Development of a PM Spherical Actuator With Tiltable Stator for High Torque Density

Spherical actuators are active joints that rotate around three directions and are more compact than conventional joints using multiple single-axis motors connected in series. However, spherical electromagnetic actuators have low torque density, particularly in tilt directions, hindering practical applications. The essential reason is that their stator and rotor are misaligned after tilting, which not only reduces the electromagnetic torque but also introduces restoring torques. To enhance torque density, this article proposes a 3-DOF permanent magnet (PM) spherical actuator with a tiltable stator. The relative positions of the stators and rotors of each actuation unit remain unchanged after tilting, maintaining high output torque and allowing large tilting angles, and rotating and tilting torques are decoupled. We introduce the working principle of the proposed actuator and establish its basic structure. Two-dimensional magnetic field finite element analysis (FEA) and conformal transformation are combined to simplify 3-D magnetic field computations. The optimized parameters for the tilting unit are obtained. Finally, a prototype is fabricated and experimentally tested. The proposed spherical actuator yields a tilt torque exceeding 8 Nm and a rotation torque of 7.34 Nm, corresponding to a torque density of 1.75 Nm/kg for tilt and 1.6 Nm/kg for rotation relative to the entire actuator.