Piezoelectric actuators

Combinations of oppositely poled piezoelectric shapes which, when electrically stimulated, effect increased linear or rotational displacement. In a typical arrangement a length of PVDF piezoelectric material poled in one direction is spatially separated from a second similarly shaped length of PVDF piezolectric material poled in the opposite direction by an electrically inert substance. When stimulated by an appropriate voltage, one length contracts while the other length expands producing an increased deflection in the combination.

FIELD OF THE INVENTION 
This invention relates to piezoelectric actuators and more particularly to 
such devices useful in applications requiring short stroke or small 
angular motion. 
BACKGROUND OF THE INVENTION 
A polymer film which is capable of exhibiting piezoelectric properties is 
prepared by stretching it to establish a preferred axis and then by 
subjecting the film to an electric field oriented transversely through the 
major surfaces of the film. This causes an average net rotation of 
molecular dipoles within the material with the dipoles being rotated 
toward alignment with the electric field. 
Thereafter, when a sheet of this material is subjected to an electric field 
transversely and preferably perpendicularly through its major surfaces, 
the dipoles tend to be rotated. This rotation causes a strain to occur 
principally along the stretch direction. If the later field which is used 
to control the strain is in the same direction as the field utilized to 
initially align the dipoles, then the material is shortened. If the field 
is opposed to the initial poling field, then it is opposed to the existing 
dipoles and causes an elongation strain in the direction of the stretch. 
The controlling electric field may be most conveniently applied by forming 
a conductive film on each of the exposed, opposite major surfaces of the 
piezoelectric polymer. This is conveniently done by applying conductive 
paint or depositing a metalized surface of nickel or aluminum, for 
example, on those major surfaces. 
Piezoelectric films such as polyvinylidene fluoride (PVDF), are 
particularly useful in applications in which short stroke and/or small 
angular motion is required. For example, as applied to mechanical 
actuation, 28 micron thick poled PVDF film at an excitation level of 10 
volts per micron (280 V) and with an active length of 1.0 meter, yields a 
theoretical displacement along the material stretch axis of 230 microns. 
This displacement can be arranged to produce an angular displacement of 
6.6 degrees when operated over a radius of 2.0 millimeters. 
BRIEF DESCRIPTION OF THE INVENTION 
To effectively utilize piezoelectric films such as PVDF, it is advantageous 
to first compact the PVDF film into cells of usable size and then to 
efficiently lever the cell displacement available into motion and force of 
a sufficient magnitude to be generally useful, doing so while 
circumventing tolerance and clearance losses associated with conventional 
mechanisms employing discrete mechanical components. In accordance with 
the principles of this invention, relatively small cell size is achieved, 
a plurality of such cells being arranged and interconnected in a manner to 
provide amplified physical movement. 
Piezoelectric devices generally draw current only while the device is 
actually in motion, contrary to electromagnetically actuated devices such 
as relays and solenoid valves which require holding current to remain 
displaced. Thus, the resulting devices described herein are relatively 
inexpensive to operate when compared to such relays and valves. 
In accordance with the principles of the present invention, incremental 
linear and rotational motions can be produced which are suitable for use 
in electromechanical relays, pneumatic and hydraulic metering valves, open 
and closed loop servo positioners, puppet animation, remote control of 
vehicles, air and spacecraft, printer hammers mechanical metering, 
moderate angle optical scanners and stepping devices, to name but a few.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1 and 2 illustrate the conversion of voltage modulated PVDF film 
length to angular displacement. The pivot comprises in this illustrative 
embodiment, an adaptation of a knife edge and anvil bearing to a system of 
crossed cylinders held on center by a soft flexible hydraulically 
incompressible elastomer (rubber) fill 3. Anvil 4, formed by a short 
length of round cross-section wire fixed against the pivoted end of output 
paddle 5, pivots against wire 1 having a round cross section. Wire 1 is 
mechanically fixed to base 2. 
Two relatively long lengths (symbolized by "L" on FIG. 1 ) of PVDF film 6 
and 7, are fixed to the upper surface 8 and lower surface 9 of paddle 5. 
PVDF film lengths 6 and 7 are joined at connector 10. Spring 11 provides 
tension between connector 10 and a fixed point. The PVDF film is suitably 
provided with electrodes and poled such that when electrically excited, 
one PVDF film length (say, length 6) decreases, and the other length 
(length 7) increases. The film stretch direction in FIGS. 1 and 2 is along 
the long dimension (left to right as shown) and is the preferred dimension 
for piezoelectric displacement. 
FIG. 2 shows the PVDF lengths 6 and 7 under bias. Length 6 is shown in the 
foreshortened state while film 7 is shown elongated. The differential 
tension thus produced, acting against the pivot causes paddle 5 to be 
deflected upward. Reversing the film bias polarity deflects paddle 5 
downward (shown in phantom in FIG. 2). 
To determine the angle of deflection, start with the relationship 
EQU dl=(D.sub.31 .times.V.times.l)/T 
where 
dl=PVDF length change 
D.sub.31 =23E--12 (piezo-strain constant) 
V=voltage 
l=length 
T=thickness. 
Under the following conditions, 
film thickness=23 microns 
active length=151 centimeters 
effective radius=2 millimeter 
bias=10 volts per micron 
dl equals 347 microns, resulting in a deflection angle of +/-10 degrees. 
Force available is a direct function of the width of PDVF lengths 6 and 7. 
FIGS. 1 and 2 are side views of the device and do not consequently 
illustrate the width. It is noted that in the arrangements of this 
embodiment, factors which affect the stretching and foreshortening of 
lengths 6 and 7 such as material age and temperature affect each length 
the same and therefore cancel one another. 
The arrangement of FIG. 3 is a variation of the arrangement of FIGS. 1 and 
2 and results in a device considerably shorter than that of FIGS. 1 and 2 
while achieving similar angles of deflection. In particular, the device of 
FIGS. 1 and 2 can be shortened by almost half (symbolized by the symbol 
.apprxeq.L/2) by implementing the principles embodied in the arrangement 
of FIG. 3. PVDF film length 18 in FIG. 3 is approximately 1/2 the length 
of PVDF element 6 in FIGS. 1 and 2. In addition, the arrangement of FIGS. 
1 and 2 has been modified to add PVDF elements 17 and 21. These compound 
element pairs pivot about a point in a plane with pivot 1a for element 17 
and pivot 1b for element 21. Elements 17 and 18 are attached at connector 
15 and connector 15 is attached, in turn, to spring 16. Bias applied to 
the device, appropriate to increasing the length of element 17 causes 
point 19 to move to the left under the influence of tension spring 16 
pulling link 18 along with it. Simultaneously link 18 is biased to 
decrease length, so that the net effect on the upper surface of paddle 5a 
is the sum of the change in length of links 17 and 18. A second element 
pair connected to the lower surface of paddle 5a comprising elements 20 
and 21, pivot 1a and tension spring 22. The second element pair is biased 
and poled to complement the length changes in elements 17 and 18. 
FIG. 4 shows a long continuous length of PVDF film 23, oriented in the 
stretch direction according to arrow 31, fan-folded and fixed between 
lower plate 24 and upper plate 25 at film ends 26 and 27. Alternate folds, 
typified by folds 28 and 29, are either poled in opposite directions or 
biased in opposite directions relative to one another. Furthermore, the 
assembly is contained by a well adhered soft flexible incompressible 
elastomer (rubber) encapsulant 30. In this manner, excitation of the film 
causes all alternate layers to contract (or expand) while interposed 
layers expand (or contract). The resulting total shear displacement 
between the upper and lower plates is the sum total of each of the 
differentially operated folds. 
FIGS. 4a and 4b show two means of implementing the alternate fold pole/bias 
polarity reversals required. In FIG. 4a the piezoelectric film is provided 
with a pair of electrodes 73 and 74. Numerals 75, 76, 77 and 78 typically 
indicate fold or bend locations. The zone between fold lines 76 and 77 is 
poled one way (+) while the 77-78 zone is oppositely (-) poled. Following 
zone poles alternate +/- throughout the active length. The end 79 is 
convenient for electrical connections. Poling of the 75-76 zone is a 
"don't care" zone and the electrodes are arranged such that there is no 
piezoelectric effect. 
Alternatively, the film is poled uniformly in FIG. 4b and the electrodes 
"wired" to effect alternate zone bias polarity. Electrodes in the 75-76 
zone are biased with identical polarity and magnitude and do not stimulate 
the piezo film, by way of the electrode arrangement. 
FIG. 5 shows in an exaggerated way displacement produced as a result of 
applied bias. Fold 28a is shortened, operating in tension and fold 29a is 
lengthened, operating in compression. Subsequent fold pairs are similarly 
operated to sum the piezo effect of each layer. Inverting bias polarity 
results in an equal movement in the opposite direction. 
An example design uses a 100 cm length of PVDF film, has 50 folds of 2.0 cm 
each, and an active area fold buildup of 2.7 mm between the plates. When 
biased at 285 V, displaces a theoretical 0.23 mm and develops an angle of 
5.0 degrees. 
The volume of the assembly is essentially constant, maintaining a fixed 
parallel spacing of the plates. Layer fold members in mechanical 
compression are restrained from collapsing by the encapsulant in 
conjunction with adjacent layer members in tension. The encapsulant need 
only accommodate the relatively small layer to layer length differential 
at the "open" ends opposite the fold. Two advantages of alternate layer 
film polarization over single polarization is overall simplification of 
the electrodes and placement of equipotential surfaces adjacent. 
FIGS. 6 and 7 illustrate a generic bending device, connecting a cell as 
previously described to a rigid armature through a volumetrically 
constrained elastic medium. Piezoelectric cell 32, a semienclosed volume 
of rubber 33 and a rigid armature 34, are coupled together by flexible 
straps 35 and 36 which are firmly adhered to opposing sides of the three 
sub components 32, 33 and 34. Exciting cell 32a, as shown, places strap 35 
in tension, pulling up on the armature 34a. Strap 35 is prevented from 
pulling away from the rubber fill 33 by adhesive 37. Simultaneously the 
lower strap 36 in the area between cell 32a and armature 34a is placed in 
compression and prevented from buckling by its adherence to rubber fill 
33a. The rubber maintains constant volume and assumes the shape 
approximating that shown in FIG. 7, resulting in the generation of an 
angle between cell 32a and armature 34a. The rubber section acts as a well 
defined distributed hinge firmly coupling the armature and cell. 
Dashed outline 38 shows cell response applied reversed polarity bias to the 
cell. 
FIGS. 8 and 9 are cross-section views of rolled film axial actuator 39. 
FIG. 8 is cross-section B--B of FIG. 9 and FIG. 9 is cross section A--A of 
FIG. 8. The device is a rolled up sandwich comprised of the divided piezo 
film half lengths 40a, 40b, the upper and lower spirals of rigid adhesive 
41, 42 respectively and connecting rubber fill 43, 44. The film 40a, 40b 
separated by the rubber fill and appropriate adhesive, is bifilar wound 
starting at film length center where it is anchored to piston 45 in slot 
46. The film stretch direction is perpendicular to the plane of the paper 
in FIG. 8 and in the direction of the arrow in FIG. 9. Outboard film ends 
48a, 48b which can advantageously serve as electrical connection points 
(areas 49a, 49b in FIG. 9), exit outer shell 50 through slots 51a, 51b, 
where the ends and spiral adhesive 41 and 42 are mechanically anchored to 
shell 50. 
Adhesive 42 placed along lower edge 53 of the film, starting at point 52 
(FIGS. 8 and 9), take on the form of a spiral, mechanically connecting 
outer surface of film turn 40a to inner surface of film turn 40b. 
Similarly, adhesive 41, placed along the upper edge 54, starting at point 
55 (FIG. 9), connects film inner surface of film 40a to outer surface of 
40b (FIG. 8). In this manner the material links the alternate ends of the 
turns so that film length changes accumulate. The zigzag philosophy of 
FIG. 4 is apparent in FIG. 9 and 10. 
Rubber fill 43, 44 placed in the interior volume not occupied by adhesive 
stripes 41, 42 inhibit film buckling and accommodates the differential 
turn to turn differential sheer displacement. 
In the variant described, the poling throughout the total length is 
constant. The required electrode polarity reversal is made on the outer 
film end 48a at pads 49a and 49b and corresponding pads on end 48b. 
Alternatively electrode polarity reversal could be done at the film center 
in the area of the slot 46. Another alternative places constant electrical 
polarity throughout the film length and inverts the poling direction of 
length 40a relative to 40b, at or near slot 46 (center). 
In either alternative, the result of applying an appropriate voltage 
results in an axial displacement of piston 45, relative to outer shell 50, 
as shown (exaggerated) in FIG. 10. 
A design option having 30 bifilar turns of PVDF film biased at 30 volts per 
micron and an active assembly length of 10 cm, yields a theoretical 
displacement of about 4.1 mm. 
FIG. 11 demonstrates a means of producing an incremental rotary actuator 63 
employing piezoelectric film. A length of film 56 is rolled up with and 
adhered to a rubber fill/spacer 57 of suitable thickness and compliance on 
a centrally located axle 58. The stretch direction of the film is shown by 
arrow 59. Film outer end 60, carried through and anchored in slot 61 of 
outer shell 62, provides a convenient place for electrical connections. 
Electronically induced change in film length exerts tangential forces on 
the radius of axle 58, turning axle 58, to yield an incremental angular 
output. 
In one embodiment in accordance with the principles of the present 
invention as illustrated by the arrangement of FIG. 11, a 1 meter long 
wrap of PVDF film biased at 10 volts per micron, on a 1.78 mm diameter 
shaft can be expected to yield about 15 degrees of angular displacement. 
The rotary actuator of FIG. 11 does not, in itself, compensate for age 
and/or temperature induced changes in film length. FIG. 12 shows in 
exploded view an embodiment that differentially joins a pair of rotary 
actuators 65 and 66 that are wound counter to each other so that common 
axle 68 is tangentially driven by complementary piezoelectric action while 
the effects of aging and temperature cancel. 
Arrows 69 and 70 show respective winding directions of actuators 65 and 66. 
Arrow 71 indicates resulting incremental rotary output of shaft 68 (shown 
broken) about a neutral position. 
Shell 63 of counterclockwise wound rotary actuator 65 is mechanically 
coupled to shell 64 of clockwise wound rotary actuator 66. Both actuators 
sharing common axle 68 cause thermal and age related film length change in 
actuator 65 to cancel that of actuator 66. The actuator pairs 65 and 66 
are poled/biased to exercise the film differentially against axle 68 so 
that incremental rotary effort is output (arrow 71). 
FIG. 13 illustrates a single cell rotary actuator. The end of the first 
half of the piezoelectric film is anchored in slot 80 of shaft 81 and in 
the example shown wound in a counterclockwise direction. The second half 
of the film, poled in the reverse direction to that of the first half, is 
folded at point 82, wound in the clockwise direction and mechanically 
anchored in slot 83. Rubber fill 84 supports the film. Electrically 
induced length modulation, acting tangentially on shaft 81, produces an 
incremental rotary output as indicated by arrow 85 which also shows the 
film stretch direction. In this arrangement aging and temperature effects 
on film length cancel. 
FIG. 14 shows a complementing pair of rectangular actuator cells 91 and 92 
which are similar to those of FIG. 4. In place of the two singular 
coupling straps 35 and 36 of FIG. 6, every film fold pair is coupled to 
corresponding and complementing film fold pair of the companion cell. The 
common connection point 95 of elements (folds) 93 and 94 for example are 
connected to the corresponding common point 96 of elements 97 and 98. This 
in addition to strengthening the hinge can produce an abrupt bend zone. 
Numeral 91a designates the film-supporting rubber fill. 
FIG. 14a shows the results of applied voltage to the compound actuator of 
FIGS. 14 in exaggerated form for illustrative purposes. 
Furthermore, by interconnecting a multiplicity of identical piezo film 
preforms in the manner shown in FIG. 15, all mechanical and electrical 
connections are simply made thereby permitting automatic assembly and 
simple electrical termination. 
FIG. 16 shows film preform 108. In the arrangement of FIG. 16, upper 
electrode area 102 and underside electrode area 103 accommodate conductive 
adhesive anchoring pad and contact area 104 and 105, respectively, on the 
upper side and contact area 106 and conductive adhesive pad 107 on the 
under side. Section 109 is folded down and under section 110 at bend line 
113. Section 112 is folded up and over section 111 at bend line 114. In 
the embodiment shown, sections 109 and 111 are poled positive up and 
sections 110 and 112 are poled positive down. 
Multiple folded preforms are stacked one above the other (FIG. 15) having 
pad 107 conductively adhered to the corresponding contact area 106 of the 
film preform above. Pad 104 is similarly conductively adhered to the 
corresponding contact area 119 and 120 are conductively adhered to 
corresponding contact areas 105 of the film preform below. 
The networked stack of film preforms are terminated by film preform 115 
(FIG. 17) on the underside and by a similar preform (not shown) at the top 
of the stack having a reversed poling pattern. Section 116 need not be 
poled. Section 117 is poled positive up and section 118 is poled positive 
down. Film interconnects contact area 119 with pad 104 of the preform 
above and interconnects and adhesive pad 102 adhered to the corresponding 
contact area 106 of the film preform above. 
Contact areas 123 (over) and 124 (under) on end 99 provide terminals for 
application of applied voltage. 
FIG. 15 shows the electrode polarities for positive voltage 100 (+) and 
negative voltage 101 (-) applied to terminals on end 99. Resultant 
polarities on the various surfaces are indicated throughout the figure. 
Film section poling direction is indicated by arrows typical of arrow 
115a. Arrow points indicate positive. 
The length expansion (E) and shrinking (S) activity of the various 
sections, with the voltage polarity 100 and 101 applied is indicated by 
imbedded "E's" and "S's", respectively, typified by numerals 121 and 122.