Analysis and control of a water hydraulic manipulator for iter divertor remote maintenance

This paper focuses on the analysis and control of a water hydraulic manipulator, which is specific for the International Thermonuclear Experimental Reactor (ITER) cassettes' (each weighing 10 tonnes) maintenance tasks. The manipulator comprises a closed-chain main body and an open-chain end-effector. Following a systematic way of analyzing differential kinematics and vibration modes, it is revealed that from a control point of view the manipulator faces two main restrictions. The first one is that the closed-chain structure degrades tracking performances, rising from kinematic coupling. Secondly, as the end-effector drives a very high inertial load instead of a gravitational load, small actuators are used, resulting in low system natural frequency and damping. To solve the first issue, a kinematic model-based decoupling controller is designed. Moreover, to improve the steady-state accuracy in spite of low frequency and damping, acceleration feedback is adopted, achieving higher damping and position loop gains. Experimental results show that decoupling controller brings five times smaller tracking errors, while acceleration feedback controller reaches three times better accuracies than proportional controller. This study also confirms that in spite of using commercially available water hydraulic components, the achieved positioning accuracies and dynamic behavior are competitive with oil hydraulics.