Machine Design

Sensor Sense: Ultrasonic sensors

Ultrasonic sensors use sound waves above 20 kHz to measure the distance from the sensor to a specified target. The sensor transmits and receives sound via a ceramic transducer that acts like a high-frequency speaker and microphone. The transducer vibrates air molecules creating an ultrasonic sound wave between the sensor face and the target. The sound waves strike the target and echo back. By measuring the length of time needed for the sound waves to travel to the target and return, the sensor can calculate the distance between itself and the target.

Because sensor operation uses sound waves rather than light, ultrasonic sensors provide an effective alternative to optical sensors in wet or dirty environments, as well as in applications where the target object is translucent or reflective. Optical properties such as target color, translucency, and reflectivity don’t affect ultrasonic sensors — a major advantage for clear object detection and liquid-level measurement.

Applications that need a precise, user-defined sensing window offer the best results for ultrasonic sensors. The user sets near and far limits to create the window. A target that falls between these limits activates the output.

Most ultrasonic sensors have an auto-window feature: If a target is caught twice at the same distance, the sensor automatically creates a near and far limit around the target (typically ±5 or 10 mm). Rather than traditional fixed-field ultrasonic sensors, many ultrasonic sensors today can be configured to handle a broad range of sensing windows.

Ultrasonic sensors are susceptible to wind and temperature fluctuations. For example, the speed of sound changes 1.8% with every 10°C temperature change, and that affects the timing of the echo. To account for this influence, many ultrasonic sensors now include temperature- compensation circuitry. Compensation uses an embedded thermocouple to measure the ambient temperature around the sensor and correct the reading. This feature eliminates about 66% of the error caused by temperature fluctuations.

Banner Engineering Corp. ( supplied information for this column.

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