Design and implementation of an energy harvesting system in a prosthetic limb

Abstract: Energy Harvesting, also known as power harvesting or ambient power, is the process of obtaining small amounts of power from secondary sources, such as vibrations, light, temperature variations and even radio-frequency emissions. These systems have been uncommon in personal and wearable electronics in the past, however they are slowly gaining traction. With the increasing sophistication of prosthetic limbs and implants, devices that in some cases require a consistent and reliable power source, the potential field of application for energy harvesting grows wider. This thesis project evaluates whether energy harvesting methods could be implemented in future prosthetic limb designs without significantly affecting weight, user comfort, complexity of design etc., and whether the gains of such an implementation would be worth the effort and cost put into it. For reference the project used the RHEO KNEE® by Össur Hf., a microcontroller controlled prosthetic knee, as a device that such a system could be integrated with. Energy harvesting is still an emerging field and is a long time away from being a viable primary power source for most electronic devices. However, it still might have potential as a supplementary source for extending charge cycles or making smaller (and therefore more lightweight) power cells viable. This master’s thesis project was broad in scope and included 3D-design; mechanical, electrical and embedded software design; and setting up a miniature kinetic power generator as well as a photovoltaic harvesting system. No amputees were available for testing the designs so the system was tested with a 3D-printed model that was moved by hand to simulate the generation process. Due to some incorrect inital assumptions, the final electronic design was not optimal for this kind of system. However, a kinetic generator that harvested power from a modeled heel striking the ground 50 times a minute produced about 23mW of power. 53cm2 of photovoltaic panels produced 42μW of power in an ambient light setting. For comparison, a low-power microcontroller needed about 119μW of power on average to do some simple processing and send Bluetooth transmissions once every two seconds.