PC based motion control is growing fast, but how do you ensure that the
high performance and installation benefits of these systems is carried
right down to motor level? Les Hunt reports on a proprietary bus system
for drive/motor interconnection that has impressed at least two leading
motion control system suppliers
Servo control systems are going through a very dynamic period at the
moment, asserts Chiew Fong, vice president of servo business at Control
Techniques, but it is a very fragmented and diverse market when compared
with variable speed drives (VSD), despite the trend towards convergence
of VSD/servo technologies. Sectors traditionally using VSD technology -
stage and scenery handling, elevators, textile and printing machines -
are now turning to servos for precision and performance. The worldwide
market for VSDs is around $6.2bn and growing - the Climate Change Levy in
the UK, at least, contributing to that growth at home. But the fastest
growing servo motion sector, currently estimated to be worth $2.7bn
worldwide, is being fuelled by machine builders seeking to satisfy the
demands of dynamic industries such as packaging.
The servo market is conveniently divided into two segments: centralised
motion control and decentralised motion control. The former is
characterised by three types of control architecture: PLC (20% of
installations), stand alone motion controller (65%) and PC based (15%).
Decentralised motion control, on the other hand, is dominated by fieldbus
in combination with PC based control. Factory automation fieldbus systems
such as Profibus and InterBus-S were developed for decentralised I/O and
PLCs, and do not meet the deterministic requirements of multi-axis motion
control, where tight synchronisation is very important. Instead these
demands are being met by high speed, purpose developed fieldbuses like
CAN and Sercos (see DPA March 2001 page XX), which are longstanding open
standards. More recently, the IEEE 1394 standard 'Firewire' technology
has emerged as an important contender. And a proprietary system, Control
Techniques' SLM technology, is poised to take on the all-important
controller/drive/motor interconnection.
Earlier this year, Control Techniques decided to make SLM more widely
available to OEMs integrating PC based, multi-axis motion control into
their systems. Designed for use with its M'Ax/MultiAx servo drives and
servomotors, the company felt SLM had prospects beyond proprietary
systems and saw it as a possible 'bridge' to the fast growing PC based
control sector. The most recent co-operation on this level is between
Control Techniques and Nyquist Industrial Control in the Netherlands.
This partnership promises a new generation of OEM systems based on open
PC motion control, integrating Nyquist's high performance NYCe series
controllers (featuring embedded Windows NT, Ethernet and Firewire) with
Control Techniques' SLM digitally linked servo drives and motors. Another
partnership has been forged with Delta Tau, which is now combining its
UMAC controller with the SLM bus for PC hosted motion control
applications.
So, what does SLM offer? Essentially a means of connecting motion
controllers to drives, it provides a four-wire connection between drive
and motor at data rates up to 2.5Mbit/s with highly deterministic updates
(50ns worse case jitter). In addition to ensuring precise synchronisation
of multi-axis systems, the technology achieves quite staggering savings
in cabling. SLM technology employs a dedicated high-speed two-wire data
link, which avoids having to route multiple noise-sensitive analogue
feedback signals. The possibility of signal degradation due to noise
injection is reduced accordingly. Furthermore, the total number of
interconnections on a typical machine tool axis is reduced by up to 57%
(from 28 to 12), when compared with a standard servo encoder solution.
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