Design and Optimization of a Two-Stage Magnetically Geared Machine Comprising Radial-Flux and Axial-Flux Magnetic Gears

Drive systems for robot joints must provide a high gear ratio within limited axial space. When two magnetic gears are axially stacked to form a two-stage transmission, the axial lengths of the individual stages accumulate. This paper therefore proposes a magnetically geared machine (MGM) comprising a radial-flux magnetic gear and an axial-flux magnetic gear. The permanent-magnet synchronous motor and the radial-flux magnetic gear occupy the inner space of the axial-flux magnetic gear, while shared rotors connect the three electromagnetic components. For this topology, the magnetic field modulation and torque relationships are derived, and the main design parameters are determined through two-stage optimization and three-dimensional transient finite-element analysis. The results show that the air gaps of both magnetic gears contain the required working harmonics and that the steady-state torques of the three rotors follow the two-stage transmission relationship. The optimized design achieves an overall gear ratio of 84.64 and a maximum transferable torque of 1098.97 N m. At this operating point, the volumetric torque density based on the overall cylindrical envelope volume is 328.00 N m L−1.