Introduction to Industrial Automation

three-wire connection.

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  Introduction to Industrial Automation

to the residual current, when the proximity sensor is connected to an electronic circuit, since it 

may activate the electronic-type load spuriously, although the sensor does not detect any object.

In Figure 2.27b the three-wire connection of a proximity sensor is shown, where the load is 

driven by an SPST independent contact output of the sensor and hence the load is not affected in 

any way by the residual current.

Effects of the target size and the metallic environment: When the detected object has an uneven-

surface, while it is approaching an inductive proximity switch, a part of the surface participates in the 

activation, as depicted in Figure 2.28a. In such a case, the maximum detection distance is reduced 

proportionally to the surface curvature. The sensing distance (SD) of an inductive proximity switch 

is also affected by the dimensions of the detected object, while the maximum detection distance, 

provided by the manufacturer, is valid when the object surface covers the whole effective area of the 

proximity switch. In general, smaller dimensions of the detecting object lead to the reduction of the 

corresponding sensing distance, as shown in Figures 2.28b and 2.28c correspondingly. The material of 

the object also affects the maximum detection distance of a proximity switch. Particularly, for the case 

of inductive proximity switches, the sensing distance is usually given for carbon steel St37 material.

In general, the inductive proximity switch may be shielded or not. The non-shielded inductive 

switches are affected by the presence of a metallic environment, due to the electromagnetic field 

scattering as shown in Figure 2.29a. This means that the switch may be activated by the metal-

lic environment and not by the target object. In such a case, the mounting of the non-shielded 

inductive switch should satisfy some safety distances, provided by the manufacturer from the adja-

cent surfaces, as presented in Figure 2.29b. The shielded inductive proximity switches create an 

electromagnetic field that is restricted just in front of the sensing area, as shown in Figure 2.29c. 

Therefore, these types of proximity switches can be mounted in a metallic environment without 

risk of pseudo activation.

Magnetic proximity switches have a different and simpler operating principle from that of the 

inductive and capacitive ones. They consist of a permanent magnet with a projected pole piece, 

a pick-up coil around a pole-piece, and its required housing. When the magnetic flow through 

the coil is varied, then a current by induction is produced, and thus a voltage output appears at 

the coil terminals, as shown in Figure 2.30. The variation of the magnetic flow may be caused by 

any ferrous material moved through the magnetic field. The magnetic proximity switch usually 

works with a ferrous gear, while its rotation produces a series of pulse-type voltages. If the gear is 

(a)

(b)

SD

max

SD

max

Sensor

effective area 

Proximity sensor

SD

Object

detected area 

(c)