Working principles of an ultrasonic sensor
The ultrasonic sensor emits an acoustic pulse in a non-audible range. It analyzes the echo of the target.
Ultrasonic sensors are sensors that convert ultrasonic signals into other energy signals (usually electrical).
Ultrasonic wave is a mechanical wave whose vibration frequency is higher than 20 kHz. It has the characteristics of high frequency, short wavelength, small diffraction phenomenon, especially good directive, and directional propagation as a ray. 
Ultrasonic waves penetrates both liquids and solids, especially in the opaque solids of sunlight.
Ultrasonic contact with impurities or boundary surfaces will produce significant reflection to form reflection echo. Touching the moving object can also produce the Doppler effect. 
Ultrasonic sensors are widely used in industry, national defense, bio-medical and other fields.
