Transmission Model and Compensation Control of Double-Tendon-Sheath Actuation System

The tendon-sheath actuation system is capable of providing remote power transmission and greatly simplifying the mechanical design of robotic systems. However, there exist many undesirable nonlinear problems in tendon-sheath transmission, such as hysteresis, dead zone, backlash, and direction-dependent behavior. Unlike most of the existing literature that focus on the analysis of single-tendon-sheath actuation, this paper proposes a general mathematical double-tendon-sheath transmission model suitable for arbitrary types of load conditions. Experimental setups composed of servo motors, tendon-sheath components, and robot joint are established to test the proposed models and gain insights into the transmission processes through sinusoidal input signal experiments. Based on the transmission models, two open-loop control strategies are developed for the friction and deformation compensation, requiring no feedback from the distal end. With the aim of obtaining the complete model parameters, an offline identification method able to calibrate the transmission model using the sensors mounted at the proximal end is developed. The performance of the proposed control methods is evaluated by torque/position-tracking experiments in different kinds of load conditions. Moreover, the frequency characteristics of the actuation system are analyzed, and the controlled bandwidth is about 10 Hz during operation.