Design and control of a compact high-power-density two-DOF hydraulic gripper

Compactness and lightweight characteristics of end-effectors are crucial for ensuring the high mobility of legged robots in complex unstructured environments. However, existing electric and pneumatic counterparts have limited output capacity, while traditional hydraulic end-effectors often suffer from excessive weight and complex piping layouts that cause motion interference and joint space limitations. This paper presents the design, modeling, and control of a compact, high-power-density 2-DOF hydraulic gripper. First, a novel annular swing actuator is coaxially nested with a linear gripping mechanism to compress axial space. Unlike typical 1-DOF designs, this 2-DOF architecture integrates independent swing (50 Nm) and gripping (1000 N) capabilities into a 1.17 kg prototype, allowing it to perform rotational tasks like valve turning without relying on the movement of upstream joints. By incorporating a tubeless embedded fluid channel design, space occupancy and physical constraints on joint movement are minimized, enhancing structural integration. Second, a state-space model incorporating servo valve dynamics, flow continuity, and load balance is established to address inherent nonlinearities and external disturbances. A sliding mode controller (SMC) is implemented to validate the tracking performance and robustness of the proposed hardware. Simulation and experimental results demonstrate steady-state errors for swing and gripping as low as 0.56° and 0.40 mm, respectively. The results demonstrate the feasibility of the compact high-power-density design, offering a potential solution to integration challenges for lightweight mobile platforms and contributing to actuator development for high-performance legged robots.