When the voltage applied to the piezoelectric ceramic, it will produce mechanical deformation with voltage and frequency changes. On the other hand, when vibrating the piezoelectric ceramic, a charge is generated. Using this principle, when a vibrator composed of two pieces of piezoelectric ceramics or a piece of piezoelectric ceramics and a metal piece, so-called bimorph element, is applied with an electric signal, ultrasonic waves are emitted due to bending vibration. In contrast, when an ultrasonic vibration is applied to a bimorph element, an electrical signal is generated. Based on the above effect, the piezoelectric ceramic can be used as an ultrasonic sensor.
Such as ultrasonic sensors, a composite vibrator is flexibly fixed to the base. The composite vibrator is a combination of a resonator and a vibrator of a bimorph element consisting of a metal piece and a piezoelectric ceramic piece. The resonator is trumpet-shaped in order to efficiently radiate the ultrasonic wave generated by the vibration and to effectively concentrate the ultrasonic wave at the central portion of the vibrator.
Ultrasonic sensors for outdoor use must have good sealing to prevent intrusion of dew, rain and dust. Piezoelectric ceramic is fixed inside the top of the metal case. The base is fixed to the open end of the box body and covered with resin. For industrial robots used in ultrasonic sensors, the accuracy required to reach 1mm, and has strong ultrasonic radiation.
With the bending vibrations of conventional bimorph vibrators, this is not possible at frequencies above 70 kHz. Therefore, in the high-frequency detection, you must use the vertical thickness vibration mode piezoelectric ceramic. In this case, the acoustic impedance of the piezoelectric ceramic matches with the air becomes very important. The piezoelectric ceramic had an acoustic impedance of 2.6 × 107 kg / m2s, while the air had an acoustic impedance of 4.3 × 102 kg / m2s. A difference of 5 power leads to a large loss on the surface of the piezoelectric ceramic vibrating radiation. A special material adheres to the piezoelectric ceramic as an acoustic matching layer to match the acoustic impedance of air. This structure allows the ultrasonic sensor to operate normally up to hundreds of kHz.