The ultrasonic sensor is a piezoelectric ceramic device that realizes the bidirectional conversion between mechanical and electrical energy through the piezoelectric effect. Its propagation speed is 344 m/s (25°). Its operating frequency generally ranges from 20 kHz to 200 kHz. The distance and relative speed of obstacles are determined by reflections and Doppler effects. The detection distance is generally 1-2 meters. It is widely used in parking sonar, anti-theft alarm, flow meter, parking timer, automatic door and other product systems.