Control of robotic joint by using antagonistic pair of Twist Drive Actuators

In this paper we present a control method for robotic finger that uses twisted strings actuator called Twist Drive. A five fingered robotic hand prototype was developed by using Twist Drive actuators to control 14 active degrees-of-freedom. This paper focuses on control of a thumb´s CM joint, which is the only joint in the robotic hand prototype that uses two Twist Drive actuators configured in an antagonistic pair of actuators. Positioning accuracy and smoothness of the joint´s motion is influenced by nonlinear characteristics of the actuators and by friction in the mechanism. To compensate for the disturbances, we used disturbance observer, and additionally enhance smoothness of motion by controlling the unwinding torque of opposing actuator. The robotic hand and the proposed control algorithm are explained in the paper, experimental results of CM joint positioning control are presented.