DAC102021

46 n PRECISION ENGINEERING AND MOTION CONTROL October 2021 www.drivesncontrols.com Using motion controls to supervise machines: an alternative to PLCs? I n the mid-1980s, the growing use of the PLCs (programmable logic controllers) looked set to revolutionise machine design. When combined with the VSD (variable-speed drive) technology that had emerged a decade earlier, newmachines with electronic controls began to replace their relay- and contactor-based predecessors. Using PLCs to control machines’switches, timers and peripheral devices, simplified and reduced the size of electrical cabinets dramatically. For machine-builders of the era who needed high speed and accuracy, especially across multiple axes, electronic alternatives emerged to fulfil the functions of line shafts, gearing and registration. These early devices led to the emergence of programmable motion controllers, typically controlling servo or stepper drives for applications that required precision as well as high throughput. They could also synchronise control over three or more axes and became known as motion coordinators. While early motion controllers offered integrated I/Os, their capabilities were limited compared to PLCs, so when operating alone they could only control machines with relatively few connected devices. Machine- builders who needed higher-performance control of speed and position of multiple axes tended to integrate motion controllers into their architectures, with overall control being provided by a PLC. The low-bandwidth, non-deterministic fieldbus technologies of the era were not capable of advanced multi-axis motion. Combined with limited development of PLCs for motion applications, this led to the evolution of intelligent drives. Often programmable and with dedicated, synchronous communications links, they could achieve line-shaft-based motion. But they were complex to apply and more costly and challenging for machine-builders to maintain, so the arrival of deterministic, high- performance Ethernet, such as EtherCat, saw an end to the need for smart drives and accelerated the evolution of motion controls. In this era, the divisions between machine logic, motion control and robotic control were clearly defined, but this separation added to capability limitations and maintenance challenges. Today, the lines of distinct capabilities between machine logic and motion control have become less clear. Some PLCs offer motion control functions, while some motion controllers also provide logic and comprehensive machine control capabilities. Many machines now incorporate robots, and while motion controllers integrate robotic kinematics and offer closely coordinated motion for entire machines, PLCs typically interface to dedicated robot controllers via a fieldbus. This has increased control options for designers, with advantages in terms of machine performance, speed of development, and cost. For applications with high demands on speed, precision, and the coordination of multiple axes, machine-builders usually have to choose between using high-performance PLCs or integrating motion controls into PLC-managed systems. An alternative for machines for motion-centric applications, such as robots, is to use motion controls that can supervise the whole machine, avoiding the need for extra controllers. Dave Greensmith and Ed Novak from Trio Motion Technology examine these approaches. Precision motion control is vital for electronic goods manufacturing, such as assembling smartphones.