Sensor device using lamb elastic waves for detecting the presence of a liquid at a predetermined level

Claim 1

A sensor using Lamb elastic waves for detecting the presence or absence of a liquid at a predetermined level N in a tank defined by a wall have a thickness e, comprising: (a) transmitter transducer means located on an external face of a portion of the wall defining the tank, which wall portion comprises a detection plate having an internal face that may come into contact with the liquid, said transmitter transducer means for locally generating Lamb waves in said wall portion; (b) receiver transducer means located on said external face of said wall portion of the tank proximate the predetermined level N, said receiver transducer means for detecting the presence or absence of Lamb waves transmitted along said wall portion; (c) electronic signal processing means for processing signals applied to said transmitter transducer means and received by said receiver transducer means, said electronic signal processing means including a high gain amplifier, an input of which is connected to said receiver transducer means, and an output of which is connected to said transmitter transducer means to, thereby, form a closed loop, a filter for allowing said closed loop to oscillate only at an excitation frequency of said transmitter transducer means, and means for detecting the presence of oscillations at said excitation frequency f when liquid in the tank remains below the predetermined level N; (d) means for adjusting said electronic signal processing means to excite said transmitter transducer means to generate first symmetrical mode So of Lamb waves in said wall portion and to excite that mode only; and (e) a prism-shaped member mounting each of said transmitter transducer means and said receiver transducer means, said prism-shaped members being attached to said wall portion to dispose said transmitter transmitter means and said receiver transducer means at an angle θ to said external face of said wall portion such that sin θ=νprism/νplate, wherein ν prism and ν plate represent the phase velocities, respectively, of longitudinal waves created in said prism-shaped members and Lamb waves created in said wall portion; (f) wherein said excitation frequency of said transmitter transducer means is determined as a function of the thickness e of said wall portion in such a manner so that the group velocity of said first symmetrical mode So of the Lamb waves is minimized.