Hierarchical Decoupling Controller With Cylinder Separated Model of Hydraulic Manipulators for Contact Force/Motion Control

Hydraulic manipulators are favored because of their high power density and strong explosive force. As more tasks interact with the environment, better force/motion control performance is demanded. However, hydraulic manipulators are usually driven by closed kinematic loops containing linear hydraulic actuators with passive resolute joints, leading to the difficulty in full dynamic modeling and parameter identification for accurate model-based force/motion control. To overcome this problem, a cylinder separated model is proposed with hydraulic cylinders separated virtually through the concept of virtual equivalent rotational joints. The decoupling model describes the complex dynamics of closed-chain structures based on the Lagrange equation, which is used to identify both inertial parameters of rigid links and cylinders in addition. Besides, a hierarchical decoupling controller is designed by splitting hydraulic cylinder control from the manipulator dynamics on the basis of the decoupling model, and then, the control problems of structure mechanisms and hydraulic system are handled independently. Based on the proposed dynamic model and control strategy, contact force/motion control is realized with the contact force estimated from cylinder pressure sensors. Experiments reveal that the position error of the end effector is reduced at least by 12.7% compared with the conventional controller that ignores coupling problems, whereas the root mean square error of contact force is within 100 N. The proposed strategy shows satisfied performance for hydraulic manipulators, which is also convenient to implement.