U.S. patent application number 11/354399 was filed with the patent office on 2007-08-16 for injection molded energy harvesting device.
This patent application is currently assigned to Robert Bosch GmbH. Invention is credited to Brian Stark.
Application Number | 20070188053 11/354399 |
Document ID | / |
Family ID | 38123866 |
Filed Date | 2007-08-16 |
United States Patent
Application |
20070188053 |
Kind Code |
A1 |
Stark; Brian |
August 16, 2007 |
Injection molded energy harvesting device
Abstract
Embodiments of an injection molded energy harvesting device are
described. In one embodiment, a piezoelectric cantilever is
produced via an injection molding method to harvest vibration
energy from an environment being sensed. The cantilever device
consists of a piezoelectric material member, a proof mass of high
density material coupled to the piezoelectric member, and a
leadframe for electrical connection. The piezoelectric member is
electrically attached to the leadframe with a standard connecting
material. The entire assembly is then injection molded with
plastic. The plastic encased piezoelectric member forms a
cantilever that generates electricity in response to vibration
exerted on the proof mass.
Inventors: |
Stark; Brian; (Palo Alto,
CA) |
Correspondence
Address: |
COURTNEY STANIFORD & GREGORY LLP
P.O. BOX 9686
SAN JOSE
CA
95157
US
|
Assignee: |
Robert Bosch GmbH
|
Family ID: |
38123866 |
Appl. No.: |
11/354399 |
Filed: |
February 14, 2006 |
Current U.S.
Class: |
310/339 |
Current CPC
Class: |
H01L 41/053 20130101;
B60C 23/041 20130101; Y10T 29/49128 20150115; H01L 41/23 20130101;
Y10T 29/49121 20150115; H02N 2/186 20130101; Y10T 29/49165
20150115; Y10T 29/42 20150115; H01L 41/1136 20130101; Y10T 29/4913
20150115; Y10T 29/49126 20150115 |
Class at
Publication: |
310/339 |
International
Class: |
H01L 41/00 20060101
H01L041/00 |
Claims
1. A device comprising: a plurality of conductive leads; a
substrate strip coupled to one or more conductive leads of the
plurality of conductive leads; a piezoelectric material coupled to
the substrate strip to form a piezo bimorph element; a proof mass
coupled to the bimorph element; a body coupled to the piezo bimorph
element and formed by an injection molding process that encases at
least a portion of the piezo bimorph element and at least a portion
of the one or more conductive leads.
2. The device of claim 1, wherein the plurality of conductive leads
are part of a lead frame comprising one or more rails connected to
the plurality of conductive leads, and wherein the body is formed
around a portion of the lead frame and electrical leads are formed
by removing the one or more rails of the lead frame after forming
the body by the injection molding process.
3. The device of claim 1, wherein the piezoelectric material
comprises Lead Zirconate Titanate (PZT), and the proof mass
comprises tungsten.
4. The device of claim 3, wherein the proof mass is encased in a
material formed by the injection molding process.
5. The device of claim 1, wherein the substrate strip comprises one
of a plastic substrate, a piezoelectric material substrate, and a
strip formed from a portion of the lead frame.
6. The device of claim 1 wherein the piezo bimorph element is
configured to deflect in a direction corresponding to a vibration
force induced onto the proof mass and generate an electric current
in response to the vibration force.
7. The device of claim 6, wherein the electric current is provided
to a sensor device mounted in a system containing the plastic body
to provide operating power to the sensor device.
8. A system comprising: a microsystem sensor including a sensor
circuit, a transmitter circuit coupled to the sensor circuit, and
an antenna coupled to the transmitter circuit; and a power circuit
coupled to the microsystem sensor, the microsystem sensor including
a plastic body, a piezo bimorph strip coupled to the plastic body,
and a proof mass coupled to the plastic body, the plastic body
formed by an injection molding process configured to encase the
plastic body and leave a portion of the piezo bimorph strip
exposed.
9. The system of claim 8, wherein the power circuit is coupled to
the microsystem sensor through a plurality of leads, and wherein
the plastic body is formed around a lead frame and the electrical
leads are formed by removing one or more rails of the lead frame
after encasing the plastic body by the injection molding
process.
10. The system of claim 9, wherein the piezo bimorph strip
comprises a piezoelectric material coupled to a substrate.
11. The system of claim 10, wherein the substrate comprises a strip
of material made of one of the following: plastic, piezoelectric
material, and metal.
12. The system of claim 10 wherein the piezo bimorph strip is
configured to deflect in a direction corresponding to a vibration
force induced onto the proof mass and generate an electric current
in response to the vibration force to provide operating power to
the microsystem sensor.
13. The system of claim 12, wherein the piezoelectric material
comprises Lead Zirconate Titanate (PZT), and the proof mass
comprises tungsten.
14. The system of claim 9, wherein the proof mass is encased within
the plastic body formed by the injection molding process.
15. The system of claim 9, microsystem sensor comprises an air
pressure sensor.
16. The system of claim 15, wherein the air pressure sensor and
power circuit are mounted inside the tire of a vehicle.
17. A method comprising: providing a lead frame comprising paired
conductive members connected to rail elements; attaching adhesive
to one or more conductive members of the lead frame; attaching a
piezoelectric element at least one conductive member of the one or
more conductive members of the lead frame; attaching a proof mass
to a first portion of piezoelectric element; forming an injection
molded plastic body around a second portion of the piezoelectric
element; and cutting the rail elements of the lead frame from the
plastic body to form separate piezoelectric devices.
18. The method of claim 17, wherein the first portion and second
portion of the piezoelectric element are coincident such that the
injection molded plastic body is formed around the proof mass.
19. The method of claim 17 further comprising the step of forming a
piezo bimorph strip by bonding the first portion of the
piezoelectric element to a substrate, the substrate comprising a
strip material made of one of the following: plastic, piezoelectric
material, and metal.
20. The method of claim 19, further comprising the step of leaving
portions of the one or more conductive members of the lead frame
exposed from the injection molded plastic body to form electrical
contacts.
Description
FIELD
[0001] Embodiments of the invention relate generally to
miniaturized electrical systems, and specifically to injection
molded devices for harvesting energy.
BACKGROUND
[0002] The use of miniaturized electrical systems (microsystems) on
the order of 1 cc has been proposed to provide distributed sensing
capability. Microsystem sensors can be used to monitor various
environmental and operational conditions and transmit signals back
to a host receiver for many different applications, such as
industrial monitoring, security applications, weather prediction,
and so on. The design and implementation of such devices and
systems requires overcoming several challenges, such as designing
small and robust packaging and providing adequate transmitter
power. A major consideration in designing such systems remains
providing adequate electrical power, and for many microsystems,
this challenge remains a significant obstacle. In general, current
miniature battery technologies cannot store enough energy to power
these systems for long periods of time, such as on the order of
months. Another disadvantage of battery use is that many sensor
applications involve harsh or limited access environments that can
limit or disable battery performance and/or render battery
maintenance virtually impossible.
[0003] One approach to overcome the problem of providing enough
battery power for microsystems is to extract energy from the
surrounding environment. This approach, which is called energy
harvesting (or scavenging) eliminates the need for an external or
stored power supply, thus allowing a system to be made fully
autonomous, that is, one that requires no external power
connections or maintenance. As long as the source of environmental
energy is available, an energy harvesting microsystem can remained
fully powered, virtually non-stop, while providing information to
the user.
[0004] Several techniques have been proposed and developed to
extract energy from the environment. The most common available
sources of energy are vibration, temperature, and stress
(pressure). In many environmental applications, vibration energy
may be the most readily available and easiest to convert into
electricity. In general, vibration energy can be converted into
electrical energy using one of three techniques: electrostatic
charge, magnetic fields, and piezoelectric materials. Piezoelectric
generation of electricity from vibration energy typically
represents the most cost-effective approach, as the electrostatic
and magnetic techniques usually require more extensive design,
packaging, and integration work to adapt to particular
applications.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] Embodiments of the present invention are illustrated by way
of example and not limitation in the figures of the accompanying
drawings, in which like references indicate similar elements and in
which:
[0006] FIG. 1 illustrates vibration energy, such as in a rotating
tire that can be used in an energy harvesting device according to
an embodiment.
[0007] FIG. 2A illustrates a piezoelectric bimorph for use in an
energy harvesting device, according to an embodiment.
[0008] FIG. 2B illustrates the piezoelectric bimorph of FIG. 2A
under a transverse loading.
[0009] FIG. 3 is a flowchart that illustrates a method of
manufacturing an injection molded energy harvesting device,
according to an embodiment.
[0010] FIG. 4 illustrates a piezoelectric element attached to a
leadframe, under an embodiment.
[0011] FIG. 5 illustrates an injection molded plastic body encasing
the piezoelectric/leadframe structure of FIG. 4, according to an
embodiment.
[0012] FIG. 6A illustrates top view of an individual piezoelectric
device formed by cutting the lead frame rails, under an
embodiment.
[0013] FIG. 6B illustrates a front view of the piezoelectric device
of FIG. 6A.
[0014] FIG. 6C illustrates a side view of the piezoelectric device
of FIG. 6A.
[0015] FIG. 7 illustrates a piezoelectric device with a proof mass
attached, according to an embodiment.
[0016] FIG. 8 illustrates a lead frame with a die paddle for use
with a piezoelectric bender, according to an embodiment.
[0017] FIG. 9 illustrates the sensor/transmitter circuit of an
intelligent tire system for use with an injection molded energy
harvesting device, under an embodiment.
DETAILED DESCRIPTION
[0018] Embodiments of an injection molded energy harvesting device
are described. In one embodiment, a piezoelectric cantilever is
produced via an injection molding method to harvest vibration
energy from an environment being sensed. The cantilever device
consists of a piezoelectric material member, a proof mass of high
density material coupled to the piezoelectric member, and a
leadframe for electrical connection. The piezoelectric member is
electrically attached to the leadframe with a standard connecting
material. The entire assembly is then injection molded with
plastic. The plastic encased piezoelectric member forms a
cantilever that generates electricity in response to vibration
exerted on the proof mass. Such a device can be used to provide
power to sensor systems deployed in various vibration intensive
environments, such as tire pressure monitoring systems, seismic
systems, and the like.
[0019] In the following description, numerous specific details are
introduced to provide a thorough understanding of, and enabling
description for, embodiments of an injection molded energy
harvesting device. One skilled in the relevant art, however, will
recognize that these embodiments can be practiced without one or
more of the specific details, or with other components, systems,
and so on. In other instances, well-known structures or operations
are not shown, or are not described in detail, to avoid obscuring
aspects of the disclosed embodiments.
[0020] Microsystem sensors can be used in a variety of different
environments to provide signals that represent one or more
characteristics or parameters of the environment being sensed. One
critical consideration in the installation of Microsystems is
providing power to the sensor. Many environments in which
microsystem sensors are deployed either produce or are subject to
vibrations. In one embodiment, an energy harvesting device uses
vibration energy present in an environment being sensed to produce
electricity to power the sensor.
[0021] Automotive applications represent one field where vibration
energy from motion of the vehicle in use is readily present and can
be used to provide power to sensor networks in a car. In one
embodiment, an energy harvesting device is used in a tire pressure
sensing module that is deployed inside of an automobile, truck or
other vehicle or machine tire to sense the air pressure inside of
the tire and transmit the air pressure information to a control or
processor module that can report low or abnormal tire pressures.
The rubber carcass of a tire as it rolls along a surface produces
vibrations that can be converted into electrical energy. FIG. 1 is
a graph 100 that illustrates the acceleration in tire rubber as the
tire rolls at a specific speed, such as 30 kilometers/hour. The
accelerative force (in g's) along axis 102 is plotted against time
(in seconds) 104 to A pressure sensor mounted within the tire, such
as embedded within or coupled to the wheel or tire carcass can be
used to monitor the pressure inside the tire. For this application,
the use of a battery is impractical because the battery size and
weight may impact the tire balance, excessively cold or warm
temperatures within the tire can significantly affect battery
performance, and replacement and disposal of the battery may be
impractical or costly. In one embodiment, a piezoelectric
cantilever or bender structure is used to provide the requisite
energy to the tire pressure sensor. The piezoelectric bender
converts the accelerative forces, such as those shown in FIG. 1, of
the tire as it rolls into electricity for powering the pressure
sensor.
[0022] In one embodiment, the energy harvesting device for use with
a tire pressure sensor comprises a piezoelectric bender that
includes a piezoelectric bimorph structure. Piezoelectric materials
are materials that convert vibration energy into electric energy. A
single piece of piezoelectric material by itself is generally a
unimorph structure that exhibits stress in equal and opposite
directions under transverse loading. Consequently, the output
voltage will be zero in the case of a sinusoidal vibration input. A
bimorph structure has stress in one direction under a transverse
load, and therefore outputs a non-zero voltage under the
application of sinusoidal vibration. To provide adequate power
output in a wide variety of different vibrating environments, a
bimorph structure is generally preferred.
[0023] FIG. 2A illustrates a piezoelectric bender for use in an
energy harvesting device and utilizing a piezoelectric bimorph
structure, according to an embodiment. The piezoelectric bender 200
consists of a plastic base 202, a plastic backing piece 204 coupled
to the plastic base 202, a piezoelectric element 206 attached to
the plastic backing 204, and a proof mass 208 attached to the
piezoelectric element 206. The proof mass could alternatively be
attached to the plastic backing 204. The piezoelectric element 206
and plastic backing piece 204 together form the piezoelectric
bimorph structure of the bender 200. When a vibration force is
induced onto bender structure 200, the bimorph strip consisting of
backing 204 and piezoelectric material 206 is deflected with a
motion proportional to the vibration force. This deflection is
converted into electrical power, amplified, and then transmitted to
other circuitry, such as that in a sensor coupled to structure 200.
In one embodiment, the piezoelectric material is Lead Zirconate
Titanate (PZT), such as a PZT-5A type ceramic, and the proof mass
is Tungsten. The body 202 and backing material 204 can be made of
plastic or any similar inactive material, such as carbon fiber,
nylon, and so on.
[0024] In an alternative embodiment, the bimorph strip can be
implemented as a piezo/metal or piezo/piezo element, or a piezo
stack comprising three or more elements in a sandwich array.
[0025] FIG. 2B illustrates the piezoelectric bimorph of FIG. 2A
under a transverse loading input, such as the vibration illustrated
in FIG. 1. As illustrated in FIG. 2B, the piezoelectric bender 200
bends from a first position x.sub.1 to a second position x.sub.2,
the amount of stress .sigma. produced depends on the displacement
of the proof mass from the first position to the second position.
For an oscillating transverse input, such as a vibration, the
piezo/plastic bimorph 210 will bend in the direction corresponding
to the phase of the vibration, thus producing a positive stress
value dependent on the magnitude of displacement caused by the
transverse load. The PZT material of the bimorph integrates the
stress to produce a power output. The proof mass serves to increase
the stress force since the force is proportional to the mass of the
bimorph strip and the induced acceleration.
[0026] In one embodiment, the piezoelectric bender is constructed
using plastic injection molding and leadframe construction
techniques to facilitate cost-effective manufacture. FIG. 3 is a
flowchart that illustrates a method of manufacturing an injection
molded energy harvesting device, according to an embodiment.
Manufacture of the piezoelectric bender under this embodiment
begins with arrangement of an appropriate leadframe 302. Conductive
epoxy, or a similar adhesive is then applied to the conductive
members of the lead frame to which a piezoelectric element is to be
attached, 304. The piezoelectric element is then attached to the
appropriate conductive members of the leadframe, as shown in block
306 of FIG. 3. FIG. 4 illustrates a piezoelectric element attached
to a leadframe, under an embodiment. The leadframe 402 consists of
a number of conductive members 404 attached to rails 406. The
piezoelectric member 408 is attached to one or more (e.g., two
pairs) of the conductive members through conductive epoxy or
similar adhesive means 410. A wirebond 412 may be provided to
connect the piezoelectric member 408 to an additional conductive
member. Once the body of the device has been formed, the conductive
members constitute leads that extend from the body and provide
electrical and mechanical contact points.
[0027] Depending upon the vibrational environment the piezoelectric
device is subject to, a proof mass may need to be attached to the
piezoelectric bimorph. In one embodiment, a proof mass, such as
proof mass 208 in FIG. 2A, is attached to the piezoelectric member,
as shown in block 307 of FIG. 3. A proof mass generally provides
force amplification for the piezoelectric bender, and may be needed
when the incident accelerative forces are relatively small.
[0028] Once the piezoelectric member is attached to the lead frame
conductors, and any proof mass is attached to the piezoelectric
member, the structure is partially encased plastic using an
injection mold process, as shown in block 308 of FIG. 3. FIG. 5
illustrates an injection molded plastic body encasing the
piezoelectric/leadframe structure of FIG. 4, according to an
embodiment. As shown in FIG. 5, the injection molded plastic body
502 encases a portion of the piezoelectric element, while a
protruding portion 504 of the piezoelectric material remains
exposed. Once the injection molded body has been formed around the
piezoelectric element, the rails are cut away from the connecting
members to expose the contact leads and produce an individual
piezoelectric device, as shown in block 310 of FIG. 3. FIG. 6A
illustrates top view of an individual piezoelectric device 602
formed by cutting the lead frame rails, under an embodiment. FIG.
6B illustrates a front view of the piezoelectric device of FIG. 6A,
and FIG. 6C illustrates a side view of the piezoelectric device of
FIG. 6A. As shown in FIG. 6C, the piezoelectric device 602
comprises a body portion 612 and a plastic/piezo bimorph element
614 that consists of a plastic backing 616 and piezo element 618.
The orientation of the bimorph structure illustrated in FIG. 6C
results in a device that responds to vibrations 620 in an up and
down direction relative to the body 612.
[0029] For the embodiment illustrated in FIG. 6C, the bimorph
element (or strip) 614 is a piezo material mounted on plastic.
Alternatively, the bimorph element may also be fabricated from two
PZT elements bonded together, or a PZT element bonded to a
different material, such as part of the lead frame itself.
[0030] In one embodiment, the proof mass is attached to the exposed
portion of piezoelectric element and is left exposed after the
injection molding process 308. Alternatively, the proof mass can be
also be encased within the injection molded plastic body. FIG. 7
illustrates a piezoelectric device with a proof mass attached,
according to an embodiment in which the proof mass is also molded
over with plastic. The encasing and orientation of the proof mass
relative to the bimorph element can be specifically configured
depending upon the application requirements of the energy
harvesting device. For the embodiment illustrated in FIG. 7, the
orientation of the bimorph structure 702 formed by the exposed
piezoelectric material and plastic backing is perpendicular to the
horizontal plane of the body 706. This results in a piezoelectric
device that responds to vibrations in a side-to-side direction
relative to the body 706. The proof mass 704 attached to the
bimorph structure 702 provides force amplification for the
piezoelectric bender. It may be attached to one side of the surface
of either the piezoelectric or plastic side of the bimorph
structure, or it may be attached to the end of the bimorph
structure. The size, weight, material, and placement of the proof
mass can be adjusted according to the requirements of the
environment in which the piezoelectric device is used.
[0031] Various different types of leadframes can be used in
conjunction with embodiments of the piezoelectric bender. Standard
lead frames, such as that shown in FIG. 3 may be used.
Alternatively, a lead frame with a die paddle may be used. FIG. 8
illustrates a lead frame with a die paddle for use with a
piezoelectric bender, according to an embodiment. In a die paddle
arrangement, one or more pairs of leads 802 are coupled to each
other through a die paddle connection 804.
[0032] An energy harvesting device utilizing a bimorph
piezoelectric bender in an injection molded body can be used in
various different applications that have long-term energy
requirements. In one embodiment, the piezoelectric bender is used
in an intelligent tire system to provide power to a circuit that
comprises a pressure sensor and transmitter circuit for mounting
within the wheel or rubber portion of the tire. FIG. 9 illustrates
the sensor/transmitter circuit of an intelligent tire system for
use with an injection molded, piezoelectric energy harvesting
device, under an embodiment. Circuit 900 consists of a pressure
sensor module 902, an oscillator 904, and a transmitter circuit
906. The transmitter may be an RF based transmitter, in which case,
a surface mount antenna 908 may be provided. The shape of the
circuit 900 may be circular, as shown, or any other suitable shape,
depending upon application. Other sensor circuits 912 can also be
provided to monitor other characteristics, such as tire air
composition, temperature, and so on. A piezoelectric bender 920
coupled to circuit 900 converts vibration energy from the rolling
tire to electricity and provides this power to the circuit. The
piezoelectric bender may be provided on a separate circuit or unit
that is coupled to the sensor circuit, or it may be incorporated on
or tightly coupled to circuit 900. The entire circuit 900
illustrated in FIG. 9 may be on the order of 10 mm, or less, in
diameter, with the piezoelectric bender 920 on the order of 5 mm,
or less, in length.
[0033] Although embodiments have been described in relation to a
tire pressure sensor system, it should be understood that these or
similar embodiments, can be utilized with respect to a wide variety
of other microsystems involving sensors or devices that require and
can operate at relatively low power. These include motion sensors,
infrared sensors, leak detectors, lubricant monitors, and other
applications that have a characteristic that can be measured and
feature a vibrating environment. For example, sensors using an
injection molded piezoelectric bender for electrical power can be
mounted within a vehicle fuel tank to monitor fuel quantity or
quality, or within an engine crankcase to monitor oil quantity and
quality. Embodiments of injection molded energy harvesting device
can be applied to many different industries, such as automotive or
aerospace applications, industrial machinery, seismic applications,
and oceanographic applications; among others.
[0034] The sensors used in conjunction with the injection molded
energy harvesting device can be equipped with any suitable sensing
and transmission circuitry, such as RF, microwave, or similar
wireless communication means. Alternatively, some applications may
be suitable for wired sensor communication.
[0035] Aspects of the injection molded energy harvesting device
described herein may be implemented as functionality programmed
into any of a variety of circuitry, including programmable logic
devices ("PLDs"), such as field programmable gate arrays ("FPGAs"),
programmable array logic ("PAL") devices, electrically programmable
logic and memory devices and standard cell-based devices, as well
as application specific integrated circuits.
[0036] Unless the context clearly requires otherwise, throughout
the description and the claims, the words "comprise," "comprising,"
and the like are to be construed in an inclusive sense as opposed
to an exclusive or exhaustive sense; that is to say, in a sense of
"including, but not limited to." Words using the singular or plural
number also include the plural or singular number respectively.
Additionally, the words "herein," "hereunder," "above," "below,"
and words of similar import refer to this application as a whole
and not to any particular portions of this application. When the
word "or" is used in reference to a list of two or more items, that
word covers all of the following interpretations of the word: any
of the items in the list, all of the items in the list and any
combination of the items in the list.
[0037] The above description of illustrated embodiments of the
energy harvesting device for use with microsystem sensors is not
intended to be exhaustive or to limit the embodiments to the
precise form or instructions disclosed. While specific embodiments
of, and examples for, the energy harvesting device are described
herein for illustrative purposes, various equivalent modifications
are possible within the scope of the described embodiments, as
those skilled in the relevant art will recognize.
[0038] The elements and acts of the various embodiments described
above can be combined to provide further embodiments. These and
other changes can be made to the energy harvesting device in light
of the above detailed description.
[0039] In general, in the following claims, the terms used should
not be construed to limit the described system to the specific
embodiments disclosed in the specification and the claims, but
should be construed to include all operations or processes that
operate under the claims. Accordingly, the described system is not
limited by the disclosure, but instead the scope of the recited
method is to be determined entirely by the claims.
[0040] While certain aspects of the energy harvesting device are
presented below in certain claim forms, the inventor contemplates
the various aspects of the methodology in any number of claim
forms. Accordingly, the inventor reserves the right to add
additional claims after filing the application to pursue such
additional claim forms for other aspects of the described
system.
* * * * *