28-01-2011, 05:15 PM
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Abstract:
We developed an electrostatic micro power generator for low frequency energy harvesting applications. This micro power generator consists of a new vibration structure and new electret electrode. The advantage of this micro power generator is the high power generation structure of both the controlled gap
between electrodes and long-range moving at low frequency. This generator shows 40+W of power output at very low frequency vibration (2Hz, 0.4G).
INTRODUCTION:
Nowadays, micro power generators that harvest vibration energy and can become power sources for wireless sensors and portable applications are drawing much attention [1-3]. There are three common mechanisms: piezoelectric, electromagnetic, and electrostatic, each of which has various examples reported in literature. The problem that each approach must address is that low frequency vibration such as human motion is only sufficient to generate an extremely low output of power.To overcome this difficult challenge, we developed a new electrostatic micro power generator that 1) can enable both separation gap control and long-range movement at low frequency, and 2) can increase the number of times power generation (electrostatic induction) is done. To achieve these we developed the following technologies: 1) a micro power generator consisting of microball bearings to roll with the separation gap control and to firmly keep the separation gap (Microactuator using microball bearing has been reported ), and 2) a new electrets structure to accommodate miniaturization.
DESIGN & FABRICATION:
2.1 Micro Power Generator for Low Frequency:-
In Fig. 1 the design of the electrostatic micro power generator is shown. The separated spring and mass manage to vibrate long-range moving (about 2 cm) at low frequency; separated microball bearings manage to keep the gap properly (10-50 μm). This generator consists of fixed parts that have two glass substrates with collector electrodes and moving parts that have proof mass and two Si substrates with new electret structure. This electret structure obtains enough surface potential to be suitable forminiaturization. A pair of collector electrodes is on the same substrate to keep wiring simple. Microballs which firmly support the moving parts can roll along each deep trench between the glass substrates and silicon substrates. Reports about current electrostatic micro power generation describe how it is difficult to vibrate at low frequency because only one or two springs keep the moving part moving (several tens Hz. level) [5,6]. Resonance frequency ωres is given by ωres = (k/m)1/2 (1) where k is the spring constant an m is the mass. To vibrate at low resonance frequency, the spring constant needs to be low. On the other hand, a spring with a low spring constant can’t retain the moving part well, and can’t keep a narrow gap to prevent an electrode break. Therefore, to keep a firm separation
Fig. 1. New structure of electrostatic micro power generator supported on microball bearings. It is possible to apply to low frequency vibration (low spring constant) because it supports a proof mass with microball bearings. gap, this design has a structure that effectively keeps the separation gap with a microball. This design is not only suitable for good gap control, but also suitable for vibration and long-range movement at low frequency with a spring that has a low spring constant.
Electret Electrode:-
Fig. 2 shows the principle of power generation with electrostatic induction [2,7,8]. The electret is a dielectric material that has a quasi-permanent electrical charge. The electrical charge is in the collector electrode facing the electret because of electrostatic induction. When the electret part moves over the other collector electrode from the electrode, electrical charges move the other electrode through the load R as power generation. Boland et al. show that load-matched output power Poptimal of the electret power generator using a rotationmodel is given by where is charge density, n is the number of electrodes, r is radius, f is frequency, ki is dielectric constant of electret, 0 is vacuum permittivity, d is thickness of electret, and g is separation gap [8]. To obtain large power output, 1) high charge density (surface potential), 2) high frequency (the number of
where σ is charge density, n is the number of electrodes, r is radius, f is frequency, ki is dielectric constant of electret, ε0 is vacuum permittivity, d is thickness of electret, and g is separation gap . To obtain large power output, 1) high charge density
(surface potential), 2) high frequency (the number of charge movements) and 3) narrowing the gap between electrodes are advantageous. For the electret electrode, 1) higher surface potential of electret, and 2) narrower electrode width are important to obtain higher power output from certain vibration energy input. Therefore, we developed a new electret structure that keeps high surface potential with minute electrode width usingSiO2.
EXPERIMENTAL SECTION:-
3.1 Experimental Setup:-
Fig. 5 is a photograph of our prototype. The dimension of the device is 20mm x 45mm, the size of the moving part is 24mm x 27mm. These electrodes keep a 40+m gap because of the 140+m depth trenches and 320+m microballs (Grade 25 (ABMA: American Bearing Manufacturers Association)). Fig. 6 is a schematic diagram of the experiment setup for power generation. Input vibration was set at 2Hz sine wave, and acceleration was 0.4G (the vibration of human walking motion at the waist).Shaker direction was the same direction as the gravity. Beginning state was the balanced state of the gravity for mass and the restoring force the spring exerts. In order to measure the output power for various external loads, a simpe measuring circuit with a voltmeter, two resistances, a capacitor, and two bridge rectifier circuits was employed.
CONCLUSION:-
We developed an electrostatic micro power generator for low frequency energy harvesting applications. This generator shows 40+W of power output at very low frequency vibration (2Hz). Technologies we developed are: 1) a micro power generator consisting of micro ball bearings to roll with the separation gap control and to keep separation gap constant, and 2) a new electric structure to accommodate miniaturization. The advantage of our design is the high power generation structure of both the controlled gap between electrodes and long-range moving, and the high surface potential electrets structure.
