WO2011079486A1

WO2011079486A1 - Electromagnetic energy harvesting device

Info

Publication number: WO2011079486A1
Application number: PCT/CN2010/000604
Authority: WO
Inventors: Sheng Lai; Ming Li
Original assignee: Siemens Aktiegesellschaft
Priority date: 2009-12-30
Filing date: 2010-04-30
Publication date: 2011-07-07
Also published as: CN102118095A

Abstract

An electromagnetic energy harvesting device includes: a first vibratory structure (224) and a second vibratory structure(226). The first vibratory structure (224) is adapted to vibrate when excited by ambient vibration with low frequency, and transfers its vibration to the second vibratory structure (226). The second vibratory structure (226) is adapted to self-vibrate in a magnetic field at its high natural frequency, and generates magnetic flux change to induce electrical power.

Description

Electromagnetic Energy Harvesting Device

Field of the Invention

This invention relates to energy harvesting technology, and more particularly, relates to an electromagnetic energy harvesting device. Background of the Invention

Most wireless sensor nodes are powered by primary or secondary batteries. These take up a large proportion of the size, weight, and often the cost, of the nodes. The need to replace or recharge these batteries creates a significant maintenance burden. Provision of maintenance-free power would therefore greatly increase the feasibility of networks with widely distributed or very large numbers of nodes.

For this reason, energy harvesting devices that extract energy from ambient lights, temperature differences, and vibrations, have attracted attention from many researchers. Table 1 compares the estimated power and challenges of those potential power sources. Light, for instance, can be a significant source of energy, but it is highly dependant on the application and the exposure to which the device is subjected. Thermal energy, on the other hand, is limited because the temperature differentials across a chip are typically low. Vibration energy is a moderate source, but seems to be the most interesting because of its applicability and abundance. Several vibration- based energy harvesting techniques based on piezoelectric, electrostatic and electromagnetic (EM) transduction have been reported, and it has been shown that the energy levels obtained using these techniques are sufficient to power-up various types of wireless sensor nodes.

Table 1 Comparison of various energy sources Maximum voltage and generated electrical power from a vibrating . mass are strongly dependent on external vibration frequency, dropping dramatically at low frequencies (< 100 Hz). However, it is at these low frequencies where most ambient vibrations exist. Most reported vibration energy harvesting devices are only capable ' of operating at frequencies of several kHz, since the generated power levels at lower frequencies are not sufficient to power wireless sensors.

Many researchers have attempted various techniques to harvest electrical energy from ambient vibrations. These techniques are based on a basic design that utilizes a mass-spring-damper system where a magnet and coil move with respect to each other under the influence of an external vibration. For these types of devices, maximum power is achieved when at resonance, i.e., the input vibration frequency matches the mechanical resonant frequency of the devices.

S. P. Beeby, R. N. Torah, and et al. (2007) presented a micro (0.15 cm3 of volume) electromagnetic generator utilizing four magnets arranged on an etched cantilever with a wound coil located within the moving magnetic field. Magnet size and coil properties of this micro generator are optimized enabling production of 46 μ\ν in a resistive load of 4ΚΩ from just 0.59 m s-2 acceleration levels at its resonant frequency of 52Hz. A voltage of 428 mV rms was obtained from the micro generator with a 2300 turn coil which had proved sufficient for subsequent rectification and voltage step-up circuitry. Nevertheless, the said power and voltage outputs of this device decrease quickly when ambient vibration frequency departs from its resonant value (52 Hz), and reach zero when the frequency is below 48Hz.

P. H. Wang, X. H. Dai, and et al. (2007) have designed a vibration-based electromagnetic micro power generator fabricated by MEMS technology. It consists of a neodymium iron boron (NdFeB) permanent magnet attached to the centre of a copper planar spring, and a lower two-layer copper coil on glass substrate. The micro generator prototype, fabricated according to this design, can generate 60 mV ac peak- peak with an input frequency of 121.25Hz and at input acceleration of 1.5g (g = 9.8 m s2). But similarly, the voltage output performance of this device deteriorates dramatically with decreases in its working frequency. For example, 10 mV with 110Hz, only 4 mV with 100 Hz, and nearly 0 mV with frequencies under 10Hz.

There are two reasons why the aforementioned micro power generators can only produce tiny power under low frequency vibrations. Firstly, the natural frequencies of the said devices are designed to be inherently high (52Hz and 121.25Hz) so that they produce more power when they resonate with ambient vibrations at those frequencies. However, such a resonant vibration cannot be achieved under lower frequency conditions. Under such conditions the excited vibrations of the devices are far from sufficient to produce an adequate power output. Secondly, if the natural frequencies of the devices are designed to be low, then even though the devices vibrate resonantly under low frequency conditions much more easily, they still produce little power output because power output decreases with the cube of the reduction of vibration frequency. Accordingly, it is really tough to decide whether these micro power generators should be set to a high or low natural frequency.

Makoto Mizuno and Derek G. Chetwynd (2003) have reported a microgenerator where cantilever beams, having coils patterned on their surfaces, resonate in the presence of a divergent magnetic field. The biggest innovation in this design is that one of two dimensional arrays of many micro-cantilevers could share the field from one large permanent magnet, thus the electrical power output could be multiplied significantly. A mesoscale test bed was constructed using an NdFeB magnet and a single, 25-mm-long cantilever with a four-turn winding. A maximum power of only 320 μν was generated for 0.64 μηι vibrations at 700Hz. For the device modeled in multi-cantilever, for instance, a two dimensional arrays of 1000 microgenerators might then provide about 0.35V. However, trying to improve generation power and mass density by using the multi-cantilever concept faces severe practical difficulties of, for example, matching the resonance of each micro-cantilever. According to the authors, no further work on this type of electromagnetic resonance microgenerator is recommended, and there is unlikely to be a satisfactory design.

Haluk Kulah and Khalil Najafi (2008) extended this idea of resonating cantilevers, and devised a method of up-converting low-frequency mechanical vibrations to induce higher frequency resonance in the cantilevers. Their design employs a bulk NdFeB permanent magnet (PM) proof-mass suspended with a compliant in-plane spring system that is designed to resonate at low frequencies. When the PM moves, it magnetically catches and releases the tips of the smaller cantilevers, exciting them into resonance at a higher frequency. Once the cantilevers begin oscillating, windings on the beam couple with the field from the PM. Test results are reported from a milliscale prototype implementation, which uses a single 50-mm-long styrene cantilever with a three-turn copper coil and an oscillating PM. For an input vibration at lHz, frequency up-conversion to the 50Hz cantilever resonance is demonstrated, and the device generates 190nW of power and 7.5 mV of voltage. A method to improve the power lever by connecting multiple cantilevers was reported, similar in concept to the Mizuno and Chetwynd device. Such an approach might become rather bulky and heavy, but the major difficulty originates from that the various cantilevers would vary in both resonance frequency and in phase of their oscillation. In the worst case, no net output would be expected from this device. Given the level of uncertainty in this design, it is far from clear whether the complexity and feasibility of such a system would be justified.

Therefore, it is desirable to provide a novel electromagnetic micro energy harvester with improved design, which has a simple and compact structure to eliminate asynchronous resonance problem.

Summary of the Invention

The present invention provides a novel vibration-based electromagnetic micro energy harvester fabricated using MEMS technology, which can convert ambient vibration energy into electric power, and more particularly, can efficiently harvest vibration energy even under low frequency conditions.

Also, the present invention can eliminate asynchronous resonance problem due to variation in natural frequencies of multiple cantilevers by introducing a simple and compact vibratory structure.

An electromagnetic energy harvesting device includes:

a first vibratory structure, adapted to vibrate when excited by an ambient vibration with a first frequency, and transfer its vibration to a second vibratory structure;

the second vibratory structure, adapted to self-vibrate in a magnetic field at a single second natural frequency, and generate magnetic flux change to induce electrical power;

wherein the first frequency is lower than the second natural frequency.

Optionally, the first vibratory structure includes: a resonating plate and a vibration actuator attached on the bottom surface of the resonating plate;

the resonating plate is adapted to vibrate in response to the first frequency ambient vibration;

the vibration actuator is adapted to vibrate together with the resonating plate. Optionally, the resonating plate is a polymer membrane with low Young's module, and the vibration actuator is a vertically polarized NdFeB magnet.

Optionally, the second vibratory structure includes: a self-vibrating planar coil spring with the second natural frequency and a vibration receiver bonded on the top surface of the self- vibrating planar coil spring; and

the vibration actuator is adapted to attract the vibration receiver at a catch point during vibration to enable the self-vibrating planar coil spring to leave balance position, and release the vibration receiver when the vibration actuator moves up together with the resonating plate;

the self- vibrating planar coil spring is adapted to self-vibrate at the second natural frequency together with the vibration receiver when the vibration receiver is released by the vibration actuator, and generate electrical power inductively inside one or more coils.

The self- vibrating planar coil spring is a Cu planar coil spring with high Young's module, and the vibration receiver is a vertically polarized NdFeB magnet.

The geometry of the one or more coils of the self-vibrating planar coil spring is optimized in terms of magnetic flux rate for tuning the second natural frequency.

The track density in the one or more coils of the self- vibrating planar coil spring is adjusted for tuning the second natural frequency.

Optionally, the device further includes: a surrounding frame, adapted to fix the first vibratory structure and the second vibratory structure, and produce a flux gradient for the second vibratory structure.

The surrounding frame includes:

a first silicon wafer, adapted to fix the first vibratory structure;

a second silicon wafer, adapted to fix the second vibratory structure;

a third silicon substrate; and

a plurality of permanent magnets, adapted to produce the flux gradient for the second vibratory structure, wherein a portion of the plurality of permanent magnets are attached between the first silicon wafer and the second silicon wafer, and another portion of the plurality of permanent magnets are attached between the second silicon wafer and the third silicon substrate.

Optionally, the plurality of permanent magnets are bonded with glues to the top and bottom surfaces of the second silicon wafer. Optionally, the second natural frequency of the second vibratory structure is tuned from hundreds to thousands of Hz.

Optionally, the second vibratory structure includes: one or more insulation films, one or more metal coils connecting with each other in series, and soft magnetic material;

wherein the one or more metal coils and the soft magnetic material are attached on the one or more insulation films.

Optionally, the first vibratory structure includes: a planar spring, and a first magnet attached on the bottom surface of the planar spring; wherein

the planar spring is adapted to capture the ambient vibration with the first frequency and drive the first magnet to vibrate;

the first magnet is adapted to attract or release the soft magnetic material according to the distance between the first magnet and the soft magnetic material, attractive force of the first magnet and stiffness of the second vibratory structure, so as to drive the one or more metal coils attached on the insulation films to vibrate with the second natural frequency.

Optionally, the one or more metal coils are distributed on one or more layers, each of which uses one insulation film as a supporting plane, and conduits are set for connecting the metal coils distributed on the different layers; and

the soft magnetic material is attached on the supporting plane nearest to the first vibratory structure.

The device further includes: a second magnet placed underneath the one or more metal coils;

wherein the one or more metal coils connected in series have two connection pads for outputting induced current generated during the movement of the one or more metal coils in a magnetic field of the second magnet.

A vibratory device for magnetic harvest includes: one or more insulation films, one or more metal coils connected with each other in series, and soft magnetic material; wherein the one or more metal coils and the soft magnetic material are attached on the one the more insulation films;

the soft magnetic material is adapted to vibrate under an external magnetic force, and drive the one or more metal coils attached on the one or more insulation films to vibrate; and the one or more metal coils are adapted to perform a high-frequency vibration in a magnetic field and generate induced electrical power.

Optionally, the one or more metal coils are distributed on one or more layers, each of which has at least one metal coil and uses one insulation film as a supporting plane, and conduits are set for connecting the metal coils distributed on the different layers; and

the soft magnetic material is attached on the supporting plane nearest to the external magnetic force.

Optionally, the one or more metal coils connected in series have two pad settings for outputting the induced electrical power generated during the movement of the one or more metal coils in the magnetic field.

A method of fabricating a vibratory device for energy harvest includes:

sputtering a first seed layer onto a first insulation film;

electroplating one or more first metal coils onto the first seed layer, and attaching soft magnetic material on the first seed layer; and

coating a second insulation film around the first seed layer, the one or more first metal coils and the soft magnetic material to form the vibratory device.

The method further includes:

before forming the vibratory device, sputtering a second seed layer onto a second insulation film;

electroplating one or more second metal coils onto the second seed layer, and electroplating conduits for connecting the one or more first metal coils and the one or more second metal coils in series; and

coating a third insulation film around the second seed layer and the one or more second metal coils.

The method further includes:

forming the first insulation film on a sacrifice layer, and removing the sacrifice layer after the vibratory device is formed.

Thus, it can be seen that in the present invention, an energy harvester able to extract sufficient energy to power wireless sensors even under low frequency ambient vibrations (< 100Hz) is provided. More specifically, a novel electromagnetic micro energy harvester fabricated with Micro-Electro-Mechanical Systems (MEMS) technology is provided, which can efficiently convert low frequency ambient vibration into high frequency relative movement between magnet and coil winds, and consequently produce electrical energy large enough to power wireless sensors.

Brief Description of Drawings

FIG. 1 is a schematic view of a generic model of electromagnetic vibration energy harvester;

FIG. 2(a) is a schematic cross-section view of the present electromagnetic vibration energy harvester;

FIG. 2(b) shows movement of the first NdFeB and the second NdFeB with respect to each other;

FIG. 3(a) is a schematic cross-section view of an electromagnetic vibration energy harvester in an embodiment of the present invention;

FIG. 3(b) is a schematic top view of a copper planar spring in an embodiment of the present invention;

FIG. 4(a) is a schematic top view of one embodiment of the planar Cu coil and the second NdFeB magnet;

FIG. 4(b) is a schematic top view of one layer of the second vibratory structure in an embodiment of the present invention;

FIG. 5 illustrates interconnection between copper coils of the bottom layer and the top layer;

FIG. 6 illustrates a fabrication process of a two-layer copper coil on a polyimide film.

Detailed Description of the Invention

To make the objective, technical solution and merits of the present invention clearer, the present invention is described hereinafter with reference to the following embodiments.

FIG. 1 shows a generic model of electromagnetic device for converting the kinetic energy of a vibrating mass to electrical power. The electromagnetic device uses a resonant mass-damper-spring arrangement mounted within an enclosure 102. The resonant mass-damper-spring arrangement includes an inertial mass 101 linked to an internal wall 103 of the enclosure 102 by a spring 104 and two dampers, electrical damper 105 and mechanical damper 106, respectively. The spring 104 and the dampers 105 and 106 are in parallel configuration. Analysis of prior art shows that it is difficult to increase the power output of a generic electromagnetic micro generator from low frequency vibrations, because the rate of flux change ^ _whi_ch decides the power output, would be strongly dependent on the ambient vibration frequency when using the mass-damper-spring model.

Then, in the present invention, dB/dt -_g designed t₀ be independent of the ambient vibration frequency. That is, only depends on the inherent natural frequency of some internal vibratory structure inside an innovative vibration energy harvester, and ambient vibrations act simply as an actuation to excite the vibratory structure's self- vibration. By doing so, the power output can be enhanced by increasing the natural frequency of the vibratory structure, and kept constantly high even when the frequency of ambient vibration is low.

Increase in the low frequency performance of a micro energy harvester by the implementation of multiple resonant structures like Haluk Kulah and Khalil Najafi's design is undesirable because the induced asynchronization can, in worst cases, cause a zero net output. It is more desirable, by virtue of the integration and precision properties of MEMS technology, to enhance the electromagnetic conversion process by optimizing the coils geometry or increasing the coils track density. The novel electromagnetic energy harvester can then be fabricated by combining standard microsystem technology processes with electroplating post-processes for the fabrication of coils with thick copper (Cu) tracks by electrochemical deposition. This could also involve an optimization of the electrochemical growth conditions to minimize residual stress in the Cu layers and to optimize their adherence, since these are critical parameters that can compromise the viability and lifetime of the devices.

Accordingly, one embodiment of the present invention provides a vibration- based energy harvester with a three-layer structure and a magnetic transmission mechanism, to solve the low frequency vibration dilemma that traditional vibration- based energy converters have faced, and improve energy efficiency under low frequency conditions. The energy harvester according to the invention includes a top layer with a resonating plate, which vibrates or resonates in response to the received low frequency vibration energy, a middle layer with a self-vibrating planar metal coil spring that is magnetically excited by the top flexible structure and then vibrates at its natural high frequency (hundreds or thousands of Hz), and a fixed substrate layer. Two NdFeB stationary permanent magnets are attached between the top layer and the middle layer, and another two are attached between the middle layer and the substrate layer. A thin and light NdFeB actuator is attached to the bottom surface of the top resonating plate, and an NdFeB receiver with small surface area is attached to the center of the middle self-vibrating planar metal coil spring, thus avoiding the production of too much magnetic force between two resonating structures when there is no ambient vibration. When ambient vibrations cause the NdFeB actuator to vibrate together with the top resonating plate, the NdFeB actuator moves close to and far from the self-vibrating planar metal coil spring, catches and releases the central NdFeB receiver at catch and release points alternately. After release, the NdFeB receiver and self-vibrating planar metal coil spring vibrate freely at an inherent high frequency, and thereby a rapid relative movement between the planar coil spring and the four stationary permanent magnets occurs.

The schematic cross-section view of the energy harvester according to the invention is shown in FIG. 2(a). Silicon wafers 210 and 212, the four stationary permanent magnets of NdFeB 214, 216, 218 and 220, together with the silicon substrate 222, are bonded by glues 232 to form the surrounding frame. The two fesonatin¾vibratory structures comprise a top resonating plate 224 and a middle planar metal coil spring 226. An NdFeB actuator 228 is attached on the bottom surface of the top resonating plate 224, and an NdFeB receiver 230 is bonded on the center of the self- vibrating planar metal coil spring 226. It should be pointed out that the NdFeB receiver 230 can also be bonded on any place of the self-vibrating planar metal coil spring 226, and is not limited to the central part.

As one embodiment of the present invention, four high energy density sintered rare earth NdFeB magnets 214, 216, 218 and 220 are bonded with glues 232 (e.g. Cyanoacrylate) to the top and bottom surfaces of the silicon wafer 212 with their magnetic poles being aligned as shown in FIG. 2(a). The arrangement produces a concentrated flux gradient through the planar metal coil spring 226 when it vibrates. The number and arrangement of these NdFeB magnets can be optimized to improve power output for different application requirements.

The top resonating plate acts as an ambient vibration receiver and is constituted by a polymer membrane 210 with Young's module of about 2.5GPa, which is significantly lower than that of silicon (Si) related materials. This makes it better suited for resonant applications in the low frequency range of few Hz. A first vertically polarized NdFeB magnet 228 is stuck on the polymer membrane, which acts as an actuation when vibrating together with the polymer membrane. The first NdFeB should be thin and light, so as to produce little magnetic force on the second NdFeB 230 at the balance position when there is no ambient vibration.

The middle flexible structure is a copper planar coil spring 226 fabricated on the

Si wafer 212. A second vertically polarized NdFeB magnet 230 is attached to the center of the Cu planar coil spring 226. Copper is selected because its Young's module is as high as 200 GPa, thus the spring's natural frequency is much higher than that of the polymer membrane. Indeed, the Cu planar spring's natural frequency is also decided by the parameters of the spring, such as the geometry of the beams, the number of the beams, and the gap between the beams, and could be tuned from hundreds to thousands of Hz, therefore it can be used in different applications. When the top resonating plate moves in response to ambient vibrations, a magnetic transmission mechanism occurs between it and the middle flexible structure. That is to say, when the first NdFeB 228 mo es down together with the polymer membrane 224, it will attract the second NdFeB 230 at the catch point and release the second NdFeB 230 when it moves up together with the polymer membrane 224. The released second NdFeB 230 will then self-vibrate together with the Cu planar coil spring 226, because the Cu planar spring 226 has left its balance position and the kinetic power of the first NdFeB 228 has been magnetically transferred to the potential power of Cu spring 226. FIG. 2(b) shows the movement of the first NdFeB 228 and the second NdFeB 230 with respect to each other, the first NdFeB vibrates with a low frequency, e.g. 2Hz, while the second NdFeB starts resonating at its high natural frequency which could be set to hundreds or thousands of Hz.

Fabrication of the copper planar coil spring involves standard microsystem manufacturing processes, such as surface micromachining, anisotropic bulk etching and wafer bonding, while other MEMS fabrication processes can also be applied. The spring is fabricated on a double-side polished silicon wafer which is thermally oxidized on both sides. Firstly, the positive photoresist is spin-coated on the backside of the silicon wafer and patterned, and then the exposed silicon dioxide (Si0₂) layer is etched in hydrofluoric acid (HF) solution. After the photoresist is stripped, a Cu seed layer is sputtered on the front side of the wafer. Secondly, another photoresist layer is coated and patterned, and then Cu planar spring is electroplated. In the next step, the photoresist layer is stripped, and the exposed seed layer is etched. Finally, the exposed silicon is etched in potassium hydroxide (KOH) solution and then Si0₂ is etched using HF solution from the backside of the silicon substrate. As a result, the copper planar spring is released.

The design of the planar coil spring 226 is based on the fact that the magnetic flux rate is a function of the coil parameters, i.e., the electromagnetic coupling can be increased by optimizing the coils geometry in terms of magnetic flux rate, or by increasing the track density in the coils while minimizing the series resistance of the coil tracks. The advantage for this planar coil design is that a multi-layer coil configuration can be implemented with MEMS manufacturing processes, which improves the electrical power generated inductively inside the coils. Furthermore, there is no asynchronization problem for the multi-layer coil structure, because it is fixed on the silicon wafer so that the magnetic flux change rate is decided only by the oscillation of the self-vibration planar coil spring 226.

As an example of optimizing coil parameters for performance, the fabrication process of a two-layer planar coil spring on a glass substrate is described as follows.

Firstly, a Cu seed layer is sputtered onto the glass substrate, and then the positive photoresist is spin-coated and patterned.

Secondly, another photoresist layer is coated and patterned after a bottom layer coil is electroplated, and then Cu conduits are electroplated as channels for connecting the two layers of the coils.

It should be mentioned that each layer of the coils can have one or more turns.

Thirdly, the photoresist layer is stripped, seed layer between the turns of the coil is etched and a polyimide layer is coated, which serves as an insulation layer.

In the next step, the polyimide layer is roasted, ground and polished. Then, a top layer coil is fabricated on the polyimide layer by repeating the above steps.

FIG. 4(a) shows a top view of a preferred embodiment of the planar Cu coil springs. Multiple turns of planar Cu coil 400 is deposited on the Si wafer 406. NdFeB magnet 402 is located at the center of the planar coil, but its vibration direction is perpendicular to the coil plane. The pad 404 is for outputting the induced electrical energy. The number and arrangement of these planar Cu coils can be chosen at the design phase according to different application requirements.

Another novel vibration-based electromagnetic energy harvester according to the invention includes two compact planar vibratory structures, and realizes vibration transmission and frequency up-conversion via a magnetic interaction between the two vibratory structures. The energy harvester can convert an ambient vibration with a first frequency to an internal vibration of the energy harvester with a second natural frequency via the magnetic interaction between the two vibratory structures, wherein the first frequency is lower than the second natural frequency. In an embodiment of the present invention, the first frequency is a low frequency in the sense of mechanical vibration, and the second natural frequency is a high frequency in the sense of mechanical vibration. Under normal circumstances, the range of low frequency in mechanical vibration is from 0 to 100Hz, while the range of high frequency is above or equal to 1000Hz. Therefore, the energy harvester presented in the present invention can efficiently harvest electrical power from ambient vibrations even in a non- resonance state, and consequently can be readily applied in a wide range of vibrations without performance deterioration.

Specifically, the function of the first vibratory structure is to pick up the low frequency ambient vibration and trigger the high-frequency self-vibration of the second vibratory structure. In a preferred embodiment, the first vibratory structure, i.e. the low-frequency structure, is made by a planar spring, and has a low natural frequency (e.g. about 100 Hz) to be easily excited by ambient vibrations. It should be pointed out that the first vibratory structure is not required to resonate with the ambient vibrations, which enables the energy harvester to work efficiently under wide range of vibration excitations. The function of the second vibratory structure is to conduct high-frequency self-vibration after triggered by the first vibratory structure and generate electrical power in a nearby magnetic field. In a preferred embodiment, the second vibratory structure, i.e. the high-frequency structure, is made of high stiffness material (e.g. polyimide) with multiple layers of copper coils and firmly fixed by two sides. The high-frequency structure has a natural frequency of about 1000 Hz. To realize the magnetic interaction between the low- frequency and high- frequency vibratory structures, magnets or ferromagnetic materials are fabricated on the two vibratory structures correspondingly. When the low-frequency vibratory structure begins to vibrate in response to ambient vibrations, it will move closer to the high-frequency vibratory structure at some time, and magnetically catch the high- frequency vibratory structure at a certain position with the magnetic materials of the two vibratory structures. Similarly, the low-frequency vibratory structure will afterwards move away from the high-frequency vibratory structure due to the vibratory characteristic of ambient vibrations, and release the high-frequency vibratory structure at another position. After this release, the high-frequency vibratory structure will vibrate at its high natural frequency, and a vibration transmission accompanied with a frequency up-conversion is realized.

FIG. 3(a) illustrates a schematic cross-section view of an electromagnetic energy harvester according to an embodiment of the present invention. As shown in FIG. 3(a), the micro electromagnetic energy harvester includes two vibratory structures. A first vibratory structure has a low natural frequency (e.g. fl<100Hz), which is composed of a copper planar spring 301 and a vertically polarized NdFeB permanent magnet (PM) 302 attached on the bottom surface of the copper planar spring 301. A second vibratory structure has a high natural frequency (e.g. fh>1000Hz), which is one to two orders higher than that of the first vibratory structure. The second vibratory structure includes a polyimide film 303, one or more copper coils 304 and a FeNi alloy 305. The number of turns of each copper coil can be adjusted according to practical requirements. The polyimide film 303 has one or more layers, and the one or more high-density copper coils 304 are patterned on two sides of the top surface of each layer. In an embodiment of the present invention, the polyimide film 303 has two layers, and the FeNi alloy 305 is attached on the center of the top surface of the top layer of the polyimide film 303. The polyimide film 303 offers good pliancy due to its relatively low Young's module (around 3 GPa), and is suitable for forming a main flexible part of the second vibratory structure. Moreover, the polyimide film 303 not only acts as the substrate for electroplating the copper coils 304, but also protects them between two layers from short circuiting. Alternatively, the polyimide film 303 can be replaced by other kind of insulation film which can prevent multiple layers of metal coils from short-circuiting.

When there is no vibration, the first and second vibratory structures are located at their respective balance positions. The initial distance between the two vibratory structures is then adjusted according to an assumption that the PM 302 will not catch the FeNi alloy 305 at a balance position. As the first vibratory structure vibrates in response to ambient vibrations, the PM 302 moves closer to the FeNi alloy 305 and catches the FeNi alloy 305 at a certain point of its movement, pulls the entire second vibratory structure up and releases the second vibratory structure at another point. As a result, the released second vibratory structure starts resonating at its high natural frequency, and hence a low- frequency to high-frequency vibration conversion via a magnetic interaction is realized. Two vertically polarized NdFeB permanent magnets 306 are placed under the second vibratory structure and fixed onto a protective casing

307 made by aluminum alloy. When the second vibratory structure vibrates, the copper coils 304 will exhibit a reciprocating movement towards and away from the magnets 306, which causes the change of magnetic flux in the copper coils 304. Therefore, induced current and voltage will be generated in the copper coils 304 according to Faraday's law of induction.

Based on the novel structural characteristic as shown in FIG. 3(a), the energy harvester presented in the present invention is then endowed with an intrinsic frequency adaptation capability to operate efficiently under low-frequency and large- bandwidth vibration environments. For example, when the frequency of ambient vibration is around 80-120Hz, the first vibratory structure having a natural frequency of 100Hz will be triggered by the ambient vibration.

As a preferred embodiment, a top view of the copper planar spring 301 shown in FIG. 3(a) is illustrated in FIG. 3(b), wherein the shadow parts of FIG. 3(b) are hollowed-out areas of the spring. From the top view, it can be seen that the copper planar spring 301 includes four spring beams 308, 309, 310 and 311, and a center rectangle platform 312. Each of the four spring beams emerges from one of four corners respectively, and finally converges at the center rectangle platform 312. By attaching a cubic permanent magnet on the bottom surface of this center rectangle platform 312, the first low-frequency vibratory structure is constructed. With this construction, there will be the biggest displacement occurring in the cubic permanent magnet when the whole low-frequency vibratory structure is excited by an ambient vibration. This construction is very helpful in allowing the cubic permanent magnet to move close enough to the FeNi alloy 305 of the second vibratory structure and attract the FeNi alloy 305 by magnetic force, especially under non-resonance condition when the energy transfer from the ambient vibration to the first vibratory structure is usually small.

FIG. 4(b) shows a schematic top view of a top layer of the second vibratory structure in another embodiment of the present invention. The top layer principally includes: a pliant supporting plane 411 made by polyimide material, two groups of electroplated copper coils 412 and 413 symmetrically distributed on two horizontal sides of the supporting plane 411 (along with X axis), and an FeNi alloy 414 located on the center of the supporting plane 411 in between the aforementioned two groups of copper coils 412 and 413. The FeNi alloy 414 is correspondingly located right below the permanent magnet of the first vibratory structure, so as to be magnetically caught as the permanent magnet moves close enough. Two vertical sides of the supporting plane 411 (along with Z axis) are fixed on inner walls 419 and 420 of a protective casing, such that when the second vibratory structure begins to vibrate after the permanent magnet releases the FeNi alloy 414, the displacement of horizontal planes away from these two fixed sides will be larger than that of horizontal planes near to these two fixed sides. Therefore, the copper coils 412 and 413 located on the supporting plane 411 will exhibit a large displacement when vibrating together with the supporting plane 411 and consequently generate a large electrical current. There are two connection pads 415 and 416 for connecting the copper coils onto a bottom layer in series to improve mechanical-to-electrical conversion performance. Another two pad settings 417 and 418 are reserved for conducting electricity to external loads, for example, wireless sensors.

Referring to FIG. 5, series interconnection between one or more copper coils of the bottom layer 500 and those of the top layer 502 is illustrated. It should be pointed out that the bottom layer has a similar structure with the top layer, and is mounted underneath the top layer. It can be seen from FIG. 5 that, the interconnection between copper coils of the top layer is different from that of the bottom layer. One difference is that the two copper coils 504 and 506 on the bottom layer 500 are connected with each other directly, while the two copper coils 508 and 510 on the top layer 502 are not connected directly with each other but are connected with the copper coils 504 and 506 on the bottom layer via four pads 512, 514, 516 and 518. Another difference is that there is no extra pad setting in the bottom layer 500, because two pad settings 520 and 522 built on the top layer are able to perform the task of outputting electricity due to the series interconnection of all copper coils on all layers. It is worthy of mention that, by virtue of the insulation characteristic of polyimide material, the copper coils on the bottom and top layers will not be short-circuited. The direction of electrical current induced in the copper coils will periodically change in accordance with the movement of the second vibratory structure. For example, when the second vibratory structure moves away from the vertically polarized permanent magnets 306 in FIG. 3(a), the current direction can be indicated by arrows as shown in FIG. 5.

Further, a process of fabricating a vibratory device is shown in FIG. 6, wherein copper coils with two layers are set on polyimide films. In step (a), a Cu seed layer 602 is sputtered onto a polyimide film 604, which is formed on a sacrifice layer 606 made of polydimethylsiloxane (PDMS) on a glass substrate 608.

In step (b), a top-layer Cu coil 610 is electroplated.

In step (c), a FeNi alloy 612 is attached on the center of the seed layer 602.

In step (d), two Cu conduits 614 and 616 are electroplated as channels for connecting the coils between the top and bottom layers. Afterwards, the seed layer 602 is etched and a second polyimide film 618 is coated around the top-layer Cu coil 610, which serves as an insulation and supporting layer.

In step (e), a second Cu seed layer 620 is sputtered on the second polyimide film

618 and a bottom-layer Cu coil 622 is electroplated.

In step (f), the second seed layer 620 is etched and a third polyimide film 624 is coated. Thereafter, by removing the sacrifice layer 606 from the glass substrate 608, two-layer Cu coils are formed.

It should be pointed out that, the track density or series resistance of the copper coils could be easily tuned by electroplating them with different thickness and width, as well as by repeating the above steps to create multiple layers of copper coils, so as to optimize the performance of the energy harvester according to practical requirements. Additionally, if only a single layer copper coil is needed in real practice, the sacrifice layer 606 could be removed from the glass substrate 608 after steps (a)-(c) are performed.

That is to say, an easily-scalable fabrication process for a vibratory device is provided in the present invention, which allows manufacturing copper coils on polyimide films with different thickness, width, and number of layers. Since power output of the energy harvester is linearly proportional to the number of coils, the power output can be adjusted by tuning the track density or number of layers of copper coils to accommodate different applications.

The invention presents a vibration-based micro energy harvester with two resonating structures. Preferably, the energy harvester according to the present invention includes a top resonating plate as ambient vibration receiver and a middle self-vibrating planar coil spring or one or more metal coils for a high frequency vibration. When the resonating plate is actuated by ambient vibrations, vibration transmission by magnetism occurs between the resonating plate and the self- ibrating planar coil spring or between the resonating plate and the one or more metal coils. The micro-machined structure and the magnetic transmission mechanism result in an innovative energy harvester that has at least one of the following merits:

1. The energy harvester has a simple structure and can eliminate the asynchronous resonance problem by using multiple vibratory cantilevers. As a result, such a compact configuration would produce electrical current with the same vibrating frequency and phase in the coils, which guarantees stable and continuous electrical power output from the device.

2. The energy harvester can be easily scaled to fulfill diverse power requirements. Manufacturing the coils with different track density and series resistance can adjust the energy harvester's performance to accommodate different applications. Also multiple layers of planar coils can be constructed with standard MEMS manufacturing technology to increase the overall power output.

3. The energy harvester is highly efficient even in low frequency vibration environments. The second vibratory structure which has high natural frequency is excited by the transferred ambient vibrations from a top resonating plate, and generates a large magnetic flux change inside the coils. Therefore, the vibration would only act as an initial actuation force, and its low frequency becomes a non-crucial factor to the device's final power output.

4. The energy harvester is adaptable to environments where vibration frequency varies. After excitation by transferred ambient vibrations, the second vibratory structure vibrates at a constant natural frequency no matter what the frequency of ambient vibration. Consequently, a constant large power output of the presented device can be expected within a large frequency bandwidth.

The foregoing descriptions are only preferred embodiments of this invention and are not for use in limiting the protection scope thereof. Any changes and modifications can be made by those skilled in the art without departing from the spirit of this invention and therefore should be covered within the protection scope as set by the appended claims.

Claims

1. An electromagnetic energy harvesting device, comprising:

wherein the second natural frequency is higher than the first frequency.

2. The device according to claim I, wherein the first vibratory structure comprises: a resonating plate and a vibration actuator attached on the bottom surface of the resonating plate;

the vibration actuator is adapted to vibrate together with the resonating plate.

3. The device according to claim 2, wherein the resonating plate is a polymer membrane with low Young's module, and the vibration actuator is a vertically polarized NdFeB magnet.

4. The device according to claim 2, wherein the second vibratory structure comprises: a self-vibrating planar coil spring with the second natural frequency and a vibration receiver bonded on the top surface of the self- ibrating planar coil spring; and

the self-vibrating planar coil spring is adapted to self-vibrate at the second natural frequency together with the vibration receiver when the vibration receiver is released by the vibration actuator, and generate electrical power inductively inside one or more coils.

5. The device according to claim 4, wherein the self- ibrating planar coil spring is a Cu planar coil spring with high Young's module, and the vibration receiver is a vertically polarized NdFeB magnet.

6. The device according to claim 4, wherein the geometry of the one or more coils of the self- vibrating planar coil spring is optimized in terms of magnetic flux rate for tuning the second natural frequency.

7. The device according to claim 4, wherein the track density in the one or more coils of the self-vibrating planar coil spring is adjusted for tuning the second natural frequency.

8. The device according to claim 1, further comprising: a surrounding frame, adapted to fix the first vibratory structure and the second vibratory structure, and produce a flux gradient for the second vibratory structure.

9. The device according to claim 8, wherein the surrounding frame comprises: a first silicon wafer, adapted to fix the first vibratory structure;

a second silicon wafer, adapted to fix the second vibratory structure;

a third silicon substrate; and

10. The device according to claim 9, wherein the plurality of permanent magnets are bonded with glues to the top and bottom surfaces of the second silicon wafer.

11. The device according to any of claims 1-10, wherein the second natural frequency of the second vibratory structure is tuned from hundreds to thousands of Hz.

12. The device according to claim 1, wherein the second vibratory structure comprises: one or more insulation films, one or more metal coils connecting with each other in series, and soft magnetic material;

13. The device according to claim 12, wherein the first vibratory structure comprises: a planar spring and a first magnet attached on the bottom surface of the planar spring; wherein

14. The device according to claim 12, wherein the one or more metal coils are distributed on one or more layers, each of which uses one insulation film as a supporting plane, and conduits are set for connecting the metal coils distributed on the different layers; and

15. The device according to any of claims 12-14, further comprising: a second magnet placed underneath the one or more metal coils;

16. A vibratory device for magnetic harvest, comprising: one or more insulation films, one or more metal coils connected with each other in series, and soft magnetic material; wherein the one or more metal coils and the soft magnetic material are attached on the one or more insulation films;

the soft magnetic material is adapted to vibrate under an external magnetic force, and drive the one or more metal coils attached on the one or more insulation films to vibrate; and

the one or more metal coils are adapted to perform a high-frequency vibration in a magnetic field and generate induced electrical power.

17. The device according to claim 16, wherein the one or more metal coils are distributed on one or more layers, each of which has at least one metal coil and uses one insulation film as a supporting plane, and conduits are set for connecting the metal coils distributed on the different layers; and

18. The device according to claim 16 or 17, wherein the one or more metal coils connected in series have two pad settings for outputting the induced electrical power generated during the movement of the one or more metal coils in the magnetic field.

19. A method of fabricating a vibratory device for energy harvest, comprising: sputtering a first seed layer onto a first insulation film; electroplating one or more first metal coils onto the first seed layer, and attaching soft magnetic material on the first seed layer; and

20. The method according to claim 19, further comprising:

21. The method according to claim 19 or 20, further comprising: