WO2018189545A1

WO2018189545A1 - A vibration-based energy harvester comprising a proof mass surrounding a central anchor

Info

Publication number: WO2018189545A1
Application number: PCT/GB2018/050974
Authority: WO
Inventors: Ashwin Arunkumar SESHIA; Sijun DU; Yu Jia; Shao-Tuan CHEN
Original assignee: Cambridge Enterprise Limited
Priority date: 2017-04-13
Filing date: 2018-04-12
Publication date: 2018-10-18
Also published as: GB201706003D0

Abstract

An apparatus is provided for harvesting energy from mechanical vibrations with high peak power, broad bandwidth, robustness and a smaller volume than conventional cantilever energy harvesters. The apparatus comprises a central anchor (110) and a vibrator structure suspended from and surrounding the central anchor. The vibrator structure comprises a proof mass (120) and a flexure (130) connected between the central anchor and the proof mass. The flexure comprises at least one piezoelectric element. The apparatus additionally comprises at least two electrodes on the piezoelectric element.

Description

A VIBRATION-BASED ENERGY HARVESTER COMPRISING A PROOF MASS SURROUNDING A CENTRAL ANCHOR

Field of the Invention

The present invention relates to vibration-based energy harvesters and in particular to micro-electrical-mechanical (MEMS) energy harvesting devices using piezoelectric materials.

Background to the Invention

Vibration-based energy harvesters are used to extract energy from mechanical vibrations in order to power local devices or in order to store that energy for later use. Piezoelectric materials are widely used in vibration-based energy harvesters. Piezoelectric materials convert mechanical strain energy into electrical energy and so are ideally suited for this purpose.

A typical MEMS piezoelectric energy harvester comprises a cantilever beam including a piezoelectric layer or portion. The cantilever beam is fixed at one end to a vibration support structure. A proof mass is suspended from the other, free end of the cantilever beam. Electrodes are provided on the piezoelectric material in areas where high strain energy is expected, between the ends of the beam. As the support structure vibrates, the cantilever beam vibrates and mechanical strain is generated within the piezoelectric material. This is converted to an electrical potential difference between the electrodes. The electrodes are coupled to a power management circuit to extract the electrical energy. The design of a typical MEMS piezoelectric energy harvester has the benefit of being simple and compact and has a moderate power output at its resonant frequency. MEMS energy harvesters can typically output a raw peak power of around 100μ\Λ/ as described in "Energy harvesting vibration sources for microsystems applications" by S.P Beeby, M.J. Tudor and N.M White in Measurement Science and Technology, vol. 17, no.12, pp. R 175, 2006. However, away from the resonant frequency, the power output drops significantly. The natural or ambient vibrations available for driving an energy harvester tend to contain a wide band of vibration frequencies and vary with time. It would therefore be desirable to provide a vibration-based energy harvester that has a significant power output over a wider band of vibrational frequencies. In addition, cantilevered structures can be very fragile and susceptible to fracture at high excitation levels. Furthermore, in order to generate a high strain in the beam, the displacement of the free end of the cantilever must be large, which requires a large cavity space. It would be desirable to provide an energy harvester with the benefits of high peak power, broad bandwidth, robustness and a smaller volume than conventional cantilever energy harvesters. Summary of the Invention

The invention is defined in the appended independent claims, to which reference should be made. Preferred aspects of the invention are defined in the dependent claims.

The invention provides an energy harvesting apparatus which compared with cantilever energy harvesters improves power response when driven at resonance, increases bandwidth and provides a higher output voltage.

In a preferred aspect, an energy harvesting apparatus comprises:

a central anchor;

a vibrator structure suspended from and surrounding the central anchor;

the vibrator structure comprising a proof mass and a flexure connected between the central anchor and the proof mass, wherein the flexure comprises at least one piezoelectric element; and

at least two electrodes on the piezoelectric element.

Surrounding in this context means surrounding in two dimensions. The proof mass may extend completely around the central anchor in one plane to form a closed structure around the central anchor. Alternatively, or in addition, the flexure may completely enclose the central anchor.

The energy harvesting apparatus may further comprise electric circuitry connected to at least one of the at least two electrodes, the electric circuitry comprising, or connected to an energy storage device. An example of electric circuitry which may be used is an active interface circuit which needs a power supply to operate. Another example of electric circuitry that may be used is a passive rectifier which may include half-bridge rectifiers, voltage doublers and Cockcroft-Walton voltage multipliers.

The energy harvesting apparatus may be vacuum packaged to minimise air dampening. As a large part of the apparatus moves, air dampening can be significant if the apparatus is not vacuum sealed. The central anchor may be cylindrical in shape. The central anchor may have a cross section which is a circle. The central anchor may be fixed to or integral with an underlying package. The flexure is connected between the central anchor and the proof mass. Strain is generated in the flexure during vibration. The flexure may be rotationally symmetrical about the central anchor to avoid strain being concentrated in particular regions of the flexure during vibration. The membrane may be annular in shape. The flexure may, for example, be a circular annulus. This ensures that the strain is evenly distributed across the flexure. An even distribution of strain reduces the peak strain density which means that the device can operate better at high vibration levels. The flexure may alternatively, for example, be a hexagonal or an oval annulus. The flexure may be any closed convex geometry that minimises strain concentration which may result in failure under low acceleration loading. The flexure may comprise a membrane extending around the central anchor.

The energy harvesting apparatus may comprise a plurality of flexures, each connected between the central anchor and the proof mass. Each flexure may comprise at least one piezoelectric element and at least two electrodes on the at least one piezoelectric element.

The flexure may comprise a substrate layer and at least one piezoelectric element on the substrate layer. Piezoelectric elements with higher charge constant are preferred as this directly affects the output power of the energy harvester. However, any piezoelectric element may be used for the harvester.

For each energy harvester, a power and a ground electrode are required. Electrical power is generated in the piezoelectric material between the two electrodes. One electrode may be a ground reference electrode and the output signal from the other electrode may be an AC signal. The electrodes may be rotationally symmetric around the central anchor and may be annular in shape. The energy harvester may have more than two electrodes positioned on top of the at least one piezoelectric element.

The energy harvesting apparatus may be a MEMS device. The energy harvesting apparatus may operate at a frequency in the range 10Hz to 10 kHz. The frequency of vibration depends on the size of the apparatus, for example, a smaller device generally has a higher operating frequency. The vibrator structure may have an area in the plane of the flexure between 1 mm² and 500mm².

The substrate layer may be formed from silicon. Furthermore, the flexure, proof mass and central anchor may be manufactured from a single crystal of silicon.

The proof mass, which surrounds the central anchor, may have a centre of mass that lies within the anchor. The proof mass may comprise one or more proof mass portions symmetrically positioned around the central anchor. The proof mass forms a closed loop surrounding the central anchor which makes the energy harvesting apparatus more robust than a conventional cantilever energy harvester. Conventional cantilever energy harvesters are fragile and often break under high excitation levels, which limits the peak output power of such designs. However, the robustness of the present energy harvesting apparatus is significantly increased so it is able to work under higher excitation

environments with a longer life-time and a higher output power.

At rest, the flexure, proof mass and central anchor may lie in a single plane. When operating in a preferred mode, the flexure vibrates in a direction normal to the single plane. For a given power output, the required displacement of the proof mass of the energy harvester apparatus of the present invention is much smaller than that required of a cantilever design. This decreases the size of the cavity required and so decreases the package volume. The device can operate in modes other than normal modes, such as torsional and longitudinal in-plane modes, which may be used to harvest large amounts of energy.

It should be clear to the skilled person that features described in the preferred aspect of the invention may vary in shape and design. For example, the proof mass may have a variety of different shapes and may have an uneven mass distribution. The central anchor may have a different shape and the electrodes may have a different shape. For example, the proof mass may comprise a plurality of separate proof mass portions that are mechanically coupled to surround the central anchor. The flexure may comprise a plurality of apertures.

Brief Description of the Drawings

Embodiments of the invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which: Figure 1a is a schematic top view of the energy harvester apparatus. Figure 1 b is a schematic side view of the energy harvester apparatus.

Figure 2 is a cross section of the energy harvester apparatus.

Figure 3 shows the strain distribution across a vibrating flexure. Figure 4 is a plan view of the energy harvester apparatus illustrating the position of the electrodes.

Figure 5 is a block diagram showing the energy harvester apparatus, interface circuit and energy storage device.

Figure 6 is a graph showing the output power of the energy harvester with a 70kQ load resistor at an excitation of approximately 0.2g.

Detailed Description

Figure 1 shows two schematic perspectives of the energy harvester apparatus 100 in accordance with an embodiment of the invention. Figure 1 a is a top perspective view of the energy harvester apparatus. Figure 1 b is a bottom perspective view of the energy harvester apparatus. The apparatus comprises a central anchor 110, a proof mass surrounding the central anchor 120 and a membrane 130 connected between the central anchor and the proof mass. The anchor, membrane and proof mass are formed together from a single piece of silicon. The central anchor 1 10 is cylindrical. The membrane is a circular annulus extending from the anchor to the proof mass. The proof mass in this example has a generally square shape.

The central anchor is fixed to an underlying package (not shown), which may also be formed from the same crystal of silicon. The membrane and proof mass are not fixed to any other structure. A piezoelectric layer is deposited on the membrane. The piezoelectric layer and membrane together form a flexure. Electrodes are placed on top of the piezoelectric layer.

Figure 1a shows that, when at rest, the flexure 130, proof mass 120 and central anchor 1 10 have top surfaces that are coplanar. When an acceleration is experienced by the proof mass in a direction normal to the plane of the top surface of the proof mass, flexure and central anchor, the flexure vibrates in a direction normal to the plane of the proof mass, flexure and central anchor. Strain is generated in the piezoelectric layer during this vibration. The highest strain is generated in the piezoelectric layer at a positions nearest to the central anchor and to the proof mass. The strain is distributed annularly around the central anchor owing to the circular symmetry of the structure. The strain close to the central anchor is of opposite sign to the strain close to the proof mass.

Figure 2 is a cross section of the energy harvesting apparatus. The energy harvester apparatus comprises a leadless chip carrier 210 and an aluminium spacer 220 of approximately 1.3mm thickness located above the leadless chip carrier. A silicon substrate 230 with an overlaying insulating oxide layer 240 is deposited on top of the aluminium spacer. The silicon substrate has a thickness of 400μηι and the insulating oxide layer has a thickness of 1 μηι. A doped silicon layer 250 of 10μηι thickness is deposited on top of the silicon substrate with the insulating oxide layer. Alternatively, the doped silicon layer 250 may be replaced by an undoped silicon layer and a bottom electrode layer. The bottom electrode layer would be positioned between the undoped silicon layer and the

piezoelectric layer 260. The piezoelectric layer 260, which is formed from aluminium nitride (AIN), is deposited on top of the silicon layer 250. The piezoelectric layer may be formed from another material such as lead zirconate titanate (PZT), zinc oxide (ZnO), barium titanate (BaTiC ), lithium niobate (LiNbOs). The piezoelectric layer is Ο.δμηι in thickness. Other processes with different layers, materials or thickness layers may also be used to prepare the energy harvester apparatus. The piezoelectric layer 260 and the doped or undoped silicon layer 250 form the flexure of the device. The leadless chip carrier 210, aluminium spacer 220, silicon substrate 230 and insulating oxide layer 240 form the anchor of the device. Pads 270 are placed on top of the doped silicon 250 and / or on top of the piezoelectric layer 260. An electrode 280 is placed on top of the piezoelectric layer 260. A proof mass 120, or portion of a proof mass, is connected below the doped or undoped silicon layer 250. The proof mass 120 is formed using an etching process. Figure 3 illustrates the strain along a centre line 140, shown in Figure 1 a, extending across the proof mass, flexure and central anchor, at a particular point in time. It can be seen that when the flexure is vibrating, the highest positive strain is located at the position in the vibrating flexure which is adjacent to the central anchor 310. The strain is reduced to zero 320 at the mid-point of the vibrating flexure. The highest negative strain 330 is located at the furthest position in the vibrating flexure from the central anchor, close to the proof mass. The electrodes are positioned in regions of highest strain.

Figure 4 is a top view photograph of a fabricated MEMS energy harvester apparatus. The MEMS energy harvester apparatus measures 11 mm x 1 1 mm. The dimensions of the energy harvester can vary depending on process and design constraints. The central anchor 1 10 is surrounded by a proof mass 120. A piezoelectric layer 260 is deposited on top of the membrane connected between the central anchor 110 and the proof mass 120. In Figure 4, two annular electrodes 410 and 420 are placed on top of the piezoelectric layer 260. A potential difference is generated in the piezoelectric layer between electrodes 410 and 420. One annular electrode is a power electrode and the other annular electrode is a ground reference electrode. Either electrode can be the ground reference and the output signal from the other electrode is an AC signal. No electrodes are positioned on top of the piezoelectric layer in regions where there is no generated strain 450, as illustrated at point 320 in Figure 3. Only covering the high strain regions of the piezoelectric layer with electrodes can increase the output power. Three pads 270 are placed on top of the anchor 1 10. Wires 440 are connected to the pads 270 to extract the output power.

Figure 5 is a block diagram showing the energy harvester, interface circuit and energy storage device. The AC signal from the power electrode is provided to an interface circuit comprising a full-bridge rectifier 510 to generate a DC output voltage and current that can either be used to power an associated device, such as a sensing device for example, or can be input to an energy storage device such as a rechargeable battery or a capacitor 520.

Figure 6 shows the measured output power while the energy harvester of Figure 4 is connected with a matched resistor of 70kQ. Figure 6 shows that the energy harvester can output a peak power of approximately 173μ\Λ/, which is greater than a typical MEMS piezoelectric energy harvester of equivalent dimensions. The present energy harvester is able to power most wireless sensor systems. This output power is achieved at 1798Hz under an excitation level of approximately 0.2g. This demonstrates a high output power of the energy harvester under low excitation levels.

It should be clear that variations to the geometry of the described embodiment are possible. For example, although the proof mass in the described example has a generally square shape with an even mass distribution, it is possible for the proof mass to have other shapes. For example, the proof mass may be generally annular. The proof mass may have an uneven mass distribution, although it is advantageous for the centre of mass of the proof mass to lie within the central anchor when at rest. The proof mass may be formed with one or more apertures formed in it. The proof mass may have a top surface that is not coplanar with the membrane.

The central anchor may have a top surface that is not coplanar with the membrane. The central anchor may be fixed to a surrounding package at a top end and at a bottom end.

The membrane may not be continuous around the central anchor. In particular, the membrane may comprise one or more apertures, or may comprise a plurality of separate membrane portions.

An energy harvesting apparatus in accordance with the invention has the advantage of being robust so that it can withstand relatively high input accelerations when compared to a cantilever design of similar dimensions. An energy harvesting apparatus in accordance with the invention also provides a higher output power over a greater bandwidth when compared to a cantilever design of similar dimensions.

Claims

1. An energy harvesting apparatus comprising:

a central anchor;

a vibrator structure suspended from and surrounding the central anchor;

at least two electrodes on the piezoelectric element.

2. An energy harvesting apparatus according to any preceding claim, further comprising electric circuitry connected to at least one of the at least two electrodes, the electric circuitry comprising, or connected to, an energy storage device.

3. An energy harvesting apparatus according to any preceding claim, further comprising electric circuitry connected to at least one of the at least two electrodes, the electric circuitry comprising an active interface circuit.

4. An energy harvesting apparatus according to claim 3, wherein the electric circuitry comprises a passive rectifier.

5. An energy harvesting apparatus according to any preceding claim, wherein the central anchor has a circular cross-section.

6. An energy harvesting apparatus according to any preceding claim, wherein the central anchor is fixed to an underlying package.

7. An energy harvesting apparatus according to any preceding claim, wherein the energy harvesting apparatus is vacuum packaged.

8. An energy harvesting apparatus according to any preceding claim, wherein the energy harvesting apparatus operates at a frequency in the range 10Hz to 10 kHz.

9. An energy harvesting apparatus according to any preceding claim, wherein the flexure comprises a substrate layer and at least one piezoelectric element on the substrate layer.

10. An energy harvesting apparatus according to claim 9, wherein the substrate layer is formed from silicon.

1 1. An energy harvesting apparatus according to any preceding claim, wherein the flexure comprises a membrane extending around the central anchor.

12. An energy harvesting apparatus according to claim 11 , wherein the membrane is annular.

13. An energy harvesting apparatus according to any preceding claim, wherein the flexure of the energy harvesting apparatus is rotationally symmetrical about the central anchor.

14. An energy harvesting apparatus according to any one of claims 1 to 4, wherein the flexure of the energy harvesting apparatus is oval in shape.

15. An energy harvesting apparatus according to any one of claims 1 to 10, comprising a plurality of flexures, each connected between the central anchor and the proof mass.

16. An energy harvesting apparatus according to any preceding claim, wherein the at least two electrodes are annular.

17. An energy harvesting apparatus according to any preceding claim, wherein the proof mass has a centre of mass that lies within the anchor.

18. An energy harvesting apparatus according to any preceding claim, wherein the proof mass forms a closed loop surrounding the central anchor.

19. An energy harvesting apparatus according to any preceding claim, wherein the proof mass comprises one or more proof mass portions symmetrically positioned around the central anchor.

20. An energy harvesting apparatus according to any preceding claim, wherein the flexure, proof mass and central anchor are manufactured from a single crystal of silicon.

21. An energy harvesting apparatus according to any preceding claim, wherein at rest, the flexure, proof mass and central anchor lie in a single plane.

22. An energy harvesting apparatus according to claim 21 , when operating in preferred modes, the flexure vibrates in a direction perpendicular to the single plane.