Introduction to Industrial Automation

Figure 2.28  The sensing distance (SD) of a proximity switch that is affected by the form and the 

dimensions of the detected object: (a) spherical object in front of the sensing surface, (b) the object 

covers the whole of the effective area, and (c) the object has smaller dimensions than needed.

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Hardware Components for Automation and Process Control 

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mounted on the shaft of a rotated machine, then the magnetic proximity switch can act as pulse 

generator, exactly synchronized with the rotation of the machine.

Magnetoresistive proximity switch: Another kind of proximity switch that bases its operation on 

magnetic field sensing technology is called magnetoresistive, because of the low or high resistance 

in a specialized chip element, depending on the existence or not of a magnetic field correspond-

ingly. The magnetoresistive element (MR) is just behind the effective surface of the proximity 

switch, as shown in Figure 2.31, and is combined with a trigger circuit and an amplifier for out-

putting a digital signal. Magnetoresistive proximity switches usually detect the magnetic field of 

a permanent magnet and are able to detect very weak magnetic fields. Since the magnetic fields 

are able to pass through many non-ferromagnetic materials, the detection process can be triggered 

without the need for direct exposure to the target object. Magnetoresistive sensors react to both 

axially and radially magnetized magnets, have significantly smaller physical size, are vibration 

resistant, and have superior noise immunity.

(a)

(b)

Metallic base 

 

Field

scattering 

base 

d

>3d

(c)

Metal

shield  

Coil

b

Metallic 

Metallic base