Contouring control of Stewart Platform-based machine tools

Machine tool manufacturers have introduced 6 degree-of-freedom machine tools with structures based on parallel linkage mechanisms and have promoted their use for the machining of sculptured surfaces. The machine tool investigated in this research is the Ingersoll Octahedral Hexapod machining center, and is based on the Stewart Platform mechanism. In the research work presented in this dissertation, attention is focused on the effects of frictional forces and torques on machine accuracy, as well as on inclusion of compensation techniques for such phenomena through nonlinear control algorithms.; In order to adequately examine the problem of friction compensation on the full scale machine, we investigate the effect of friction on the problem of position control of a test strut which comprises of a single strut mechanically identical to the six-struts of the full scale machine, because the test setup is available for control experimentation. A test strut dynamic model is presented and forms the basis for the development of two types of robust adaptive control algorithms which have the potential to improve drive positioning accuracy significantly over that of well-tuned PID controllers. In particular, the proposed controllers are capable of compensating for frictional effects in the strut joints which are considered to be a significant source of positioning error, the friction compensation algorithms developed here accommodating frictional effects which may depend on time varying normal forces in addition to velocity. Also, we use the nonparametric structure of fuzzy systems and their ability to approximate arbitrary nonlinear mappings to represent friction.; The adaptive friction compensation schemes developed here are evaluated on the test strut experimentally. Experimental and simulation results are also seen to agree well with each other. The performance of the two proposed robust adaptive controllers with friction compensation is evaluated experimentally and compared with the response of well tuned PID. The large tracking errors caused by friction at the velocity reversals are reduced greatly by the adaptive controllers.; The adaptive controllers developed for the test stand were extended to the case of six degrees of freedom in order to effectively control the Hexapod machining center—a multivariable system with 6 inputs and 6 outputs with interactions between the inputs and outputs, and nonlinear effects. The proposed controllers clearly outperform the Cartesian space computed torque PID controller. The large tracking errors usually caused by friction at velocity reversals are reduced greatly by the adaptive controllers.