Proximity sensors utilizing an evanescent acoustic field formed by flexural plate waves

A proximity sensor utilizing an evanescent acoustic field formed by flexural plate waves is proposed. When an object comes into the evanescent field, the acoustic radiation admittance of the bending vibrator may change. Hence, the proximity sensing can be achieved by measuring the electrical admittance of the piezoelectric flexural vibrator. To confirm the characteristics of the proximity sensor, some experiments were carried out using a flexural vibrator consisting of a 140° rotated Y-cut LiNbO₃ plate with a polarization-inverted layer. When an object was approached to the major surface of the vibrator, the admittance at a resonance of the bending vibrator decreased drastically according as the distance d between the object and the vibrator became short. On the other hand, when the object was approached to the tip of the vibrator, the Q-factor decreased in the range d>0.2 mm but increased again unexpectedly in the range d<0.2 mm