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Title: Electrical tuning of electromagnetic energy harvesters with switched mode power electronics
Author: Bowden , James Anthony
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2013
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Energy Harvesting is a key enabling technology for highly distributed electronic systems such as wireless sensor networks, One of the most commonly described harvesting techniques is vibration harvesting, where a base-excited resonant mass/spring system is damped via an electromechanical transducer. A significant drawback of the resonant mechanical system, required to amplify low-level base vibrations, is the narrow bandwidth over which the system can operate, This thesis describes work carried out towards a method of increasing the bandwidth of resonant vibration harvesters by synthesising a variable complex load impedance using highly efficient switched-mode converters: loading the harvester with an optimum complex impedance effectively tunes the complete electromechanical system to the excitation frequency. This tuning effect is described analytically and demonstrated in practice with linear impedance emulating circuits. To benchmark the electrically tuned system against a more conventional converter, a micro power resistance emulator was developed, featuring an average quiescent power consumption of 56.6uW and a peak efficiency of 85.4%. The prototype harvester for which this tuning system was developed generates approximately 20mW, presenting a challenge for • converter design. This is further exacerbated when synthesizing large reactive loads at tuning frequencies far from mechanical resonance where the apparent power is much larger than the real power delivered from the harvester. To achieve the desired performance a custom micropower VSC power stage was implemented using discrete components, having an average quiescent power consumption of 454uW. The complete switched-mode tuning system requires current and voltage sensing, control compensation and PWM generation. A range of analogue and miJ5'ed signal implementations of these subsystems were investigated and it was concluded that at this power level the most significant challenge arises from balancing gate drive losses against the requirement for ultra low conduction losses in the power devices. Experimental results demonstrate that electrical tuning can extend the bandwidth over which a resonant vibration harvester can provide useful DC power, to almost three times that achieved with a simple unity power factor converter synthesising a fixed load resistance.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available