Power Consideration in a Piezoelectric Generator

A piezoelectric generator converts mechanical energy into electricity and is used in energy harvesting devices. In this paper, synchronisation conditions in regard to the excitation vibration are studied. We show that a phase shift of ninety degrees between the vibration excitation and the bender’s displacement provides the maximum power from the mechanical excitation. However, the piezoelectric material is prone to power losses; hence the bender’s displacement amplitude is optimised in order to increase the amount of power which is converted into electricity. In the paper, we use active energy harvesting to control the power flow, and all the results are achieved at a frequency of 200？Hz which is well below the generator’s resonant frequency. 1. Introduction A piezoelectric generator (PEG) [1] can be used to extract energy from ambient vibrations. For that purpose, a proof mass is firmly attached to one end of a bender, while the other end is fixed onto a vibrating case [2]. The voltage supplied to the piezoelectric material produces internal stresses, which create active damping [3]. The power which brakes the movement of the proof mass is not dissipated into heat but is converted into electrical power and fed back to an electrical load. For a given design of the PEG, the harvested power depends on the operating conditions. First, the resonant behaviour of the bender and its proof mass lead to a strong dependency of harvested power on vibrations [2]. Moreover, the impedance of the PEG and of the load should match [4], and, then, there exists an optimal electric load to be connected to the PEG connection [5, 6]. Optimal energy harvesting can be achieved using active solutions. For example, [7] proposes to use a full H-bridge in order to accurately control the voltage supplied to the PEG. For an excitation close to the resonant frequency of the PEG, semiactive (or semipassive) solutions can also be used. The SSHI technique, for example, uses a switched inductor to reverse the voltage across the piezoelectric generator and synchronises the voltage on maxima and minima of the displacement [8]. The SECE technique [9] uses a flyback topology to extract the charges for the piezoelectric generator at each maximum of the voltage. In this way, energy scavenging is obtained at any load value and can be used to charge up a battery, in a wireless communication application [10], for example. In this work, we are using active energy harvesting, because we control the instantaneous voltage across the piezoelectric material. However, this voltage is not synchronised