Inductive proximity sensor for embedded mounting and method of design

Abstract

The invention relates to an inductive proximity sensor for embedding in a mild steel mounting plate, comprising an enclosure with a front wall of synthetic material forming a sensing face at a front end of the enclosure, an oscillator comprising a sensor coil with a core made of a material with a relative magnetic permeability larger than 1, typically a ferrite, which is arranged within the enclosure behind the front wall so that an open side of the core is directed towards the sensing face in order to direct the magnetic field of the coil towards a target in front of the sensing face, a hollow cylindrical metal member arranged perpendicular to the sensing face and surrounding the core, and a measuring circuit for measuring an attenuation of the oscillator due to eddy currents. The core is radially surrounded by a metal layer with a low electrical resistivity of less than 15 μΩ·cm and with a thickness of less than 40 μm, in order to improve the embeddability of the sensor. Alternatively, this thin metal layer may be omitted, if the electrical resistivity of the metal member is adjusted to a value in the range of 15 μΩ·cm-50 μΩ·cm.