Ultrasonic Sensors

Ultrasonic sensors use sound waves at frequencies above the range of human hearing, typically above 20 kilohertz, to detect objects and measure distance, motion and level. Most are built around a piezoelectric transducer, which converts an electrical drive signal into mechanical vibration to radiate a sound pulse, and conversely converts a returning echo back into an electrical signal. The same element can often serve both to transmit and to receive.

The most common operating principle is echo ranging, sometimes called time-of-flight. The sensor emits a short burst of ultrasound and then measures the time taken for the pulse to travel to an object and reflect back. Because the speed of sound in air is roughly constant for given conditions, the round-trip time gives the distance to the object. This technique is used in parking sensors, robotics, liquid-level measurement and non-contact distance gauging. It works on a wide range of surfaces because it depends on acoustic reflection rather than colour or optical properties.

Ultrasonic motion sensing works on a related principle: a continuous ultrasonic field fills a space, and movement within it changes the reflected signal, often by a small frequency shift caused by the Doppler effect. The detector senses this change and registers motion, an approach used in some intruder alarms and automatic door openers. Performance depends on factors such as air temperature, which affects the speed of sound, and on the size and orientation of the target.

Frequently asked questions

How does an ultrasonic sensor measure distance?

It emits a sound pulse and times how long the echo takes to return; multiplying the round-trip time by the speed of sound gives the distance.

What type of transducer do ultrasonic sensors use?

Usually a piezoelectric transducer, which vibrates to produce sound when driven electrically and generates a voltage when it receives an echo.

How does ultrasonic motion detection work?

Movement within an ultrasonic field alters the reflected signal, often shifting its frequency through the Doppler effect, which the detector senses as motion.