Design and Implementation of Energy Harvesting for Digital Badges and Signage

With improvements in power harvesting transducers' power density, and power reduction in communication transceiver systems, displays, sensors and energy-aware microprocessors, smart wireless network nodes are becoming ubiquitous throughout our daily life. Digital signage has gained popularity with the integration of smart wireless network nodes into the application space replacing traditional signage and badges. Primary battery sources traditionally supply energy for digital signage, however, that generates waste and maintenance costs that are counterproductive to using digital signage. Therefore, a digital signage prototype called the wireless display sensor node (WDSN) with a micro-power photovoltaic energy harvesting system was developed at UC Davis and is presented as an alternative in this work. The WDSN and node management system is comprised of an electrophoretic display, Wi-Fi radio, photovoltaic and vibration power transducers, internet connected management system, sensors and power harvesting power electronics. A holistic energy approach was used to drive the development of the proposed digital badge and signage. This approach encompasses the characterization of vibrational and photovoltaic energy sources, analyzing the energy requirement from typical digital signage and developing a power harvesting energy management system that will maximize the lifetime and allow for self sufficiency of the digital signage. To bridge the gap between the energy source and the required peripheral supplies, a multiple input and multiple outputs (MIMO) H-bridge DC to DC converter was designed to harvest and regulate photovoltaic energy, and deliver energy to the various continuously active, and charge-and-execute loads of the WDSN. The H-bridge DC to DC converter comprising of a single inductor, two input power FETs from both primary and secondary power sources, and five symmetric output power FETs to create the various supply rails, supply the regulated energy required for the radio, the display, the sensors and the microcontroller. For the charge-and-execute supplies, a constant current ramp charging was developed to transfer charge at the maximum power point current of the photovoltaic cell to the supply capacitors of the peripherals that support the charge-and-execute supply generation. The MIMO H-bridge DC to DC converter presented supports active regulation using pulse width modulation for high current loads, pulse frequency modulation for light current loads, and ramp charging for capacitive loads. The controller was designed using digital logic and the entire MIMO H-bridge DC to DC converter occupies an area of 0.36 mm2 to 1.63 mm2 depending on the power transistor size selection. The measured instantaneous peak power efficiency is 86% while driving a 63 ?A load with a transient energy efficiency delivered to the load is 81%. The prototype WDSN dissipates 933 mJ to complete a server data synchronization and display refresh. The WDSN update energy when supplied with vibrational and photovoltaic power harvesting is equivalent to 7.52 hours of casual continuous walking (34.44 ?W), 1,230 laboratory door toggles (open and close at 986 ?W) and 12 hours of continuous office lighting (7 am to 7 pm with a daily total of 958 mJ under an average of 538 Lux of CFL lighting)