U.S. patent application number 09/837773 was filed with the patent office on 2002-04-25 for packaged strain actuator.
Invention is credited to Crawley, Edward, Lazarus, Kenneth B., Lundstrom, Mark E., Moore, Jeffrey W..
Application Number | 20020047499 09/837773 |
Document ID | / |
Family ID | 22691931 |
Filed Date | 2002-04-25 |
United States Patent
Application |
20020047499 |
Kind Code |
A1 |
Lazarus, Kenneth B. ; et
al. |
April 25, 2002 |
Packaged strain actuator
Abstract
A modular actuator assembly includes one or more plates or
elements of electro-active material bonded to an electroded sheet,
preferably by a structural polymer to form a card. The card is
sealed, and may itself constitute a practical device, such as a
vane, shaker, stirrer, lever, pusher or sonicator for direct
contact with a solid or immersion in a fluid, or may be bonded by a
stiff adhesive to make a surface-to-surface mechanical coupling
with a solid workpiece, device, substrate, machine or sample. The
structural polymer provides a bending stiffness such that the thin
plate does not deform to its breaking point, and a mechanical
stiffness such that shear forces are efficiently coupled from the
plate to the workpiece. In further embodiments, the card may
include active circuit elements for switching, powering or
processing signals, and/or passive circuit elements for filtering,
matching or damping signals, so that few or no connections to
outside circuitry are required. The actuator assembly can be
manufactured in quantity, to provide a versatile actuator with
uniform mechanical and actuation characteristics, that introduces
negligible mass loading to the workpiece. The cards themselves may
be arranged as independent mechanical actuators, rather than
strain-transfer actuators, in which the induced strain changes the
position of the card. Various arrangements of pinned or
cantilevered cards may act as a pusher, bender or other motive
actuator, and structures such as powered bellows may be formed
directly by folding one or more suitably patterned cards.
Inventors: |
Lazarus, Kenneth B.;
(Concord, MA) ; Lundstrom, Mark E.; (Seattle,
WA) ; Moore, Jeffrey W.; (Arlington, MA) ;
Crawley, Edward; (Cambridge, MA) |
Correspondence
Address: |
TESTA, HURWITZ & THIBEAULT, LLP
HIGH STREET TOWER
125 HIGH STREET
BOSTON
MA
02110
US
|
Family ID: |
22691931 |
Appl. No.: |
09/837773 |
Filed: |
April 18, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09837773 |
Apr 18, 2001 |
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08943646 |
Oct 3, 1997 |
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08943646 |
Oct 3, 1997 |
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08188145 |
Jan 27, 1994 |
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Current U.S.
Class: |
310/330 ;
29/25.35; 29/831 |
Current CPC
Class: |
H01L 41/094 20130101;
Y10T 29/49126 20150115; H01L 41/0953 20130101; Y10T 29/49146
20150115; G03F 7/70725 20130101; H01L 41/0926 20130101; H01L 41/23
20130101; H01L 41/047 20130101; H04R 17/08 20130101; Y10T 29/49128
20150115; G03F 7/709 20130101; H01L 41/053 20130101; H01L 41/0993
20130101; H01L 41/0933 20130101; Y10T 29/42 20150115; G03F 7/70758
20130101 |
Class at
Publication: |
310/330 ;
29/25.35; 29/831 |
International
Class: |
H05K 003/20; H02N
002/00 |
Claims
What is claimed is:
1. An electro-active device comprising a plurality of flex
circuits, each having a sheet of film and an electrode on at least
one surface of the film, said plurality including at least first
and second flex circuits, means forming a recess between said first
and second flex circuits, and an electro-active element in said
recess bonded to the flex circuits being mechanically and
electrically coupled thereto.
2. An electro-active device according to claim 1, constituting a
card wherein said element is bonded within the card by a thin layer
of curable material.
3. An electro-active device according to claim 2, wherein the
curable material is a structural polymer.
4. An electro-active device according to claim 1, wherein the
electro-active element is a piezoelectric plate having a thickness
under approximately one millimeter.
5. An electro-active device according to claim 4, wherein the
piezoelectric plate has a thickness, and first and second cross
dimensions, each cross dimension being greater than about ten times
the thickness.
6. An electro-active device according to claim 1, wherein the
electrodes have an electrode pattern, and said element is bonded to
the flex circuit by a planarizing layer of curable material having
a pattern complementary to the electrode pattern.
7. An electro-active device according to claim 4, wherein the
electro-active element has a surface plane, and electrodes of said
first and second flex circuits are patterned for applying an
electric field which varies in said plane.
8. An electro-active device according to claim 4, wherein the
electro-active element has a surface plane, and electrodes of said
first and second flex circuits apply an electric field which varies
in a direction normal to said plane.
9. An electro-active device according to claim 1, wherein the
electrodes have a comb pattern.
10. An electro-active device according to claim 1, comprising two
different electro-active elements in two different respective
recesses and oriented to produce torsional actuation.
11. An electro-active device according to claim 1, further
comprising a circuit element within the device.
12. An electro-active device according to claim 11, wherein the
circuit element includes at least one of a shunt, a filter, an
impedance matcher, a storage element, a power source, an amplifier,
and a switch.
13. An electro-active device according to claim 11; wherein the
circuit element includes a controller.
14. An electro-active device according to claim 1, wherein first
and second electro-active elements are connected in different
layers of the assembly for moving in different senses.
15. An electro-active device according to claim 1, constituting a
device selected from among vanes, airfoils, shakers, steppers,
stirrers and sonicators.
16. An electro-active device according to claim 1, having a
thickness less than twice a combined thickness of electro-active
elements stacked in the device.
17. An electro-active device according to claim 1, wherein the
element is selected from among a stack, flexure, shell, plate and
bender.
18. An electro-active device according to claim 1, configured as
one of a pusher, vane, flap, lever, bender, bellows and combination
thereof.
19. An actuator comprising a flex circuit having conductors, and a
sheet strain element wherein the flex circuit is assembled with at
least some of its conductors in electrical contact with the sheet
strain element and is bonded together therewith by a structural
polymer into a flat card having an output face with a substantially
shear-free mechanical coupling to the flat strain element.
20. A method of perturbing a device, such method comprising the
steps of (i) cementing a card in contact with a region of the
device, the card enclosing a sheet of electro-active ceramic
material with actuation electrodes, and (ii) applying an electrical
signal to the actuation electrodes to create strain energy in the
electro-active ceramic material, whereby the strain energy from the
electro-active ceramic material is coupled across a face of the
card into said region to perturb the device.
21. A method of forming an actuator, such method comprising the
steps of forming a flex circuit having conductors arranged in a
pattern bonding an electro-active ceramic sheet in contact with at
least some of said conductors, and assembling the flex circuit and
the electro-active ceramic sheet with a stiff structural polymer so
as to constitute a card such that the sheet has a non-shear
coupling to an outer face of the card and is electrically coupled
over a region to electrodes of said flex circuit.
22. The method of claim 21, wherein the step of assembling includes
assembling circuit elements in said card.
23. A method of forming an electro-active device, such method
comprising the step of: preparing first and second flex circuits
with first and second electrodes and a recess therebetween, and
bonding at least one electro-active element in the recess in
mechanical and electrical contact with said flex circuits over its
surface area to form a unitary electro-active structure.
24. The method of claim 23, further comprising the step of
attaching circuit elements on said first and second flex
circuits.
25. The method of claim 23, wherein the step of bonding includes
bonding plural pairs of electro-active elements.
26. The method of claim 23, wherein said flex circuits are pliable
in a region away from said recess.
27. The method of claim 23, wherein the step of preparing includes
preparing at least three flex circuits.
28. The method of claim 23, further comprising the step of bonding
a surface of one of said flex circuits to an object whereby the
device mechanically acts on said object through the flex circuit
when signals are applied to the electrode.
29. The method of claim 23, wherein the step of bonding is
performed with a patterned layer of bonding material co-planar with
at least some of said electrodes.
30. The method of claim 23, wherein the step of bonding hardens the
flex circuits and bonded electro-active element into a card.
31. The method of claim 23, wherein the device constitutes a simple
mechanical device selected from among pushers, vanes, flaps,
levers, benders, bellows and combinations thereof.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to actuator elements such as
may be used for active vibration reduction, structural control,
dynamic testing, precision positioning, motion control, stirring,
shaking, and passive or active damping. More particularly, the
present invention relates to a packaged actuator assembly that is
electronically controllable and may be used separately or adapted
to actively suppress vibration, actuate structures, or damp
mechanical states of a device to which it is attached. As described
in a subsequent section below, the assembly may be bonded or
attached to a structure or system, thereby integrating it with the
system to be actuated, controlled or damped.
[0002] Smart materials, such as piezoelectric, electrostrictive or
magnetostrictive materials, may be used for high band width tasks
such as actuation or damping of structural or acoustic noise, and
also for precision positioning applications. Such applications
frequently require that the smart material be bonded or attached to
the structure that it is to control. However, general purpose
actuators of these materials are not generally available, and
typically a person wishing to implement such a control task must
take raw, possibly non-electroded, smart material stock, together
with any necessary electrodes, adhesives and insulating structures
and proceed to fasten it onto, or incorporate it into, the article
of interest.
[0003] For such applications, it becomes necessary to connect and
attach these materials in such a way that the mechanical and
electrical connections to the smart material are robust and capable
of creating strain within the smart member or displacing or forcing
the system, and to couple this strain, motion or force to the
object which is to be controlled. Often, it is required that the
smart material be used in a non-benign environment, greatly
increasing the chances of its mechanical or electrical failure.
[0004] By way of example, one such application, that of vibration
suppression and actuation for a structure, requires attachment of a
piezoelectric element (or multiple elements) to the structure.
These elements are then actuated, the piezoelectric effect
transforming electrical energy applied to the elements into
mechanical energy that is distributed throughout the elements. By
selectively creating mechanical impulses or changing strain within
the piezoelectric material, specific shape control of the
underlying structure is achievable. Rapid actuation can be used to
suppress a natural vibration or to apply a controlled vibration or
displacement Examples of this application of piezoelectric and
other intelligent materials have become increasingly common in
recent years.
[0005] In a typical vibration suppression and actuation
application, a piezoelectric element is bonded to a structure in a
complex sequence of steps. The surface of the structure is first
machined so that one or more channels are created to carry
electrical leads needed to connect to the piezoelectric element.
Alternatively, instead of machining channels, two different epoxies
may be used to make both the mechanical and the electrical
contacts. In this alternative approach, a conductive epoxy is
spotted, i.e., applied locally to form conductors, and a structural
epoxy is applied to the rest of the structure, bonding the
piezoelectric element to the structure. Everything is then covered
with a protective coating.
[0006] This assembly procedure is labor intensive, and often
involves much rework due to problems in working with the epoxy.
Mechanical uniformity between different piezoelectric elements is
difficult to obtain due to the variability of the process,
especially with regard to alignment and bonding of the
piezoelectric elements. Electrical and mechanical connections
formed in this way are often unreliable. It is common for the
conductive epoxy to flow in an undesirable way, causing a short
across the ends of the piezoelectric element. Furthermore,
piezoelectric elements are very fragile and when unsupported may be
broken during bonding or handling.
[0007] Another drawback of the conventional fabrication process is
that after the piezoelectric element is bonded to the structure, if
fracture occurs, that part of the piezoelectric element which is
not in contact with the conductor is disabled. Full actuation of
the element is thereby degraded. Shielding also can be a problem
since other circuit components as well as personnel must generally
be shielded from the electrodes of these devices, which may carry a
high voltage.
[0008] One approach to incorporating piezoelectric elements, such
as a thin piezoelectric plate, a cylinder or a stack of discs or
annuli, into a controllable structure has been described in U.S.
Pat. No. 4,849,668 of Javier de Luis and Edward F. Crawley. This
technique involves meticulous hand-assembly of various elements
into an integral structure in which the piezoceramic elements are
insulated and contained within the structure of a laminated
composite body which serves as a strong support. The support
reduces problems of electrode cracking, and, at least as set forth
in that patent, may be implemented in a way calculated to optimize
structural strength with mechanical actuation efficiency.
Furthermore, for cylinders or stacked annuli the natural internal
passage of these off-the-shelf piezo forms simplifies, to some
extent, the otherwise difficult task of installing wiring.
Nonetheless, design is not simple, and fabrication remains
time-consuming and subject to numerous failure modes during
assembly and operation.
[0009] The field of dynamic testing requires versatile actuators to
shake or perturb structures so that their response can be measured
or controlled. Here, however, the accepted methodology for shaking
test devices involves using an electromechanical motor to create a
linear disturbance. The motor is generally applied via a stinger
design, in order to decouple the motor from the desired signal.
Such external motors still have the drawback that dynamic coupling
is often encountered when using the motor to excite the structure.
Furthermore, with this type of actuator, inertia is added to the
structure, resulting in undesirable dynamics. The structure can
become grounded when the exciter is not an integral part of the
structure. These factors can greatly complicate device behavior, as
well as the modeling or mathematical analysis of the states of
interest. The use of piezoelectric actuators could overcome many of
these drawbacks, but, as noted above, would introduce its own
problems of complex construction, variation in actuation
characteristics, and durability. Similar problems arise when a
piezoelectric or electrostrictive element is used for sensing.
[0010] Thus, improvements are desirable in the manner in which an
element is bonded to the structure to be controlled or actuated,
such that the element may have high band width actuation
capabilities and be easily set up, yet be mechanically and
electrically robust, and not significantly alter the mechanical
properties of the structure as a whole. It is also desirable to
achieve high strain transfer from the piezoelectric element to the
structure of interest.
SUMMARY OF THE INVENTION
[0011] An actuator assembly according to the present invention
includes one or more strain elements, such as a piezoelectric or
electrostrictive plate or shell, a housing forming a protective
body about the element, and electrical contacts mounted in the
housing and connecting to the strain element, these parts together
forming a flexible card. At least one side of the assembly includes
a thin sheet which is attached to a major face of the strain
element, and by bonding the outside of the sheet to an object a
stiff shear-free coupling is obtained between the object and the
strain element in the housing.
[0012] In a preferred embodiment, the strain elements are
piezoceramic plates, which are quite thin, preferably between
slightly under an eighth of a millimeter to several millimeters
thick, and which have a relatively large surface area, with one or
both of their width and length dimensions being tens or hundreds of
times greater than the thickness dimension. A metallized film makes
electrode contact, while a structural epoxy and insulating material
hermetically seal the device against delamination, cracking and
environmental exposure. In a preferred embodiment, the metallized
film and insulating material are both provided in a flexible
circuit of tough polymer material, which thus provides robust
mechanical and electrical coupling to the enclosed elements.
[0013] By way of illustration, an example below describes a
construction utilizing rectangular PZT plates a quarter millimeter
thick, with length and width dimensions each of one to three
centimeters, each element thus having an active strain-generating
face one to ten square centimeters in area. The PZT plates are
mounted on or between sheets of a stiff strong polymer, e.g., one
half, one or two mil polymide, which is copper clad on one or both
sides and has a suitable conductive electrode pattern formed in the
copper layer for contacting the PZT plates. Various spacers
surround the plates, and the entire structure is bonded together
with a structural polymer into a waterproof, insulated closed
package, having a thickness about the same as the plate thickness,
e.g., 0.30 to 0.50 millimeters. So enclosed, the package may bend,
extend and flex, and undergo sharp impacts, without fracturing the
fragile PZT elements which are contained within. Further, because
the conductor pattern is firmly attached to the polymide sheet,
even cracking of the PZT element does not sever the electrodes, or
prevent actuation over the full area of the element, or otherwise
significantly degrade its performance.
[0014] The thin package forms a complete modular unit, in the form
of a small "card", complete with electrodes. The package may then
conveniently be attached by bonding one face to a structure so that
it couples strain between the enclosed strain element and the
structure. This may be done for example, by simply attaching the
package with an adhesive to establish a thin, high shear strength,
coupling with the PZT plates, while adding minimal mass to the
system as a whole. The plates may be actuators, which couple energy
into the attached structure, or sensors which respond to strain
coupled from the attached structure.
[0015] In different embodiments, particular electrode patterns are
selectively formed on the sheet to either pole the PZT plates
in-plane or cross-plane, and multiple layers of PZT elements may be
arranged or stacked in a single card to result in bending or shear,
and even specialized torsional actuation.
[0016] In accordance with a further aspect of the invention,
circuit elements are formed in, or with, the modular package to
filter, shunt, or process the signal produced by the PZT elements,
to sense the mechanical environment, or even to locally perform
switching or power amplification for driving the actuation
elements. The actuator package may be formed with pre-shaped PZT
elements, such as half-cylinders, into modular surface-mount shells
suitable for attaching about a pipe, rod or shaft.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] These and other desirable properties of the invention will
be understood from the detailed description of illustrative
embodiments, wherein:
[0018] FIG. 1A is a system illustration of a typical prior art
actuator;
[0019] FIG. 1B and 1C are corresponding illustrations of two
systems in accordance with the present invention;
[0020] FIGS. 2A and 2B show top and cross-sectional views,
respectively, of a basic actuator or sensor card in accordance with
the present invention;
[0021] FIG. 2C illustrates an actuator or sensor card with circuit
elements;
[0022] FIG. 3 illustrates another card;
[0023] FIGS. 4A and 4B show sections through the card of FIG.
3;
[0024] FIGS. 5 and 5A show details of the layer structure of the
card of FIG. 3;
[0025] FIG. 6 shows an actuator package comb electrodes for
in-plane actuation;
[0026] FIG. 7 illustrates a torsional actuator package using the
cards of FIG. 6;
[0027] FIGS. 8A and 8B show actuators mounted as surface mount
actuators on a surface or rod, respectively; and
[0028] FIG. 9 shows actuators mounted as mechanical elements.
DETAILED DESCRIPTION OF THE INVENTION
[0029] FIG. 1A illustrates in schema the process and overall
arrangement of a prior art surface mounted piezoelectric actuator
assembly 10. A structure 20, which may be a structural or machine
element, a plate, airfoil or other interactive sheet, or a device
or part thereof has a sheet 12 of smart material bonded thereto by
some combination of conductive and structural polymers, 14, 16. An
insulator 18, which may be formed entirely or in part of the
structural polymer 16, encloses and protects the smart material,
while conductive leads or surface electrodes are formed or attached
by the conductive polymer. An external control system 30 provides
drive signals along lines 32a, 32b to the smart material, and may
receive measurement signals from surface-mounted instrumentation
such as a strain gauge 35, from which it derives appropriate drive
signals. Various forms of control are possible. For example, the
strain gauge may be positioned to sense the excitation of a natural
resonance, and the control system 30 may simply actuate the PZT
element in response to a sensor output, so as to stiffen the
structure, and thereby shift its resonant frequency. Alternatively,
a vibration sensed by the sensor may be fed back as a processed
phase-delayed driving signal to null out an evolving dynamic state,
or the actuator may be driven for motion control. In
better-understood mechanical systems, the controller may be
programmed to recognize empirical conditions, i.e., aerodynamic
states or events, and to select special control laws that specify
the gain and phase of a driving signal for each actuator 12 to
achieve a desired change.
[0030] For all such applications, major work is required to attach
the bare PZT plate to its control circuitry and to the workpiece,
and many of the assembly steps are subject to failure or, when
quantitative control is desired, may require extensive modeling of
the device after it has been assembled, in order to establish
control parameters for a useful mode of operation that are
appropriate for the specific thicknesses and mechanical stiffnesses
achieved in the fabrication process.
[0031] FIG. 1B shows an actuator according to one embodiment of the
present invention. As shown, it is a modular pack or card 40 that
simply attaches to a structure 20 with a quick setting adhesive,
such as a five-minute epoxy 13, or in other configurations attaches
at a point or line. The operations of sensing and control thus
benefit from a more readily installable and uniformly modeled
actuator structure. In particular, the modular pack 40 has the form
of a card, a stiff but bendable plate, with one or more electrical
connectors preferably in the form of pads located at its edge (not
shown) to plug into a multi-pin socket so that it may connect to a
simplified control system 50. As discussed in greater detail below
with respect to FIG. 2C, the modular package 40 may also
incorporate planar or low-profile circuit elements, which may
include signal processing elements, such as weighting or shunting
resistors, impedance matchers, filters and signal conditioning
preamplifiers, and may further include switching transistors and
other elements to operate under direct digital control, so that the
only external electrical connections necessary are those of a
microprocessor or logic controller, and a power supply.
[0032] In a further embodiment particularly applicable to some low
power control situations, a modular package 60 as shown in FIG. 1C
may include its own power source, such as a battery or power cell,
and may include a controller, such as a microprocessor chip or
programmable logic array, to operate on-board drivers and shunts,
thus effecting a complete set of sensing and control operations
without any external circuit connections.
[0033] The present invention specifically pertains to piezoelectric
polymers, and to materials such as sintered metal zirconate,
niobate crystal or similar piezoceramic materials that are stiff,
yet happen to be quite brittle. It also pertains to
electrostrictive materials. As used in the claims below, both
piezoelectric and electrostrictive elements, in which the material
of the elements has an electromechanical property, will be referred
to as electro-active elements. High stiffness is essential for
efficiently transferring strain across the surface of the element
to an outside structure or workpiece, typically made of metal or a
hard structural polymer, and the invention in its actuator aspect
does not generally contemplate soft polymer piezoelectric
materials. While the terms "stiff" and "soft" are relative, it will
be understood that in this context, the stiffness, as applied to an
actuator, is approximately that of a metal, cured epoxy, high-tech
composite, or other stiff material, with a Young's modulus greater
than 0.1.times.10.sup.6, and preferably greater than
0.2.times.10.sup.6. When constructing sensors, instead of
actuators, the invention also contemplates the use of low-stiffness
piezoelectric materials, such as polyvinylidene difluoride (VDF)
film and the substitution of lower cure temperature bonding or
adhesive materials. The principal construction challenges, however,
arise with the first class of piezo material noted above, and these
will now be described.
[0034] In general, the invention includes novel forms of actuators
and methods of making such actuators, where "actuator" is
understood to mean a complete and mechanically useful device which,
when powered, couples force, motion or the like to an object or
structure. In its broad form, the making of an actuator involves
"packaging" a raw electro-active element to make it mechanically
useful. By way of example, raw electro-active piezoelectric
materials or "elements" are commonly available in a variety of
semi-processed bulk material forms, including raw piezoelectric
material in basic shapes, such as sheets, rings, washers, cylinders
and plates, as well as more complex or composite forms, such as
stacks, or hybrid forms that include a bulk material with a
mechanical element, such as a lever. These materials or raw
elements may have metal coated on one or more surfaces to act as
electrical contacts, or may be non-metallized. In the discussion
below, piezoelectric materials shall be discussed by way of
example, and all these forms of raw materials shall be referred to
as "elements", "materials", or "electro-active elements". As noted
above, the invention further includes structures or devices made by
these methods and operating as transducers to sense, rather than
actuate, a strain, vibration, position or other physical
characteristic, so that where applicable below, the term "actuator"
may include sensing transducers.
[0035] Embodiments of the invention employ these stiff
electrically-actuated materials in thin sheets--discs, annuli,
plates and cylinders or shells--that are below several millimeters
in thickness, and illustratively about one fifth to one quarter
millimeter thick.
[0036] Advantageously, this thin dimension allows the achievement
of high electric field strengths across a distance comparable to
the thickness dimension of the plate at a relatively low overall
potential difference, so that full scale piezoelectric actuation
may be obtained with driving voltages of ten to fifty volts, or
less. Such a thin dimension also allows the element to be attached
to an object without greatly changing the structural or physical
response characteristics of the object. However, in the prior art,
such thin elements are fragile, and may break due to irregular
stresses when handled, assembled or cured. The impact from falling
even a few centimeters may fracture a piezoceramic plate, and only
extremely small bending deflections are tolerated before
breaking.
[0037] In accordance with the present invention, the thin
electrically actuated element is encased by layers of stiff
insulating material, at least one of which is a tough film which
has patterned conductors on one of its surfaces, and is thinner
than the element itself. A package is assembled from the piezo
elements, insulating layers, and various spacers or structural fill
material, such that altogether the electrodes, piezo element(s),
and enclosing films or layers form a sealed card of a thickness not
substantially greater than that of the bare actuating element.
Where elements are placed in several layers, as will be described
below, the package thickness is not appreciably greater than the
sum of the thicknesses of the stacked actuating elements.
[0038] FIG. 2A illustrates a basic embodiment 100 of the invention.
A thin film 110 of a highly insulating material, such as a
polyimide material, is metallized, typically copper clad, on at
least one side, and forms a rectangle which is coextensive with or
slightly larger than the finished actuator package. A suitable
material available for use in fabricating multilayer circuit boards
is distributed by the Rogers Corporation of Chandler, Ariz. as
their Flex-I-Mid 3000 adhesiveless circuit material, and consists
of a polyimide film formed on a rolled copper foil. A range of
sizes are available commercially, with the metal foils being of 18
to 70 micrometer thickness, integrally coated with a polyimide film
of 13 to 50 micrometer thickness. Other thicknesses may be
fabricated. In this commercial material, the foil and polymer film
are directly attached without adhesives, so the metal layer may be
patterned by conventional masking and etching, and multiple
patterned layers may be built up into a multilayer board in a
manner described more fully below, without residual adhesive
weakening the assembly or causing delamination. The rolled copper
foil provides high in-plane tensile strength, while the polyimide
film presents a strong, tough and defect-free electrically
insulating barrier.
[0039] In constructions described below, the film constitutes not
only an insulator over the electrodes, but also an outer surface of
the device. It is therefore required to have high dielectric
strength, high shear strength, water resistance and an ability to
bond to other surfaces. High thermal resistance is necessary in
view of the temperature cure used in the preferred fabrication
process, and is also required for some application environments. In
general, polyamide/imides have been found useful, but other
materials, such as polyesters with similar properties, may also be
used.
[0040] In the present constructions, the foil layer is patterned by
conventional masking and etch techniques (for example, photoresist
masking and patterning, followed by a ferric chloride etch), to
form electrodes for contacting the surface of piezo plate elements.
Electrodes 111 extend over one or more sub-regions of the interior
of the rectangle, and lead to reinforced pads or lands 111 1a, 111b
extending at the edge of the device. The electrodes are arranged in
a pattern to contact a piezoelectric element along a
broadly-turning path, which crosses the full length and width of
the element, and thus assures that the element remains connected
despite the occurrence of a few cracks or local breaks in the
electrode or the piezo element. Frame members 120 are positioned
about the perimeter of sheet 110, and at least one piezoelectric
plate element 112 is situated in the central region so that it is
contacted by the electrodes 111. The frame members serve as edge
binding, so that the thin laminations do not extend to the edge,
and they also function as thickness spacers for the hot-press
assembly operation described further below, and as position-markers
which define the location of piezo plates that are inserted during
the initial stages of assembling the laminated package.
[0041] FIG. 2A is a somewhat schematic view, inasmuch as it does
not show the layer structure of the device which secures it
together, including a further semi-transparent top layer 116 (FIG.
2B), which in practice extends over the plate 112 and together with
the spacers 120 and sheet 110 closes the assembly. A similar layer
114 is placed under the piezo element, with suitable cut-outs to
allow the electrodes 111 to contact the element. Layers 114, 116
are preferably formed of a curable epoxy sheet material, which has
a cured thickness equal to the thickness of the metal electrode
layer, and which acts as an adhesive layer to join together the
material contacting it on each side. When cured, this epoxy
constitutes the structural body of the device, and stiffens the
assembly, extending entirely over a substantial portion of the
surface of the piezo element to strengthen the element and arrest
crack growth, thereby enhancing its longevity. Furthermore,
applicant has found that epoxy from this layer actually spreads in
a microscopically thin but highly discontinuous film, about 0.0025
mm thick, over the electrodes, bonding them firmly to the piezo
plate, but with a sufficient number of voids and pinholes so that
direct electrical contact between the electrodes and piezo elements
still occurs over a substantial and distributed contact area.
[0042] FIG. 2B shows a cross-sectional view, not drawn to scale, of
the embodiment of FIG. 2A. By way of rough proportions, taking the
piezoelectric plate 112 as 0.2 -0.25 millimeters in thickness, the
insulating film 110 is much thinner, no more than one-tenth to
one-fifth the plate thickness, and the conductive copper electrode
layer 111 may have a thickness typically of ten to fifty microns,
although the latter range is not a set of strict limits, but
represents a usefull range of electrode thicknesses that are
electrically serviceable, convenient to manufacture and not so
thick as to either impair the efficiency of strain transfer or
introduce delamination problems. The structural epoxy 114 fills the
spaces between electrodes 111 in each layer, and has approximately
the same thickness as those electrodes, so that the entire assembly
forms a solid bock. The spacers 120 are formed of a relatively
compressible material, having a low modulus of elasticity, such as
a relatively uncrosslinked polymer, and, when used with a
pressure-cured epoxy as described below, are preferably of a
thickness roughly equivalent to the piezoceramic plate or stack of
elements, so that they form an edge binding about the other
components between the top and bottom layers of film 110.
[0043] A preferred method of manufacture involves applying pressure
to the entire package as the layer 116 cures. The spacers 120 serve
to align the piezoceramic plates and any circuit elements, as
described below with reference to FIGS. 3-5, and they form a frame
that is compressed slightly during assembly in the cure step, at
which time it may deform to seal the edges without leaving any
stress or irregularities. Compression eliminates voids and provides
a dense and crack-free solid medium, while the curing heat effects
a high degree of cross-linking, resulting in high strength and
stiffness.
[0044] An assembly process for the embodiment of FIGS. 2A, 2B is as
follows. One or more pieces of copper clad polyimide film, each
approximately 0.025 to 0.050 millimeters thick in total, are cut to
a size slightly larger than the ultimate actuator package
dimensions. The copper side of the film is masked and patterned to
form the desired shape of electrodes for contacting a piezo element
together with conductive leads and any desired lands or access
terminals. A pitchfork electrode pattern is shown, having three
tines which are positioned to contact the center and both sides of
one face of a piezo element, but in other embodiments an H- or a
comb-shape is used. The patterning may be done by masking, etching
and then cleaning, as is familiar from circuit board or
semiconductor processing technology. The masking is effected by
photoresist patterning, screening, tape masking, or other suitable
process. Each of these electroded pieces of polyimide film, like a
classical printed circuit board, defines the positions of circuit
elements or actuator sheets, and will be referred to below simply
as a "flex circuit."
[0045] Next, uncured sheet epoxy material having approximately the
same thickness or slightly thicker than the electrode foil layer is
cut, optionally with through-apertures matching the electrode
pattern to allow enhanced electrical contact when assembled, and is
placed over each flex circuit, so it adheres to the flex circuit
and forms a planarizing layer between and around the electroded
portions. The backing is then removed from the epoxy layers
attached to the flex circuits, and pre-cut spacers 120 are placed
in position at corner and edges of the flex circuit. The spacers
outline a frame which extends above the plane of the electrodes,
and defines one or more recesses into which the piezo elements are
to be fitted in subsequent assembly steps. The piezo element or
elements are then placed in the recesses defined by the spacers,
and a second electroded film 111, 112 with its own
planarizing/bonding layer 114 is placed over the element in a
position to form electrode contacts for the top of the piezo
element. If the device is to have several layers of piezo elements,
as would be the case for some bending actuator constructions, these
assembly steps are repeated for each additional electroded film and
piezoelectric plate, bearing in mind that a polymide film which is
clad and patterned on both sides may be used when forming an
intermediate electrode layer that is to contact actuator elements
both above and below the intermediate sheet.
[0046] Once all elements are in place, the completed sandwich
assembly of patterned flex circuits, piezo sheets, spacers and
curable patterned epoxy layers is placed in a press between heated
platens, and is cured at an elevated temperature and pressure to
harden the assembly into a stiff, crack-free actuator card. In a
representative embodiment, a cure cycle of thirty minutes at
350.degree. F. and 50-100 psi pressure is used. The epoxy is
selected to have a curing temperature below the depoling
temperature of the piezo elements, yet achieve a high degree of
stiffness.
[0047] The above construction illustrates a simple actuator card
having a single piezo plate sandwiched between two electroded
films, so that the plate transfers shear strain efficiently through
a thin film to the surface of the actuator card. The measure of
transfer efficiency, given by the shear modulus divided by layer
thickness squared, and referred to as gamma (.GAMMA.), depends on
the moduli and thickness of the epoxy 114, the rolled foil
electrodes 111, and the polyimide film 110. In a representative
embodiment in which the epoxy and copper electrode layers are 1.4
mils thick and the epoxy has a modulus of 0.5.times.10.sup.6, a
gamma of approximately 9.times.10.sup.10 pounds/inch.sup.4 is
achieved. Using a thinner epoxy layer and film with 0.8 mil foil,
substantially higher .GAMMA. is achieved. In general, the gamma of
the electrode/epoxy layer is greater than 5.times.10.sup.10
pounds/inch.sup.4, while that of the film is greater than
2.times.10.sup.10 pounds/inch.sup.4.
[0048] It should be noted that using PZT actuator plates ten mils
thick, a card having two PZT plates stacked over each other with
three flex circuit electroded film layers (the middle one being
double clad to contact both plates) has a total thickness of 28
mils, only forty percent greater than the plates alone. In terms of
mass loading, the weight of the actuator elements represents 90% of
the total weight of this assembly. Generally, the plates occupy
fifty to seventy percent of the package thickness, and constitute
seventy to ninety percent of its mass, in other constructions.
Thus, the actuator itself allows near-theoretical performance
modeling. This construction offers a high degree of versatility as
well, for implementing benders (as just described) as well as
stacks or arrays of single sheets.
[0049] Another useful performance index of the actuator constructed
in accordance with the present invention is the high ratio of
actuator strain .epsilon. to the free piezo element strain
.LAMBDA., which is approximately (0.8) for the two layer embodiment
described herein, and in general is greater than (0.5). Similarly,
the ratio of package to free element curvatures, K, is
approximately 0.85-0.90 for the described constructions, and in
general is greater than 0.7.
[0050] Thus, overall, the packaging involved in constructing a
piezo element embedded in a flex circuit impairs its weight and
electromechanical operating characteristics by well under 50%, and
as little as 10%, while profoundly enhancing its hardiness and
mechanical operating range in other important respects. For
example, while the addition of sheet packaging structure to the
base element would appear to decrease attainable K, in practical
use the flex card construction results in piezo bender
constructions wherein much greater total deflection may be
achieved, since large plate structures may be fabricated and high
curvature may be repeatedly actuated, without crack failure or
other mechanical failure modes arising. Several Figures will
illustrate the variety of constructions to which such enhanced
physical characteristics are brought.
[0051] First, applicant notes that the structure of an
electro-active element embedded between flex circuits not only
provides a low mass unified mechanical structure with predictable
response characteristics, but also allows the incorporation of
circuit elements into or onto the actuator card. FIG. 2C shows a
top view of one device 70 of this type, wherein regions 71, 73 each
contain broad electro-active sheets, while a central region 72
contains circuit or power elements, including a battery 75, a
planar power amplification or set of amplifiers 77, a
microprocessor 79, and a plurality of strain gauges 78. Other
circuit elements 82a, 82b may be located elsewhere along the path
of circuit conductors 81 about the periphery. As with the other
embodiments, spacers 120 define layout and seal edges of the
device, while electrodes 111 attach the electro-active elements to
the processing or control circuitry which is now built-in. The
circuit elements 82a, 82b may comprise weighting resistors if the
device is operated as a sensor, or shunting resistors to implement
passive damping control. Alternatively, they may be filtering,
amplifying, impedance matching or storage elements, such as
capacitors, amplifiers or the like. In any case, these elements
also are located away from electro-active plates 84. The components
collectively may sense strain and implement various patterns of
actuation in response to sensed conditions, or perform other
sensing or control tasks.
[0052] Returning now to the actuator aspect of the invention, FIG.
3 shows a top view of an actuator package 200 having dimensions of
about 1.25.times.9.00.times.0.030 inches and assembled with two
layers of piezoelectric plates of four plates each. A rectangular
polyimide sheet 210 with an end tab 210a carries an electrode 211
in the form of a lattice of H-shaped thin copper lines
interconnected to each other and to a single runner 211a that leads
out to the tab, thus providing a low impedance connection directly
to each of four rectangular regions which hold the piezo
plates.
[0053] Spacer elements 220a, 220b of H-shape, or 220c of L-shape
mark off corners and delineate the rectangular spaces for location
of the piezo plates 216. In this embodiment, a plurality of gaps
230, discussed further below, appear between adjacent the H- or
L-spacers. As will be apparent from the description below, the use
of these small discrete spacer elements (I-, T- or O-shaped spacers
may also be convenient) is enhanced because they may be readily
placed on the tacky bonding epoxy layer 114 during assembly to mark
out assembly positions and form a receiving recess for the piezo
elements. However, the spacer structure is not limited to such a
collection of discrete elements, but may be a single or couple of
frame pieces, formed as a punched-out sheet or molded frame, to
provide all, or one or more, orienting and/or sealing edges, or
recesses for holding actuation of circuit components.
[0054] FIG. 5 illustrates a top view of each of the three sheet,
electrode and piezo plate layers separately, while FIG. 5A
illustrates the general layering sequence of the film, conductor,
and spacer/piezo layers. As shown, the spacers 220 and piezo plates
216 constitute a single layer between each pair of electrode
layers.
[0055] FIGS. 4A and 4B (not drawn to scale) illustrate the layer
structure of the assembled actuator along the vertical sections at
the positions indicated by "A" and "B" in FIG. 3. As more clearly
shown in FIG. 4A, a patterned bonding layer of epoxy sheet 214 is
coplanar with each electrode layer 211 and fills the space between
electrodes, while the spacer 220c is coplanar with the piezo plate
216 and substantially the same thickness as the plate or slightly
thicker. Illustratively, the piezo plate 216 is a PZT-5A ceramic
plate, available commercially in a five to twenty mil thickness,
and has a continuous conductive layer 216a covering each face for
contacting the electrodes 211. The spacers 220 are formed of
somewhat compressible plastic with a softening temperature of about
250.degree. C. This allows a fair degree of conformability at the
cure temperature so the spacer material may fill slight voids 214a
(FIG. 4A) during the assembly process. As shown in FIG. 4B, the
gaps 230 (when provided) between spacers result in openings 214b
which vent excess epoxy from the curable bonding layers 214, and
fill with epoxy during the cure process. As illustrated in that
FIGURE, a certain amount of epoxy also bleeds over into patches of
film 215 between the electrodes 211 and the piezo plate 216.
Because of the large and continuous extent of electrode 211, this
patchy leakage of epoxy does not impair the electrical contact with
the piezo elements, and the additional structural connection it
provides helps prevent electrode delamination.
[0056] It will be appreciated that with the illustrated
arrangements of electrodes, each vertically stacked pair of piezo
plates may be actuated in opposition to each other to induce
bending, or more numerous separate electrodes may be provided to
allow different pairs of plates to be actuated in different ways.
In general, as noted above, the invention contemplates even quite
complex systems involving many separate elements actuated in
different ways, with sensing, control, and power or damping
elements all mounted on the same card. In this regard, great
flexibility in adapting the card to practical tasks is further
provided by its flexibility. In general, it has a supple
flexibility comparable to that of an epoxy strip thirty mils thick,
so that it may be bent, struck or vibrated without damage. It may
also be sharply bent or curved in the region of its center line CL
(FIG. 3) where no piezo elements are encased, to conform to an
attaching surface or corner. The elements may be poled to change
dimension in-plane or cross-plane, and the actuators may therefore
be attached to transmit strain to an adjacent surface in a manner
effective to perform any of the above-described control actions, or
to launch particular waveforms or types of acoustic energy, such as
flexural, shear or compressional waves into an adjacent
surface.
[0057] FIG. 6 shows another actuator embodiment 300. In this
embodiment, illustrated schematically, the epoxy bonding layer,
film and spacer elements are not shown, but only electrode and
piezo sheets are illustrated to convey the operative mechanisms. A
first set of electrodes 340 and second set 342 are both provided in
the same layer, each having the shape of a comb with the two combs
interdigitated so that an electrical actuation field is set up
between the tooth of one comb and an adjacent tooth of the other
comb. A parallel pair of combs 340a, 342a is provided on the other
side of the piezo sheet, with comb electrode 340 tied to 340a, and
comb electrode 342 tied to 342a, so as to set up an electric field
with equipotential lines "e" extending through the piezo sheet, and
in-plane potential gradient between each pair of teeth from
different combs. The piezoceramic plates are not metallized, so
direct electrical contact is made between each comb and the plate.
The plates are poled in-plane, by initially applying a high voltage
across the combs to create a field strength above one two thousand
volts per inch directed along the in-plane direction. This orients
the piezo structure so that subsequent application of a potential
difference across the two-comb electrodes results in in-plane
(shear) actuation.
[0058] As shown in FIG. 7, two such actuators 300 may be crossed to
provide torsional actuation.
[0059] In discussing the embodiments above, the direct transfer of
strain energy through the electrode/polyimide layer to any
adjoining structure has been identified as a distinct and novel
advantage. Such operation may be useful for actuation tasks or
diverse as airfoil shape control actuation and noise or vibration
cancellation or control. FIGS. 8A and 8B illustrates typical
installations of flat (FIG. 8A) and hemicylindrical (FIG. 8B)
embodiments 60 of the actuator, applied to a flat or slightly
curved surface, and a shaft, respectively.
[0060] However, while the electromechanical materials of these
actuators operate by strain energy conversion, applications of the
present invention extend beyond strain-coupling through the
actuator surface, and include numerous specialized mechanical
constructions in which the motion, torque or force applied by the
actuator as a whole is utilized. In each of these embodiments, the
basic strip- or shell-shaped sealed actuator is employed as a
robust, springy mechanical element, pinned or connected at one or
more points along its length. As shown in FIG. 9, when electrically
actuated, the strip then functions, alone or with other elements,
as a self-moving lever, flap, leaf spring, stack or bellows. In the
diagrams of FIGS. 9(a)-9(q), the elements A, A', A" . . . are strip
or sheet actuators such as shown in the above FIGURES, while small
triangles indicate fixed or pinned positions which correspond, for
example, to rigid mounting points or points of connection to a
structure. Arrows indicate a direction of movement or actuation or
the contact point for such actuation, while L indicates a lever
attached to the actuator and S indicates a stack element or
actuator.
[0061] The configurations of FIGS. 9(a)-9(c) as stacks, benders, or
pinned benders may replace many conventional actuators.
[0062] For example, a cantilevered beam may carry a stylus to
provide highly controlled single-axis displacement to constitute a
highly linear, large displacement positioning mechanism of a pen
plotter. Especially interesting mechanical properties and actuation
characteristics are expected from multi-element configurations 9(d)
et seq., which capitalize on the actuators having a sheet extent
and being mechanically robust. Thus, as shown in FIGS. 9(d) and
(e), a pin-pin bellows configuration may be useful for extended and
precise one-axis Z-movement positioning, by simple face-contacting
movement, for applications such as camera focusing; or may be
useful for implementing a peristalsis-type pump by utilizing the
movement of the entire face bearing against a fluid. As noted in
connection with FIG. 3, the flex circuit is highly compliant, so
hinged or folded edges may be implemented by simply folding along
positions such as the centerline in FIG. 3, allowing a closed
bellows assembly to be made with small number of large,
multi-element actuator units. The flex circuit construction allows
strips or checkerboards of actuator elements to be laid out with
fold lines between each adjacent pair of elements, and the fold
lines may be impressed with a thin profile by using a contoured
(e.g. waffle-iron) press platen during the cure stage. With such a
construction, an entire seamless bellows or other folded actuator
may be made from a single flex circuit assembly.
[0063] Applicant has further utilized such actuators to perform
simple mechanical motions, such as bending, twisting or wiggling,
applied to portions of a face mask or puppet for theatrical
animation, and has found the actuators to have excellent actuation
characteristics and versatile mounting possibilities for
small-load, medium displacement actuation tasks of this type.
[0064] In general, tasks capable of implementation with pneumatic
actuators of small or medium displacement may be addressed using
the flex circuit actuator cards of the invention as discrete
mechanical elements, and where the task involves a structure such
as a sheet, flap or wall, the flex circuit itself may constitute
that structural component. Thus, the invention is suited to such
functions as self-moving stirring vanes, bellows or pump walls,
mirrors, and the like. In addition, as noted above, tasks involving
surface coupling of small displacement acoustic or ultrasonic band
frequency are also readily implemented with the low mass highly
coupled flex circuit actuators.
[0065] As noted above, the piezo element need not be a stiff
ceramic element, and if the flex circuit is to be used only as a
sensor, then either a ceramic element, or a soft material such as
PVDF may be employed. In the case of the polymer, a thinner more
pliant low temperature adhesive is used for coupling the element,
rather than a hard curable epoxy bonding layer.
[0066] The foregoing description of methods of manufacture and
illustrative embodiments is presented to indicate the range of
constructions to which the invention applies. The invention having
overcome numerous drawbacks in the fragility, circuit configuration
and general utility of strain actuators, strain activated
assemblies and sensors, other variations in the physical
architecture and practical applications of the modular flex circuit
actuators and sensors of the invention will occur to those skilled
in the art, and such variations are considered to be within the
scope of the invention in which patent rights are asserted, as set
forth in the claims appended hereto.
* * * * *