A Compact Actuator for Lower-Limb Exoskeletons With High Torque Density and High Backdrivability

Lower-limb exoskeletons demand high torque density and high backdrivability for flexible assistance, which poses a challenge to the available actuators. To handle this issue, this article proposes a compact actuator which mainly consists of a hollow single-stator-single-rotor (SSSR) axial flux permanent magnet motor and a built-in planetary reducer with a low ratio. Owing to its unique axial flux path, the SSSR motors share the merits of superior torque density, high efficiency, and flat shape. The integration principle of the proposed joint is outlined, and the key components, namely the SSSR motor and planetary reducer, are designed to meet the design objectives. Subsequently, the coupling model of the proposed actuator is developed based on the electromagnetic modeling of the SSSR motor and transmission modeling of the planetary reducer. The accuracies of the electromagnetic and transmission models are verified by finite-element analysis and experiment, respectively. By matching the motor and reducer, the primary parameters of the actuator are determined, and its torque characteristics are theoretically analyzed. Finally, the actuator prototype is fabricated and the experimental platform is built to further validate the proposed design. The experimental results indicate that the proposed actuator achieves a torque density of 28.8 N·m/kg and a backdrivability of 18.2.