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Research improves servo dynamics

11 November 2008

The University of Manchester, UK, demonstrated servo systems with improved dynamics through research led by Dr Nigel Schofield, of the Power Conversion Group. The research is said to have shown that increased performance was achieved with a Sensor Technology torque transducer.

Based on Surface Acoustic Wave (SAW) technology, the TorqSense transducers provide a low-cost torque measurement solution for brushless servo drive-system applications, providing active damping and/or resonance ratio control. The Manchester research looked into this particular measurement system.

Using the SAW based TorqSense transducer, it is possible to accurately measure instantaneous shaft torque from the various mechanical components of the drive train induced by the fast transients from the controller.

The Surface Acoustic Wave based transducer is essentially a ‘frequency dependent’ strain gauge that measures the change in resonant frequency caused by an applied shaft strain. Two SAW devices embedded on a shaft form part of a high frequency oscillator circuit. When the shaft is twisted, the resulting deformation of the substrate creates a frequency difference between two embedded SAW devices. The two frequencies produced by the SAW devices are mixed together to produce difference and sum signals. The difference signal is a measure of the induced strain due to the twisting moment, and from this the torque can be derived. The sum signal is a measure of the shaft temperature.

Coupling of the signals to the outside world is via an electromagnetic coupling device, allowing non-contact torque measurement.

The primary frequency of oscillation can be chosen anywhere from 100 to 1000MHz, with the difference frequency varying up to 1MHz. Operating at such high frequencies, the transducers are much less susceptible to electrical interference than conventional torque sensors. And this high immunity to magnetic fields in particular makes them eminently suitable for use with motors.

Sensor Technology TorqSense transducers, built on SAW technology, are designed to operate direct from a PLC or a PC, making it easy to interface them with standard controllers, and so further reducing the overall cost of integration in servo drive systems. And being compact and wireless, they simplify the mechanical design.

Easily embedded within the drive system, the TorqSense transducers can withstand heat, dirt and mechanical vibration that can represent problems for optical sensors. Their non-contact coupling between the shaft and the controller eliminates any issues of mechanical compliance.

The system integration and control studies at the University of Manchester are highlighting major benefits for tomorrow’s servo drive systems, offering the potential of servo drive trains that are ‘intelligently rigid’ and so free from torsional losses. The result could soon be commercial servo products that deliver improved performance and vastly superior system dynamics with even the most demanding mechanical loads.

The ongoing research is focused on developing the control algorithms that will enable the servo controllers to take full advantage of the torque feedback.


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