The Influence of Dynamics Model Simplifications on an Electrohydraulic Stewart Platform Model-based Control Process

A simulational model of 6 dof Stewart platform type parallel manipulator is presented. The manipulator is driven by electrohydraulic actuators. A novel invariant error dynamics controller is used. The developed control scheme employs the dynamic and hydraulic model of the system. The manipulator model consists of the rigid body equations of motion and the hydraulic dynamics of the main elements. Friction is included in the model. Two well-known software packages, one designed to perform control system simulation and the other dedicated to multibody simulation, are simultaneously used to conduct the study. The aim of the study is to check, what is the impact of dynamics model accuracy on the quality of control process. The simplifications of the model and the problems with finding accurate values of its parameters (especially friction parameters) are considered. The study helps to predict what are the possible results of inaccurate determination of the crucial model parameters. The simulation results show, that inverse dynamics model simplifications consisting in neglecting the mass of actuators have relatively little influence on the control quality. The effects of actuators masses neglecting can be reduced by appropriate changes in the modeled mass of the platform. The study shows that friction effects should be introduced to the inverse dynamics model and the friction parameters should be identified with possibly big accuracy. The parameters describing stiction-friction transition effects are the crucial ones.