We take a closer look at a new truly distributed digital servo control
system, which apparently achieves a 20-fold increase in position feedback
resolution while reducing the cable burden, typical of standard servo
solutions, by as much as 65%
At the In typical machining applications, the quality of the process
depends largely on the smooth running of the motor and on stable, dynamic
response during system disturbances. The ability to achieve smooth
rotation of a motor depends, in turn, mainly on the quality of the
feedback signal, mechanical non-linearities, the torque ripple introduced
by the motor and the closed loop controller algorithms.
By increasing the resolution of the position feedback signal from a
motor, controller gain can generally be increased, yielding reduced
position deviations due to system disturbances. In addition, the
resulting high resolution in position information also leads to the
possibility of implementing a realistic torque compensation algorithm by
computing torque (acceleration) feedback. Since acceleration feedback
involves the double differential of position information, a stable
response from an acceleration control loop can only be realised if the
initial position information has sufficient resolution and the control
algorithm has a sufficiently high sampling rate.
The key, then, to achieving a significant performance enhancement is to
obtain high quality, non-degraded position feedback. Help in this respect
has arrived in the shape of Sine/Cosine (SinCos) encoders, which are now
commonly available and which provide a resolution in the region of four
million pulses per revolution. However, the problem with these components
is that their signals have to be conditioned for transmission to drives
and controllers. If there were no need to transmit this data - often over
long distances - then the conditioning issue could be avoided. Another
way is to locate the position and velocity control loops in, or alongside
the encoder. With the launch earlier this year of Control Techniques'
M'Ax digital servo control system comes a new concept in servo system
technology - Speed Loop Motor (SLM) - that achieves this very aim. SLM
uses a combination of high resolution, motor-mounted SinCos encoders and
DSP technology to provide a position feedback resolution of more than 8.3
million points per turn, plus an active torque compensation capability.
The latter is achieved by integrating speed and position control within
the feedback system onboard the servomotor.
As a result, the usual degradation in performance that is experienced
when synchronising multiple servo axes at higher operating speeds, is
overcome. The availability of small signal processing components, such as
DSPs and A/D converters, enables the information to be processed at
source, directly behind the motor-mounted SinCos encoder. This both
reduces the quantity and cost of interconnections and eliminates any
noise injection associated with systems that transport analogue signals
The Speed Loop Motor (Unimotor SLM) is a combination comprising a
permanent magnet servomotor, SinCos encoder plus the necessary
electronics to process and close the position and speed loops. In order
for it to operate as an ultra-high performance drive, a power stage,
MultiAx three axis or M'Ax single axis drive plus a programmable motion
controller are required in addition to the Unimotor SLM. When coupled to
the new M'Ax range of drives, however, no motion controller is necessary
and the M'Ax/SLM motor combination functions as a stand alone drive
system. Significantly, the system employs a dedicated high speed,
two-wire data link, based on the RS485 physical layer, which means that
you can dispense with all those analogue signal cables and their inherent
noise problems. In fact, the total number of interconnections per machine
axis