Introduction to Industrial Automation

2.2.2   Proximity  Switches

Proximity switches are, in general, sensors for the non-contact detection of the presence of various 

objects, whether metallic or not, in front of the switches’ effective area, while the form of a typi-

cal proximity switch is shown in Figure 2.22. There are three basic types of proximity switches 

with respect to their operating principles: inductive, capacitive, and magnetic. The operation of 

inductive proximity switches is based on the variation of the magnetic inductance. The capacitive 

proximity switches base their operation on the variation of capacitance and the magnetic proxim-

ity switches on the variation of the magnetic flow. The most widely-used proximity switches in 

industrial applications are the inductive and the capacitive ones. The substantial advantages of 

using proximity switches over other detectors (e.g., limit switches) are as follows:

 

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Long electrical life

 

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Durability to vibrations, accelerated motion, and toxic environments

 

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Operation under DC or AC voltage

 

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Effective response in objects’ high speed approaching and high frequency excitation

 

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Absence of any kind of movable elements

The basic parameters of the proximity switches on the basis of which their selection is per-

formed are the following:

Detection or sensing distance is the maximum detection distance between the target object and 

the effective area of the switch, as shown in Figure 2.23. The presence of an object in front 

of the effective area of the switch is detected when the distance between the object and the 

effective area is not larger than the detection distance.