CA1066345A - Linear piezoelectric actuator with liner controlling frictional wear - Google Patents
Linear piezoelectric actuator with liner controlling frictional wearInfo
- Publication number
- CA1066345A CA1066345A CA258,792A CA258792A CA1066345A CA 1066345 A CA1066345 A CA 1066345A CA 258792 A CA258792 A CA 258792A CA 1066345 A CA1066345 A CA 1066345A
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- electromechanical actuator
- actuator according
- piezoelectric
- transducer
- electromechanical
- Prior art date
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Abstract
LINEAR PIEZOELECTRIC ACTUATOR WITH LINER
CONTROLLING FRICTIONAL WEAR
Abstract of the Disclosure An electromechanical actuator is described having a movable member, a first piezoelectric transducer imparting discrete displacements to second and third piezoelectric transducers which either grip the movable member and transmit to it the displacements or provided sliding bearing surfaces for the movable member. An inert liner is interposed between the second and third transducers and the movable member for reducing wear. Such an actuator is, for example, useful as a linear positioner requiring a high precision of travel and/or a high degree of uniformity of motion.
CONTROLLING FRICTIONAL WEAR
Abstract of the Disclosure An electromechanical actuator is described having a movable member, a first piezoelectric transducer imparting discrete displacements to second and third piezoelectric transducers which either grip the movable member and transmit to it the displacements or provided sliding bearing surfaces for the movable member. An inert liner is interposed between the second and third transducers and the movable member for reducing wear. Such an actuator is, for example, useful as a linear positioner requiring a high precision of travel and/or a high degree of uniformity of motion.
Description
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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 ..
.
~: :
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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 ~
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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 . .
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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 . .
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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, . ' 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 .:
.:
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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 , .
.~ . . .
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~ ~ ~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 . ~'~`.
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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 ':
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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.
.
.'.`', .. .
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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 . .
. .
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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 -' ;
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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, . ' 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 ':
': ' '. ,' : . . ' ~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 ~
:'. ' ' ~' ' ;:
Claims (26)
1. All electromechanical actuator comprising:
a support member, a first member;
first electromechanical transducer means fixed to said support member and responsive to first 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 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.
a support member, a first member;
first electromechanical transducer means fixed to said support member and responsive to first 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 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.
2. An improved electromechanical actuator of the type comprising:
a support member;
a linearly movable member;
first electromechanical transducer means fixed to said support member and responsive to first electrical signals for producing discrete displacements of said first transducer means with respect to said support member in a predetermined direction; and second electromechanical transducer means rigidly coupled to said first transducer means, and responsive to second electrical signals for contacting said movable member, thereby transmitting said discrete displacements to said movable member, wherein the improvement comprises means interposed between said second transducer means and said movable member for controlling the amount of frictional wear therebetween.
a support member;
a linearly movable member;
first electromechanical transducer means fixed to said support member and responsive to first electrical signals for producing discrete displacements of said first transducer means with respect to said support member in a predetermined direction; and second electromechanical transducer means rigidly coupled to said first transducer means, and responsive to second electrical signals for contacting said movable member, thereby transmitting said discrete displacements to said movable member, wherein the improvement comprises means interposed between said second transducer means and said movable member for controlling the amount of frictional wear therebetween.
3. An electromechanical actuator according to claim 2 wherein the first and second electromechanical transducer means respectively comprises a first piezoelectric element and a second and third piezoelectric elements.
4. An electromechanical actuator according to claim 3 wherein the first, second and third piezoelectric elements are lead zirconate titanate ceramic transducers.
5. An electromechanical actuator according to claim 3 wherein the second and third piezoelectric elements of said second transducer means comprise at least one electrode each, and wherein the frictional wear control means comprises a pair of ceramic liners respectively interposed between the movable member and said electrode.
6. An electromechanical actuator according to claim 5 wherein the liners are made of lead zirconate titanate ceramic material.
7. An electromechanical actuator according to claim 6 wherein the movable member is made of sintered tungsten carbide material having a smooth surface finish.
8. An electromechanical actuator according to claim 6 wherein the movable member is made of dense Al2O3 ceramic material having a smooth surface finish.
9. An improved electromechanical actuator of the type comprising:
a cylindrical support member;
an axially movable cylindrical plug;
a first annular transducer element fixed to said support member and surrounding said plug, being responsive to a first electrical signal for producing discrete displacements in the axial direction; and second and third annular transducer elements, rigidly coupled to said first element and adapted to alternately clamp said plug in response to a second electrical signal, thereby transmitting said axial displacements to said plug wherein the improvement comprises an annular liner member interposed between the second and third elements and the plug for controlling the amount of frictional wear therebetween.
a cylindrical support member;
an axially movable cylindrical plug;
a first annular transducer element fixed to said support member and surrounding said plug, being responsive to a first electrical signal for producing discrete displacements in the axial direction; and second and third annular transducer elements, rigidly coupled to said first element and adapted to alternately clamp said plug in response to a second electrical signal, thereby transmitting said axial displacements to said plug wherein the improvement comprises an annular liner member interposed between the second and third elements and the plug for controlling the amount of frictional wear therebetween.
10. An electromechanical actuator according to claim 9 wherein the actuator further comprises first and second rings for rigidly coupling the first element to the second and third elements, respectively.
11. An electromechanical actuator according to claim 10 wherein the first, second and third annular transducer elements are piezoelectric elements.
12. An electromechanical actuator according to claim 11, wherein the first piezoelectric element is made of a cylindrical lead zirconate titanate ceramic material poled in the radial direction and having a pair of electrodes for applying said first electrical signal thereto.
13. An electromechanical actuator according to claim 11 wherein the second and third piezoelectric elements are lead zirconate titanate (PZT) ceramic rings poled in the radial direction and having first and second electrodes on their respective inner and outer surfaces for applying said second electrical signal thereto.
14. An electromechanical actuator according to claim 13 wherein the second and third PZT rings are poled in opposite radial directions.
15. An electromechanical actuator according to claim 13 wherein the second and third PZT rings are poled in the same radial direction, and wherein said second electrical signal comprises a pair of separately timed electrical signals respectively applied to the second and third rings.
16. An electromechanical actuator according to claim 11 wherein the annular liner member is made of lead zirconate titanate ceramic material.
17. An electromechanical actuator according to claim 16 wherein the axially movable cylindrical plug is made of sintered tungsten carbide material.
18. An electromechanical actuator according to claim 16 wherein the axially movable cylindrical plug is made of dense Al2O3 ceramic material.
19. An electromechanical actuator according to claim 11 wherein the first piezoelectric transducer element comprises a plurality of axially poled stacked ceramic discs each having a pair of electrodes for applying said first electrical signal thereto.
20. An electromechanical actuator according to claim 11 wherein each second and third piezoelectric transducer element comprises:
a first piezoelectric ring radially poled in one direction;
a second piezoelectric ring radially stacked to said first ring and poled in the opposite direction thereof;
an outer metallic electrode coupled to the outer surface of said first ring;
an intermediate electrode coupled to the inner surface of said first ring and to the outer surface of said second ring; and an inner electrode coupled to the inner surface of said second ring, whereby said inner and outer electrodes are commonly connected together for applying said second electrical signal between said common connection and said intermediate electrode.
a first piezoelectric ring radially poled in one direction;
a second piezoelectric ring radially stacked to said first ring and poled in the opposite direction thereof;
an outer metallic electrode coupled to the outer surface of said first ring;
an intermediate electrode coupled to the inner surface of said first ring and to the outer surface of said second ring; and an inner electrode coupled to the inner surface of said second ring, whereby said inner and outer electrodes are commonly connected together for applying said second electrical signal between said common connection and said intermediate electrode.
21. An electromechanical actuator according to claim 20 wherein said annular liner member is interposed between said movable plug and said inner electrode.
22. An electromechanical actuator according to claim 20 wherein the first and second piezoelectric rings are made of lead zirconate titanate ceramic material.
23. An electromechanical actuator according to claim 22 wherein the annular liner member is made of lead zirconate titanate ceramic material.
24. An electromechanical actuator according to claim 23 wherein the axially movable cylindrical plug is made of sintered tungsten carbide material.
25. An electromechanical actuator according to claim 23 wherein the axially movable cylindrical plug is made of dense Al2O3 ceramic material.
26. An improved electromechanical actuator of the type comprising:
a fixed member;
a linearly movable structure;
first electromechanical transducer means fixed to said structure and responsive to first electrical signals for producing discrete displacements in a predetermined direction; and second electromechanical transducer means rigidly coupled to said first transducer means, and responsive to second electrical signal for contacting said fixed member, thereby transmitting said discrete displacements to said movable structure, wherein the improvement comprises means interposed between said second transducer means and said fixed member for controlling the amount of frictional wear therebetween.
a fixed member;
a linearly movable structure;
first electromechanical transducer means fixed to said structure and responsive to first electrical signals for producing discrete displacements in a predetermined direction; and second electromechanical transducer means rigidly coupled to said first transducer means, and responsive to second electrical signal for contacting said fixed member, thereby transmitting said discrete displacements to said movable structure, wherein the improvement comprises means interposed between said second transducer means and said fixed member for controlling the amount of frictional wear therebetween.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US60900175A | 1975-08-29 | 1975-08-29 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1066345A true CA1066345A (en) | 1979-11-13 |
Family
ID=24438962
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA258,792A Expired CA1066345A (en) | 1975-08-29 | 1976-08-10 | Linear piezoelectric actuator with liner controlling frictional wear |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1066345A (en) |
Cited By (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4468583A (en) * | 1982-10-22 | 1984-08-28 | Hitachi, Ltd. | Piezoelectric rotary actuator |
| US4570096A (en) * | 1983-10-27 | 1986-02-11 | Nec Corporation | Electromechanical translation device comprising an electrostrictive driver of a stacked ceramic capacitor type |
| US4602702A (en) * | 1983-12-28 | 1986-07-29 | Jidosha Kiki Co., Ltd. | Brake apparatus |
| US4623044A (en) * | 1983-12-22 | 1986-11-18 | Jidosha Kiki Co., Ltd. | Brake apparatus |
| US4777398A (en) * | 1984-08-31 | 1988-10-11 | Tokyo Juki Industrial Co., Ltd. | Piezoelectric motor |
| US4779018A (en) * | 1985-10-24 | 1988-10-18 | Canon Kabushiki Kaisha | Vibration wave motor |
| US4914338A (en) * | 1986-12-17 | 1990-04-03 | Canon Kabushiki Kaisha | Vibration wave motor |
| US4947077A (en) * | 1986-12-03 | 1990-08-07 | Jgc Corporation | Drive apparatus and motor unit using the same |
-
1976
- 1976-08-10 CA CA258,792A patent/CA1066345A/en not_active Expired
Cited By (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4468583A (en) * | 1982-10-22 | 1984-08-28 | Hitachi, Ltd. | Piezoelectric rotary actuator |
| US4570096A (en) * | 1983-10-27 | 1986-02-11 | Nec Corporation | Electromechanical translation device comprising an electrostrictive driver of a stacked ceramic capacitor type |
| US4623044A (en) * | 1983-12-22 | 1986-11-18 | Jidosha Kiki Co., Ltd. | Brake apparatus |
| US4602702A (en) * | 1983-12-28 | 1986-07-29 | Jidosha Kiki Co., Ltd. | Brake apparatus |
| US4777398A (en) * | 1984-08-31 | 1988-10-11 | Tokyo Juki Industrial Co., Ltd. | Piezoelectric motor |
| US4779018A (en) * | 1985-10-24 | 1988-10-18 | Canon Kabushiki Kaisha | Vibration wave motor |
| US4947077A (en) * | 1986-12-03 | 1990-08-07 | Jgc Corporation | Drive apparatus and motor unit using the same |
| US4914338A (en) * | 1986-12-17 | 1990-04-03 | Canon Kabushiki Kaisha | Vibration wave motor |
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