On-line Joint Limit Avoidance for Torque Controlled Robots by Joint Space Parametrization

TL;DR

This paper introduces a control approach for torque-controlled robots that ensures joint trajectory tracking while avoiding joint limits through joint space parametrization, validated on humanoid robot experiments.

Contribution

It presents a novel joint space parametrization method enabling joint limit avoidance in torque-controlled manipulators, with stability proofs and experimental validation.

Findings

Successful joint limit avoidance demonstrated on iCub robot.

Stable tracking of time-varying trajectories confirmed by Lyapunov analysis.

Control laws effectively prevent joint limit violations during operation.

Abstract

This paper proposes control laws ensuring the stabilization of a time-varying desired joint trajectory, as well as joint limit avoidance, in the case of fully-actuated manipulators. The key idea is to perform a parametrization of the feasible joint space in terms of exogenous states. It follows that the control of these states allows for joint limit avoidance. One of the main outcomes of this paper is that position terms in control laws are replaced by parametrized terms, where joint limits must be avoided. Stability and convergence of time-varying reference trajectories obtained with the proposed method are demonstrated to be in the sense of Lyapunov. The introduced control laws are verified by carrying out experiments on two degrees-of-freedom of the humanoid robot iCub.