Force Soft Measurement Based on Cylinder Decoupling Model and Stepwise Parameter Identification for Hydraulic Manipulators

Force monitoring or feedback on the end-effector is a key component for hydraulic manipulators to perform heavy-duty tasks such as handling and dismantling. However, the force sensors used in heavy-duty applications are expensive and fragile under complicated conditions. As an alternative, soft measurement methods are used to obtain the external force indirectly, but this is challenging for multi-DOF hydraulic manipulators due to the complex parallel-serial characteristics of the closed-chain structures with highly nonlinear frictions. To overcome the obstacles, a novel force soft measurement method for hydraulic manipulators is proposed using the cylinder decoupling model with LuGre friction compensation. In this method, the hydraulic manipulator is decoupled by the main chain consisting of manipulator links and several branch chains consisting of hydraulic cylinders, of which the inertia effect can be fully considered and modeled. The friction in each joint is also compensated by the LuGre friction model. Besides, the lumped inertia parameters and the LuGre friction parameters are acquired through the stepwise parameter identification method, which mitigates the complexity of the full model by first determining steady friction parameters and then solving the remaining parameters with different excitation trajectories. Compared with the traditional method without considering the closed-chain structures, the proposed force soft measurement method shows higher force estimation accuracy, which is conducive to the implementation of force-based strategies.