SAW Sensor sees Servo Resonance.
Servo control systems that reduce drive-train
torsional resonances are being researched at
the University of Sheffield. Underpinning the
research is a radically new, non-contact, low
cost, high bandwidth, torque transducer based
on Surface Acoustic Wave (SAW) techniques
developed by Sensor Technology in Banbury,
Oxfordshire and trade marked Torqsense.
Torsional resonance is a significant limiting factor to servo system
performance, but to date it has proved virtually impossible to measure,
model and control. The discrete rotating masses in a drive train tend to
induce torsional oscillations, particularly in interconnecting shafts and
most notably at start-up, but also during speed or load change dynamics
typical of fast acting servos. The oscillations manifest themselves in a
variety of ways, for example, in workpiece signatures, or chatter in
machine tool cutting, drive-train noise and component fatigue.
Damping of these oscillations via feedback of shaft torque information is
expected to lead to a major improvement in the dynamic performance of future
servo drive systems. Additionally, the measurement can be used to enhance
the bandwidth of shaft position measurement.
Dr Nigel Schofield, who is leading the research team at the University of
Sheffield explains: The ideal servo drive-train would be completely stiff
so that the entire mechanical system reacts instantly and predictably to
changes in speed, while load torque perturbations would not initiate
mechanical resonances back through the drive line.
A lot of drive system designers try to achieve this by using over-sized
shafts and couplings, but this is neither elegant nor efficient and is often
simply impractical. In practise, the torsional resonance inherent in every
drive-train reduces stability margins, forcing servo gains down and hence
reducing machine performance.
The goal of the research programme is to give machine designers servo drive
trains that are intelligently rigid and thus free from torsional losses.
This will allow them to realise higher performance servo drive systems,
which are more compact, yet are completely predictable and highly efficient
in terms of dynamic performance.
Until recently it has not been feasible to directly measure shaft torque due
to the difficulties in obtaining sufficiently reliable and accurate signals
with currently available transducer technologies. In fact most torque
transducers are too mechanically compliant to provide an accurate dynamic
measurement of torsional excitation during speed or load changes.
Additionally, attempts to derive the torque from the electromagnetic
characteristics of the drive machine do not account for the various inertias
of the rotating loads in the drivetrain, or torque perturbations.
By using an embedded SAW based Torqsense transducer, it is possible to
accurately measure and hence provide improved control of the instantaneous
shaft torque from all possible sources, including machine generated torque
ripple, high speed control loops and load/torque disturbances, at different
points in the drive-train. The Torqsense torque measurement system is
relatively small, self-contained, non-contact and is unaffected by magnetic
fields. Thus it can be mounted either inside a machine or as a discrete
add-on measurement device. With this additional control data, control
algorithms for the servo controller can be improved.
The principle behind SAW devices used in Torqsense transducers is that
they are essentially frequency dependent strain gauges, which measure a
change in SAW resonant frequency caused by an applied shaft strain. The
transducer signal is transmitted via an RF couple from the rotating shaft to
a fixed pick-up. By using a frequency-based concept, the signal bandwidth is
increased, and electronic interference common with other analogue based
technologies, for example strain gauge and inductive devices, is eliminated.
Torqsense using the SAW concept was developed, trade marked and
patented by Anthony and Bryan Lonsdale, Directors of Sensor Technology,
which is also working on applications for SAW technology in large machine
Sensor Technologys Torqsense Transducers interface directly with a PC or
PLC, eliminating the need for extra instrumentation hardware. The company
has developed them to be low cost, robust and simple so that the technology
is suitable for use through out a wide range of industrial applications.
As part of an on-going evaluation of the SAW based Torqsense transducer
technology, Sensor Technology have supplied seven experimentation stations
incorporating Torqsense transducers to the University for use in its
electrical machines teaching laboratory for first and second year
undergraduate students. The transducer feeds straight to a PC running a
LabViewä software control platform and the students undertake a number of
experiments to assess different drive control strategies. Additionally many
third, fourth year and post-graduate students also use the rigs for their
own thesis and project work.
I generally reckon that equipment which can stand up to undergraduate
students can take any thing that industry can throw at it, jokes Schofield.
The results we are getting are building my confidence in SAW-based torque
measurement systems by the day.
For more information, please contact :-
Sensor Technology Ltd.
Tel: +44(0)1295 730746 Fax: +44(0)1295 738966