AN2679

Sensor overview

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1

Sensor overview

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

manufacturing processes. Typically, they utilize an oscillator driver 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 enters within the generated magnetic field. The oscillation

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

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 to further increase the sensing

distance. The oscillator typically 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 are:

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more reliable operation thanks to the sensor self-diagnostics

●

cheap and easy sensor trimming in the production line

●

digital temperature compensation

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linearization of the sensor characteristic

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simple implementation of an analog or PWM output

Figure 2.

Smart inductive proximity switch block diagram