Super-Twisting Algorithm Gain Adjustment Strategy for Manipulator Position Control

Build-in controllers of industrial manipulators ensure high repeatability of actions even under unknown changes of external payload. Most manipulator manufacturers do not provide access to joint torque control, and only position/velocity control is available in presence of disturbances and system imperfections. Thus, an external control loop design for compensation of system perturbations may enhance the position accuracy of manipulators using encoder feedback. Then, the Super-twisting algorithm (STA) appears as a reasonable choice since it generates continuous control signal and provides the theoretically exact compensation for Lipschitz perturbations. However, unmodeled dynamics and signal processing delays existing in the system exhibit chattering that cannot be eliminated. The paper provides the strategy for designing and adjusting external control loop with STA controller to enhance position accuracy. Namely, two-steps Super-twisting algorithm (STA) gains adjustment strategy is proposed. Firstly, it is found that for ABB IRB 1600 the sine mixture signal provides the system response allowing to identify the plant structure as first-order system with time delay. At the second step, Harmonic Balance (HB) method is used to estimate system output oscillation amplitude. Since HB method considers the first harmonic of the periodic motion, Pade approximation of time delay was chosen leading to the HB close-from solution. The proposed STA gain adjustment strategy was tested on simulation and verified through an experiment. Experiments with industrial manipulator ABB IRB 1600 demonstrated the increased position accuracy of the proposed methodology compare to LQR-predictor model and build-in control system.