DAC102021

Since the days of simple point-to-point control, PLC vendors have added extra motion control functions. PLCs can now provide coordinated motion – often by adding motion hardware or higher-performance CPUs – although they typically cannot match the flexibility or performance of dedicated motion controllers. The increased hardware cost of a motion PLC is also likely to make a motion controller a more cost-effective option for many applications. A PLC – or even a PC-based system – involving high processing power could be used but, with a purpose-designed motion controller, the same performance can be achieved with the equivalent of a mobile phone processor in a compact footprint, at a fraction of the cost. The common barrier for PLC motion users is the programming interface and languages used. Developments such as PLCopen motion were an attempt to unify commands between controllers and vendors, providing a common library of function blocks. Many OEMs continue to develop motion using IEC languages originally designed for logic programming. Compared to high-level languages, logic-based languages can take longer to program, especially for complex motion tasks. For this reason, PLC users often switch to structured text (ST). This is similar in principle to the plain English language commands used, for example, in Trio’s dedicated, but optional, programming language. IEC languages can still be used effectively though, especially for low-axis- count, less complex motion applications, as well as hierarchical machine and logic functions. Conversely, one of the most significant developments to enable machine control using motion controllers has been the incorporation of IEC languages such as ladder, function block, sequential function chart and structured text. This has encouraged developers who are used to logic languages to switch to dedicated motion controllers to optimise machine performance. It has also enabled familiar programming of logic functions when integrating peripheral machine devices. While EtherCat allows the highest performance motion control, today’s motion controllers also integrate other Ethernet protocols, as well as control of analogue and step and direction signals. Like PLCs, modern motion controllers using technologies such as EtherCat also offer scalability of decentralised I/O expansion and support for other Ethernet- based communications to enable flexible machine integration. PLC or motion controller? As PLC and motion control technologies converge, the fundamental difference is whether the machine control requirement is logic- or motion-centric. For an application with high demand on logic, extensive I/O, or where widespread machine safety integration is needed, a PLC may be preferable. A PLC system could also be the preferred option to cover aspects of motion control if demands include the coordination of relatively simple motion, sequential motion, or motion limited to PLCopen. In principle, PLCs can resolve the needs of many motion control applications, however as well as having higher hardware costs, it is generally more difficult to use PLCs to achieve complex motion demands and it takes longer. A motion controller benefits from a motion-centric development environment, a library rich in motion commands and kinematic models, as well as motion-specific hardware functions. As machines grow in complexity, including multiple processes, I/O, and robotic integration, motion controllers can be combined with PLC-managed systems to add the advantages of a motion-centric environment, although the machine’s scale must be sufficient to ensure a cost-effective approach. Many OEMs build machines for more than one part of the world, or end-users may dictate that a specific PLC brand must be used. In such cases, OEMs can use an additional, dedicated motion layers common across their machines to boost the efficiency of motion development and maintenance. For applications with high motion dependency, dedicated motion coordinators provide the optimum control of accuracy and throughput. This includes applications such as CNC machines, electronic device assembly and packaging machines, where precision and high productivity, combined with synchronised multi-axis control, are required. For machines of this scale, a motion coordinator can manage the whole machine. For example, with a packaging machine performing wrapping, sealing and labelling, a motion coordinator can synchronise the servo or stepper motors, as well as providing automated control of any Scara or Delta robots used in the process. Future machines The demand for OEMs to deliver higher quality manufacturing, with increased productivity and lower costs, is being joined by innovations such reducing packaging and eliminating plastics. These demands will increasingly rely on robots being integrated into machines at all levels, not just the most high-tech production facilities. While PLCs continue to interact with robots for large machine architectures, more linear manufacturing processes, such as food production, will increasingly require smaller robots such as Delta and Scara types. Because robots are based on kinematic structures – a central concept of motion control – today’s motion coordinators can manage both a machine and its integrated robots as one coordinated system. The advantage is a single, integrated controller, simplifying and speeding up programming with motion and robotic development from a common software environment that also cuts machine costs. For simple motion applications, or for machines with the high levels of I/O integration, a PLC might provide the optimum design. But applications with the highest demands for control and performance will be optimised by using motion controllers. For many applications, these devices could also improve machine control. n PRECISION ENGINEERING AND MOTION CONTROL n