Experimental Analysis and Optimization of a Contactless Eddy-Current-Based Speed Sensor for Smooth Conductive Surfaces

Abstract

© 1982-2012 IEEE. Speed sensing is an essential part in all closed-loop systems. There exist some situations in industry where the speed has to be measured without touching the target object, for example, the accurate speed measurement of the solid metal wheels with smooth surfaces of freight wagons. In this article, a contactless, eddy-current-based speed sensor is proposed for applications where the speed of a smooth conductive surface is to be measured; but contact to or modification of this target surface is prohibited. The proposed speed sensor is composed of a permanent magnet (PM) rotor that is free to rotate above the target surface. The relative motion of the surface with respect to the PM field induces eddy currents in the surface, which leads to a torque being applied on the rotor. Consequently, the PM rotor speeds up until it reaches a steady rotational speed that is proportional to the speed of the target surface. Three models are proposed. They are a two-dimensional (2-D) finite-element model, a 2-D analytical model, and a three-dimensional (3-D) combined numerical/analytical model. Measurements are taken on multiple hardware prototypes to validate the analysis. Finally, a multiobjective (PM volume vs. dynamic performance) Pareto optimization is conducted for the proposed speed sensing system. The results show that smaller rotors with lower pole-pair numbers generally have better dynamic performance as well as lower costs.