Second-order Sliding Mode Control with Adaptive Control Authority for the Tracking Control of Robotic Manipulators

Abstract

AbstractIn this work, the joint position tracking control problem of industrial robots is tackled. To cope with the model uncertainties and external disturbances affecting the robot, the Inverse Dynamic Controller (IDC) is combined with an approach based on higher order Sliding Mode Control (SMC) technique. We make use, in particular, of the so-called “Twisting” Second Order Sliding Mode Controller. Higher order SMC techniques transfer the inherent discontinuities to the time derivative of the input torque and this allows to obtain a continuous profile for the input torque, which is computed through integration of an appropriate discontinuous switching signal. Despite the chattering phenomenon is strongly attenuated, some residual problems (vibration and acustical noise) are still observed during the experimental implementation of such an approach in its standard formulation. To improve the system performance we suggest in this work an adaptation mechanism to adjust on-line the authority of the SMC. The logic is driven by a “sliding-mode indicator” that detects, on line, the occurrence of a sliding mode behaviour and uses this information for adaptation purposes. When large and fast control activity is demanded (e.g. to track fast reference trajectories) the adaptation unit reacts by automatically increasing the control authority of the SMC. On the other hand when small control authority is sufficient the control magnitude is lowered. Such a bidirectional adaptation logic significantly reduces the chattering. The proposed technique is theoretically analyzed and experimentally tested, and the results of comparative experiments are discussed in the paper.