Controllers for motion control and beyond

The main task of any motion controller is tocommand the motion of some machine or system, including any motors and other mechanical components such as actuators. Whether control is needed for simple point-to-point positioning or for multi-axis interpolated moves, choosing the right motion controller can have lasting effects on the final designed system. For instance, a controller with insufficient computing power or capabilities that fails to meet a machine’s motion control needs can degrade the machine’s performance and abilities. On the other hand, overly powerful or robust controllers can add unnecessary system cost.

More specifically, motion controllers calculate and generate the commands for the motor or actuator to move. These are known as motion trajectories and involve calculating the right speed and position. These commands can be sent to a drive which develops the necessary current to drive the motor. In a closed-loop system, there will also be some type of feedback from the process that helps to adjust the commands in order to meet system requirements. Controllers can use a number of algorithms needed to generate commands including control loops, interpolators and step generators.

Choosing the right motion controller can be challenging because they range in complexity and capability — from simple indexing to full-fledged motion control systems. But knowing as much as possible about the immediate application and of the range of controllers on offer can vastly simplify the task.

The iQ-R Series from Mitsubishi Electric is a fully integrated controller designed to support a host of manufacturing applications. It integrates motion, safety, process, redundant, and other control disciplines on a single platform, interfaces with enterprise and visualization systems, and conforms with global standards such as IEC 61131-3 programming languages.

CATEGORIZING CONTROLLERS

Controllers can be categorized in a number of different ways depending on what is prioritized. For instance, they can be divided by the physical format (plug-in board, stand-alone, embedded,) or by function (PLC, PAC, motion controller) or even by architecture (distributed or centralized.)

If we focus first on size, we can start from the smallest motion controllers; embedded controllers. These are so-called because the control is literally embedded into the device or system being controlled. Most often these are circuit-board-sized controllers with a main processor on board, memory, and control circuitry. They also include various bus interfaces as well as software for controlling actuators or receiving data from sensors. Embedded motion controllers can be connected to or built into drives or amplifiers or even onto the motor unit itself with drive circuitry to form what are often called integrated motors.

STAND-ALONE AND PC-BASED CONTROLLERS

Standalone controllers are complete systems that include all electronics, power supplies and external connections that mount to one physical enclosure. These controllers fit into machines to command applications consisting of either a single motion axis or multiple motion axes.

PC-based controllers include a basic PC motherboard or ruggedized industrial PC as well as PC-type hardware components and a high speed dedicated bus that transmits information to and from the processor. PC control requires the same inputs and outputs as a basic PC, as well as interfaces to factory floor devices. Here, typical I/O includes the electric motors and other actuators, as well as discrete sensors and other mechanical components. One key advantage of PCbased controllers is that they provide a readymade graphical user interface (GUI) for easier programming and tuning.

PC-based controller software includes an operating system to manage internal processing and resources. Where once OS instability issues were a real concern, today with the proliferation of real-time operating systems, professional grades of Windows, and Linux application software, those concerns have largely been dealt with.

Control programming languages include common general-purpose languages including C++ or Visual Basic but also more control oriented languages such as IEC-61131- recognized Ladder Diagram, Instruction List, Function Block Diagram, Structured Text and Sequential Function Chart languages.

PLCs

Programmable logic controllers (PLCs) are microprocessor-based electronic components that serve as the programmable smarts for simple or isolated applications. Now they’re also finding increased use on integrated machines and more complex automation setups.

PLCs have evolved from their early days into controllers with a reputation for unbeatable reliability. Today, their logic and memory boards, backplane interfaces to I/O modules, and generally heavy-duty circuitry help make PLCs indispensable.

The basic parts of any PLC system include the processor, I/O modules to handle inputs to the controller and outputs to the controlled devices, and some type of user interface which could be as simple as a keypad or a touchscreen interface or a programming link via a PC. The PLC’s processor is programmed via the user interface. The I/O modules are used to bring input signals into the PLC’s CPU and output control signals to controlled devices such as motors, valves, sensors and actuators.

Small servo control modules, such as the TMCM-1617 single-axis servo controller with integrated field-oriented control implemented in hardware from TRINAMIC Motion Control, illustrate the idea of embedding controllers into machines and devices. They’re designed for servo drives with 18 A RMS and feature EtherCAT, CAN and RS485 interfaces all in a compact size of 36.8 mm long x 26.8 mm wide x 11.1 mm high.

One core PLC strength is that they let engineers write application code to suit specific applications in which they’ll operate — on assembly lines and in machinery and elsewhere on factory floors, often times with other electrical, mechanical, and electronic equipment. Here, the original form of PLC programming known as ladder logic is still dominant.

Another fact about PLCs is their durability. Despite industry predictions to the contrary, PLCs are still by far the most common choice for standalone or singleaxis motion. Such applications are seeing more inroads from motor drives sporting controller functions. These motor drives excel in machine designs that still need PLC functions with multiple interfaces — including Ethernet communications and digital I/O, for example. Because such motor drives can also incorporate motion controls delivering S-curve, camming, and freeform motion profiles, OEMs are more likely to pick them than programmable automation controllers (PACs) in otherwise-simple designs eliminating standalone PLCs.

Part of the PLC’s endurance is related to cost. At least on the hardware side, the cost has consistently gone down over the decades, even as their capabilities have grown. For example, some PLCs are just a few hundred dollars. One caveat here is that PLC software sold through license agreements can increase overall design costs.

PACs

A programmable automation controller (PAC) is an industrial controller that combines the functionality of a PLC with the processing capability of a PC. The term “Programmable Automation Controller” is generally accepted as having been coined by the ARC Advisory Group, which specified five characteristics that define a PAC:

• Multi-domain functionality

• A single, multi-discipline development platform

• Flexible software tools that maximize process flow across machines or processes

• An open, modular architecture

• Compatibility with enterprise networks

PAC, the distinction between PACs and PLCs is blurry. Higher-end PLCs now incorporate some of the characteristics described above and are encroaching on what was once considered PAC territory. In fact, many PLCs now include standard programming languages, the ability to expand functionality through add-on modules, and connectivity to various bus systems.

However, PACs still differentiate themselves from PLCs by employing a more open architecture and modular design. They’re also more capable than PLCs at monitoring and controlling a large number of I/O, such as in a large processing plant or a complex automation system. They do this because data can be exchanged between devices and applications in different domains, such as motion and process control. And, a programmable automation controller can send and receive data to and from other PACs, creating a distributed control system of PACs. With large memory capacity, the ability to handle complex or high-speed analog I/O, and high-speed communication capabilities, PACs are well- suited for vision applications, including vision-guided motion.

The ability of PACs to gather, store, and track large amounts of data means they can handle predictive maintenance and operations monitoring. Data is often stored and accessed through an Ethernet network or a USB storage device.

PACs can operate in multiple domains simultaneously – such as motion control, process control, sequential control, logic, data management, and communication – using a single platform. Image courtesy of Opto 22

PACs also use the IEC 61131-3 programming languages (ladder diagram, function block diagram, sequential function chart, instruction list, or structured text), and some include standard PC programming languages such as C/C++, so familiarity is retained and the learning curve for programming is low, regardless of prior experience or expertise.

PACs are best suited for applications that require complex controls — in automation, this often means multiaxis, coordinated motion or circular interpolation — while PLCs generally work well for simpler applications such as single-axis motion control.

PLC VS. DISTRIBUTED CONTROL

While there are differences between a PLC and a distributed control system (DCS), there has also been a move towards greater convergence of the two technologies.

In the early days of automation, PLCs dominated machine control – that is, applications calling for discrete control of machine processes. Meanwhile, a DCS was the predominant choice for process control in applications such as oil and gas and chemical plants. For the most part, the PLC/ DCS split still follows this historical pattern. PLCs still are used to control individual machines, while a DCS can control a larger number of machines or processes within a factory or plant.

One hallmark of PLCs is their relatively simple operating system, designed to do a minimum of tasks like scanning inputs and updating outputs. This simple OS structure means they can execute programs quickly because there are not many other processes running in the background and demanding time from the processor. PLC processing times are also quicker because they are naturally closer to the devices they control (motors, pumps, switches, etc.), so they are more responsive than a larger DCS would be because it is controlling much more than a single machine. PLCs are generally flexible and more readily customizable as well.

On the other hand, a DCS can control many more machines and processes at the same time. In fact, they are often used to control entire systems within a factory. A DCS is likely to be more reliable as well, ensuring the continuous operation of a process or system.

However, with more powerful PLCs and PACs available today, the line between a PLC and DCS continue to blur. For instance, PLCs are faster and can handle more complex control. They’ve also become more reliable, making them more like a DCS than ever before.

As for the DCS side of the equation, they have become more nimble and adaptable like PLCs and are sometimes used in place of a PLC. So in terms of functionality, the two are growing more similar than dissimilar. The cost of a DCS is still more than a typical PLC, but even here the difference in cost is shrinking not expanding.