Power Converter Design for Maximum Power Transfer and Battery Management for Vibration-Based Energy Harvesting on Commercial Railcars
O'Connor, Thomas Joseph III
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Although the locomotive of a train is energized, in general, other railcars are not. This prevents commercial rail companies from installing sensor equipment on the railcars. Thus, several different solutions have been proposed to provide energy for commercial railcars. One such solution is a vibration-based energy harvester which can be mounted in the suspension coils of the railcar. The harvester translates the linear motion of the suspension vibration into rotational motion to turn a 3-phase AC generator. When subjected to real-world suspension displacements, the harvester is capable of generating peak energy levels in excess of 70 W, although the average energy harvested is much lower, around 1 W. A battery pack can be used to store the useful energy harvested. However, a power conditioning circuit is required to convert the 3-phase AC energy from the harvester into DC for the battery pack. The power converter should be capable of extracting maximum power from the energy harvester as well as acting as a battery manager. Experimental results with the energy harvester conclude that maximum power can be extracted if the harvester is loaded with 2 . In order to maintain a constant input impedance, the duty cycle of the power converter must be fixed. Conversely, output regulation requires the duty cycle to change dynamically. Consequently, there is a tradeoff between extracting maximum power and prolonging the battery life cycle. The proposed converter design aims to achieve both maximum power transfer and battery protection by automatically switching between control modes. The proposed converter design uses an inverting buck-boost converter operating in discontinuous conduction mode to maintain a constant input impedance through a fixed duty cycle. This constant input impedance mode is used to extract maximum power from the harvester when the battery is not close to fully charged. When the battery is near fully charged, extracting maximum power is not as important and the duty cycle can be controlled to regulate the output. Specifically, one-cycle control is used to regulate the output by monitoring the input voltage and adjusting the duty cycle accordingly. Finally, the converter is designed to shut down once the battery has been fully charged to prevent overcharging. The result is a power converter that extracts maximum power from the energy harvester for as long as possible before battery protection techniques are implemented. Previous related studies are discussed, tradeoffs in converter design are explained in detail, and an experimental prototype is used to confirm operation of the proposed control scheme.
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