Decoupled torque control of tendon-driven fingers with tension management

To facilitate human assembly tasks, Robonaut 2 is equipped with a dexterous, compact hand featuring fingers driven remotely by tendons. This work outlines the force-control strategy for the fingers, which are actuated by an “n +1” tendon arrangement. Existing tendon-driven fingers have applied force control through independent tension controllers on each tendon, in other words, in the tendon space. The coupled kinematics of the tendons, however, cause such controllers to exhibit a transient coupling in their response. This problem can be resolved by alternatively framing the controllers in the joint space of the finger. A joint-space torque control law is proposed here that demonstrates a decoupled response with a faster settling time than an equivalent tendon-space formulation. In addition, a tension distribution algorithm is presented here to translate joint torque commands into tendon tensions. It guarantees that each tendon tension respects both an upper and a lower bound, using an efficient, finitely-convergent algorithm. These two contributions provide for a compliant, well-controlled hand, aptly suited for unstructured interaction.