Vibration-driven energy harvesting device and vibration-driven energy harvester

A vibration-driven energy harvesting device includes: a first vibration-driven energy harvesting assembly constituted with a first fixed comb tooth portion and a first movable comb tooth portion that are interdigitated with each other; a second vibration-driven energy harvesting assembly constituted with a second fixed comb tooth portion and a second movable comb tooth portion that are interdigitated with each other; a first output electrode connected to the first vibration-driven energy harvesting assembly; and a second output electrode connected to the second vibration-driven energy harvesting assembly, wherein: an output of the first vibration-driven energy harvesting assembly is different from an output of the second vibration-driven energy harvesting assembly.

TECHNICAL FIELD

The present invention relates to a vibration-driven energy harvesting device and a vibration-driven energy harvester.

BACKGROUND ART

Among the energy harvesting technologies through which energy in the environment is harvested, a method whereby power is generated from an environmental vibration by a vibration-driven energy harvester has been attracting a great deal of attention in recent years (see, for instance, PTL1). The vibration-driven energy harvester described in PTL1 includes a fixed electrode and a movable electrode both having a comb tooth structure and generates power as vibration of the movable electrode causes a change in the area over which the fixed electrode and the movable electrode face opposite each other.

CITATION LIST

Patent Literature

PTL 1: Japanese Laid Open Patent Publication No. 2017-070163

SUMMARY OF INVENTION

Technical Problem

There is an issue yet to be effectively addressed in that when a power supply circuit, via which electric power is to be extracted from the vibration-driven energy harvester is connected to the vibration-driven energy harvester, the presence of the power supply circuit causes a change in the waveform of the output from the vibration-driven energy harvester, resulting in difficulty in accurately ascertaining the vibrating condition of the vibration-driven energy harvester.

Solution to Problem

According to a 1st aspect of the present invention, a vibration-driven energy harvesting device, comprises: a first vibration-driven energy harvesting assembly constituted with a first fixed comb tooth portion and a first movable comb tooth portion that are interdigitated with each other; a second vibration-driven energy harvesting assembly constituted with a second fixed comb tooth portion and a second movable comb tooth portion that are interdigitated with each other; a first output electrode connected to the first vibration-driven energy harvesting assembly; and a second output electrode connected to the second vibration-driven energy harvesting assembly, wherein: an output of the first vibration-driven energy harvesting assembly is different from an output of the second vibration-driven energy harvesting assembly.

According to a 2nd aspect of the present invention, a vibration-driven energy harvester used in the vibration-driven energy harvesting device according to the 1st aspect, comprises: the first vibration-driven energy harvesting assembly constituted with the first fixed comb tooth portion and the first movable comb tooth portion; and the second vibration-driven energy harvesting assembly constituted with the second fixed comb tooth portion and the second movable comb tooth portion, wherein: comb teeth included in the first fixed comb tooth portion and the second fixed comb tooth portion are each electrically insulated, with the first output electrode connected to the first fixed comb tooth portion and the second output electrode connected to the second fixed comb tooth portion, or comb teeth included in the first movable comb tooth portion and the second movable comb tooth portion are each electrically insulated, with the first output electrode connected to the first movable comb tooth portion and the second output electrode connected to the second movable comb tooth portion.

According to a 3rd aspect of the present invention, a vibration-driven energy harvester used in the vibration-driven energy harvesting device according to the 1st aspect, comprises: the first vibration-driven energy harvesting assembly constituted with the first fixed comb tooth portion and the first movable comb tooth portion; and the second vibration-driven energy harvesting assembly constituted with the second fixed comb tooth portion and the second movable comb tooth portion, wherein: the first fixed comb tooth portion and the second fixed comb tooth portion are electrically insulated, with the first output electrode connected to the first fixed comb tooth portion and the second output electrode connected to the second fixed comb tooth portion, or the first movable comb tooth portion and the second movable comb tooth portion are electrically insulated, with the first output electrode connected to the first movable comb tooth portion and the second output electrode connected to the second movable comb tooth portion.

According to a 4th aspect of the present invention, in the vibration-driven energy harvester according to the 3rd aspect, it is preferable that at least either of the first fixed comb tooth portion and the second fixed comb portion, which are electrically insulated from each other, includes at least two electrically insulated fixed comb tooth groups, or at least either of the first movable comb tooth portion and the second movable comb tooth portion, which are electrically insulated from each other, includes at least two electrically insulated movable comb tooth groups.

According to a 5th aspect of the present invention, in the vibration-driven energy harvester according to any one of the 2nd through 4th aspects, it is preferable to comprise: the first vibration-driven energy harvesting assembly constituted with the first fixed comb tooth portion and the first movable comb tooth portion; and the second vibration-driven energy harvesting assembly constituted with the second fixed comb tooth portion and the second movable comb tooth portion, wherein: a total quantity of comb teeth included in the first fixed comb tooth portion and the first movable comb tooth portion is different from a total quantity of comb teeth included in the second fixed comb tooth portion and the second movable comb tooth portion.

According to a 6th aspect of the present invention, in the vibration-driven energy harvester according to any one of the 2nd through 4th aspects, it is preferable to comprise: the first vibration-driven energy harvesting assembly constituted with the first fixed comb tooth portion and the first movable comb tooth portion; and the second vibration-driven energy harvesting assembly constituted with the second fixed comb tooth portion and the second movable comb tooth portion, wherein: a comb tooth height of at least either of the first fixed comb tooth portion and the first movable comb tooth portion, which are interdigitated with each other, is different from a comb tooth height of the second fixed comb tooth portion and the second movable comb tooth portion, which are interdigitated with each other.

According to a 7th aspect of the present invention, in the vibration-driven energy harvester according to any one of the 2nd through 4th aspects, it is preferable to comprise: the first vibration-driven energy harvesting assembly constituted with the first fixed comb tooth portion and the first movable comb tooth portion; and the second vibration-driven energy harvesting assembly constituted with the second fixed comb tooth portion and the second movable comb tooth portion, wherein: a distance between comb teeth in the first fixed comb tooth portion and the first movable comb tooth portion, which are interdigitated with each other, is different from a distance between comb teeth in the second fixed comb tooth portion and the second movable comb tooth portion, which are interdigitated with each other.

According to an 8th aspect of the present invention, in the vibration-driven energy harvester according to any one of the 2nd through 4th aspects, it is preferable to comprise: the first vibration-driven energy harvesting assembly constituted with the first fixed comb tooth portion and the first movable comb tooth portion; and the second vibration-driven energy harvesting assembly constituted with the second fixed comb tooth portion and the second movable comb tooth portion, wherein: a comb tooth length of the second fixed comb tooth portion or the second movable comb tooth portion is set smaller than a comb tooth length of the first fixed comb tooth portion and the first movable comb tooth portion, and the second fixed comb tooth portion and the second movable comb tooth portion are set to be a non-interdigitated state when no vibration is occurring.

According to a 9th aspect of the present invention, in the vibration-driven energy harvester according to any one of the 2nd through 8th aspects, it is preferable that electrets are formed at at least either of comb teeth in the first vibration-driven energy harvesting assembly and comb teeth in the second vibration-driven energy harvesting assembly that are interdigitated with each other.

According to a 10th aspect of the present invention, a vibration-driven energy harvester used in the vibration-driven energy harvesting device according to the 1st aspect, comprises: the first vibration-driven energy harvesting assembly constituted with the first fixed comb tooth portion and the first movable comb tooth portion; and the second vibration-driven energy harvesting assembly constituted with the second fixed comb tooth portion and the second movable comb tooth portion, wherein: electrets are formed at at least either of comb teeth in the first vibration-driven energy harvesting assembly and comb teeth in the second vibration-driven energy harvesting assembly that are interdigitated with each other; and an amount of electret charge per unit area in the first vibration-driven energy harvesting assembly is different from an amount of electret charge per unit area in the second vibration-driven energy harvesting assembly.

Advantageous Effects of Invention

According to the present invention, the vibrating condition of a vibration-driven energy harvester can be ascertained with accuracy.

DESCRIPTION OF EMBODIMENTS

The following is a description of an embodiment of the present invention, given in reference to drawings.FIG. 1schematically illustrates the structure of a vibration-driven energy harvesting device1achieved in the embodiment of the present invention, withFIG. 1(a)showing it in a plan view andFIG. 1(b)showing it in a sectional view taken over A-A inFIG. 1(a). The vibration-driven energy harvesting device1inFIG. 1comprises a vibration-driven energy harvester10with two vibration-driven energy harvesting assemblies11A and11B disposed thereat, a power output electrode20A connected to the vibration-driven energy harvesting assembly11A and a monitor electrode20B connected to the vibration-driven energy harvesting assembly11B. The vibration-driven energy harvester10is formed through a standard MEMS (micro-electromechanical systems) machining technology by using, for instance, an SOI (silicon-on-insulator) substrate. Such an SOI substrate may be formed by laminating a lower Si layer where a handle layer is formed, a SiO2layer where a BOX layer is formed and an upper Si layer where a device layer is formed, one on top of another.

The vibration-driven energy harvester10includes a base12, a fixed portion13at which fixed comb tooth electrodes13a,13b,13cand13dare formed, a movable portion14at which movable comb tooth electrodes14a,14b,14c,14dand14eare formed, and elastic support portions15that elastically support the movable portion14. The movable portion14is elastically supported relative to the base12via a pair of elastic support portions15. It is to be noted that the numbers of fixed comb tooth electrodes and movable comb tooth electrodes are not limited to those shown inFIG. 1.

As the A-A sectional view inFIG. 1(b)indicates, the movable portion14that includes the movable comb tooth electrodes14athrough14eand the fixed comb tooth electrodes13athrough13dare formed with an upper Si layer120of an SOI substrate, with aluminum layers131and141, which are electrically conductive layers, formed at the top surfaces of the movable portion14and the fixed comb tooth electrodes13athrough13drespectively. It is to be noted that the aluminum layers131and141do not need to be formed so as to cover the entire top surfaces of the movable portion14and the fixed comb tooth electrodes13athrough13d. The fixed comb tooth electrodes13athrough13dare fixed, via an SiO2layer110, to the base12constituted with a lower Si layer100.

At least at either of the fixed comb tooth electrode13athrough13dand the movable comb tooth electrodes14athrough14ethat are interdigitated, electrets are formed near the surfaces facing opposite the surfaces of the opposing electrodes. Thus, at least the surfaces of either the fixed comb tooth electrodes13athrough13dor the movable comb tooth electrodes14athrough14ethat face the opposing electrodes are individually electrically charged. It is to be noted that electrets may be formed by adopting, for instance, the method disclosed in Japanese Laid Open Patent Publication No. 2016-149914.

The vibration-driven energy harvester10generates power as an external vibration causes the movable comb tooth electrodes14athrough14eto vibrate along an x axis relative to the fixed comb tooth electrodes13athrough13d. The power output electrode20A is connected to the fixed comb tooth electrodes13a,13band13cconstituting the vibration-driven energy harvesting assembly11A, and is used as an output terminal through which an electric current generated by the vibration-driven energy harvesting assembly11A is output. The monitor electrode20B, connected to the fixed comb tooth electrode13d, is used as an output terminal through which an electric current generated by the vibration-driven energy harvesting assembly11B is output.

It is to be noted that in the embodiment illustrated inFIG. 1, the fixed comb tooth electrodes13athrough13din the vibration-driven energy harvester10are formed as elements separate from one another and are each electrically insulated. The comb tooth electrode group made up with the fixed comb tooth electrodes13athrough13cconstitutes a first fixed comb tooth portion whereas the fixed comb tooth electrode13dconstitutes a second fixed comb tooth portion. While the single fixed comb tooth electrode13dby itself constitutes the second fixed comb tooth portion in the example presented inFIG. 1, the second fixed comb tooth portion may be made up with a plurality of fixed comb tooth electrodes and in such a case, the monitor electrode20B will be connected to all these fixed comb tooth electrodes.

The fixed comb tooth electrodes13athrough13cin the first fixed comb tooth portion are each electrically connected to the power output electrode20A either inside the vibration-driven energy harvester10or outside the vibration-driven energy harvester10. In this context, “electrical connection inside the vibration-driven energy harvester10” means that wiring is formed entirely within the vibration-driven energy harvester10. “Electric connection outside the vibration-driven energy harvester10” means that the fixed comb tooth electrodes13athrough13care not electrically connected within the vibration-driven energy harvester10and that the fixed comb tooth electrodes13athrough13care electrically connected via wiring at a circuit substrate on which the vibration-driven energy harvester10is mounted. The fixed comb tooth electrode13din the second fixed comb tooth portion is connected to the monitor electrode20B.

While the fixed comb tooth electrodes13athrough13dare formed as separate elements in the example presented inFIG. 1, the fixed comb tooth electrodes13athrough13cmay instead be formed as an integrated unit, as illustrated inFIG. 2. In the example presented inFIG. 2, the fixed comb tooth electrodes13athrough13cconstituting the first fixed comb tooth portion are connected via a connecting portion13e, and the power output electrode20A is connected to the connecting portion13e. Other structural features are similar to those of the vibration-driven energy harvesting device1inFIG. 1. The structure inFIG. 1makes it possible to adjust the number of fixed comb tooth electrodes in the second fixed comb tooth portion, which is connected to the monitor electrode20B, in correspondence to the required level of monitor signal output or the purpose of use of the monitor signal.

FIG. 3presents a comparison example to the embodiment. While two outputs can be extracted via the power output electrode20A and the monitor electrode20B in the vibration-driven energy harvesting device1in the embodiment, a vibration-driven energy harvesting device30inFIG. 3(a)assumes a single-output structure that includes a single vibration-driven energy harvesting assembly, constituted with a set of a movable comb tooth portion31and a fixed comb tooth portion32, and an electric current is output from the single vibration-driven energy harvesting assembly.FIG. 3(b)indicates an amplitude x of the vibration of the movable comb tooth portion31, whereasFIG. 3(c)indicates an open-circuit voltage V present between the movable comb tooth portion31and the fixed comb tooth portion32in the vibrating condition shown inFIG. 3(b).

As the movable comb tooth portion31moves and the size of the area over which the movable comb tooth electrodes and the fixed comb tooth electrodes in the interdigitated state changes, a voltage is generated between the movable comb tooth electrodes and the fixed comb tooth electrodes due to movement of electric charges in the comb tooth electrodes. As a result, the open-circuit voltage V assumes a waveform synchronous with the amplitude x of the vibration of the movable comb tooth portion31, which has an amplitude proportional to the amplitude x. As the voltage output from a rectifier circuit rises to the level of the rated capacitor voltage when a power supply circuit33is connected to the vibration power generation device30, a behavior whereby an electric current flows via a resistor while the voltage is constant is observed. As a result, the voltage becomes saturated once it reaches the rated capacitor voltage level, and the voltage V between the terminals does not achieve a waveform having an amplitude proportional to the amplitude x at the movable comb tooth portion31, asFIG. 3(d)indicates. This means that the vibrating condition of the movable comb tooth portion31cannot be accurately ascertained based upon the voltage signal V inFIG. 3(d).

FIG. 4illustrates a state achieved by connecting the power supply circuit33to the vibration-driven energy harvesting device1shown inFIG. 2. AsFIG. 2indicates, the movable comb tooth electrodes in the vibration-driven energy harvesting assembly11A, which includes the power output electrode20A, and the movable comb tooth electrodes in the vibration energy harvesting assembly11B, which includes the monitor electrode20B, are disposed in the single movable portion14. Thus, even with the power supply circuit33connected, a voltage V2at the monitor electrode20B indicated inFIG. 4(d)assumes a waveform with an amplitude that is proportional to the amplitude x at the movable portion14indicated inFIG. 4(b). A voltage V1at the power output electrode20A, on the other hand, has a waveform such as that indicated inFIG. 4(c)due to the influence of the power supply circuit33.

While the fixed comb tooth portion32includes four fixed comb tooth electrodes in the structure described in reference toFIG. 3(a), the vibration-driven energy harvesting assembly11B inFIG. 4(a)includes a single fixed comb tooth electrode. This means that the amplitude of the voltage waveform inFIG. 4(d)is smaller than the amplitude of the voltage waveform inFIG. 3(c). In other words, in the vibration-driven energy harvesting device1according to the embodiment a smaller number of comb tooth electrodes is disposed in the vibration energy harvesting assembly11B for the monitor signal in comparison to the number of fixed comb tooth electrodes in the vibration energy harvesting assembly11A for electric power. In this manner, a monitor signal can be obtained while minimizing the extent to which the power output from the power output electrode20A is lowered.

FIG. 5is a block diagram of a power supply system achieved in conjunction with the vibration-driven energy harvesting device1, and presents an example of an application for the output signal provided through the monitor electrode20B. The output from the vibration-driven energy harvesting assembly11A for electric power is input to a voltage conversion circuit21in a power supply unit2. The power supply unit2includes an amplitude detection circuit22and a charge unit23in addition to the voltage conversion circuit21. The output from the vibration-driven energy harvesting assembly11B for the monitor signal is input to the amplitude detection circuit22and amplitude information indicating the amplitude detected by the amplitude detection circuit22is input to the voltage conversion circuit21. Based upon the amplitude information input thereto from the amplitude detection circuit22, the voltage conversion circuit21executes voltage conversion so that power from the vibration-driven energy harvesting assembly11A is output, for instance, with high efficiency, and outputs the voltage resulting from the conversion to the charge unit23. While accurate amplitude information on the output from the vibration-driven energy harvesting assembly11A must be obtained for these purposes, accurate amplitude information can be obtained by utilizing the output from the vibration-driven energy harvesting assembly11B in the embodiment.

FIG. 6illustrates Variation 1 of the vibration-driven energy harvesting device1.FIG. 6(a)provides a schematic diagram illustrating structures adopted for movable comb tooth electrodes and fixed comb tooth electrodes disposed in a vibration-driven energy harvester10. In the structure illustrated inFIG. 4, the number of fixed comb tooth electrodes in the vibration-driven energy harvesting assembly11B is set smaller than the number of fixed comb tooth electrodes in the vibration-driven energy harvesting assembly11A so as to ensure that the amplitude of the monitor signal obtained via the vibration-driven energy harvesting assembly11B is smaller than the amplitude of the output signal obtained via the vibration-driven energy harvesting assembly11A.

In Variation 1, different outputs are achieved for the vibration-driven energy harvesting assembly11A and the vibration-driven energy harvesting assembly11B by setting the distance between the fixed comb tooth electrodes and the movable comb tooth electrodes in the vibration-driven energy harvesting assembly11A different from the distance between the fixed comb tooth electrodes and the movable comb tooth electrodes in the vibration-driven energy harvesting assembly11B, thus achieving different capacitances at the vibration-driven energy harvesting assembly11A and the vibration-driven energy harvesting assembly11B, instead of disposing different numbers of fixed comb tooth electrodes in the vibration-driven energy harvesting assemblies11A and11B.FIG. 6(a)shows that a lateral width W2of the fixed comb tooth electrodes13cand13din the vibration-driven energy harvesting assembly11B is set smaller than a lateral width W1of the fixed comb tooth electrodes13aand13bin the vibration-driven energy harvesting assembly11A. As a result, distances d1and d2between the fixed comb tooth electrodes and the movable comb tooth electrodes in the vibration-driven energy harvesting assemblies11A and11B achieve a relationship expressed as; d1<d2. It is to be noted that instead of setting the lateral width of the fixed comb tooth electrodes13cand13din the vibration-driven energy harvesting assembly11B to W2, the lateral width of movable comb tooth electrodes14dand14emay be set to W2.

It is to be also noted that the vibration-driven energy harvesting assembly11A and the vibration-driven energy harvesting assembly11B include equal numbers of fixed comb tooth electrodes in the illustration provided inFIG. 6(a)so as to clearly demonstrate that the outputs from the vibration-driven energy harvesting assembly11A and the vibration-driven energy harvesting assembly11are differentiated by having different lateral widths set for the respective fixed comb tooth electrodes. However, even when different lateral widths are set for the fixed comb tooth electrodes, it is desirable to dispose fixed comb tooth electrodes in the vibration-driven energy harvesting assembly11B in a quantity such as that inFIG. 4(a), in order to maximize the output of the vibration-driven energy harvesting assembly11A.

When the lateral with W2of the fixed comb tooth electrodes13cand13dis set so that W2<W1as illustrated inFIG. 6(a), the distance between the side surfaces of the fixed comb tooth electrodes13cand13dand the side surfaces of the movable comb tooth electrodes14cthrough14eincreases, and under such circumstances, the capacitance at the vibration-driven energy harvesting assembly11B is smaller in comparison with the capacitance at the vibration-driven energy harvesting assembly11A. Since the extent of electric charge movement occurring as the movable portion14vibrates is greater where the capacitance is greater, an open-circuit voltage V4at the vibration-driven energy harvesting assembly11B inFIG. 6(c)has a smaller amplitude in comparison with the amplitude of an open-circuit voltage V3at the vibration-driven energy harvesting assembly11A shown inFIG. 6(b). The amplitudes of the open-circuit voltages V3and V4change proportionally to the amplitude of the vibration of the movable portion14.

It is to be noted that while an explanation has been given in reference toFIG. 6on an example in which a plurality of fixed comb tooth electrodes are divided into two groups as indicated inFIG. 2, the output of the vibration-driven energy harvesting assembly11A and the output of the vibration-driven energy harvesting assembly11B can be differentiated from each other when a plurality of fixed comb tooth electrodes are separated from one another as illustrated inFIG. 1, as well, by varying the distances between the comb teeth in the vibration-driven energy harvesting assembly11A and in the vibration-driven energy harvesting assembly11B.

FIG. 7illustrates Variation 2 of the vibration-driven energy harvesting device1.FIG. 7provides schematic diagrams illustrating structures adopted for movable comb tooth electrodes and fixed comb tooth electrodes disposed in a vibration-driven energy harvester10.FIG. 7(a)shows the vibration-driven energy harvester10in a plan view,FIG. 7(b)shows it in a sectional view taken over D1-D1andFIG. 7(c)shows it in a sectional view taken over D2-D2.

In Variation 2, the fixed comb tooth electrodes in the vibration-driven energy harvesting assembly11A assume a height different from the height of the fixed comb tooth electrodes in the vibration-driven energy harvesting assembly11B so as to achieve different size areas over which the movable electrodes and the fixed electrodes face opposite each other in the vibration-driven energy harvesting assembly11A and in the vibration-driven energy harvesting assembly11B, and ultimately to differentiate the outputs from the vibration-driven energy harvesting assemblies11A and11B. In the example presented inFIG. 7, a height H2(seeFIG. 7(c)) of fixed comb tooth electrodes13cand13din the vibration-driven energy harvesting assembly11B is set smaller than a height H1(seeFIG. 7(b)) of fixed comb tooth electrodes13aand13bin the vibration-driven energy harvesting assembly11A.

It is to be noted that the vibration-driven energy harvesting assembly11A and the vibration-driven energy harvesting assembly11B include equal numbers of fixed comb tooth electrodes in the illustration provided inFIG. 7so as to clearly demonstrate that the outputs from the vibration-driven energy harvesting assembly11A and the vibration-driven energy harvesting assembly11B are differentiated by having different heights set for the respective fixed comb tooth electrodes. However, even when different heights are set for the fixed comb tooth electrodes, it is desirable to dispose fixed comb tooth electrodes in the vibration-driven energy harvesting assembly11B in a quantity such as that inFIG. 4(a), in order to maximize the output of the vibration-driven energy harvesting assembly11A.

When the height H2of the fixed comb tooth electrodes13cand13dis set so that H2<H1, the area over which the fixed comb tooth electrodes and the movable comb tooth electrodes face opposite each other in the vibration-driven energy harvesting assembly11B is smaller than the area over which the fixed comb tooth electrodes and the movable tooth electrodes face opposite each other in the vibration-driven energy harvesting assembly11A, as illustrated inFIG. 7. This means that as the movable portion14becomes displaced, the area over which the fixed comb tooth electrodes and the movable comb tooth electrodes face opposite each other in the vibration-driven energy harvesting assembly11B changes to a smaller extent in comparison with the extent to which the area over which the fixed comb tooth electrodes and the movable comb tooth electrodes face opposite each other changes in the vibration-driven energy harvesting assembly11A. This results in the vibration-driven energy harvesting assembly11B having an open-circuit voltage lower than the open-circuit voltage at the vibration-driven energy harvesting assembly11A, as in Variation 1. The amplitudes of both open-circuit voltages change proportionally to the amplitude of the vibration of the movable portion14.

While the height of the fixed comb tooth electrodes is set to different values for the vibration-driven energy harvesting assembly11A and the vibration-driven energy harvesting assembly11B in the example presented inFIG. 7, different heights may be set for the movable comb tooth electrodes instead. In addition, whileFIG. 7(c)only shows the height of the fixed comb tooth electrode13dset to H2in the vibration-driven energy harvesting assembly11B inFIG. 7(c), the height of the movable comb tooth electrodes14dand14e, too, may be set to H2.

It is to be noted that while an explanation has been given in reference toFIG. 7on an example in which a plurality of fixed comb tooth electrodes are divided into two groups as indicated inFIG. 2, the output of the vibration-driven energy harvesting assembly11A and the output of the vibration-driven energy harvesting assembly11B can be differentiated from each other when a plurality of fixed comb tooth electrodes are separated from one another as illustrated inFIG. 1, as well, by differentiating the comb tooth height in the vibration-driven energy harvesting assembly11A from the comb tooth height in the vibration-driven energy harvesting assembly11B.

FIG. 8illustrates Variation 3 of the vibration-driven energy harvesting device1.FIG. 8(a)provides a schematic diagram illustrating structures adopted for movable comb tooth electrodes and fixed comb tooth electrodes disposed in a vibration-driven energy harvester10.FIG. 8(b)shows the waveform of an open-circuit voltage V5at the vibration-driven energy harvesting assembly11A, whereasFIG. 8(c)shows the waveform of an open-circuit voltage V6at the vibration-driven energy harvesting assembly11B. In Variation 3, fixed comb tooth electrodes take on different lengths in the vibration-driven energy harvesting assembly11A and in the vibration-driven energy harvesting assembly11B, instead of fixed comb tooth electrodes disposed in different numbers in the vibration-driven energy harvesting assembly11A and in the vibration-driven energy harvesting assembly11B, so as to differentiate the timing with which the movable electrodes and the fixed electrodes start to become interdigitated with each other and to ultimately achieve different outputs for the vibration-driven energy harvesting assemblies11A and11B.

InFIG. 8(a)a length L2of fixed comb tooth electrodes13cand13din the vibration-driven energy harvesting assembly11B is set smaller than a length L1of fixed comb tooth electrodes13aand13bin the vibration-driven energy harvesting assembly11A. It is to be noted that the vibration-driven energy harvesting assembly11A and the vibration-driven energy harvesting assembly11B include equal numbers of fixed comb tooth electrodes in the illustration provided inFIG. 8(a)so as to clearly demonstrate that the outputs from the vibration-driven energy harvesting assembly11A and the vibration-driven energy harvesting assembly11are differentiated by having different lengths set for the respective fixed comb tooth electrodes. However, even when different lengths are set for the fixed comb tooth electrodes, it is desirable to dispose fixed comb tooth electrodes in the vibration-driven energy harvesting assembly11B in a quantity such as that inFIG. 4(a), in order to maximize the output of the vibration-driven energy harvesting assembly11A.

When the lengths L1and L2of the fixed comb tooth electrodes13athrough13dare set so that L2<L1as indicated inFIG. 8(a), the open-circuit voltage V5at the vibration-driven energy harvesting assembly11A takes on a waveform such as that shown inFIG. 8(b)and the open-circuit voltage V6at the vibration-driven energy harvesting assembly11B takes on a waveform such as that shown inFIG. 8(c), both in synchronization with the amplitude of the vibration of the movable portion14.

The position taken by the movable portion14along the x direction inFIG. 8(a)represents a state in which the electret-induced electric power is in balance with the elastic force imparted from the elastic support portions15. In the state illustrated inFIG. 8(a), the fixed comb tooth electrodes13cand13dand the movable comb tooth electrodes14cthrough14eare not interdigitated in the vibration-driven energy harvesting assembly11B. x0denotes the amplitude of the vibration of the movable portion14. When t=0 inFIGS. 8(b) and 8(c), the movable portion14is at the in-balance position.

Following the time point t=0, as the movable portion14starts moving toward the positive side along the x-axis, the open-circuit voltage V5at the vibration-driven energy harvesting assembly11A starts to rise, as indicated inFIG. 8(b). However, a charge migration does not occur in the vibration-driven energy harvesting assembly11B, where the fixed comb tooth electrodes13cand13dand the movable comb tooth electrodes14cthrough14eare not interdigitated and thus, the open-circuit voltage V6at the vibration-driven energy harvesting assembly11B remains at 0.

As the movable portion14reaches a position indicated with the reference sign t1inFIG. 8(a)at a time point t1, the fixed comb tooth electrodes13cand13dand the movable comb tooth electrode14cthrough14ebecome interdigitated in the vibration-driven energy harvesting assembly11B. Following the time point t1, the areas over which the fixed comb tooth electrodes and the movable comb tooth electrodes face opposite each other increase both in the vibration-driven energy harvesting assembly11A and in the vibration-driven energy harvesting assembly11B, causing both the open-circuit voltage V5and the open-circuit voltage V6to rise.

As the movable portion14moves upward in the figure, the amplitude of its displacement reaches x0at a time point t2. At this time, the open-circuit voltages V5and V6at the vibration-driven energy harvesting assemblies11A and11B each climb to a positive peak. Beyond the time point t2, the movable portion14moves toward the negative side along the x axis, and thus, the areas over which the fixed comb tooth electrodes and the movable comb tooth electrodes face opposite each other decrease both in the vibration-driven energy harvesting assembly11A and in the vibration-driven energy harvesting assembly11B. As a result, the open-circuit voltages V5and V6decrease.

After the movable portion14starts moving downward in the figure, the area over which the fixed comb tooth electrodes and the movable comb tooth electrodes face each other in the vibration-driven energy harvesting assembly11B is reduced to 0 at a time point t3, and since the area remains at 0, the open-circuit voltage V6remains at 0 beyond the time point t3. The area over which the fixed comb tooth electrodes and the movable comb tooth electrodes face opposite each other continues to decrease in the vibration-driven energy harvesting assembly11A, which causes the open-circuit voltage V5to decrease while sustaining a positive value. Then, at a time point t4, the movable portion14reaches an in-balance position, and beyond the time point t4, the direction along which the electric charge migrates is reversed and the open-circuit voltage V5thus takes on a negative value.

Since the length L2of the fixed comb tooth electrodes13cand13din the vibration-driven energy harvesting assembly11B is set smaller than the length L1of the fixed comb tooth electrodes13aand13bin the vibration-driven energy harvesting assembly11A as described above, V6≠0 is true for the open-circuit voltage V6only over a range near the highest peak of the open-circuit voltage V5. Namely, a monitor signal is output only over a range around the maximum output peak, in which the fixed comb tooth electrodes13cand13dand the movable comb tooth electrodes14cthrough14eare in an interdigitated state in the vibration-driven energy harvesting assembly11B.

While the comb tooth length at the fixed comb tooth electrodes is set to L1in the vibration-driven energy harvesting assembly11A and the comb tooth length at the fixed comb tooth electrodes is set to L2in the vibration-driven energy harvesting assembly11B in the structure illustrated inFIG. 8, comb tooth lengths at the movable tooth electrodes may instead be set to L1and L2. For instance, the length of the movable comb tooth electrodes14athrough14cmay be set to L1and the length of the movable comb tooth electrodes14dand14emay be set L2.

It is to be noted that while an example in which a plurality of fixed comb tooth electrodes are divided into two groups, as indicated inFIG. 2, has been explained in reference toFIG. 8, the output from the vibration-driven energy harvesting assembly11A and the output from the vibration-driven energy harvesting assembly11B can be differentiated in a structure with a plurality of fixed comb tooth electrodes formed as elements separate from one another as illustrated inFIG. 1, as well, by setting the comb tooth length of the fixed comb tooth electrodes in the vibration-driven energy harvesting assembly11B smaller than the comb tooth length of the fixed comb tooth electrodes in the vibration-driven energy harvesting assembly11A and by setting the vibration-driven energy harvesting assembly11B to be in a non-interdigitated state when there is no vibration.

FIG. 9illustrates Variation 4 of the vibration-driven energy harvesting device1. While the movable comb tooth electrodes in the vibration-driven energy harvesting assemblies11A and11B are all disposed at the movable portion14on the right side thereof in the structure illustrated inFIG. 2, movable comb tooth electrodes14dand14ein the vibration-driven energy harvesting assembly11B are disposed on the left side of the movable portion14as indicated in the figure in Variation 4. The vibration-driven energy harvesting assembly11A is configured with movable comb tooth electrodes14athrough14ddisposed on the right side of the movable portion14and fixed comb tooth electrodes13athrough13cthat are interdigitated with the movable comb tooth electrodes14athrough14d. The vibration-driven energy harvesting assembly11B is configured with the movable comb tooth electrodes14eand14fdisposed on the left side of the movable portion14and a fixed comb tooth electrode13dthat is interdigitated with the movable comb tooth electrodes14eand14f.

As the movable portion14becomes displaced toward the positive side along the x axis, the area over which the movable comb tooth electrodes14athrough14dand the fixed comb tooth electrodes13athrough13cface opposite each other in the vibration-driven energy harvesting assembly11A increases and the area over which the movable comb tooth electrodes14eand14fface opposite the fixed comb tooth electrode13din the vibration-driven energy harvesting assembly11B decreases. In contrast, as the movable portion14becomes displaced toward the negative side along the x axis, the area over which the movable comb tooth electrodes14athrough14dand the fixed comb tooth electrodes13athrough13cface opposite each other in the vibration-driven energy harvesting assembly11A decreases and the area over which the movable comb tooth electrodes14eand14fface opposite the fixed comb tooth electrode13din the vibration-driven energy harvesting assembly11B increases. This means that while the phase of the waveform of the open-circuit voltage at the vibration-driven energy harvesting assembly11B is offset by 180° relative to the waveform of the open-circuit voltage at the vibration-driven energy harvesting assembly11A, the waveforms of the open-circuit voltages at the vibration-driven energy harvesting assemblies11A and11B are both synchronous with the amplitude of the movable portion14. Namely, a signal output from the monitor electrode20B in the vibration-driven energy harvesting assembly11B can be utilized as a monitor signal for monitoring the power output from the vibration-driven energy harvesting assembly11A, as in the embodiment described earlier.

It is to be noted that while a plurality of fixed comb tooth electrodes are divided into two groups in the example described in reference toFIG. 9, a plurality of fixed comb tooth electrodes may instead be formed as elements separate from one another, as inFIG. 1, and the separate fixed comb tooth electrodes may then be disposed, one group on the right side and the other group on the left side of the movable portion14in the figure.

FIG. 10illustrates Variation 5 of the vibration-driven energy harvesting device1. While the vibration-driven energy harvester10in the embodiment and the variations thereof described above adopts a structure in which the movable comb tooth electrodes are displaced relative to the fixed comb tooth electrodes within a single plane, an alternative structure, in which movable comb tooth electrodes undergo out-of-plane vibration, as shown inFIG. 10, may be adopted.

On a base12formed with a lower Si layer100of an SOI substrate, a first fixed comb tooth portion41A, a second fixed comb tooth portion41B and a cantilever42, all formed with an upper Si layer120, are disposed via an SiO2layer110inFIG. 10. At the cantilever42, a first movable comb tooth portion40A to interdigitate with the first fixed comb tooth portion41A and a second movable comb tooth portion40B to interdigitate with the second fixed comb tooth portion41B are formed. A power output electrode20A is connected to the first fixed comb tooth portion41A, whereas a monitor electrode20B is connected to the second fixed comb tooth portion41B. It is to be noted that the figure does not include an illustration of the conductive layers (equivalent to the aluminum layers131and141inFIG. 1) formed at the upper surfaces of the first and second movable comb tooth portions40A and40B, the first and second fixed comb tooth portions41A and41B and the cantilever42.

A vibration-driven energy harvesting assembly11A is configured with the first movable comb tooth portion40A and the first fixed comb tooth portion41A and a vibration-driven energy harvesting assembly11B is configured with the second movable comb tooth portion40B and the second fixed comb tooth portion41B. As in the first embodiment, electrets are disposed at electrode surfaces. As an external vibration is applied to the vibration-driven energy harvester10, the cantilever42becomes flexed along the z direction, causing the first and second movable comb tooth portions40A and40B to vibrate along the z direction relative to the first and second fixed comb tooth portions41A and41B. Since the number of comb tooth electrodes disposed in the vibration-driven energy harvesting assembly11B is smaller than the number of comb tooth electrodes disposed in the vibration-driven energy harvesting assembly11A, the output from the vibration-driven energy harvesting assembly11B is smaller than the output from the vibration-driven energy harvesting assembly11A.

It is to be noted that while the amplitudes of the outputs are differentiated by disposing the fixed comb tooth electrodes and movable comb tooth electrodes in the vibration-driven energy harvesting assembly11A in a total quantity different from the total quantity of the fixed comb tooth electrodes and movable comb tooth electrodes disposed in the vibration-driven energy harvesting assembly11B in the structure illustrated inFIG. 10, the structure in which out-of-plane vibration is induced allows the output amplitudes to be differentiated instead by setting different lengths for the fixed comb tooth electrodes and the movable comb tooth electrodes in the vibration-driven energy harvesting assemblies11A and11B. It will be also obvious that different output amplitudes will be achieved by setting the distance between the fixed comb tooth electrodes and the movable comb tooth electrodes that are interdigitated with each other in the vibration-driven energy harvesting assembly11A different from distance between the fixed comb tooth electrodes and the movable comb tooth electrodes that are interdigitated with each other in the vibration-driven energy harvesting assembly11B, as in Variation 1 (seeFIG. 6).

The levels of the outputs from the vibration-driven energy harvesting assemblies11A and11B are each dependent upon the electrode distance between the fixed comb tooth electrodes and the movable comb tooth electrodes, the area over which the fixed comb tooth electrodes and the movable comb tooth electrodes face opposite each other and the amounts of electric charges in the electrets formed at the fixed comb tooth electrodes or the movable comb tooth electrodes. In the embodiment and the variations thereof described above, the areas over which the fixed comb tooth electrodes and the movable comb tooth electrodes face opposite each other in the vibration-driven energy harvesting assemblies11A and11B are differentiated by setting different lengths or heights for the fixed comb tooth electrodes, and the distances between the comb tooth electrodes in the vibration-driven energy harvesting assemblies11A and11B are differentiated from each other by setting different widths for the fixed comb tooth electrodes. As an alternative, the output from the vibration-driven energy harvesting assembly11A and the output from the vibration-driven energy harvesting assembly11B can be differentiated from each other by differentiating an electret charge per unit area in the vibration-driven energy harvesting assembly11A from an electret charge per unit area in the vibration-driven energy harvesting assembly11B.

In the embodiment and the variations thereof described above, a plurality of fixed comb tooth electrodes are divided into two electrically insulated fixed comb tooth portions. As an alternative, a plurality of movable comb tooth electrodes disposed in a single movable portion14may be divided into two electrically insulated movable comb tooth portions, or a plurality of movable comb tooth electrodes disposed at a single movable portion14may be formed so that they are electrically insulated from one another.

In the example presented inFIG. 11, movable comb tooth electrodes14athrough14c, among movable comb tooth electrodes14athrough14eformed at the movable portion14, constitute a movable comb tooth portion belonging to the vibration-driven energy harvesting assembly11A and the movable comb tooth electrodes14dand14econstitute a movable comb tooth portion belonging to the vibration-driven energy harvesting assembly11B. At the movable portion14, an upper Si layer120that forms the movable comb tooth electrodes14athrough14cis separated from an upper Si layer120that forms the movable comb tooth electrodes14dand14e, and the movable comb tooth electrodes14athrough14care electrically insulated from the movable comb tooth electrodes14dand14e. A power output electrode20A is connected to the movable comb tooth electrodes14athrough14cand a monitor electrode20B is connected the movable comb tooth electrodes14dand14e.

FIG. 12presents an example in which a plurality of movable comb tooth electrodes14athrough14edisposed at a single movable portion14are formed so that they are electrically insulated from one another. The movable comb tooth electrodes14athrough14eare formed upon a lower Si layer100via an SiO2layer110, and the movable comb tooth electrodes14athrough14eare formed with upper Si layers120separated from one another and electrically insulated from one another. A power output electrode20A is connected to the movable tooth electrodes14athrough14cand a monitor electrode20B is connected to the movable comb tooth electrodes14dand14e. The power output electrode20A and the monitor electrode20B may be connected within the vibration-driven energy harvester10or their connections may be achieved outside the vibration-driven energy harvester10. The structure shown inFIG. 12, too, allows the number of movable comb tooth electrodes, to which the monitor electrode20B is connected, to be adjusted in correspondence to the output level required of the monitor signal or the purpose of use for the monitor signal, as does the structure shown inFIG. 1.

The vibration-driven energy harvesting device1and the vibration-driven energy harvester10having been described in reference to the embodiment and the variations thereof above may be summarized as follows.

(1) AsFIG. 1andFIG. 2illustrate, the vibration-driven energy harvesting device1includes a vibration-driven energy harvester10having fixed comb tooth electrodes13athrough13cand movable tooth electrodes14athrough14d, together constituting a vibration-driven energy harvesting assembly11A, and a fixed comb tooth electrode13dand movable comb tooth electrodes14dand14e, together constituting a vibration-driven energy harvesting assembly11B. The number of fixed comb tooth electrodes13athrough13cto which a power output electrode20A is connected is set different from the number of fixed comb tooth electrodes13dto which a monitor electrode20B is connected, i.e., the total number of fixed comb tooth electrodes and movable comb tooth electrodes disposed in the vibration-driven energy harvesting assembly11A is differentiated from the total number of fixed comb tooth electrodes and movable comb tooth electrodes disposed in the vibration-driven energy harvesting assembly11A, so as to achieve different outputs for the vibration-driven energy harvesting assembly11A and the vibration-driven energy harvesting assembly11B. The expression “different outputs” in this context means that the amplitude of a monitor signal output from the vibration-driven energy harvesting assembly11B is different from the amplitude of an output signal from the vibration-driven energy harvesting assembly11A.

The structure that allows a monitor signal, synchronous with the amplitude of the vibration of the movable portion14, to be output independently from the power output as described above, makes it possible to accurately ascertain the vibrating condition of the vibration-driven energy harvester10based upon the monitor signal. For instance, the use of such a monitor signal makes it possible to utilize the generated power with high efficiency, as has been explained in reference toFIG. 5.

It is to be noted that while an explanation has been given on an example in which a monitor signal is utilized for the purpose of making use of generated power efficiently, the monitor signal may instead be used as a trigger signal for failure detection or as an input signal provided to a protection circuit in the event of an error.

(2) If the individual fixed comb tooth electrodes13athrough13ddisposed in the vibration-driven energy harvesting assemblies11A and11B are electrically insulated from one another in the vibration-driven energy harvester10as illustrated inFIG. 1, the power output electrode20A is connected to the fixed comb tooth electrodes13athrough13cin the vibration-driven energy harvesting assembly11A and the monitor electrode20B is connected to the fixed comb tooth electrode13din the vibration-driven energy harvesting assembly11B. In addition, if the individual movable comb tooth electrodes14a through14edisposed in the vibration-driven energy harvesting assemblies11A and11B are electrically insulated from one another as illustrated inFIG. 12, the power output electrode20A is connected to the movable comb tooth electrodes14athrough14din the vibration-driven energy harvesting assembly11A and the monitor electrode20B is connected the movable comb tooth electrodes14dand14ein the vibration-driven energy harvesting assembly11B.

In comparison to the structure illustrated inFIG. 2, in which the vibration-driven energy harvester10is configured with comb tooth electrodes separated into a group of comb tooth electrodes to be connected to the power output electrode20A and a group of comb tooth electrode(s) to be connected to the monitor electrode20B, a structure such as that shown inFIG. 1with all the fixed comb tooth electrodes formed so that they are electrically insulated from one another better facilitates adjustment of the number of comb tooth electrodes, to which the monitor electrode20B is connected, in correspondence to the level of output required of the monitor signal or in correspondence to the purpose of use of the monitor signal.

It is to be noted that in the example presented inFIG. 1orFIG. 12, the individual fixed comb tooth electrodes13athrough13dor the individual movable comb tooth electrodes14athrough14eare electrically insulated from one another. In the example presented inFIG. 2, the fixed comb tooth electrodes13athrough13cin the vibration-driven energy harvesting assembly11A are formed as an electrically integrated unit, whereas in the example presented inFIG. 11, the movable comb tooth electrodes14athrough14cin the vibration-driven energy harvesting assembly11A and the movable comb tooth electrodes14dand14ein the vibration-driven energy harvesting assembly11B are formed as electrically integrated units. However, the present invention is not limited to these structural examples, and it may adopt alternative structures such as those illustrated inFIG. 13(a)andFIG. 13(b).

In the example presented inFIG. 13(a), a power output electrode20A is connected to fixed comb tooth electrodes13athrough13c, i.e., a fixed comb tooth electrode13cand a fixed comb tooth group G1made up with fixed comb tooth electrodes13aand13bformed as an electrically integrated unit. It is to be noted that the fixed comb tooth electrode13c, too, may be considered to be a fixed comb tooth group constituted with a single fixed comb tooth electrode. Fixed comb tooth electrodes13dand13eto which a monitor electrode20B is connected are electrically insulated from each other, and the fixed comb tooth electrode13dand the fixed comb tooth electrode13e, too, may each be regarded to constitute a fixed comb tooth group.

In the example presented inFIG. 13(b), among movable comb tooth electrodes14athrough14dto which a power output electrode20A is connected, the movable comb tooth electrodes14aand14b(fixed comb tooth group G2) are formed as an electrically integrated unit, whereas the movable comb tooth electrodes14cand14dare each electrically insulated. The movable comb tooth electrodes14cand14deach constitute a movable comb tooth group made up with a single movable comb tooth electrode. Movable comb tooth electrodes14eand14f, to which a monitor electrode20B is connected, are electrically insulated from each other. The movable comb tooth electrodes14eand14f, too, each constitute a movable comb tooth group made up with a single movable comb tooth electrode.

As described above, at least either of a first fixed comb tooth portion (the fixed comb tooth electrodes13athrough13c) and a second fixed comb tooth portion (the fixed comb tooth electrodes13dand13e), which are electrically insulated from each other, includes at least two fixed comb tooth groups electrically insulated from each other in the example presented inFIG. 13(a). In addition, at least either of a first movable comb tooth portion (the movable comb tooth electrodes14athrough14d) and a second movable comb tooth portion (the movable comb tooth electrodes14eand14f), which are electrically insulated from each other, includes at least two movable comb tooth groups electrically insulated from each other in the example presented inFIG. 13(b).

(3) The output of the vibration-driven energy harvesting assembly11A and the output of the vibration-driven energy harvesting assembly11B may be differentiated from each other by disposing fixed comb tooth electrodes and movable comb tooth electrodes in the vibration-driven energy harvesting assembly11A in a total quantity different from the total quantity of fixed comb tooth electrodes and movable comb tooth electrodes disposed in the vibration-driven energy harvesting assembly11B as illustrated inFIGS. 1, 2 and 9 through 13, by setting different comb tooth heights for at least either of the fixed comb tooth electrodes and the movable comb tooth electrodes that are interdigitated in the vibration-driven energy harvesting assembly11A and in the vibration-driven energy harvesting assembly11B as has been explained in reference to Variation 2 (seeFIG. 7), or by setting different distances between the fixed comb tooth electrodes and the movable comb tooth electrodes that are interdigitated in the vibration-driven energy harvesting assembly11A and in the vibration-driven energy harvesting assembly11B, as has been explained in reference to Variation 1 (seeFIG. 6).

(4) In addition, in the vibration-driven energy harvester10adopting a structure such as that illustrated inFIG. 8, in which movable comb tooth electrodes are induced to engage in in-plane vibration relative to fixed comb tooth electrodes, the comb tooth length of the fixed comb tooth electrodes13dand13din the vibration-driven energy harvesting assembly11B may be set smaller than the comb tooth length of the fixed comb tooth electrodes13aand13bin the vibration-driven energy harvesting assembly11A, or the comb tooth length of the movable comb tooth electrodes14dand14ein the vibration-driven energy harvesting assembly11B may be set smaller than the comb tooth length of the movable tooth electrodes14aand14bin the vibration-driven energy harvesting assembly11A such that the fixed comb tooth electrodes13dand13dand the movable comb tooth electrodes14dand14emay be set to be in a non-interdigitated state when there is no vibration.

The structure described above allows a monitor signal to be output only over a range near the maximum output peak at which the fixed comb tooth electrodes13cand13dand the movable comb tooth electrodes14cthrough14eenter an interdigitated state in the vibration-driven energy harvesting assembly11B. This means that the timing with which the output from the power output electrode20A peaks can be detected based upon the monitor signal.

(5) Furthermore, the amplitudes of the outputs of the vibration-driven energy harvesting assembly11A and the vibration-driven energy harvesting assembly11B can be differentiated from each other by ensuring that the amount of electret charge per unit area at the vibration-driven energy harvesting assembly11A is different from the amount of electret charge per unit area at the vibration-driven energy harvesting assembly11B, as has been explained in reference to Variation 5. In this case, a waveform with an amplitude proportional to the amplitude of the vibration of the movable unit14is achieved with respect to the output from the vibration-driven energy harvesting assembly11B, which makes it possible to utilize the output of the vibration-driven energy harvesting assembly11B as a monitor signal.

It is to be noted that while comb tooth electrodes are formed by using the upper Si layer120of the SOI substrate and electrical insulation is achieved by physically separating the fixed comb tooth electrodes in the embodiment described in reference toFIGS. 1 and 2, electrical insulation may be achieved through a method other than this. For instance, comb teeth may be formed with quartz, a silicon layer may be formed at the comb teeth and comb tooth electrodes may then be formed by rendering the silicon layer into electrets. In such a case, the silicon layer may be formed in correspondence to each comb tooth as shown inFIG. 1, so as to form comb tooth electrodes that are electrically insulated from one another, or silicon layers may be formed in correspondence to two separate groups, as illustrated inFIG. 2, so as to form two groups of comb tooth electrodes electrically insulated from each other.

While an explanation has been given on an embodiment and variations thereof, the present invention is in no way limited to the particulars of these examples. Other modes conceivable within the scope of the technical teachings of the present invention are also within the scope of the present invention.

The disclosure of the following priority application is herein incorporated by reference: Japanese Patent Application No. 2018-105434, filed May 31, 2018

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