Design and Experimental Validation of a Controller for Bowden-Cable Actuators Subject to Friction Variation
and Nathanaël Jarrassé
AI summary
Problem
Bowden-cable transmissions in flexible exosuits suffer from nonlinear friction that varies with bending angle, causing tracking errors and control instability. Existing compensation methods are either too complex, require unmeasurable parameters, or degrade performance at high bending angles.
Approach
The authors design a continuous modified twisting controller paired with a super-twisting velocity observer to reject unknown friction disturbances without direct measurement. This output-feedback scheme ensures finite-time stability for the cable-driven system modeled as a perturbed double integrator.
Key results
- Modified twisting controller eliminates chattering while maintaining robustness to friction variations
- Super-twisting observer accurately estimates joint velocity without additional hardware sensors
- Experimental validation confirms stable tracking and improved energy efficiency across 0° to 360° bending angles
- Closed-loop system achieves global finite-time stability under varying disturbance conditions
Why it matters
Provides a practical, sensor-light control strategy for reliable exosuit and cable-driven robotic actuation in dynamic real-world environments.
Abstract
Exoskeleton robots hold great potential for both in- dustrial applications and assisting patients with locomotor dis- abilities. 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. How- ever, 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 ac- tuation 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.