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.

Electro-hydrostatic actuators (EHAs) have been widely utilized in the aerospace and modern manufacturing industries owing to their small installation space, high energy efficiency, and extreme force generation capability. Recently, the model-based compensation controller has been designed to enhance the control quality of hydraulic systems. Meanwhile, some modified models focusing on friction, nonlinear flow, and flow leakage have been developed to further improve the control performance. However, the nonlinear characteristic of fluid bulk modulus reflecting oil elasticity has not been considered which still limited transient control accuracy. The existing studies on modeling of bulk modulus result in models too complicated to be used in control design. So a control-oriented model needs to reflect the real nonlinear behavior of the bulk modulus while feasible for control design. In this study, a control-oriented model for the effective bulk modulus is synthesized, which is not only able to reflect the nonlinear behavior of the real-world bulk modulus but also feasible for model compensation control design. By introducing such a model into the system dynamics, a model-based motion controller is developed for EHAs. The performance of the closed-loop system is evaluated in theory and also in comparative experiments where the proposed method further improves the trajectory tracking performance, especially during transient processes.

Hydraulic manipulators play an irreplaceable role in many heavy-duty applications. Currently, there are stronger demands for the hydraulic manipulator to achieve high precision, as well as high force/power. However, due to the inherent nonlinearities of its high-order dynamics, the precision of the manipulator has been a common weakness compared with electrically driven ones. Thus, in this paper, a nonlinear adaptive robust control method for the hydraulic manipulator is proposed. To make the controller more applicable to practical engineering projects, this study tried to control each joint independently instead of directly based on the complicated multi-degree high-order dynamics, while guaranteeing the control precision by the adaptive nonlinear model compensation, as well as a robust feedback design. The closed-loop control performance was theoretically verified. Besides, several sets of comparative motion tracking experiments were conducted, and the proposed closed-loop system achieved high precision under different trajectories and postures.