U.S. patent application number 11/652272 was filed with the patent office on 2010-10-07 for energy harvester utilizing external magnetic field.
This patent application is currently assigned to FERRO SOLUTIONS, INC.. Invention is credited to David C. Bono, Jiankang Huang, Robert C. O'Handley.
Application Number | 20100253089 11/652272 |
Document ID | / |
Family ID | 42797757 |
Filed Date | 2010-10-07 |
United States Patent
Application |
20100253089 |
Kind Code |
A1 |
Huang; Jiankang ; et
al. |
October 7, 2010 |
ENERGY HARVESTER UTILIZING EXTERNAL MAGNETIC FIELD
Abstract
Apparatus and method for harvesting energy from the environment
and/or other external sources and converting it to useful
electrical energy. The harvester does not contain a permanent
magnet or other local field source but instead relies on the
earth's magnetic field of another source of a magnetic field that
is external to the sensing device. One advantage of these new
harvesters is that they can be made smaller and lighter than energy
harvesters that contain a magnet and/or an inertial mass.
Inventors: |
Huang; Jiankang;
(Roslindale, MA) ; O'Handley; Robert C.; (Andover,
MA) ; Bono; David C.; (Wellesley, MA) |
Correspondence
Address: |
RISSMAN HENDRICKS & OLIVERIO, LLP
100 Cambridge Street, Suite 2101
BOSTON
MA
02114
US
|
Assignee: |
FERRO SOLUTIONS, INC.
Roslindale
MA
|
Family ID: |
42797757 |
Appl. No.: |
11/652272 |
Filed: |
January 11, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10730355 |
Dec 8, 2003 |
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11652272 |
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60431847 |
Dec 9, 2002 |
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60758042 |
Jan 11, 2006 |
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60790921 |
Apr 11, 2006 |
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Current U.S.
Class: |
290/1R ;
307/104 |
Current CPC
Class: |
H01L 41/00 20130101;
G01R 33/18 20130101 |
Class at
Publication: |
290/1.R ;
307/104 |
International
Class: |
F03G 7/08 20060101
F03G007/08; H01F 38/00 20060101 H01F038/00 |
Claims
1-3. (canceled)
4. An energy harvester without a local field source comprising: a
magnetic field sensing element including one or more layers of
magnetostrictive material having a magnetization vector that
responds to variations in an applied magnetic field by generating a
stress, and one or more layers of electroactive material,
mechanically bonded to the layer of magnetostrictive material, that
responds to the stress by generating a voltage; and a circuit
coupled to the sensing element that converts the voltage to
electrical power for immediate use or storage; wherein the sensing
element is mounted to an object that moves relative to the applied
magnetic field, such that changes in orientation of the sensing
element with respect to the applied field generates the
voltage.
5-7. (canceled)
8. The energy harvester of claim 4, wherein the sensing element
includes electrodes for measuring the voltage generated and wherein
the electrodes are configured such that the distance between the
electrodes and cross sectional area between the electrodes are
tailored to produce a desired electrical power; and wherein the
magnetization vector rotates relative to the electrode axis due to
changes in the orientation of the sensor in the applied external
field.
9. The energy harvester of claim 8, wherein local vibrations also
change the orientation of the sensor with respect to the applied
external field.
10-21. (canceled)
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of priority to U.S.
Provisional Application No. 60/758,042 filed Jan. 11, 2006 entitled
"Novel Energy Harvester Without Moving Parts" by J. Huang et al.,
U.S. Provisional Application No. 60/790,921 filed Apr. 11, 2006
entitled "Wireless Transfer of Electrical Power From Outside a Body
to Inside a Body" by J. Huang et al., and U.S. Ser. No. 10/730,355
filed 8 Dec. 2003 entitled "High Sensitivity, Passive Magnetic
Field Sensor and Method of Manufacture" by J. Huang et al., which
claims priority to U.S. Provisional Application No. 60/431,487
filed 9 Dec. 2002, the disclosures of which are hereby incorporated
by reference in their entirety.
FIELD OF THE INVENTION
[0002] This invention relates to apparatus and methods for
harvesting energy from the environment and other sources external
to the harvester and converting it to useful electrical energy.
BACKGROUND OF THE INVENTION
[0003] Energy harvesters are known that convert vibrational energy
into electrical energy. The electrical energy produced can then be
stored or used by other devices. For example, the vibrations of an
air conditioning duct can be converted into electrical energy by an
energy harvester and the electrical energy then used to power a
sensor that measures the air temperature in the duct. The sensor
does not require electrical wiring to a remote source of power or
periodic battery changes.
[0004] There are a variety of such devices for generating
electrical power from vibrations, oscillations or other mechanical
motions. Generally such devices are categorized as inductive,
capacitive, and/or piezoelectric devices. While each of the known
types of vibrational energy harvesters have different advantages,
they also have drawbacks such as: the need for heavy, powerful
permanent magnets (in the case of inductive energy harvesters) to
produce a sufficiently large flux density; an auxiliary source of
power, such as a battery (in the case of capacitive harvesters);
the need for large vibration frequencies and/or a heavy inertial
mass to generate sufficient vibrational energy for harvesting (for
all types of vibration energy harvesters); undesirable levels of
damping and noise generation from interaction between the locally
generated magnetic field and nearby metallic parts (for inductive
harvesters); and other disadvantages such as size or weight that
make them unsuitable for use in a remote or generally inaccessible
location.
[0005] As shown in FIG. 1, a typical magnetic device 2 to harvest
vibrational energy consists of a permanent magnet 3 (an internal
local field source) attached to a housing 7 and connected by a
spring 4 to a passive magnetic field sensor 5 (e.g., an induction
coil or a passive magnetostrictive/electroactive field sensor).
External vibrations cause a relative motion of the magnet 3 and
sensor 5, producing an electrical voltage across a load 8 and a
current through the load. In addition to requiring a local magnetic
field source (e.g., a permanent magnet disposed adjacent to the
field sensor), these devices also typically include an inertial
mass 6 (also known as a proof mass) to increase the vibrational
energy generated. The inertial mass may be rigidly attached to the
sensor (as shown), may be the same or separate from the permanent
magnet, or may comprise part of the sensor itself. As a further
alternative, the magnet may comprise part of the moving proof mass,
as opposed to being fixed to the housing. In each case, the
inertial mass and the permanent magnet increase the size and weight
of the device.
[0006] It would be desirable to provide a magnetic energy harvester
that does not require a local magnetic field source as part of the
device itself. It would also be beneficial to provide a magnetic
energy harvester that does not require a source of vibrational or
other mechanical motion.
SUMMARY OF THE INVENTION
[0007] In accordance with various embodiments of the present
invention, an apparatus is provided for harvesting energy from the
environment or other remote sources and converting it to useful
electrical energy. The harvester does not contain a permanent
magnet or other local field source but instead relies on the
earth's magnetic field or another source of a magnetic field that
is external to the sensing device. One advantage of these new
harvesters is that they can be made smaller and lighter than energy
harvesters that contain a magnet. Another advantage is that they do
not require vibrational energy to function.
[0008] According to various embodiments of the invention disclosed
herein, the harvester differs from those of the prior art by the
absence of a permanent magnet or other local (internal) field
source. In these new devices, a change in the state of
magnetization of the sensing element may be achieved in one (or
both) of two general ways: [0009] 1. The magnetic flux density in
the sensing element may be altered by changes in the orientation of
the sensor (movement of the sensor) with respect to a static
(non-changing) external field. For example, the magnetization
vector M of the sensing element may rotate due to changes in the
orientation of the sensing element with respect to the earth's
field. Such movement of the sensing element may be achieved by
attaching the sensing element to a piece of rotating machinery or a
rotating part on a vehicle. Alternatively, the sensing element may
be suspended on its axis and allowed to rotate (due to external
vibrations), the rotation causing a change in its orientation in
the earth's field. [0010] 2. Alternatively, the sensing element may
remain stationary and be operated on by a remote changing magnetic
field from any of a variety of sources. The remote changing
magnetic field can be produced by an electrical transformer, motor,
electronic device, moving machinery or inductive wire or coil which
is relatively remote (acting at a distance or through a
non-magnetic barrier) on the sensing element. The changing (e.g.,
alternating) magnetic field source can be designed to couple with a
remote sensing element efficiently in terms of frequency, distance,
field orientation and magnitude to deliver power remotely to the
sensing element.
[0011] When operating from the earth's magnetic field, the power
harvested may be less than that achieved with a prior art
vibrational energy harvester having a built-in magnetic field. The
power harvested from a remote field source will be measured in
microwatts per centimeter cubed (.mu.W/cm.sup.3), as opposed to
milliwatts per centimeter cubed (mW/cm.sup.3) for energy harvesters
that include a strong magnet. However, for certain applications the
power delivered by such a small, lightweight and simple energy
harvester will be sufficient and enable new applications.
[0012] When operating near some man-made external sources of
alternating magnetic fields, or when such an external field source
is brought near to the energy harvester, the power harvested can be
considerably greater because the strength of such fields is often
considerably greater than the earth's magnetic field (H.sub.earth
is approximately 0.3 Oe or 3 micro-Tesla).
[0013] A preferred sensing element for use in the present invention
is based on a class of passive magnetostrictive electroactive (PME)
magnetic field sensors that produce a voltage when exposed to a
changing magnetic field. The sensing element is preferably a
layered structure (e.g., sandwich) of magnetostrictive material
bonded to an electroactive material, the latter being poled in a
direction preferably parallel to the plane of the magnetostrictive
layer(s). An external magnetic field causes a magnetization change
in the magnetostrictive layer(s), which respond(s) with a
magnetoelastic stress. Part of the stress is transferred to the
electroactive layer that responds by producing a voltage given by
V.sub.i=g.sub.ij.sigma..sub.jL.sub.i. Here, L.sub.i is the distance
between the electrodes across which the voltage V.sub.i is
measured, .sigma..sub.j is the stress transferred to the
electroactive component, and g.sub.ij is the stress-voltage
coupling coefficient. The voltage is greatest when the direction
i=j. However, in different applications the principal stress and
induced voltage may lie in orthogonal directions (e.g., 1-3
operation), or the principal stress and voltage may act along
different axes (e.g., 1-5 operation).
[0014] The energy harvester of the present invention is more than a
simple passive magnetostrictive/electroactive (PME) field sensor. A
simple PME field sensor is comprised of materials and dimensions
designed preferably to produce a large voltage across a high
impedance circuit, the voltage being indicative of the field of
interest. The electronic circuit for a simple PME sensor is
designed to register a field value. In contrast, the PME energy
harvester of the present invention is comprised of materials and
dimensions designed preferably to produce a voltage and current
that match the impedance of the load to be driven. The PME energy
harvester is coupled to an electronic circuit that converts the PME
output to power for immediate use or storage. The PME element is
preferably optimized to respond to the field strength of the
intended environment, which would generally be much greater than
that of a pure field sensor.
[0015] This new type of energy harvester can be simpler, lighter
and/or more compact than those requiring a permanent magnet as a
field source, and also those requiring an inertial mass for
enhancing vibrational energy. For example, suitable applications
may include wireless monitoring applications, wherein wireless
monitoring is meant to include self powered sensing of local
conditions and processing of the sensor output and self powered
wireless communication to a central data processing point. Other
suitable applications might include wireless transfer of electrical
power over a small distance to a location inaccessible via
electrical leads or not convenient for battery replacement. More
specifically, these applications may include supplying power for:
[0016] wireless health monitoring or condition based maintenance;
[0017] supplementing power or recharging batteries without
physically accessing them; [0018] elimination of wiring of
electrical devices remote from a power source; [0019] wireless
monitoring of temperature, airflow, humidity and gas content in
heating, ventilation and air conditioning (HVAC) systems; [0020]
wireless monitoring of traffic flow, turbulence, noise or personnel
movement; [0021] wireless, self powered security systems; [0022]
powering of mobile electronic instruments; [0023] passive detection
of creep or crack propagation in structures for condition based
maintenance; and [0024] powering of devices implanted in a living
body (or to another inaccessible location) for purposes of sensing,
transmitting, or actuating (e.g., motors, pumps, switches, valves,
electrodes), as well as for accomplishing therapy or other
functions that require a voltage and/or current.
[0025] Furthermore, applications of the new energy harvester are
not limited to vibration rich environments. A simple repetitive
motion or rotation of the more sensitive PME devices as described
herein allows them to operate from the earth's magnetic field. They
can be placed in physically inaccessible locations and activated
from a remote field source that is either present in the
environment or placed there for the purpose of energizing the
device.
[0026] In one embodiment of the invention: an energy harvester
without a local field source comprises:
[0027] a magnetic field sensing element including one or more
layers of magnetostrictive material having a magnetization vector
that responds to variations in an applied magnetic field by
generating a stress, and one or more layers of electroactive
material, mechanically bonded to the layer of magnetostrictive
material, that responds to the stress by generating a voltage;
and
[0028] a circuit coupled to the sensing element that converts the
voltage to electrical power for immediate use or storage, wherein
the sensing element either: [0029] a) moves relative to a remote
static external magnetic field, such that changes in orientation of
the sensing element with respect to the external field generates
the voltage; or [0030] b) is stationary with respect to a remote
changing external magnetic field, wherein the changing external
field causes the sensing element to generate the voltage.
[0031] In various embodiments:
[0032] the electrical power comprises a voltage and current
suitable for an intended application;
[0033] the magnetostrictive material layer has a magnetization
vector that responds to variations in the magnetic field by
rotating in a plane and wherein the electroactive material is poled
in a direction substantially parallel to the plane in which the
magnetization vector rotates;
[0034] the sensing element is mounted to an object that moves
relative to the applied magnetic field;
[0035] the variations in the applied external field are in one or
more of magnitude and direction of the field;
[0036] the sensing element is mounted such that local vibration
changes its orientation with respect to the applied magnetic
field;
[0037] the sensing element includes electrodes for measuring the
voltage generated and wherein the electrodes are configured such
that the distance between the electrodes and cross sectional area
between the electrodes are tailored to produce a desired electrical
power;
[0038] the magnetization vector rotates relative to the electrode
axis due to changes in the orientation of the sensor in the applied
external field;
[0039] local vibrations also change the orientation of the sensor
with respect to the applied external field;
[0040] the remote magnetic field is generated by one or more of an
electrical transformer, motor, actuator, switch, electronic device,
moving machinery or inductor;
[0041] the inductor is a wire or coil through which an alternating
current is flowing, to produce the remote changing external
magnetic field;
[0042] the sensing element is rigidly attached to an inertial
mass;
[0043] the sensing element includes an inertial mass;
[0044] the changing external field or sensing element movement is
at vibration or power transmission frequencies of no greater than 1
kHz;
[0045] the changing external field is at a resonance frequency in
the range of that of the sensing element;
[0046] the changing external field is in a range of 20 to 50
kHz;
[0047] the external field frequency is equal to or close to the
resonance frequency of the sensor, which varies roughly according
to the equation
fr .apprxeq. 1 2 L E eff .rho. eff ##EQU00001##
where L is a characteristic length of the sensor and E.sub.eff and
.rho..sub.eff are the elastic modulus and mass density appropriate
to describe the composite magnetostrictive/electroactive sensor
properties;
[0048] the changing external field and sensing element are within a
resonant frequency range;
[0049] the circuit is within the resonant frequency range;
[0050] the external changing field is outside a human or other
animal body and the sensing element is inside the body.
[0051] In another embodiment of the invention, a method of
harvesting energy comprises:
[0052] providing a magnetic field sensing element including one or
more layers of magnetostrictive material having a magnetization
vector that responds to variations in an applied magnetic field by
generating a stress, and one or more layers of an electroactive
material, mechanically bonded to the layer of magnetostrictive
material, that responds to the stress by generating a voltage;
[0053] wherein the voltage is generated by either: [0054] moving
the sensing element relative to a remote static external magnetic
field, such that changes in orientation in the sensing element with
respect to the external field generates the voltage; or [0055] the
sensing element is stationary with respect to a remote changing
external magnetic field, and the changing external field causes the
sensing element to generate the voltage; and [0056] converting the
generated voltage to electrical power for immediate use or
storage.
[0057] These and other advantages of the present invention may be
better understood by referring to the following detailed
description and accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0058] FIG. 1 is a block schematic diagram of a prior art energy
harvester device having a permanent magnet and inertial mass;
[0059] FIG. 2 is a schematic diagram of an energy harvester
according to one embodiment of the invention, in which the
harvester is attached to a moving part and is actuated by its
motion in the earth's magnetic field;
[0060] FIG. 3 is a schematic diagram of another embodiment of the
energy harvester of the invention, in which a stationary harvester
is actuated by a changing external field (H.sub.earth-modified),
the external field being that of the earth's field changed in
magnitude or direction by a moving object in its path;
[0061] FIG. 4 is a schematic diagram of another embodiment of the
invention in which a stationary harvester device is activated by a
remote external changing magnetic field H.sub.ext, where H.sub.ext
is due to a permanent magnet affixed to a moving part exterior to
the harvester;
[0062] FIG. 5 is a schematic diagram of one construction of a
sensing element useful in the present invention;
[0063] FIG. 6 is a schematic diagram of a circuit for converting
and changing electrical output of the harvester to a DC electrical
signal, conditioning, storing and providing the resulting
electrical energy to a load;
[0064] FIG. 7 is a graph of PME voltage output versus field at 20
Hz for one embodiment of the invention;
[0065] FIGS. 8a-8b are schematic diagrams of two embodiments of an
energy harvester wherein an external changing magnetic field
generated by a coil is transmitted through tissue to an embedded
PME sensor;
[0066] FIG. 9 is a graph of PME power versus load at 1 Oe and 29
kHz according to another embodiment of the invention; and
[0067] FIG. 10 is a block schematic diagram of a passive
magnetostrictive/electroactive sensor constructed in accordance
with one embodiment of the invention.
DETAILED DESCRIPTION
[0068] FIG. 2 illustrates a first embodiment of an energy harvester
device according to the invention, wherein a static external
magnetic field, here H.sub.earth (the earth's magnetic field), acts
at a distance (or through a non-magnetic barrier) on a sensor whose
orientation in the field changes with time. Thus, the flux density
in the sensor is altered by changes in the physical orientation of
the sensor with respect to the direction of the earth's field (or
other substantially static field). FIG. 2 illustrates this with a
non-magnetic barrier 14 separating the energy harvester 12 on the
left, attached to a moving object (rotating machinery part 16),
from the source of external field 18 on the right. The arrow 26
extending across the width of sensor 12 represents the plane of the
magnetization vector M. The arrow 20 illustrates rotation of sensor
12 from a first position 22 (labeled X) to a second position 24
(labeled X'). Alternatively, the sensor can be attached to a
rotating part on a vehicle, a door, or other object that moves
relative to the earth's field. As a further alternative, the sensor
may also be suspended on its axis such that linear vibration acting
on the sensor changes its orientation in the earth's field. The
change in sensor orientation requires some asymmetry in the
suspension for the vibrations to cause rotation of sensor about its
axis. It is preferable that the change in sensor orientation be
such that the axis between its electrodes changes orientation
relative to the static field direction. In other words, the sensor
motion should preferably not be rotation about the axis of the
applied field.
[0069] A second embodiment of the invention is shown in FIG. 3,
wherein a changing external field, H.sub.earth-modified, acts at a
distance (or through a non-magnetic barrier) on a stationary sensor
32. Here the orientation of the field at the sensor location
changes with time. In FIG. 3, a moving magnetic object (rotating
disk 34) in the path of the earth's ambient field causes a change
in that field, and this changing field (H.sub.earth-modified) then
reaches (acts on) the static sensor 32.
[0070] In a further alternative embodiment, shown in FIG. 4, a
permanent magnet 42 is affixed to a rotating or moving object 44
remote from the sensor 48. The change in position of the magnet 42
relative to the stationary sensor 48 causes a changing field (see
field lines 46 of H.sub.ext) that acts on the static sensor 48.
Arrow 45 illustrates rotation of the magnet to a second position
(shaded area 42' at the bottom of rotating disk 44) and the
changing field as dashed lines 46'. Other sources of changing or
alternating magnetic field can be found near electrical
transformers, motors, actuators, switches, many electronic devices,
inductor wires or coils, and near areas of high vehicle traffic or
moving machinery. The remote changing magnetic field source can be
designed to couple with the sensor efficiently in terms of
frequency, distance and magnitude to deliver power remotely to the
sensor.
[0071] FIG. 5 is a schematic illustration of a preferred sensor
configuration 50 for use in the present invention. In this
embodiment, a central layer of an electroactive (e.g., ceramic,
polymer or single crystal piezoelectric; or a relaxor
ferroelectric) material having a polarization vector P is shown,
sandwiched between two layers 54, 56 of magnetostrictive material
(e.g., of a soft ferromagnetic material having a non-zero
magnetostriction) on opposing faces of central layer 52. Each
magnetostrictive layer has a magnetization vector M which is caused
to rotate in the plane of the magnetostrictive layer by an applied
field H. A pair of electrodes 58 are disposed at opposite ends of
the piezoelectric, the axis between the electrodes being parallel
to the plane in which the magnetization vectors rotate. The voltage
V generated in the piezoelectric, resulting from the magnetoelastic
stress generated in the magnetic layers and transferred to the
piezoelectric, can be measured in a circuit coupled to the
electrodes attached to the piezoelectric layers.
[0072] The materials and configuration of the sensor may vary
depending upon the particular application. While it is generally
desirable to use a magnetic material with large magnetostriction
for the magnetic layer(s), it is generally more important (for
optimum power delivery) that the magnetostrictive material have a
large product of a magnetostrictive stress and stiffness modulus
(see "Novel Sensors Based on Magnetostrictive/Piezoelectric
Lamination," J. K. Huang, D. Bono and R. C. O'Handley, Sensors and
Actuators 2006). This insures that the magnetic layer(s) more
effectively transfer stress to the piezoelectric material. For
example, while FeCo (Hyperco) shows a relatively large
magnetostriction (approaching 100 ppm) and is extremely stiff, the
product of these parameters translates to a magnetostrictive stress
of 1.2 MPa. A high-magnetostriction material such as
Fe.sub.2(Dy.sub.2/3Tb.sub.1/3) (known as Terfenol-D) on the other
hand, is mechanically softer than FeCo but shows a much larger
magnetostrictive strain and its magnetostrictive stress approaches
6 MPa.
[0073] It is also important (for optimum power generation) that the
magnetostrictive stress changes by the largest possible amount
under the influence of the changing field strength available at the
sensor. For example, the magnetization vector of FeCo can be
rotated in a field of a few tens of Oe (Oersteds) while the
magnetization vector of Terfenol-D can be rotated in a field of
several hundreds of Oe, provided in each case they are properly
annealed and the aspect ratio of the material in the magnetizing
direction is favorable.
[0074] The class of magnetostrictive materials that can be
magnetized in the weakest fields consist of a variety of amorphous
alloys based on iron (Fe) (optionally in combination with nickel
(Ni)) and with glass formers such as boron (B) (optionally with
silicon (Si)).
[0075] Electro-active materials, such as the commercially available
piezoelectric lead-zirconate-titanates (PZT) have stress-voltage
coefficients, g.sub.13 and g.sub.33, with values approximately
equal to 10 and 24 mV/(Pa-m), respectively. Thus, a stress applied
to the piezoelectric parallel to the direction across which the
voltage is measured is more effective in generating a voltage than
a stress transverse to this direction (out of the plane in which
the vector is rotated by the field). Further, relaxor
ferroelectrics have g.sub.ij values that can be three to four times
those of piezoelectrics. Also useful in these applications are
piezo fibers or manufactured piezo fiber composites. They may have
interdigitated electrodes with various spacings to produce electric
fields along the piezo fibers or they may be electroded across the
thickness of the fibers. Polymeric piezoelectric material(s) (e.g.,
poly-vinylidene-difluoride PVDF) may be advantageous in some
applications.
[0076] There are a number of ways to increase the strength of the
earth's field entering the magnetostrictive layers so as to enhance
the power harvested. One way is to use flux concentrators (e.g.,
fan-shaped soft magnetic layers) placed in series with the sensing
element in the presence of the field.
[0077] The sensor output can be adapted for immediate use or
storage by coupling the sensor to an electronic circuit. One such
circuit 70 is shown in FIG. 6. On the left hand side, a PME energy
harvester Y1 is shown. A diode bridge D1 is disposed in parallel
across the harvester output. The full wave diode bridge converts
the AC electric charge on the harvester to a DC charge. Connected
in parallel to the diode bridge is an energy storage capacitor C1
which stores the harvested energy as a voltage across it. Parallel
to the capacitor is a limiter zener diode D2 which prevents
overcharging of the capacitor C1 beyond its breakdown voltage. Next
provided in parallel to the capacitor and diode bridge is a voltage
regulator U1. The voltage regulating circuit reduces the capacitor
voltage to a useful level for a load. The voltage regulated output
across J1 is applied to the load, here represented as a load
impedance Z1, which typically includes resistive and capacitive
elements, and which uses the harvested energy to do useful
work.
[0078] FIG. 7 is a comparison of the PME output voltage signal (RMS
voltage in millivolts) versus magnetic field strength (H in telsa).
In this embodiment the changing external magnetic field is at a low
frequency of 20 Hz. The PME voltage output linearly increases from
0 to 650 millivolts with increasing magnetic field strength from 0
to 2 Oe. Alternatively, the magnetic field can be static and the
position of the PME varying. The substantially linear relationship
between the PME voltage output and magnetic filed strength is
representative for low frequency applications (where the field
changes or the sensor motions are at low vibration frequencies or
power transmission frequencies).
[0079] Alternatively, a higher frequency external field can be used
to obtain a greater level of power from the PME (compared to the
low frequency operation of FIG. 7). This is illustrated with the
embodiment and resulting power output shown in FIGS. 8-9. FIG. 8a
illustrates a means of delivering power inside a living organism
(or any inaccessible or difficult to access location) without
requiring the use of electrical wiring between the source of the
power and the target device and without requiring (or with
diminished need for) batteries. FIG. 8a shows an external loop
antenna 80 generating an alternating magnetic field outside of the
body. The magnetic field 84 generated by this loop antenna is
transmitted through the skin and other tissue 82 to an embedded PME
sensor 86 producing a resulting output voltage V. The power
transmission here is achieved not by a high frequency microwave, RF
or other electromagnetic wave, but rather by means of a relatively
low frequency, benign, alternating magnetic field. Microwaves and
other electromagnetic waves having a wavelength comparable to or
less than the distance between the source and receiver, are rapidly
attenuated by water or metals, and thus would not be suitable in
this application. Instead, the loop antenna produces a
low-frequency, magnetic-rich waves which are left essentially
unattenuated by tissue (assuming no intervening magnetic material),
and which do not have problems with tissue heating that accompanies
microwaves. Alternatively, instead of a loop antenna (with no core)
the external source could be a core-filled coil antenna such as a
solenoid coil with core 90 (FIG. 8b), wherein the core may
significantly enhance the field 92 in the body
[0080] The field generated by a loop, solenoid or core-filled coil
antenna is richer in magnetic field strength than electric field
strength within a range comparable to the wavelength of the
radiation. The wavelength is given by the equation .lamda.=c/f,
where c is the speed of light in the medium, and f is the frequency
of the radiation. At 1 MHZ (megahertz) in air, .lamda. equals 300 m
(meters); at 100 MHZ, .lamda. equals 3 m. Thus, there is a wide
range of frequencies over which to transmit a magnetic-field rich
electromagnetic wave without significant attenuation.
[0081] The implanted passive magnetostrictive/electroactive (PME)
sensor/transducer 86 receives and converts the AC magnetic field 84
to an AC voltage that can be processed to produce power needed for
a particular application. For example, this apparatus can be used
in powering internal pumps, sensors, valves and transponders in
human and animals. More generally, it can be used to power devices
which monitor health, organ function or medication needs, and for
performing active functions such as pumping, valving, stimulation
of cell growth or accelerated drug or radiation treatment locally.
The described means of delivering power inside a living organism
can be achieved without the use of electrical wires in between the
source of the power and the target device and without the need, or
diminished need, for batteries.
[0082] The wireless power harvested for the remote application can
be optimized, for example, if resonance is achieved at each stage
of transduction. Thus the external power source and the transmit
antenna should be in resonance. The PME device should also be in
resonance with the field it responds to, and the PME device should
also be in resonance with the part of the circuit that receives the
signal from the PME device. By careful design and material
selection, it is possible for all three resonances to closely
coincide. FIG. 9 illustrates one example of a PME power output
(mW/cm.sup.3) versus load (ohm) for one such resonant system
operating at a frequency of 29 kHz and a field of one (1) Oe.
[0083] The remote sensor can be used not only for powering internal
pumps, sensors and transponders in humans and animals, but can be
used to monitor the flow of things (people or inanimate objects)
past gates (either for security or tracking purposed).
[0084] There will now be described in more detail alternative
sensor configurations and sensor materials which may be useful in
various embodiments or the present invention.
[0085] FIG. 10 is a block schematic diagram of one embodiment of a
passive magnetostrictive sensor useful in the practice of the
invention. The sensor 400 comprises a magnetic layer 402 that is
bonded to a piezoelectric layer 404 by a suitable non-conductive
means, such as non-conductive epoxy glue. Although only one
magnetic layer 402 is shown bonded to a single piezoelectric layer
404, those skilled in the art would understand that two or more
magnetic layers can be used. The magnetization vector 415 (M) of
the magnetic material 402 rotates in the plane 416 of the magnetic
layer 402 when an external magnetic field (H) is applied as shown
by the arrow 414. The rotation of the magnetization vector M causes
a stress in the magnetostrictive layer 402 which is, in turn,
applied to the piezoelectric layer 404 to which the magnetic layer
402 is bonded. In this design the direction of magnetization, M,
rotates in the preferred plane of magnetization, changing direction
from being parallel to perpendicular (or vice versa) to a line
joining the electrodes. This maximizes the stress change
transferred to the electroactive element.
[0086] The stress-induced voltage in the piezoelectric material 404
is measured across a pair of electrodes 406 and 407 of which only
electrode 406 is shown in FIG. 10. The magnitude of the voltage
developed across electrodes 406 and 407 is a function of the
magnetic field strength for H<H.sub.a, the anisotropy field (at
which M is parallel to the applied field) and can be utilized to
power a device 410 that is connected to electrodes 406 and 407 by
conductors 412 and 408, respectively.
[0087] The sensor is constructed so that stress-induced voltage is
measured in a direction that is parallel to the plane 416 in which
the magnetization rotates. The stress is generated in the magnetic
material 402, which responds to an external magnetic field 414 (H)
with a magnetoelastic stress, .sigma..sub.mag, that has a value in
the approximate range of 10 to 60 MPa. Because the magnetic
material 402 is bonded to a piezoelectric layer 404, the layer 404
responds to the magnetostrictive stress with a voltage proportional
to the stress, .sigma..sub.mag, transmitted to it. Piezoelectric
materials respond to a stress with a voltage, V, that is a function
of the applied stress, a voltage-stress constant, and the distance,
l between the electrodes. In particular,
.delta.V=g.sub.ij.sup.piezof.delta..sigma..sub.magl
[0088] Here .delta..sigma..sub.mag is the change in magnetic stress
that is generated in the magnetic material by the field-induced
change in its magnetization direction. A fraction, f of this stress
is transferred to the electroactive element. .delta.V is the
resulting stress-induced change in voltage across the electrodes on
the electroactive element.
[0089] If the voltage is measured in a direction orthogonal to the
direction in which the stress changes, then g.sub.ij=g.sub.13. As
mentioned previously, typically piezoelectric values for g.sub.13
are 10 millivolt/(meter-Pa). However, if the voltage is measured in
a direction parallel to the principal direction in which the stress
changes in accordance with the embodiment of FIG. 4, then
g.sub.ij=g.sub.33. Thus, the sensor operates in a g.sub.33 or
d.sub.33 mode. For a typical piezoelectric material g.sub.33=24
millivolt/(meter-Pa)=0.024 volt-meter/Newton. In this case, a
stress of 1 MPa generates an electric field of 24 kilovolt/meter.
This field generates a voltage of 240 V across a 1 cm (l=0.01 m)
wide piezoelectric layer.
[0090] The stress generated by the magnetic material 402 depends on
the extent of rotation of its magnetization, a 90 degree rotation
producing the full magnetoelastic stress. The extent of the
rotation, in turn, depends of the angle between the magnetization
vector 415 and the applied magnetic field direction 414 and also
depends on the strength of the magnetic field and on the strength
of the magnetic anisotropy (magnetocrystalline, shape and
stress-induced) in the magnetic layer. The fraction, f of the
magnetostrictive stress, .sigma..sub.mag, transferred from magnetic
to the piezoelectric layer depends on the
(stiffness.times.thickness) product of the magnetic material, the
effective mechanical impedance of the bond between the magnetic and
electric elements (proportional to its stiffness/thickness), and
the inverse of the (stiffness.times.thickness) of the piezoelectric
layer.
[0091] A quality factor may be defined from the above equation to
indicate the sensitivity of the device, that is, the voltage output
per unit magnetic field, H (Volts-m/A):
.differential. V .differential. H = g 33 piezo f ( .differential.
.sigma. mag .differential. H ) l ##EQU00002##
[0092] The characteristics of a suitable magnetostrictive material
are preferably large internal magnetic stress change as the
magnetization direction is changed. This stress is governed by the
magnetoelastic coupling coefficient, B.sub.1, which, in an
unconstrained sample, produces the magnetostrictive strain or
magnetostriction, .lamda., proportional to B.sub.1 and inversely
proportional to the elastic modulus of the material. It is also
important that the magnetization direction of the magnetic material
can be rotated by a magnetic field of magnitude comparable to the
applied field. In general, the magnetic material should also be
mechanically robust, relatively stable (not prone to corrosion or
decomposition), and receptive to adhesives. In addition, if the
magnetic material is electrically non-conducting, it can be bonded
to the electroactive element with the thinnest non-conducting
adhesive layer that provides the needed strength without danger of
shorting out the stress-induced voltage developed across the
electroactive element. For PME devices in which the voltage is
measured across electrodes that are not the same as the
magnetostrictive layers, care must be taken that the
magnetostrictive layers not short out the voltage between the
measuring electrodes. This can be accomplished by using a
non-conducting adhesive to insulate the magnetostrictive layer(s)
from the electroactive element(s).
[0093] Many known magnetostrictive materials can be used for the
magnetic layer 402. These include various magnetic alloys, such as
amorphous-FeBSi or Fe--Co--B--Si alloys, as well as polycrystalline
nickel, iron-nickel alloys, or iron-cobalt alloys such as
Fe.sub.50Co.sub.50 (Hyperco). For example, amorphous iron and/or
nickel boron-silicon alloys of the form
Fe.sub.xB.sub.ySi.sub.1-x-y, where 70<x<86 at %,
2<y<20, and 0<z=1-x-y<8 at % are suitable for use with
the invention with a preferred composition near
Fe.sub.78B.sub.20Si.sub.2. Also suitable are alloys of the form
Fe.sub.xCo.sub.yB.sub.zSi.sub.1-x-y-z where 70<x+y<86 at %
and y is between 1 and 46 at %, 2<z<18, and
0<1-x-y-z<16 at %, with a preferred composition near
Fe.sub.68Co.sub.10B.sub.18Si.sub.4. Iron-nickel alloys with Ni
between 40 and 70 at % with a preferred composition near 50% Ni can
be used. Similarly, iron-cobalt alloys with Co between 30 and 80%
and a preferred composition near 55% Co (such as
Fe.sub.50Co.sub.50.) are also suitable.
[0094] Another magnetostrictive material that is also suitable for
use with the invention is Terfenol-D.RTM.
(Tb.sub.xDy.sub.1-xFe.sub.y), an alloy of rare earth elements
Dysprosium and Terbium with the transition metal iron, manufactured
by ETREMA Products, Inc., 2500 N. Loop Drive, Ames, Iowa 50010,
among others. Terfenol-D.RTM. can generate a maximum stress on the
order of 60 MPa for a 90-degree rotation of its magnetization. Such
a rotation can be accomplished by an external applied magnetic
field on the order of 400 to 1000 Oersteds (Oe). Also useful are
highly magnetostrictive alloys such as Galfenol.RTM.,
Fe.sub.1-xGa.sub.x. (ETREMA Products). Softer magnetic materials,
such as certain Fe-rich amorphous alloys mentioned above, may
achieve full rotation of magnetization in fields of order 10 Oe,
making them suitable for the magnetic layer in a sensor for sensing
weaker fields. Finally, it is possible to use certain so-called
nanocrystalline magnetic materials. In these polycrystalline
materials, it is generally that case that the magnetization can be
rotated as easily as it can be in amorphous materials. But
nanocrystalline materials can sometimes be engineered to have
larger magnetoelastic coupling coefficients than amorphous
materials.
[0095] The preferred characteristics of a suitable electroactive
layer for the sensor devices are primarily that they have a large
stress-voltage coupling coefficient, g.sub.33. In addition, they
preferably should be mechanically robust, receptive to adhesives,
not degrade the metallic electrodes that must be placed on them
(this is most often easily achieved when the electrodes are made of
noble metals, such as silver or gold). Generally, the electroactive
material is chosen on the basis of having a value of g.sub.ij
greater than 10 mV/(Pa-m).
[0096] The electroactive layer can be a ceramic piezoelectric
material such as lead zirconate titanate
Pb(Zr.sub.xTi.sub.1-x)O.sub.3, or variations thereof, aluminum
nitride (AlN) or simply quartz, SiO.sub.x. In some applications a
single crystal (as opposed to a ceramic or polycrystalline)
piezoelectric material may be advantageous. Alternatively, a
polymeric piezoelectric material such as polyvinylidene difluoride
(PVDF) would be suitable for applications where the stress
transferred from the magnetostrictive material is relatively weak.
The softness of the polymer will allow it to be strained
significantly under weaker applied stress to produce a useful
polarization, or voltage across its electrodes. It is also
advantageous in some applications to use another electroactive
material, such as an electrostrictive material (for example,
(Bi.sub.0.5Na.sub.0.5).sub.1-xBa.sub.xZr.sub.yTi.sub.1-yO.sub.3) or
a relaxor ferroelectric material (for example,
Pb(Mg.sub.1/3Nb.sub.2/3).sub.3). Collectively, the piezoelectric,
ferroelectric, electrostrictive and relaxor ferroelectric layers
are called "electroactive" layers.
[0097] Piezoelectric materials typically have
g.sub.33.about.4.times.g.sub.31 and g.sub.33.apprxeq.20 to 30
mV/(Pa-m) which is about 10.times.d.sub.31. For PVDF,
g.sub.33.apprxeq.100 mV/(Pa-m) and some relaxor ferroelectrics can
have g.sub.22.apprxeq.60 my/(Pa-m).
[0098] Model predictions and experimental results shown in Table 1
compares the parameters g.sub.ij, in mV/m-Pa, the electrode spacing
l in meters, the maximum stress per unit field
(B.sub.1/.mu..sub.oH.sub.a) in Pa/T, and calculated field
sensitivity in nV/nT and the observed field sensitivity, dV/dB. The
values tabulated for a g.sub.33 device using a relaxor
ferroelectric are based on the data observed with a piezoelectric
based sensor and using a ratio of g.sub.33 for typical
relaxors/piezoelectrics.
TABLE-US-00001 TABLE 1 max. Sensitivity g.sub.ij l stress Calc.
Obs. Piezo/magnetic sensors: d.sub.31 sensor 11 10.sup.-3 10.sup.8
10.sup.4 280 d.sub.31 sensor 11 10.sup.-3 10.sup.8 10.sup.4 1,200
d.sub.33 sensor 24 10.sup.-2 10.sup.9 2 .times. 10.sup.5 1.5
.times. 10.sup.4 Relaxor/magnetic sensors: d.sub.33 relaxor/mag
sensor 60 10.sup.-2 10.sup.9 10.sup.6 .sup. (10.sup.5)
[0099] The calculated sensitivity in the table is defined with
perfect stress coupling, namely f=1 in MKS units (V/Tesla) as
.differential. V .mu. o .differential. H .apprxeq. g 33 piezo l B 1
.mu. o H a ##EQU00003##
[0100] Here B.sub.1 is the magnetoelastic coupling coefficient, a
material constant that generates the magnetic stress in the
magnetostrictive material, .sigma..sub.m, which was used in earlier
equations.
[0101] Other useful sensor embodiments are disclosed in U.S. Ser.
No. 10/730,355 filed 8 Dec. 2003 entitled "High Sensitivity,
Passive Magnetic Field Sensor and Method of Manufacture," by J.
Huang, et al., the subject matter of which is incorporated by
reference herein in its entirety.
[0102] Although exemplary embodiments of the invention have been
disclosed, it will be apparent to those skilled in the art that
various changes and modifications can be made which will achieve
all or some of the advantages of the invention.
* * * * *