BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is directed to a drive mechanism for accurately positioning a work stage along two axes with three degrees of freedom.

2. Related Art

Mechanisms for positioning a work stage for locating a work piece are well known in the art. For example, U.S. Pat. No. 4,528,490 of Hansen for “Two Axis Drive for Stage” includes a base plate and an additional pair of stacked plates, one known as a stage plate and the other as an intermediate plate. Each plate is driven by a drive. The intermediate plate is driven by drive bar along a linear direction with respect to the base plate. A drive means for the stage plate includes a motor driven capstan, and a drive bar has angular freedom of movement, i.e. is pivotally mounted under stage plate. The stage plate is thus free to move along a second path and to rotate.

J. Reed et al “High Speed Precision X-Y Stage”, J. Vac. Sci. Technol. B, Vol. 3, No. 1 pp. 112 et seq., (Jan./Feb. 1985) describe conventional linear ways and ball bearings but “incorporates a unique capstan/swinging drive bar design to couple the servomotors and the X and Y stage elements.” The stage comprises three stacked plates coupled to one another by crossed linear bearings. The bottom plate is affixed to a base. The center plate is fixed rigidly to its drive bar through a preloaded duplex bearing. The drive bars are driven by a servomotor via a friction-drive capstan.

S. Ido et al. “Precision X-Y Stage for Electron Beam Lithography System” pp 267-268 Bull. Japan Soc. of Prec. Eng. Vol. 18, No. 3 (September 1984) describe a stacked X-Y stage configuration with hydrostatic bearings.

Kallmayer et al “X-Y Table” IBM Technical Disclosure Bulletin Vol. 30, No. 7 (December 1987), pp. 376-377 show three rigidly mounted spindle drives with stators affixed to a rigid support so they do not pivot relative to the table 4, and so the flexibility of the drives is limited by their rigid mounting to a restricted range of motions afforded by the guides and in the slots in the table 4. In addition, two of the spindle drives are parallel to each other. Laser interferometers and mirrors are used to measure displacement.

Tsuyuzaki et al. J. Vac. Sci. Technol. B, Vol. 4, No. 1 p28X, (January/February 1986) describe a plate structure or “planar” stage with X-Y motion accuracy imparted via machined guide slots in the base and a substrate positioning table. An X-Y cross structure is placed within these slots between the top table and the base. A low friction polymeric material such as PTFE is employed as a bearing surface.

Constant Download Friction Drive System, IBM Technical Disclosure Bulletin, Vol. 32, No. 8A, (January 1990) pages 120-121 describes a method and apparatus for canceling the variation in download exerted by the end of a drivebar system by preloading the drivebar as shown in

FIG. 3

herein.

Ball Joint Pivot with Dynamic Preload

DISCUSSION OF RELATED ART

Ball type pivot joints provide multiple degrees of freedom about a point at the center of a ball of spherical shape. Heretofore, ball joints have included two basic types including as follows:

(1) Spring-loaded joints and

(2) Spherical bearings.

Spring-loaded ball joints include a pair of cones or sockets which are tightened against the ball to eliminate backlash. A disadvantage of that system is that when using high preloads for high linear stiffness, there is considerable friction and wear between the ball and the cones or sockets.

The spherical bearing ball joint relies on closely matched inner and outer spherical bearing elements. This type of ball joint has high linear stiffness and low friction, but by the nature of this design must operate with a small clearance between the inner and outer bearings. Such a small clearance increases with wear and the backlash associated with this clearance makes it unsuitable for micropositioning applications.

U.S. Pat. No. 5,140,242 to Doran et al., which is herein incorporated by reference in its entirety, discloses a servo guided stage system having integrated dual axis plane mirror interferometers for sensing stage position. U.S. Pat. No. 5,052,844 to Kendall, which is herein incorporated by reference in its entirety, discloses a ball joint with an adjustable preload for use in a stage positioning system.

Conventional methods for positioning a substrate, however, use costly or ineffective means to measure the yaw angle of the stage. What is needed in the art is a stage position system having simplified yaw angle measuring device.

BRIEF SUMMARY OF THE INVENTION

Now, according to the present invention, the above-described and other disadvantages of the prior art are overcome or alleviated by the positioning stage comprising a base having rectilinearly disposed x and y axes, a stage plate slideably supported on said base, at least three linear drive means rotatably engaging both said stage plate and rotatably engaging said base for moving said stage plate on a path with at least three separate drive displacements along said rectilinearly disposed x and y axes and rotation of said plate on said base substantially parallel to the surface of said base, said plate moving upon the surface of said base, whereby combined x, y rectilinear and rotary motions with respect to said base can be achieved, an x interferometer and receiver for determining x position, a y interferometer and receiver for determining y position, and, a yaw sensor device for determining yaw angle comprising a light emitting source, a cylinder lens, and a position sensing detector. In this manner, the x position measurement, the y position measurement, and the yaw measurement can be used to close mechanical servo loops to accurately position the stage plate and eliminate yaw errors.

A method for determining yaw angle of a positioning stage also is provided comprising directing an incident beam of light from a light emitting source at a reflective surface on said positioning stage to create a reflected beam of light, passing said reflected beam of light through a cylinder lens to vertically focus said reflected beam of light on a position sensing detector, and generating a signal from said position sensing detector, wherein said signal is dependent upon the lateral position of said reflected beam on said position sensing detector.

The above-described and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example only, with reference to the accompanying drawings, which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several FIGURES, in which:

FIG. 1

is a perspective view of an x, y, theta stage plate and a plurality of linear drives including drivebars driven by friction drive units and a control system for positioning the stage plate.

FIG. 2

shows an example of a prior art drivebar system.

FIG. 3

shows a preload roller which has been added to the drivebar system of FIG.

2

.

FIG. 4

shows a detailed plan view of a capstan and pinch rollers for driving a drivebar in a first diagonal position.

FIG. 5

is a variation of

FIG. 4

in which the drivebar is shown in a vertical position after rotation of the capstan and pinch rollers about the axis of the capstan.

FIG. 6

is a further variation of the drive of

FIG. 5

where the drivebar has been rotated to the opposite diagonal position.

FIGS. 7-10

show plan views of the stage of

FIG. 1

which have been simplified for the purpose of illustrating the way in which drivebars in accordance with this invention can be driven linearly to locate the stage plate anywhere on the base supporting it.

FIG. 10

also shows rotation of the stage plate about an angle Theta.

FIGS. 11

,

12

and

13

show three alternative arrangements for a ball joint in accordance with this invention with conical bearings therefor for use in the linkage between one of the drivebars and the stage plate.

FIG. 11

shows a partially sectional view of a ball joint linkage with an automatically controlled actuator for preloading the ball joint.

FIG. 12

is a modification of the ball joint linkage of FIG.

11

.

FIG. 13

is another embodiment of the ball joint linkage analogous to

FIG. 12

but the structure for applying force to the cones and the ball in the joint is modified.

FIG. 14

shows the electrical schematic diagram of the control system for the drives employed to position the stage of FIG.

1

.

FIG. 15

is a plan view of the yaw sensor comprising a position sensing detector and light source with the mirrored surface at a proximal and distal Y-axis position.

FIG. 16

is a partial cross section of the yaw sensor comprising a position sensing detector and light source of

FIG. 15

with the mirrored surface at the distal Y-axis position along the line A—A of FIG.

15

.

FIG. 17

is a partial cross section of the yaw sensor comprising a position sensing detector and light source of

FIG. 15

with the mirrored surface at the proximal Y-axis position along the line A—A of FIG.

15

.

FIG. 18

is a plan view of the yaw sensor comprising a position sensing detector and light source with the mirrored surface at a yaw angle of &thgr;.

DETAILED DESCRIPTION OF THE INVENTION

Described herein is a simplified system for measuring and controlling the X-Y stage plate of a stage positioning system. Specifically, two interferometers are used to detect X and Y position, while a position sensing detector is used to determine the yaw angle of the stage.

Referring to

FIG. 1

, a base

10

supports a stage plate

11

, adapted for carrying a work piece

8

slideably supported for moving on the upper surface of the base

10

. The plate moves along the rectilinear x and y axes which rest substantially parallel with the flat surface of base

10

. Base

10

is preferably a very flat, massive stable table comprising a material such as granite, ceramic or steel with a highly polished, extremely flat planar upper surface

9

which carries the X-Y stage plate

11

, with plate

11

supporting on its upper surface a work piece

8

. The lower surface of stage plate

11

is slideably supported on the upper surface

9

of base

10

by low friction supports such as feet

12

composed of a low friction polymeric material such as PTFE (polytetrafluoroethylene) or, alternatively, the feet

12

can be replaced by equivalent support bearings such as air bearings or roller bearings.

To the X-Y stage plate

11

there are pivotably secured three linear, friction drive units X

1

, Y

1

and Y

2

. The drive unit X

1

includes a drive motor M

1

which is located along one side of base

10

and the Y

1

and Y

2

include drive motors M

2

and M

3

which are located along the adjacent side of base

10

spaced well apart from each other to provide three degrees of freedom to the stage plate

11

. The transmission of power for drive unit X

1

on one side of the X

1

-drivebar

16

(comprising ceramic or steel material) is provided by two pinch rollers

13

and

14

and a capstan

15

driven by motor M

1

. The X

1

drive capstan

15

is located on the opposite side of X

1

-drivebar

16

from pinch rollers

13

and

14

so that a friction drive is provided by the capstan

15

and the two rollers

13

and

14

. Preload roller

20

presses down on the top of X

1

-drivebar

16

. Pinch rollers

13

and

14

as well as drive capstan

15

and roller

20

are mounted on carriage

18

to rotate about pivot P

1

along the axis of the shaft of the motor of drive unit X

1

to permit pivoting of drivebar

16

. The rollers

13

and

14

on one side, and the capstan

15

on the other side exert opposing forces which act together to provide friction drive engagement of the capstan

15

with drivebar

16

for reciprocating it longitudinally as capstan

15

turns while concomitantly permitting the drivebar

16

and the carriage

18

carried on the shaft of capstan

15

and rollers

13

and

14

to pivot about the axis P

1

of the shaft of the capstan

15

, thereby permitting rotation of the drivebar

16

on carriage

18

and the shaft of capstan

15

. Drivebar

16

is offset at its inner end

17

where it connects to a linkage including pin

19

secured to stage plate

11

to secure the drivebar

16

to stage plate

11

.

The X and Y position of the stage plate

11

is measured by a laser interferometer system with a pair of bars

50

and

51

secured to two orthogonal sides of plate

11

opposite from the drive units X

1

, Y

1

and Y

2

. Each of bars

50

and

51

has a mirrored surface

73

and

52

respectively for measuring the X-axis and Y-axis displacements of stage plate

11

. Laser beam

75

passes through interferometer

110

, which produces beams

111

and

112

. Together, beams

111

and

112

are grouped as reference

202

for clarity. Beams

111

and

112

are reflected from mirror

73

to interferometer

110

, which produces an output beam

46

directed at receiver

45

.

Laser beam

300

is provided to interferometer

306

, which produces beams

307

and

308

. Together, beams

307

and

308

are grouped as reference

310

for clarity. Beams

307

and

308

emanate from interferometer

306

towards mirror

52

, and are reflected back from mirror

52

to interferometer

306

. Interferometer

306

then produces output beam

304

, which passes to receiver

302

from the interferometer

306

.

Receiver

302

and receiver

45

are optical-to-electrical transducers for converting the laser signal to electronic signals. The receivers

302

and

45

include a lens which focuses the laser beam onto an active chip of a silicon photodiode. Each receiver

302

,

45

(which can be a commercially available product such as the Hewlett Packard 10780A receiver) includes a photodetector, an amplifier and level translator, a line driver, a level sensor (comparator) and local voltage regulators. The receivers

302

and

45

convert the Doppler-shifted laser light into electrical signals that can be processed by the electronic system to determine the X and Y positions of the mirror.

Theta Angle Measurement Apparatus

The &thgr; angle, or “yaw” angle, of the stage plate

11

is determined by directing an incident light beam

314

from a light source in a yaw sensor

312

against mirror

52

at a slightly downward vertical angle, &agr;, (which is not the yaw angle) relative to the plane of the stage plate

11

. A reflected beam

316

is directed back into yaw sensor

312

, where it strikes a position sensing detector. Together, beams

314

and

316

are grouped as reference

318

for clarity. Based upon the yaw angle of the stage

11

, the position sensing detector generates an analog signal, which is sent to the control electronics. The yaw sensor

312

, light source, position sensing detector, and yaw angle are discussed in more detail below within the discussion of

FIGS. 15 through 18

.

Constant Download Friction Drive System

FIGS. 2 and 3

illustrate how the variation in download exerted by the ends of drivebars

16

,

26

and

36

is minimized.

Friction drive units are frequently used in mechanical systems for highly precise positioning of components such as work pieces and the like. A problem with such systems is that the download of the drivebar upon the driven component varies with the cantilevered extension of the drivebar along its path.

FIG. 2

shows an example of a prior art drivebar system. If a drivebar

16

of length L and weight W is extended by a distance B beyond where it is supported by a friction drive unit X with a capstan

15

′ contacting drivebar

16

at the drive point, it will exert a force of W(B-L/2)/B on the driven object: stage plate

11

. The stage plate

11

is shown supported on rollers

12

′ in this modified embodiment. It is assumed that the center of gravity of the drivebar

16

is at L/2, but this is not necessary. The variation of the download force causes a variation in the strain or distorting of the driven object. If the object is driven off center, a varying pitching moment is also applied to the load. It should be noted that a download force at the end of the drivebar

16

is not necessarily a disadvantage because it can be used as a preload. It is the variation in the download force as a function of the extension of the drivebar

16

which causes the problem.

FIG. 3

shows a preload roller

20

′ which is added to the drivebar system of FIG.

2

. The preload-roller

20

′ under the force of a spring

20

″ exerts a force P preloading the drivebar

16

at a distance C from the drive point and exerts a force PC/B on the driven object. (While a spring

20

″ is preferably employed as shown in

FIG. 3

, in the embodiment of

FIG. 1

the spring has been omitted from

FIG. 1

, for convenience of illustration.) If the product PC equals WL/2, then the download force at the distal end of drivebar

16

where it connects to the driven object: stage plate

11

does not change materially as the drivebar

16

extends and retracts. The product PC is a torque exerted about the drive point. Means other than a preload-roller

20

′ can be used to create this torque.

FIGS. 4

,

5

and

6

are simplified illustrations of the rotatable carriages

28

′ (similar to carriages

18

,

28

and

38

in

FIG. 1

) carrying elements of a friction drive adapted to produce linear motion of one of the drivebars

16

,

26

, and

36

showing one of the rotatable drive capstans

15

,

25

and

35

. In this case capstan

25

, which is turned by motor M

2

, is holding the drivebar

26

between the side of capstan

25

and the sides of pinch rollers

23

and

24

. The capstan

25

is supported on a shaft (not shown) which has a bearing within carriage

28

′ which has been modified in that the preload roller

30

has been omitted for ease of illustration. Carriage

28

′ rotates about the shaft carrying capstan

25

so that the drivebar

26

can rotate from the position in

FIG. 4

where it is rotated from a diagonal orientation to a vertical position in

FIG. 5

to a reverse diagonal position in FIG.

6

. It should be noted that for the X

1

-drivebar

16

,

FIG. 4

is analogous to

FIG. 9

,

FIG. 5

is analogous to FIG.

7

and

FIG. 6

is analogous to FIG.

8

.

X, Y, Theta, Three-Linear-Drivebar System

X

1

-drivebar

16

reciprocates in general in parallel with the X axis as indicated in

FIG. 1

, with rotation about P

1

axis away from parallel with the x axis to afford enhanced flexibility of being able to provide positioning of pin

19

and stage plate

11

anywhere within predetermined boundaries of base

10

. As can be seen in

FIGS. 7-10

, the plate

11

can be rotated through an angle theta (as shown in

FIG. 10

) with respect to the X and Y axes using the three drive assemblies of

FIG. 1

in cooperation, where the displacement of drivebars

26

and

36

is unequal.

FIGS. 7-10

show plan views of the stage of

FIG. 1

which have been simplified for the purpose of illustrating the way in which drivebars

16

,

26

and

36

in accordance with this invention can be driven linearly to locate the stage plate

11

anywhere on the base

9

supporting it.

FIGS. 7-9

illustrate the capacity of the drive to position the stage plate

11

in the four remote positions on the base

10

.

FIG. 10

shows rotation of the stage plate

11

about an angle Theta.

FIGS. 7-10

show plan views of the table of

FIG. 1

which have been simplified for the purpose of illustrating the way in which the three drivebars

16

,

26

and

36

can be driven linearly to locate the stage plate

11

anywhere on the base

10

. The work piece

8

shown represents a wafer with chips indicated by squares on the work piece

8

. Rotation through angle Theta is shown by FIG.

10

.

In

FIG. 7

, all the drivebars

16

,

26

and

36

are approximately half way extended with the X

1

-drivebar

16

horizontally oriented, parallel to the X axis, and the Y-drivebars

26

and

36

vertically oriented parallel to the Y axis.

FIGS. 8 and 9

illustrate movement of plate

11

and work piece

8

to two opposite corners of the base

10

, and it is obvious that the other two corners can be reached by analogous operation of the drivebars. To move the plate

11

to the left, the X

1

-drivebar

16

is driven to the left. To move plate

11

to the right, X

1

drivebar

16

is driven to the right. In each case, the drivebars

26

and

36

will pivot about the pivots P

2

and P

3

.

In

FIG. 8

, drive unit X

1

and drivebar

16

has been driven to the left, and drivebars

26

and

36

have been rotated counter-clockwise. All three drivebars

16

,

26

and

36

have been extended to their fullest extension to move the stage plate

11

to its illustrated position—i.e., furthest from the three drive units X

1

, Y

1

, and Y

2

.

In addition, to move the plate

11

to the upper left, drivebar

16

is driven to the left and drivebars

26

and

36

are driven upwardly, with the drivebar

16

rotating clockwise about axis P

1

, and drivebars

26

and

36

rotating counter-clockwise about pivots P

2

and P

3

, as shown in FIG.

8

.

To move the plate

11

to the lower right of the base

10

as shown in

FIG. 9

, the drivebars

26

and

36

are retracted downwardly to near their lowest excursion with the drivebar

16

rotated counter-clockwise from its positions in

FIGS. 7 and 8

, and with drivebars

26

and

36

rotated to their clockwise extreme positions. To move to the lower left of base

10

, the drivebars

26

and

36

will be rotated to their counter-clockwise extremes as the drivebar

16

is moved to the left along the X-axis, rotating slightly clockwise to move the plate

11

to its proximal position.

In

FIG. 9

, drive unit X

1

and drivebar

16

have been rotated counter-clockwise. Drive units Y

1

and Y

2

as well as drivebars

26

and

36

have been rotated clockwise. All three drivebars

16

,

26

and

36

have been retracted to their shortest extension to move the stage plate

11

to its proximal position, i.e. nearest to the three drive units X

1

, Y

1

, and Y

2

.

In

FIG. 10

the rotation from the position in

FIG. 7

has been achieved by driving the second Y-axis drive

36

downwardly while holding the first Y-axis drivebar

26

and the X

1

-axis drivebar

26

stationary.

Pinch rollers

23

and

24

cooperate with capstan

25

to provide friction drive engagement with drivebar

26

which is secured at inner end

27

to pin

29

which connects the drivebar

26

to stage plate

11

. Pinch rollers

23

and

24

as well as drive capstan

25

and roller

30

are mounted on carriage

28

to rotate about pivot P

2

along the axis of the shaft of the motor M

2

of drive unit Y

2

.

Pinch rollers

33

,

34

cooperate respectively with capstan

35

to engage with drivebar

36

which is secured at inner end

37

to pin

39

which connects the drive unit Y

2

to stage plate

11

. Pinch rollers

33

and

34

as well as drive capstan

35

and roller

40

are mounted on carriage

38

to rotate about pivot P

3

along the axis of the shaft of the motor of drive unit Y

1

.

Ball Joint Pivot with Dynamic Preload

A ball-type pivot joint in accordance with this invention employs dynamic preload adjustment of the ball joint. The ball joint includes a ball and a pair of cones which are dynamically preloaded by servo (feedback) control of a preload actuator. Alternate designs are limited by manufacturing and assembly tolerances as well as wear of bearing surfaces or are subject to change due to thermal fluctuations.

Ball type pivot joints provide multiple degrees of freedom about a point at the center of a ball of spherical shape. Heretofore, ball joints have included two basic types including spring loaded joints and spherical bearings.

Spring loaded ball joints include a pair of cones or sockets in which the springs press the cones or sockets against the ball to eliminate backlash. A disadvantage of that system is that when using high preloads for high linear stiffness, there is considerable friction and wear between the ball and the cones or sockets.

The spherical bearing ball joint relies on closely matched inner and outer spherical bearing elements. This type of ball joint has high linear stiffness and low friction, but by the nature of its design must operate with a small clearance between the inner and outer bearings. Such a small clearance increases with wear and the backlash associated with this clearance makes it unsuitable for micropositioning applications.

The joint in accordance with a preferred embodiment of this invention provides the ability to alter the preload between the cones and the ball dynamically to desired levels. This is a very significant advantage for use in micro-positioning applications such as an X-Y stepper stage. When the stage or other device is in motion, the preload can be reduced to provide low friction and wear. When the stage or other device is not in motion, then the preload can be increased to provide zero backlash and high stiffness.

This is achieved using the arrangement depicted in

FIGS. 11-13

. Referring to

FIG. 11

, a drive rod

51

(having a round cross section at the end) with a ball end

50

is located between two cones or sockets

67

and

68

in a fixed member

52

and a preload bar

53

, respectively. The cone

67

, shown in the preferred embodiment in

FIG. 11

as a fixed cone

67

, is affixed to an object to be moved. The preload cone

68

is formed in the lower surface of preload bar

53

. Preload bar

53

is connected to flange

64

, i.e. pivot end

64

. The opposite ends of bar

53

are the flange

64

secured to base

63

and the main body

53

which are flexibly connected together by flexure strip

54

formed between the elongated pair of transverse slots

154

and

254

. In summary, flexure strip

54

provides from an elevational point of view a flexure “point” formed between body

53

and flange

64

which forms the pivot end

64

of the preload bar

53

.

Flange

64

is affixed to base

63

of fixed member

52

by threaded fasteners

65

. Base

63

is a mounting plate having holes therethrough for fastening to the stage plate

11

by threaded fasteners (not shown.) The flexure pivot

54

permits preload cone

68

in member

53

to be moved through a small angle about the flexure pivot

54

. Preload actuator

55

is mounted so as to provide a compressive preload force acting to press down against the preload member

53

and preload cone

68

. Preload actuator

55

comprises a piezoelectric or equivalent actuator such as a pneumatic, hydraulic, voice coil actuator, or the equivalent.

The compressive preload force applied to the cones

67

and

68

and to the ball

50

by using a preload tensioning rod

57

and a preload tensioning nut

59

through a hole in the center of preload actuator

55

. In addition rod

57

passes through a hole

70

in preload member

53

through the center of the preload cone

68

, through ball

50

and through the fixed member

52

and the center of fixed cone

67

as well as a hole

74

in the center of a preload sensor

56

comprising a strain gauge or the equivalent such as a force gauge.

Load sensor

56

is mounted so as to detect the load on the tensioning rod

57

. Line

62

connects the strain drive electronics

58

whose output is connected to the input of the piezoelectric preload actuator

55

. To increase the preload force upon the ball joint, the actuator

55

is extended or increases in thickness to compress the ball joint. To reduce the preload force, the actuator is retracted or made thinner, with the servo

58

having a predetermined level programmed into it electronically. The preload force can be altered at a frequency limited only by the bandwidth of the actuator

55

. Servo

58

has another input

61

from a system controller for the purpose of setting the desired level of preload.

Although

FIG. 11

shows a system using a tensioning rod

57

and a closed-loop servo system, other arrangements can be employed to provide dynamic preload adjustment forces on a ball-type pivot joint.

FIG. 12

shows a modified embodiment of this aspect of the invention where a drive rod

51

′ with a ball end

50

′ is located between two cones or sockets

67

′ and

68

′ in fixed member

52

′ and wedge-shaped preload lever

53

′. Those two cones are shown in

FIG. 12

as a fixed cone

67

′ formed in the upper surface of fixed member

52

′, which are affixed to the object to be moved and a preload cone

68

′.

The preload cone

68

′ is an integral part of wedge-shaped lever

53

′ connected integrally through a flexure pivot

54

′ to pivot end

64

′ which is integral with base

63

′ eliminating the need for a separate preload bar secured to the base

63

′. The flexure pivot

54

′ permits preload cone

68

′ in lever

53

′ to be moved through a small angle about the flexure pivot

54

′.

Preload actuator

55

′ is mounted so as to provide a preload force acting to press laterally against the preload lever

53

′ which drives preload cone

68

′ down against ball end

50

′. The compressive preload is achieved without using a preload tensioning rod and nut

59

since wedge-shaped lever

53

′ is integral with fixed member

52

′. Preload sensor

56

′ is mounted so as to detect the load on the wedge-shaped lever

53

′ from actuator

55

′. Line

62

′ connects the electrical output from strain gauge

56

′ into the input of the preload servo and drive electronics

58

′ whose electrical output signal is connected to the input of the piezoelectric preload actuator

55

′.

To increase the preload force upon the ball joint, the actuator

55

′ is extended or increases in thickness and to reduce the preload force, the actuator is retracted or made thinner, with the servo

58

′ having a predetermined level programmed into it electronically. The preload force can be altered at a frequency limited only by the bandwidth of the actuator

55

′. Servo

58

′ has another input

61

′ from a system controller for the purpose of setting the desired level of preload.

FIG. 13

shows another embodiment analogous to

FIG. 12

where like elements have like functions, but the structure for applying force to the cones and the ball is modified.

In this embodiment drive rod

51

″ has a tall end

50

″ located between two cones or sockets

67

″ and

68

″ in fixed member

52

″ and preload lever

53

″, respectively. Fixed cone

67

″ is formed in the upper surface of fixed member

52

″ which is affixed to the object to be moved. Preload cone

68

″ is formed in the lower surface of preload bar

53

″ connected integrally through a flexure pivot

54

″ to pivot end

64

″ which is integral with base

63

′ eliminating the need for a separate preload bar secured to the base

63

″. The flexure pivot

54

″ permits preload cone

68

″ in member

53

″ to be moved through a small angle about the flexure pivot

54

″.

Preload actuator

55

″ is mounted so as to provide a compressive preload force acting to press down against the preload member

53

″ and preload cone

68

″. The compressive preload is achieved using a preload tensioning arm

72

integral with base

63

″. Preload sensor

56

″ is mounted so as to detect the load on the bar

53

″ from actuator

55

″.

Line

62

″ connects the electrical output from strain gauge

56

″ into the input of the preload servo and drive electronics

58

″ whose electrical output signal is connected to the input of the piezoelectric preload actuator

55

″. To increase the preload force upon the bearing, the actuator

55

″ is extended or increases in thickness and to reduce the preload force, the actuator is retracted or made thinner, with the servo

58