Force-controlled Compensation Scheme for P-Q Valve-controlled Asymmetric Cylinder used on Hydraulic Quadruped Robots

Under the requirement of the force controller of hydraulic quadruped robots, the goal of this work is to accurately track the force commands at the level of the hydraulic drive unit. The main contribution focuses on the development of a force-controlled compensation scheme, which is specifically aimed at the key issues affecting the hydraulic quadrupedal locomotion. With this idea, based on a P-Q valve-controlled asymmetric cylinder, we first establish a mathematical model for the hydraulic drive unit force control system. With the desired force commands, a force feed-forward algorithm is presented to improve the dynamic performance of the system. Meanwhile, we propose a disturbance compensation algorithm to reduce the influence induced by external disturbances due to foot-ground impacts. Afterwards, combining with a variable gain PI controller, a series of experiments are implemented on a force control performance test platform to verify the proposed scheme. The results demonstrate that the force-controlled compensation scheme has the ability to notably improve the force tracking accuracy, reduce the response time and redundant force.