David Audigier

Buck-Boost Converter for Sensorless Power Optimization of Piezoelectric Energy Harvester

This paper presents a comprehensive model for miniature vibration-powered piezoelectric generators and analyses modes of operation and control of a buck-boost converter for the purpose of tracking the generators optimal working points. The model describes the generators power dependence with the mechanical acceleration and frequency, and helps in the definition of the load behaviour for power optimization. Electrical behaviour of the input of buck-boost converter in discontinuous current mode turns out to be in perfect agreement with the considered optimization criteria with a very simple, sensorless control. Experimental results show that the converter controlled by a very low consumption circuit effectively maximizes the power flow into a 4.8 V rechargeable battery connected to the converter output. The converters efficiency is above 84% for input voltages between 1.6 and 5.5 V, and for output powers between 200 muW and 1.5 mW. The presented circuit and control can be used as well for power optimization of electromagnetic energy harvesting devices.

Enhanced semi-passive damping using continuous switching of a piezoelectric device on an inductor

The SSD technique proposed here addresses the problem of resonance damping on a mechanical structure. SSD stands for Synchronized Switch Damping. Apart from active techniques, passive ones consist in connecting a piezoelectric insert attached to the structure to a passive electric network in which the energy generated by the piezoelectric inserts is degraded. In the semi passive approach, the piezoelectric inserts are continuously switched from open circuit to short circuit synchronously to the structure motion. Due to this switching mechanism, a phase shift appears between the piezoelectric strain and the resulting voltage, thus creating energy dissipation. For the new technique proposed here, instead of discharging the piezoelectric inserts during a brief short circuit, they are connected on a small inductor, allowing the inversion of the voltage and then released to open circuit. In this case the voltage amplitude is optimized and is 90 degrees out of phase with the motion then enhancing the damping mechanism. The technique is applicable at any frequency without the need for a large tuned inductor, especially for low frequency applications. There is no need for external power supply unless for the low power circuitry of the switch device. The implementation of the switch drive with a very cheap micro-controller is described. Experimental results measured on cantilever beams made with different materials are proposed. Damping ability ranges from 6 dB on a very viscoelastic epoxy beam to nearly 20 dB on a steel beam. Harmonic excitation and transient results are both proposed and compared. Finally, an electromechanical model is proposed, giving an interpretation of the damping mechanism. Theoretical predictions are in good agreement with the experiments.

Dielectric and piezoelectric properties of (La, Mg, F) and (Mg, Mn, F) doped PZT ceramics under low and high sollicitations

Abstract The hysteretic behavior of the charge coefficient d 33 under high mechanical stress and of frequency constant N 33 during the temperature cycle −40°C–+80°C can be cancelled by fluorine oxygen substitution in the lattice of Mg-doped PZT. These fluorine doped materials also exhibit a high mechanical quality factor and low dielectric losses. The drawback is that they have a lower d 33 coefficient and are difficult to pole. Mn doping in B site or La doping in A site, in the ABO 3 type perovskite structure are known to increase the d 33 coefficient and the poling ability. This result is observed on (Mg, F) co-doped PZT with Mn and with La content of 1.5 mol%. In the same manner, Mn doping enhances the hysteretic behavior and the losses while La doping does not significally changes the (Mg, F) co-doped PZT characteristics except the fluorine content for which the losses and the hysteresis are the lowest.

Effect of (Mn, F) co-doping on PZT characteristics under the influence of external disturbances

Abstract This study concerns the reversibility of (Mn, F) co-doped PZT characteristics under thermal disturbances. PZT formulations with high piezoelectric coefficients (d 33 320 pC/N) and exhibiting a non hysteretic frequency response between -40C and +85C have been obtained. A phenomenological model is proposed to explain these results. This model is based upon an analogy of the dopants behaviour with the fundamental laws of the redox phenomena approach in aqueous solutions. This also permits one to easily differentiate the mechanisms leading to soft and hard PZT materials

PZT uniaxial stress dependence: experimental results

Abstract The main characteristics of a PZT ceramic, such as the piezoelectric charge coefficient or the permittivity, may change drastically when the ceramic undergoes a high uniaxial stress along its polar axis in the 10 to 150 MPa range. A special device has been built to measure accurately and directly the d33 and ϵT 33 parameters of a PZT rod with a superimposed controlled uniaxial stress. Experimental results are proposed showing the uniaxial stress dependence on several PZT samples near the morphotropic phase boundary. These results point out the influence of the Zr/Ti ratio on the uniaxial stress and show that according to a given working stress range, it could be possible to optimize the evolution of the piezoelectric coefficients such as d33 or ϵT 33.

Electron Transfer Mechanism in PZT Ceramics—Application to Squeeze Ignition

The aim of this paper is to clarify the fundamental difference occurring between a soft PZT ceramic and a hard one. The first type presents singly ionised and doubly ionised cation (lead) vacancies and the second one exhibits singly and doubly ionised anion (oxygen) vacancies. In this sense, this work also explains the mechanism that rales piezoelectric igniters and the reason why some materials lead to higher performances. For igniters, two types of systems can be used: the first one corresponds to a slow application of the stress on the piezoelectric element (squeeze) and the second one implies a mechanical shock produced on the element. In the case of “squeeze” igniters, it is clearly evidenced that spark creation only occurs when hard PZTs are used and this effect originates from the crucial role of the effective charge of oxygen vacancies in hard PZTs.

Mechanical damping induced by switching of piezoelectric elements

The proposed technique is based on an intermittent switching of piezoelectric elements bonded on the structure to be d amped. As a result of the switching, the global losses coefficient of the structure is increased by a significant factor. From a physical point of view, the damping results from the energy dissipation due to the discharge of the piezoelement capacitance in the switch resistance. The switch has to be controlled and thus requires an electrical power about a few milliwatts for be activated. Consequently, the described approach is considered to be a semi-passive technique. For enhanced effects, the switching sequence has to be optimized. No tuning elements such as inductors or resistor1 are required, consequently the switching method can operate at any frequency, in particular in the low frequency regime, and is inherently broadband. Transient or continuous vibrations are damped with a comparable efficiency. A theoretical model is proposed to interpret the experimental results, to give a comprehensive understanding of the underlying physics and to optimize the switching sequence. It is show that, unlike standard passive techniques, the added damping in non-newtonian but, indeed exhibits a dry friction behavior. Numerous experimental results are given for flexural damping of steel cantilever beam and aluminum plate. It is shown that the damping efficiency can be up to 20 dB for the steel beam configuration. Harmonic and transient regimes of the beams are considered and compared. The design of electronic switching board and power requirements of the micro-controller are discussed.

Fluoridated piezoelectric ceramics for power transducer

The fluoridation of anionic sites in the perowskite lattice ABO3 can greatly stabilize the remanent polarization under high stresses level. The dielectric losses are smaller than oxygen deficient materials. The mechanism for stabilization is always allocated to sites which can act either as electron donors or acceptors. The charge transfer induces a polarization which is sufficient to cancel the distribution of bound charge ((rho) equals -divP) in the poled phase. The B3+ - F1- couples constitute dipoles which have a better stability than the B3+ - V0 couples only present in perowskite ceramics with oxygen vacancies.