Magnetic shield optimization for the multi-motor assembly

Electric motor influences its surroundings by stray magnetic fields. Their intensity is usually small compared to the intensity inside the motor due to a closed magnetic circuit. However, there are other open motor constructions where the influence of stray magnetic fields cannot be neglected. This paper analyses a particular case of influence of stray magnetic fields of a motor in the multi-motor assembly. Three bipolar two-phase stepping motors with permanent magnets are arranged close to each other. The rotor position of each motor is affected by stray magnetic fields of nearby motors. Therefore the magnetic shielding is placed between rotors to reduce the position shift of the rotor due to the rotation of neighbouring rotors. The dimensions, namely diameter and thickness, of the shielding element are design variables of optimization. The objective function is a sum of squared deviations of a rotor position from each desired step angle and should be minimized. The value range of diameter and thickness is restricted by motor dimensions. Step angles are calculated by Finite element method. For this purpose, a MAXWELL ANSYS 3D model of multi-motor assembly was built, and a dynamic rotor angle response was simulated. The steady state angle for each half step is retrieved from simulation and is used for the objective function evaluation. Response surface of the objective function, as well as optimal dimensions of the magnetic shield subject to optimization constrains, is showed.