A multi-degree-of-freedom water hydraulic manipulator in nuclear applications: Design and experiments

Abstract

In nuclear applications, water hydraulic manipulators present notable advantages in terms of safety and environmental compatibility compared to oil hydraulic alternatives. However, traditional Valve-Controlled Hydraulic Cylinder Systems (VCHCS) suffer from severe leakage, and existing new designs add system complexity. These design flaws adversely affect the control accuracy of the manipulator. To address these issues, a novel multi-degree-of-freedom water hydraulic manipulator for nuclear applications is proposed in this study, including system design, controller design and experiments. In the system design, the servo valve in VCHCS is replaced by two ball-type proportional valves per joint, which avoids severe leakage due to their sealing properties. Additionally, a pressure-reducing valve and a relief valve shared by three joints achieve fewer valves, minimizing potential leakage points while reducing system complexity. Further, to improve the control accuracy of the new system, a nonlinear control strategy based on virtual decomposition control (VDC) is designed. The VDC strategy addresses the hydraulic-kinematic coupling and nonlinearities by decomposing the manipulator into closed-chain subsystems and integrating fluid dynamics into the rigid-body dynamics model. Experimental results demonstrate tracking accuracy improvements of 29.2% in the X direction and 70.2% in the Y direction, where X is defined as the forward direction and Y is defined as the upward direction of the end clamp. Finally, underwater task operation experiments show that the tracking errors are confined within 3 mm (X) and 4 mm (Y), demonstrating the practical application of the proposed system and control strategy.