Don’t lack control over motion
Anyone brave, or foolhardy, enough to have tried to build motion control
solutions from disparate components will know the pitfalls. Simon Smith
demonstrates why ill-fated multi-vendor schemes rarely succeed.
Most engineers are aware that products from a single source are likely to
be compatible with each other when installed in a system. This begs the
question why therefore specify components from a variety of sources? The
usual answer is because the cheapest products for each component part of
a system may come from different sources. The second is where a single
supplier appears to have a technical shortfall in one area that might be
compensated for by opting for a higher specification product from another
supplier. Both are dangerous premises.
There are many considerations when specifying motion control products
that extend beyond price and specification. Linear motor ‘A’, for
example, may give greater force than another of similar price; therefore,
‘A’ is better value. It’s a simple deduction. But what if motor ‘A’ is
more expensive? Then other factors may come into consideration, such as
ease of fitting and alignment, which can make enormous savings, over and
above the price of the product, in the installation and build costs.
A further factor is the cost of procurement. The time taken to identify
and locate suitable products, test samples, make evaluations of
performance, set up the supply paperwork and so on, can render the actual
purchase price of the product but a small part of the overall cost.
Single vendor solutions can reduce these costs of procurement
dramatically.
It is the technical issues, however, that outweigh all the other factors.
Consider a recent example encountered by the author’s company. A
manufacturer of automotive testing machines used motion control products
from various sources and their machines would not work properly.
Diagnosing the problem, moreover, proved impossible. It was clear that
there were incompatibility problems between the various motion components
- commonplace enough, since different manufacturers use varying
parameters by which to set performance.
The company’s latest design, a four-axis machine, used linear motors but
the force output was significantly below the rated specifications from
the motor manufacturer. Secondly, the velocity regulation of the system
was poor, leading to jerky movements. Since the process required
contoured motion, these shortcomings were critical. The remainder of the
motion platform consisted of a PC-bus motion system with controllers and
amplifiers coming from different suppliers. Resolving the performance
issue was complicated by the fact that there were so many component
vendors and none was able to analyse the machine at the system level.
The machine’s launch date had already passed when engineers from the
author’s company were called in. Two fundamental problems were
identified. In the case of the linear motor, it was discovered that the
amplifiers could not deliver the rated power due to incompatibility with
the resistance characteristics of the motors. in the case of the rough
contouring, it was revealed that the motion controller selected could not
sinusoidally commutate. While the motion controller was capable of
generating a contoured profile, the lack of sinusoidal commutation
resulted in a torque profile with significant disturbances. A matched
system of motion controllers, amplifiers and brushless linear motors not
only easily met the system’s performance requirements, but also meant the
machine builder had a single source with full accountability for that
performance, delivery and service.
In another example, a manufacturer of electronic fabrication machines was
forced to introduce a machine with direct drive linear motors when a
competitor’s machine based on this configuration was shown to offer a
higher rate of throughput and lower cost of ownership compared with ball
screw driven designs. Migrating to a direct drive linear solution from
traditional mechanical drive train technology was, understandably,
something of a culture shock for this particular machine builder. The
competitor’s market share was growing and there was a sense of real
urgency about the new machine development. This was more than just the
replacement of ball screws with linear motors; controller, amplifier,
motor and mechanical design issues all had to be taken into consideration.
The previous ball screw driven system used rotary motors with tachometer
feedback, as well as belt and pulley mechanisms to provide linear motion.
The existing controller had no support for commutated motors, nor could
it sustain the higher speeds required by the new machine design. Clearly,
for this customer time was of the essence. The machine had to be got to
market quickly and it had to be right first time. The author’s company
was able to apply its experience of linear motion system building,
including cable management systems, integrating linear encoders and
taking the heating effects of the linear motor into account. Using three
brushless linear motors and a motion controller, the new machine was able
to operate at up to 43m/min at a maximum 1.3g acceleration. this was
nearly double the previous best throughput of the ball screw driven
machine.
The prospect of being able to identify all the various parameters of
disparate components in a motion system is a daunting one, even for the
best experts in the business. using single vendor sourcing for motion
control systems enables compatibility to be assured, gives a true partner
in the development stages and enables reliable support and service. Simon
Smith is with Aerotech.