. 2. Hydraulic drive:Hydraulic drive is preferred over electric or pneumatic drive in spray painting applications. In electric drive there is danger that a spark in the electric motor system may ignite the paint fumes in the spray booth environment. The motions generated in pneumatic drive are generally to jerky to be suitable for spray-coating applications. 3. Manual lead through programming:In most spray coating applications, the moat convenient method of teaching the robot involves lead through programming in which the robot arms is manually pulled through the desired motion pattern by a human operator who is skilled in the techniques of spray painting. During the programming procedure, a “teach arm� which is light and maneuverable, is often substituted for the actual robot arm, which tends to be heavy and difficult to manipulate smoothly. 4. Multiple program storage:The need of multiple program storage arises in paint production in more than one part style are presented to the robot for spraying. The capability to quickly access the program for the current part is a requirement for these lines. Either the robot itself must have sufficient memory for the programs required or it must be interfaced to the cell controller (computer or programmable controller) for random access to this memory capacity. Another requirement for consistent quality in the finishing operation is that the spray gum must be periodically cleaned. This can be accomplished without significant loss of production time by programming a cleaning cycle into the work cell operation at regular intervals. The cleaning operation takes only a few seconds to complete and consists of the robot placing the spray nozzle under cleaning jets which spray solvent into the nozzle opening. Incorporating the cleaning operation into the work cycle should be planned to minimize the impact on the productive portion of the cycle.
Benefits of Robot Spray Coating:Use of robots to perform spray-finishing operations provides a number of important advantages. These advantages include:1. Removal of operators from hazardous environment 2. Lower energy consumption 3. Consistency of finish 4. Reduced coating material usage 5. Greater productivity Removing the human workers from the kinds of hazards which characterize the manual spray finishing environment (i.e. fumes, fire hazards, etc.) is a significant health benefit of using robots. Also, because humans are not in the spray booth, the ventilation requirements are reduced below the levels needed when humans are present. Therefore less energy is needed to control the environment. ASSEMBLY FINISHINING
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
99
The term assembly is defined here to mean the fitting together of two or more discrete parts to form a new subassembly. The process usually consists of the sequential addition of components to a base part or existing subassembly. To create a more complex subassembly or a complete product. As such, assembly operation involves a considerable amount of handling and orienting of parts to mate them together properly. The difference between assembly tasks and other material-handling tasks is value is added to the product through the assembly operation. Also, there are often interactions that take place between the two parts being assembled, between the gripper and the part, and between other elements of the work cell. When parts are fastened together (called parts joining), there are often additional interactions with the medium used to join the components (e.g. adhesive). All of these potential interactions can make assembly operations considerably more complex compared to the simpler task of moving a part from one location to another. There is a variety of assembly processes used in industry today. These include mechanical fastening operations (using screws, nuts, bolts, rivets, swaging etc.), welding, brazing and bonding by adhesives. Some of these processes are more adaptable to automatic assembly. In our coverage of the application of robotics to assembly, we will divide the subject into three areas as suggested by the preceding discussions: 1. Parts presentation methods 2. Assembly tasks 3. Assembly cell design Other assembly applications performed by robots include putting together scissors, pliers and other simple hand tools, the fabrication of small electric motors, and the assembling of electrical plugs and switches. In most of these examples, the robot is taught the desired points and the sequence of operations. The only external sensory information that is normally utilized is whether or not a part or subassembly is at a particular location within the work cell. As mentioned above, some applications depend on the robot wrist being compliant. This is especially important in certain assembly operations, for example, insertion of shafts or rods into small clearances holes or the screwing of a screw into a threaded hole. To prevent the binding and/or bending of the rods or cross threading of the screws, an RCC is often used between the end effectors and the robot’s wrist flanges. Alternatively, force and/or tactile feedback can be utilize to provide better external sensing capability , thereby permitting the robot to adapt better to any positional errors ADOPTING ROBOTS TO WORK STATION In addition to the problem of designing the physical layout of the robot cell, another problem is concerned with coordinating the various activities that occur in the cell. Most of these activities occur sequentially, but simultaneously occur in the cell. There are other factors such as safety of human personnel which must also be considered. Co ordinates of these various activities are accomplished by a device called the work cell controller or work station controller. The functions performed by the work cell 11 controller can be divided into three categories as suggested by thomas . 1. Sequence control 2. Operator interface 3. Safety monitoring These functions are accomplished either by the robot controller itself or by higher-level control devices, such as a programmable controller. The robot controller usually has a limited input/output capability to permit interfacing with other pieces of equipment in the cell. If the control requirement to operate the cell become at all complicated, then a higher-level controller is needed. We shall discuss the use of programmable controllers in a subsequent section of this chapter. Also, the implementation of an effective work cell controller is dependent largely on the programming capabilities of the robots used. Sequence Control
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
100
Sequence control is the primary function of the work station controller during regular automatic operation of the work cell. It includes the following kinds of control functions: Control of the sequence of activities in the work cell Control of simultaneous activities Making decisions to proceed with the work cycle based on events that occur in the cell Making decisions to stop or delay the work cycle based on events that occur in the cell These functions occur during the normal operation of an average robot work cell. The following example illustrates these control requirements. Example 1:This example in intended to demonstrate the importance of controlling the sequences of activities in the work cell during regular operation. The cell consists of a large robot, a numerically controlled machining center (which operates on an automatic cycle), and a belt conveyor for delivering raw work parts into the cell. Finished parts are placed on a pallet. The layout shown in below figure. The work cycle consists of the following sequences.
Machine tool
Output Conveyor
Input Conveyor Robot
1. Robot picks up raw work parts from conveyor which has delivered the parts to a known pickup position (machine idle). 2. Robot loads part into fixture at machining center (machine idle). 3. Machining center begins automatic machining cycle (robot idle). 4. Machine completes automatic machining cycle. Robot unloads machine and places part on the pallet (machine idle). 5. Robot moves back to pickup point (machine idle). In this cell operation, nearly all of the activities occur sequentially. The only significant exception is that the conveyor would probably operate continuously to deliver raw parts into the cell during the work cycle. The purpose of the workstation controller in this example would be to make sure that the activities occurred sequence and that each step in the sequence has been completed before the next is initiated. Fro example, the start of the automatic machining cycle must not occur until the robot has loaded the raw work part into the machining future. Since all the relevant activity in the proceeding example is sequential, either the robot or the machine tool is idle for a significant portion of the cycle. The productivity of this work cell could be improved by using a double gripper. We have considered the different types of grippers in previous. A double gripper is a gripper with two mechanisms for independent holding of two separate work parts. Use of a double gripper in our example would permit unloading and loading of the machine to be combined into a sing (but more complicated) step. It would also allow the automatic machining cycle to occur at the same time that the robot was performing a portion of its motion. Operator Interface The purpose of the operator interface in workstation control is to provide a means for human operators to interact with the operation of the cell. There are several situations where this would be required. Among the most important cases are the following:
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
101
1. The human is an integral part of the work cell. 2. Emergency stop conditions. 3. Program editing or data input by operator. The first situations where the human worker plays an integral role in the operation of the cell. The human performs a portion of the work cycle and the robot performs a portion of the cycle. In this situations, there is need to allow for variations in the time required by the operator to perform the manual part of the cycle. In a robotic cell, this can be easily accomplished by means of stop/start controls placed conveniently for the operator. The operator uses these controls to regulate the robot cycle as required by the situation. The following example illustrates the need for using operator controls to allow for variability in operating pacing. Example:In this example, a human operator is used with a robot to perform a hot working operation on a metal billet. Hot forging us a common operation in this category. The operator’s task in the cycle is to remove hot billets from random positions in a furnace and place them in a nest from which the robot can retrieve the parts for the hot working operation. There would probably be variations in the time required for the worker to perform the manual portion of the cycle because of differences in the locations of properly heated parts in the furnace. At a particular time during the day, it might turn out that none of the parts in the furnace are heated to a sufficient temperature for the operation. This would be a reason for delaying the next operation cycle. For these and various other reasons, there is a need for the operator to be able to signal the robot when to start its portion of the cycle. The second situation involves an emergency in which a human worker needs to prevent continued operation of the robot cycle. The reason for the emergency may be a safety problem or an irregularity in the work cycle that is potentially destructive to the robot or other equipment in the cell. A safety problem might arise when some person walking through the plant has unwittingly intruded into the robot’s space. An irregular event in the work cycle might be that the robot has grasped the work part improperly for loading into a machine and this would cause damage to the machine or the associated tooling during processing. In either case, the human worker located in the cell would have reason to 11 interrupt the operation of the robot cycle until the emergency situation was corrected. Thomas stresses the need for the operator controls to respond in a consistent and predictable manner to emergency stop conditions. The third requirement for operator control is to perform editing of the program or other similar input functions. Some robot controller requires that the robot be in a nonoperational mode when changes are made in the program. The more flexible controllers allow for editing to be accomplished while the robot is performing its regular cycle.
Safety Monitoring In addition to the operator’s ability to override the regular work cycle in the event of an observed safety hazard, the work cell controller should also be capable pf monitoring its own operation for unsafe or potentially unsafe conditions in the cell. This function is called safety monitoring of hazard monitoring.
ROBOT CHARACTERISTICS The following definitions are used to characterize robot specifications:Payload Payload is the weight a robot can carry and still remain within its other specifications. For example a robot’s maximum load capacity may be much larger than its specified payload, but at maximum level, it may become lass accurate, may not follow its intended path accurately, or may have excessive deflections. The payload of robots compared with their own weight is usually very small. For
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
102 TM
example, Fanuc robotics LR Mate robot has a mechanical weight of 86 lbs and a payload of 6.6 lbs TM and the M-16i robot has a mechanical weight of 594 lbs and a payload of 35 lbs. Reach Reach is the maximum distance a robot can reach within its work envelope. As we will see later, many points within the work envelope of the robot may be reached with any desired orientation (called dexterous). However, for other points, close to the limit of robot’s reach capability, orientation cannot be specified as desired (called nondexterous point). Reach is a function of the robot’s joint length and its configuration. Precision (validity) Precision is defined as how accurately a specified point can be reached. This is a function of the resolution of the actuators, as well as its feedback devices. Most industrial robots can have precision of 0.001” or better.
Repeatability (variability) Repeatability is how accurately the same position can be reached if the motion is repeated many times. Suppose that a robot is driven to the same point 100 times. Since many factors may affect the accuracy of position, the robot may not reach the same point every time, but will be within a certain radius from the desired point. The radius of a circle that is formed by this repeated motion is called repeatability. Repeatability is much more important that precision. If a robot is not precise, it will generally show a consistent error, which can be predicated and thus corrected through programming. As an example, suppose that a robot is consistently off 0.05” to the right. In that case, all desired points can be specified at 0.05” to the left, and thus the error can be eliminated. How ever, if the error is random, it cannot be predicated and thus cannot be eliminated. Repeatability defines the extent of this random error. Repeatability is usually specified for a certain number of runs. More test yield larger (bad for manufacturers) and more realistic (good for the users) results. Manufacturers must specify repeatability in conjunction with the number of tests, the repeatability of an arm in a vertical direction will be different from when the arm is tested in a horizontal configuration. Most industrial robots have repeatability in the 0.001” range.
STAGES IN SELECTING ROBOT FOR INDIVIDUAL APPLICATION After the application has been selected, the applications engineer must choose which robot to use for the job from the many commercial models available. Indeed, selection of the application must often be done with consideration of available robots in mind. The robot selected should possess an appropriate combination of technical features (e.g number of axes, types of control systems, work volume, ease of programming, precision of motions, load carrying capacity, etc,) for the application being consideration. If a deviation from the needed capabilities rather than less. It might turn out that when the current application is completed and another application is being sought for the robot, the new application may require greater technological capabilities than the current application. Below table presents a listing of the kinds of typical robot technical features needed in common applications. It should be emphasized that the data included in this table are consider to be represented of current robot application practice, but exceptions to the recommendations in the table can be found in successful installation in industry today. However, the use of this table might help the applications engineer to reduce the number if alternative robot models to select from.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
Sl.No. 1
Applications Material transfer
2
Machine loading
3
Spot welding
4
Arc welding
5
Sprat coating
6
Assembly
103
Typical technical feature required Number of axes: 3 to 5 Control system: limited sequence or point to point playback Drive system: pneumatic or hydraulic (for heavy loads) Programming: manual, powered lead through Anatomy: Polar, cylindrical, jointed arm Number of axes: 4 or 5 Control system: limited sequence or point to point playback Drive system: electric or hydraulic (for heavy loads) Programming: powered lead through Anatomy: Polar, jointed arm Number of axes: 5 or 6 Control system: point to point playback Drive system: electric or hydraulic Programming: powered lead through Anatomy: Polar, jointed arm, Cartesian Number of axes: 5 or 6 Control system: continuous path playback Drive system: electric or hydraulic Programming: manual or powered lead through Anatomy: Jointed arm Number of axes: 6 or more Control system: continuous path playback Drive system: hydraulic Programming: manual lead through Anatomy: Jointed arm, Cartesian(box), SCARA Number of axes: 3 to 6 Control system: point to point or continuous path Drive system: electric Programming: powered lead through, textual language Accuracy and repeatability: high
To make the final selection of the robot, the following decision procedure is suggested. The procedure consists of preparing a detailed listing of the technical features of the particular applications and then systematically comparing these features against the specifications of the alternative models under considerations. It is advantageous to divide the list of technical features into two categories: “must” and “desirable”. The “must” features are ones that must be satisfied by the robot in order to perform the application. If any of the candidates do not satisfy the “must” features, then that robot model is excluded from further consideration.
The “desirable” features are ones that are necessarily required to accomplish the application but would be highly beneficial during installation and/or operation. The specifications of each robot candidate would be compared to each of the desirable features and a rating score would be assigned to the candidate to indicate how well the robot satisfies the particular features. There may be difference in relative importance among the various features, and this would be taken into account by giving each feature a maximum possible point score. Determination of rating score for the different robot models in each feature category would be a judgment call that the applications engineers would have to make based on the relative merits of each candidate. PRECAUTION OF ROBOT
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
104
There are two aspects of the safety issue in robotics. The first deals with the justification of robots. Historically, one of the fundamental reasons for using robots in industrial applications is to remove human operators from potentially hazardous work environments. The hazards in the workplace include heat, noise, fumes, and other discomforts, physical dangers (potential injuries or even loss of limbs), radiation, toxic atmospheres, and other health hazards. The problem of removing or reducing these hazards from the workplace has provided one of the important justifications for industrial robots in applications such as welding, forging, spray painting, and die casting. Since the Occupational Safety and Health Act (OSHA) was enacted in 1971, worker safety has become a significant factor in promoting the substitution of robots for human labor in these kinds of dangerous jobs. The second aspect of the safety issue involves the potential hazards to humans posed by the robot itself. The use of robots presents a new set of possible dangers to the human worker fro which precautions must be taken. It is this second aspect of the safety issue which we will address in this chapter. In considering the potential hazards that are encountered the use of the robot are robot, it seems reasonable to question when during the use of the robot are humans in contact or close proximity to it. There are three occasions when humans are close enough to the machine to be exposed to danger There are: During programming of the robot During operation of the robot cell when humans work in the cell During maintenance of the robot The types of risks encountered during these times include physical injury from collision between the humans and the robot, electrical shock, objects (parts or tools) dropped from the robot gripper, and loose power cables or hydraulic lines on the floor. Some of these risks can be reduced with straight forward safety measures such as proper grounding of electrical cables to prevent shock, and raised floor platforms to cover power cables and hydraulic lines. In other cases, operator safety is improved by requiring certain common sense procedures to be followed. For example, when the robot is being programmed, the speed of the arm should be set at a low level during teaching and testing of the program. Another example would be when the maintenance personnel are servicing the robot. During maintenance, the power to the machine should be turned off under normal circumstances. More extensive safety measures must be taken to guard against hazards that arise during robot operation. Many of these safety measures must be designed into the work cell, wither as part of the work cell, either as part of the work place design or as part of the work cell control system. Workplace design consideration for safety Certain safety features can e designed into the robot work cell. These include physical barriers to limit intrusion into the cell, emergency stop buttons to hall the cell operation, and laying out the equipment in the cell for maximum safety. The most common approach is to construct a physical barrier around the periphery of the robot work cell. The periphery of the robot must be defined to be out side the farthest reach of the robot in all directions with end effectors attached to the wrist. The work cell would also include any equipment in the cell which operates with the robot. The barrier should not be designed only for the programmed work cycle envelope because a malfunction of the controller may cause the robot to follow a trajectory different from its normal program. The barrier has the effect of preventing human intruders from entering the vicinity of the robot while it is operating. The barrier often consists of a fence with a gate fro access to the work cell. The gate is equipped with an interlock device so that the work cycle is interrupted when the gate is open. A positive restart procedure is designed into the cell for resumption of the cycle, rather than using the gate closure for restarting. Other possible physical barriers include safety rails and chains, although these are not as effective as a full fence. Another approach that has been used in industry as a physical barrier is a steel post in the floor at the limits of the programmed motion cycle, so that a cut of control robot arm crashes into the post
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
105
rather than g beyond its allowed space. However, there are certain undesirable aspects to this type of barriers that make it unsuitable. First, if the robot arm crashes into the steel post at high speed, it will no doubt be damaged or destroyed. This kind of mishap could occur during programming of the robot rather than as a result of a malfunction of the robot itself. Thus, although the steel post is intended to protect against a robot control failure, it is possible that the robot arm could be ruined due to human error instead. A second features which makes the post unsatisfactory is that it dose not prevent intruders from entering the robot cell. The third undesirable aspect of the steel post is that a human could be pinned between the robot arm and the post. This would probably result in greater injury to the human than by simply being struck in open space by the robot arm.
In a robot cell designed to operate with humans as coworkers in the production process, certain features must be designed into cell layout to protect the workers. This is typically encountered when human workers are employed to load and unload work parts in the cell and the robot is used to perform a processing operation such as arc welding or grinding. In these cases, some form of two-position parts manipulator can be used to exchange parts between the robot and the worker. One possible work cell configuration for accomplishing this exchange illustrated below. It uses a rotary indexing table to move raw work parts from the human operator’s position to the robot position for processing, and simultaneously moves completed parts from the robot position to the operator’s position for unloading. This arrangement prevents inadvertent collision between the worker and the robot. Some from of operator interface is required to permit the worker to index the table for the next cycle and to perform other functions such as emergency stop. In addition to its safety features, this type of layout has the additional advantage of production efficiency since it allows two operations to be performed simultaneously rather than sequentially. The loading and unloading of parts by the human operators takes place simultaneously with the processing of parts by the robot. Human worker Rotary table Position 2 Safety Barrier Position 1
Robot
Fig:- Cell layout using part manipulator to separate human worker from robot for safety and production efficiency Safety sensors In addition to these various approaches for designing safety into the robotic work cell, other safety provisions can be made as well. We will desirable some of the possible safety monitoring schemes that can be utilized in robot work cells in this subsection, and other measures, including emergency stop buttons and “dead man switches”. The National Bureau of Standards defines three levels of safety sensor systems in robotics. Level 1 – Perimeter penetration detection Level 2 – Intruder detection inside the work cell Level 3 – Intruder detection in the immediate vicinity of the robot. The first level systems are intended to detect that an intruder has crossed the perimeter boundary of the work cell without regard to the location of the intruder. In effect this would operate much the same as the fence surrounding the cell.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
106
Level 2 systems are designed to detect the presence of an intruder in the region between the work cell boundary and the limit of the robot work volume. The exact definition of this region would depend on the cell layout and the strategy used to ensure the safety of the intruder. Level 3 systems provide intruder detection inside the work volume of the robot. These sensor systems are intended to protect workers who must be in close proximity to the robot during operation of the robot (e.g during programming of the robot). This third category must be capable of detecting an imminent collision between the worker and the robot, and of executing a strategy for avoiding the collision. The below figure illustrates the three sensor levels. Level 1
Level 2
Work cell perimeter Machine Robot Work Volume
Machine Level 3
Level 2
Level 2
Fig:- Three levels of safety sensor systems
ECONOMIC ANALYSIS In addition to the technological considerations involved in applications engineering for a robotics project, there is also the economic issue. Will the robot justify itself economically? The economic analysis for any proposed engineering project is of considerable importance in most companies because management usually decides whether to install the project on the basis of this analysis. In the present chapter, we consider the economic analysis of a robot project. We discuss the various costs and potential benefits associated with the robot installation, and we describe several methods for analyzing these factors to determine the economic merits of the project. Basic Data required To perform the economic analysis of a proposed robot project, certain basic information is needed about the project. This information includes the type of project being considered, the cost of the robot installation, the production cycle time and the savings and benefits resulting from the project. Type or robot Installation There are two basic categories of robot installations that are commonly encountered. The first involves a new application. This is where there is no existing facility. Instead, there is a need fro a new facility, and a robot installation represents one of the possible approaches that might be used to satisfy that need. In this case, the various alternatives are compared and the best alternatives are selected, assuming it meets the company’s investment criteria. The second situation is the robot installation to replace a current method of operation. The present method typically involves a production operation that is performed manually, and the robot would be used somehow to substitute for the human labor. In this situation, the economic justification of the robot installation often depends on how inefficient and costly manual method is, rather than the absolute merits of the robot method.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
107
In either of these situations, certain basic cost information is needed in order perform the economic analysis. The following subsection discusses the kinds of cost and operating data that are used to analyze the alternative investment projects. The methods by which the analysis is accomplished are explained later in the chapter. Cost Data required for the analysis The cost data required to perform the economic analysis of a robot project divide into two types: Investment costs and operating costs. A. Investment costs 1. Robot purchase costs – The basic price of the robot equipped from the manufacturer with the proper options (excluding end effectors) to perform the applications. 2. Engineering costs – The costs of planning and design by the user company’s engineering staff to install the robot. 3. Installation costs – This includes the labor and material needed to prepare the installation site (provision for utilities, floor preparation, etc.,) 4. Special tooling – This includes the cost of the end effectors, parts positioners, and other fixtures and tools required to operate the work cell. 5. Miscellaneous costs – This covers the additional investment costs not included by any of the above categories. (e.g. other equipment needed for the cell)
B. Operating costs and savings 6. Direct labor cost – The direct labor cost associated with the operation of the robot cell. Fringe benefits are usually included in the calculation of direct labor rate, but other overhead costs are excluded. 7. Indirect labor cost – The indirect labor cost than can be directly allocated to the operation of the robot cell. These costs include supervision, setup, programming, and other personnel costs not included in category 6 above. 8. Maintenance – This covers the anticipated costs of maintenance and repair for the robot cell. These costs are included under this separate heading rather than in category 7 because the maintenance costs involve not only indirect labor (the maintenance crew) but also materials (replacement parts) and service calls by the robot manufacturer. A reasonable “rule of thumb” in the absence of better data is that the annual maintenance cost for the robot will be approximately 10 % of the purchase price (category 1). 9. Utilities – This includes the cost of utilities to operate the robot cell (e.g electricity, air pressure, gas). These are usually minor costs compared to the above items. 10. Training – Training might be considered to be an investment cost because cost because much of the training required for the installation will occur as a first cost of installation. However training should be a continuing activity, and so it is included as an operating cost. METHOD OF ECONOMIC ANALYSIS We shall describe three methods for analyzing investments and comparing investment alternatives that are in common use in industry. The three methods are. 1. Payback (or payback period) method 2. Equivalent uniform annual cost (EUAC) method 3. Return on investment (ROI) method Each of the methods accomplishes the analysis using a slightly different twist. Ideally, the same decision should be reached no matter which method is used; however, this is not always the case. We assume that the reader has some familiarity with the principles of engineering economy. Payback method
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
108
The payback method uses the concept of the payback period as shown. The payback period is the length of time required for the net accumulated cash flow to equal the initial investment in the project. Under the assumption that the net annual cash flows are equal from year to year, this notion can be reduced to the following simple formula
n=
IC / NACF
Where n = the payback period IC = the investment cost NACF = the net annual cash flow In most investment projects it would be unlikely that the cash flows would be exactly equal each year. When there are year to year differences in the cash flows, equation. Must be altered slightly to account for the differences. In this equation, the vale of n is determined so that the sum of the annual cash flows is equal to the initial investment cost. In the special case when the net annual cash flows are equal 0 = -(IC) + n(NACF) The reader should note that we have adopted the logical convention that costs are treated as negative values and revenues or savings (as well as profits) are treated as positive values in these equations. The NACF is assumed to be a positive cash flow since revenues derived from the robot project would be greater than the operating costs (we hope). We have also assumed that all cash flows occur either at the beginning of the year or at the end of the year. Any investments are assumed to be transactions that occur at the beginning of the year, while the net annual cash flows are assumed to be end-of year transactions. Most companies today require payback of no more than two or three years. As investment whose cash flows pays back the investment in less than one year is considered excellent Equivalent uniform annual cost (EUAC) method Equivalent uniform annual cost (EUAC) method converts all of the present and future investments and cash flows into their equivalent uniform cash flows over the anticipated life of the project. It does this by making use of the various interest factors associated with engineering economy calculations. We present a tabulation of these interest factors in an appendix to this chapter, and we save a considerable amount of explanation by assuming that the reader is familiar with their use. To begin with, the company must select a minimum attractive rate of return (MARR) which is used as a criterion to decide whether a potential investment project should be funded. Today, MARR values of 20 to 50% are not unusual for robot projects. Using the interest factors for the MARR to make the conversions, the uniform annual cost method then sums up the EUAC values for each of the various investment and cash flows associated with the project. If the sum of EUACs I greater than zero, this is interpreted to mean that the actual rate of return associated with the investment is greater than the MARR used by the company as the criterion. If the EUAC sum is less than zero, then the project is considered unattractive.
Return on investment (ROI) method:The return on investment (ROI) method determines the rate of return for the proposed project based on the estimated costs and revenues. This rate of return is then compared with the company’s
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
109
minimum attractive rate of return involves setting up an equivalent uniform annual cost equation similar to the one used above. The difference is that the EAUC sum of the left hand side of the equation is made equal to zero. Then the values of the interest factors (and correspondingly, the interest rates) are found that make the right hand side of the equation sum to zero.
SOCIAL AND LABOR ISSUES The growing number of robot installations will have an impact on our society in general and the labor force in particular. In addition to robotics, other manufacturing automation technologies (CAD/CAM, flexible automation, computer integrated production management systems, etc.,) will have similar effects. The obvious impact on the labor force will be the displacement of human workers by robots. There will also be other effects and in the natures of the work performed by humans. Not only will direct labor operators be affected by the introduction of robotics and related technologies: professional and semiprofessional staffs will be affected as well. Indeed, the field of robotics will have an impact that goes beyond the immediate work force. Society in general will be affected by this technology in areas such as productivity, education, and international economic competition. The applications of robotics will gradually extend beyond manufacturing into service sector. The spectacle of robots at work in blanks, hospitals, grocery stores, and fast food restaurants will not be uncommon in the future. When in the future these kinds of applications will begin is difficult to predict, but the impact on society will be significant. In July 1981, the Office of Technology Assessment (OTA) held a work shop to explore the social impact of robotics. A summary and statement of the issues was published in an OTA report in Feb 1982. Five categories of social issues are identified in that report. Productivity and capital formation Labor Education and training International impact Other applications (beyond current industrial applications) Productivity and capital formation In our previous discussions of applications and applications engineering, we have described how robots can reduce cost and increase productivity in manufacturing. Productivity improvement in the United States is an important social issue and a major national concern. The conventional definition of productivity if the following: Productivity = Units of output / Units of input The units of output can be reduced to monetary terms for purposes of comparing the products of different industries. Labor hours have traditionally been used as the units of input in productivity measurements, and the resulting ratio gives an indication of labor productivity only. However, capital (equipment), technical knowledge, and various other inputs are also considered to be ingredients contributing to productivity. When these other inputs are combined, the ratio is called the total factor productivity. Manufacturing management practice has been improved, for example, by computerized methods such as materials requirements planning (MRP), shop floor control, and other computer integrated manufacturing techniques. Robotics is an input to the productivity ratio which represents both capital and technical knowledge. As an input, it is a substitute for human labor in determining productivity. Presumably by making this substitution, productivity is improved. In the bargain, human workers are denied work opportunities with potentially serious financial and emotional consequences to themselves ad their families. Whether or not the productivity improvement is worth the sacrifice by the workers constitutes social issue.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
110
Robotics technology is a form of programmable automation. With programmable automation, the production system is designed with the capability to change its sequence of operations so as to process different product configurations. This type of automation is therefore ideally suited to batch manufacturing. The opportunity offered by the use of robotics and other forms of programmable automation is to increase the productivity and equipment utilization by significant margins in the batch manufacturing industries. If these systems could be made sufficiently independent of human labor, they could be operated 24 hours per day for 7 days per week. The resulting nonproductive time would be significantly reduced, and the economics of the production process would be substantially improved.
Table: -Time allocations in three basic types of discrete parts manufacturing Plant activity Plant shutdown Losses from second and third shift Equipment failure Inadequate storage Tool changes Load/unload, non cutting, etc., Setups, gauging etc., Setups, gauging and loading etc., Losses from nonoptimal cutting Other idle time Work standards, allowances, etc., Productive cutting
High Production %
Batch production %
27 -
28 40
7 7 7 14
6 7 4
Job shop production % 34 44 -
7 12 2 2 16 22 100
8 100
6 100
Capital formation is another social issue that is related to productivity. Economists often attribute the capability to create new investment capital to growth of productivity. The important social questions concerned with capital formation, as developed in the OTA exploratory workshop, seem to be the following. 1. Will there be sufficient capital available to fund the construction o few plants and the modernization of existing plants that will use robotics and other automation technologies? 2. Will there be sufficient capital to build the factories that will produce robots in the quantities needed for the new plants? 3. Will there be enough capital to fund research and development by entrepreneurs who wish to develop new types of robots and related products? The answers to these questions will depend largely on how robotics is perceived by business managers and the investment community to be a promising technology in which to invest. Investment capital in the United Stated is generally allocated to those opportunities that seem most attractive compared to the alternatives. If robotics is perceived as a technology that will generate a return on
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
111
investment for those who want to buy robots, build robots, or develop robots, then the capital will be available.
Robotics and labor The effect of robotics on direct labor Nearly all robot installations involve the substitution of the robot for one or more human workers. When robots are installed in existing production operations, this involves a direct displacement of the workers. And when robots are installed in new facilities, the direct labor operator who would have performed the work in these facilities is not hired. The rate of substitution would typically be one to three human workers for each robot that is installed. The number of robots compare with a present total human work force of about 103 million, according to the Bureau of Labor Statistics (1983). This total is expected to increase into the early 1990s but not with nearly the growth rate of robots. Of this 103 million person work force, approximately 19 million are employed directly or indirectly in manufacturing. This number decline over the next decade because of the introduction of robotics and other computer automated production technologies into current industrial operations. On the other hand, some economics forecasts suggest that there will be a net gain in the number of jobs in manufacturing due to the growth of new industry, including the robotics industry itself. Indirect labor jobs are not subjected to the same high level of the routine and structure as direct labor jobs. These jobs usually require higher skill and knowledge levels than direct labor tasks. Examples of these jobs that will tend to grow with number of robot installations include maintenance personnel, programmers, setup workers, and other technical support specialists. In general, the jobs will tend to possess higher technologies content in such areas as computers and electronics. A category of job called robot technician will likely develop, and the responsibilities of this position will include functions such as testing, programming, installing, troubleshooting, and maintaining industrial robots. Effect of robotics on labor unions The effect of these shifts in the structure of factory jobs is a matter of concern to the labor unions. Unless economic growth results in a general increase in employment, the labor displacement and shifts in job structure described above will mean a small population for workers who will be candidates for union membership. The unions acknowledge that these are likely to be the effects of robotics and other automation technologies. Instead of resisting the introduction on these technologies, they are attempting to deal with their membership problems in positive ways. Two approaches being used by the unions are:
1. Trying to recruit among indirect labor and semiprofessional workers in additions to their regular direct labor sources of membership. 2. Trying of participate in the benefits of automation and robotics on behalf of their members. The first approach is the obvious strategy for increasing membership. If the traditional sources of membership are reduced in size, then it seems reasonable to seek membership growth from new sources that are readily accessible. Although some unions have had success in this area, the success has not generally been sufficient to offset declines in regular union membership sources. The problem for the unions has been the tendency on the part of the technically trained indirect personnel to be less interested in union membership than direct labor factory workers. Highly technical and semiprofessional personnel are likely to identify and associate their own interests with those of management and professional staff, much more so than direct labor. They are likely to see a career path in the organization leading to professional promotion and financial reward that cannot be realized from trade union membership.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
112
The second approach reflects recognition of the economic merits of the new manufacturing technologies and the inevitability of their eventual adoption in industry. In this approach unions are bargaining for contracts aimed at participating in the benefits of automation. The ways of participating include retraining, reemployment, and sharing of the cost savings between management and labor. The examples which follow do not pertain exclusively to robotics, but to the broader field of automation and related new technologies in manufacturing. Education and training Several issues related to education and training is raised by the mass introduction of robotics into society. Many of these issues are identified in the two reports published by the “Office of Technology Assessment”. Listed among the references. They include: Shortage of trained technical staff in robotics and other programmable automation technologies. Need for a more technologically literate work force. Shortage of technical instructor and state of the art laboratory equipment in the schools. Need for retraining and job counseling programs designed for the displaced workers Some of the additional education and training requirements cited by OTA were the need for the development of multiple skills and “cross training” of workers who must perform a variety of different functions in the factory production environment, and the increasing need for using analytical and problemsolving skills as opposed to strictly manual skills. At the engineering level, there is the need for engineering specialists to comprehend the complete sequence of activities that must take place in the design to manufacturing process and how computers, machines, and people must be integrated to achieve optimum production systems. Finally, there is a need to institute more programs for retaining and job counseling of displaced workers. The rapid advances that are taking place in the technology of manufacturing suggest that there will be continuing need for these kinds of program. Past experiences with retaining program has shown that participation rates are low and dropout rates are high, often because the programs have failed to build on the existing strengths of the students and to provide opportunities to increase basic skills applicable to a wide variety of technical areas. One of the important needs in the job counseling field is access to dependable up to date information on job market trends and developing vocational opportunities. FUTURE OR ROBOTICS The approach adopted in the preceding chapters of this book has been to describe the state of the art in the technology, programming, and applications of industrial robots. However, the state of the art in this field is changing rapidly, and it is difficult for us to be satisfied with merely providing a description of the present status. The purpose of this chapter is to describe some of the research and development that is currently taking place and to forecast some of the future advances in robotics technology that will result from these efforts. We include robot programming and related software developments within the scope of robotics technology. We can hypothesize a likely profile of the future robot based on the various research activities that are currently being reported. The features and capabilities of the futures robot will include the following. Intelligence The future robot will be an intelligent robot, capable of making decisions about the tasks it performs based on high-levels programming commands and feedback data from its environment. Sensor capabilities The robot will have a wide array of sensor capabilities, including vision, tactile sensing, and others.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
113
Telepresense It will possess a telepresense capability, the ability to communicate information about its environment (which may be unsafe for humans) back to a remote “safe” location where humans will be able to make judgment and decisions about actions that should be taken by the robot.
Mechanical design The basic design of the robot manipulator will be mechanically more efficient, more reliable and with improved power and actuation systems compared to present day robots. Some robots will have multiple arms with advanced control systems to coordinate the actions of the arms working together. The design of the robot is also likely to be modularized, so that robots for different purposes can be constructed out of components that are fairly standard. Mobility and navigation Future robots will be mobile, able to move under their own power and navigation systems. Universal gripper Robot gripper design will be more sophisticated and universal hands capable of multiple tasks will be available. Systems integration and networking Robots of the future will be “user friendly” and capable of being interfaced and networked with other systems in the factory to achieve a very high level integration. The following sections will be present a review of these areas of robotics development. Robot intelligence The concept of an intelligent robot is not a new concept. The “intelligent robot” was defined as one of four basic control systems categories for industrial robots. Although this type of robot must be considered relatively primitive at the present time, the level of sophistication of the intelligent robot will evolve in the future as the prerequisite technological components (e.g., sensors, mobility, programming languages, networking, etc.,) are developed. The concept of the intelligent robot includes the capability to receive high level instructions into a set of actions that must be followed to accomplish the task. The future intelligent robot will also base in part environment. We will present a summary of alternative approach to robot control developed at the National Bureau of Standards (NBS). The industrial systems division of NBS has designed the framed work for a real time hierarchical control system that will be capable of responding to goal oriented instruction and will receive sensor data regarding its environment. The following summary of the NBS system will provide a narrative of the likely way in which the future intelligent robot will operate.
In the NBS Real Time Control System (RCS), a hierarchical control structure is used. In the operation of RCS goals expressed as commands at the highest level are decomposed at each successively lower level into simpler commands. The levels at the top of the hierarchical pyramid would correspond to factory shop orders, and these orders would be decomposed respectively into work cell instructions, and workstation commands. In turn, these commands would be subdivided into tasks, moves and primitives. The flow of commands and status information at these lower levels of the RCS are shown below. At the lowest level, the signals to drive the robot, gripper, and other equipment are generated.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
114
Each control level in the hierarchical structure has a clearly defined and bounded control function with a limited number of inputs and outputs. The configuration of each control module is the same, regardless of its level in the hierarchical structure. The module
Status
Commands
Work Station Status
Commands
Task Status
Commands Commands Status
Move
Status
Go to
Commands
Vision system
Open (Close) Status Primitive
status information in the NBS Real Time Control System. JointFig:- The flow of commands and Gripper control control configuration is illustrated below. The input portion of the module receives status information from feedback sensors and from the control module at the level immediately below. It also receives commands from the next higher level in the control structure. The module preprocesses the input data, combining and converting it into the proper format for the processing which is to be accomplishes at that level. The processing is done by mean of a state table, consisting of list output procedures which are determined according to the status conditions represented by the input data. A post processor in the control module is utilized to convert the output procedures and relevant other data from the state tables into formats that are required by components external to the immediate control module. These components would consist of other control levels of pieces of equipment that are controlled by the module.
Level above
Commands
Status Input state
Output procedures
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
115
Sensory Feedback
Preprocess
State-table
Status
Post process
Commands
Level below Fig:- Configuration of each control module, irrespective of the level in the control hierarchy Advanced sensor capabilities It is anticipated that the typical robot of the future will be much more richly endowed with sensor capabilities than today’s machines. These sensor capabilities would permit the robot to be more aware of its environment, to higher level of intelligence described in the previous section. Included among the advanced sensor capabilities will be three dimensional vision and tactile sensing. Mechanical design features Future robots will be designed to be mechanically more efficient, and will use improved power and actuation systems compared to current robots. Some future robots may have more than a single arm and will require advanced control systems to coordinate the actions of the arm and working together. The design of the robot is also likely to be modularized, so that robots for different purposes can be constructed out of components that are fairly standard. One of the improvements in the area of mechanical design is the direct drive robot.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
116
UNIT QUESTIONS
UNIT-I QUESTIONS
1. 2. 3.
Explain the product life cycle. Explain central processing unit. Explain computer programming language.
Self Assessment Questions
1. 2. 3. 4. 5.
CAD/CAM is a term which means___________________. CPU is a term which means _______________________. Define Automation. BCD stands for __________________. Define High level language.
UNIT-II QUESTIONS
1. 2. 3.
Explain designing process. Explain Benefits of Computer-Aided Design. Explain Input Devices.
Self Assessment Questions 1. CRT is a term which means___________________. 2. Give types of Pen Plotters 3. List the secondary device.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
117 Unit-III
Questions: 1. Define CAM. 2. What is the need of CAM? 3. What are the elements of CAM systems 4. Define Simultaneous Engineering. 5. What are the advantages of CAM? 6. Define Total Quality Approach 7. Briefly explain the life cycle of a product.
Answer For Self Assessment Questions UNIT I ANS1. : Computer-aided design and Computer-aided manufacturing.
ANS2. : Central Processing Unit ANS3. : Automation was defined as the technology concerned with the application of complex mechanical, electrical, and computer-based systems in operation and control of production. ANS4. : Binary-Code Decimal ANS5. : High level language consists of English-like statements and traditional mathematical symbols. Each high-level statement is equivalent to many instructions in machine language.
IMTSINSTITUTE.COM
CAD,CAM AND ROBOTICS
118
UNIT II ANS1. : Cathode Ray Tube. ANS2. : Two types of pen plotters Drum plotters Flat-bed plotters ANS3. : Secondary storage devices are 1. Magnetic disk 2. Magnetic tape
------------------------THE END-------------------------
IMTSINSTITUTE.COM
CAD, CAM ANDROBOTI CS
Publ i s he dby
I ns t i t ut eofManage me nt& Te c hni c alSt udi e s Addr e s s:E4 1 , Se c t o r 3 , No i da( U. P) www. i mt s i ns t i t ut e . c o m| Co nt a c t :+9 1 9 2 1 0 9 8 9 8 9 8