Advanced Motion Control of Hydraulic Manipulator With Precise Compensation of Dynamic Friction

Multiple degrees-of-freedom (multi-DOF) hydraulic manipulators are usually recognized as hard-to-control systems to achieve dynamic trajectory tracking because of strong nonlinearities, uncertainties, and complex couplings within the dynamics. In practice, when the end-effector of the multi-DOF hydraulic manipulator is tracking a given trajectory continuously, some joints may experience frequent stop-and-go or low-speed motions due to its kinematics. In such situations, dynamic friction becomes one of the main factors that affect the control performance. Inadequate compensation for the dynamic friction can result in undesired crawling or oscillatory behaviors of the manipulator. However, it is challenging to make effective compensation of the dynamic friction in control design due to its complicated and nonlinear properties. In this article, motion control of a multi-DOF hydraulic manipulator with extra consideration on the nonlinear friction for dynamic trajectory tracking is proposed. First, to make a more precise compensation of the nonlinear friction force, an improved LuGre model ensuring continuity and differentiability is developed, along with the method for obtaining nominal values of internal friction state based on desired trajectories. Then, a model-based adaptive robust motion controller is developed for the multi-DOF hydraulic manipulator. The nonlinearities and uncertainties of the high-order dynamics are well addressed in the closed-loop system, and the transient and asymptotic tracking performance can be guaranteed in theory. Finally, experimental validation was conducted, and the comparison with existing methods showed the improved tracking performance.