Robust fractional-order integral terminal sliding mode control with adaptive uncertainty observation for a VCM-driven X–Y motion stage

In this study, a fractional-order integral terminal sliding mode control (FITSMC) system is proposed to control a new voice coil motor (VCM)-driven X–Y motion stage with different thrust forces in the X–Y axes for the tracking of reference contours. First, the operation principle and dynamics of the stage are conducted based on the equivalent electrical circuit and magnetic analysis of the VCM. Subsequently, an integer-order integral terminal sliding mode control (ITSMC) approach is designed to control the stage for contour tracking in finite time. To enhance the positioning accuracy of the ITSMC, a novel FITSMC system is further proposed by bringing in a new fractional-order integral terminal sliding surface. However, the hitting control gain for the FITSMC is difficult to design because of the system uncertainties. To address this issue, an intelligent adaptive uncertainty observer (AUO) is proposed to directly observe the system uncertainties, thereby eliminating the need for hitting control term and attenuating the chattering phenomena of control force. Stability analysis of the proposed FITSMC and FITSMC-AUO systems is conducted on the basis of the Lyapunov stability theory. The experimental results demonstrate the effectiveness and superiority of the proposed control systems in comparison with the existing ITSMC system in the contour tracking tests.