Torque transducers aid development of improved servos

Ease of set-up for modern servo drive systems, combined with steadily reducing costs, has made servo control attractive for motion control applications in a wide range of industries. Despite the developments in inverter technology that promise near-servo performance, in the most demanding applications requiring the fastest response and the most precise control of position, velocity and torque, true servo systems are required.

The ease of access to servo technology and the increasing capabilities of servo controllers have resulted in faster system response becoming an increasingly common requirement. With a theoretically ideal load, this would not represent a problem. But no 'real world' load is ideal. Practical mechanical drive systems can be complex, incorporating several non-still interconnecting shafts and elastic couplings, all characterised by a dominant resonant frequency. Fast impulse signals from the servo controller excite torsional resonances through the drive train, leading to controller instability - particularly as the servo controller bandwidth approaches the resonant frequency of the load.

This torsional resonance is a significant factor that limits the ultimate dynamic performance achievable by servo systems. In looking to damp these oscillations, the obvious answer is to measure and control the torsional resonance using shaft torque feedback techniques, but traditionally this has proved something of a challenge. The lack of reliable, low-noise, low-cost shaft torque transducers that are non-invasive to the mechanical drive system have precluded the use of direct torque feedback in all but a minority of specialised closed-loop servo drive systems.

Conventional torque sensors employ a torsion bar to translate torque into a relative large mechanical movement that can be measured by a potentiometer, capacitive, inductive, magnetic or optical angular position sensor. Alternatively, the strain produced by the torque on the surface of the torsion bar can be measured using piezoresistive or piezoelectric strain gauges. The need for the torsion bar is a compromise in itself on the system design, making the transducers too mechanically compliant in many servo drive applications. A further issue is that many designs require some form of mechanical stop, while a number of the sensors need a mechanical connection between the shaft and the interrogation unit. This adds to the complexity and cost of the sensor.

Non-contact torque measurement

Now, though, research in the Power Conversion Group at Manchester University, led by Dr Nigel Schofield, is focusing on the use of a non-contact torque measurement system developed by Sensor Technology. Based on Surface Acoustic Wave (SAW) technology, the TorqSense transducers provide low-cost torque measurement for brushless servo applications, providing active damping and/or resonance ratio control.

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

The SAW-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 give 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 100-1000MHz, 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 makes them eminently suitable for use with motors.

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

Easily embedded within the drive system, the TorqSense transducers can withstand heat, dirt and mechanical vibration; these are problematic for optical sensors in particular. Their non-contact coupling between the shaft and the controller eliminates any issues of mechanical compliance.

Studies at the University of Manchester are already highlighting major benefits for future servo drive systems, offering the potential of servo drive trains that are 'intelligently rigid' and therefore 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.

Ongoing research is focused on developing the control algorithms that will enable the servo controllers to take full advantage of the torque feedback. With this done, and with the availability of the TorqSense transducers as discrete add-on measurement devices as well as embedded feedback products, there is potential for all servo systems to benefit, giving new impetus to a whole spread of industries to significantly boost the performance of existing machinery as well as new equipment.

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