WO1998053509A1

WO1998053509A1 - Piezoelectric motor

Info

Publication number: WO1998053509A1
Application number: PCT/IL1997/000167
Authority: WO
Inventors: Izhak Rafaeli
Original assignee: Nanomotion Ltd.
Priority date: 1997-05-22
Filing date: 1997-05-22
Publication date: 1998-11-26
Also published as: AU2786797A

Abstract

A piezoelectric micromotor for rotating a shaft comprising a piezoelectric plate having edges and planar faces where the piezoelectric plate has specially shaped surfaces for coupling to the shaft. In one embodiment part of an edge is formed with a cylindrical surface and the shaft is resiliently pressed to the cylindrical surface. In another embodiment a hole is formed through the plate perpendicular to a planar face. The shaft is inserted into the hole and couples with the surface of the hole. One or a multiplicity of the micromotors according to the invention are coupled to a shaft. When a multiplicity of micromotors according to the invention are coupled to the shaft different advantagous configurations of phase differences between the vibration modes of the micromotors are possible.

Description

PIEZOELECTRIC MOTOR FIELD OF THE INVENTION

This invention relates to micromotors and in particular to the conversion of vibratory motion of piezoelectric motors to rotational motion. BACKGROUND OF THE INVENTION

Piezoelectric motors use piezoelectric ceramic vibrators to convert electrical energy into mechanical motion. The motors are used in many and varied applications. They have been designed for, among other things, automotive fuel injectors, video cassette recorders, automatic cameras, computer disc drives, and precision microscopes. In almost all cases, the piezoelectric ceramic vibrator is excited to vibrate at or near to the frequency of an impressed alternating electric field. Most of the frequencies of vibration are in the range from 20,000 Hz to 150,000 Hz. Amplitudes of vibration have magnitudes that range from magnitudes on the order of nanometers to magnitudes on the order of microns.

Useful work is extracted from the vibrator by alternately coupling it to a movable element and uncoupling it. When the vibrator and moveable element are coupled, the motion of the vibrator is transmitted to the moveable element. The vibrator is coupled to the movable element during that part of its vibration cycle that contributes to a desired displacement or acceleration of the moveable element. It is uncoupled from the moveable element during the part of the vibration cycle in which the motion of vibration is in a direction that does not contribute to the desired displacement or acceleration.

Displacements of the moveable element that result from coupling it to the vibrator for a part of a vibration cycle are on the order of the amplitude of vibration. A given desired displacement of the moveable body is achieved by accumulating the contributions to displacement or energy from many vibration cycles. Since the coupling and uncoupling of the vibrator and moveable element are done at the same phase points of each of N vibration cycles, the displacement and energy transfer from N vibration cycles is N times the displacement and energy transfer from one of the vibration cycles.

Coupling of the moveable element to the vibrator is generally achieved by friction forces between the two. A resilient biasing force presses a planar surface of the vibrator, usually extended with a hard material such as ceramic, against a parallel planar surface of the moveable body. The biasing force provides enough force so that friction between the two planar surfaces is high. As a result, the moveable body is either locked to the motion of the vibrator, and motion of the vibrator surface parallel to the two friction coupled surfaces moves the moveable element along with it, or if there is slippage the moveable element is forced in the direction of motion of the vibrator by the force of friction acting between the two planar surfaces. Uncoupling the moveable element from the vibrator is accomplished by a vibration of the vibrator with a displacement perpendicular to the friction locked surfaces. The relaxation time of the flexible biasing force is much longer than the vibration period of the vibrator. As a result, the vibration perpendicular to the friction locked surfaces causes the friction locked surfaces to separate for a part of each cycle. The fraction of the cycle that the friction locked surfaces are separated is usually much larger than that during which the surfaces are friction locked.

It is thus seen that the displacement and transfer of energy to the moveable element is not smooth. It is achieved by repeated pulsed transfer of motion at relatively low duty cycles.

In many instances it is advantageous to convert the vibration of a piezoelectric vibrator to rotary motion of a circular shaft. In the prior art, the piezoelectric vibrator is coupled to the shaft in the manner described above, with the shaft as the movable element. A resilient biasing force presses the circular surface of the shaft against a planar surface of the vibrator (or a hard extender of the surface) to provide periodic frictional locking between the shaft and the vibrator. The vibrator vibrates with a component of displacement perpendicular to the axis of the shaft and to the surface of the vibrator against which the shaft is pressed. This vibration uncouples the shaft from the vibrator for the greater part of the vibration cycle. A second component of vibration displacement lies parallel to the surface of the vibrator against which the shaft is pressed. This component applies a torque to the shaft once each vibration cycle for the short period of time during the cycle when the shaft and vibrator are coupled together by friction.

For a given load, rpm and frictional loss, the average instantaneous torque applied to the shaft during a cycle multiplied by the length of time during the cycle that torque is applied to the shaft is a constant. Therefore, the magnitude of the instantaneous torque applied to the shaft could be reduced if torque could be applied to the shaft for a longer period of time during each cycle. This would reduce the strains on all the surfaces and structures of the piezoelectric motor and the shaft. As the wear on conventional piezoelectric motors of this type when coupled to circular shafts is generally high, this would be a desirable improvement.

When the circular shaft and planar surface of the vibrator are in contact, the area of contact is very small. The shape of the area of contact is a long narrow rectangle (almost a line). The length of the rectangle is generally on the order of millimeters. The width of the rectangle is on the order of tenths of a millimeter.

The small area of contact between the shaft and the planar surface is a major factor leading to high wear rates on the parts and surfaces of piezoelectric motors that are coupled to circular shafts according to the prior art. The force of the applied torque distributed over the small surface area produces large pressures on the surface. Furthermore, in the prior art motors, the shaft makes contact with the vibrator when the torque is applied to the shaft on the same small area of the vibrator surface during every vibration cycle. Not only is the operative contact area of the vibrator that applies the torque to the shaft small, but it is the same area of the vibrator surface that is always used for transfer of energy. This leads to aggressive abrasion of the vibrator surface. It would be desirable to distribute the contact between the shaft and the vibrator over a larger area of the vibrator (or extender) surface.

Descriptions of the coupling of piezoelectric vibrators to circular shafts typical of the prior art are found in U.S. patent 5,121,025; U.S. patent 5,200,665; and U.S. patent 5,453,653 .

SUMMARY OF THE INVENTION It is an object of one aspect of the present invention to provide a piezoelectric motor coupled to a circular shaft, wherein the area of the vibrator surface which is operative in applying torque to the shaft is increased over that in the prior art.

In many preferred embodiments of the present invention a portion of an edge surface of the vibrator of a piezoelectric motor or of an extension of the edge is formed as a concave cylindrical surface. A length of a circular shaft to be coupled to the motor is preferably resiliently pressed into the cylindrical surface with the axis of the shaft parallel to the generators of the cylindrical surface. Preferably, the directrix of the cylindrical surface is an arc of an ellipse or a circle, (referred to collectively herein as "an ellipse"). Preferably the radius of curvature of the directrix, at every point on the directrix, is greater than the radius of the shaft, although in some embodiments of the invention it may closely approach the radius of the shaft. The shaft is coupled to the vibrator by frictional coupling of the surface of the shaft with the cylindrical surface. The area of contact between the surface of the shaft and the cylindrical surface is the area which is operative in applying torque to the shaft.

The area of contact between a circular shaft and a surface against which it is pressed is a function of the shape of the surface and the force pressing the shaft against the surface.

However, for two identical circular shafts with the same length, pressed by the same force, one onto a cylindrical surface and the other to a plane surface, the area of contact between the one shaft and the cylindrical surface will be greater than the area of contact between the other shaft and the plane surface. The source of this difference is geometrical and can be understood by considering the forces and geometries of the two examples. Consider a first and a second circular shaft which are identical and made from a compressible material. Let the first circular shaft be tangent to a concave cylindrical surface which is a boundary surface of another compressible material. Let the radius of curvature of the cylindrical surface be everywhere greater than the radius of the shaft. The locus of contact between the first shaft and the cylindrical surface is a line. Let the second circular shaft be tangent to a plane which is a boundary surface of the same material which is bounded by the cylindrical surface. The locus of contact between the second shaft and the plane is also a line.

Let the first shaft be pressed into the cylindrical surface by a first compressing force so that the material of the shaft and cylindrical surface are compressed and the surface of the shaft and the cylindrical surface deform. The locus of contact between the first shaft and the cylindrical surface, which was a line of contact, broadens into a locus of contact which is a surface, a "first surface of contact". Let the second shaft be pressed into the plane surface by a second compressing force so that the material of the second shaft and the plane surface are compressed and the surface of the second shaft and the plane surface deform. The locus of contact between the second shaft and the plane broadens from a line of contact into a surface of contact, a "second surface of contact".

Assume that the area of the first surface of contact is equal to the area of the second surface of contact. Then volume of material of the first shaft and the cylindrical surface that is deformed by compression to form the first surface of contact is less than the volume of material of the second shaft and the plane that is deformed by compression to form the second surface of contact. Therefore, also the first compressing force is less than the second compressing force. This is a result of the geometries of the cylindrical surface and the plane surface in relation to the geometry of the circular shape of the shaft. The cylindrical surface, because of its curvature, is everywhere closer to the surface of the circular shaft than is the plane surface. Less deformation and less force is needed to bring an area of the cylindrical surface into contact with the circular shaft than is needed to bring an equal area of the plane into contact with the circular shaft.

Therefore by coupling a length of a circular shaft to a cylindrical surface of a vibrator in accordance with a preferred embodiment of the present invention, the force pressing the shaft to the vibrator surface is distributed over an area of contact between the shaft and the vibrator that is larger than the area of contact obtained according to the prior art by coupling the same length of the shaft with the same force to a planar surface of the vibrator. This results in lower pressures acting on the surfaces of the shaft and the vibrator that are in contact with each other and less deformation of the vibrator surface when torque is transmitted to the shaft. Surface wear of the vibrator and the shaft is thereby reduced. While in absolute terms the area of contact is still small, in relative terms the increase can be significant.

It is an object of another aspect of the present invention to provide a piezoelectric motor coupled to a circular shaft wherein the length of the time that the motor applies a torque to the shaft during a vibration cycle is increased over that in the prior art.

In preferred embodiments of the present invention the vibrator of a piezoelectric motor is provided with a cylindrical surface for coupling to a circular shaft and the cylindrical surface preferably has a directrix which is an arc of an ellipse as described above. As indicated above this surface can be an edge of the vibrator or an extension of the edge.

Preferably the vibrator has a plurality of electrodes in configurations such as described in U.S. patent 5,453,653 which is incorporated herein by reference. Preferably, different ones of the plurality of electrodes may be excited by applied voltages to cause the vibrator to vibrate in different vibration modes by methods described in U.S. patent 5,453,653.

The vibrator is preferably caused to vibrate in an elliptical vibration mode so that the motion of mass points on the cylindrical surface trace out ellipses. Preferably, the ellipses of motion of mass points on the cylindrical surface are similar and parallel to the ellipse of the directrix of the cylindrical surface. Preferably, but not necessarily, the amplitudes of motion of the mass points on the cylindrical surface are close to the differences between the radius of the shaft and the semi-axes of the ellipse of the directrix of the cylindrical surface. When the cylindrical surface of a vibrator is matched to the vibration mode of the vibrator as described above the shaft will roll along the surface of the cylindrical vibrator for a portion of the vibration cycle.

Because of the rolling effect, the area of the vibrator which is used to transmit torque to the shaft is increased further, beyond the increase achieved by the geometric effect described above. Additionally, the contact time per vibration cycle between the shaft and the cylindrical surface will be increased over that which is obtained in prior art. Therefore the duty cycle for the transmission of torque to the shaft is increased over that of the prior art.

The increase in duty cycle allows for a desired amount of energy per vibration cycle to be transmitted to a circular shaft at lower magnitudes of instantaneous torque than is possible according to the prior art. The lower magnitudes of instantaneous torque result in reduced wear of motor and shaft parts and surfaces.

There is thus provided, in accordance with a preferred embodiment of the present invention, a method for producing a piezoelectric micromotor for driving a rotatable shaft comprising: providing a piezoelectric plate with plane surfaces and edges; and forming at least one electrode on at least one plane surface, wherein, a cylindrical contact surface is formed on an edge of the piezoelectric plate or on an extension thereof for contact between the rotatable shaft and the piezoelectric plate prior to the first use of the micromotor for driving the rotatable shaft.

Preferably the cylindrical contact surface is formed as a concave surface. The cylindrical contact surface is preferably formed using an arc of an ellipse or a circle as a directrix for the cylindrical contact surface. Preferably, at every point on the cylindrical contact surface the radius of curvature of the directrix is greater than or substantially equal to the radius of the shaft.

In preferred embodiments of the invention a vibration mode of the piezoelectric plate is matched to the geometry of the cylindrical contact surface.

Preferably, an elliptical motion of vibration of mass points on the cylindrical surface is excited. Preferably, the major axis of the elliptical motion of vibration is parallel to the major axes of an ellipse, an arc of which is a directrix of the cylindrical surface. Preferably, the major axes of the elliptical motion is substantially equal to the difference between the major axis of an ellipse, an arc of which is a directrix of the cylindrical surface, and the diameter of the shaft. Preferably, the minor axes of the elliptical motion is substantially equal to the difference between the minor axis of an ellipse, an arc of which is a directrix of the cylindrical surface, and the diameter of the shaft.

There is further provided a micromotor produced by any of the methods described above.

There is further provided, in accordance with a preferred embodiment of the present invention, a micromotor for rotating a shaft comprising: a piezoelectric plate with plane surfaces and edges; and at least one electrode on at least one plane surface, wherein an edge of the piezoelectric plate or on an extension thereof has a cylindrical contact surface for contact with the shaft.

Preferably, the cylindrical contact surface is concave. Preferably, the directrix of the cylindrical contact surface is an arc of an ellipse or a circle.

In a preferred embodiment of the present invention the surface of the piezoelectric plate or an extension thereof that comes into contact with the surface of the shaft has mass points that vibrate with an elliptical motion. Preferably, the major and minor axes of the elliptical motion are parallel respectively to the major and minor axes of an ellipse an arc of which is a directrix of the cylindrical contact surface. Preferably, the major axis of the elliptical motion is substantially equal to the difference between the major axis of an ellipse, an arc of which is a directrix of the surface of the piezoelectric plate or an extension thereof that comes into contact with the surface of the shaft, and the diameter of the shaft. Preferably, the minor axis of the elliptical motion is equal to the difference between the minor axis of an ellipse, an arc of which is a directrix of the surface of the piezoelectric plate or an extension thereof that comes into contact with the surface of the shaft, and the diameter of the shaft. There is further provided in accordance with a preferred embodiment of the present invention a method for rotating a shaft comprising: resiliently pressing a plurality of micromotors according to preferred embodiments of the present invention to a same shaft; and exciting vibrations in the plurality of micromotors such that they apply torque to said same shaft.

In a preferred embodiment of the invention, at least two of the plurality of micromotors are identical. In a preferred embodiment of the present invention resiliently pressing a plurality of micromotors to a same shaft comprises pressing identical micromotors to the same shaft in a circularly symmetric configuration.

In a preferred embodiment of the present invention resiliently pressing a plurality of micromotors to a same shaft comprises pressing micromotors of the plurality of micromotors to the same shaft by equal forces.

In a preferred embodiment of the present invention exciting vibrations in the plurality of micromotors comprises exciting the micromotors to vibrate in the same vibration mode. 25.

A method for rotating a shaft according to any of claims 21 - 24 wherein exciting vibrations in the plurality of micromotors comprises exciting the micromotors to vibrate in the same vibration mode.

In a preferred embodiment of the present invention the phase differences between vibrations in the plurality of micromotors is set to zero. In another preferred embodiment of the present invention the phase difference between the vibration mode of any one of the plurality of micromotors and the vibration mode of any of the other micromotors is set equal to (n x 360 degrees/N), where N is the number of the plurality of micromotors and n is an integer with 0<n<N, and wherein n is different for each of the other micromotors.

In a preferred embodiment of the present invention the phase difference between the vibration modes of micromotors situated on opposite sides of the shaft is zero. Preferably, the phase difference between the vibration modes of one of a plurality of pairs of micromotors situated opposite to each other and having the same phase, and any of the other pairs of micromotors situated opposite to each other and having the same phase is set equal to (n x 360 degrees/N), where N is the number of pairs of micromotors which are opposite each other and n is an integer with 0<n<N, and n is different for each of the other pairs of micromotors situated opposite to each other.

There is further provided in accordance with a preferred embodiment of the present invention a piezoelectric micromotor for rotating a shaft comprising: a piezoelectric plate with plane surfaces and edges having a hole formed in the piezoelectric plate perpendicular to a plane surface into which the shaft is inserted; and means for exciting the plate in a vibration mode such that mass points on the surface of the hole vibrate with an elliptical motion. Preferably, the hole is an elliptical hole. In a preferred embodiment of the present invention the major and minor axis of the hole are equal. Preferably, the major axis of the elliptical motion of the mass points on the surface of the hole is parallel to the major axis of the elliptical hole. The major axis of the elliptical motion is preferably substantially equal to the difference between the radius of the shaft and the major axis of the elliptical hole. Preferably, the minor axis of the elliptical motion is substantially equal to the difference between the radius of the shaft and the minor axis of the elliptical hole.

In a preferred embodiment of the invention the axis of the shaft and the hole are substantially congruent when the piezoelectric plate is not vibrating. Alternatively, a resilient biasing force preferably presses the shaft to a part of the surface of the hole. There is also provided in accordance with a preferred embodiment of the present invention a method for rotating a shaft comprising: forming a hole in a piezoelectric plate; inserting the shaft into the hole; and exciting the piezoelectric plate so that mass points on the surface of the hole vibrate so as to transmit torque to the shaft. Preferably, the hole is an elliptical hole. In a preferred embodiment of the invention the major and minor axis of the hole are equal.

Preferably, the piezoelectric plate is excited such that mass points on the surface of the hole vibrate with an elliptical motion. The major axis of the elliptical motion of mass points on the surface of the hole is preferably parallel to the major axis of the elliptical hole. Preferably, the major axis of the elliptical motion of mass points on the surface of the hole is substantially equal to the difference between the major axis of the hole and the radius of the shaft. The minor axis of the elliptical motion of mass points on the surface of the shaft is preferably substantially equal to the difference between the minor axis of the elliptical hole and the radius of the shaft.

In a preferred embodiment of the invention inserting the shaft into the hole comprises stabilizing the shaft in the hole such that the axis of the shaft and the hole are substantially congruent when the piezoelectric plate is not vibrating. Alternatively, inserting the shaft into the hole preferably comprises pressing the shaft with a resilient force to a part of the surface of the hole.

The invention will be more clearly understood by reference to the following description of preferred embodiments thereof in conjunction with the figures in which: BRIEF DESCRIPTION OF FIGURES

Fig. 1 shows a schematic illustration of vibrator of a piezoelectric motor coupled to a circular shaft in accordance with prior art;

Fig. 2 schematically shows a typical vibration and torque transmission cycle for a vibrator coupled to a circular shaft according to the prior art as shown in Fig. 1 ;

Fig. 3 schematically shows a piezoelectric vibrator coupled to a circular shaft in accordance with a preferred embodiment of the present invention;

Fig. 4A shows a magnified view of the circular shaft of Fig. 3 and the area that the shaft contacts on the vibrator; Fig. 4B shows a comparison of coupling a circular shaft to a planar surface of a vibrator in the prior art with coupling of a circular shaft to a cylindrical surface of a vibrator in accordance with a preferred embodiment of the present invention;

Fig. 5 A shows a perspective view of a shaft coupled to a vibrator in accordance with a preferred embodiment of the present invention; Fig. 5B shows a cross section view of a vibrator formed with a surface for coupling to a shaft in accordance with a preferred embodiment of the present invention;

Fig. 5C shows a representation of a vibration and torque transmission cycle for a circular shaft coupled to a vibrator in accordance with a preferred embodiment of the present invention. Fig. 6 shows a shaft coupled to two vibrators according to a preferred embodiment of the present invention.

Fig. 7 shows a shaft coupled to a single vibrator by coupling the shaft to a hole in the vibrator.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Fig. 1 shows a circular shaft 20 coupled to a plane surface 22 of a piezoelectric vibrator

24 in accordance with prior art. Vibrator 24 represents only the small central portion of an extended ceramic vibrator which is close to the area of surface 22 that contacts shaft 20. Alternatively, 24 represents the surface of a ceramic or other extension of the edge, which performs the same motion as the edge to which it is attached. Shaft 20 is held in position by bearings which allow shaft 20 to rotate freely but prevent the displacement of the axis of rotation 21, of shaft 20. Surface 22 is pressed to the surface of shaft 20 by a resilient biasing force 26 in a direction perpendicular to surface 22 which is applied to vibrator 24 by methods known in the art. Biasing force 26 causes surface 22 and the surface of shaft 20 to deform and come into contact with each other at a common surface of contact 28 (shown very much enlarged for clarity).

A directrix of contact surface 28 is an arc 34 with endpoints 33 and 35. The area of contact of surface 28 is equal to the product of the length of arc 34 and the length of shaft 20 that is in contact with surface 22. Arc 34 is generally very small. As a result the area of surface 28 is also very narrow and small. The forces which operate between shaft 20 and surface 22 are distributed over the area of surface 28. The small area of surface 28 therefore results in the existence of very large forces per unit area on parts of surface 22 and the surface of shaft 20. The large forces are a major factor in the rapid degradation of the surfaces and parts of piezoelectric motors. While in Fig. 1, the deformation is shown, for simplicity, as occurring only on the edge of the vibrator, in actuality, this deformation will be distributed between the shaft and the edge. This joint deformation does not substantially affect the explanation given of this and the other examples described herein. Fig. 2 shows a diagram of the motion (exaggerated for clarity) of mass points on surface

22 of vibrator 24 which is shown in Fig. 1. As vibrator 24 oscillates through a typical vibration cycle the majority of mass points on surface 22 trace out an approximately elliptical path represented in the figure by an ellipse 44 (for an extension, all of the mass points on the surface of the extension trace out the same path). Ellipse 44 has a major axis 46 and a minor axis 48. Minor axis 48 is peφendicular to surface 22 and represents the amplitude of motion in the direction peφendicular to surface 22 of mass points on surface 22. Major axis 46 represents the amplitude of motion of mass points on surface 22 in a direction parallel to surface 22.

Points on ellipse 44 represent different phase points of a vibration cycle of vibrator 24. The displacement of a point on ellipse 44 from the origin of ellipse 44 represents the displacements of mass points on surface 22 at the phase point of the vibration cycle represented by the point. The displacement of a mass point on surface 22 is measured relative to the position of the mass point when vibrator 24 is not vibrating.

Velocities of mass points on surface 22 at a phase point in the vibration cycle of vibrator 24 are represented by vectors parallel to the tangent at the point on ellipse 44 representing the phase point. The direction of the velocity is in the direction of increasing phase. The relative disposition of shaft 20 and vibrator 24 are shown in Fig. 2, at different phase points 50, 52, 54, 56, 58, and 60 of a vibration cycle. As mentioned above, the position of axis of rotation 21 of shaft 20 is fixed and shaft 20 is only allowed to rotate. Therefore, changes in relative position between shaft 20 and vibrator 24 are due only to displacements of surface 22 which result from the motion of mass points on surface 22 represented by ellipse 44. A vertical line 75 is a fiducial mark on vibrator 24. It indicates the center of the area of surface 22 which makes contact with shaft 20. The direction of motion of mass points on surface 22 at phase points 50, 52, 54, 56, 58, and 60 are indicated by arrows 62, 64, 66, 68, 70, and 72, respectively. Biasing force 26 and the motion of mass points on surface 22 parallel to minor axis 48 are responsible for coupling shaft 20 to vibrator surface 22 at contact surface 28 for a short period at and near to phase point 50 during a vibration cycle. Motion of mass points on surface 22 parallel to major axis 46 is responsible for transmitting torque to shaft 20 when the surface of shaft 20 and vibrator surface 22 are in contact. At phase point 50 surface 22 is displaced a maximum distance in the direction toward the axis 21 of shaft 20. The force pressing shaft 20 to surface 22 is therefore at maximum for the vibration cycle. The biasing force causes a frictional force to couple surface 22 to the surface of shaft 20 at contact surface 28. The frictional force tends to prevent surface 22 from sliding on the surface of shaft 20. Mass points on surface 22 that are in contact with shaft 20 at surface 28 are moving with a velocity shown by an arrow 62 which is a maximum for the vibration cycle in a direction parallel to surface 22. Since the mass points on surface 22 that are in contact with shaft 20 are coupled by frictional forces to the surface of shaft 20, they transmit their motion to shaft 20 resulting in a torque which rotates shaft 20 in the direction shown by a circular arrow 74. As the vibration cycle proceeds from phase point 50 to phase point 52, surface 22 recedes from contact with shaft 20 and torque ceases to be transmitted to shaft 20. The distance between shaft 20 and surface 22 of vibrator 24 increases to a maximum at phase point 56 and begins to decrease as the cycle begins to return to phase point 50. However, contact between shaft 20 and surface 22 is realized only very close to phase point 50. Therefore, no torque is transmitted to shaft 20 except at phase points of the vibration cycle which are very close to phase point 50. As a result torque is transmitted to shaft 20 only for a short fraction of the period of a vibration cycle. Fig. 3 shows a schematic of a circular shaft 76 coupled to a cylindrical surface 78 of a piezoelectric vibrator 80 in accordance with a preferred embodiment of the present invention. Shaft 76 is preferably pressed to surface 78 by a resilient biasing force 82 produced by methods known in the art. Surface 78 is preferably cylindrical. Shaft 76 is pressed onto surface 78 with the axis of shaft 76 parallel to generators of surface 78. Preferably, the directrix of cylindrical surface 78 is an arc of an ellipse or circle. Preferably, the semi -major and semi- minor axes of the ellipse which generates the directrix of surface 78 have extents greater than but close to the length of the radius of shaft 76. When shaft 76 is forcibly pressed onto surface 78 a surface of contact 83 results. Fig. 4A illustrates schematically in great magnification a view of shaft 76 and contact surface 83 where shaft 76 is pressed into contact with surface 78. As indicated above, it has been assumed that the material of shaft 76 is much harder than the material of surface 78. Therefore, when force 82 presses surface 78 to shaft 76 shaft 76 is not deformed. Only surface 78 deforms to produce contact surface 83. As a result contact surface 83 has a radius of curvature very close to that of shaft 76. The area of contact surface 83 is approximately equal to the product of the length of arc 84 defined by points 86 and 88 and the length of shaft 76 that is in contact with surface 78. Points 86 and 88 are the endpoints of the lines which are the intersections of the surface of shaft 76 with surface 78 if surface 78 were not deformed. For a given magnitude of force 82, contact surface 83 increases as the radius of curvature of surface 78 decreases towards the radius of shaft 76. The cross section of the volume of vibrator material that is compressed in the formation of surface 83 is shown as shaded area 90.

Fig. 4B compares the contact between shaft 76 and surface 78 as shown in Fig. 4A with the contact between shaft 76 and a planar surface 92 of a similar vibrator. To have the same contact surface 83 in both cases, shaft 76 must compress more material when it is pressed into planar surface 92 than when it is pressed into cylindrical surface 78. The cross section of the added volume of material to be compressed is indicated in shaded area 94 of the figure. As a result, more force must be applied to depress shaft 76 to result in a given contact surface 83 when shaft 76 is pressed into a planar surface 92 than when it is pressed into cylindrical surface 78. A similar analysis will show that, for a given force, as the radius of surface 78 decreases contact surface 83 increases. These results are also true for quite general concave cylindrical surfaces including circular cylindrical surfaces. The same results hold true if we drop the simplifying assumption that there is little or no deformation of shaft 76 when surface 78 is pressed to shaft 76 by force 82. Quite generally, to the extent that the curvature of surface 78 decreases and approaches the curvature of shaft 76 there is less material of shaft 76 and surface 78 that has to be deformed to create a contact surface 83 of given area. Therefore, also, the force needed to create a contact surface 83 of a given area decreases as the radius of surface 78 approaches the radius of shaft 76. The prior art case where shaft 76 is pressed to a plane surface is equivalent to having the radius of surface 78 equal to infinity.

Therefore, if two identical circular shafts are pressed by equal forces, a first circular shaft onto a cylindrical surface of an edge of a first vibrator or its extension according to a preferred embodiment of the present invention, and a second circular shaft onto a plane surface of an identical vibrator as in prior art, then the first circular shaft will have a greater surface of contact with the first vibrator than the second circular shaft will have with the second vibrator. The forces per unit area of surface contact between the first circular shaft and the first vibrator will be less than the forces per unit area of contact between the second circular shaft and the second vibrator. Therefore, by coupling a circular shaft to a vibrator of a piezoelectric motor according to a preferred embodiment of the present invention wear and damage to the vibrator and shaft surfaces will be reduced.

Fig. 5 A, Fig. 5B, and Fig. 5C illustrate schematically the coupling of a circular shaft 76 to a cylindrical surface 78 of a vibrator 80 in accordance with a preferred embodiment of the present invention and (in an exaggerated manner) transmission of torque to the shaft that results from a vibration cycle of the vibrator.

Fig. 5A shows a perspective view of shaft 76 and vibrator 80 which is the part of an extended vibrator near to a surface 78 to which shaft 76 is coupled. A line 125 is a fiducial mark indicating the center of surface 78. Arrow 82 indicates a resilient biasing force which presses surface 78 to the surface of shaft 76.

Fig. 5B is a cross section view of vibrator 80. An arc 104 is the directrix of surface 78 and forms a portion of an ellipse 102. Ellipse 102 has a major axis 110 and a minor axis 112.

Preferably, the vibrators of the invention have a plurality of electrodes (not shown) on their large surfaces in configurations such as described in U.S. patent 5,453,653. Preferably, different ones of the plurality of electrodes may be excited by applied voltages to cause vibrator 80 to vibrate in different vibration modes by methods described in U.S. patent 5,453,653. Alternatively, other excitation methods known in the art may be used.

Vibrator 80 is preferably made to vibrate so that the motion of mass points on surface 78 of vibrator 80 trace out an approximately elliptical path represented in Fig. 5C by an ellipse 100. The size of ellipse 100 is greatly exaggerated in comparison to the actual motion of mass points on surface 78 for the sake of clarity of presentation. Preferably, the phase differences between the cycles of motion of different mass points on surface 78 of vibrator 80 are small. Generally, a point on ellipse 100 which traces the motion of mass points on surface 78 can be made to advance either clockwise or counter clockwise around ellipse 100 by changing the way electrodes on vibrator 80 are electrified. Ellipse 100 is preferably parallel to ellipse 102.

Ellipse 100 has a major axis 106 and a minor axis 108. Minor axis 108 represents the amplitude of motion of mass points on surface 78 parallel to minor axis 112 of ellipse 102. Major axis 106 represents the amplitude of motion of mass points on surface 78 parallel to major axis 110 of ellipse 102. In embodiments of the present invention, where minor axis 108 represents a distance equal to the difference between minor axis 112 of ellipse 102 and the diameter of shaft 76 and where major axis 106 represents a distance equal to the difference between major axis 110 of ellipse 102 and the diameter of shaft 76, the match between the vibration mode of vibrator 80 and the shape of surface 78 is optimum. A match between surface 78 and the vibration mode of vibrator 80 that is less than optimum will result in contact surfaces and contact times between surface 78 and the surface of shaft 76 that are less than those obtained when surface 78 and the vibration mode of vibrator 80 are optimally matched. However, even significantly less than optimum matches result in increased contact times and larger contact surfaces between shaft 76 and surface 78 than are obtained in prior art. It is noted, that due to the small extent of the vibrations as compared to the size of the shaft, the optimal condition may be difficult to achieve in practice.

Points on ellipse 100 represent different phase points of a vibration cycle of vibrator 80. As vibrator 80 vibrates through a vibration cycle mass points on surface 78 are displaced relative to the positions of their locations when vibrator 80 is at rest. The displacements of mass points on surface 78 at a phase point of a vibration cycle are represented by the displacement from the origin of ellipse 100 of the point on ellipse 100 which represents the phase point. A line tangent to ellipse 100 is parallel to the velocity of mass points on surface 78 at the phase point of a vibration cycle represented by the point on ellipse 100 at which the line is tangent. The direction of the velocity is in the direction of increasing phase.

The relative disposition of shaft 76 and vibrator 80 are shown on ellipse 100 in Fig. 5C at different phase points 114, 116, 118, 120, 122, and 124 of a vibration cycle. The position of shaft 76 is fixed in space by bearing structures which support it and allow it to rotate. Therefore, changes in relative position between shaft 76 and vibrator 80 are due to displacements of surface 78 which result from the motion of mass points on surface 78. As vibrator 80 vibrates, fiducial mark 125 moves to the right and left, and towards and away from axis 85 of shaft 76. The direction of motion of mass points on surface 78 at phase points 114, 116, 118, 120, 122, and 124 are indicated by arrows 128, 130, 132, 134, 136, and 138 respectively.

Biasing force 82, the shape of surface 78 and the motion of mass points on surface 78, in accordance with a preferred embodiment of the present invention, cause shaft 76 to contact vibrator surface 78 during a vibration cycle at and near to a central contact phase point 114 of the cycle. Contact between surface 78 and the surface of shaft 76 during a vibration cycle occur only at phase points near phase point 114 that are represented by points on ellipse 100 that are between contact phase points 116 and 124. To the extent that the matching of the shape of surface 78 and the vibration mode of vibrator 80 approaches the optimum matching as discussed above, contact phase point 124 moves further to the right of contact phase point 114 and contact phase point 116 moves further to the left of contact phase point 114 and arc 124-

114-116 increases in length. The increase in length of arc 124-114-116 represents an increase in contact time and contact surface during a vibration cycle between the surface of shaft 76 and surface 78. The maximum length for arc 124-114-116 occurs when contact phase point 124 corresponds to point 103 of ellipse 102 and contact phase point 116 corresponds to point 105 on ellipse 102. This occurs when a vibration mode of vibrator 80 is optimally matched to surface 78.

In practical situations optimal matching is not achieved. Contact phase points 124, 114,

116 are clustered closely together. Contact time and contact area between surface 78 and the surface of shaft 76 during a cycle remain small compared to both the period of a vibration cycle and the full area of surface 78. However, improvements by factors of 2 and 3 or more in contact time and contact surface are very significant, and are possible in preferred embodiments of the present invention. In Fig. 5C the distance between contact phase point 124 and contact phase point 116 is shown greatly exaggerated in the interest of clarity of presentation.

At each contact phase point of a vibration cycle, when shaft 76 comes in contact with surface 78, a contact surface similar to contact surface 83 shown in Fig. 4A is formed between shaft 76 and surface 78. Contact surfaces 140, 142, and 144 which are formed respectively at contact phase points 124, 114, and 116 in a vibration cycle are shown in Fig. 5C and are emphasized with bold line segments. At contact phase points 124, 114, and 116, and respective contact surfaces 124, 114, and 116 torque 146, 148, and 150 respectively are transmitted to shaft 76.

At contact phase point 114 of a vibration cycle, under the action of biasing force 82, shaft 76 and surface 78 are in contact at contact surface 142 located at the center of surface 78. Axis 85 of shaft 76 is in line with fiducial mark 125. Mass points on surface 78 move with velocity 128 parallel to the tangent to ellipse 100 at phase point 114. Velocity 128 is also parallel to a tangent to the surface of shaft 76 at a point of contact between the surface of shaft 76 and surface 78 at a point in contact surface 142. Torque impulse 148 is transmitted to shaft 76.

As the vibration cycle continues from contact phase point 114 towards contact phase point 116 fiducial mark 125 moves left and downwards away from axis 85 of shaft 76. If surface 76 did not curve upwards to the right of fiducial mark 125, shaft 76 and surface 78 would separate. However, in a preferred embodiment of the present invention as discussed above, the vibration mode of vibrator 80, represented by ellipse 100, is at least partly matched to the shape of surface 78 which is generated by an arc of ellipse 102. As the vibration of vibrator 80 causes fiducial mark 125 to move left and away from shaft 76 the part of surface 78 to the right of fiducial mark 125 moves up by an amount required to maintain contact with shaft 76.

Therefore, at contact phase point 116 shaft 76 is in contact with surface 78 at contact surface 144. Mass points on surface of contact 144 are moving with velocity 130 which is parallel to the tangent to ellipse 100 at phase point 116. Velocity 130 is also parallel to a tangent to the surface of shaft 76 at a point of contact between the surface of shaft 76 and surface 78 at a point in contact surface 144. Torque 150 is transmitted to shaft 76. As the vibration cycle continues, fiducial 125 moves left and downwards away from axis 85 of shaft 76, and at some point before the maximum left displacement of fiducial mark

125 in the vibration cycle surface 78 separates from the surface of shaft 76. For less than optimal matching of the vibration mode of vibrator 80 to the shape of surface 78, as the cycle proceeds from phase point 114 the deformation of surface 78 is reduced, until contact is lost.

The distance between surface 78 and the surface of shaft 76 continues to increase until phase point 120 when the vibration of vibrator 80 in the direction parallel to minor axis 108 reverses itself. Surface 78 and the surface of shaft 76 remain separated until at contact phase point 124 surface 78 and the surface of shaft 76 again make contact and torque impulse 146 is transmitted to shaft 76. Contact occurs at contact surface 140 which is well to the left of fiducial mark 125.

As the vibration cycle continues from contact phase point 124 to return to contact phase point 114 and complete the vibration cycle, fiducial mark 125 moves left towards the axis of shaft 76. As the vibration of vibrator 80 moves fiducial mark 125 left, towards the axis of shaft 76 it also moves surface 78 parallel to minor axis 108 upwards towards the axis of shaft 76 by an amount required to maintain contact between the surface of shaft 76 and surface 78. When contact phase point 114 is reached torque impulse 148 is again transmitted to shaft 76 and a new vibration cycle begins.

The area of surface 78 which is operative in transferring torque to shaft 76 is the area of the contact surface that is formed between shaft 76 and surface 78 during various parts of a vibration cycle. While this area is generally small in comparison to the total area of surface 78 it is still significantly increased over the operative area of surface 78 that would result if surface 78 were planar as in prior art. The increase is a result of the geometric effect shown in magnified view in Fig 4A and Fig. 4B and discussed above and by having contact between the surface of shaft 76 and surface 78 over an extended range of contact phase points in a vibration cycle between contact phase point 124 and contact phase point 116.

Total torque transmitted to shaft 76 during a vibration cycle is the integral of the torque transmitted to shaft 76 during contact with surface 78. The time period over which torque is transmitted to shaft 76 during a vibration cycle is equal to the sum of the periods of time that the surface of shaft 76 and surface 78 remain in contact at contact phase points of the vibration cycle. The time period over which torque is transmitted to shaft 76 during a vibration cycle in preferred embodiments in accordance with the present invention is larger than the time period over which torque would be transmitted to shaft 76 in the prior art where surface 78 is planar and contact is made between shaft 76 and surface 78 only in a very restricted range of phase points in a vibration cycle.

In preferred embodiments of the invention contact between the surface of shaft 76 and surface 78 will be nearly continuous between contact phase point 124 and contact phase point 116 of a vibration cycle. Shaft 76 will roll onto surface 78 at contact phase point 124 and roll off surface 78 at contact phase point 116. Shaft 76 will make contact with all of surface 78 once every vibration cycle. All of surface 78 will be operative in transmitting torque to shaft 76 during a vibration cycle. Torque will be transmitted to shaft 76 continuously during a vibration cycle for a time equal to the time it takes for the vibration cycle to move from phase point 124 to phase point 116.

It is clear from the above discussion that preferred embodiments of the present invention result in increases of the areas of piezoelectric vibrators which are operative in the transmission of torque to shafts over that which is obtained in the prior art. Preferred embodiments of the present invention result also in increased duty cycles for the transmission of torque to a shaft over that obtained in prior art. It is also clear from the discussion that the increases in operative areas and duty cycles can be very large in comparison to the operative areas and duty cycles obtained in prior art.

The benefits described in the above discussion result from use of a single piezoelectric ceramic vibrator in accordance with a preferred embodiment of the present invention to drive a shaft. The benefits are further increased by coupling a plurality of vibrators to drive a shaft.

By way of example Fig. 6 shows a schematic of a plurality of two piezoelectric micromotor vibrators 162 and 164 coupled to a shaft 166 in accordance with a preferred embodiment of the present invention. Vibrators 162 and 164 are respectively pressed by preferably equal resilient forces 168 and 170 to preferably opposite parts of the surface of shaft 166. Preferably, vibrator 162 vibrates in the same mode as vibrator 164. Preferably, vibrators 62 and 164 are driven in phase or 180 degrees out of phase.

Preferably, a phase difference between vibration of vibrator 162 and vibration of vibrator 164 can be adjusted and set to a desired phase difference by methods known in the art. The phase difference determines the timing of torque transmission to shaft 166 by vibrators 162 and 164. By way of example, if the phase of vibration of vibrators 162 and 164 are the same, vibrator 162 and vibrator 164 will apply torque to shaft 166 simultaneously during a portion of their vibration cycle. By way of a second example, if the phase difference between vibration of vibrator 162 and vibration of vibrator 164 is 180 degrees, then vibrators 162 and 164 will alternately apply torque in the same direction to shaft 166 at periods 180 degrees out of phase in the vibration cycle. For a given load, φm and frictional loss, when vibrators 162 and 164 are configured, in accordance with a preferred embodiment of the present invention as described above, the peak instantaneous torque applied to shaft 166 by either vibrator 162 or 164 is half the peak instantaneous torque applied to shaft 166 by a single vibrator driving shaft 166 under the same conditions of load, φm and frictional loss. Furthermore, by tuning phase angle 176, vibrator 162 and vibrator 164 can be synchronized to apply torque to shaft 166 at the same time or at different times during a vibration cycle.

It is clear from the above discussion that a plurality of three, four and more vibrators can be coupled to a single shaft to transmit torque to the shaft in accordance with a preferred embodiment of the present invention. It is also clear that the plurality of micromotors can be coupled to the shaft in different physical configurations e.g. micromotors can be positioned at different positions along the length of the shaft and at different angular positions around the circumference of the shaft.

Furthermore, different configurations for the phase differences between vibrators in the plurality of vibrators are advantageous. By way of an example in accordance with a preferred embodiment of the present invention, all the phase differences between vibration modes of the micromotors of the plurality of micromotors are set to zero. All the micromotors will then apply torque to the shaft simultaneously. The instantaneous torque applied to the shaft will be a maximum for the configuration of the micromotors and equal to the torque applied to the shaft by a single micromotor times the number of the plurality of micromotors. In another example in accordance with a preferred embodiment of the present invention, the phase difference between the vibration modes of one of the plurality of micromotors and any other one of the plurality of micromotors is set equal to (n x 360 degrees/N), where N is the number of the plurality of micromotors and n is an integer with 0<n<N, and where n is different for each different other one of the plurality of micromotors. For example, if there are 4 identical micromotors driving a shaft, the phase differences between the vibration mode of one of the micromotors and the vibration modes of the other micromotors are set to 90 degrees,

180 degrees and 270 degrees. In this case a same torque, equal to the torque applied to the shaft by a single one of the identical micromotors, will be applied to the shaft at equal time intervals, four times during the vibration cycle of the vibrators. Total torque applied to the shaft during a vibration cycle will be four times the torque applied to the shaft during the vibration cycle by one of the micromotors. Total torque in this example is equal to the total torque that would be applied to the shaft if all four micromotors acted simultaneously. However, maximum instantaneous torque applied to the shaft will be equal to the instantaneous torque applied to the shaft by only a single micromotor, one quarter the instantaneous torque that results when all four micromotors act simultaneously during a vibration cycle. Torque applied to the shaft in the phase configuration of this example is more uniform over a vibration cycle than is the torque applied to the shaft when all four micromotors act simultaneously. It should be noted that quite generally, for a given desired rate of torque transfer to a shaft, as the frequency at which torque impulses are imparted to the shaft increases the average of the magnitude of the torque impulses decreases and the applied torque becomes more uniform.

By way of a further example in accordance with a preferred embodiment of the present invention, six identical micromotors are symmetrically placed around a shaft and resiliently pressed to the shaft to drive it. The phase difference between opposite micromotors is set to zero. The phase difference between the vibration mode of one of the three pairs of opposite micromotors and the second and third pairs of opposite micromotors is set to 120 degrees and 240 degrees respectively. As a result of this phase configuration each pair of opposite micromotors will sequentially apply torque to the shaft every one third of the period of the vibration cycle of the micromotors. Since opposite micromotors are in phase, there will be little or no force on the shaft that tends to bend it and there will be a minimum of stress on the bearings holding the shaft in place.

The method of coupling a shaft to a piezoelectric vibrator in accordance with a preferred embodiment of the invention by matching the geometry of the surface of the vibrator that contacts the surface of the shaft to the vibration mode of the vibrator and the radius of the shaft leads to still other advantageous configurations for coupling a shaft to a micromotor. Fig. 7 illustrates a configuration for coupling a shaft 180 to a piezoelectric vibrator 182 in accordance with a preferred embodiment of the present invention where the surface of shaft 180 couples to the surface of a hole 184 cut through vibrator 182.

Hole 184 is preferably elliptical or circular. The axis of hole 184 is preferably peφendicular to the large planar surfaces of vibrator 182. Vibrator 182 preferably vibrates so that mass points on the surface of hole 184 move in elliptical or circular paths of motion. Preferably, the major axis and minor axis of the paths of motion of mass points on the surface of hole 184 are parallel respectively to the major and minor axis of hole 184. Preferably, the differences between the radius of shaft 180 and the minor axis and major axis of hole 184 are slightly less than the major and minor amplitudes of motion of mass points on the surface of hole 184. Preferably, shaft 180 and vibrator 182 are fixed with respect to each other so that when vibrator 182 is quiescent the axis of shaft 180 is substantially congruent with the axis of hole 184. Under these conditions the geometries of shaft 180 and hole 184 are matched ideally to the vibration mode of vibrator 182. The surface of hole 184 will be in contact with the surface of shaft 180 almost continuously during a vibration cycle of the vibrator and torque will be transmitted to shaft 180 smoothly and continuously during the vibration cycle.

Under less than ideal matching of the geometries of shaft 180, hole 184 and the vibration mode of vibrator 182 a resilient biasing force is required to press shaft 180 to a part of the surface of hole 184. Torque will be transferred to shaft 180 only during part of the vibration cycle of vibrator 182 and only part of the surface of hole 184 will be active in transmitting the torque. However, by periodically changing the direction of the biasing force, the active area of the surface of hole 184 can be changed and wear on the surface of hole 184 can be distributed substantially evenly over the surface of hole 184.

While the invention has been described utilizing an edge having a concave depression, it is to be understood that the invention is applicable and includes the case where the depression is formed in an extension of the edge, such as a ceramic extension.

Variations of the above described preferred embodiment will occur to persons of the art. The above detailed description is provided by way of example and is not meant to limit the scope of the invention which is limited only by the following claims.

Claims

1. A method for producing a piezoelectric micromotor for driving a rotatable shaft comprising: providing a piezoelectric plate with plane surfaces and edges; and forming at least one electrode on at least one plane surface, wherein, a cylindrical contact surface is formed on an edge of the piezoelectric plate or on an extension thereof for contact between the rotatable shaft and the piezoelectric plate prior to the first use of the micromotor for driving the rotatable shaft.

2. A method according to claim 1 comprising forming the cylindrical contact surface as a concave surface.

3. A method according to claims 1 or 2 comprising forming the cylindrical contact surface using an arc of an ellipse as a directrix for the cylindrical contact surface.

4. A method according to claim 3 wherein at every point on the contact surface the radius of curvature of the directrix is greater than the radius of the shaft.

5 A method according to claim 3 wherein at every point on the contact surface the radius of curvature of the directrix is substantially equal to the radius of the shaft.

6. A method according to any of claims 3 through 5 wherein the major and minor axes of the ellipse are equal.

7. A method according to any of the preceding claims comprising matching the motion of said edge to the geometry of the cylindrical contact surface.

8. A method according to any of the preceding claims and comprising exciting an elliptical motion of vibration of mass points on the cylindrical surface.

9. A method according to claim 8 wherein the major axis of the elliptical motion of vibration is parallel to the major axes of an ellipse, an arc of which is a directrix of the cylindrical surface.

10. A method according to claims 8 or 9 wherein the major axes of the elliptical motion is substantially equal to the difference between the major axis of an ellipse, an arc of which is a directrix of the cylindrical surface, and the diameter of the shaft.

11. A method according to any of claims 8 through 10 wherein the minor axes of the elliptical motion is substantially equal to the difference between the minor axis of an ellipse, an arc of which is a directrix of the cylindrical surface, and the diameter of the shaft.

12. A micromotor produced according to any of claims 1 through 11.

13.. A micromotor for rotating a shaft comprising: a piezoelectric plate with plane surfaces and edges; and at least one electrode on at least one plane surface, wherein an edge of the piezoelectric plate or on an extension thereof has a cylindrical contact surface, for contact with the shaft, formed prior to contact of the surface and the shaft.

14. A micromotor according to claim 13 wherein the cylindrical contact surface is concave.

15. A micromotor according to claim 13 wherein the directrix of the cylindrical contact surface is an arc of an ellipse.

16. A micromotor according to claim 15 wherein the major and minor axis of the ellipse are equal.

17. A micromotor according to any of claims 13 through 16 wherein the cylindrical contact surface of the piezoelectric plate or an extension thereof that comes into contact with the surface of the shaft has mass points that vibrate with an elliptical motion.

18. A micromotor according to claim 17 wherein the major and minor axes of the elliptical motion are parallel respectively to the major and minor axes of an ellipse an arc of which is a directrix of the cylindrical contact surface.

19. A micromotor according to claim 17 or 18 wherein the major axis of the elliptical motion is substantially equal to the difference between the major axis of an ellipse, an arc of which is a directrix of the cylindrical contact surface of the piezoelectric plate or an extension thereof that comes into contact with the surface of the shaft, and the diameter of the shaft.

20. A micromotor according to any of claims 17, through 19 wherein the minor axis of the elliptical motion is equal to the difference between the minor axis of an ellipse, an arc of which is a directrix of the cylindrical contact surface of the piezoelectric plate or an extension thereof that comes into contact with the surface of the shaft, and the diameter of the shaft.

21. A method for rotating a shaft comprising: resiliently pressing at least one micromotor according to any of claims 12 through 20 to a same shaft; and exciting vibrations in the at least one micromotor such that the at least one micromotor applies torque to said shaft.

22. A method according to claim 21 wherein at least two of a plurality of micromotors are identical.

23. A method according to claim 21 or claim 22 wherein resiliently pressing a plurality of micromotors to a same circular shaft comprises pressing identical micromotors to the shaft in a circularly symmetric configuration.

24. A method according to any of claims 21 - 23 wherein resiliently pressing a plurality of micromotors to the circular shaft comprises pressing micromotors of the plurality of micromotors to the shaft with equal forces.

25. A method for rotating a shaft according to any of claims 21 - 24 wherein exciting vibrations in the plurality of micromotors comprises exciting the micromotors to vibrate in the same vibration mode.

26. A method according to claim 25 comprising setting the phase differences between the vibration modes of each of the micromotors of the plurality of micromotors to zero.

27 A method according to claim 25 comprising setting the phase difference between the vibration mode of any one of the plurality of micromotors and the vibration mode of any of the other micromotors equal to (n x 360 degrees/N), where N is the number of the plurality of micromotors and n is an integer with 0<n<N, and wherein n is different for each of the other micromotors.

28. A method according to claim 26 comprising setting to zero the phase difference between the vibration modes of micromotors situated on opposite sides of the shaft.

29. A method according to claim 28 comprising setting the phase difference between the vibration mode of one of a plurality of pairs of micromotors situated opposite to each other and any of the other pairs of micromotors situated opposite to each other equal to (n x 360 degrees/N), where N is the number of pairs of micromotors which are opposite each other and n is an integer with 0<n<N, and n is different for each of the other pairs of micromotors situated opposite to each other.

30. A piezoelectric micromotor for rotating a shaft comprising: a piezoelectric plate with plane surfaces and edges having a hole formed in said piezoelectric plate peφendicular to a plane surface into which the shaft is inserted; and means for exciting the plate in a vibration mode such that mass points on the surface of the hole vibrate with an elliptical motion.

31. A piezoelectric micromotor according to claim 30 wherein the hole is an elliptical hole.

32. A piezoelectric micromotor according to claim 31 wherein the major axis of the elliptical motion of the mass points on the surface of the hole is parallel to the major axis of the elliptical hole.

33. A piezoelectric micromotor according to claim 32 wherein the major axis of the elliptical motion is substantially equal to the difference between the radius of the shaft and the major axis of the elliptical hole.

34. A piezoelectric micromotor according to claim 33 wherein the minor axis of the elliptical motion is substantially equal to the difference between the radius of the shaft and the minor axis of the elliptical hole.

35. A piezoelectric micromotor according to any of claims 30 -34 wherein the major and minor axis of the hole are equal.

36. A piezoelectric micromotor according to any of claims 30- 35 wherein the axis of the shaft and the hole are substantially congruent when the piezoelectric plate is not vibrating.

37. A piezoelectric micromotor according to any of claims 30 - 35 wherein a resilient biasing force presses the shaft to a part of the surface of the hole

38. A method for rotating a shaft comprising: forming a hole in a piezoelectric plate; inserting the shaft into said hole; and exciting the piezoelectric plate so that mass points on the surface of the hole vibrate so as to transmit torque to the shaft.

39. A method according to claim 38 wherein forming a hole comprises forming an elliptical hole.

40. A method according to claims 38 or 39 comprising exciting a vibration mode in the piezoelectric plate such that mass points on the surface of the hole vibrate with an elliptical motion.

41. A method according to claim 40 wherein the major axis of the elliptical motion of mass points on the surface of the hole is parallel to the major axis of the elliptical hole.

42. A method according to claim 41 wherein the major axis of the elliptical motion of mass points on the surface of the hole is substantially equal to the difference between the major axis of the hole and the radius of the shaft.

43. A method according to claim 42 wherein the minor axis of the elliptical motion of mass points on the surface of the shaft is substantially equal to the difference between the minor axis of the elliptical hole and the radius of the shaft.

44. A method according to any of claims 37 - 43 wherein the major and minor axis of the hole are equal.

45. A method according to any of claims 38 - 44 wherein inserting the shaft into the hole comprises stabilizing the shaft in the hole such that the axis of the shaft and the hole are substantially congruent when the piezoelectric plate is not vibrating.

46. A method according to any of claims 38 -44 wherein inserting the shaft into the hole comprises pressing the shaft with a resilient force to a part of the surface of the hole

AMENDED CLAIMS

[received by the International Bureau on 20 May 1998 (20.05.98) ; original cl aims 1 -46 replaced by amended claims 1 -43 (6 pages)]

1. A method for producing a piezoelectric micromotor for driving a rotatable shaft comprising: providing a piezoelectric plate with plane surfaces and edges; and forming at least one electrode on at least one plane surface, wherein, a cylindrical contact surface is formed on an edge of the piezoelectric plate or on an extension thereof for contact between the rotatable shaft and the piezoelectric plate prior to the first use of the micromotor for driving the rotatable shaft, and wherein the radius of curvature of said cylindrical contact surface is everywhere greater than the radius of the shaft.

2. A method according to claim 1 comprising forming the cylindrical contact surface using an arc of an ellipse as a directrix for the cylindrical contact surface.

3. A method according to claim 2 wherein at every point on the contact surface the radius of curvature of the directrix is greater than the radius of the shaft.

4. A method according to claim 2 or claim 4 wherein the major and minor axes of the ellipse are equal.

5. A method according to any of the preceding claims comprising matching the motion of said edge to the geometry of the cylindrical contact surface.

6. A method according to any of the preceding claims and comprising exciting an elliptical motion of vibration of mass points on the cylindrical surface.

7. A method according to claim 6 wherein the major axis of the elliptical motion of vibration is parallel to the major axes of an ellipse, an arc of which is a directrix of the cylindrical surface.

8. A method according to claims 6 or 7 wherein the major axes of the elliptical motion is substantially equal to the difference between the major axis of an ellipse, an arc of which is a directrix of the cylindrical surface, and the diameter of the shaft.

9. A method according to any of claims 6 through 8 wherein the minor axes of the elliptical motion is substantially equal to the difference between the minor axis of an ellipse, an arc of which is a directrix of the cylindrical surface, and the diameter of the shaft.

10. A micromotor produced according to any of claims 1 through 9.

11. A micromotor for rotating a shaft comprising: a piezoelectric plate with plane surfaces and edges; and at least one electrode on at least one plane surface, wherein an edge of the piezoelectric plate or on an extension thereof has a cylindrical contact surface, for contact with the shaft, formed prior to contact of the surface and the shaft and wherein the radius of curvature of said cylindrical contact surface is everywhere greater than the radius of the shaft.

12. A micromotor according to claim 13 wherein the directrix of the cylindrical contact surface is an arc of an ellipse.

13. A micromotor according to claim 12 wherein the major and minor axis of the ellipse are equal.

14. A micromotor according to claim 11 or claim 13 wherein the cylindrical contact surface of the piezoelectric plate or an extension thereof that comes into contact with the surface of the shaft has mass points that vibrate with an elliptical motion.

15. A micromotor according to claim 14 wherein the major and minor axes of the elliptical motion are parallel respectively to the major and minor axes of an ellipse an arc of which is a directrix of the cylindrical contact surface.

16. A micromotor according to claim 14 or 15 wherein the major axis of the elliptical motion is substantially equal to the difference between the major axis of an ellipse, an arc of which is a directrix of the cylindrical contact surface of the piezoelectric plate or an extension thereof that comes into contact with the surface of the shaft, and the diameter of the shaft.

17. A micromotor according to any of claims 14 through 16 wherein the minor axis of the elliptical motion is equal to the difference between the minor axis of an ellipse, an arc of which is a directrix of the cylindrical contact surface of the piezoelectric plate or an extension thereof that comes into contact with the surface of the shaft, and the diameter of the shaft.

18. A method for rotating a shaft comprising: resiliently pressing at least one micromotor according to any of claims 12 through 20 to a same shaft; and exciting vibrations in the at least one micromotor such that the at least one micromotor applies torque to said shaft.

19. A method according to claim 18 wherein at least two of a plurality of micromotors are identical.

20. A method according to claim 18 or claim 19 wherein resiliently pressing a plurality of micromotors to a same circular shaft comprises pressing identical micromotors to the shaft in a circularly symmetric configuration.

21. A method according to any of claims 18 - 20 wherein resiliently pressing a plurality of micromotors to the circular shaft comprises pressing micromotors of the plurality of micromotors to the shaft with equal forces.

22. A method for rotating a shaft according to any of claims 18 - 21 wherein exciting vibrations in the plurality of micromotors comprises exciting the micromotors to vibrate in the same vibration mode.

23. A method according to claim 22 comprising setting the phase differences between the vibration modes of each of the micromotors of the plurality of micromotors to zero.

24 A method according to claim 22 comprising setting the phase difference between the vibration mode of any one of the plurality of micromotors and the vibration mode of any of the other micromotors equal to (n x 360 degrees/N), where N is the number of the plurality of micromotors and n is an integer with 0<n<N, and wherein n is different for each of the other micromotors.

25. A method according to claim 23 comprising setting to zero the phase difference between the vibration modes of micromotors situated on opposite sides of the shaft.

26. A method according to claim 25 comprising setting the phase difference between the vibration mode of one of a plurality of pairs of micromotors situated opposite to each other and any of the other pairs of micromotors situated opposite to each other equal to (n x 360 degrees/N), where N is the number of pairs of micromotors which are opposite each other and n is an integer with 0<n<N, and n is different for each of the other pairs of micromotors situated opposite to each other.

27. A piezoelectric micromotor for rotating a shaft comprising: a piezoelectric plate with plane surfaces and edges having a hole formed in said piezoelectric plate peφendicular to a plane surface into which the shaft is inserted; and means for exciting the plate in a vibration mode such that mass points on the surface of the hole vibrate with an elliptical motion.

28. A piezoelectric micromotor according to claim 27 wherein the hole is an elliptical hole.

29. A piezoelectric micromotor according to claim 28 wherein the major axis of the elliptical motion of the mass points on the surface of the hole is parallel to the major axis of the elliptical hole.

30. A piezoelectric micromotor according to claim 29 wherein the major axis of the elliptical motion is substantially equal to the difference between the radius of the shaft and the major axis of the elliptical hole.

31. A piezoelectric micromotor according to claim 30 wherein the minor axis of the elliptical motion is substantially equal to the difference between the radius of the shaft and the minor axis of the elliptical hole.

32. A piezoelectric micromotor according to any of claims 27 -31 wherein the major and minor axis of the hole are equal.

33. A piezoelectric micromotor according to any of claims 27- 32 wherein the axis of the shaft and the hole are substantially congruent when the piezoelectric plate is not vibrating.

34. A piezoelectric micromotor according to any of claims 27 - 32 wherein a resilient biasing force presses the shaft to a part of the surface of the hole

35. A method for rotating a shaft comprising: forming a hole in a piezoelectric plate; inserting the shaft into said hole; and exciting the piezoelectric plate so that mass points on the surface of the hole vibrate so as to transmit torque to the shaft.

36. A method according to claim 35 wherein forming a hole comprises forming an elliptical hole.

37. A method according to claims 35 or 36 comprising exciting a vibration mode in the piezoelectric plate such that mass points on the surface of the hole vibrate with an elliptical motion.

38. A method according to claim 37 wherein the major axis of the elliptical motion of mass points on the surface of the hole is parallel to the major axis of the elliptical hole.

39. A method according to claim 38 wherein the major axis of the elliptical motion of mass points on the surface of the hole is substantially equal to the difference between the major axis of the hole and the radius of the shaft.

40. A method according to claim 39 wherein the minor axis of the elliptical motion of mass points on the surface of the shaft is substantially equal to the difference between the minor axis of the elliptical hole and the radius of the shaft.

41. A method according to any of claims 35 - 40 wherein the major and minor axis of the hole are equal.

42. A method according to any of claims 35 - 41 wherein inserting the shaft into the hole comprises stabilizing the shaft in the hole such that the axis of the shaft and the hole are substantially congruent when the piezoelectric plate is not vibrating.

43. A method according to any of claims 35 -41 wherein inserting the shaft into the hole comprises pressing the shaft with a resilient force to a part of the surface of the hole.