Model-based control of a hexapod with linear direct drives

Abstract

To fulfil the increasing demands on accuracy and dynamics of modern machine tools a highly dynamic hexapod design has been developed at the Institute of Production Engineering and Machine Tools (IFW). This handling and processing machine PaLiDA allows very high dynamics, owing to the combination of a parallel kinematics design with linear direct drives as struts variable in lengths. The hexapod is operated by a single-joint control, based on the linear direct drives, that leads to a stable and robust handling but lacks dynamic accuracy. To raise the precision of motion, a model-based feedforward control is applied to calculate the rigid body and friction forces. This feedforward control is added to any applied independent drive controller and thereby reduces the tracking error significantly. Furthermore the pose-dependent mass matrix is considered in order to decouple the dynamics of the struts. This linearization allows operating the complex parallel structure with single-joint controllers in contrast to the implemented control. Furthermore the consideration of the diagonal elements of the mass matrix allows an adaptation of the PID controller's parameters within the workspace.