A Low-vibration Controller for the Series Elastic Tendon-driven Robots

Series elastic tendon-drive robots have been proposed intended for safe physical human-robot interaction. While the tendon elasticity enables a safe task execution under hand contact conditions, it poses overshoot and vibration problems in tasks without hand contact. In this paper, we install an electromagnetic brake in the pulley at the middle of the cable path, in order to stabilize the vibration. This method enables a wide range of viscosity changes, because the elements are arranged so that the brake force is directly transmitted to the arm. Moreover, redesign of the joint structure is unnecessary. The elastic tendon with brake can be modeled as a parallel viscoelastic element. We propose a sliding mode controller by using the electromagnetic brake. High-speed friction control by the electromagnetic brake allows an oscillation-free positioning controller. Unlike conventional sliding mode controllers, softness (low stiffness) of the controlled arm is not lost because high-gain position feedback is not used. Reachability condition of the sliding surface is expressed in terms of the equilibrium and target positions of the arm angles. Control results are demonstrated experimentally.