Ultrasonic sensors are developed by using the characteristics of ultrasonic wave. Ultrasonic wave is a mechanical wave with higher vibration frequency than sound wave, which is produced by vibration of transducer chip under excitation of voltage, it has the characteristics of high frequency, wave length, small diffraction phenomenon, especially directional good, can become ray and directional propagation.
Ultrasonic penetration of liquids and solids is great, especially in opaque solids, which can penetrate dozens of of meters in depth. The ultrasonic wave encounters the impurity or the interface will produce the remarkable reflection to form the echo, the movement object can produce the Doppler effect. The sensor developed based on ultrasonic characteristics is called "ultrasonic sensor", which is widely used in industry, national defense and biomedicine.