Plate-mode ultrasonic structure including a gel

An ultrasonic sensor comprising:

• a composite structure comprising (a) a supporting frame of a first material, and (b) a propagation medium comprising a thin planar sheet of a second different material capable of supporting Lamb waves, supported peripherally by said supporting frame, said sheet having a thickness very much thinner than the wavelength of Lamb waves to be propagated thereon and having some physical characteristics determined by the value of a measurand acting thereon, said physical characteristics determining the propagation characteristics of Lamb waves to be propagated along the medium, said propagation medium including on a surface thereof a layer of a gel;
• electrical means coupled to the thin planar sheet of said propagation medium for producing Lamb waves in the propagation medium;
• output means on said sheet for producing an electrical signal representative of the determined propagation characteristics of the Lamb waves propagating along the propagation medium;
• and
• measuring means for measuring selected characteristics of said electrical signal,
• whereby when said sensor is acted on by a measurand, the physical characteristics of said propagation medium are determined and said electrical signal characteristics are also determined, so that said measuring means indicates the value of the measurand.

The sensor as recited in claim 38, wherein the output means includes a receiving transducer, coupled to the propagation medium, for producing an electrical signal representative of the Lamb waves propagating along said propagation medium,

• the electrical means includes a launching transducer, coupled to the propagation medium, for generating Lamb waves in the propagation medium,
• a feedback path between the receiving and launching transducers for providing feedback from the receiving transducer to the launching transducer, and
• a feedback amplifier in the feedback path for amplifying said electrical signal produced by the receiving transducer and fed back to the launching transducer, the sensor forming an oscillator in which Lamb waves are continuously propagated along the propagation medium,
• the measuring means includes frequency measuring means for measuring the oscillation frequency.