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A Primer on SAWs

Surface acoustic waves were quantitively described by Lord Rayleigh in 1885, when he showed theoretically that waves can be propagated over the plane boundary between an elastic half-space and a vacuum, or a sufficiently rarefied medium (eg. air), where the amplitude of the waves decay rapidly with depth. They are mechanical (acoustic) rather than electromagnetic. Much of an earthquake's destructive force is carried by this type of wave.

Surface waves achieved little recognition for their application in RF until less than two decades ago when SAW devices began to be developed for spread spectrum use in military radarequipment. From these beginnings an exciting new technology has evolved for RF signal processing applications.

Drawing of SAW element

In its simplest form, a SAW transducer consists of two interdigital arrays of thin metal electrodes deposited on a highly polished piezoelectric substrate such as quartz. The electrodes that comprise these arrays alternate polarities so that an RF signal of the proper frequency applied across them causes the surface of the crystal to expand and contract. This generates the Rayleigh wave, or surface wave, as it is more commonly called. These interdigital electrodes are generally spaced at ½ or ¼ wavelength of the operating centre frequency. Since the surface wave or acoustic velocity is 10-5 of the speed of light, an acoustic wavelength is much smaller than its electromagnetic counterpart. For example, a CW signal at 100 Mhz with a free space wavelength of three metres would have a corresponding acoustic wavelength of about 30 microns.

This results in the SAW's unique ability to incorporate an incredible amount of signal processing or delay in a very small volume. As a result of this relationship, physical limitations exist at higher frequencies when the electrodes become too narrow to fabricate with standard photolithographic techniques and at lower frequencies when the devices become impractically large. Hence, at this time, SAW devices are most typically used from 10 Mhz to about 3 Ghz.

The operation of a SAW transducer for strain measurement depends on the choice of a suitable piezoelectric substrate which can be attached to the material to be stressed. The stress results in a strain which can be in tension or compression. The sensitive axis of the transducer is longitudinal in the direction of wave propagation. Strain will change the spacing of the interdigital electrodes and hence the operating frequency. For an excitation frequency of 500 Mhz, 1000 µ-strain of tension will decrease the frequency by 500 kHz; conversely a compressive strain will increase the frequency by the same amount.

Shaft with RF couples and electronic interface box

To function as an oscillator, the element is used as amplifier feedback. The Q factor of the transducer is high typically 104. Therefore by meeting the phase and gain requirement, the circuit will oscillate with very high stability typically one part in 109.

From the technique described it is apparent that the output signal will be in the frequency domain. This has many advantages from an application viewpoint, particularly in variable speed electrical machines where low level signals can be easily contaminated by drive electronic noise.

The diagram above shows an arrangement of SAW elements mounted on a shaft which is subject to torsional forces. The excitation and pick-off signals are transmitted via non-contact RF couples to and from an electronic interface box, which then outputs control and display data.

The photograph above shows a typical member of the new TorqSense line of in-line rotary non-contact torque sensors using the novel SAW technology. Through their use of the SAW technology, these sensors deliver the following key benefits over conventional torque measurement techniques:

  • High Stiffness - typically 2x or more higher,
  • Low Inertia - lightweight plastic couples and no on-shaft processing,
  • High Bandwidth - typicaly >1kHz (opticals offer higher bandwidth),
  • Short Length - <2" shaft length required for active sensor,
  • Drift Stability - robust, frequency domain system using ultra-stable quartz elements virtually eliminates system drift,
  • EMI Immunity - suitable for operation inside electrical machinery e.g. motors.

Combined with an attractive cost vs. performance balance, TorqSense promises new opportunities for Rotary Torque sensor application!

© 2002 by WEN Technology Inc., Raleigh, NC 27616 USA
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