In the realm of legged robot system, the idea of compliance is gaining substantial attention from its researchers and engineers for its requirement.
Physical contact of legged robot is crucial as there is interaction between hydraulic system and environment around them. When it comes to legged robot system, controlling interaction force is necessity to deal with physical contact. In same meaning, controlling the robots compliance is significant. This paper presents hydraulic compliance control algorithm with the concept of hydraulic stiffness. The hydraulic actuators are widely used for its high power, fast response and high stiffness. However, high stiffness of hydraulic actuator is not always suitable for robots that are contacting with human or any diverse environment. Without adding compliance to hydraulic system,human or hydraulic system itself could be damaged by its high stiffness condition. By controlling the hydraulic stiffness, it is able to design compliance system without adding any spring or damping elements. In this paper, we introduce actively compliance control algorithm for hydraulic system, also dynamic model is applied for its high tracking performance In terms of compliance control, unexpected impact force is inevitable as the rigid body system is contacting with diverse external environment conditions. So, in this paper, compliance and torque control of an hydraulic system is introduced that shows robustness in interaction forces that occurs between
robots and unexpected environment. Inverse dynamic control has been applied to satisfy tracking performance of rotational angle and torque. In view of this unfavorable working condition, pressure sensor is required to handle durability problem. For instance, torque cell has chance to be damaged by transferred impact from interaction forces. Pressure sensor has strength in durability matter, as the sensor is located on the actuator.