Gravity Balancing Flexure Spring Mechanisms for Shoulder Support in Assistive Orthoses

Abstract

Passive shoulder supports show large potential for a wide range of applications, such as assisting activities of daily living and supporting work-related tasks. The rigid architectures of currently available devices, however, may pose an obstacle to finding designs that offer low protrusion and close-to-the-body alignment. This study explores the use of mechanisms that employ a flexible element which connects the supported arm to an attachment at the back and acts as energy storage, transmission and part of the load bearing structure. Based on the synthesis method explained in this paper, we conducted a large scope investigation into possible flexure-based mechanism topologies. Many potential designs were discovered and are presented, categorized and compared. Two promising designs were developed into prototypes that were built and tested on a dedicated test bench. These mechanisms reduce the necessary moment to lift the arm by more than 80 % throughout 85 % of the range of motion, while staying within 18 cm and 10 cm distance from the body, respectively. Our study indicates that, due to its lower protrusion and interface loads, a design with a tapered flexure connecting the upper arm via a hinge to a spring loaded slider at the back offers the most promising solution.

A Multi-objective Simulation-Optimization Framework for the Design of a Compliant Gravity Balancing Orthosis

“…While Designs A exhibits extreme protrusions, its compensation capabilities align with experimental results presented in [7] and [8], where errors in gravity compensation of the elbow and shoulder (both presented as 1 DoF systems) are less than 20%. Simulated compensation presented in [7] can be as low as 2%, however, indicating that the framework presented here may require additional optimization iterations to find such a solution, or that such extreme compensation is only possible by compromising the minimization of protrusion.…”

“…The damaged CNS, upon exhausting all intact voluntary control pathways during functional task performance, will begin to rely on undamaged pathways that are non-specific to voluntary function [2]. This can result in the involuntary activation in distal limb flexors during shoulder motion, often termed "flexion synergy", which can significantly inhibit independent joint control [2], [4], [5], [6], [7], [8], [9], [10].…”

“…A common flexion synergy, that between shoulder abductors and elbow flexors, reduces active range of motion (ROM) when reaching against gravity [2], [4], [5], [6], [7], [8], [9], [10]. However, ROM can be restored by relieving the shoulder muscles through compensation of the upper-limb's weight, decoupling the flexion synergy [6], [7], [8], [10], [11], [12].…”

“…A common flexion synergy, that between shoulder abductors and elbow flexors, reduces active range of motion (ROM) when reaching against gravity [2], [4], [5], [6], [7], [8], [9], [10]. However, ROM can be restored by relieving the shoulder muscles through compensation of the upper-limb's weight, decoupling the flexion synergy [6], [7], [8], [10], [11], [12]. In clinical settings, this can be achieved using gravity-balancing technologies such as wearable robotics and exoskeletons [12], [13], [14], [15]; however, these devices typically possess inherent drawbacks associated with portability and load on the user, and therefore paradigms to decouple flexion synergies outside of controlled settings are far less common [7], [8].…”

“…While spring-based, passive devices, such as the occupational exoskeletons presented in [16], [17], [18], [19], [20], and [21], have been successful at providing wearable solutions to gravity compensation, such devices require rigid links to enforce spring extension [7], [8]. To avoid the use of rigid bodies, passive designs implementing compliant beams as flexure springs have been increasingly investigated due to their high customization [6], [7], [8].…”