Abstract
Despite a large increase in robotic exoskeleton research, there are few studies that have examined human performance with different control strategies on the same exoskeleton device. Direct comparison studies are needed to determine how users respond to different types of control. The purpose of this study was to compare user performance using a robotic hip exoskeleton with two different controllers: a controller that targeted a biological hip torque profile and a proportional myoelectric controller. We tested both control approaches on 10 able-bodied subjects using a pneumatically powered hip exoskeleton. The state machine controller targeted a biological hip torque profile. The myoelectric controller used electromyography (EMG) of lower limb muscles to produce a proportional control signal for the hip exoskeleton. Each subject performed two 30-min exoskeleton walking trials (1.0 m/s) using each controller and a 10-min trial with the exoskeleton unpowered. During each trial, we measured subjects' metabolic cost of walking, lower limb EMG profiles, and joint kinematics and kinetics (torques and powers) using a force treadmill and motion capture. Compared to unassisted walking in the exoskeleton, myoelectric control significantly reduced metabolic cost by 13% (p = 0.005) and biological hip torque control reduced metabolic cost by 7% (p = 0.261). Subjects reduced muscle activity relative to the unpowered condition for a greater number of lower limb muscles using myoelectric control compared to the biological hip torque control. More subjects subjectively preferred the myoelectric controller to the biological hip torque control. Myoelectric control had more advantages (metabolic cost and muscle activity reduction) compared to a controller that targeted a biological torque profile for walking with a robotic hip exoskeleton. However, these results were obtained with a single exoskeleton device with specific control configurations while level walking at a single speed. Further testing on different exoskeleton hardware and with more varied experimental protocols, such as testing over multiple types of terrain, is needed to fully elucidate the potential benefits of myoelectric control for exoskeleton technology.
Highlights
Engineers and scientists have been developing powered exoskeleton devices to augment normal human strength or aid individuals with disabilities since the 1960s (Laurence et al, 1969; Fick and Makinson, 1971; Vukobratovic et al, 1974; Zoss et al, 2006; Dollar and Herr, 2007, 2008; Yan et al, 2015)
By reducing the net joint torque required by the muscles, it may be possible to reduce the amount of mechanical power that muscles must provide to help reduce the metabolic cost of walking (Farris et al, 2013)
The current study investigates a state machine-based controller that was designed to provide a biological hip torque profile during stance phase based on Winter’s biomechanical data from humans (Winter et al, 1996), which is a common strategy in the field (Ding et al, 2016a; Seo et al, 2016)
Summary
Engineers and scientists have been developing powered exoskeleton devices to augment normal human strength or aid individuals with disabilities since the 1960s (Laurence et al, 1969; Fick and Makinson, 1971; Vukobratovic et al, 1974; Zoss et al, 2006; Dollar and Herr, 2007, 2008; Yan et al, 2015). Many lower limb exoskeletons are designed to assist walking such that the user lowers their muscle activity but remains in control over the movement (Lenzi et al, 2013). This results in the user modifying their joint torque to make the sum of the robotic assisted torque and biological muscle torque to be similar in magnitude to the torque during unassisted walking (Lewis and Ferris, 2011). The purpose of this study was to compare user performance using a robotic hip exoskeleton with two different controllers: a controller that targeted a biological hip torque profile and a proportional myoelectric controller
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