Inductive Proximity Sensors

What is inductive proximity sensor?

Inductive proximity sensor is the sensor which detects metal object without physical contact with metal. (opposite to Limit Switch)

An inductive proximity sensor

An inductive proximity sensor

The sensor incorporates an electromagnetic coil which is used to detect the presence of a conductive metal object. The sensor will ignore the presence of an object if it is not metal.

Block Diagram


inductive proximity sensor working principle

Inductive proximity sensor working principle

inductive proximity sensor working principle 1

inductive proximity sensor when an object is detected

Working Principle

When a metal target enters the field, eddy currents circulate within the target. This causes a load on the sensor, decreasing the amplitude of the electromagnetic field. As the target approaches the sensor the eddy currents increase, increasing the load on the oscillator and further decreasing the amplitude of the field. The trigger circuit monitors the oscillator’s amplitude and at a predetermined level switches the output state of the sensor from its normal condition (on or off). As the target moves away from the sensor, the oscillator’s amplitude increases. At a predetermined level the trigger switches the output state of the sensor back to its normal condition (on or off).

Inductive proximity sensor waveform of oscillator

Inductive proximity sensor waveform of oscillator

Operating Voltage

There are many Inductive proximity sensors available in market depending on the operating voltage. Inductive Proximity sensors comes in AC,DC or AC/DC (universal) modes.Inductive Proximity sensor range from 10VDC to 320VDC and 20VAC to 265VAC.

Typical connection diagram

Inductive proximity sensor wiring

Inductive proximity sensor wiring

Normally Open (NO) and Normally Closed (NC)

Outputs of Inductive proximity sensor are considered normally open (NO) or normally closed (NC) based on the condition of the transistor when a target is absent. If, for example, the PNP output is off when the target is absent then it is a normally open device. If the PNP output is on when the target is absent it is a normally closed device.

Target Size

A standard target is defined as having a flat, smooth surface, made of mild steel that is 1 mm (0.04”) thick. Steel is available in various grades. Mild steel is composed of a higher content of iron and carbon. The standard target used with shielded sensors has sides equal to the diameter of the sensing face. The standard target used with unshielded sensors has sides equal to the diameter of the sensing face or three times the rated operating range,whichever is greater. If the target is larger than the standard target, the sensing range does not change. However, if the target is smaller or irregular shaped the sensing distance (Sn) decreases. The smaller the area of the target the closer it must be to the sensing face to be detected.

Inductive proximity sensor with different target size

Inductive proximity sensor with different target size

This post is one of our sensor series posts.

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