Vibration powered generators for self-powered microsystems

Abstract

Methods are examined for deriving energy from vibrations naturally present around sensor systems. Devices of this type are described in the literature as self-powered. This term is defined as describing systems that operate by harnessing ambient energy present within their environment. Traditionally, remote devices have used batteries to supply their energy, which offer only a limited life span to a system. The recent rapid advances in integrated circuit technology have not been matched by similar advances in battery technology, thus, power requirements place important limits on the capability of modern remote microsystems. Self-power offers a potential solution to power requirement, and when combined with some form of wireless communications, can produce truly wireless autonomous systems.

A generator based on the thick-film piezoelectric material, PZT, is produced. The resulting device is tested, and methods are devised to measure the material properties of its constituent layers. Power output is low at only 3μW. Modelling shows that the low power output is due to the low electromagnetic coupling of thick-film PZT. The modelling includes the development of a new method of a resistively shunted piezoelectric element undergoing pure bending. Numerical optimisation is used to predict the power output from piezoelectric generators of arbitrary dimensions and excitation conditions.

Experiments have been devised to assess the long-term stability of thick film PZT materials. A technique for measuring the ageing rate of the d₃₁ and K₃₃ coefficients of a PZT thick-film sample is presented. The d₃₁ coefficient is found to age at -4.4% time decade, and K₃₃, at -1.34% per time decade (PZT-5H).

An electrical equivalent circuit model of a generator based on electromagnetic induction has been described, and verified by producing a prototype generator. The prototype could produce 4.9mW in a volume of 4cm³ at a resonant frequency of 99Hz. A typical configuration is modelled, and numerical methods used to find optimum generator dimensions, and predict power output for various excitations. The model is used to compare this type of generator to piezoelectric generators, and hence evaluate the two technologies. Graphs are produced to permit estimates of how much power could be produced by either generator type under arbitrary excitation conditions. It is concluded that neither generator type is superior under all excitation conditions, but that severe manufacturing difficulties with piezoelectric generators mean that they are unlikely to be commonly used in future applications.

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