PLC Programming by Arvydas Gordejevas

Microprocessors and PLC Applications (DCL_2_246)

GROUP MEMBERS: Arvydas Gordejevas Jacobus van Schalkwyk

PLC PROJECT 2: Control of an Assembly Line

SUBMISSION DATE: 11 January 2009

TABLE OF COTENTS _____________________________________________________________ Title

Page

1. Project Brief

2. Program Structure

3. System Analysis

4. The Program

The Ladder Program

5. Problems Encountered

6. Conclusion

1. PROJECT BRIEF For this project our group chose the control of an assembly line. The reasons for our choice of project was that assembly lines are a big part of the manufacturing industry, the project seemed challenging and would teach a lot about the possible uses of PLC systems, including the range of input sensors and output solenoids and actuators available on the assembly line. A miniature assembly line was already present in the laboratory (T405) in the form of the Industrial Control Trainer by Bytronic Associates. With the hardware system already present more time could be spent on the writing and testing of the assembly line program. The idea of the application we would use was to utilise of all the inputs and outputs available on the system. The application was to use the hardware present to assemble two parts, one plastic ring and a metal component, into a final product. See Figure 1. The plastic is smaller than the metal and half the size. And Figure 2 shows the assembly line used.

Figure 1: An illustration of the metal and plastic components and the final assembled unit

Figure 2: The Industrial Control Trainer assembly line by Bytronic Associates

4 2. PROGRAM STRUCTURE Sorting The structure of the program was to firstly initialise the two conveyer belts present on the system: the chain belt for carrying the separate components and the belt conveyer used for assembly and transport of the final unit. On the chain belt the separate components go to the sorting area where the plastic is separated from the metal. This is done by means of two sensors and a sorting solenoid. The one sensor is located on the belt level and the other is higher up. If only the lower sensor is activated the program reads the part as a plastic ring and this is sorted down a chute by means of the solenoid. If both sensors are activated, when a metal component passes though, the solenoid is not activated and the part goes through to go down a separate chute to the belt conveyer, from there the metal component will move to the assembly area. See Figure 3 for the sorting area.

Sensors

Sorting Solenoid

Figure 3: The sorting area

Assembly The metal component goes down the far end chute, after sorting, to the assembly area via the belt conveyer. Once the plastic ring has gone down the chute it is held by a rotary solenoid. This rotary solenoid is activated shortly after the rejection solenoid by means of a timer. With the opening of the rotary solenoid the plastic ring is moved to the assembly area. See Figure 4 on the next page for the assembly area and rotary solenoid. Once the plastic ring is in the assembly area it activates the sensor present to stop the rotary solenoid from admitting any more plastic rings. As the metal component passes on the belt, through the assembly area, the plastic ring is fitted on top and is carried away with the metal component on the belt conveyer. This empties the assembly area and the sensor activates the rotary solenoid to admit one more plastic ring for assembly.

5 Quality Control From the assembly area the assembled unit moves on the belt conveyer through several sensors to check various aspects of the unit. Using the sensor in the capacitive area and the capacitive proximity sensor the program can check if the two pieces have been successfully assembled. See Figure 5. If both sensors are activated the product is deemed acceptable. If only the capacitive area sensor is activated the product is deemed incomplete and this will activate the rejection solenoid at the end of the belt conveyer.

The rotary solenoid

The assembly area sensor

Figure 4: Assembly area and Rotary solenoid

The capacitive area sensor

The capacitive proximity sensor

Figure 5: The capacitive proximity sensor (yellow) and the capacitive area sensor

Between the capacitive sensors and the rejection solenoid there is a gauge controller used to measure the diameter of the metal components. But this was not included in our project as this was added later and is not part of the main assembly line. See Figure 6.

The gauge control

The gauge control sensor

Figure 6: The gauge control and sensor

At the end of the belt conveyer, after the gauge control, is the rejection solenoid. This is coupled with a sensor and is used to reject parts not successfully assembled from the belt conveyer. The belt conveyer ends after this and there is another small chute for collecting the correctly assembled pieces.

Additional Sensors Along the belt conveyer there are two extra sensors that we use in our program for counting the components passing along. The one is an inductive metal detector and the other is a through beam. See Figures 7 and 8.

Figure 7: The inductive metal detector

Figure 8: Through beam sensor

3. SYSTEM ANALYSIS Before we could start with the project all the sensors, solenoids and actuators had to be identified according to inputs and outputs, as connected to the controller unit. The PLC and circuit boards were used to do this. When a sensor is activated it also activates a specific light emitting diode (LED) on the input circuit board. First we programmed a single input, X0, and a single output, Y0, into the PLC. By trying all the sensors the input and output was identified. Then the output was changed on the PLC program to Y1, Y2 and so on, and the respective output solenoid noted with each instance. In this fashion the different outputs were identified. We used the same procedure to identify the input sensors. One output was programmed into the PLC and the inputs changed from X1, X2 and so on, until all the input sensors have been identified as well. The respective LED of each sensor on the circuit boards was also noted. See Table 1 for the complete list of inputs and outputs and Figure 9 for the location of each sensor and solenoid on the assembly line. It was noted that the gauge control on the assembly line acted independently from the PLC program. This was linked to a separate circuit board and was not integrated into the overall system.

X4 Y5 X1 X6

Y1 Y4

X2 X0

X5 Y2 X12

Figure 9: Location of all the sensors and solenoids on the assembly line

Inputs (Contacts) Outputs (Coils)

Name

LED

Assembly area full infrared (IR) sensor

Object detected in sort area (sorting lower sensor)

Object detected in reject area

Object detected at capacitive area

Metal peg detected (sorting upper sensor)

Object detected in through beam

Object detected at capacitive proximity (yellow sensor)

Object detected at inductive metal detector

X12

Start/Stop switch (Emergency stop hardwired)

Sorting solenoid

Rotary solenoid

Reject solenoid

Chain conveyer belt

Belt conveyer

Gauge control

Table 1: The inputs and outputs of the assembly line system

4. THE PROGRAM The program we wrote for the PLC follows the structure as outlined in section 2 of this report (Page 4). See Page 9 for the ladder program produced. We decided to include a number of optional counters in the program, see Page 10. This could be used to count the number of plastic rings and metal components going into the line and also to count the number of successful assemblies and rejected parts. These counters can be utilized to monitor production rates, success as well as stock movement and may also be used to switch off the system once a set amount of units have been assembled (although we did not include that particular feature in the program).

The Assembly Line Ladder Program

X12 Y3

Y4 X1

X4 Y0

If X1 is ON and X4 is OFF, connecting output Y0 (sorting solenoid sorts plastic from metal)

If X1 is ON and X4 is OFF, Relay connection used for timing of the rotary solenoid

Input X12 is always on connecting Outputs Y3 and Y4 (Chain & Belt conv. Motor)

X0 T0.0015

If T1 and X0 is OFF, putting the timer T0 ON for 1.5 sec

T0 connecting output Y1 (Rotary solenoid)

T1.0020

Activation of Rotary sol. Starts T1 to reset T0

X6 connecting Relay1

R1 Relay1 is ON until T2 is OFF R1 T2.0020 X3

Y2 R2

Resetting X6 input in 2 sec

If X3 is ON and R1 is OFF connecting Relay2. Y2 resets X3

R2 Relay2 is ON until Y2 is OFF R2 T3.0025

R2 connects the timer T3 for 2.5 sec (The timer for T3 depends on the distance between X3 and Y2)

T3 Y2

T3 connects Y2 to reject the faulty part

Optionally, the ladder program can be extended to include the following counters for monitoring the material usage and assembly success of the line. A single input (X8) can be used to reset all the counters.

Assembly Line Program Counters

Y0 C0.1000

Counter for monitoring plastic rings sorted

X8 RC0 X7 C1.1000

Resets the Counter

Counter for monitoring the number of metal components passed on the belt conveyer

X8 RC1

Resets the Counter

X6 C2.1000

Counter for monitoring successful assemblies

X8 RC2

Resets the Counter

Y2 C3.1000

Counter for monitoring the number of parts rejected

X8 RC3

Resets the Counter

11 5. PROBLEMS ENCOUNTERED During the process of identifying the inputs and outputs we found that the sorting solenoid did not always eject fully and not with a consistent force. The ejection arm seemed to be slightly bent, which might affect its function, and/or solenoid might require some lubrication. Through this project it was also noted that the belt conveyer did not always start with the system or might stop randomly. The exact reason for that could not be found. It may get stuck somewhere or the power input is interrupted somewhere. Also the plastic rings sometimes had difficulty in sliding down the chute, and got stuck along the way. The reason could be that the surface is not clean and it was also noted that the paint on the surface was a bit worn. At the end of the metal component chute the piece is supposed to be held in place using the chute shape, waiting for the following metal piece to move it onto the belt conveyer. It was found that sometimes the metal component will move directly to the belt instead of being held. Also on the odd occasion the metal components got stuck at the end of the chute and would pile up. Another thing noted was that the second metal piece coming down the chute is supposed to push the first one onto the conveyer taking its place, but these sometime came out together. One more problem that we encountered was that the distance between the capacitive sensors (X3 and X6) and the rejecting sensor (X2). Due to the nature of our program, it is time dependent on the moment that the capacitive sensors detect a faulty assembly until the moment it reaches the rejection part. To optimise our program the distance between the capacitive and rejection sensors has to be minimised. But with the gauge control between these two it was a bit difficult, and it could not be completely moved during the demonstration as the assembly line was being used by other people as well. Due to the distance and the program, the parts or assembled units directly after the faulty part will sometimes reset the timer for the rejection solenoid if following the faulty assembly too closely. This causes the rejection solenoid to reject properly assembled parts. To eliminate this problem, the capacitive sensors and rejection sensor have to be moved as close together as possible and the timer value adjusted accordingly.

6. CONCLUSION Using the assembly line, and the associated sensors, solenoids and circuitry, it was possible to learn how a system functions and to write a program for it to utilise the hardware to perform a given function. This is, in a sense, reverse engineering of a system.