Surface Acoustic Wave Accelerometer

This invention relates to accelerometers of the type in which a resilient beam is secured in a mount so that movement of the mount results in inertial bending of the beam, the beam being provided with surface acoustic wave (SAW) devices, the propagation characteristics of which change as the beam bends.

Such an accelerometer is described in our co-pending British application No. 8208434 (published British application No. 2,117,115A). In particular, there is described therein an embodiment in which a pair of SAW devices are used, one on either side of the beam together with two separate oscillators, the frequencies of which are controlled by a respective SAW device. This differential SAW accelerometer provides a low cost convenient method of measuring acceleration for sub-inertial navigation purposes. However, any differences in the two oscillator maintaining amplifiers will give rise to errors as a function of temperature and time.

According to the present invention there is provided a differential SAW accelerometer of the type in which a resilient beam, secured in a mount so that movement of the mount results in inertial bending of the beam, is provided with a pair of SAW devices, one on either side of the beam, operating in a differential mode, wherein the SAW devices are connected such that their outputs are in antiphase and are balanced in amplitude under conditions of zero acceleration, the accelerometer including means for setting the beam into a mechanical flexural oscillation which is superposed on the movement of the beam due to acceleration forces.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

FIG. 1 illustrates a vibrating cantilever differential SAW accelerometer,

and FIG. 2 illustrates a convenient means of generating a suitable R.F. signal synchronous with the devices.

In the arrangement shown in FIG. 1, which illustrates a first embodiment of the invention, a resilient quartz beam 1 is clamped at one end in a support 2 so that the beam forms a cantilever or semi-beam structure. Electroacoustic transducers 3,4 and 6,7 are provided on both opposite surfaces or faces of the beam 1 whereby surface acoustic waves can be propagated between transducer 3 in the surface region of the quartz beam and transducer 4, and similarly propogated between transducers 6 and 7. Transducers 3 and 6 are driven by a stable oscillator 5. The outputs of the SAW devices are summed in antiphase by a summing circuit 8. A variable attenuator 9 is included for calibration purposes. Surface acoustic wave absorption may be provided at each end of the beam. Preferably, the SAW devices formed by transducers 3, 4 and transducers 6, 7 are substantially of the same nominal frequency and delay. The SAW devices may be incorporated into electrical circuits as either delay lines or resonators. In a balance condition, the outputs S.sub.1 (t) and S.sub.2 (t) of the SAW devices are arranged to be in antiphase and equal in amplitude, so that the summed output signals will cancel. If the support 2 is moved in a direction normal to the plane of the beam 1, with an acceleration a, the beam 1 bends. This bending causes the surface acoustic wave path length between the transducers on each side of the beam to change. A differential strain will then be introduced in the two delay lines formed by the two transducer pairs 3, 4 and 6, 7, and an unbalance in the relative phase .phi..sub.1 and .phi..sub.2 of the output signals will occur. This phase difference is difficult to measure directly with high accuracy.

Therefore means are introduced for setting the beam into mechanical flexural oscillation at a low frequency, which is superposed on the movement of the beam due to acceleration forces. Typically this may be achieved by mounting the support 2 on a vibrating acoustic device 12. Each time the beam passes through the position where the two SAW devices are of equal length then .phi..sub.1 =.phi..sub.2 and no RF signal will appear at the summed output. On either side of this position .phi..sub.1 .noteq..phi..sub.2 and there will be a differential output produced. Under conditions of no acceleration the null points in the output waveform will occur at equal intervals of time. This signal can be detected in an envelope detector and converted to a rectangular waveform which will provide a square wave signal, the "mark-to-space" ratio, i.e., the ratio of pulse duration to the time between successive pulses, of which is 1:1 in the balanced condition, i.e. no acceleration. Under conditions of acceleration .phi..sub.1 .noteq..phi..sub.2 and the mark-to-space ratio will change, the time intervals between successive null points becoming unequal. Digital means (not shown) can be used to determine the mark-to-space ratio with high accuracy and can provide an output signal with a known relationship to acceleration.

In the alternative arrangement shown in FIG. 2 the SAW device constituted between transducers 3 and 4 is incorporated in a feedback loop of an amplifier 10 to generate an RF signal of frequency W.sub.o. This RF signal is also applied to transducer 7. The outputs from transducers 3 and 6 are summed in antiphase and detector 11 converts the output to a rectangular waveform.

As an alternative to the single mount cantilevered beam arrangement, the resilient beam 1 may be clamped at both ends in support members 2, 2a, as shown in FIG. 2, and the support members may be mounted on corresponding vibrating acoustic devices 12, 12a, which are arranged such that movement may be imparted to both supports in unison to set the beam into said superposed flexural oscillation.