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Harvesting energy from low-frequency and non-harmonic excitations typical of human motion presents specific challenges. While resonant devices do have an advantage in environments where the excitation frequency is constant, and while they can make use of the entire proof mass travel range in the case of excitation amplitudes that are smaller than the internal displacement limit, they are not suitable for body applications since the frequencies are random and the amplitudes tend to be larger than the device size. In this paper a piezoelectric, impulse-excited approach is presented. A cylindrical proof mass actuates an array of piezoelectric bi-morph beams through magnetic attraction. After the initial excitation these transducers are left to vibrate at their natural frequency. This increases the operational frequency range as well as the electromechanical coupling. The principle of impulse excitation is discussed and a centimetre-scale functional model is introduced as a proof of concept. The obtained data show the influence of varying the frequency, acceleration and proof mass. Finally, a commercially available integrated circuit for voltage regulation is tested. At a frequency of 2 Hz and an acceleration of 2.7 m s−2 a maximal power output of 2.1 mW was achieved.

In the field of energy harvesting from low-frequency random excitation, a technique known as piezoelectric beam plucking or frequency up-conversion has seen increasing interest. This paper presents an experimentally validated model to calculate the voltage response and power output when actuating piezoelectric beams via a pair of magnets. Using magnetic coupling avoids impact on the brittle piezoceramic material. The relevant equations are derived for the piezoelectric beam, and two different approaches to include the magnetic interaction are presented and discussed. After comparing the models to experimental results, their use in predicting the response to changes in the system, e.g. using thicker magnets, and determination of the optimal load resistance and power output with regards to frequency, are investigated.

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Abstract.Piezoelectric energy harvesting is an attractive alternative to battery powering for wireless sensor networks. However, in order for it to be a viable long term solution the fatigue life needs to be assessed. Many vibration harvesting devices employ bimorph piezoelectric bending beams as transduction elements to convert mechanical to electrical energy. This paper introduces two degradation studies performed under symmetrical and asymmetrical sinusoidal loading. It is shown that besides a loss in output power, the most dramatic effect of degradation is a shift in resonance frequency which is highly detrimental to resonant harvester designs. In addition, micro-cracking was shown to occur predominantly in piezoelectric layers under tensile stress. This opens the opportunity for increased life time through compressive operation or preloading of piezoceramic layers.