Hydraulic manipulators can provide a great power-to-weight ratio compared to their electrical counterparts. However, their closed-loop control is challenging due to their highly nonlinear dynamic behavior. In contact tasks, which are essential for hydraulic manipulators, a need for rigorously addressed manipulator dynamics is substantial. This is because contact dynamics between the manipulator and its environment can be drastic and one identified reason for unstable system behavior is that the manipulator dynamics are not considered rigorously.
This paper proposes a novel variable impedance control method. While the proposed variable impedance law (external control for the manipulator) is generic, it has been developed especially to improve the Cartesian free-space position-tracking accuracy in force-sensorless contact force control with hydraulic manipulators. With the proposed method, the manipulator can be made stiff in free-space motions (enabling accurate free-space position tracking) and compliant in constrained motions. To address the manipulator’s highly nonlinear dynamic behavior, the manipulator’s internal control is designed on the virtual decomposition control (VDC), which has been proven to perform the state-of-the-art control in hydraulic robotic systems. The rigorous stability proof for the proposed method is provided, covering both free-space and constrained motions. The experiments demonstrate the efficiency of the proposed method.