Diffuse photoelectric sensors have been and are used to successfully solve numerous applications in automation. However, there are some applications that are too difficult or impossible to solve with standard diffuse sensors. In some cases, these difficult applications can be solved with a background suppression sensor that is also based on the diffuse operation principal. So the question is then raised, what makes the background suppression sensor capable of solving these difficult applications?
This may be a good time to review… Diffuse sensors operate on the principal that when a light source is shined on a surface, the light is scattered or diffused in many directions. A small portion of the light is reflected back to the sensor receiver. The receiver used in this style of sensor is designed to be sensitive to a smaller or larger amount of light, depending on the sensor configuration, that is reflected back from the target surface. There are a number of factors that affect how well diffuse sensors operate including, but not limited to, surface finish, color, texture or surface irregularities, target size, dirty or dusty environment and the background of the application.
Background sensors, sometimes referred to as BGS, actually have two receivers built into the sensor. These two receivers detect the angle of the light reflected back from the target, referred to as triangulation. If the target is between the focal point and the receiver the light is reflected to one receiver and if the target is beyond the focal point the light is reflected to the second receiver. The sensor compares the amount of light on each receiver and sets the output accordingly.
You can purchase BGS as either fixed or adjustable. The adjustable versions are available in two versions, mechanical and electronic. The mechanical version utilizes two receivers, if the target is between the focal point and the receiver the light falls on receiver one and if the target moves beyond the focal point then the light falls on receiver two. The potentiometer on the sensor adjusts the focal point.
The electronic versions use a Position Sensitive Device, also known as a PSD, instead of the two receivers. When the light is reflected back to the PSD the beam will fall on two points, one from the target and one from the background. The electronics in the sensor then determines the angle of the light which in turn determines the distance.
The first application that I used a BGS on was a conveyor in a facility that had people walking by the back of the conveyor. The conveyor originally had diffuse sensor and when personnel in light colored clothes walked by the system false triggered. Other applications that I have solved with background sensors include, detecting a gray cookie pan on a gray link belt conveyor, white sound deadening fabric on a shiny stainless steel surface, and as an error proofing sensor to insure a rubber washer on a valve stem. As you can see unlike standard diffuse sensors background suppression sensors are typically not tricked by like colors and target texture.
So now that we know background suppression sensors solve more and difficult applications why not use them everywhere? These sensors are typically more expensive than standard diffuse sensors because of the PSD and required circuitry. Background suppression sensors are worth every cent to solve those difficult applications.