Design and Optimization of an Outer-Rotor PMSM for Knee Assistive Exoskeletons Based on Gait Cycle

High torque density and low torque ripple are critical requirements for actuators in knee assistive robots to ensure comfortable wearing and efficient power transmission. This paper presents the design and multi-objective optimization of a high-torque outer-rotor Permanent Magnet Synchronous Motor (PMSM), specifically based on human gait cycle analysis. First, biomechanical requirements are analyzed, mapping the root-mean-square (RMS) and peak torque demands from gait data to the motor shaft, resulting in a rated torque specification of 3 Nm and a peak of 6 Nm. Based on these needs, a 36-slot/42-pole outer-rotor topology with Fractional Slot Concentrated Winding (FSCW) is designed to maximize the air-gap radius and torque output within a compact volume. To resolve the conflict between maximizing average torque and minimizing torque ripple, a multi-objective optimization framework is implemented based on genetic algorithms and Finite Element Analysis (FEA). Finally, the optimal trade-off design was determined based on the Pareto optimization strategy. FEA results verify that the optimized motor achieves a rated torque of 3.17 Nm with a torque ripple of only 1.86%. Furthermore, the cogging torque is suppressed to 0.055 Nm, and the motor demonstrates a peak torque capability of over 6 Nm, validating the effectiveness of the proposed multi-objective optimization method for wearable robot applications.