Performance Evaluation of Posture Measurement System for a Rotor of a Spherical Motor

Abstract

　An electromagnetic-type spherical motor rotates by the electromagnetic force worked between a rotor with permanent magnets and a stator with electromagnets. The torque map control method proposed by Kasashima, et. al., can control a spherical motor to rotate in any direction by adjusting the currents to excite electromagnets. The measurement of relative rotor posture to stator is necessary for the torque map control method because the currents to excite electromagnets to rotate the rotor around a specified rotation axis depend on relative rotor posture. A posture measurement technique of the rotor using Hall sensors is studied. A prototype posture measurement device developed is arranged Hall sensors in a mesh format to cover some part of the rotor in order to measure magnetic field generated by permanent magnets. It is necessary to estimate the position of permanent magnets of the rotor for the measurement of rotor posture. However, the permanent magnet position may not be estimated if there is an error in the distance measurement between Hall sensors and permanent magnets calculated from the output values of the Hall sensors.

　This study addresses the problem of the distance measurement error by applying least squares method to trilateration. The developed rotor posture measurement system is evaluated by rotor posture measurement experiments at various rotational speeds not only around the orthogonal coordinate axes but also an oblique axis.