Modeling and Control of Fluidic Robotic Joints with natural compliance

This paper reports on basic properties, dynamic modeling, pressure and position control of a newly developed fluidic actuator with rotary elastic chambers (REC-actuator), intended for robots working in human environment. This actuator with natural compliance can be operated by gas or oil. Pressure control of the actuator in pneumatic realization is designed using fast on-off valves, suitable for lightweight robotic joints. A phenomenological model of the actuator is considered, in which spring, damper and torque generating element are connected in parallel. The joint stiffness and damping as functions of pressures in the actuator chambers are identified in free oscillations experiments at constant pressures. The torque-generating element is further identified in a separate series of step responses experiments, using thereby previously obtained stiffness and damping. A sliding mode control law is applied for the position control of the naturally compliant actuator.