Introduction to Industrial Automation

Figure 2.22  Typical external view of a proximity sensor.

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Hardware Components for Automation and Process Control 

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Object placement distance: The motion of an object in front of the effective area of the proxim-

ity switch may be perpendicular to the switch axis, as shown in Figure 2.23a, or parallel, 

as shown in Figure 2.23b. The object placement distance has meaning only in the first case 

and is defined as the constant distance between the passing object and the effective area of 

the proximity switch. The object placement distance should always be less than the detection 

distance. Usually, the object placement distance must be precisely adjusted during mount-

ing of the switch. For this reason, the cylindrical type proximity switches have a threaded 

cylindrical surface for micro-adjusting their location.

Power supply is the supplied voltage range that the proximity switch will operate at. This volt-

age should not be confused with the operating voltage of the automation circuit where the 

output of the proximity switch, usually a SPDT contact, is inserted.

Response time is the time elapsed between the object detection instant and the switch output 

activation.

Operating frequency is the maximum number of ON/OFF changes per second that the proxim-

ity switch, and particularly its SPDT output, is able to perform.

Residual current is the current that flows through the proximity switch when it is not activated 

by an object. The residual current has a particular importance when the proximity switch 

is connected in series with the load and, furthermore, when it is connected as input in a 

programmable logic controller, as is explained in Chapter 6.

Switch OFF Switch ON

Sensing

distance

Sensing surface

Detected object

Proximity sensor

(b)

Parallel to sensor’s axis movement

(a)

Perpendicular to sensor’s axis movement

Switch ON

area

Switch OFF

area

Proximity sensor

Detected object

Object placement

distance 

Boundary curve of

effective area