Note: Descriptions are shown in the official language in which they were submitted.
6~
Background of the Inven-tion
This invention relates to an electromechanical
actuator, and more particularly to a piezoelectric actuator.
A known piezoelectric actuator comprises a piezo-
electric hollow cylindrical clamping transducer and a cylin-
drical moving member. The cylindrical member is fitted inside
the piezoelectric transducer, and serves as the inner electrode
of the piezoelec-tric hollow transducer. An outer electrode
is bonded on the outer cylindrical surface of the
transducer. The piezoelectric transducer acts either as a
clamp or as a bearing for the axial motion of the
cylindrical member depending on the voltage applied between
the outer electrode and the cylindrical member. This known~
piezoelectric actuator presents the possibility of ion
bombardment or sparking on the interface between the moving
member and the clamping cylindrica:L transducer. Such ion
bombardment evidences itself in an oxidation of the moving
member, a slight roughening of its surface finish, and a
buildup of material on the moving member that interferes and
disrupts the proper operation of the piezoelectric actuator.
` Another electromechanical actuator is described in
~ U.S. Patent 3,551,764 to E.B. Evans, wherein the basic
. concepts of a piezoelectric actuator are.disclosed. In this
known arrangement, a pair of piezoelectric elements are
cyclically energized to provide a series of small
incremental displacement~ causing a mennber to move in a
..
.
~: :
3~ ~' "''
;.~ ' .
cumulative motion. One of the piezoelectric elements
provides either a controlled frictional restraint to clamp
the moving member while the other piezoelectric element
imparts an incremental motion thereto or it permits the
moving member to slide freely relative to it. A sliding
interface, which is characteristic of this kind of
piezoelectric actuator, exists between the moving member
and the piezoelectric clamping element. In the known
piezoelectric actuator, an electrode separates the moving
10 member from the clamping element to which it is attached,
and thereb~ forms together with the moving member the
sliding interface. This known arrangement eliminates wear
by ion bombardment but presents a wear problem due to the
sliding of the moving member on the electrode, the latter
being not sufficienty resistant to frictional wear.
Another source of wear in known actuators is due to
the fact that when a potential is applied to the clamping
element, it deflects radially as well as axiall~. The
axial deflection results in sliding on the interface under
a rather high pressure causing interfacial wear tending to
- reduce the clamping force.
Brief Descri~ of the Invention
In accordance with an aspect of the present invention
there is provided an electromechanical actuator comprising: -
a support member; a first member; first electromechanical
transducer means fixed to said support member and
responsive to ~irst electrical signals for producing
discrete displacements of said first transducer means with
respect to said support member in a predetermined
direction; second electromechanical transducer means
rigidly coupled to said first transducer means, and ~
. ..':
,
~/ - 2 -
3~
responsive to second electrical signals for contacting
said first member and displacing one member relative to
the other; and means interposed between said second
transducer means and said first member for controlling the
amount of frictional wear therebetween.
The foregoing problems are solved in accordance with
an embodiment of the present invention wherein special
materials are selected for the moving member and a liner
member is interposed between the clamping element and the
moving member to reduce wear at the sliding interface. In
an illustrative embodiment of the present invention, the
piezoelectric actuator comprises first, second and third
piezoelectric transducers and a moving member, the latter
preferably made of very hard material. The first and second
. .
. .
: ~,
.:, :
'' . ' '' .
- 2a -
''~'' .
::
6~
transducers, denoted clamping transducers or elements, are
respectively attached at each end of the third piezoelectric
transducer. The three piezoelectric transducers are
electrically timed to move the clamping elements back and
~orth while the clamping ~orce on the moving member
alternates from one clamp to the other~ thereby moving the
member in very small displacement steps.
In accordance with one illustrative embodiment of
the invention, each o~ the piezoelectric transducers, as
well as the moving member, are of cylindrical shape. The
two piezoelectric clamping transducers are preferably made
of lead zirconate titanate (PZT) ceramic and are poled to
move in the radial direction. The third transducer is also
pre~erably made of lead zirconate titanate (PZT) ceramic and
is poled to move in the axial direction.
In accordance with another illustrative embodiment
of the invention, a clamping transducer comprises two
radially stacked ceramics having an outer, intermediate, and
- inner cylindrical electrode. An inert ceramic liner is
; 20 ~itted between the cylindrical member and the inner
electrode.
In a further illustrative embodiment o~ the
invention, the third piezoelectric cylinder preferably
;` comprises a plurality of axially poled stacked transducers
designed to give as large an axial displacement as possible.
One ob~ect of the present invention is to eliminate
the wear and ion bombardment problems in piezoelectric
actuators, thereby rendering the actuators useful and
practical.
Another object of the present invention is to
realize a piezoelectric actuator having a very long
`; ~.' ~
,
, :
.. .
. .
,: . . ....
service-free operational life.
A still fur-ther object of the present invention is
to increase the operational thermal stability of a
piezoelectric actuator by an optimum choice of materials for
the moving member, the clamping transducers, and the liner
member.
These and other objects and advantages will appear
more fully upon consideration of the various illustrative
embodiments now to be described in detail in connection with
the accompanying drawings.
: Brie~ Description of the Drawings
FIG. 1 is a schematic illustration of a clamping
transducer of a known piezoelectric actuator;
. FIG. 2 is a schemakic illustration of one
illustrative embodiment of the present invention;
FIG. 3 shows another illustrative embodiment of a
~; piezoelectric actuator according'to the present i.nvention;
FIGS. 4A and 4B show wave diagrams of ~roltages
applied to the piezoelectric actuators of FIGS. 2 and 3 in
accordance with one mode of operation of the piezoelectric
: actuators o~ the present invention; and : .:
- FIGS. SA and 5B show wave diagrams of voltages
applied to the piezoelectric actuators` of FIGS. 2 and 3 in - : :
accordance with an alternative mode of operation of the
. piezoelectric actuators of the present invention. - -
Detailed Description ' . . :
. Referring to FIG. 2, there is illustrated a
piezoelectric actuator in accardance with one illustrative
embodiment of the present invention. The actuator comprises
30 a moving membe.r 1, which is preferably of cylindrical shape. :
However, moving member 1 could also be rectangular or even
~ .
.
, . : , ' ' ' ' ` ` ,~ . .:
triangular. For illustration purposes only~ the
pie20electric actuator of the present invention will be
hereunder described with reference to different embodiments
using a cylindrical moving member 1 denoted a plug or plug
gauge. The plug can be either a hollow cylinder or a solid
cylinder. Although plug 1 could be made of any material, it
: advantageously comprlses a sintered tungsten carbide
material or a dense A1203 ceramic provided with a very
smooth surface finish. The actuator further comprises a
radially poled piezoelectric transducer 2 mounted around the
plug 1, and capable of being radially as well a~' axially
expanded or contracted by the application or removal of a
potential Vl to electrodes 11 and 12 thereof. The
piezoelectric axial transducer 3 is of cylindrical shape and
has an inner diameter larger than the outer diameter of the
plug 1 to enable a free axial movement of the plug. The
midplane of the axial transducer is fixed via a xing 6 to a
support 5. ~ pair of clamping transducers 3 and 4 are
. . . . .
~` attached to the axial displacement transducer 2 at each end
thereof by means of rings 9 and 10, respectively. The
rings 9 and 10 are designed to be axially stiff and radially
soft in order to transmit only the axial displacement of
transducer 2 to clamping transducers 3 and 4.
As illustrated in FIG. 2, the axial transducer 2 -
and the clamping transducers 3 and 4 are coaxial and mounted
around plug 1. The three transducers 2, 3 and 4 are
` preferably made o~ lead zirconate titanate (PZT) ceramic,
although any other piezoelectric material can be used
without af~ecting the proper operation of the present
3Q ~piezoelectric actuator. In the illustrative embodiment
shown in FIG. 2, clamping transducers 3 and 4 each have a
. .
, _ 5 -
'
;
~. ' ,'~ . :. ! .
3~i
pair of cylindrical electrodes 13, 14 and 15, 16,
respectively. Both clamping -transducers 3 and 4 are
radially poled such that an application of a voltage V2
between their respectively inner electrodes 14, 16 and outer
electrodes 13, 15 ei-ther radially expands or contacts the
clamping transducers. The expansion and contraction of the
clamping transducers depend on their direction of
polarization and on the sign of the applied voltage.
The expansion of a clamping transducer 3 or 4
achieves a bearing function by deforming the clamping
transducer such that sufficient clearance exists between the
transducer and the plug 1 to allow unimpeded relative
motion~ During contraction of a clamping transducer, the
latter is deformed to remove the interfacial clearance and
cause a sizeable pressure to exist on the interface thereb,y
preventing relative motion. The deformation of the clamping
transducers is very small. Therefore, to make the clamping
pressure on the interface sufficiently large when the
' gripping function is selected, the interfacial clearance
must necessarily be very small when the bearing function is
selected. In the embodiments hereunder described this
clearance cannot be more than 10 to 20 microinches.
Therefore, wear or other action causlng either small
material removal or build up, or roughening of the bearing
surfaces will,interfere with one or the other function of
the clamping transducers. To avoid these actions special
~ materials are selected both for the movable member and a
- liner that is interposed between the movable member and the
clamping transducers for reducing detrimental wear or other
action on the critical interfaces.
In accordance with one embodiment of the invention,
-- 6
. '
wear at the sliding interface between the plug 1 and the
clamping transducers 3 and 4 ls virtually eliminated by
inserting a s~litable liner between the moving plug 1 and the
clamping transducers. The liner is shown in FIG. 2 as
rings 7 and 8. In this illustrative embodiment, the sliding
interface becomes the interface between the plug and the
liner. Although the liner members 7 and 8 could be made of
any material having thermal expansion characteristics
matching those of the plug and the clamping transducers and
not tending to adhere to the plug, by way o~ example,
liners 7 and 8 are made o~ an inert ceramic material iuch as
lead zirconate titanate (PZT) material. Whereas, as
indicated above, any hard plug material that can be given a
smooth surface finish and having a similar thermal expansion
is satisfactory, it appears that high density A12O3 ceramic
has excellent properties for this application and its -
coe~ficient of expansion is well mat:ched to that of PZT. In
actual operation the PZT liner ~ A12O3 ceramic interface
shows no evidence of wear after months of continuous
reciprocating operation with an accumulated axial travel of
' more than one mile.
The operation of the piezoelectric actuator of ~ ~-
FIG. 2 can be accomplished by the application of either two
or three separately timed voltagesi. The first mode of
~, operation i5 described with reference to FIGS. 4A and 4B. -
As already mentioned, the piezoelectric axial transducer 2
axially expands and contracts upon application or removal of
a potential Vl. The clamping transducers 3 and 4 are in
this case made such that normally one ~its loosely on the
moving plug 1, while the other one is tiyht on the plug.
Clamping transducers 3 and 4 are oppositely poled such that
the application of a potential V2 thereto reverses their
respective clamping actions. The piezoelectric actuator
according to the invention operates in a push-pull fashion.
In other words, if one clamping transducer (e.g.,
transducer 3) is clamped while the axial transducer 2
expands, the clamping transducer 3 pushes the cylindrical
plug 1 to the right in FIG. 2. When the axial transducer is
fully expanded, the clamping force is transferred to the
other clamping transducer (i.e., transducer 4) which now
~0 pushes the plug also in the same direction while the axial
-transducer 2 is contracting. The just described push-pull
operation is accomplished by alternately increasing and
decreasing Vl in a sawtooth pattern, while V2 fc)llows a
square wave + 90 degrees out-of-phase, as shown in FIGS. 4A
and 4B. The choice of one or the other wave diagram
determines the direction of motion of the cylindrical
plug 1. For example, dependiny on the direction of
polarization o~ clamping transducers 3 and 4, FXC7. 4A can
illustrate a movement of the plug t:o the right, while
FIG. 4B illustrates a movement of the plug to the left. It
is noted that clamping transducers 3 and 4 should not be
free to slide at the same instant, i.e., should not be open
at the same time to avoid an uncontrolled sliding motion by
external forces acting on the plug.
The second mode of operation emplo~ing three
voltages is described with reference to FIGS. 5A and 5B
where in addition to a ~irst voltage Vl, already described,
second and third voltages V2(3) and V~(4) respectively
represent the voltages applied to clamping transducers 3 and
4. In this case both clamping transducers are normally
unclamped and clamp upon the application of the voltages
.:
.:
' ' :
:, ', ~ ', ,' , '~ ' ., : . ' :
V2(3) and V2(4). FIG. 5A represents a fine-step mode of
operation of the plezoelectric actuator, while FIG. 5B shows
the coarse-step mode of operation. In the fine mode when
one o the clamplng transducers is clamped and the other
unclamped, the potential V1 on the axial transducer 2 varies
in small successive steps in an ascending or descending
staircase. Thus, several fine displacement steps o~ the
actuator occur between each instant when the clamping force
; is changed from one clamp to the other. It is noted that
during periods tl ~ t2, t3 - t4~ t5 - t6 and t7 - t8 both
transducers are clamped to avoid having them opened
simultaneously. In the coarse-step mode, the staircase
pattern of Vl is replaced by a square wave, as shown in
FI~. 5B. Only a large step occurs between each instant when
the clamping force is changed. This operation yields the
greatest speed of the actuator. As shown in the wave
diagram, appropriate time delays are provided to prevent any
change in Vl during the brie~ periods when the clamping
force is being transferred from one clamping transducer to
the other clamping transducer, i.e , when both clamps are
engaged simultaneously.
The displacement steps of a piezoelectric actuator
- made in accordance with the principles of the present
invention are of the order of 0.2 ~m for the aforedescribed
fine mode of operation, and 2 ~m for the coarse mode. By
way of example, the step rate could vary from zero to 200G
steps/second for both the fine and the coarse mode. The
spee~ o~ operation of the piezoelectric actuator according
to the present invention depends upon the type o~ electronic
; 30 circuitry used to generate the electrical signals Vl and V2.
In actual operation, the speed could be varied from zero to
, .
.~ . . .
., ,:'
" ' ' ' ', ', ~ , :, ~ ' :'
.
~ ~ ~6
4,000 ~m/second.
Referring to FIG. 3, -there is shown another
illustrative embodiment of the present invention. Identical
numerals corresponding to the numerals of FIG. 2 are
utilized to illustrate the similarities of both embodiments~
Each clamping transducer 3 and 4 comprises a pair of
radially stacked ceramic rings 31, 32 and 41, 42,
respectively. The rings of each pair are radially poled in
` opposite directions, and intermediate cylindrical
electrodes 30 and 40 separate the rings of each pair. Since
electrodes 14 and 16 are not suitable as low-wear sliding
surfaces on plug 1, the inert liners 7 and 8 are interposed
between these electrodes and the plug.
In the illustrative embodiment of FIG~ 3 the inner
and outer electrodes of a clamping transducer are coupled
together and to ground, for example, whereas the potential
of the intermediate electrode varies from zero to V2. The
piezoelectric axial transducer 2 comprises a plurality of
` axially poled stacked discs, e.g., 2-1, 2-2, 2-3, ... 2-12
20 designed to give as large an axial displacement as possible
~or a given applied voltage. Both piezoelectric disc
` comprises a pair of electrodes, as shown by 21, 22, 23, 24,
etc. The odd numbered electrodes, e.g., 21, 23, etc. are
coupled together to a first input terminal 51, and the even
` numbered electrodes (e.g., 22, 24, etc.) are coupled `
together to a second input terminal 52. The pstential V
- for axially expanding and contracting the piezoelectric
axial transducer 2 is applied between terminals 51 and 52.
In accordance with this illustrative embodiment, each
30 ceramic disc is, for example, 0.045 inches thick and the
total length of the axial displacement transducer is of the
s -- 1 0
. ~'~`.
'., ' ~
.
I,'~'' ' `
. .
34~i
order of 0.55 inches, its axial displacement at a 500 volt
potential being approximately 2 ~m.
The piezoelectric actuator of the present invention
can push loads up to 5 pounds, and can hold loads up to
20 pounds when operated with a 500 volt potential. Such a
piezoelectric actuator dissipates average power of the order
o~ one milliwatt at full speed.
An actuator made according to the present invention
is particularly suitable for use as a micropositioner to
operate arrangements where accurate optical alignment is
desired. By combining two actuators in an x, y, plane, it
is possible to achieve a planar positioning of one object
with reference to another. By further modifying such
arrangements, e.g., by combining an x, y actuator and a z
actuator or a ~ actuator, thxee-dimensional positioning can
be achieved. The piezoelectric acl:uator of the present
invention can be controlled either by a computer for
automatic alignment or by an operat.or for manual alignment.
Moreover, such an actuator can operate in any normal room
environment or in a vacuum.
Although primary emphasis herein ~as been directed
to embodiments each including two clamping transducers, it
is apparent that in some applications of practical
~`~ importance only one such transducer-is sufficient.
- The embodiments described herein are intended to be
illustrative of the features o~ the present invention. It
is recognized that modifications and variations are possible
within the spirit and scope o~ the invention.
For example, plug 1 could be fixed to a base (not
shown in the drawings), while a linearly moving structure
comprising clamping transducers 3 and 4 and axial
-- 11 --
':
': ' '. ,' : . . '
~6~
transducer 2 attached to the support member 5 could move
relative to the fixed plug. Moreover, a hollow cylindrical
moving member around the clamping transducers could be used
instead of moving plug 1. Furthermore, piezoelectric
ceramics poled in di~ferent directions than the ones
described could be substituted therefor, provided that
control potentials with the proper polarities are used.
.
.'.`', .. .
-
' ''~
. - 12 ~
:'. ' ' ~'
' ;:
