Design and Experimental Validation of a Controller for Bowden-Cable Actuators Subject to Friction Variation

Exoskeleton robots hold great potential for both industrial applications and assisting patients with locomotor disabilities. Among them, flexible exoskeletons, known as “exosuits”, have attracted a great deal of interest from researchers. Those are usually made up of flexible components such as cables and pieces of fabric, which are much lighter than the rigid exoskeletons. By using a Bowden-cable transmission in their design, the actuators can be placed away from the end-effectors of exosuits and thus are considered an effective transmission solution for reducing the weight and inertia felt by operators wearing the exosuit. However, a critical issue of Bowden-cable transmission is the complex and nonlinear friction between the inner cable and outer sheath, which affects the control robustness by introducing a time delay and inaccuracy in position tracking. Besides, friction along the cable varies with the accumulated bending angle of the sheath. A control synthesis approach is proposed for the Bowden-cable actuation system, utilizing twisting and super-twisting algorithms to ensure finite time stability and robustness properties of a perturbed double- integrator model. The experimental results demonstrate the effectiveness of the proposed method across various bending angle conditions.