This paper reviews the state-of-the art in vibration energy harvesting for wireless, self-powered microsystems. Vibration-powered generators are typically, although not exclusively, inertial spring and mass systems. The characteristic equations for inertial-based generators are presented, along with the specific damping equations that relate to the three main transduction mechanisms employed to extract energy from the system. These transduction mechanisms are: piezoelectric, electromagnetic and electrostatic. Piezoelectric generators employ active materials that generate a charge when mechanically stressed. A comprehensive review of existing piezoelectric generators is presented, including impact coupled, resonant and human-based devices. Electromagnetic generators employ electromagnetic induction arising from the relative motion between a magnetic flux gradient and a conductor. Electromagnetic generators presented in the literature are reviewed including large scale discrete devices and wafer-scale integrated versions. Electrostatic generators utilize the relative movement between electrically isolated charged capacitor plates to generate energy. The work done against the electrostatic force between the plates provides the harvested energy. Electrostatic-based generators are reviewed under the classifications of in-plane overlap varying, in-plane gap closing and out-of-plane gap closing; the Coulomb force parametric generator and electret-based generators are also covered. The coupling factor of each transduction mechanism is discussed and all the devices presented in the literature are summarized in tables classified by transduction type; conclusions are drawn as to the suitability of the various techniques.

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Energy harvesting vibration sources for microsystems applications

Power consumption is forecast by the International Technology Roadmap of Semiconductors (ITRS) to pose long-term technical challenges for the semiconductor industry. The purpose of this paper is threefold: (1) to provide an overview of strategies for powering MEMS via non-regenerative and regenerative power supplies; (2) to review the fundamentals of piezoelectric energy harvesting, along with recent advancements, and (3) to discuss future trends and applications for piezoelectric energy harvesting technology. The paper concludes with a discussion of research needs that are critical for the enhancement of piezoelectric energy harvesting devices.

/pdf/powering-mems-portable-devices-a-review-of-non-regenerative-30r5zxggdp.pdf

Powering MEMS portable devices—a review of non-regenerative and regenerative power supply systems with special emphasis on piezoelectric energy harvesting systems

http://ieeexplore.ieee.org/iel5/5/31047/01444179.pdf

Fundamentals of acoustics

This review provides a detailed overview of the energy harvesting technologies associated with piezoelectric materials along with the closely related sub-classes of pyroelectrics and ferroelectrics. These properties are, in many cases, present in the same material, providing the intriguing prospect of a material that can harvest energy from multiple sources including vibration, thermal fluctuations and light. Piezoelectric materials are initially discussed in the context of harvesting mechanical energy from vibrations using inertial energy harvesting, which relies on the resistance of a mass to acceleration, and kinematic energy harvesting which directly couples the energy harvester to the relative movement of different parts of a source. Issues related to mode of operation, loss mechanisms and using non-linearity to enhance the operating frequency range are described along with the potential materials that could be employed for harvesting vibrations at elevated temperatures. In addition to inorganic piezoelectric materials, compliant piezoelectric materials are also discussed. Piezoelectric energy harvesting devices are complex multi-physics systems requiring advanced methodologies to maximise their performance. The research effort to develop optimisation methods for complex piezoelectric energy harvesters is then reviewed. The use of ferroelectric or multi-ferroic materials to convert light into chemical or electrical energy is then described in applications where the internal electric field can prevent electron–hole recombination or enhance chemical reactions at the ferroelectric surface. Finally, pyroelectric harvesting generates power from temperature fluctuations and this review covers the modes of pyroelectric harvesting such as simple resistive loading and Olsen cycles. Nano-scale pyroelectric systems and novel micro-electro-mechanical-systems designed to increase the operating frequency are discussed.

/pdf/piezoelectric-and-ferroelectric-materials-and-structures-for-39myuuqvo7.pdf

Piezoelectric and ferroelectric materials and structures for energy harvesting applications

This paper presents a state-of-the-art review on a hot topic in the literature, i.e., vibration based energy harvesting techniques, including theory, modelling methods and the realizations of the piezoelectric, electromagnetic and electrostatic approaches. To minimize the requirement of external power source and maintenance for electric devices such as wireless sensor networks, the energy harvesting technique based on vibrations has been a dynamic field of studying interest over past years. One important limitation of existing energy harvesting techniques is that the power output performance is seriously subject to the resonant frequencies of ambient vibrations, which are often random and broadband. To solve this problem, researchers have concentrated on developing efficient energy harvesters by adopting new materials and optimising the harvesting devices. Particularly, among these approaches, different types of energy harvesters have been designed with consideration of nonlinear characteristics so that the frequency bandwidth for effective energy harvesting of energy harvesters can be broadened. This paper reviews three main and important vibration-to-electricity conversion mechanisms, their design theory or methods and potential applications in the literature. As one of important factors to estimate the power output performance, the energy conversion efficiency of different conversion mechanisms is also summarised. Finally, the challenging issues based on the existing methods and future requirement of energy harvesting are discussed.

A comprehensive review on vibration energy harvesting: Modelling and realization

Microfabricated electric generators, scavenging ambient mechanical energy, are potential power sources for autonomous systems. Described presently are the design, modeling, and implementation of a single-wafer floating-electrode electric microgenerator, integrating a micromechanical resonator and a number of electronic devices. Forming a plate of a variable capacitor, the resonator is responsible for converting mechanical vibration to electricity. A sense transistor and a diode bridge are integrated, respectively, for monitoring the "charging" of the floating electrode and for rectification. A lumped electromechanical model of the generator is developed and expressed in terms of a set of nonlinear coupled state equations that are numerically solved. For small-amplitude excitation, a circuit based on a set of linearized equations is developed. The generator is realized using a compatible combination of standard complementary metal-oxide-semiconductor (CMOS) "floating gate" process and a post-CMOS photoresist molded electroplating process. Adequate agreement between model predictions and measurement results was obtained

http://repository.ust.hk/ir/bitstream/1783.1-3044/1/integ.pdf

An Integrated Floating-Electrode Electric Microgenerator

Micro-structured cellular polydimethylsiloxane (PDMS) materials were prepared by a low-cost molding process allowing us to control geometry and sample size. Cellular structures are charged with a triangular quasi-static voltage with amplitudes between 1 kV and 4 kV and a frequency of 0.5 Hz fixed after having evaluated the conditions enhancing the piezoelectric response of the cellular PDMS. The piezo-electret PDMS material charged at room temperature has a piezoelectric coefficient d 33 of 350 pC/N, which is ten times larger than that of polyvinylidene fluoride. The high piezoelectric coefficient with a very low elastic modulus of 300 kPa makes these materials very useful for wearable device applications. The piezoelectric coefficient d 33 of the samples poled at high temperatures improves thermal stability but reduces PDMS piezo-electret piezoelectricity, which is explained by the structure's stiffness. These results are useful and allow us to set the conditions for the preparation of the piezo-electret materials according to desired applications.

http://iopscience.iop.org/article/10.1088/0964-1726/25/10/105027/pdf

Micro-structured PDMS piezoelectric enhancement through charging conditions

This work reports the bias and pressure sensitivity of AlGaN/GaN High Electron Mobility Transistors (HEMTs) sensing elements strategically placed on a pressure sensitive diaphragm clamped at its edges. The sensitivity was over 150 times greater in the weak inversion regime than in the strong inversion regime of the HEMT, leading to a drain current change of >38% when a pressure of 50 bar was applied. The sensitivity of the HEMT to pressure followed an exponential dependence from atmospheric pressure up to 80 bar, behaviour explained by the response of the density of a two-dimensional electron gas to pressure induced changes in the HEMT threshold voltage in the weak inversion regime. Finally, it was found that the sensitivity of the HEMT was maximum when it was situated in the middle of the diaphragm, whereas a device mounted over the clamping point showed less than 0.02% change in drain current when pressure change of 50 bar was applied.

Effect of bias conditions on pressure sensors based on AlGaN/GaN High Electron Mobility Transistor

This paper presents a Built-In-Self-Test (BIST) implementation of pseudo-random testing for MEMS. The technique is based on Impulse Response (IR) evaluation using pseudo-random Maximum--Length Sequences (MLS). The MLS approach is capable of providing vastly superior dynamic range in comparison to the straightforward technique using an impulse excitation and is thus an optimal solution for measurements in noisy environments and for low-power test signals. The use of a pseudo-random sequence makes the practical on-chip implementation very efficient in terms of the extra hardware required for on-chip testing. We will demonstrate the use of this technique for an on-chip fast and accurate broadband determination of MEMS behaviour, in particular for the characterisation of cantilever MEMS structures, determining their mechanical and thermal behaviour using just electrical tests.

Libor Rufer

Papers

An Integrated Floating-Electrode Electric Microgenerator

Micro-structured PDMS piezoelectric enhancement through charging conditions

Effect of bias conditions on pressure sensors based on AlGaN/GaN High Electron Mobility Transistor

On-Chip Pseudorandom MEMS Testing

Built-in-self-test techniques for MEMS