Modeling and Balance Control of Supernumerary Robotic Limb for Overhead Tasks

Overhead manipulation tasks often require collaborations between two operators, which becomes challenging in confined spaces such as in a compartment. Supernumerary Robotic Limb (SuperLimb), as a promising wearable robotic solution, can provide assistance in terms of broader workspace, wider manipulation functionalities and safer working conditions. However, the safety concerns of human-centered SuperLimb interaction mechanisms are rarely studied to date, particularly regarding human standing balance. This study proposes a balance controller by which one individual operator can accomplish overhead tasks with the assistance of SuperLimb via tunable interaction force and supporting force regulation. The SuperLimb-human interaction is modeled and a dynamics control method based on QR decomposition (also known as QR factorization, in which a matrix is factorized into an orthogonal matrix and an upper triangular matrix) is adopted to decouple joint torques of the SuperLimb and the interaction forces. Therefore, the supporting forces can be regulated independently to guarantee the operator-SuperLimb interaction forces in a safe region. Force plate is used for measuring the CoP position as an evaluation method of the standing balance. The critical horizontal push force is learned through experiment to guide the balance controller. This method is implemented on a SuperLimb prototype worn on the operator's back, to provide necessary supporting forces on overhead object while allowing the operator to move freely underneath.

I. Introduction

Superlimb, i.e., Supernumerary Robotic Limb, as a pivotal branch of wearable robots, is an extension to exoskeletons and prostheses [1]–[3]. SuperLimb has the merit of extending the operator's workspace, augmenting human strength and allowing diverse postures beyond the human limb movement. Specifically, SuperLimb is capable of assisting overhead tasks, especially those in constrained workspaces [4]. These tasks include a series of operations with intensive workload and flexibility requirements in industrial set-ups, such as the compartment in the aircraft [5]–[7]. The working space limitation impedes the participation of large-size robots or coexisted multi-operator collaboration.