Piezoelectric resonant contact sensor

A piezoelectric resonant contact sensor, comprising

• an enclosure in the form of a hollow body of revolution, said enclosure having an external surface and an outer diameter;
• a hollow piezoelectric vibration exciter which reproduces the shape of said enclosure and is disposed within said enclosure in a manner providing an acoustic contact, said piezoelectric exciter having an inner surface;
• a grounded electrode interposed between said enclosure and said piezoelectric exciter;
• an exciting electrode disposed on the inner surface of said piezoelectric exciter;
• a sine-wave generator having an output connected to said exciting electrode;
• a phase detector having a signal input and a control input connected to the output of said sine-wave generator;
• at least two rods disposed on said enclosure in such a manner that their axes are normal to said external surface thereof;
• at least two contact lugs secured to the free ends of said rods in a manner providing acoustic contact with said enclosure and forming in conjunction with said rods, said enclosure and said piezoelectric exciter a common oscillatory system wherein the self-oscillation frequency of said enclosure with said rods and said contact lugs is equal to the oscillation frequency of a sine-wave electrical signal;
• and
• a measuring electrode disposed on the inner surface of said piezoelectric exciter and connected to the signal input of said phase detector, said measuring electrode being adapted to provide an output electrical signal, a variation of whose phase relative to the phase of said sine-wave signal fed from the output of said sine-wave generator is indicative of the moment at which said contact lug contacts a part to be measured, each of said rods having a length L determined by the expression:
• L=l.sub.0 +l.sub.1
• wherein
• l.sub.0 is the distance between the external surface of said enclosure and the nodal point "a" of oscillations of said rod, ##EQU3## C is the speed of sound in the material of said rods, f.sub.0 is the self-oscillation frequency of said common oscillatory system, and
• n=0, 1, 2 . . . .