Introduction to Industrial Automation

Figure 2.36  Diffuse-reflective photoelectric switch.

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  Introduction to Industrial Automation

2.2.4   Limit  Switches

Limit switches are purely mechanical constructions, where the only electrical parts they have are 

their electric contacts. These types of switches have the ability to detect the “end or limit of a 

motion” through the rectilinear movement and force of a plunger that is transferred to the inter-

nal mechanism and converted to open or closed electric contacts, as shown in Figure 2.37a. The 

return spring resets the switch to its initial normal position, when the cause of the plunger move-

ment is eliminated. The switch housing may be made from aluminum or plastic material, but in 

all cases, it protects the internal mechanism with superior electrical insulation and mechanical 

strength. Figure 2.37a shows the manual actuation of the limit switch by hand in a similar way 

as a push button switch, but in reality, limit switches are mechanically actuated by a movable 

object, e.g., a piston rod or a movable shaft. Whereas the movement of the plunger during actua-

tion is possible in only one direction, the motion of the object actuating the switch may have 

any direction with respect to the plunger. From a practical point of view, it is very difficult for 

the two movements, of the plunger and the actuating object, to be identified. For this reason, an 

actuator head is adapted to the body of the limit switch, as shown in Figure 2.37b, in order for a