FEATURE Machine Building & Frameworks
Motion control miniaturisation means smaller, better machines
The move toward more mobile and compact machines is forcing motion control systems manufacturers to keep up. Now, there’s a shift toward embedded, decentralised architectures, says Gerard Bush of INMOCO
T
he growing need for applications that are mobile, lightweight and compact has passed these same requirements on to designers of
miniature motion control systems. This in turn has facilitated the reorganisation of machine architectures, enabling OEMs to decentralise motion control to local, embedded positions. As a result, in addition to machines with highly- compact footprints, this trend is improving performance, increasing reliability and reducing costs.
Reducing motor size Brushless DC motors remain in favour for their high power-to-weight ratio and the ability to generate a constant torque to high RPM. Alternatively, stepper motors are frequently used in portable applications as a result of their high torque-to-weight ratio. Despite the once- detracting eff ect of a lowered eff ective torque because of the tendency to oscillate, the stepper servo technique has resolved the issue by using an encoder to enable higher torque across the operating range of the motor, eliminating mid-range instability.
Particularly for steppers, as motor size
reduces, it becomes harder to add sensors without impacting the overall footprint. This explains the trend in sensorless operation for some applications using control techniques such as electronic stall detection for stepper motors, as well as back EMF for brushless DC motors. However, the challenge for sensorless techniques in applications such as electronic stall detection can be to match the required level of reliability. This is pushing the trend for even the small motors to have their sensors upgraded, or include encoders where they were not previously used.
This demand for more robust sensors 16 March 2021 | Automation
The closer the better The more compact the motion control device, the greater its potential to fi t locally and embed the motor and controller into the host machine. These embedded controllers can take the form of cable- connected modules and application- specifi c control boards. The advantage of the devices, particularly those that integrate analogue sensors such as sin/ cos encoders, is more accurate signals whereas the reduced length of the physical communications means higher speed servo loops. These gains are augmented for ultra-high accuracy systems where performance is measured in nano or
has also seen an increased demand for magnetic rather than optical encoders, especially in harsh conditions and medical applications.
Whether using a motion-control IC or module, the most signifi cant contributor to the size reduction in electronic motion control is the performance increase of digital switching amplifi ers. Most frequently this means the metal-oxide semiconductor fi eld-eff ect transistor switch, or MOSFET. Today, MOSFETs are reliable and compact, with signifi cant improvements in energy losses for increased effi ciency. Another signifi cant advance is in heat-
fl ow management. A new, specialised PCB, known as a metal-clad printed circuit board (MCPCB), is gaining prominence in motion control amplifi ers, too. It involves a traditional copper PCB that holds the integrated circuits onto a relatively thick aluminium or copper substrate. This layer acts as a dielectric which effi ciently transfers heat from the PCB. The MCPCB thermal-transfer technology has accelerated the power density of motion- control amplifi ers and the latest generation devices achieve results of over 100W/cc3
.
picometers. As a result, the trend shifts closer towards the advantage of embedded motion control solutions versus those that are remotely mounted. Even though the current technology may not represent signifi cant performance gains for large machines, it’s clear that a single PCB-based embedded motion solution creates a more compact footprint compared to an external device.
Cost-wise, embedded control can be cheaper than module-based control, providing a suffi ciently many devices are involved but, at the same time, a new motion control device is easing the barrier to building application-specifi c embedded control boards. Known as either intelligent amplifi ers, programmable amplifi ers or motion-control modules, these PCB- mountable devices simplify custom-control board design and development. PCB-mounted motion control modules
can run user application code, generate motion profi les, control servo positioning and drive the motor with an on-board amplifi er. These devices can also interface via serial, CAN and Ethernet links. Crucially, they are of small size, measuring just 3-5cm in length, and can be interconnected, plug-and-play devices.
Shrinking systems
Compact and mobile applications continue to grow in number while the machines themselves decrease in size. Along with miniature motors, motion-control electronics have not only reduced in size and weight but increased in performance capability. Performance has also been able to advance with the greater potential of a reliable and cheaper distributed architecture.
CONTACT:
INMOCO
www.inmoco.co.uk
automationmagazine.co.uk
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