U.S. patent application number 15/404084 was filed with the patent office on 2020-08-06 for ferrofluid liquid spring with magnets between coils inside an enclosed chamber for vibration energy harvesting.
The applicant listed for this patent is University of Southern California. Invention is credited to Eun Sok Kim, Yufeng Wang, Qian Zhang.
Application Number | 20200251973 15/404084 |
Document ID | 20200251973 / US20200251973 |
Family ID | 1000004970237 |
Filed Date | 2020-08-06 |
Patent Application | download [pdf] |
United States Patent
Application |
20200251973 |
Kind Code |
A1 |
Kim; Eun Sok ; et
al. |
August 6, 2020 |
FERROFLUID LIQUID SPRING WITH MAGNETS BETWEEN COILS INSIDE AN
ENCLOSED CHAMBER FOR VIBRATION ENERGY HARVESTING
Abstract
A vibration energy harvester includes a proof mass that is a
magnetic array or a coil array. The magnetic array has multiple
magnets. The coil array has one or more coils. The vibration energy
harvester includes an enclosed chamber. The enclosed chamber has
the other of the coil array or the magnetic array that is not the
proof mass. The one or more copper coils and the multiple magnets
are configured to generate the electrical energy from a relative
movement between the one or more copper coils and the multiple
magnets. The vibration energy harvester includes a liquid
suspension that suspends the proof mass within the enclosed
chamber.
Inventors: |
Kim; Eun Sok; (Rancho Palos
Verdes, CA) ; Wang; Yufeng; (Los Angeles, CA)
; Zhang; Qian; (Methuen, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
University of Southern California |
Los Angeles |
CA |
US |
|
|
Family ID: |
1000004970237 |
Appl. No.: |
15/404084 |
Filed: |
January 11, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62279449 |
Jan 15, 2016 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02K 15/0407 20130101;
H02K 35/02 20130101; H02K 3/02 20130101; H02K 1/02 20130101 |
International
Class: |
H02K 35/02 20060101
H02K035/02; H02K 1/02 20060101 H02K001/02; H02K 3/02 20060101
H02K003/02; H02K 15/04 20060101 H02K015/04 |
Goverment Interests
STATEMENT REGARDING GOVERNMENT RIGHTS
[0002] This invention was made with Government support under Grant
No. N66001-13-1-4055 awarded by the Defense Advanced Research
Projects Agency (DARPA) and under Grant No. ECCS-1308041 awarded by
the National Science Foundation (NSF). The Government has certain
rights in this invention.
Claims
1. A vibration energy harvester that converts kinetic energy to
electrical energy, comprising: a proof mass that is a magnetic
array having a plurality of magnets or a coil array having one or
more copper coils; an enclosed chamber having the other of the coil
array or the magnetic array, wherein the one or more copper coils
and the plurality of magnets are configured to generate the
electrical energy from a relative movement between the one or more
copper coils and the plurality of magnets; and a liquid suspension
that completely encloses and suspends the proof mass within the
enclosed chamber.
2. The vibration energy harvester of claim 1, wherein the plurality
of magnets includes 2-10 Neodymium (NdFeB) magnets with alternating
north and south poles.
3. The vibration energy harvester of claim 1, wherein the liquid
suspension is a ferrofluid liquid suspension that becomes
magnetized in a presence of a magnetic field and is attracted by a
magnet.
4. The vibration energy harvester of claim 3, wherein the
ferrofluid liquid suspension suspends the magnetic array in a
middle of the enclosed chamber so that attractive forces of the
ferrofluid liquid suspension counteract each other when there is no
applied acceleration.
5. The vibration energy harvester of claim 3, wherein when the
magnetic array is displaced from a middle of the enclosed chamber,
and a portion of the ferrofluid liquid suspension that has no
symmetric counterpart attracts the magnetic array toward the middle
of the enclosed chamber.
6. The vibration energy harvester of claim 5, wherein an amount of
displacement between the displaced magnetic array from the middle
of the enclosed chamber is proportional to a force applied by the
portion of the ferrofluid liquid suspension that has no symmetric
counterpart to pull the displaced magnetic array back into the
middle of the enclosed chamber.
7. The vibration energy harvester of claim 1, wherein the enclosed
chamber is formed by bonding micromachined silicon with the one or
more copper coils and a laser-machined acrylic frame.
8. The vibration energy harvester of claim 1, wherein a center of a
respective copper coil of the one or more copper coils is aligned
with a boundary between two magnets of the plurality of magnets due
to the liquid suspension.
9. The vibration energy harvester of claim 1, wherein the
ferrofluid liquid suspension is configured to reduce a resonant
frequency of the vibration energy harvester to 1-500 Hz.
10. The vibration energy harvester of claim 1, wherein the one or
more copper coils are shaped in a rectangular shape.
11. A vibration energy harvester, comprising: a magnetic array
having a first magnet and a second magnet; an enclosed chamber
having a plurality of copper coils, the plurality of copper coils
and the magnetic array configured to generate electrical energy
from a movement of the magnetic array in a first direction parallel
to the plurality of copper coils; and a ferrofluid liquid
suspension within the enclosed chamber that is configured to
suspend the magnetic array within the enclosed chamber, the
ferrofluid liquid suspension having a portion that has no symmetric
counterpart and that attracts the magnetic array laterally in a
second direction that is opposite the first direction toward a
center of the enclosed chamber.
12. The vibration energy harvester of claim 11, wherein the
ferrofluid liquid suspension is further configured to align a
copper coil of the plurality of copper coils with a boundary
between the first magnet and the second magnet.
13. The vibration energy harvester of claim 11, wherein the
ferrofluid liquid suspension is self-aligning and the portion of
the ferrofluid liquid that has no symmetric counterpart draws or
pulls the magnetic array to attract the magnetic array laterally in
the second direction when an acceleration is applied in the second
direction and displaces the magnetic array in the first
direction.
14. The vibration energy harvester of claim 11, wherein an amount
of the electrical energy that is generated is based on a velocity
of a relative movement between the plurality of copper coils and
the magnetic array and a magnetic flux density of the plurality of
copper coils.
15.-20. (canceled)
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to and the benefit of U.S.
Provisional Application 62/279,449, titled "FERROFLUID LIQUID
SPRING FOR VIBRATION ENERGY HARVESTING," filed on Jan. 15, 2016,
and the entirety of which is hereby incorporated by reference
herein.
BACKGROUND
1. Field
[0003] This specification relates to a system for converting
vibration energy to electrical energy and a method for fabricating
the vibration energy harvester.
2. Description of the Related Art
[0004] Vibrations are found in many places and objects, such as
building walls, bridges, automobiles, airplanes, a human body, etc.
These ubiquitous vibration sources provide significant amount of
renewable energy that can be harvested and used to power electronic
devices including sensors, actuators, and/or wireless transceivers.
Vibration energy harvesters are typically built on resonant
structures with a rigid suspension, such as a membrane, a
cantilever or a spring. Moreover, the fabrication process of the
vibration energy harvesters are difficult, especially if the
fabrication process requires a low resonant frequency. A rigid
suspension is prone to breakage or failure due to strong vibrations
or continual usage. Accordingly, there is a need for a more durable
suspension structure with a low resonant frequency.
SUMMARY
[0005] In general one aspect of the subject matter described in
this specification is embodied in a vibration energy harvester. A
vibration energy harvester includes a proof mass that is a magnetic
array or a coil array. The magnetic array has multiple magnets. The
coil array has one or more coils. The vibration energy harvester
includes an enclosed chamber. The enclosed chamber has the other of
the coil array or the magnetic array that is not the proof mass.
The one or more copper coils and the multiple magnets are
configured to generate the electrical energy from a relative
movement between the one or more copper coils and the multiple
magnets. The vibration energy harvester includes a liquid
suspension that suspends the proof mass within the enclosed
chamber.
[0006] These and other embodiments may include one or more of the
following features. The enclosed chamber may be formed by bonding
micromachined silicon with the one or more copper coils and a
laser-machined acrylic frame. The multiple magnets may include 2-10
Neodymium (NdFeB) magnets with alternating north and south poles.
The one or more coils may be shaped in a rectangular shape. The
liquid suspension may be a ferrofluid liquid suspension that
becomes magnetized in the presence of a magnetic field and is
attracted by a magnet. The ferrofluid liquid suspension may be
configured to reduce a resonant frequency of the vibration energy
harvester to 1-500 Hz.
[0007] The ferrofluid liquid suspension suspends the magnetic array
in the middle of the chamber, as the attractive forces of the
ferrofluid liquid suspension counteracts each other when there is
no applied acceleration. Consequently, the center of a respective
copper coil of the one or more copper coils may be aligned with a
boundary between two magnets of the multiple magnets.
[0008] The magnetic array may be displaced from the middle of the
chamber. A portion of the ferrofluid that has no symmetric
counterpart may attract the magnetic array toward the middle of the
chamber. The amount of displacement between the displaced magnet
array from the middle of the chamber may be proportional to a force
applied by the portion of the ferrofluid liquid suspension that has
no symmetric counterpart to pull the displaced magnetic array back
into the middle of the chamber.
[0009] In another aspect, the subject matter is embodied in a
vibration energy harvester. The vibration energy harvester includes
a magnetic array that may have a first magnet and a second magnet.
The vibration energy harvester may include a copper coil. The
copper coil may be aligned with a boundary between the first magnet
and the second magnet. The copper coil and the magnet may be
configured to generate electrical energy from a relative movement
between the copper coil and the magnetic array. The vibration
energy harvester may include a ferrofluid liquid suspension within
the enclosed chamber that suspends the magnetic array. The
ferrofluid liquid suspension may suspend the magnetic array in the
middle of the enclosed chamber.
[0010] In another aspect, the subject matter is embodied in a
method for fabricating the vibration energy harvester. The method
may include forming a coil plate having a silicon substrate layer,
one or more connection electrodes, a thin film of silicon nitride
and two or more coils. Forming the coil plate may include etching a
portion of the silicon substrate layer to form the thin film of
silicon nitride and one or more trenches and forming the one or
more connection electrodes using one or more metal portions that
are positioned between the two or more coils to be deposited.
Forming the coil plate may include evaporating or sputtering the
one or more metal portions, and electroplating a metal to form the
two or more coils of a coil plate. The method may include
assembling multiple magnets in a laser-cut acrylic chamber. The
method may include bonding the laser-cut acrylic chamber with the
coil plate so that the multiple magnets are positioned within the
one or more trenches. The method may include filling the acrylic
chamber with ferrofluid through an inlet hole. The method may
include removing gases or air cavities entrapped in the ferrofluid
through one or multiple steps of subjecting the
ferrofluid-containing chamber in a vacuum system. The method may
include closing the inlet hole through conformal deposition of a
polymeric material such as Parylene, acrylic, silicone, urethane,
etc.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Other systems, methods, features, and advantages of the
present invention will be or will become apparent to one with skill
in the art upon examination of the following figures and detailed
description. It is intended that all such additional systems,
methods, features, and advantages be included within this
description, be within the scope of the present invention, and be
protected by the accompanying claims. Component parts shown in the
drawings are not necessarily to scale, and may be exaggerated to
better illustrate the important features of the present
invention.
[0012] FIGS. 1A-1B illustrate exploded views of an example
vibration energy harvester according to an aspect of the
invention.
[0013] FIG. 2A illustrates a square shaped coil according to an
aspect of the invention.
[0014] FIG. 2B illustrates two rectangular shaped coils according
to an aspect of the invention.
[0015] FIGS. 3A-3B illustrate the self-alignment of the magnetic
array within the ferrofluid liquid suspension according to an
aspect of the invention.
[0016] FIGS. 4A-4B illustrate graphs of the measured output
voltages of the example vibration energy harvester of FIG. 1A
according to an aspect of the invention.
[0017] FIGS. 5A-5B illustrate graphs of the measured output voltage
of the example vibration energy harvester of FIG. 1B according to
an aspect of the invention.
[0018] FIG. 6 illustrates a flow diagram of an example process for
fabricating a vibrating energy harvester according to an aspect of
the invention.
[0019] FIGS. 7A-7F illustrate cross-sectional views of an example
vibration energy harvester formed by the process of FIG. 6
according to an aspect of the invention.
[0020] FIG. 8A illustrates an interior view of a chamber of the
vibration energy harvester showing a liquid suspension within the
chamber according to an aspect of the invention.
[0021] FIG. 8B illustrates an interior view of a chamber of the
vibration energy harvester showing a partially filled chamber
according to an aspect of the invention.
[0022] FIG. 8C illustrates an interior view of the chamber of the
vibration energy harvester after being filled, vacuumed and
refilled according to an aspect of the invention.
DETAILED DESCRIPTION
[0023] Disclosed herein are systems for a vibration energy
harvester that converts kinetic energy into electrical energy and
methods for fabricating the vibration energy harvester. Particular
embodiments of the subject matter described in this specification
may be implemented to realize one or more of the following
advantages. A vibration energy harvester having a liquid
suspension, as a spring, is able to convert kinetic energy, such as
vibration energy, into electrical energy.
[0024] Typically, a vibration energy harvester has a resonant
structure with a solid or rigid suspension. A rigid suspension
results in a higher resonant frequency which results in the
difficulty of capturing vibrations having a lower resonant
frequency. Most vibrations occur at the lower resonant frequency,
e.g., between 0 and 99 Hz. Thus, the size or volume of the resonant
structure with the rigid suspension is increased to reduce the
resonant frequency of the resonant structure to capture the
vibrations at the lower resonant frequency. Thus, a resonant
structure with a rigid suspension has difficulty in capturing
vibrations at the lower resonant frequency.
[0025] A resonant structure with a liquid suspension, however, has
a lower resonant frequency, and so, the size or the volume of the
resonant structure with the liquid suspension may remain the same
or may be reduced to capture the vibrations that occur at the lower
resonant frequency. Thus, the resonant structure with the liquid
suspension may be incorporated into smaller devices, such as smart
watches or other wearable devices, and more efficiently capture and
convert the vibration energy into electrical energy. Additionally,
since the resonant structure with the liquid suspension resonates
at the lower resonant frequency and matches the vibrations at the
lower resonant frequency, the vibration energy harvester more
efficiently captures and converts the kinetic energy of the
vibrations into electrical energy.
[0026] Other benefits and advantages include the liquid suspension
acting as a lubricant between the chamber and the proof mass which
reduces the amount of friction and/or heat generated as a result of
the movement or displacement of the proof mass. Additionally, a
vibration energy harvester that has a resonant structure with a
liquid suspension is more durable than one with a rigid solid
suspension due to the molecular composition of a liquid in
comparison to a solid. The increased durability and reduced
friction decreases the amount of wear and aging to the vibration
energy harvester.
[0027] FIGS. 1A and 1B are exploded views of a vibration energy
harvester 100 that converts kinetic energy, such as vibration
energy, into electrical energy. The vibration energy harvester 100
has a resonant structure that includes a liquid suspension 108, a
frame 110, a proof mass, such as a magnetic array 106 or a coil
array, and a plate, such as a coil plate 102 or a plate with
magnets. The proof mass may be the magnetic array 106 or the coil
array. If the magnetic array 106 is the proof mass, the one or more
coils 104a-c are mounted on the plate to form the coil plate 102.
If the coil array is the proof mass, the one or more magnets of the
magnetic array 106 are mounted on the plate. The coil plate 102 may
have multiple layers. Thus, the magnets or the coils may be the
proof mass so long as the other is mounted on the plate.
[0028] The liquid suspension 108 is formed in an enclosed chamber
made by bonding the plate with a frame 110. The plate, such as the
coil plate 102, may be a micromachined silicon with one or more
electroplated copper coils 104a-c, and the frame 110 may be a
laser-machined acrylic frame. The position of the plate and/or the
shape of the one or more coils 104a-c impacts the generation of the
electromotive force by the vibration energy harvester 100. The
plate may be positioned either above or below the proof mass.
[0029] The position of the plate impacts the generation of the
electromotive force because a gravitational force acts downward on
the proof mass that is suspended within the chamber by the liquid
suspension 108. FIG. 1A is an exploded view of the vibration energy
harvester 100 having the coil plate 102 positioned above the
magnetic array 106. FIG. 1B is an exploded view of the vibration
energy harvester 100 having the coil plate 102 positioned below the
magnetic array 106.
[0030] The downward gravitational force increases the distance
between the plate and the proof mass when the plate is located
above the proof mass and decreases the distance between the plate
and the proof mass when the plate is located below the proof mass.
When the plate is below the proof mass and the downward
gravitational force reduces the distance between the plate and the
proof mass, the vibration energy harvester 100 more efficiently
converts kinetic energy into electrical energy. An increase in the
distance between the plate and the proof mass, when the plate is
above the proof mass, decreases the conversion efficiency of the
vibration energy harvester 100.
[0031] The electromotive force between the plate and the proof mass
is also based on the number of coil turns in each of the one or
more coils and the shape of the one or more coils 104a-c. More coil
turns result in a larger electromotive force produced from the
relative movement between the one or more coils 104a-c and the
magnetic array 106. The one or more coils 104a-c may be shaped as a
square coil, a rectangular coil, a circular coil or any other
shape.
[0032] FIGS. 2A and 2B show a square coil 202 and two rectangular
coils 204a-b, respectively. When the polarization is along the Z
axis and the vibration or displacement is along the X axis, the
electromotive force depends on the Z component of the magnetic flux
density. Therefore, only wires 206 along the Y axis cut the
magnetic field line and contribute to the electromotive force while
the wires 208 along the X axis only add resistance that needs to be
minimized. Since the two rectangular coils 204a-b occupy the same
area as the square coil 202, as long as the coils have the same
wire width and spacing, a larger percentage, i.e., 67% of the wires
in the rectangular coil contribute to the electromotive force while
only half (50%) of the wires in the square coil contribute to the
electromotive force.
[0033] The magnetic array 106 may be formed by multiple magnets
110a-d, e.g., 2-10 Neodymium (NdFeB) magnets. The multiple magnets
110a-d may be arranged with alternating north and south poles to
provide a larger magnetic field gradient. The relative movement
between the one or more coils 104a-c and the magnetic array 106 due
to an applied acceleration produces the electromotive force. The
liquid suspension 108 aligns the center of each of the one or more
coils 104a-c with the boundary between two magnets of the magnetic
array 106 where the magnetic flux gradient peaks to maximize the
electromotive force. For example, the first coil 104a may be
centered at the boundary of the first magnet 110a and the second
magnet 110b.
[0034] The liquid suspension 108 may be water or other liquid, such
as a ferrofluid. The liquid suspension 108 surrounds and encloses
the proof mass within the enclosed chamber. A ferrofluid is a
liquid that has magnetic nanoparticles and becomes strongly
magnetized in the presence of a magnetic field, and is attracted by
a magnet. The ferrofluid liquid suspension works as a mechanical
spring. The attractive forces of the ferrofluid liquid suspension
that surrounds the proof mass counteract each other when the proof
mass is in the center of the enclosed chamber, i.e., in a balanced
position, when there is no applied acceleration. When the proof
mass is displaced, the part of the ferrofluid suspension which has
no symmetric counterpart draws or pulls the proof mass in the
opposite direction. A non-ferrofluid liquid suspension, however,
does not self-align the proof mass in the center of the enclosed
chamber.
[0035] FIGS. 3A-3B illustrate the self-alignment of the magnetic
array 106 within a liquid suspension 108 that is a ferrofluid
liquid suspension. When an acceleration, e.g., due to vibration, is
applied in the direction 308 and displaces the magnetic array 106 a
distance 304, a portion 306 of the ferrofluid liquid suspension
applies a restoring force in the direction 310 along the x-axis to
the magnetic array 106 to pull the magnetic array 106 back into the
balanced position 302. The larger the acceleration that is applied
in the direction 308, the larger the displacement of the magnetic
array 106 from the balanced position 302. The restoring force or
force to pull back the magnetic array 106 is proportional and/or
directly correlated to the deviated distance 304 from the balanced
position 302. Moreover, the restoring force is further based on the
ferrofluid's nanoparticle density and dimensions of the chamber,
i.e., the height and the width of the chamber. When no acceleration
is applied, the ferrofluid liquid suspension automatically keeps
the magnetic array 106 situated in the middle of the chamber in the
balanced position 302 and aligned with the one or more coils
104a-c.
[0036] The magnetic array 106 is aligned with the one or more coils
104a-c when the center of each coil of the one or more coils 104a-c
is aligned with a boundary between two magnets 110a-d. For example,
if there are four adjacent magnets 110a-d and three coils 104a-c
where the center of a first coil 104a is aligned with the boundary
of the first magnet 110a and a second magnet 110b that is adjacent
to the first magnet 110a, the second coil 104b is aligned with the
boundary of the second magnet 110b and a third magnet 110c that is
adjacent to the second magnet 110b, and the third coil 104c is
aligned with the boundary of the third magnet 110c and a fourth
magnet 110d that is adjacent to the third magnet 110c.
[0037] The equivalent spring constant of the ferrofluid liquid
suspension is proportional to the particle density of the
ferrofluid and the dimensions of the chamber. That is, the
restoring force needed to extend or compress the ferrofluid liquid
suspension to displace the proof mass depends on the viscosity of
the ferrofluid. Thus, the viscosity of the liquid suspension is
directly proportional to the equivalent spring constant which is
directly correlated with the resonant frequency so a less condensed
or viscous liquid has a lower spring constant and a lower resonant
frequency than a highly viscous liquid. The magnetic properties of
the magnetic nanoparticles in the ferrofluid also effect the spring
constant and the resonant frequency. The strength of the magnetic
field generated by the magnetic nanoparticles also directly
correlates with the spring constant and corresponding frequency.
That is, a stronger magnetic field has a greater spring constant
and corresponding resonant frequency, and a weaker magnetic field
has a smaller spring constant and corresponding resonant frequency.
By having a lower resonant frequency, the resonant structure is
able to more efficiently capture and convert vibrations that are at
a lower resonant frequency to electrical energy.
[0038] The vibration energy harvester 100 may be tuned to more
efficiently capture and convert vibration energy into electrical
energy by altering and/or adjusting the dimensions, parameters
and/or configurations of the resonant structure including shape,
position and/or size of the coils, the number and magnetic
characteristics of the magnets and/or the composition of the liquid
suspension, e.g., the viscosity, density and/or magnetic
characteristics of the liquid suspension. The vibration energy
harvester 100 in FIG. 1A has, for example, the following dimensions
and parameters with the coil plate 102 located above the magnetic
array 106:
TABLE-US-00001 TABLE 1 Dimensions and Parameters of the Vibration
Energy Harvester Magnet size 6.4 .times. 3.2 .times. 0.8 mm.sup.3
Coil width 100 .mu.m Surface field 2,186 Gauss Coil spacing 100
.mu.m Total weight 1 g Coil thickness 30 .mu.m Total volume 18.4
.times. 11.times. 1.7 mm.sup.3 Resistance 2.3 .OMEGA.
[0039] The measured output voltages as a function of frequency
under various accelerations are shown in FIG. 4A for a vibration
energy harvester 100 having the dimensions and parameters of Table
1 and filled with a ferrofluid liquid suspension, such as Ferrotec
APG 1123. At a fixed acceleration, the voltage depends on the
vibration frequency and peaks at a resonant frequency. The measured
resonant frequency is approximately 340 Hz. The resonant frequency
decreases as the input acceleration increases which indicates that
the ferrofluid liquid suspension becomes softer as the vibrational
amplitude increases.
[0040] The root mean square (rms) of the output voltage and power
that may be delivered to a matched load are shown in FIG. 4B. The
output voltage depends linearly on the applied acceleration, and 36
nW is delivered into a load of 2.3.OMEGA. from 7 g acceleration
which corresponds to 17 .mu.m of vibrational amplitude at 320
Hz.
[0041] In another example, the vibration energy harvester 100 of
FIG. 1B has the following dimensions and parameters with the coil
plate 102 located below the magnetic array 106:
TABLE-US-00002 TABLE 2 Dimensions and Parameters of the Vibration
Energy Harvester Magnet size 6.4 .times. 3.2 .times. 0.8 mm.sup.3
Coil width 100 .mu.m Surface field 2,186 Gauss Coil thickness 30
.mu.m Total weight 1 g # of layers 6 Total volume 17 .times. 11
.times. 2.5 mm.sup.3 Resistance 4.5 .OMEGA.
[0042] The vibration energy harvester 100 with the dimensions and
parameters of Table 2 may be filled with a less dense, water-based
ferrofluid, such as Ferrotec, EMG 705, to further reduce the
resonant frequency. The frequency response is shown in FIG. 5A and
is approximately 15 Hz. The output voltage power delivered into a
matched load is shown in FIG. 5B, and 176 nW is delivered into a
load of 4.5.OMEGA. from 3.5 g acceleration which corresponds to 3.9
mm of vibrational amplitude at 15 Hz. The resonant frequency has
been reduced due to the adoption of a less dense ferrofluid, and
the conversion efficiency has been increased through a multilayer
coil plate and having the coil plate below the magnetic array which
results in a gravitational force pushing the magnetic array closer
to the coil plate.
[0043] FIG. 6 is a flow diagram of an example process 600 for
fabricating a vibration energy harvester. A microfabrication system
may have one or more controllers or processors, appropriately
programmed, to implement the process 600 to form the vibration
energy harvester 100. FIGS. 7A-7F show cross-sectional views of the
fabrication of the vibration energy harvester. The vibration energy
harvester 100 has a coil plate 102, a magnetic array 106 and a
chamber 714.
[0044] The coil plate 102 includes a silicon substrate layer 702
having at trench 706, one or more connection electrodes and at
least one coil. The microfabrication system etches a portion of the
silicon substrate layer (or silicon wafer) 702 to form a thin film
of silicon nitride (SiN) (or SiN micro-diaphragms) 704 through low
pressure chemical vapor deposition (LPCVD) and one or more trenches
706 (602). FIG. 7A shows a cross-sectional view of the vibration
energy harvester being formed after the silicon wafer or substrate
layer is etched. The microfabrication system may use potassium
hydroxide (KOH) to etch the silicon substrate layer 704. The trench
706 may be 200 .mu.m deep.
[0045] The microfabrication system forms the one or more connection
electrodes (604). The microfabrication system deposits and/or
patterns a first set of one or more metal portions 708a-b to form
the one or more connection electrodes. FIG. 7B shows a
cross-sectional view of the vibration energy harvester being formed
after the one or more metal portions are deposited on the thin film
of SiN to form the one or more connection electrodes. The one or
more metal portions 708a-b are positioned in between the coils that
are to be deposited, and may be formed from titanium (Ti) or copper
(Cu).
[0046] The microfabrication system may deposit and pattern a
parylene isolation layer 710 (605). FIG. 7C shows a cross-sectional
view of the vibration energy harvester being formed after the
parylene isolation layer 710 is deposited. The parylene isolation
layer 710 may have a thickness of 1 .mu.m, and provide for
electrical insulation. The parylene isolation layer 710 may be
deposited on top of the thin-film of SiN, the silicon substrate
layer 702 and the first set of the one or more metal portions
708a-b. The microfabrication system may pattern the parylene
isolation layer 710 so that there are one or more gaps that allow
the first set of the one or more metal portions 708a-b to be
accessible to allow for an electrical connection with a second set
of one or more metal portions 712a-d.
[0047] The microfabrication system may perform evaporation
deposition to deposit and pattern the second set of one or more
metal portions 712a-d on top of the parylene isolation layer 710
(606). The one or more metal portions 712a-d may be formed from Ti
or Cu. The one or more metal portions 712a-d may be deposited as a
seed layer.
[0048] After evaporation deposition of the second set of the one or
more metal portions 712a-d, the microfabrication system may
spin-coat and pattern a photoresist for a mold for the coils (607).
The photoresist may have a thickness of approximately 30 .mu.m. The
microfabrication system electroplates a metal, such as Cu, to form
one or more coils of the coil plate using the mold (608). FIG. 7D
shows the vibration energy harvester being formed after evaporation
and electroplating of the second set of the one or more metal
portions 712a-d. The Cu metal may have a thickness of approximately
30 .mu.m. After electroplating, the seed layer and the photoresist
may be removed (609) forming the coil plate 701.
[0049] The microfabrication system may assemble one or more magnets
716 to form a magnetic array 106 within a recessed region of a
laser-cut acrylic plate or chamber 714 (610). FIG. 7E shows a
cross-sectional view of the vibration energy harvester being formed
after assembly of the one or more magnets 716. The one or more
magnets may be arranged with alternating north and south poles and
may be NdFeB magnets. After the one or more magnets 716 are
assembled, the microfabrication system may bond the chamber 714
with the coil plate 102 formed from the silicon substrate layer 702
that has the one or more trenches 706 (612).
[0050] The microfabrication system fills the chamber 714 that
contains the magnetic array 106 with a liquid, e.g., a ferrofluid,
to suspend the magnetic array 106 within the chamber 714 (614). The
chamber 714 is filled with the liquid suspension 718 through an
inlet hole 802 in the chamber 714. FIG. 7F shows a cross-sectional
view of the vibration energy harvester being formed after the
chamber 714 is filled with the liquid suspension 718. FIG. 8A shows
a chamber 714 filled with the liquid suspension 718.
[0051] The microfabrication system determines whether the chamber
714 is fully filled or if there is air, bubbles or other gas within
the chamber 714 (616). If the chamber 714 is not fully filled, the
microfabrication system vacuums the chamber 714 to remove any
gaseous bubbles or air and then refills the chamber 714 with the
liquid suspension 718 (618). FIG. 8B shows the chamber 714 with a
vacant portion 806 after a portion of the air or bubbles 808 has
been vacuumed or removed.
[0052] The microfabrication system continues to refill and vacuum
the chamber 714 with the liquid until the chamber is fully filled.
A chamber 714 is fully filled when there is no air or gas remaining
within the chamber 714. FIG. 8C shows the chamber 714 after several
cycles of filling the chamber 714 with liquid suspension and then
vacuuming the air or bubbles 808 resulting in a smaller vacant
portion 806.
[0053] The microfabrication system seals the inlet hole 802 when
the chamber 714 is fully filled (620). The microfabrication system
seals the inlet hole 802 with a parylene layer to contain the
liquid suspension 718 inside the chamber 714. The parylene layer is
coated over the chamber 714 at room temperature to seal the inlet
hole 802. The parylene is coated conformally over the chamber 714
and may surround the entire chamber 714.
[0054] Exemplary embodiments of the invention have been disclosed
in an illustrative style. Accordingly, the terminology employed
throughout should be read in a non-limiting manner. Although minor
modifications to the teachings herein will occur to those well
versed in the art, it shall be understood that what is intended to
be circumscribed within the scope of the patent warranted hereon
are all such embodiments that reasonably fall within the scope of
the advancement to the art hereby contributed, and that that scope
shall not be restricted, except in light of the appended claims and
their equivalents.
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