Contouring control of Stewart Platform based machine tools

In this paper, two novel robust adaptive Cartesian space control algorithms are proposed for friction compensation in the six degrees of freedom high performance Stewart Platform based machine tools. The first controller utilizes an adaptive friction compensation scheme based on a postulated linear-in-the-parameters friction model. The proposed friction compensation algorithm explicitly accounts for time varying normal forces as well as dependence of the friction coefficient on velocity. The Stribeck friction characteristic and varying spherical joint static friction are treated as bounded disturbances, and compensated by a sliding mode robust controller. In the second controller, a new form of Takagi-Sugeno multi-input multi-output fuzzy system is developed to adaptively learn unknown friction behavior and compensate for it. This approach assumes that no a priori knowledge about frictional effects in the strut joints is available. The simulation results indicate that large contouring errors caused by friction at the velocity reversals when conventional control algorithms are used, are reduced greatly by the adaptive controllers.