Background suppression
A photoelectric retro- reflective sensor with background suppression consists of a light transmitter and several light receivers. The distance of an object can be determined by the principle of triangulation . The proximity switch switches on or off depending on this distance.
functionality
With these optical proximity switches , the light beam emitted by the transmitter is reflected on the object. With purely energetic light barriers (without background suppression), the energy of the reflected light is evaluated on a receiving element. In the case of light barriers with background suppression, it is not the energy but the energy difference on at least two light receivers that is used for the evaluation. A signal is triggered if more energy is detected on a receiver for the foreground than on a receiver for the background.
Adjustable background suppression
In addition to a fixed area for the foreground and background, there are also buttons with adjustable areas. This can either be mechanical, e.g. B. done by moving the lens, or electronically with the help of a position sensitive device .
advantages
The black-and-white difference, i.e. the distance difference in the detection of white, highly reflective and black, hardly reflective objects, is very small.
disadvantage
If an object is significantly smaller than the light beam of the sensor, it will not be recognized.
Pepita effect: If an object with very different contrasts (e.g. black and white) passes over the transmitter and receiver one after the other, the light point of the transmitter is located on the two high-contrast colors at the same time. If the reflection from the better reflective part hits the background receiver and that of the less reflective part hits the foreground receiver, the sensor switches off. This results in a short switch-off pulse when swept over. This can be avoided by rotating the sensor or object by 90 ° and simultaneously sweeping the high-contrast edge over the transmitter and receiver.