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Sensors: Inductive proximity vs displacement vs eddy-current

04 Apr 2016  | Patrick Mannion

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It seems like engineering 101, but can you actually tell specific differences between inductive proximity, inductive displacement, and eddy-current sensors? I asked a few friends and everyone had a pretty good idea, to varying degrees, but if you aren't immersed in the topic the nomenclature can throw you off, as they're all reliant on eddy currents.

Eddy currents, also called Foucault current, are loops of electrical current induced within a conductor by a time-varying magnetic field in that conductor (figure 1). This phenomenon of induction was first observed by Michael Faraday way back in 1831, and he summed up his experiments in Faraday's Law. This of course states that the induced electrical current (electromotive force, or EMF) in a closed circuit is equal to the negative of the rate of change of the magnetic flux, or:



where is the EMF and ΦB is the magnetic flux.


Figure 1: Eddy currents are generated in a conductor by a time-varying magnetic field. When induced in a nearby conductor, the induced eddy currents oppose those of the originating magnetic field. (Image courtesy of Microwave Soft.)


Heinrich Lenz contributed by adding that the direction of the induced EMF always opposes the change that induced it.

These combined phenomena of induction, eddy current generation, and opposition are the fundamental principles of proximity, inductive displacement, and eddy-current sensors. The primary differences are structure and the accompanying electronics. So let's start with the humble proximity sensor, the most basic embodiment and application of these principles.

The basic but well-loved proximity sensor is a binary device that simply tells whether or not a metallic object (the "target") is present – or not. It comprises a wire coiled around a ferromagnetic core and an oscillator to generate an alternating current to create the time-varying magnetic field (figure 2).


Figure 2: The basic proximity sensor is binary in that that it uses the opposing eddy currents induced in the target metal to detect absence or presence of the target. There's no relative or absolute position information. (Image courtesy of robotpark.com.)



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