Control-Oriented Modeling of Effective Bulk Modulus and Online Compensation for Hydraulic Manipulator Motion Control

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.