An axial flux eddy current sensor with perpendicular coils for speed measurement

This paper presents an axial flux eddy current sensor with a compact and simple structure for measuring the rotating speed of iron shafts. The sensor structure is optimized for high sensitivity with a novel configuration of the coils positioned perpendicularly to each other. The sensor comprises two D-shaped excitation coils and two D-shaped pick-up coils in a double-layer structure. A disc-shaped magnetic shield or core shields the coils. Two iron shafts with different material properties are considered. A new cup-shaped configuration of nonmagnetic copper and aluminum caps mounted on iron shafts improves sensitivity and suppresses susceptibility to shaft material properties and airgap variation. 2D and 3D finite element methods are utilized for the performance analysis of the sensor. The measurements are performed at speeds up to +/- 3000 rpm and different excitation frequencies from 400 Hz to 4 kHz. The eddy current speed sensor has excellent linearity characteristics with a nonlinearity error of 0.15%. The geometry of the coils is further optimized and improved for maximum sensitivity and compactness. The fault-tolerant capability of the sensor is also evaluated.