In retroreflective mode, a permanently installed reflector constantly reflects the ultrasonic signal. An aligned metal or plastic panel can be used as a reflector. An existing background such as a wall, conveyor belt, or the floor can also be used. As long as there is no object between the ultrasonic sensor and the reference reflector, the sensor receives a constant echo from the reflector. When a detected object enters the detection range, the reflection of the ultrasound signal changes and the ultrasonic sensor detects the presence of the object. In general, the following three scenarios trigger switching.
- A small object that is located in front of the reference reflector is detected by the sensor. The ultrasonic sensor receives an extra echo from the reference reflector.
- A large object is detected and completely obscures the reference reflector. The ultrasound sensor receives an echo only from the object.
- A large, slanted object in front of the reference reflector is not detected but obscures the reflector. The ultrasonic sensor does not receive an echo from the object or the reference sensor.
All three scenarios trigger switching at the ultrasonic sensor output. Retroreflective sensors are especially recommended for reliable detection of sound-absorbing objects. They are also suitable for objects without reliably detectable surfaces, e.g. smooth, slanted surfaces such as a car windshield. There are no blind zones in this operating mode.
- Easy to install, only one sensor head
- Reliably detects problematic objects (sound-absorbing, slanted surfaces)
- Reference object/background is used as a fixed reflector; the object disturbs/breaks the beam
- Switching frequency is lower compared to a thru-beam sensor