S. Matova

VIBRATION ENERGY HARVESTING WITH ALUMINUM NITRIDE-BASED PIEZOELECTRIC DEVICES

This paper describes the measurement results of piezoelectric energy harvesters with aluminum nitride (AlN) as a piezoelectric material. AlN was chosen for its material properties and for its well-known sputter deposition process. For AlN devices a high optimum load resistance is required, which is favorable due to the high resulting voltage level. The output power harvested from mechanical vibrations has been measured on micromachined harvesters with different geometries. The resonance frequencies ranged from 200 up to 1200 Hz. The packaged devices had limited output powers and quality factors due to air damping caused by the package. A maximum output power of 60 µW has been measured on an unpackaged device at an acceleration of 2.0 g and at a resonance frequency of 572 Hz. The package of the harvester requires special attention, since air damping can significantly decrease the maximum power output.

Vacuum-packaged piezoelectric vibration energy harvesters: Damping contributions and autonomy for a wireless sensor system

This paper describes the characterization of thin-film MEMS vibration energy harvesters based on aluminum nitride as piezoelectric material. A record output power of 85 μW is measured. The parasitic-damping and the energy-harvesting performances of unpackaged and packaged devices are investigated. Vacuum and atmospheric pressure levels are considered for the packaged devices. When dealing with packaged devices, it is found that vacuum packaging is essential for maximizing the output power. Therefore, a wafer-scale vacuum package process is developed. The energy harvesters are used to power a small prototype (1 cm\3 volume) of a wireless autonomous sensor system. The average power consumption of the whole system is less than 10 μW, and it is continuously provided by the vibration energy harvester.

Shock induced energy harvesting with a MEMS harvester for automotive applications

In this work we report shock induced measurement and simulations on AlN based piezoelectric vibration energy harvesters. We compare the result with sinusoidal input vibrations, where we obtain a record power of 489 µW. The vacuum packaged harvesters have high quality factors and high sensitivity. We validate the potential of piezoelectric vibration harvesters for car tire applications by measurements and simulations.

First autonomous wireless sensor node powered by a vacuum-packaged piezoelectric MEMS energy harvester

This paper describes the experimental characterization of piezoelectric harvesters with different dimensions. We present a record level of generated power of 85 µW obtained from an unpacked device. We have developed a wafer-scale vacuum package which shows a 100–200 fold increase in power output compared with packaged devices under atmospheric pressure. A wireless sensor node was fully powered by a piezoelectric harvester. The average power consumption was less than 10 µW while it was operating at 15 seconds duty cycle.

Harvesting energy from airflow with a michromachined piezoelectric harvester inside a Helmholtz resonator

In this paper we report an airflow energy harvester that combines a piezoelectric energy harvester with a Helmholtz resonator. The resonator converts airflow energy to air oscillations which in turn are converted into electrical energy by a piezoelectric harvester. Two Helmholtz resonators with adjustable resonance frequencies have been designed - one with a solid bottom and one with membrane on the bottom. The resonance frequencies of the resonators were matched to the complementing piezoelectric harvesters during harvesting. The aim of the presented work is a feasibility study on using packaged piezoelectric energy harvesters with Helmholtz resonators for airflow energy harvesting. The maximum energy we were able to obtain was 42.2 νW at 20 m s\-1.

Effect of length/width ratio of tapered beams on the performance of piezoelectric energy harvesters

Tapering of the beams as a way to increase the generated output power of cantilever piezoelectric energy harvesters has gained popularity in recent years. The tapering increases the average strain in the beam and consequently the charge generated by the piezoelectric material. Different authors claim an improvement of up to 30% in the generated output power. We have investigated the possibility of using tapered beams in MEMS piezoelectric energy harvesters. Numerical simulations did not suggest any increase in the generated output power and the lack of improvement was confirmed in practice. With the help of the numerical simulations it was further found that the tapering does work but only for certain design configurations, namely for cantilevers with long slender beams. For cantilevers with short wide beams, the tapering has no significant effect on the output power of the harvester.

Shock Reliability of Vacuum-Packaged Piezoelectric Vibration Harvester for Automotive Application

This paper reports for the first time the shock reliability of vacuum-packaged piezoelectric vibration harvesters for automotive application. Experimental study on >80 devices shows that the failure is induced by the impact between seismic mass and glass package. The statistical results obtained for various types of harvesters have shown a trend of higher shock reliability for a higher resonance frequency. The influence of beam thickness on the shock reliability is then discussed. The trade-off between the power output and shock reliability is elaborated. To further improve the reliability to the level of automotive application, the idea of stepped cavity is discussed and implemented. The experimental results confirmed a significantly enhanced reliability increased by ~50%, up to maximum 2000 g, for the same type of device under shock excitations.

A piezoelectric vibration harvester based on clamped-guided beams

The paper addresses the design, modeling, fabrication and experimental results of a piezoelectric energy harvester based on clamped-guided beams. The design is featured by shorter mass displacement and higher reliability than cantilever beams. Two separate sets of capacitors allow exploiting both tensile and compressive stress at the same time. The maximum output power reaches about 20 μW at an input acceleration of 1.2 g. A tuning range of 3 Hz is demonstrated by varying the voltage across the capacitors. Nonlinear behavior observed at larger mass displacement is also discussed.

Improved mechanical reliability of MEMS piezoelectric vibration energy harvesters for automotive applications

This paper addresses the issue of the mechanical reliability of MEMS piezoelectric vibration harvesters aimed at powering tire pressure monitoring systems. These harvesters generate sufficient power for the targeted application. However, for bringing them to the automotive market, their mechanical reliability has to be optimized, particularly in terms of shock resilience. Experimentally verified methods for improving the mechanical reliability of such devices are showcased in this article. These methods concern both the design of the harvesters (introduction of stoppers in the package) and their manufacturing process (release method of the MEMS structure).

Large power amplification of a piezoelectric energy harvester excited by random vibrations

This paper reports the method and results of amplifying the power output of a piezoelectric energy harvester excited by random vibrations. The amplification is achieved by applying a dual-mass-spring system. A maximum power amplification of 80 times has been experimentally demonstrated. The generated power output from a piezoelectric energy harvester, when excited by as-measured random vibrations, amounts to 28.9 μW. This is sufficient to operate a battery-free Tire Pressure Monitoring System (TPMS) for wireless sensing of pressure and temperature in car tires. Thus, the piezoelectric energy harvester with power amplification is proved to be a viable solution to replace batteries in the TPMS application.