Abstract：Aiming at the requirements of collaborative robots for high load-to-weight ratio and high-precision speed control, a joint module is designed, which integrates a double-stator permanent magnet synchronous motor and a harmonic drive reducer. The characteristics of the double-stator structure are utilized to increase the torque density of the motor. In order to improve the control accuracy of joint speed, an equivalent input torque disturbance to the joint system is used to describe the lumped effect of all factors acting on the joint speed ripple, including the low-frequency nonlinearities such as friction, the motor torque ripple, and the transmission error of the harmonic drive reducer, and the joint speed ripple is suppressed by compensating this equivalent input torque disturbance. In order to improve the disturbance estimation precision, a method is proposed which utilizes double disturbance observers and an adaptive algorithm. A performance test on the double-stator motor demonstrates that the torque density of the double-stator motor is increased by 11% compared with the single-stator structure. The joint speed control experiment verifies that compared with the conventional proportional-integral (PI) speed controller, the root mean square (RMS) of the joint low-speed steady-state fluctuation errors is reduced by 40%~60%, while the RMS of medium-speed and high-speed steady-state fluctuation errors is reduced by about 30%~40% with the proposed method, and the speed control accuracy is improved.
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