Electro hydrostatic actuator (EHA) has been successfully developed for flight control applications to replace the cumbersome centralized hydraulic system. It also has excellent potential for ocean applications due to its advantages on miniaturization and energy-savings. One of the special technologies for EHA’s underwater application is pressure compensation, which is used to equalize the return pressure of the hydraulic system and the seawater pressure. This paper investigates the modeling and control design of underwater EHA to improve performance, especially considering the effect of additional pressure compensator and uncertain external load. The nonlinear hydraulic model is extended by the dynamic characteristics of the pressure compensator. Two low-order extended state observers were constructed to cope with the external load fore and the effect of the pressure compensator, respectively. The backstepping methods were designed to guarantee the robust stability of the entire high-order nonlinear hydraulic system. Finally, the theoretical proving and simulation on Matlab/Simulink are conducted to demonstrate the high tracking performance of the proposed control strategy.

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.

This paper presents a disturbance-observer-based sliding mode control strategy for an underwater electro-hydrostatic actuator, particularly considering that electro-hydrostatic actuators (EHAs) significantly suffer from sea pressure disturbance, which makes it hard to achieve high-precision position control. Therefore, a nonlinear disturbance observer was designed to aim at the matched and mismatched disturbance caused by sea pressure disturbance. Then, a nonlinearities model for an underwater EHA was established, and a related non-singular fast terminal sliding mode (NFTSM) controller was designed by changing the conventional sliding mode surface to further improve the control accuracy. In addition, the backstepping tool was used to guarantee the robust stability of the entire three-order hydraulic dynamic system. Finally, a comparative simulation was conducted with different load forces in AMESim and Simulink, which effectively verified the high tracking performance of the proposed control strategy.

Force tracking control for hydraulic series elastic actuators (SEAs) is the demand in robots interacting with the surrounding world. However, the inherent nonlinearities and uncertainties of the hydraulic system, as well as the unknown environment, make it difficult to achieve precise contact force control of hydraulic SEAs. Therefore, in this study, force tracking impedance control of hydraulic SEAs is developed considering interaction with an unknown environment in which the force tracking performance can be guaranteed in theory. Based on the typical force tracking impedance frame, the force tracking performance is improved by introducing backstepping control into the inner position controller to deal with the high-order nonlinear dynamics of the hydraulic SEA. In addition, the environment parameters are also estimated online by the adaptive method. Finally, comparative simulation is conducted with different interacting environments, which verifies the advantages of the proposed method.

Model-based control methods have been acknowledged as powerful solutions for hydraulic manipulators through compensating nonlinear dynamics. Hydraulic oil, as the working medium for energy transfer, significantly affects the system’s rigidity and control performance. In most studies, the effective bulk modulus of oil is either treated as a constant or estimated as an unknown parameter. However, it is closely related to working pressure and can vary several times under different pressures. Although several theoretical models for the effective bulk modulus exist, their complexity and dependence on specific measurement equipment limit models’ application in hydraulic manipulator control. In this study, a control-oriented model for the effective bulk modulus is developed, balancing the feasibility of control design and the accuracy of the model description. A model-based controller is then synthesized for a multi-degree-of-freedom hydraulic manipulator. Through an especially designed X-swapping scheme, the primary parameters of the manipulator, including those in the bulk modulus model, can be updated online without additional hardware dependencies. Theoretical analysis and experiment results demonstrate that the method improves the dynamic performance of hydraulic manipulators under varying pressures. Notably, this represents the first instance where a hydraulic manipulator controller accounts for the nonlinear characteristics of the effective bulk modulus.