Patent GB-1432818-A (GB)
Title: Transducer means
IPC: G01L1/16
Filed: 1973-09-28
Abstract: 1432818 Flexure transducer; surface wave devices SPERRY RAND CORP 28 Sept 1973 [24 Oct 1972] 45453/73 Headings H4D and H3U A measure of the flexure of a cantilever device, and hence a measure of a related parameter (such as acceleration, strain, displacement or fluid pressure) is obtained by sensing variations in the propagation time of a surface acoustic wave along a predetermined path in a flexible surface layer on the cantilever device. The Fig. 7 device comprises a cantilever 11 with one end clamped at 12. Transmitting transducer 14, receiving transducer 15 and oscillator 60 provide a feedback system for propagating a surface acoustic wave in layer 10. A similar system 14a, 15a, 60a propagates a surface wave in layer 10a. The two oscillation frequencies change in opposite senses when the cantilever is flexed, and the difference frequency is fed to a frequency-counter or via a frequencyto-voltage converter 63 to display 30. In a second embodiment (Fig. 8, not shown) both transducers 14, 14a are energized by a common CW oscillator and the outputs of transducers 15, 15a are phase-compared to indicate the flexure of the cantilever. In further embodiments transducers 14a and 15a are omitted and travel time variations over a single surface wave path are determined, by a phase-comparison of the transmitted and received signals if CW transmission is used (Fig. 4, not shown) by a timecomparison if the transmission is pulsed (Fig. 5, not shown) or by a feedback arrangement to vary an oscillator frequency (Fig. 3, not shown). The transducers in the single-path embodiments are arranged-one at each end of the cantilever or, as in Fig. 13, are both at the clamped end 12, the free end of the cantilever in the latter case having a wave reflector formed by boundaries 130, 131, by an inductively-shunted interdigitated electrode array (Fig. 12, not shown) or by grooves in the surface layer 10 (Fig. 16, not shown). An inertial mass is added to the free end when the embodiment is to be used as an accelerometer. Alternatively, the transducers are arranged near the rim of a rim-clamped diaphragm constituting the cantilever in an embodiment for measuring the fluid pressure acting on the diaphragm (Fig. 19, not shown). Embodiments involving two surface wave paths and a comparison of respective propagation times are inherently compensated for temperature-induced changes in the path length. For single-path embodiments the flexure measurement may be temperature-compensated by use of a bimetal temperature sensor and by forming the cantilever from an ST cut quartz bar having a substantially zero temperature coefficient of velocity. Surface wave generation and reception may be by means of interdigitated electrode arrays. Two arrays energized with a mutual phase-shift (Fig. 10, not shown) launch a wave propagating away from cantilever end 12, any undesired reverse wave being absorbed by, e.g. wax or rubber filling slots cut in the surface layer 10. The cantilever is made of quartz, lithium niobate, bismuth germanium oxide, bismuth germanate or lithium germanate. Alternatively, magnetostrictive transducers may be used.