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SAW torque sensors in dynamometers

At the moment, most dynamometer torque sensors use either strain gauges or, in more advanced applications, magneto-elastic sensors to measure output torque. However, in recent years new sensors for torque measurement based on Surface Acoustic Wave (SAW) transducers have been developed that have potential uses in dynamometer applications.

The sensors work by measuring changes in SAW properties across a substrate as the stress within the substrate varies. In its simplest form, a SAW transducer consists of two arrays of thin metal electrodes deposited on a piezoelectric substrate such as quartz. The polarities of the electrodes alternate, and when an RF signal of the right frequency is applied across them it causes the surface of the crystal to expand and contract, generating the surface wave.

The surface velocity of the acoustic waves is one ten-thousandth the speed of light, so for example, a signal at 100 MHz with a free-space wavelength of 3 m would have a corresponding acoustic wavelength of about 30 microns. The upshot is that SAW transducers can incorporate a very large signal bandwidth in a small volume, allowing for high-resolution measurements.  

In a SAW transducer designed for strain measurement, the piezoelectric substrate is attached to the material to be stressed – for example, the output shaft of a dynamometer. What is known as an input interdigitated transducer (IDT) is mounted on one side of the surface of the substrate, and a second, output IDT on the other side of the substrate. The acoustic wave created at the input IDT travels across the substrate to the output transducer, which converts the wave back into an electrical signal. Any changes in the frequency of the mechanical wave between the input and the output can then be recorded.

The transmitted waves’ frequency depends on the distance between the electrodes on each electrode array, and the waves travel at right angles to the electrodes. Therefore, any change in shaft length alters the electrode spacing and thus operating frequency. Tension increases the frequency while compression reduces it.

Using a suitable signal processing device, these changes in the wave’s frequency can be used to determine torque as a function of strain in the surface substrate. In effect, the transducer acts as a ‘frequency dependent’ strain gauge. One of the biggest benefits is that SAW transducers are not affected by strong magnetic fields, as the wave is ‘mechanical’ rather than electromagnetic, making them particularly useful for measuring electromagnetically noisy devices such as electric motors.

The signals generated by SAW transducers can be transmitted using a non-contact capacitive coupling, which also supplies sufficient power for the transducer to operate. The coupler consists of a stator and a rotor that may be a pair of discs or coaxial cylinders. The rotor is mounted directly to the sensing component – in the case of a dynamometer, the output shaft – and rotates with it. The rotor carries a 360° microstrip wired to the SAW sensor, while the stator is mounted statically and also carries a microstrip wired into the signal processing unit.

Although still a relatively new technology, the use of SAW transducers on dynamometers has a number of benefits. The most notable is the ability to conduct contactless measurement, removing the need for devices such as slip rings, which are necessary for signal transmission when using traditional strain gauges. SAW sensors can also be used on any material, unlike magneto-elastic sensors which require the test components to be made from specific metals with suitable magnetic properties. Other benefits include the compact nature of the sensors and the fact that they are inherently rugged, having few parts that can be damaged or displaced.

Overall, this emerging technology is likely to have a number of useful applications both in both the powertrain testing and wider automotive worlds.

Written by Lawrence Butcher

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