Abstract: In order to overcome the system over-damping caused by the conventional hinged form commonly-used in parallel manipulators in industry，a modal space controller with positive pressure feedback is presented． In this controller，the highly-coupled multiple-input multiple-output systems in the physical space are transformed into six independent single-input single-output decoupled systems in the modal space，and the positive pressure feedback technology is employed to tune each degree of freedom (DOF) with over-damping independently and to broaden the bandwidth of each DOF to the eigenfrequencies． In the investigation，first，the influence of passive joint damping on the control characteristics of a hydraulically-driven Stewart platform is analyzed based on the modal decoupling theory． Then，the structure and the design method of the modal space controller with pressure feedback are presented． Finally，an experiment is carried out to testify the effectiveness of the proposed controller． The results indicate that the designed modal space controller effectively broadens the system bandwidth，thus reducing the coupling effect between each two DOFs and improving the response speed．

Key words: passive joint damping, modal space control, Stewart platform, positive pressure feedback