Tank based unified torque/impedance control for a pneumatically actuated antagonistic robot joint.

In this paper the concept of a unified torque/impedance controller is applied to a pneumatically actuated, antagonistic robot joint. The investigated control algorithm consists of a cascaded structure in which the outer torque/impedance controller commands a desired torque to two cylinder-based force controllers. The torque-/impedance controller is equipped with a virtual tank that ensures passivity of the control loops above force level. Additionally, a shaping function provides a continuous transition to impedance control in case of sudden contact loss during a torque control operation. External torques used in the feedback loop for contact force regulation are estimated by a momentum observer. Experimental and simulation results show a maximum deflection of 9.7° over 130 ms in case of contact loss until a resting position is reached. Additionally, step and sinusoidal torque tracking up to 5 Hz was successfully tested at good performance.