Reflections from the target eliminate the need for a secondary device such as a reflector. However, targets in everyday applications vary greatly in color and reflectivity. Sensors for targets that have dark colors or poor reflectivity must be quite sensitive to operate correctly. One drawback, though, is that the sensor can respond to light reflected by the background as well as the desired target.
Diffused-mode sensors with background suppression use principles of triangulation to overcome these limitations. An LED beams light through a lens in a straight line toward the target. The target reflects the light back to the receiver lens at some angle. The distance between the sensor and the point of reflection determines the angle of reflection back to the receiver. The closer the target to the sensor, the greater the angle.
Diffused mode with background suppression operates at a fixed or variable range and can take either mechanical or electronic form. Mechanical background suppression uses two receiving elements in the photoelectric sensor. The closer target makes the reflected light hit the near receiver element. As the reflection point moves away, the angle of light reflected from the background decreases. The smaller angle of the reflected light from the background makes it shine on the far receiver element. Thus the sensor differentiates between the target and background according to whichever element receives the reflected light. Sensor output is based purely on the reflection distance from the sensor, not on the amount of received light.
Electronic background suppression uses the same triangulation principles except a position-sensitive device (PSD) replaces the two separate near and far receiver elements. The PSD gives an output corresponding to where the reflected light strikes its surface. As the distance between the sensor and the reflection point changes, the focal point moves up or down the PSD. The output of the PSD is compared to a preset value to determine the switch point that triggers the output of the sensor.
Pepperl+Fuchs (am.pepperlfuchs.com) provided information for this column.