Disturbance-observer-based adaptive feedforward cancellation of torque ripples in harmonic drive systems

This paper proposes a scheme for controlling the output torque of a harmonic drive actuator equipped with a torque sensor. The control scheme consists of the internal model control (IMC) and an adaptive feedforward cancellation (AFC) based on a disturbance observer (DOB). The relationship between the IMC and the DOB is presented in this paper, and the IMC is adopted as a feedback compensator for its ease in design and implementation. The DOB, on the other hand, is suitable for estimating an unknown disturbance, and its output is applied to the AFC resonator that generates an adaptive dither to compensate for the torque ripples induced by harmonic drives. Compared with the conventional AFC, the salient features of the proposed DOB-based AFC include the independence of designing the AFC's adaptation gain from the plant and the feedback compensator, fast convergence of the disturbance-cancellation error, and no influence of aperiodic reference changes upon the adaptive dither. The effectiveness of the proposed scheme is demonstrated through experimental results, in which its performance is shown to be superior to that of the conventional AFC.