Sensor Sense: Ultrasonic sensors

Jan. 12, 2006
Ultrasonic sensors can measure distance without contact.

Typical ultrasonic sensors emit highfrequency sound waves that are reflected by the target. A target in the path of the sound wave reflects the sound back to the sensor where it is detected, triggering the sensor output.

Special programmable sensors compensate for irregular surfaces, such as differing levels of material in a bin. Here, the sensor output turns on when the level drops below the Low Level indication, and turns off when it detects levels above the High Level mark, to keep the bin properly filled. Additional setpoints monitor if levels become too low (dry-up) or too high (overflow.)

can measure distance without contact. They bridge the gap between proximity and photoelectric sensing. They possess a longer sensing distance than inductive or capacitive proximity sensors and remain unaffected by target color, ambient noise, or dusty atmospheric conditions. They handle applications considered difficult such as areas too dirty for photoelectric sensors or that involve unusually shaped targets.

Transmitters in ultrasonic sensors generate acoustic waves at frequencies from 65 to 400 kHz, well beyond the hearing range of most animals. typically operate in one of two ways: reflection and thru-beam. Reflection is the most common method of ultrasonic sensing. Targets reflect sound waves back to receivers in the sensors as an echo. The receiver detects the reflected sound waves and activates the sensor output.

The thru-beam ultrasonic sensor is less common. Here the transmitter beams the ultrasonic wave to a separate receiver. The target disrupts the beam, triggering the receiver output.

Like all sensors, proper operation depends upon a good target. Ultrasonics function best when detecting and monitoring objects with a relatively high density. Solid, liquid, or granular media make ideal targets because of their high acoustic reflectivity.

Porous targets such as felt, cloth, or foam rubber make poor targets because of their high sound-absorption properties. Liquids typically are excellent targets for ultrasonic detection because of their smooth, reflective surface. However, a surface covered in bubbles or foam may render it undetectable.

A less-obvious factor affecting reflectivity is surface temperature. Heat waves radiating from objects with surface temperatures above 212°F distort the ultrasonic beam, causing erratic outputs.

Standard ultrasonic sensors can also generate erroneous output signals when monitoring turbulent or unstable targets. The brief peaks and valleys in target levels do not represent true target position. Special programmable sensors provide a method for correcting such potential instabilities.

Pepperl+Fuchs ( provided information for this article.

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