Methods of frequency tuning vibration based micro-generator.
University of Southampton, School of Electronics and Computer Science,
A vibration based micro-generator is an energy harvesting device that couples a certain transduction mechanism to the ambient vibration and converts mechanical energy to electrical energy. In order to maximize available power, micro-generators are typically inertial devices that operate at a single resonant frequency. The maximum output power is generated when the resonant frequency of the generator matches the ambient vibration frequency. The output power drops significantly if these two frequencies do not match due to the high Q-factor of the generator. This thesis addresses possible methods to overcome this limit of vibration based micro-generators, in particular, method of tuning the resonant frequency of the generator to match the ambient vibration frequency.
This thesis highlights mechanical and electrical methods of resonant frequency tuning of a vibration based micro-generator. The mechanical frequency tuning is realized by applying an axial tensile force to strain the cantilever structure of the generator. A tunable micro-generator with a tuning range from 67.6 Hz to 98Hz and a maximum output power of 156.6μW at a constant low vibration acceleration level of 0.59m·s-2 was designed and tested. The tuning mechanism was found not to affect the damping of the generator. A closed loop frequency tuning system as well as the frequency searching algorithms has been developed to realize automatic frequency tuning using the proposed mechanical tuning method. The model of duty cycle of the system was established and it was proved theoretically that a reasonable duty cycle can be achieved if the generator and tuning system is designed properly.
The electrical tuning method is realized by changing the load capacitance of the generator. Models of piezoelectric and electromagnetic generators using electrical tuning methods were derived. The model of the electromagnetic generator has also been experimentally verified. The electrically tunable generator tested has a maximum 3dB bandwidth of 4.2Hz.
In conclusion, resonant frequency tuning using mechanical methods presented in the thesis have larger tuning range than that using electrical methods. However, frequency tuning using electrical tuning methods consumes less power than that using mechanical methods for the same amount of tuning range.
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