Edited by Robert Repas

The options when specifying sensors for long-range object detection or positioning usually boil down to two alternatives: optical sensors using either infrared or visible-red light and ultrasonic. Optical sensing is often the first choice. Optical sensors have excellent long-ranThe options when specifying sensors for long-range object detection or positioning usually boil down to two alternatives: optical sensors using either infrared or visible-red light and ultrasonic. Optical sensing is often the first choicge detection qualities and comparatively low cost. Direct-detecting ultrasonics are a reliable second option when dust, dirt, color variance, or ambient-light contamination limit optical effectiveness.

However, there are times when neither sensor will work. Fortunately, a third option does exist: the retroreflective ultrasonic sensor.

Traditional ultrasonic sensor designs determine sensor-to-target distances by timing the propagation delay of high-frequency sound pulses as they travel from the sensor to the target and return. But problems occur when the target reflects the sound pulses away from the sensor, making the target acoustically invisible. Retroreflective ultrasonic sensors compare the echo return time from a known, fixed acoustic reflector, such as a wall or ceiling. If that time changes, or the echo disappears, the sensor knows there’s a target present.

Vehicle detection, as when counting cars in a parking deck, crafts an excellent example of the benefits in using retroreflective technology. Optical technologies can display problems caused by varying paint colors and transparent glass windshields. Meanwhile, the smooth, sloped contour of the car body and windshield deflect ultrasonic echoes away from the transducer, rendering the target ultrasonically invisible.

An ultrasonic sensor mounted overhead learns the distance to the floor below. Anytime the sensor detects a change in that distance or loses the echo entirely, it signals the presence of a vehicle.

Beyond vehicle detection, retroreflective ultrasonic sensing can also detect humans, whose soft clothing is both randomly colored and acoustically absorbent. Colors along with dusty, fiber-laden environments such as those found in textile and carpet manufacturing can also quickly defeat a standard optical or ultrasonic sensor, giving the retroreflective ultrasonic sensor an advantage in that environment.

Pepperl+Fuchs supplied information for this column.

© 2011 Penton Media, Inc.