Introduction to Industrial Automation

Figure 2.23  Object movement in front of the sensing area of a proximity sensor perpendicular 

to the sensor’s longitudinal axis (a), and parallel to the sensor’s longitudinal axis (b).

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  Introduction to Industrial Automation

The main types of proximity switches, with respect to their external form and shape, are the 

cylindrical, rectangular, and the slot ones, as shown in Figure 2.24. The double cycle area in all 

of them is the effective area for the detection of an object’s presence. From the internal structure 

point of view and the used operating principle, the proximity switches include three basic stages, 

the LC oscillator, the Schmitt trigger unit, and the amplifier-output switching circuit, as shown 

in Figure 2.25.

Inductive proximity switch is where the LC oscillator creates, through the projected coil, a high 

frequency electromagnetic field which is extended in front of the whole effective area. If a metal-

lic object enters inside the zone of the electromagnetic field, then eddy currents are generated 

in the object’s body. These eddy currents generate their own electromagnetic field that opposes 

the field of the proximity switch and draws energy from the oscillating circuit, reducing thus 

the oscillation amplitude. The rise or fall of the circuit oscillation triggers the amplifier and the 

output switching circuit, where the oscillation change is converted to a SPDT contact-output 

change.

Capacitive proximity switches are similar to inductive proximity switches, since they have the 

same external form and size but a different operating principle. In general, the capacitive switch 

produces an electrostatic field instead of an electromagnetic field, and can detect metal as well as 

nonmetallic materials such as paper, glass, wood, liquids, and granular materials. In the capaci-

tive proximity switch, the capacitor of the LC oscillator, shown in Figure 2.25, takes the place of 

the coil, just behind the effective surface of the cylindrical switch. The capacitor is formed by two 

concentrically shaped metal electrodes like two circular sectors (plates) as shown in Figure 2.26. 

It is known that the capacitance is proportional to the surface area of the electrode plates and the