Hybrid motor

A hybrid actuator includes strain actuated elements that displace fluid to move a piston, with the elements forming a fluid housing, and being oriented and actuated to optimize force, stroke or bandwidth. In one preferred embodiment the elements are cylinders enclosing the fluid, with radial and axial components of strain-induced dimensional change adding together to enhance displacement. In another preferred embodiment, piezo bender elements produce large stroke, high bandwidth movements. Strokes of up to fifty percent of actuator length, and bandwidths above 500 Hz are achieved in light weight electrically actuated devices free of external piping or hydraulics. The actuator is readily integrated into a gate valve, sub-woofer, or other driven device.

BACKGROUND OF THE INVENTION 
The present invention relates to actuators or motors for producing motion. 
By far the most commonly used actuators for motion control are inductive in 
nature. Examples are linear and rotary solenoids, brushed and brushless dc 
motors, brushless linear dc motors, and stepper motors. These inductive 
actuators are appropriate in velocity control applications with relatively 
low torque requirements. However, even there, the stiffness and bandwidth 
of an inductive motor are limited by properties of the magnetic coupling 
between the permanent magnet and the windings. 
In many applications, a high force, quasi-static position actuator is 
desired. Achieving high torque outputs from an electric motor presents a 
variety of mechanical and electrical problems. Often, to obtain high 
torque outputs, some form of mechanical transmission is employed. However, 
the transmission reduces the actuator bandwidth and contributes to 
mechanical losses and backlash. In addition to transmission concerns, the 
motor itself has limitations, in that significant currents have to pass 
through the motor windings to increase motor torque outputs when the motor 
is operated close to stall. This causes high power dissipation through the 
winding resistance and results in a corresponding need to transfer the 
generated heat away. Further, the design and operation of inductive motors 
is complicated by the need to commutate the magnetic field. Commutation 
introduces significant torque ripple at low velocity and degrades overall 
torque output. Electrical commutation, as used in brushless motors, 
requires a motor position sensor whose output is fed back to a relatively 
complex controller. In a brushed motor, high currents at low velocities 
cause arcing of the commutation brushes and greatly reduce motor life. 
Thus, electric motors have inherent limitations. 
From a purely theoretical point of view, capacitive devices such as 
piezoceramic actuators exhibit much more desirable mechanical and 
electrical characteristics. They have a very efficient coupling of energy 
from applied charge to mechanical strain, which results in a high 
bandwidth, a large force output and negligible resistive heating. The 
actuator stiffness is determined by the modulus of the ceramic material 
used for the actuator, rather than by an inherently weak magnetic 
coupling. Because these elements are capacitive in nature, they draw their 
least current at low or zero rate of displacement. Furthermore, a direct 
correspondence exists between actuator voltage and resultant position, 
without the need for commutation. Piezoceramic actuators, however, have 
historically been limited to extremely low displacement precision 
applications such as mirror control, ink jet nozzles, ultrasonic medical 
devices, high frequency audio speakers and miniature valves, where motions 
of only a few thousandths of an inch are needed. 
Piezoceramics are commercially available in a variety of configurations, 
such as plates, tubes and stacks. Composite actuators, such as bimorphs, 
can be made by sandwiching a metal shim between two thin piezoceramics 
which are oppositely poled. When a voltage is applied to the bimorph, one 
piezoceramic expands while the other contracts, introducing a bending 
motion and/or bending moment of greatly amplified displacement into the 
composite element. 
Several prior patents have been issued for hybrid devices, wherein 
electrically actuated elements that change dimension in response to an 
applied electrical drive signal are used to displace fluid for driving a 
hydraulic ram. Among such patents are U.S. Pat. Nos. 3,501,099 of Benson 
and 5,055,733 of Eylman. Other patents, such as U.S. Pat. No. 4,995,587 of 
Alexius show mechanical arrangements for amplifying the displacement so 
produced. However, to the applicant's knowledge, this prior art has not 
specifically addressed the particular mechanical properties of 
piezoelectric elements, other than, for example, their general benefit of 
electrical actuation and their usual limitation of small actuation 
displacement. In addition, this art has not achieved constructions which 
optimize the efficiency of a hybrid actuator, or which substantially 
outperform a conventional actuator. 
Accordingly it would be desirable to provide a hybrid electrohydraulic 
actuator construction of enhanced electro-mechanical efficiency and 
performance characteristics. 
SUMMARY OF THE INVENTION 
A hybrid actuator, or motor, in accordance with the present invention 
combines strain actuated and fluid actuated elements to achieve, in a 
single unit, the advantages of a capacitive actuator, the force and stroke 
characteristics of a small hydraulic device, and the bandwidth of an 
electric motor. One embodiment of the actuator produces a displacement on 
the order of 20-30% of the actuator length, and has a bandwidth of 
approximately 500 Hz. Briefly, the actuator uses strain actuated 
piezoceramic elements with a mechanical advantage to magnify the 
piezoceramic actuation strain, with the mechanical advantage being 
obtained by displacing fluid to drive a piston. That is, the small 
piezoceramic displacements are amplified by a hydraulic transmission. The 
piezoceramic displaces the fluid in a master cylinder that acts on a 
smaller diameter actuator piston in the same cylinder or a slave cylinder. 
In preferred embodiments, the piezoceramic elements are shaped and 
oriented in special housing configurations to maximize a property such as 
the stroke or volume displacement of the fluid, and the piston may be 
round or have a shape and dimensions which are optimized based on the 
housing and actuator geometry. Preferably the actuator is instrumented to 
provide collocated displacement and load information. In further 
embodiments, a local feedback loop enhances the accuracy attained in 
reaching commanded displacements or forces. 
Solid-state hydraulic actuators of the invention are suited to a variety of 
applications. The actuator is small compared to an electric motor and 
dissipates minimal power. It requires low currents to produce significant 
static loads. Controllers for the device are small and simple, not 
requiring complicated switching and timing for commutation. The controller 
can be packaged integrally with the actuator, to provide a modular design. 
Unlike electric motors which produce a velocity output for a commanded 
voltage, and require controllers to produce displacement profiles, this 
solid state hydraulic actuator has a direct correspondence between stroke 
and voltage, and its response may be programmed with a voltage profile. 
When instrumented, the actuator provides local information about 
displacement and applied load and hence lends itself to effective 
controller designs. A preferred embodiment of the actuator is extremely 
modular, with no external hydraulic or pneumatic lines, and yields 
significant stroke and force output for its size. It is also easily scaled 
to provide larger strokes and/or force outputs as required.

DETAILED DESCRIPTION 
The invention will be best understood after a brief discussion of prior art 
motors or actuators, and a theoretical discussion of the actuation 
characteristics of hybrid actuators. 
FIG. 1 shows, for a variety of different actuator types, a plot of the 
specific load of the actuator against its operating bandwidth. Specific 
load is a performance metric which is the product of the average force 
produced by the actuator times its average stroke capability scaled by the 
actuator weight and characteristic length. Hydraulic actuators are shown 
to have a high specific load but low bandwidth. So are shape memory alloys 
(SMA), a type of temperature controlled transducer that returns to an 
initial shape on heating. On the other hand, piezoceramics have very high 
bandwidth but small stroke capability. Electric motors have significantly 
less specific load capability than hydraulics, but greater bandwidth. 
Overall, there is a trade-off, in that actuators with high specific load 
capability are generally of low bandwidth, and actuators with high 
bandwidth have relatively low specific load capability. Of the seventeen 
prior art actuators examined to compile the chart of FIG. 1, all lie below 
the trade-off line L, and most occupy only a small region of the total 
range spanned by the chart. By contrast, a representative hybrid 
motor/actuator 10 is seen to lie well above line L, representing almost an 
order of magnitude improvement in specific load and in bandwidth, in the 
highly useful central region of the chart. 
The specific constructions for achieving such enhanced performance will be 
discussed after modeling the behavior of several illustrative 
constructions of hybrid actuators in accordance with the present 
invention. Schematics of a few solid-state actuator architectures in 
accordance with the present invention are shown in FIGS. 2A--2D, and will 
be discussed below. Other possible configurations and detailed 
implementations are shown in the Figures, which is not a comprehensive 
list of actuator configurations, but illustrate a number of different 
embodiments of the invention which applicant has determined to have high 
expected performance or particular useful characteristics of force, 
stroke, size or physical shape. In the Figures, piezoceramic components 
are shown shaded, whereas other structural components such as housings, 
pistons and the like are illustrated by simple outline drawing. The 
configurations use piezoceramics in a variety of architectures, each 
within or bordering an oil filled cavity. Further, as will be understood 
from the following descriptions of a number of representative embodiments, 
the piezoelectric elements may in many cases constitute all or a major 
portion of the fluid housing, or body of the device. 
As is well known, a voltage applied to a piezoelectric element produces an 
actuation strain which deflects the piezoceramic element. In all 
embodiments of the present invention, this deflection creates a fluid 
pressure which acts against a piston. Depending on the stiffness against 
which the actuator pushes, this fluid pressure may produce a force, or the 
fluid pressure may simply lead to a fluid displacement if the piston is 
free to move unhindered. In general, the small piezoceramic displacements 
are amplified by the hydraulic transmission, that is by the displaced 
hydraulic fluid acting on the piston structure. 
As shown in FIGS. 2A-2D, different geometries of the basic elements are 
employed to form four configurations A-D having conceptually different 
properties. Configuration A uses bimorphs which are each pinned (i.e., 
fixed) at the piston end and rear of the actuator, and are free to bulge 
on the sides. When a voltage is applied, the bimorphs flex out, 
effectively bowing the broad faces inwardly to squeeze the oil filled 
cavity. Configuration B uses a piezoceramic tube which is preferably poled 
to constrict in the radial direction when a voltage is applied, reducing 
the cross sectional area of the oil filled center. Configuration C uses a 
piezoceramic stack acting against an oil filled cavity. Configuration D 
uses opposed plates, sealed at the edges, with a piezoceramic stack at 
each corner. The stacks are actuated to push the plates closer together 
and force oil out of the cavity. 
In each case a piston element P is shown having a relatively small 
cross-sectional area perpendicular to its direction of motion. This may be 
understood in a rough sense as enhancing the stroke of the device. In 
addition, the piezoelectric elements are preferably specially shaped and 
poled, as in the embodiments of Configurations A and B, to enhance fluid 
displacement with good response characteristics. 
Applicant has developed a model to predict the anticipated stroke 
capability of the configuration shown in FIG. 2A as a function of actuator 
geometry and piezoceramic constants, which will now be discussed in detail 
for a better understanding of the novel constructions of the present 
invention. 
FIG. 3 shows the actuator height a, width b, and length L of a rectangular 
sandwich-shaped squeezer type hybrid actuator. The upper and lower faces, 
f.sub.1 and f.sub.2, are each formed by a bimorph having thickness h, and 
the piston has thickness 5h. The displacement, w, of the bimorph at any 
position x along the length of the actuator is given by 
##EQU1## 
where .kappa. is the local curvature of the bimorph. For a thin shim, the 
curvature may be accurately expressed as a function of the local strain 
.epsilon.. 
##EQU2## 
Integrating this displacement over the length and width of the bimorph 
yields a volume V which is equivalent to the volume of oil displaced by 
the flexure of one bimorph, i.e., by actuation of one side of the fluid 
housing: 
##EQU3## 
If this volume of oil drives a piston of width b, and height 5h, the 
piston travel .DELTA.L when both bimorphs are actuated is obtained by 
equating the volume displaced by the piston's travel to the volume of oil 
displaced by the flexure of the two bimorphs. The travel is given by: 
##EQU4## 
or, as percent of the overall length of the actuator, 
##EQU5## 
Assuming a typical value of 450 ppm strain for piezoceramics in a d.sub.31 
configuration, piston travel as a function of the actuator geometries is 
readily predicted. FIG. 4 shows the stroke which is calculated for two 
standard catalog piezo components. It will be seen that the available 
stroke from such an actuator is quite large. With the thinner actuator of 
the two, the stroke increases dramatically, but less force is produced. 
Applicant's analysis performed to predict the forces produced by such an 
actuator yields preliminary estimates for these forces on the order of two 
pounds, which is a magnitude suitable for a great number of practical 
actuator applications, including applications such as driving gate valves, 
operating low frequency (woofer or sub-woofer) audio speakers, and various 
indicating or mechanical switching operations. 
In a similar manner, models are constructed to predict the force and stroke 
capabilities of the three other architectures of hybrid actuators shown in 
FIGS. 2B-2D. Using the published characteristics of a few commercially 
available piezoelectric cylinders, and stacks of discs or annuli, the 
models combine electrical, mechanical and hydraulic properties of the 
system components such that they predict piston deflection when a voltage 
is applied, or operating as transducers, they predict the induced 
piezoceramic voltage when the piston is deflected. The losses to be 
expected from motion of the viscous oil are evaluated analytically, and 
can be minimized, for the designs shown in FIGS. 2A-2D, by constructions 
which have the piezoelectric elements directly constitute or act on one or 
more walls of the piston housing, rather than acting in a separate fluid 
displacement chamber. 
This modeling predicts strokes on the order of thirty percent of actuator 
length, with a bandwidth of 500 hz. Moreover, the actuators are readily 
scaled, so that they may be used for diverse applications, ranging from 
such medium-scale movements as animating puppets or mannequins, to 
micro-applications such as operating a shutter assembly of a camera. 
With regard to details of construction of the actuators in accordance with 
the present invention, applicant has found certain approaches to appear 
more reliable, in terms of effective maintenance-free operation, in common 
with all constructions described below. Among such details it is necessary 
to provide a seal about the piston assuring that fluid does not leak from 
within. This is preferably accomplished using either a linear bushing or 
bearing through which the piston passes, or using an O-ring or scraper 
arrangement about the piston. For low force applications, a diaphragm seal 
rather than an O-ring or other seal frictionally contacting the piston is 
preferred. FIG. 3A illustrates the physical structure of the 
sandwich-shaped squeezer actuator schematically shown in FIG. 3. Each of 
the piezobender elements is mounted with both ends fixedly secured to a 
surrounding (rectangular or cylindrical) housing, and the housing secures 
an O-ring or bushing seal, through which the piston rod slides. 
It is also in general necessary to ensure that arcing of the actuator 
electrodes does not occur through the surrounding fluid or oil. One 
approach to this problem is to employ a robust but thin elastic insulating 
layer such as a polyurethane coating over the piezoceramic elements. The 
polyurethane is electrically insulating, and, being considerably more 
elastic than the piezoceramic, does not impede its motion. It is also 
necessary to connect the electrodes to the piezoceramic elements. In 
general each element will undergo a relatively small motion and it is 
therefore feasible to employ electrical wire feed throughs through the 
actuator housing to connect, via a short pigtail, to a metallized 
electrode surface of the actuator. In making the electrode contacts, it is 
also desirable to avoid elements such as sharp spring loaded tabs which 
might introduce stress concentration on the electrode surfaces of the 
actuators. Two way travel of the actuator is achieved in some embodiments 
with a return spring design, and in others is achieved with a dual action 
actuator having two similar piezoceramic mechanisms pushing oil into two 
separate chambers located on either side of the piston rod. The return 
spring construction will generally reduce the overall stroke capability of 
the actuator, while the use of dual piezoceramic mechanisms and dual oil 
chambers would complicate and raise the cost of overall design and 
fabrication. In preferred embodiments, applicant achieves a bidirectional 
actuator by using step or tapered piston bodies in a single fluid chamber, 
with the bodies extending out of the fluid chamber at each end. The 
actuator is preferably biased in a mid position to be able to either 
increase or decrease its volume upon actuation. Preferably, various 
additional steps are also taken to avoid delamination or cracking of the 
piezoceramic elements, such as might occur with bimorphs, tubes, stacks or 
any elements that extend for any substantial length in a direction, or are 
subjected to high pressure pulses in the fluid surroundings. 
In controlling both the shape and the disposition of the strain actuated 
elements within the fluid system, an actuator sensor or motor according to 
the present invention may be configured to optimize any of the operating 
parameters and, in some cases, several of the parameters for specialized 
tasks. The great bandwidth achieved, and efficiency have each been noted 
above. However, it is also possible to optimize or enhance the stroke for 
actuating mechanical displays or objects and motion amplification; or to 
optimize or enhance the force produced by the device, a trait which is 
useful in a specialized feedback system for controlling small localized 
displacements, or for controlling instantaneous pressure in a fluid system 
such as a braking system; or to optimize or enhance the motion or force 
signal resolution for sensing and amplification of motion or force. 
These desirable traits will be understood with reference to the following 
discussions of illustrative embodiments. In each of the FIGS. 5 through 
22, three views are shown, a first, conceptual, view labeled with the 
numeral, e.g., FIG. 5, followed by a FIG. 5A and a FIG. 5B. The "A" view 
represents an end view taken along the direction of the axis of the piston 
actuation for a rectangular or square embodiment, whereas the "B" view 
represents a similar view of a cylindrical embodiment. The first view in 
each case is a section taken along the axis of piston movement, which, as 
explained further below, may be either aligned with or aligned 
perpendicular to the axis of actuation of the elements involved. 
Taking FIG. 5 for an illustrative example, the Figure shows a section 
through an actuator housing 10 having a piston 12 and filled with a fluid 
13. An actuating body 14 seals the end of the housing, and as described 
above is actuated to induce local strain which causes a change in its 
dimensions, i.e., a change in its length, width, thickness or the like. In 
the Figures, an isosceles triangle is placed on the actuator element to 
indicate the poling direction of the element, denoted P, while a line 
through the triangle indicates the actuation axis or direction of 
principal movement of the element when actuated. Thus, briefly looking 
ahead, the configurations shown in FIGS. 5 and 6 are actuated along 
D.sub.31, whereas the configurations shown in FIGS. 7 and 8 are actuated 
along D.sub.33. Returning to FIG. 5, the actuating body 14 closes the end 
of the housing, and is actuated to expand lengthwise across the housing 
(D.sub.31) so that it flexes forward in the center displacing an amount of 
fluid swept by the diaphragm-like bending displacement of the element. As 
shown in FIGS. 5A and 5B, the housing may be either rectangular or 
circular in cross-section. Similarly, the piston may be of a rectangular 
or circular cross-section; however, for purposes of economic bearing and 
seal fabrication, a circular cross-section is generally preferred for the 
piston shaft. 
FIG. 6 shows a somewhat similar embodiment which differs in having a 
movable pusher panel 15 displaced by the diaphragm. In this case, a 
greater displacement is obtained since panel 15 is moved by the center or 
maximum displacement of the diaphragm. A plurality of springs, shown 
schematically, returns the panel to its zero or resting position when 
actuation is discontinued. This return mechanism is desirable when the 
actuator is configured for relatively small force applications, in which 
frictional hysteresis might otherwise prevent reliable return. 
FIG. 7 shows a related embodiment, wherein the movable panel is replaced by 
a bendable panel 16, which may for instance, take the form of a hard 
rubber diaphragm. Bendable panel 16 is fastened about its circumference to 
the housing, while its center is free to flex forwardly. The panel is 
actuated by a stack 17 of strain actuating elements (e.g., piezoceramic 
discs) which are preferably actuated for extension in the D.sub.33 
direction. The stack may consist of square, round or other planar 
elements, with a multiplication or amplification of extension obtained by 
stacking a plurality of elements one on top of the other. 
The embodiment shown in FIG. 8 eliminates the panel and relies on a stack 
17 of piezoelectric plates residing directly in the body of fluid. In this 
embodiment, again, the stack is actuated in the D.sub.33 direction. Since 
it resides directly in the fluid, the expansion in the axial direction is 
somewhat offset by a contraction in the in-plane direction of each 
element. That is, the change in length times the cross-sectional area will 
displace fluid, but this displacement is partially offset by a decrease in 
fluid displaced at the edges of the stack due to coupling between the 
directions of strain introduced in the device. This decrease will be 
proportional to the perimeter times the height times the change in 
cross-sectional dimension. As noted above, these latter two embodiments 
are actuated in the D.sub.33 direction, so the D.sub.31 change is 
relatively slight. However, to optimize piston displacements, some form of 
surface seal across the top of the stack to the housing wall is preferred, 
to prevent fluid from reaching the side region. Furthermore, the area of 
each plate in the stack 17 is preferably a substantial portion of the 
cross-sectional area of the fluid chamber. 
A different situation is shown in FIG. 9, wherein the stack 17 is replaced 
by a thin shell 18 which, as in the embodiment of FIG. 6, supports a fixed 
plate 15. In this case, the thin shell may be made of a piezoceramic 
cylinder, or a series of plates which are fastened together and sealed. As 
illustrated, actuation is by extension in the D.sub.31 direction. As 
before, rectangular or circular embodiments are possible. 
Variations of these basic approaches are also contemplated by the 
invention. For example the embodiment of FIG. 6, in which a movable stiff 
panel 15 is pushed by a bender element, is modified in a straight-forward 
way by replacing the bender with a stack 17 that is activated to lengthen 
and push the panel. All of these described embodiments may be 
characterized in having actuable strain elements for moving a piston along 
the direction of actuation. The basic elements are quite flexible in 
design, and similar units may be placed not just at the bottom of the 
housing, but also at the top surrounding the cylinder so as to displace up 
to twice as much fluid with the same actuating signal. In all of the 
foregoing constructions, fluid displacement is axial, in the direction of 
piston travel, so that losses from hydrodynamic effects due to movement of 
the displaced fluid are minimal. However, the invention is not restricted 
to such embodiments, but also includes constructions wherein fluid 
displacement (or more precisely, volumetric expansion of the displacing 
element) occurs in a direction perpendicular to the direction of piston 
travel. 
FIG. 10 shows such an embodiment, wherein the external lateral walls 
forming a fluid-bounding structure are formed by a thin strain-actuated 
shell 9 such as a cylinder or a series of plates. In this case polling is 
radial, and actuation is preferably along the radial direction in the 
D.sub.31 sense so that when the element is actuated it squeezes or 
constricts radially to move the piston 2. If the external walls are made 
of separate plates, a single plate, a pair of opposed plates or all four 
sides may be made of the strain-actuated elements. 
FIG. 11 shows a hybrid motor wherein the strain actuators move in a 
direction perpendicular to that of piston motion. In this case, the 
actuators 11 are piezobenders actuated in the D.sub.31 direction to flex 
inwardly along their lengths. This embodiment is preferably implemented 
with a pair of opposed sides in a more or less rectangular configuration 
with a piston having a rectangular or oblong cross-section. That is, the 
piston has a greater width in a direction normal to motion of the 
actuator, than in a direction parallel to motion of the actuator. This 
allows a thin flat "sandwich" aspect to the housing. FIGS. 12 and 13 show 
constructions for this perpendicularly actuated embodiment which bear the 
same relationship to FIG. 11 as the constructions of FIGS. 6 and 7 do to 
that of FIG. 5. Specifically, FIG. 12 shows spring-loaded pusher plates 5 
which are displaced in accordance with the maximum displacement of the 
center portion of the piezobenders 11. FIG. 13 shows a bendable panel 13, 
analogous to the stiff diaphragm-like panel 16 of FIG. 7, having its 
center displaced perpendicularly to the piston axis by a stack of strain 
actuators 12. 
Among the foregoing constructions, that of FIG. 11 employing bending walls 
to displace fluid will be appreciated by those skilled in piezoceramics to 
displace a relatively large amount of fluid upon actuation. This property 
may be further enhanced by the embodiment shown in FIG. 14, wherein the 
fluid chamber is annular, and the inside bounding walls 11 and outside 
bounding walls 11 are each formed of a strain-actuated bending element. In 
this case, the inside walls flex outwardly while the outside walls flex 
inwardly; the piston 14 is formed as an annular or U-shaped body (for the 
cylindrical or rectangular configurations respectively) so that a full 
circumference (cylindrical configuration) or a pair of legs (rectangular 
configuration) are acted upon between the respective sets or regions of 
the inner and outer walls. 
FIG. 15 shows an embodiment wherein plates 7 are actuated through their 
thickness in a direction perpendicular to the axis of the piston to 
displace fluid. This construction advantageously provides a large fluid 
displacement with only small compensating coupled shrinkage at the end of 
plates. 
FIG. 16 shows a similar arrangement wherein pusher plates 5 are supported 
by plate or cylinder strain actuators 8. The latter are polled through 
their thickness, and actuated in the D.sub.31 direction. 
FIG. 17 shows a similar arrangement but supported by stacks actuated in the 
D.sub.33 direction. In each of these latter two embodiments, large 
displacements are obtained with only small coupled counter effects at the 
end supports. Coupled shrinkage effects are especially small in the 
embodiments of FIG. 16 and 17, which do not involve large continuous 
strain elements, and which use pusher plates to amplify the fluid volume 
displaced. The region below each pusher plate is sealed against fluid 
communication with the rest of the housing. 
FIG. 18 shows a two chamber embodiment (rectangular) or annular embodiment 
(cylindrical configuration) wherein inner and outer walls are each poled 
and actuated in their thickness dimension. Illustratively, an end plate 16 
closes the ends of the fluid housing formed by the strain elements, and 
slidably supports the piston 14. In this embodiment, and in other 
embodiments employing cylinders described herein, stroke is greatly 
enhanced by virtue of the fact that a thickness expansion displacing fluid 
is coupled by Poisson coupling to a decrease in cylinder or plate length. 
This draws the end cap in, further displacing fluid from a length 
contraction as well as from the width expansion. Thus both of the two 
strain-induced motions of the element additively displace fluid resulting 
in an enhanced stroke. A similar result is achieved with a simple 
cylindrical embodiment using a piezo ceramic cylinder polled across its 
thickness. 
Applicant has modeled the force and displacement available from a simple 
half-inch diameter actuator with a 1/32-inch diameter piston rod, the 
results being shown in FIG. 19. As illustrated, a simple 3-inch long 
piezoceramic cylinder provides piston displacement of approximately one 
inch with forces up to twenty pounds. Longer cylinders provide longer 
stroke, and a higher force profile over extended portions of the stroke. 
In general, the piston diameter will determine the actuator blocked force, 
with larger pistons providing larger blocked forces, so that either large 
stroke or large force may be obtained. When frequency of actuation becomes 
a concern, a minimum force requirement is set by the mass and bandwidth 
specifications, since a specific force is needed to accelerate a given 
mass at a particular frequency, where the force F=m A .OMEGA..sup.2. In 
general, all of the foregoing embodiments have the property that very low 
electrical current levels generate significant static loads. Furthermore, 
controllers for the actuators can be small and simple since they require 
no complicated power or mechanical switching, or timing for commutation. 
FIG. 19A shows the force versus displacement characteristics of another 
embodiment, employing a stack of piezoelectrically actuated plates. Here, 
a three and one-half inch diameter stack of disc elements, six inches 
tall, was assumed. As shown, forces of over three hundred pounds over a 
half-inch extension, and one hundred pounds at a one-inch extension were 
obtained, with a sharp drop-off above one inch. Thus, high force, high 
displacement output is achieved in this embodiment, although the relative 
displacement as a function of actuator size, is less than for the 
embodiment discussed earlier. 
These characteristics may be varied somewhat by the selection of 
appropriate piezoelectric elements. In general, the strength and stiffness 
will be important for hybrid devices wherein the piezoelectric element 
forms the device housing, while properties such as actuation bandwidth may 
be a primary consideration for devices that require high frequency or 
short response actuation characteristics. FIG. 19B shows the physical 
characteristics of a number of piezoelectric materials including the 
piezoceramic PZT, lead magnesium niobate (PMN), the polymer Terfenol, and 
shape memory alloy (PMA). PZT actuators provide good overall 
characteristics for high bandwidth actuators, while at the other extreme 
SMA actuators may provide high strength, but at the price of very low 
bandwidth. 
FIG. 20 shows design curves for a hybrid linear displacement actuator of 
the type shown in FIG. 8. The curve illustrates the force versus stroke 
characteristics for six different hybrid actuators having cylindrical 
casing diameters of either 2.0 or 3.0 centimeters, and three different 
piston diameters. The length of each actuator is assumed fixed at two 
centimeters. As illustrated, strokes of approximately 30% of the actuator 
length are obtained for the smallest piston, and approximately 5% for the 
intermediate size piston, with good force characteristics. 
FIGS. 21 through 25 illustrate further embodiments of the invention with 
different poling or actuation arrangements in actuators of rectangular or 
cylindrical cross-section, and with simple or annular chambers. As before, 
poling and actuation directions are indicated schematically. 
In addition to the foregoing constructions, actuators in accordance with 
the present invention may be configured as single acting, double acting or 
differentially double acting cylinders as illustrated in FIGS. 26 through 
29. A representative single acting cylinder has a piston extending out one 
end of the housing, preferably returned via a constant force spring as 
shown in FIG. 26. FIG. 27 illustrates an embodiment with a piston 
extending at each end out of a single housing. In such embodiments, the 
pistons are preferably tied together with a common constant force spring 
and each is equipped with a limit stop to prevent excessive extension. In 
each case, the spring bias may be replaced by electrically biasing the 
piezo elements into a mid-operating region. Since the cylinder is 
hydraulically full, motion of the piston then tracks increase or decrease 
in fluid volume. 
FIGS. 28 and 29 show embodiments wherein a single piston passes entirely 
through the housing, to extend from either end depending on the volume of 
fluid displaced. As shown in FIG. 28, the piston is a step piston having a 
small diameter at one end, a large diameter at the other end and a step in 
an intermediate portion that moves within the fluid housing. In the 
embodiment shown in FIG. 29, the contour of the piston within the fluid 
housing is a taper rather than a step. In these two embodiments, decrease 
of the volume available for fluid moves the piston to the right, while 
increase of the available volume drives the piston assembly to the left. 
It will be appreciated that the foregoing embodiments have been described 
generally with respect to hybrid systems employing strain-actuated 
elements in the form of stacks, plates or cylinders that either constitute 
the fluid housing, or act on pusher plates constituting the fluid housing, 
thus providing a very lightweight construction of hydromechanical 
simplicity. In general, the piezoceramic elements produce a force or 
displacement characteristic that is directly related to applied voltage 
for a given actuator, and therefore these devices may be driven and 
controlled in a quite simple manner with microprocessor controllers using 
simple circuit interface elements for amplification. They thus offer a 
versatile approach to many production line and manufacturing applications 
previously tied to pneumatic actuator technology. They further offer 
myriad uses in areas as diverse as medical products, window displays, 
puppets, motor vehicle subsystems, locks, and numerous signaling or 
control applications. 
The compact housings of such hybrid actuators allow them to be readily 
integrated into or attached to various actuated devices. For example, one 
area in which the hybrid actuator of the present invention offers a 
substantial improvement in operational characteristics is as a 
low-frequency audio speaker, a woofer or sub-woofer. In such a device, a 
fluid reservoir with a thin cylindrical piston as shown, for example, in 
FIGS. 24 and 24B may attach directly to the apex of a speaker cone, or 
even to a flat plate, directly replacing the magnet and coil of a 
conventional speaker assembly. Such a hybrid speaker driver may achieve 
high efficiency, large displacement, linear motions (e.g., of one to five 
inches) at frequencies from DC to several hundred hz or more, with an 
exceptionally flat frequency response. Such a sub-woofer 30 is shown in 
FIG. 30. As shown, a hybrid actuator 1 as shown in any of the foregoing 
Figures has its piston 2 connected to drive one end of a large plate, 
illustrated as a speaker cone 30, the outer perimeter 32 of which is 
suspended, by springs or elastic material 33 from a frame 40 to which the 
actuator 1 is rigidly attached. Operation of the actuator thus drives an 
extended air column defined by the large diameter cone 30. 
The foregoing description of the operation of several illustrative and 
preferred embodiments of the invention has been undertaken to show the 
principles thereof and details of construction for enhanced performance 
hybrid actuators. Being thus disclosed, further variations and 
modifications will occur to those skilled in the art, and all such further 
variations and modifications are intended to be with the scope of the 
invention, as defined in the claims appended hereto.