A Linear Differential Inductive Displacement Sensor With Dual Planar Coils

A non-contact displacement sensor employing a couple of planar coils etched on printed circuit boards (PCBs) and an E-type soft ferromagnetic core is proposed here. The E-type soft ferromagnetic core moves over two triangular shaped spiral coils etched on a couple of PCBs. The variation in the inductances of the two coils has a cubic polynomial relationship with the displacement of the core. An appropriate signal conditioning technique operates on the inductances and provides an output that varies linearly with the displacement. The symmetrical structure of the sensor provides negligible cross axis sensitivity. Simulation studies and results obtained from a prototype sensor having a 70-mm range and a resolution of
$2~\mu \text{m}$
, built and tested, establish the efficacy of the proposed technique. The worst-case nonlinearity observed from the prototype is 0.4%.