Automating Manufacturing Systems - Process Control and ...

discrete sensors - 4.13
components, as shown in Figure 4.12. This arrangement works well with opaque and
reflective objects with the emitter and detector separated by distances of up to hundreds of
feet.
emitter object detector
Figure 4.12
Opposed Mode Optical Sensor
Having the emitter and detector separate increases maintenance problems, and
alignment is required. A preferred solution is to house the emitter and detector in one unit.
But, this requires that light be reflected back as shown in Figure 4.13. These sensors are
well suited to larger objects up to a few feet away.
emitter
reflector
detector
reflector
emitter
object
detector
Note: the reflector is constructed with polarizing screens oriented at 90 deg. angles. If
the light is reflected back directly the light does not pass through the screen in front
of the detector. The reflector is designed to rotate the phase of the light by 90 deg.,
so it will now pass through the screen in front of the detector.
Figure 4.13
Retroreflective Optical Sensor