Introduction to Industrial Automation

Figure 2.24  Basic exterior forms of proximity sensors for various applications.

Sensing metal object

Induced eddy currents

oppose field 

Electromagnetic

detection field 

LC oscillator

electronics

Schmitt

trigger

Inductive coil

Amplifier and

output switching

circuit

Inductive proximity switch

L

C

Figure 2.25  Internal elements of an inductive proximity sensor and object-electromagnetic 

field interaction.

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Hardware Components for Automation and Process Control 

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dielectric constant of the material between them, while it is inversely proportional to the distance 

between electrodes. When an object approaches the sensing surface, it enters the electrostatic field 

of the electrodes and changes the capacitance of the LC oscillator circuit, since the object plays the 

role of a new dielectric material. As a result, the oscillator begins oscillating and continues until 

the object is removed. The existence or absence of the oscillation is subsequently converted to the 

SPDT contact-output change, in a similar way to the case of the inductive proximity switch.

Electric connection of proximity sensors: Inductive and capacitive proximity switches can be 

 connected to an AC or DC power supply source in a series or separately with a load, the so-called 

two-wire or three-wire connections. The connected load may be the coil of a power relay, the elec-

tronic circuit of a control unit, or the digital input of a programmable logic controller. The two-

wire connection of a proximity switch is shown in Figure 2.27a, where the load is connected in 

series under a 24 V DC power supply. In the two-wire connection of an electronic-type proximity 

sensor, a residual current flows through the load, even when the sensor is in the non-activated 

state. Due to the residual current, a rotary switch (RS) is necessary for the load isolation during 

the rest period. When the sensor is in the activated state, a voltage drop occurs among it and the 

flowing current increases to a level that is being determined by the load. Attention should be paid 

Plastic object

Electrostatic field

RC oscillator

electronics

Schmitt

trigger

Ground electrode

Amplifier and

output switching

circuit

Capacitive proximity switch

C

Main electrode

R

19

Effective area

front view

Main electrode

Ground electrode

Figure 2.26  Internal elements of a capacitive proximity sensor and object-affecting dielectric 

constant.

Sensing surface

Proximity sensor

Load  

RS

+24 V DC

N

Sensing surface

Proximity sensor

Load

 

RS

+24 V DC

N

(a)

(b)

Figure 2.27  Electric circuit of a proximity switch with (a) a two-wire connection, and (b) a