Abstract
A hybrid neuroprosthesis that uses an electric motor-based wearable exoskeleton and functional electrical stimulation (FES) has a promising potential to restore walking in persons with paraplegia. A hybrid actuation structure introduces effector redundancy, making its automatic control a challenging task because multiple muscles and additional electric motor need to be coordinated. Inspired by the muscle synergy principle, we designed a low dimensional controller to control multiple effectors: FES of multiple muscles and electric motors. The resulting control system may be less complex and easier to control. To obtain the muscle synergy-inspired low dimensional control, a subject-specific gait model was optimized to compute optimal control signals for the multiple effectors. The optimal control signals were then dimensionally reduced by using principal component analysis to extract synergies. Then, an adaptive feedforward controller with an update law for the synergy activation was designed. In addition, feedback control was used to provide stability and robustness to the control design. The adaptive-feedforward and feedback control structure makes the low dimensional controller more robust to disturbances and variations in the model parameters and may help to compensate for other time-varying phenomena (e.g., muscle fatigue). This is proven by using a Lyapunov stability analysis, which yielded semi-global uniformly ultimately bounded tracking. Computer simulations were performed to test the new controller on a 4-degree of freedom gait model.
Highlights
Each year, approximately 5100 people in the USA alone are diagnosed with paraplegia due to a spinal cord injury (The National SCI Statistical Center, 2014), impairing their ability to walk again
In our proposed adaptive control scheme, we showed that the synergy-based approach can be modified to provide a lower dimensional feedforward controller and combined with a feedback controller to control a hybrid walking neuroprosthesis
An adaptive synergy-based controller was presented for a hybrid walking neuroprosthesis
Summary
Approximately 5100 people in the USA alone are diagnosed with paraplegia due to a spinal cord injury (The National SCI Statistical Center, 2014), impairing their ability to walk again. Functional electrical stimulation (FES) and powered orthoses are two viable technologies that have the potential to restore the walking function in persons with SCI (Kralj and Bajd, 1989; Kobetic et al, 1997; Farris et al, 2011; Neuhaus et al, 2011; Esquenazi et al, 2012; del-Ama et al, 2014a; Ha et al, 2015). The use of FES for gait restoration is limited by the rapid onset of muscle fatigue (Binder-Macleod and Snyder-Mackler, 1993), and powered exoskeletons require batteries and larger actuators to generate the torques necessary to produce the gait motion. Combining the two technologies may provide the benefits of both powered exoskeletons and FES-based devices and overcome their limitation when used alone. A hybrid device composed of FES and electric motors may have smaller motors because
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