Evaluation of Force Controllers for a Gravity-Compensating Upper Limb Soft Exoskeleton

Abstract

For assistive exoskeletons, researchers have introduced soft exoskeletons (exosuits) because they tend to be lightweight, easily worn and mobile for daily use. However, most existing devices have not introduced closed-loop control methods that users can intuitively learn and use during activities of daily living. The goal of this pilot study is to design and test adequate controllers on the shoulder exosuit for the main study with human trials. In this paper, three controllers, namely a direct force (DF) controller, a direct force controller with a friction compensation (DFF) and an indirect force (IF) admittance controller, are introduced and tested for their feasibility and performance on the exosuit on a human. The controllers were initially tested on the test bench and the DF and IF controllers were selected and compared to a no-controller condition as a pilot study on the exosuit with four participants. Three different conditions did not show significant difference in tracking error, smoothness and bandwidth in arm elevation. However, the IF controller showed better performance in force than the DF controller for the tracking error of 2.95±0.15N (mean±standard deviation) and 9.34±2.99N, and for the smoothness of movements (SPectral ARC length) of -4.70±0.61 and -7.07±0.89, respectively. The force error between reference and measured force noticeably increased from 0.5Hz of elevation movements which is similar to their force bandwidth of 0.55Hz for the IF controller and 0.4Hz for the DF controller. Despite the fact that the obtained results may not suffice to verify that the controllers provide sufficient assistance to a user, the indirect force admittance controller with the given experiment procedure shows promising results and performance with the exosuit which can later be used for further studies and human trials.