Validation of a weight compensation for wearable upper extremity support system

Abstract

DMD (Duchenne muscular dystrophy) is a genetic disorder characterized by progressive muscle weakness, leading to the eventual loss of muscle function. After losing lower extremity function, DMD patients lose the ability to use their arms. To assist individuals with DMD, an upper extremity exoskeleton is being developed to provide support.

The device is required to provide proper weight compensation for the weight of both the user and the support system. Previous literature review has highlighted three strategies for achieving compensation: Model, Calibration dynamic and Calibration static. However, the review lacks a conclusive understanding of the differences between these strategies. Thus, this study aims to validate a weight compensation model by comparing the joint torque with two different measurements: Calibration static and Calibration dynamic on non-disabled participants with a one DOF (Degree of freedom) elbow support system.

The weight compensation model was designed to accurately account for the weight of the user's arms and the exoskeleton device itself. It incorporates multiple inputs, including shoulder flexion, shoulder abduction, elbow joint angle, arm mass, and arm center of mass to calculate the required compensation torque for the motor located at the elbow joint.

The model was validated by a dead weight experiment. The weight compensation model was validated by comparing the joint torque measurements from Calibration dynamic and Calibration static results. Afterwards, experiments were performed on 12 male non-disabled participants. The weight compensation model results does not align well with the measurements from non-disabled participants. Analysis suggests that joint impedance caused these discrepancies. However, even after accounting for joint impedance, the weight compensation model still exhibits a tendency to overestimate the required compensation torque. A fitted model was used to decrease the product value of mass and center of mass to decrease overestimation. Furthermore, the comparative analysis indicates that dynamic and static measurements yielded similar mean values for joint torque.

While the weight compensation model demonstrates accuracy in a dead weight experiment under the assumption of an accurate estimation of its mass and center of mass, its performance is sub-optimal during the human experiment. This is attributed to the absence of joint impedance consideration and the overestimation of the forearm plus hand mass and center of mass for the user. Comparison between dynamic and static measurements the mean difference between dynamic and static measurements obtained from non-disabled participants indicates no substantial disparity in terms of joint torque.

Future plans for this research involve expanding the current one DOF configuration to a four DOF configuration and incorporating an inertial measurement unit (IMU) for more accurate angle measurements. Additionally, different compensation strategies will be compared in terms of task performance using metrics such as external interaction force or sEMG (surface electromyography). Lastly, the overestimation of forearm plus hand mass and center of mass will be further investigated.