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Publication# 27.2 An Adiabatic Sense and Set Rectifier for Improved Maximum-Power-Point Tracking in Piezoelectric Harvesting with 541% Energy Extraction Gain

Résumé

Piezoelectric energy harvesters (PEHs) convert mechanical energy from vibrations into electrical energy. They have become popular in energy-autonomous IoT systems. However, the total energy extracted by a PEH is highly sensitive to matching between the PEH impedance and the energy extraction circuit. Prior solutions include the use of a full-bridge rectifier (FBR) and a so-called synchronous electric-charge extraction (SECE) [1], and are suitable for non-periodic vibrations. However, their extraction efficiency is low since the large internal capacitance C p (usually 10's of nF) of the PEH (Fig. 27.2.1) prevents the output voltage from reaching its maximum power point (MPP) under a typical sinusoidal and transient excitation (V MPP = 1/2·I p R p ). A recently proposed technique [2,3,4], called bias-flip, achieves a higher extraction efficiency by forcing a predetermined constant voltage at the PEH output, V p , which is then flipped every half-period of the assumed sinusoidal excitation (Fig. 27.2.1, top left). To flip V p , the energy in capacitor C p is extracted using either a large external inductor [2,3] or capacitor arrays [4]. It is then restored with the opposite polarity (Fig. 27.2.1, top). However, V MPP of the PEH varies with sinusoidal current I p ; hence, the two fixed values of V p in the flip-bias technique either over or underestimate V MPP for much of the oscillation cycle (pattern filled regions in Fig. 27.2.1, top right). In addition, none of the prior approaches compensate for V MPP -waveform amplitude changes, due to input intensity variations or decaying oscillations after an impulse, further degrading efficiency.

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La piézoélectricité (du grec πιέζειν, piézein, presser, appuyer) est la propriété que possèdent certains matériaux de se polariser électriquement sous l’action d’une contrainte mécanique et réciproque

Piezoelectric energy harvesters (PEHs) are widely deployed in many self-sustaining systems, and proper rectifier circuits can significantly improve the energy conversion efficiency and, thus, increase the harvested energy. Various active rectifiers have been proposed in the past decade, such as synchronized switch harvesting on inductor (SSHI) and synchronous electric charge extraction (SECE). This article presents a sense-andset (SaS) rectifier that achieves maximum-power-point-tracking (MPPT) of PEHs and maintains optimal energy extraction for different input excitation levels and output voltages. The proposed circuit is fabricated in the 0.18-μm CMOS process with a 0.47-mm 2 core area, a 230-nW active power, and a 7-nW leakage power. Measured with a commercial PEH device (Mide PPA-1022) at 85and 60-Hz vibration frequency, the proposed circuit shows 512% and 541% power extraction improvement [figure of merit (FoM)] compared with an ideal full-bridge rectifier (FBR) for ON-resonance and OFF-resonance vibrations, respectively, while maintaining high efficiency across different input levels and PEH parameters.

Danick Briand, Nico de Rooij, Simon Nessim Henein, Don Isarakorn, Pattanaphong Janphuang, Robert Andrew Lockhart

This paper presents an analytical and experimental study of a compact configuration to harvest energy from a rotating gear using piezoelectric microelectromechanical system harvesters. The reported configuration realizes a contact-type frequency up-conversion mechanism in order to generate useful electrical energy. The up-conversion mechanism was achieved using an atomic force microscope (AFM)-like piezoelectric cantilever plucked by the teeth of the rotating gear that could be eventually driven by an oscillating mass. This paper describes relevant design guidelines for harvesting energy from the low-frequency mechanical movement of a rotating gear through analytical modeling and finite element method (FEM) simulation followed by experimental validation. Different harvester configurations are investigated to identify the optimal configuration in terms of the output energy and energy conversion efficiency. The latter results are reported for the first time because of the implementation of an original concept based on the coupling of the harvester with a rotational flywheel. The experimental results reveal that free vibrations of the harvester after plucking contribute significantly to the output energy and efficiency. By adding a proof mass, the efficiency of the system can be greatly improved. For plucking speeds between 3 and 19 r/s, average output powers in the order of tens of microwatts were obtained for continuous plucking. By combining interaction energy, friction, and energy absorption, between the harvester and inertial mass, the maximum efficiency of the impact piezoelectric harvesters was found to be 1.4%. The efficiency results obtained were compared with the noncontact magnetic plucking approach further demonstrating the potential of our concept. Finally, different tip-gear materials combinations were evaluated showing the importance of their nature on the reliability of the presented configuration.

Electric motors consume over half of the electrical energy produced by power stations, almost the three-quarters of the electrical consumption in industry and almost the half of commercial electrical consumption in developed countries. Motors are by far the most important type of electric charges, and so constitute the main targets to achieve energy saving. Owing to their simple and robust construction, the asynchronous motors and especially those of squirrel-cage types, represent about 90-95% of the electrical energy consumption of electric motors, which is equivalent to about 53% of total electrical energy consumption. They are widely used as electrical drives in industrial, commercial, public service, traction and domestic applications. Owing to the importance of induction motors, this thesis is aimed at contributing to energy saving efforts, more specifically in the field of low power induction motors. A contribution is kept in perspective by taking into consideration the energy saving potential during the motor design stage as well as during its operation. Every effort to save energy in motor application can be made by always attempting to use energy only as much as what needed during its operation. The best way is to exploit the saving potential during motor design, while at the same time taking into account its intended application. It can be achieved either through the improvement of motor design or through the reduction of its input electrical energy when the motor has already been built. These two efforts are studied, elaborated and worked out thoroughly in this thesis. To attain this objective, a synthesis has been started with the description of how to model an induction motor. To obtain a better model, an improvement is proposed by using the Schwarz-Christoffel mapping to calculate the slot leakage inductance in induction motor. With such method, slot-leakage inductance can be determined more precisely, resulting in more accurate prediction of motor characteristics. It is based on the stored magnetic energy calculation using two-directional field distribution in the slot. The air gap influence can be observed easily, so that a reasonable slot leakage definition can be adopted. Unlike the conventional method, which is only suitable for rectangular slots (otherwise empirical corrections are required), the proposed general slot form can be extended to any desired polygonal slot form. Consideration of saturation is also indispensable because ignoring it could result in inaccuracy in motor performance prediction. Considering the saturation is essential owing to its important role in self-excitation phenomenon to establish voltage build-up in induction generator. However, the self-excitation phenomenon is undesirable in certain group of capacitor motors as it may hinder the switching-off process and mechanical braking at a desired moment. The undesirable switching-off failure condition is to be avoided by properly designing the capacitor motor. Like in this capacitor motor special application, where a proper design is useful from the point of view of operation safety, designing properly a motor is also very important in energy saving efforts. Motor design and optimization to minimize losses as well as to make possible wide speed-range motor operation are some of the efforts. However, when induction motor has already been built, saving energy is only possible by managing its supplying electrical energy. Various strategies are possible and a particular emphasis on the use of triac to reduce motor input voltage is presented. Besides, a brief economic saving evaluation is given to draw attention to the energy saving potential.