Autonomous Control of a Tendon-driven Robotic Limb with Elastic Elements Reveals that Added Elasticity can Enhance Learning

TL;DR

This study shows that adding elastic elements to tendon-driven robotic limbs can improve autonomous learning and performance, suggesting co-development of robot bodies and controllers can leverage elasticity benefits.

Contribution

It demonstrates that increased tendon stiffness enhances learning accuracy and robustness in autonomous control of tendon-driven limbs, revealing benefits of elastic elements in robotic systems.

Findings

Higher tendon stiffness requires more learning iterations but improves task accuracy.

The system adapts quickly to changes in muscle stiffness within five attempts.

Elastic elements contribute to improved locomotion performance.

Abstract

Passive elastic elements can contribute to stability, energetic efficiency, and impact absorption in both biological and robotic systems. They also add dynamical complexity which makes them more challenging to model and control. The impact of this added complexity to autonomous learning has not been thoroughly explored. This is especially relevant to tendon-driven limbs whose cables and tendons are inevitably elastic. Here, we explored the efficacy of autonomous learning and control on a simulated bio-plausible tendon-driven leg across different tendon stiffness values. We demonstrate that increasing stiffness of the simulated muscles can require more iterations for the inverse map to converge but can then perform more accurately, especially in discrete tasks. Moreover, the system is robust to subsequent changes in muscle stiffnesses and can adapt on-the-go within 5 attempts. Lastly, we test the system for the functional task of locomotion, and found similar effects of muscle stiffness to learning and performance. Given that a range of stiffness values led to improved learning and maximized performance, we conclude the robot bodies and autonomous controllers---at least for tendon-driven systems---can be co-developed to take advantage of elastic elements. Importantly, this opens also the door to development efforts that recapitulate the beneficial aspects of the co-evolution of brains and bodies in vertebrates.