Effect of Hip Assistance Modes on Metabolic Cost of Walking With a Soft Exoskeleton

Abstract

Understanding the effects of different hip assistance modes is a fundamental step in the process of designing hip assistance devices and controllers that can provide better performance in terms of metabolic cost. We have developed and tested a soft exoskeleton for hip assistance, which includes three assistance modes: hip extension assistance (HEA), hip flexion assistance (HFA), and hip extension and flexion assistance (HEFA). A proportional derivative (PD) iterative learning controller based on the feedforward model was proposed to control the assistive force accurately. The three hip assistance modes were evaluated on seven male subjects walking on a treadmill at a speed of 5 km/h in two scenarios-first with a 15-kg backpack and then without any backpack. The net metabolic costs could be reduced during the loaded condition, compared with those under no exoskeleton condition, by 9.95%, 6.25%, and 15.28% for HEA, HFA, and HEFA, respectively. The reductions were found significant in HEA ( p=0.048) and HEFA ( p=0.005) modes, while the HFA mode ( p=0.202) was not found statistically significant. It indicates that the HEA and HEFA modes with the soft exoskeleton provide more benefit to the net metabolic cost compared with the HFA mode. The net metabolic costs reduced during the unloaded condition were 9.21%, 2.58%, and 13.05% for HEA, HFA, and HEFA, respectively. The improvements in the walking efficiency during both the conditions with the developed soft exoskeleton are demonstrated. Note to Practitioners-This article was motivated by the problem that how to reduce the metabolic cost most appropriately of walking by hip assistance of soft exoskeleton. In this article, we conduct a comparison of three hip assistance modes to discuss the balance of system weight and assistance efficiency. We then propose a PD iterative learning controller based on the feedforward model to track the desired assistive force accurately. Preliminary experiments suggest that the hip extension assistance (HEA) is more suitable than hip flexion assistance (HFA) for hip assistance during single motion assistance. Multiple motion assistance is more beneficial for metabolic cost reduction when the weight of the soft exoskeleton is the same. The experimental tests show that the proposed soft exoskeleton and the control algorithm are effective for walking assistance during the loaded condition. However, the performance of the hip assistance device is tested based on the treadmill walking only. In future research, we will conduct a performance evaluation of the soft exoskeleton on a complex road environment.