Tendon-based transmission systems for robotic devices: Models and control algorithms

Tendon-based transmission systems present many positive aspects and greatly simplify the mechanical design of small robotic devices, such as robotic fingers. On the other hand, they introduce several nonlinear effects that must be properly considered by the control algorithms to achieve a suitable performance level in the regulation of the finger joint torques. In this paper, the model of the tendons-based driving system and of the nonlinear effects arising from the use of sliding paths instead of pulleys for the tendon routing are discussed, and control algorithms aiming at compensating these nonlinearities are presented. Both models and control algorithms have been validated by experiments. In particular, in order to gain a better insight on the force distribution along the tendon, an experimental setup for the measurement of the tension in some intermediate points has been developed. After the identification of the tendon characteristics, a suitable control law for the compensation of the nonlinear effects due to the friction acting on the transmission system has been applied. The proposed compensation scheme is based on a sliding-mode controller with boundary layer, where the boundary threshold is modulated as a function of the desired tendon tension.