Control System for Outdoor Use of Powered Portable Ankle-Foot Orthosis

Abstract

Technological advancements and the decreasing cost of sensors and controllers have fueled a growing interest in gait analysis and support systems, especially for use outside of a laboratory environment. This thesis presents a control system for a lower-limb exoskeleton, the Powered Portable Ankle-Foot Orthosis (PPAFO), designed to assist both individuals with motor disabilities and healthy subjects in various outdoor settings. The work begins by reviewing relevant literature and prior work in the field of gait analysis, embedded systems, and powered exoskeletons. It then details the implementation of the control system, outlining both the hardware components, such as IMUs, encoders, and industrial computers, and the software components, including the control algorithms and communication protocols. A central feature of the system is the Zero Velocity Update (ZUPT) method, which is used to correct for sensor drift and accurately detect when the foot is stationary during the gait cycle. This allows for the precise, real-time estimation of foot orientation and tibia angle. The second part of the thesis presents a numerical evaluation of the control algorithms through simulation studies and self-collected data. The performance and adaptability of the system are assessed by comparing the output of a Look-Up Table (LUT) against gait variations in simulated subjects. The results show that while a general LUT offers some compatibility, performance significantly improves with condition-specific LUTs that incorporate factors like terrain inclination. A sensitivity analysis on the ZUPT algorithm and IMU characteristics further demonstrates the system's robustness to noise and varying parameters. The findings also reveal that the angular velocity of the foot can substitute for the tibia's angular velocity at critical moments, suggesting a potential for sensor reduction. Finally, the thesis provides recommendations for future work to enhance the PPAFO's capabilities and robustness in real-world conditions.