AN2863

Sensor overview

Doc ID 15277 Rev 1

5/13

2

Sensor overview

Proximity switches are generally applied to sense the position of a moving object in

manufacturing processes. Typically, they utilize an oscillator drive circuit in combination with

an induction tank circuit. The tank circuit includes an induction coil as a means for sensing

the presence of an object such as metal. The magnetic field induces eddy currents in

a conductive object which comes within the generated magnetic field. The oscillation

amplitude is attenuated due to the energy drawn from the induction coil. The amount of the

oscillation attenuation is directly related to the distance between the metal object and the

induction coil.

A typical inductive proximity switch employs a ferrite cup core as the sensing element.

It allows the flux field to be focused in front of the cup and further prolong the sensing

distance. The oscillator usually operates between 100 kHz and 800 kHz, where the eddy

current losses are significant.

Some benefits of the MCU approach compared with a traditional solution include:

●

More reliable operation thanks to the sensor self diagnostics

●

Easy and cheap sensor trimming in the production line

●

Digital temperature compensation

●

Linearization of the sensor characteristic

●

Easy realization of an analog or a PWM output.

Figure 2.

Smart inductive proximity switch block diagram