For machines that depend on high-performance control, the emerging trend of integrating logic and PLC functions into a motion controller is simplifying machine design, as well as set-up and maintenance.
Machine builders who need to optimize the performance of a machine’s motion cycle typically rely on a dedicated motion controller. A motion controller achieves the fast and precise control of servo motors, often involving the coordination of three or more axes, demanded in applications ranging from packaging to electronics manufacture.
Dedicated motion controllers feature a real-time motion engine, executing motion commands and the coordination of servo motors over a high-speed, deterministic communications protocol. This also enables motion controllers to resolve interrupts at high speed – an approach referred to as touch-trigger. When there’s an event on the machine that needs to be addressed, the reaction to the signal happens in microseconds.
Controllers designed for motion
Alternatively, a typical PLC scans the programme in a loop, addressing each event in sequence. This approach is ideal for dealing with logic commands, but it lacks the dynamism required for performance motion control.
A typical PLC scans in milliseconds, not microseconds, which makes a significant difference to machine performance. This can impact precision, resulting in sub-optimal manufacturing quality. It can also cause more downtime too. Instead, a motion controller can run the machine much more quickly, with much greater reliability, which ultimately increases productivity.
Yet, for machine builders of discrete machines with a focus on performance, integrating a PLC as well as a dedicated motion controller has often been required. From a machine control perspective, a PLC might be necessary to manage logic functions, as well as enabling the integration of wider devices. The ubiquitous PLC also provides a comfortable development environment for many machine builders, and crucially, from a machine user’s perspective, a PLC presents a more familiar interface for a maintenance engineer.
The complexity of interfacing two controllers
However, the main challenge of interfacing a motion controller with a PLC is design complexity. It takes significant time to ensure that the two controllers can reliably communicate, and maintenance issues experienced when the machine has been commissioned are more difficult to resolve. Two controllers also mean extra cost and consume more energy and space.
The familiarity of PLCs means that some machine builders might rely on them for motion control as well as logic. While this approach is suitable for many applications, it means a compromise for machines that demand optimal performance.
In response, an emerging trend is to take the performance platform of the motion controller and add the functionality of a PLC. A single controller is much faster and easier to program, commission, and maintain, and it brings down cost. So how is this new type of controller achieved?
Retaining motion control performance
As motion control performance remains the priority, a motion controller is the fundamental basis. This means a motion engine with a wide range of motion commands. This is supported by highly capable processors, as well as a real-time, deterministic communications network that can optimise the speed and reliability of response times.
It’s increasingly clear that EtherCAT is the dominant communications protocol to optimise performance at machine level between a controller and servo drives. EtherCAT also offers high-speed performance to connect wider devices to the machine, and another feature of emerging motion and PLC hybrid controllers is the inclusion of I/O slice integration with a range of digital and analogue inputs and outputs.
Yet, to ensure seamless, upward connectivity between a machine and the operational technology (OT) level, a controller still needs to integrate communications protocols such as Profinet and Ethernet I/P that remain dominant above the machine level. However, another advantage of including a dedicated, machine-level network, such as EtherCAT, is that it is not subject to the high volume of traffic flowing over the OT-level Ethernet network. This isolates the machine from external communications disruption without hindering machine performance.
Ease of use
While this new trend of controllers provides the performance of a traditional motion controller, motion commands can be accessed via PLC programming languages based on the IEC 61131-3 standard. For machine builders more familiar with PLCs, this can speed up development, recreating the programming environment they’re used to.
There are also advantages for maintenance engineers, as logic functionality can be presented in common, clear to understand forms, such as Ladder Diagram, which enables quick machine debugging without the need to get involved in programming.
In the important arena of price, a clear advantage of a single controller that can handle motion and logic is that it achieves a significant cost reduction. Many motion-specific automation vendors are price-competitive compared to the most established automation players, including providing software and licencing that is free of charge.
A motion-capable controller can also reduce overall machine cost by removing the need for intelligent drives. Historically, to resolve the unreliability of digital networks, a PLC has often been combined with intelligent drives. However, taking a motion and PLC-combined controller, operating over a robust network like EtherCAT, machine coordination can be centred on the controller, without the need for complex drives. Instead, drives can focus purely on achieving performance at the motor shaft, significantly simplifying design and set-up as well.
The future of motion control
To design a machine with a focus on motion performance, this emerging breed of controllers means that for many applications, there’s no longer a reason to specify a pure PLC. A controller based on optimised motion coordination, with added PLC functionality, can provide the best of both worlds.
However, this doesn’t mean that all motion controllers will integrate PLC functionality in future. Specific applications that require a concentration on pure motion control are set to remain. For example, PC-based motion projects will still be useful to support the development of richer user interfaces, such as custom HMIs.
But for many machine applications that rely on performance, the future of motion control is set to integrate further PLC functionality, and simplify machine design as a result. The key will be the fundamental reliance on a capable motion engine.
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(Originally Posted at: https://blog.triomotion.com/combining-motion-performance-with-plc-functionality/)