| Neuromuscular electrical stimulation (NMES) or functional electrical stimulation (FES) is a widely used technique for rehabilitation and restoration of motor function. Millions of people suffering from disability and paralysis caused by neural disorders such as a stroke, spinal cord injury, multiple sclerosis, cerebral palsy, or traumatic brain injury can benefit from NMES. Open-loop methods, using grouped electrical pulses with fixed parameters, are widely accepted in clinical settings, primarily for strength training related rehabilitation treatments. The development of closed-loop NMES can provide new rehabilitation treatments where accurate limb movement is essential.;Contributions of this dissertation result from the development of robust closed-loop NMES controllers that account for uncertainties and nonlinearities in the muscle and activation dynamics. Specifically, this dissertation examines an optimal trade-off between performance and muscle fatigue, the effects of modulating the control input, the effects of time delay in the muscle contraction, and switching controllers during the gait cycle.;In Chapter 2 and 3, inverse and direct optimal NMES controller are designed which consider potential overstimulation by NMES controllers. Muscle fatigue is a multiple-factor problem which affects all aspects of NMES. Overstimulation is an avoidable fact that leads to early onset of muscle fatigue. An optimal control framework provides practitioners a useful tool to balance between stimulation dosage and tracking performance. Experiments are provided to illustrate the performance.;In Chapter 4, a muscle activation model with a pulse modulated control input is developed to capture the discontinuous nature of muscle activation, and a closed-loop NMES controller is designed for the uncertain pulse muscle activation model. The pulse modulated control input in the model results in an explicit condition that relates performance, pulse magnitude, and pulse frequency. Higher frequency results in more rapid muscle fatigue. Given the important role of modulation frequency in managing muscle fatigue, this contribution illustrates how stimulation frequency can be included in the analysis of the closed-loop controller design, which provides a starting point for designing more frequency efficient NMES controllers. Experiments are provided to illustrate the performance.;In Chapter 5, an identifier based control structure is developed. Muscle force output from electrical stimulation exhibits large time delays from the muscle contraction dynamics. Previous results in literature had to use known (or estimated) model parameters to compensate for the muscle contraction dynamics. By using acceleration feedback uncertain muscle contraction model can be used for closed-loop controller design. The use of limb acceleration is problematic for control implementation due to noise. In this chapter, a closed-loop controller is developed which can be implemented only using position and velocity signals.;In Chapter 6, by combining the approaches in Chapter 4 and Chapter 5, an identification-based controller is developed which includes an uncertain muscle contraction dynamics with pulse modulated control input. The controller is implemented only using position and velocity signals and the pulse modulation effect is included in the analysis. Experiments are provided to illustrate the performance.;Ankle motion is important for maintaining normal gait. For individuals who lost their ability to control the ankle, NMES can be used to help restore normal gait. In Chapter 7, a sliding mode based controller is developed to control ankle motion during gait. Ankle motion is modeled as a hybrid system and a switched sliding mode controller is designed to enable the ankle to track desired trajectories during gait. |
