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Retroreflective Mode: When Ultrasonic Sensors Rise to the Highest Challenges

Posted by Patricia Stafford on Fri, Aug 07, 2015

What is the principle behind ultrasonic retroreflective mode sensors?  How do they work?

Retroreflective Mode: When Ultrasonic Sensors Rise to the Highest Challenges

Retroreflective mode uses the same type of design as diffuse mode sensors, such that a single housing contains both the transducer and the evaluation electronics, allowing one sensor to operate as both an emitter and receiver. Much like diffuse mode sensors, retroreflective sensors emit a three-dimensional sound cone which creates a beam angle that defines the sensing range. However, instead of calculating the distance to a target object, retroreflective sensors are set to a specific constant value based on an  unchanging distance to a reflective surface like a wall, plate, or conveyor belt. This distance between the sensor and the surface serves as a reference. The sensor measures the amount of time between the emission of a sound pulse and its return echo from the reflective surface. If there is a change in the amount of time or if the echo disappears entirely, this indicates that a target object is present.

How do I mount a retroreflective mode sensor?

Ultrasonic retroreflective mode sensor

Align it like you would a photoelectric sensor to a reflector. Retroreflective mode sensors easily mount on a bracket or by using a through hole. This mounting method works just fine for most applications where the target to be sensed is far away, regardless of whether the sensor sticks out several inches from its mounted base.

When should I use a retroreflective mode sensor instead of a photoelectric sensor?

Ultrasonic sensors in general can detect objects of any color or transparency. They can also handle the dusty and dirty environments where photoelectric sensors don’t work.

Why should I use retroreflective mode instead of thru-beam or diffuse mode ultrasonic sensors?

Thru-beam mode is powerful in that it offers longer detection ranges and shorter response times than other ultrasonic sensors. However, a thru-beam sensor is more expensive since it requires both an emitter and receiver, as well as power for both. Diffuse mode works well with many common, simple applications. However, the biggest drawback of diffuse mode is that it can lose the echo when a target is present. Retroreflective mode is for objects which would otherwise be difficult to sense, including objects that are highly absorbent (clothing, textiles, dust, sand) or that have a smooth, angled surface.

What can you do with ultrasonic retroreflective mode sensors? What kinds of applications are they used in?


Ultrasonic retroreflective mode sensors are used to detect cars at the car wash, vehicle assembly line, or parking garage. With a traditional ultrasonic sensor, the ultrasonic beam might be deflected off the slanting windshield so that no car is detected. By teaching in the distance from an ultrasonic retroreflective sensor to the floor, the car is detected even if the ultrasonic beam is deflected.

Ultrasonic sensing at a car wash

Overhead conveyor belt

Ultrasonic diffuse mode sensors are ideal for use on overhead conveyors because once these sensors learn the conveyor belt, they will identify anything that is not the conveyor belt as a target to be detected—even if the sound is absorbed by an actual target object or material. Objects (for example, carpet samples) can be detected regardless of color, angle of orientation, dirt, or whether they are sound absorbent.

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Topics: Ultrasonic Sensors

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