For applications combining traditional automation, multi-axis coordinated motion control, and a need for data handling, a motion-optimized PLC platform provides an elegant and streamlined solution.
By Kevin McClelion and Joe Scoccimaro, AutomationDirect
Anyone who has had the chance to see a modern factory in action, whether in person or perhaps via an episode of the “How It’s Made” documentary television series, is aware of just how impressive machine automation can be. This is especially the case for robotic arms and other machine motion shuttling rapidly and precisely from point to point, typically controlled using servo motors.
Industrial motion control is somewhat of a design specialty, but depending on the application, its intricacy ranges from straightforward operations up to very high-performance and complicated maneuvers. For simpler systems, designers today enjoy more options than ever before for easily accessible and cost-effective motion control products. At the other end of the application complexity spectrum, high-performance motion control platforms are available, but they can be difficult to configure and expensive to implement.
But there is now a viable happy medium for industrial motion control, suitable for many applications demanding more than basic functionality, but without the cost and complexity of high-end implementations. An industrial programmable logic controller (PLC) with significant built-in motion-specific features can operate multiple coordinated axes and perform a wide range of motion functionality, and also perform direct control of associated field equipment. Because of the wide range of capabilities incorporated by such a controller, in conjunction with external servo drives connected via a motion-specific fieldbus, designers are finding this type of PLC to be a cost-effective, complete solution for automated equipment requiring substantial motion control capabilities.
Stepping up to next-level motion control
Certain servo drives incorporate logic controllers on board, and a few of these drives can be connected to coordinate functionality. Alternatively, it is possible to select a small industrial PLC that includes essential motion control capabilities to command external servo drives using hardwired signals. Using either of these approaches, designers can control a few axes of motion and some associated input/output (I/O) signals. A prominent limitation of these hard-wired systems is the intensive and time-consuming wiring work necessary to handle various commands and data signals between the servo drives and PLC.
However, there are applications where even more automation and motion functionality are needed. For these cases, a motion-optimized PLC is often the best choice (Figure 1). This type of PLC uses industrial-grade hardware, and it also provides wired I/O and Ethernet network ports on the base PLC controller and via expansion with modules. Although the PLC includes specific motion instructions, they are implemented via dedicated motion-control modules, improving performance and providing connectivity to servo drives via a standard motion control fieldbus.
Figure 1: A motion-optimized PLC, like the AutomationDirect LS Electric XGB shown installed here, combines proven automation control and data connectivity with multi-axis servo controllers in a compact platform.
Designers must evaluate whether their application calls for any of the following requirements, which would prompt the need to choose a motion-optimized PLC platform:
• Need to command more than two or three coordinated axes, up to four or eight.
• Need to integrate with servo drives using a motion-capable fieldbus for more effective data communication, instead using just hardwired signals.
• Requirement to incorporate particular advanced motion control operations, such as:
o Speed-limited torque control to prevent overspeed condition while in torque control mode.
o Registration, as used for printing and bagging operations.
o More specialized interpolation movements, like circular (helical) and multi-point linear.
o Use of a “virtual” axis as a “perfect master” signal for driving other physical axes.
• Preference to use additional I/O and IEC-compliant programming languages to coordinate other equipment monitoring and control functionality.
Many motion projects need only one-, two-, or three-axis coordinated motion control, such as conveyors, plotters, and gantries. While these can be handled by basic motion controls, a motion-optimized PLC with fieldbus can provide real advantages even in these cases. Simplified Ethernet cabling—instead of multiple multi-wire cables—saves installation time and troubleshooting. Advanced data between the servo and PLC provides a higher level of feedback and better control of the application. Once the software is developed, deployment of new systems becomes much easier.
But numerous larger applications—such as fillers, assembly machines, and even multiple-stage production line equipment—need even more coordinated axes. A motion-optimized PLC is designed to natively handle these situations, much like a dedicated high-end motion controller. Using expansion motion controller modules, which can commonly handle four or eight axes, the PLC can be configured with as many or as few axes as needed, up to a practical limit of eight axes coordinated on one motion module.
Another benefit of this architecture is that the motion cards can communicate to external servo drives using motion-optimized industrial protocols. One of the most popular options is Ethernet for Control Automation Technology (EtherCAT) connectivity. Standardized under IEC 61158, the EtherCAT protocol works over standard Ethernet media and architectures much more efficiently than other general industrial protocols by delivering deterministic cycle times. In some applications, this is essential for maintaining very accurate synchronization among all communicating devices (Figure 2).
Figure 2: A motion-optimized PLC platform consolidates traditional control and motion via hardwired devices, industrial fieldbuses, and EtherCAT connectivity for high-performance servo motor control, providing a convenient way to aggregate data and communicate it to higher-level visualization and computing systems.
This type of control system architecture allows designers to select almost any commercially available servo drives that support EtherCAT, however, in many cases, users will prefer a controller vendor that also offers a full line of compatible servo drives to simplify the product ordering experience.
When selecting a controller, users should make sure it supports EtherCAT slave information (ESI) file usage for storing the drive configuration information, so this info can be easily downloaded into new and replacement drives. EtherCAT provides a massive advantage compared with hardwired high-speed I/O for controller-to-drive connectivity in terms of speed, coordination among multiple axes, data handling, and other areas. For example, EtherCAT minimizes field wiring, and it allows the controller complete access for reading and writing information from and to each drive.
A motion-optimized PLC with motion control modules and EtherCAT will offer advanced configuration options. Users can specify speed-limited torque control at the controller level, useful for maintaining proper material tension among many servo-operated motors in a web-machine, for instance. Registration operations, where a controller must recognize a moving target on material and then perform a perfectly aligned in-flight operation—such as printing, cutting, or sealing—call for a controller and servo system with exceptionally fast response times. Finally, machines needing sophisticated movements, coordinated among many motors and sensors, will benefit from a motion-optimized PLC architecture.
A control system architecture based on a motion-optimized PLC is typically the best option for these types of applications because systems with complex motion requirements are likely to be associated with many other non-motion devices, such as sensors, actuators, and on/off motors. A PLC is a natural high-speed platform for integrating these I/O points, and for programming the other functionality necessary to integrate these signals with the motion control. For this type of work, many users prefer a PLC based on IEC 61131-3 compliant programming languages so they can choose the right language for each task, mixing and matching as needed.
Users should look for IEC-capable PLCs, preferably those offering free programming software. With the right software, users may choose a sequential function chart (SFC) for the supervisory arrangement, with ladder diagram (LD) and function block diagram (FBD) for detailed I/O and device handling, for example. For certain tasks like repetitive looping operations, conditional execution, and math, many users find that structured text (ST) is often the best solution. Only a motion-optimized PLC provides all these logic and motion control capabilities consolidated into a single platform.
Data as a differentiator
The importance of using digital connectivity for motion control systems cannot be overstated. EtherCAT between the PLC controller and the associated motion drives simplifies installation and enables functionality to be fully orchestrated in the PLC, and permits extensive data communication between the controller and drives which is not possible with traditional hardwired control.
Not only can the PLC use ESI files to download configuration data to the drives, but it can also constantly monitor drive information and diagnostics, and then present it in multiple formats, including trend charts (Figure 3). Motion systems can be notoriously difficult to fine-tune, so the ability to gather all motion data into a centralized PLC for monitoring is a clear advantage during commissioning, troubleshooting, and other activities.
Figure 3: Extensive data connectivity is demanded by modern automated systems, and trending is essential for developing and commissioning complex motion applications.
Drive data is useful at the PLC level for orchestrating control strategies. The PLC program can use servo drive speed, position, and other status information for supporting thorough integration with other automation elements, and for operator visualization.
This type of PLC can also concentrate operational and performance data, and then deliver it to higher-level systems using a common industrial protocol, such as EtherNet/IP. Examples of higher-level systems are PC-based visualization software and enterprise resource planning software. Therefore, users should look for motion-optimized PLCs offering one or more onboard or expansion Ethernet ports for data handling and other purposes.
Getting the most out of motion
Motion control applications have developed a reputation for being costly and complex. Some high-end applications have earned this status, although there are also simple applications which can be solved with basic product solutions. However, many designers are looking to create automation and motion designs which exist somewhere between the two extremes. For mid-tier motion control—with several coordinated axes, other sensing and automation devices, and a need for data handling—designers will find that a motion-optimized PLC is likely to meet or exceed all their technical needs, while providing high performance-to-cost ratio.
AutomationDirect
www.automationdirect.com
* Figures all courtesy of AutomationDirect unless otherwise noted
About the Authors
Kevin McClelion is a product manager at AutomationDirect. Over his 25-year career he has held various power and controls engineering positions in the consumer products industry. He has led manufacturing facilities around the globe to utilize lean manufacturing concepts to ideate and deliver energy savings projects. Kevin has worked at AutomationDirect since 2019 supporting new PLC, drives, and motor products. He holds a bachelor’s degree in Electrical Engineering from N.C. State University.
Joe Scoccimaro is a product engineer at AutomationDirect. Over his 19-year career he has held positions at an automation supplier and as a controls engineer for an OEM in the automotive industry. He has experience launching advanced motion control systems and implementing those systems on machines purchased by end-users. Joe has worked at AutomationDirect since 2023 supporting motion-optimized PLCs. He holds a bachelor’s degree and master’s degree in Computer Science from Georgia Institute of Technology.
Filed Under: PLCs + PACs
