As part of his doctoral dissertation, Dr Waleed al Ashtari (Dr.-Ing.) investigated the model-based integration of a piezoelectric ‘energy harvester’ into an autonomous system. This device converts vibrational energy from the environment into useful electrical energy. To calculate the electromechanical characteristics of a piezoelectric ‘energy harvester’, an analytical model based on material properties, geometry and boundary conditions was developed. This serves as the basis for a further model describing the operation of an autonomous system. The theoretical work was validated by laboratory experiments. It was found that, in steady state, the piezoelectric harvester experiences two alternating load conditions due to the rectification process. This leads to non-linear behaviour of the harvester, particularly when the connected load has low impedance. Furthermore, the results show that such an autonomous system operates efficiently when connected to a high-impedance load and is excited at a frequency corresponding to the anti-resonance frequency of the piezoelectric harvester.

The model of the autonomous system was extended to a system comprising several piezoelectric transducers. For the practical implementation of such a system, a frequency-tuning technique was introduced, as the optimal operating frequencies of the individual transducers must be coordinated with one another. The adjustment is carried out by altering the distance between two permanent magnets and thus their attractive force, which influences the stiffness of the harvester. This frequency-tuning technique was modelled and experimentally validated. The results show that frequency tuning using permanent magnets represents a simple yet effective solution to the problem of frequency matching in piezoelectric ‘energy harvesters’.