A Rotary Encoder With an Eccentrically Mounted Ring Magnet

A novel design of a magnetic rotational sensor is presented in this paper. It contains a ring magnet simply magnetized through its central axis, which is deliberately displaced a distance from the axis of rotation. Hall effect sensors fixed at a distance to the magnet will detect a near sinusoidal variation in the magnetic-flux density as the magnet rotates a cycle, even though the flux is symmetrically distributed about the central axis of ring magnet. Finite-element analyses and physical measurement on an experimental device confirm the smoothness of the flux-density curve, based on which a harmonic fitting algorithm is used to estimate the rotational angle. Relation between measurement accuracy and eccentricity of the magnet is established by extensive testings on the experimental device. It is found that, using two differential pairs of Hall effect sensors, a fifth-order harmonic fitting yields a measurement error of less than 0.14 degrees for a certain amount of eccentricity.