High-order variable-structure disturbance estimators with applications to harmonic drive systems

This paper describes the design of a high-order variable-structure disturbance estimator (VSDE) that provides asymptotic estimation of unknown disturbances described by polynomial functions of time with arbitrary degrees. In comparison with conventional VSDEs, the high-order VSDE requires only a small switching gain for ensuring the existence of a sliding mode, thus alleviating the chattering problem. Moreover, the transfer function of a high-order VSDE from an unknown disturbance to its estimate can have a high relative degree to enable the estimation of time derivatives of an unknown disturbance while providing adequate high-frequency attenuation. Two high-order VSDEs were implemented to simultaneously observe the motor-side and the load-side disturbances in a harmonic drive system. Experimental results show that the compensation offered by the high-order VSDEs effectively deals with the disturbances induced by harmonic drive gearing.