Towards Human-Powered Lower-Limb Exoskeletons

Abstract

Most of the current commercially available exoskeletons use rechargeable Li-Ion batteries, which require frequent charging to meet the operational needs. The battery charging is a serious bottleneck, when the person, wearing the exoskeleton, needs to go for trip outdoors or for extended excursions such as trekking. To reduce the reliance on battery power, more reliable and alternative energy sources are required. In this respect, human-powered products (HPPs) are emerging as useful emergency electric power sources, when regular power supplies are unavailable. Human power is defined as the use of human work for energy generation. The energy is harvested from the user's everyday actions (walking, breathing, body heat, blood pressure, finger motion etc.). Once the power is harvested, it must be stored in a suitable device like capacitors, rechargeable batteries, etc. Being economical and environment friendly, these devices can also act as a boon for under-developed countries, since batteries are expensive and the mains power supply can be unreliable. The energy generation in these devices is broadly based on five methods―piezoelectric, vibrations, radio frequency (RF), electrostatic and electromagnetic, and each method produces different amount of electrical energy. All these methods are reviewed in this work; the first four methods produce relatively small amounts of energy, which is inadequate to charge the battery of assistive exoskeletons. Therefore, the focus here is on electromagnetic devices and how this can be used to power assistive exoskeletons. Some human-powered products are also reviewed and the report compares conventional and alternate methods to charge lower-limb exoskeletons used for assisting elderly persons. However, in this thesis objective presented are focused on charging exoskeleton with upper-body motion, the use of a hand-crank generator for this purpose is proposed in this work. A mathematical model is developed to describe the complete process of energy generation and deployment. Further, experimental test-rig is developed to validate the mathematical model. Along with this, the Matlab/Simulink based PID controller is used to suppress the fluctuations in the output of a hand-crank generator.