Positioning controller for mechanical systems with a mini harmonic drive servo actuator

Harmonic drives (HD) are high-ratio, compact torque transmission systems. However, due to frictional effects and internal flexibility, a HD shows unique (torsional) stiffness behavior characterized by a stiffening spring in parallel with a hysteresis element, which is modeled using the Maxwell-slip model. As a result of this complex nonlinear behavior, a classical linear control strategy obviously does not perform very well, in a system with HD component, for achieving accurate positioning. This paper aims to characterize the dynamic behavior in mechanical system with HD and, more specifically, to use this knowledge to design effective control schemes. Modeling of the system is achieved via the Describing Function approach, applied to the hysteresis element, to yield the stiffness and damping in function of the amplitude of motion. Afterwards, a nonlinear feedback control strategy, utilizing a gain scheduling obtained from the aforementioned model-based identification, is implemented for compensating the error. The results show high performance in regard to rise time, positioning error and robustness as compared to a conventional cascade controller.