Introduction to Industrial Automation

Figure 2.33  Typical exterior view of a photoelectric sensor.

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  Introduction to Industrial Automation

shown in Figure 2.34. When an object breaks, the light beam causes the receiver to change the state 

of the output contacts, which have been inserted in the automation circuit. The sensing distance is 

the maximum operating distance between the emitter and receiver, and is achieved when the two 

units have perfect alignment and hence the maximum amount of the emitted energy reaches the 

receiver. This means that the placement angle of the emitter that is relative to the receiver must 

not exceed a predefined value given by the constructor. This type of photoelectric switch has the 

advantage of a longer sensing distance, in comparison to other types of photoelectric switches.

Retro-reflective photoelectric switches: In this type of photoelectric switch, the emitter and 

receiver are embedded in the same housing unit, and a reflector is used for the reflection of the 

light beam, as shown in Figure 2.35. Any object situated between the sensor and the reflector, 

interrupting the light beam from reaching the receiver, activates the output of the photoelectric 

switch, such as in the case of the through-beam mode of sensing. Reflectors are usually made up of 

many small, corner-cube prisms that reflect the beam back to the receiver, almost in parallel to the 

entering beam. A simple mirror can also be used as reflector, but precise positioning of it is needed 

in order for the emitted beam to strike almost perpendicular to the mirror surface, while reflec-

tors may have a small deviation angle. When the detected object is opaque with a highly reflective 

Receiver

Emitter

Side view

Top view

Detecting

object

Effective

placement

angle

Sensing distance

Emission beam