JPH07318341A - Rectilinear motion apparatus equipped with deformation-amount detection means and with displacement scale - Google Patents

Abstract

PURPOSE:To obtain a rectilinear motion apparatus in which a change in the temperature of a rectilinear shaft member due to a change in an environmental temperature, due to heat generated by a motor and the like is obtained as information and in which a tool, a table or the like is positioned precisely by a method wherein a deformation-amount detection means is installed at the rectilinear shaft member. CONSTITUTION:A guide device 10 for rolling-contact-type rectilinear motion is constituted of a track rail 11 as a rectilinear shaft member and of a sliding stand 13 which is installed on the track rail 11 so as to be freely movable via a rolling body 12 such as a ball, a roller or the like. Then, a deformation-amount detection means 1 which detects the absolute deformation amount of the track rail 11 is installed at the track rail. The deformation-amount detection means 1 is constituted of a criterion member 2 which is installed in parallel with a part, where the deformation amount of the track rail 11 is to be detected, and which is held in a state that no force acts and of a displacement sensor 3 which detects the relative displacement amount of the criterion member 2 and that of the track rail 11. Thereby, only when the criterion member 2 and the track rail 11 are arranged in parallel and their relative displacement is detected, the absolute displacement amount in the axial direction of the track rail 11 can be recognized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a linear motion device such as a ball screw device, a ball spline device, a rolling contact type linear motion guide device, or the like, and particularly to detecting the absolute deformation amount of a linear shaft member. The present invention relates to a linear motion device with a possible deformation amount detecting means.

A second aspect of the present invention relates to a linear motion device used in, for example, semiconductor manufacturing equipment, machine tools, various inspection devices, and the like, and particularly to a linear motion device with a displacement scale for detecting relative displacement between a track rail and a slide. Regarding

[0003]

2. Description of the Related Art In recent years, there has been an increasing demand for higher precision in fine processing as represented by semiconductor technology. Since NC (Numerical Control) was developed, position control has dramatically improved the machining accuracy. In such position control, since the table position and tool position are all determined by the product shape, the feed amount and procedure of the processing table and tool are determined in advance, and the position of the drive mechanism side, that is, the feed side is monitored by a rotary encoder or the like. Most of them were controlled.

[0004]

However, what monitors the position of the sending side in this way merely estimates the actual position of the tool or table being sent, and the actual position of the tool or table. The location information of does not come out. For example, the control system on the sending side may malfunction due to disturbance such as lightning, and if it malfunctions, the actual position and the control of the drive system may not match.

Even if the feeding side is controlled with high precision, backlash of the feeding mechanism of the tool or table,
For example, the actual position of the tool or the table changes due to backlash or the like when the ball screw is reversed. In particular, the heavier the load, the greater the effect of backlash.

Further, even if mechanical backlash and the like can be completely eliminated, the actual positions of the tool and the table will change due to changes in environmental temperature and frictional heat generation of various parts of the machine during work.

From this point of view, it is required to know the absolute position of the tool, table, etc. on the sending side.

In order to know the absolute position, it is necessary to know the amount of deformation of each part, but all structural members are deformed by heat and force, and if the absolute amount of deformation of each part is not detected sequentially, the absolute position Can't know.

On the other hand, attention has been focused on a rolling contact type linear motion guide device for a linear guide portion such as a tool or a table, and positioning control is performed by detecting the position of a guided member such as a table by a displacement scale such as a linear scale. Is going.

However, conventionally, a special occupying space for the displacement scale is required, and the space is restricted.

Further, it is difficult to make both the track rail and the displacement scale parallel to each other with required accuracy. That is, it is necessary to set both the displacement scale and the inclination of the mounting surface of the track rail.

The present invention has been made to solve the above-mentioned problems of the prior art, and the first object of the present invention is to linearly move a ball screw, a ball spline, a linear guide device, or the like, which has a great influence on the positional accuracy. An absolute deformation amount detecting means is provided on the linear shaft member of the apparatus so that the actual deformation amount can be monitored.

A second object of the present invention is to provide a displacement scale integrally with the track rail or the slide base itself so that a special occupied space is not required and the displacement amount can be output as information with high accuracy. It is to provide a linear motion device with a displacement scale that can be used.

[0014]

[Means for Solving the Problems]

-First Invention- To achieve the above object, in the first invention,
In a linear motion device including a linear shaft member and a moving member that is movable along the linear shaft member, a deformation amount detection unit that detects an absolute deformation amount in the axial direction of the linear shaft member. It is characterized by being provided.

The deformation amount detecting means is provided in parallel with the portion of the linear shaft member where the deformation amount is to be detected, and is held in a state in which no force acts, and the relative displacement between the reference member and the linear shaft member. Displacement sensor for detecting the amount, composed of one end of the reference member fixed to one end of the portion to detect the deformation amount of the linear shaft member in a cantilever state to hold in a free state other than the fixed end, Further, the displacement detecting means detects the relative displacement between the free end of the reference member and the other end of the portion where the deformation amount of the linear shaft member is to be detected, and the absolute deformation amount of the linear shaft member is calculated based on the detected relative displacement. Characterized by seeking.

The reference member is a member that does not have thermal expansion.

Further, the reference member is a member having a known coefficient of thermal expansion, the length of the reference member is obtained from the temperature of the reference member at the time of detection, and the absolute deformation of the linear shaft member is obtained from the reference length of the reference member. It is characterized by determining the quantity.

The displacement sensor is characterized by being constituted by a proximity sensor.

Further, the displacement sensor includes an elastic member which is elastically deformable in the axial direction, and displacement or strain detecting means for converting the axial displacement or strain of the elastic member into an electric signal for detection.
It is characterized by comprising.

It is preferable that the elastic member has a structure that is elastically deformable in the axial direction and is rigid in the other directions, and that it has a thin wall portion that can be displaced in the axial direction. Further, it is preferable to attach a strain gauge to the thin portion as the strain detecting means.

-Second Invention- In the second invention, a linear motion guide device comprising a track rail and a slide base slidably provided on the track rail via rolling elements. In at least one of the track rail and the slide base, a displacement scale for measuring the relative displacement between the track rail and the slide base is provided, and the displacement scale is integrated with the track rail or the slide base itself. It is characterized by

Further, it is characterized in that a reading means for reading the displacement scale is provided on the slide base or the track rail so as to face the displacement scale.

The displacement scale is a pattern as position information attached to the track rail or the slide, and the pattern is composed of portions having different physical characteristics, and can be identified by the difference in the physical characteristics. To do.

The physical property is a magnetic property.

It is preferable that the pattern utilizing magnetic properties is formed by portions having different eases of entering magnetic flux.

The portions where the easiness of entering the magnetic flux is different are
It is characterized in that minute irregularities are formed on the surface of the base material of the displacement scale, which is a magnetic body, so that the magnetic flux easily passes through the convex portions and the magnetic flux hardly passes through the concave portions.

The surface of the fine irregularities is characterized by being coated with a non-magnetic material to form a smooth surface.

Further, the invention is characterized in that the non-magnetic material coating surface is coated with an abrasion resistant material.

The portions having different magnetic flux penetration easiness may be constituted by partially modifying the metal structure of the same base material into different structures.

The metal structure of the base material is austenite,
It is preferable to perform quenching with a laser beam to give a pattern of martheinsite having different magnetic properties.

The physical property may be an electrical property such as an inductance or an optical property such as a moire fringe.

[0032]

In the linear motion device with the deformation amount detecting means having the above structure, the absolute deformation amount of the linear shaft member can be obtained as the detection information by providing the deformation amount detecting means.

More specifically, the absolute amount of deformation of the linear shaft member is detected by detecting the relative displacement between the reference member and the linear shaft member.

Further, if the reference member is made of a member having no thermal expansion, the absolute deformation amount due to heat can be detected.

Further, by using a member having a known coefficient of thermal expansion as the reference member, even if the reference member thermally expands, the absolute deformation amount at the time of detection can be detected if the temperature at the time of detection is known. it can.

In the linear motion device with the displacement scale having the above-mentioned structure, the displacement scale is integrally provided, so that the occupied space for mounting the displacement scale becomes unnecessary. And the magnetic of the pattern attached to the track rail or slide,
Position information is detected by the difference in physical characteristics such as optical or electrical characteristics.

[0037]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to illustrated embodiments.

-First Invention- FIG. 1 (a) shows a linear motion device according to an embodiment of the first invention, and this embodiment is applied to a rolling contact type linear motion guide device 10. It is a thing. The rolling contact type linear motion guide device 10 includes a track rail 1 as a linear shaft member.
1 and a slide base 1 as a moving member movably provided on the track rail 11 via rolling elements 12 such as balls and rollers.
3 and. Further, a deformation amount detecting means 1 for detecting an absolute deformation amount of the track rail 11 is provided on the track rail 11 as a linear shaft member.

The example shown in FIG. 1 (b) is the deformation amount detecting means 1
Is a reference member 2 that is provided in parallel with a portion of the track rail 11 where the deformation amount is to be detected and is held in a state in which no force acts, and a displacement sensor that detects the relative displacement amount of the reference member 2 and the track rail 11. 3 and.

The reference member 2 is inserted into the through hole 14 provided in the track rail 11, and one end thereof is fixed to a portion where the deformation amount of the track rail 11 is to be detected, in this embodiment, one end of the entire length of the track rail 11, It is in a cantilever state where it is held in a free state except for the fixed end. The displacement sensor 3 is attached to the other end of the track rail 2 to detect the relative displacement between the reference member 2 and the displacement sensor 3.

As the reference member 2, a member whose length at the time of detection is known is used, but in consideration of the influence of thermal expansion, a superinvar rod having no coefficient of thermal expansion or a negligibly small superinvar bar is used. Is preferred.

Even if the super invar rod is not used, the length of the reference member 2 at the time of detection may be known, and if the length at a predetermined temperature and the coefficient of thermal expansion are known, the temperature at detection may be If known, the amount of thermal expansion of the reference member at the time of detection can be known. If this amount of thermal expansion is corrected, the track rail 1
It is possible to know the absolute deformation amount of 1.

Further, if the thermal expansion coefficients of the reference member 2 and the track rail 11 are made different, if the lengths of both members at a predetermined temperature are known in advance, from the relative displacement detected by the displacement sensor 3, It is also possible to determine the temperature. Therefore, the absolute deformation amount of the track rail 11 in the axial direction can be known only by arranging the reference member 2 and the track rail 11 in parallel and detecting the relative displacement thereof.

FIGS. 1 (c) to 1 (f) show various modifications of holding means for holding the reference member 2 in a state where no force is applied.

In FIG. 6C, a leaf spring 41 is used, and in FIG. 7D, an elastic member 42 such as silicon rubber is used to hold the reference member 2 so that it can be displaced only in the axial direction without being displaced vertically. It was done. As the leaf spring 41, as shown in FIG. 2 (e), a thin annular flat plate structure is used to fix the outer end portion 41a to the inner circumference of the through hole 14 of the track rail 11 and the inner end portion 41b to the outer circumference of the reference member 2. It is fixed.

In FIG. 6 (f), the liquid 43 is put in the through hole 14, and the pipe-shaped reference member 2 is floated in the liquid 43 and held so as to be displaceable only in the axial direction.

As the displacement sensor 3, in addition to the proximity sensor, a displacement sensor combining an elastic member 3a and a strain gauge 3b attached to the surface of the elastic member 3a as shown in FIG. 1 (g) is used. You can also That is, the elastic member 3a has an annular flat plate structure similar to the plate spring 41 shown in FIG. 1 (c), and the strain amount of the elastic member 3a that deforms in accordance with the relative displacement between the reference member 2 and the track rail 2 is Strain gauge 3b
The relative displacement amount is obtained by detecting the relative displacement amount.

FIG. 2 is a schematic diagram showing the basic structure of the displacement sensor shown in FIG. 1 (f). That is, the displacement sensor 3 includes an elastic member 3a that is elastically deformable in the axial direction, and a detecting unit 102 that detects the axial displacement or strain of the elastic member 3a by converting it into electric vibration. The elastic members 3a are fixed members 100 arranged to face each other.
And the movable member 101, the fixed member 100 and the movable member 101 are connected to each other. Fixed member 1
00 is fixed to one end of the reference member 2, and the movable member 101 is fixed to the track rail 11. And the reference member 2
Elastic member 3a according to the relative displacement between the track rail 11 and
Is elastically deformed, and the strain or displacement of the elastic member 3a is converted into an electric signal by the detection means 102 to detect the displacement.

As the elastic member 3a, a leaf spring 41 for holding the reference member 2 as shown in FIG. 1 (c) can be used. That is, the relative displacement between the reference member 2 and the track rail 11 can be known by detecting the displacement and strain of the leaf spring 41.

3 (a) to 3 (e) show various modified examples of the detection means 102 of the displacement sensor 3 described above.

As shown in FIG. 3 (a), the strain gauge 3b is used as the detecting means 102 to detect the strain of the elastic member 3a, as in FIG. 1 (g).

In the structure shown in FIG. 3 (b), the displacement of the elastic member 3a is detected by using a voltage type sensor 3c which detects the displacement as a voltage change by using a piezoelectric element or an electrostrictive element as the detecting means. It is a thing.

FIG. 3C shows an electromagnetic induction type sensor 3d such as a differential transformer or an eddy current sensor as a detecting means.
Is used to detect the displacement of the elastic member 3a.

In FIG. 3 (d), a capacitance type gap sensor 3e is used as a detecting means to detect the displacement of the elastic member 3a by converting it into an electric capacitance.

In FIG. 3 (e), the displacement of the elastic member 3a is detected by using the optical fiber type sensor 3f of the optical interference type as the detecting means.

In the above embodiment, the linear motion device is applied to the rolling contact type linear motion guide device as an example, but the linear motion device is not limited to the rolling contact linear motion guide device. 4 and a feed screw device 3 as shown in FIG.
The same applies to 0.

That is, the spline device 20 includes a spline shaft 21 as a linear shaft member and the spline shaft 2
An outer cylinder 23 is movably fitted to a roller shaft 1 such as a ball or a roller, and a ball screw device 30 includes a screw shaft 31 as a linear shaft member and a ball shaft or the like on the screw shaft 31. The nut 33 is movably fitted through the moving body 32.

The deformation amount detecting means is provided on the spline shaft 21 and the screw shaft 31 as the linear shaft members.

-Second Invention- FIGS. 6 (a) to 6 (c) show the basic structure of a linear motion guide device with a displacement scale according to the present invention. That is, the linear motion guide device includes a track rail 101 and a slide base 103 slidably provided on the track rail 101 via rolling elements 102 such as balls and rollers.

The track rail 101 is integrally provided with a displacement scale 104 for measuring the relative displacement between the track rail 101 and the slide base 103. The displacement scale 104 is formed over the entire length of the track rail 101, and a reading means 106 for facing the displacement scale 104 and reading the positional information attached to the displacement scale 104 is provided on the sliding base 103 side. There is.

The displacement scale 104 may be provided on the upper surface of the track rail 101 as shown in FIG.
It may be provided on the side surface of the track rail 101 as shown in (c).

Further, as shown in FIG. 6D, the displacement scale 104 may be provided on the slide base 103 side and the reading means 106 may be provided on the track rail 101.

The positional information attached to the displacement scale 104 is a pattern 105. The pattern 105 is composed of portions having different physical characteristics, and can be identified by the difference in the physical characteristics. The information that can be identified by the difference in physical characteristics includes, for example, a normal scale that can be visually identified by the difference in reflectance, and other information such as magnetic characteristics, optical characteristics, and electrical characteristics. Various detectable properties can be applied.

The physical characteristics to be used are selected depending on the specific use such as the place of use, but in the case of a machine tool, it is expected that the surface of the displacement scale 104 will be contaminated by dust or oil. It is preferable to use a magnetic property or the like that is disadvantageous in optical characteristics and is not affected by dirt and the like.

As the magnetic properties, as shown in FIG.
As the pattern 105 of the displacement scale 104, it is preferable that the pattern 105 is configured by a portion having a different ease of entering the magnetic flux φ into the scale surface. In the portion where the magnetic flux φ is different in ease of entering, fine unevenness is formed on the surface of the base material of the track rail 101 which is a magnetic body, and the magnetic flux φ easily passes through the convex portion 105A and the magnetic flux φ passes through the concave portion 105B. Use a difficult configuration. Then, the difference in the magnetic property of the surface of the displacement scale 104 is detected by the reading means 106 to obtain the position information.

In the illustrated example, the Hall element 1061 is used as the reading means 106, the Hall element 61 is provided so as to face the surface of the displacement scale 104, and the bias magnet 62 is provided on the back side of the Hall element 61. Also, FIG. 7 (b)
A flux concentrator 63 is provided between the Hall element 61 and the displacement scale 104 to concentrate the magnetic flux to enhance the sensitivity.

As shown in FIG. 8, the surfaces of the fine projections 105A and the recesses 105B have a non-magnetic material 10 such as copper.
It is preferable to coat 7 in advance. further,
In this embodiment, the coating surface of the non-magnetic material 107 is coated with a wear resistant material 108 such as chromium, and the coating surface of the wear resistant material 108 is a smooth surface.

In FIG. 9, the metal structure of the same base material 109 is partially transformed into a different structure in order to form the above-mentioned patterns having different magnetic properties.

For example, if the metal structure of the base material 109 is austenite and quenching is performed by laser light, a pattern of the marthein site 109a having different magnetic properties can be formed very easily, and the surface is smooth. It is possible to perform processing extremely simply and with high precision, as the processing for

Other physical properties may be electrical properties such as inductance, or optical properties such as moire fringes.

In the linear movement guide device with the displacement scale having the above structure, the displacement scale 104 is integrally provided, so that the occupied space for mounting the displacement scale 104 becomes unnecessary.

Then, the pattern 1 attached to the track rail 101
The positional information is detected by the difference in the physical properties such as the magnetic properties of 05.

[0073]

According to the first aspect of the present invention, by providing the temperature detecting means on the linear shaft member, the temperature change of the linear shaft member due to the environmental temperature change, the heat generation of the motor or the frictional heat generation of each portion during the work is informed. As a result, it is possible to determine the position of the tool, the table, etc. and perform the precision more accurately.

According to the second aspect of the present invention, by disposing the displacement scale integrally on at least one of the track rail and the slide base, a special occupied space for the scale as in the conventional case is unnecessary.

Occasionally, it is not necessary to consider the parallelism between the scale and the track rail or the slide as in the conventional case, and the displacement amount can be output as information with high accuracy.

[Brief description of drawings]

FIG. 1 shows various structural examples of a deformation amount detecting means by taking a rolling contact type linear motion guide device as an example.
(a) is a basic configuration diagram, (b) is a sectional view showing a specific configuration example of a track rail provided with a deformation amount detecting means, (c),
(d) is a cross-sectional view showing an example of the holding means for the reference member,
(e) is a plan view of the leaf spring shown in (c) of the same figure, (f) is a cross-sectional view showing another configuration example of the holding means, and (g) is a schematic view showing an example of a displacement sensor. .

FIG. 2 is a sectional view schematically showing a basic configuration of the displacement sensor shown in FIG. 1 (g).

3 (a) to 3 (e) are schematic cross-sectional views showing various modifications of the displacement sensor of FIG.

FIG. 4 is a cross-sectional view showing an example in which a spline device is provided with deformation amount detection means.

FIG. 5 is a sectional view showing an example in which a feed screw device is provided with a deformation amount detecting means.

6 (a) and 6 (b) are schematic perspective views of a linear motion guide device with a displacement scale according to the present invention.

7 (a) and 7 (b) are diagrams schematically showing an example of a configuration of a displacement scale and a reading unit.

8 is a sectional view schematically showing a state in which the surface of the displacement scale of FIG. 7 is coated.

FIG. 9 is a cross-sectional view showing another configuration example of the pattern on the surface of the displacement case.

[Explanation of symbols]

 1 Deformation amount detection means 2 Reference member 3 Displacement sensor 41 Leaf spring 42 Elastic member 43 Liquid 10 Rolling contact type linear motion guide device 11 Track rail (linear shaft member) 12 Rolling body 13 Sliding table 20 Spline device 21 Spline shaft (linear line) Shaft member) 22 Rolling body 23 Outer cylinder 30 Feed screw device 31 Screw shaft 32 Rolling body 33 Nut 101 Track rail 102 Rolling body 103 Sliding table 104 Displacement scale 105 Pattern 105A, 5B Convex / concave 106 Reading means 107 Magnetic material 108 wear resistant material 109 base material 109a austenite φ magnetic flux

Claims (23)

[Claims] 1. A linear motion device including a linear shaft member and a moving member movably provided along the linear shaft member, wherein an absolute deformation amount of the linear shaft member in the axial direction is detected. A linear motion device with deformation amount detecting means, characterized in that deformation amount detecting means is provided. 2. A deformation amount detecting means is provided in parallel with a portion of the linear shaft member where the deformation amount is to be detected, and is held in a state in which no force acts, and the reference member and the linear shaft member. A cantilever state in which one end of the reference member is fixed to one end of a portion where the deformation amount of the linear shaft member is to be detected and the rest is held in a free state other than the fixed end. Further, the displacement detecting means detects the relative displacement between the free end of the reference member and the other end of the portion where the deformation amount of the linear shaft member is to be detected, and the absolute deformation of the linear shaft member is detected based on the detected relative displacement. The linear motion device with deformation amount detection means according to claim 1, wherein the amount is obtained. 3. The linear motion device with deformation amount detecting means according to claim 2, wherein the reference member is a member having no thermal expansion. 4. The reference member is a member having a known coefficient of thermal expansion, the length of the reference member is obtained from the temperature of the reference member at the time of detection, and the absolute deformation of the linear shaft member is calculated from the reference length of the reference member. The linear motion device with deformation amount detecting means according to claim 2, wherein the amount is obtained. 5. The linear motion device with deformation amount detecting means according to claim 1, 2, 3 or 4, wherein the displacement sensor is a proximity sensor. 6. The displacement sensor comprises an elastic member that is elastically deformable in the axial direction, and displacement or strain detection means for detecting the axial displacement or strain of the elastic member by converting it into an electrical signal. The linear motion device with deformation amount detecting means according to claim 1, 2, 3, or 4. 7. The linear motion device with deformation amount detecting means according to claim 6, wherein the elastic member is elastically deformable in the axial direction and is rigid in the other directions. 8. The linear motion device with deformation amount detecting means according to claim 7, wherein the elastic member is provided with a thin portion that is displaceable in the axial direction. 9. The linear motion device with deformation amount detecting means according to claim 8, wherein a strain gauge is attached to the thin portion. 10. A linear motion device comprising a track rail and a slide base slidably provided on the track rail via rolling elements, wherein at least one of the track rail and the slide base is provided. ,
A linear motion guide device with a displacement scale, wherein a displacement scale for measuring relative displacement between the track rail and the slide base is provided, and the displacement scale is integrated with the track rail or the slide base itself. 11. A linear motion guide device with a displacement scale according to claim 10, wherein the slide base or the track rail is provided with a reading means for reading the displacement scale by facing the displacement scale. 12. The displacement scale is a pattern as position information attached to a track rail or a slide, and the pattern is composed of portions having different physical characteristics, and can be identified by the difference in the physical characteristics. 10. A linear motion guide device with a displacement scale according to 10. 13. The linear motion device with a displacement scale according to claim 12, wherein the physical property is a magnetic property. 14. The linear motion device with a displacement scale according to claim 13, wherein the pattern utilizing magnetic properties is formed by portions having different easiness of entering magnetic flux in a direction perpendicular to the displacement scale surface. 15. The portion where the easiness of entering the magnetic flux is different,
The linear motion guide device with a displacement scale according to claim 14, wherein fine irregularities are formed on the surface of the base material of the displacement scale that is a magnetic body, and the magnetic flux easily passes through the convex portions and the magnetic flux hardly passes through the concave portions. . 16. The linear motion device with a displacement scale according to claim 15, wherein the surface of the fine irregularities is coated with a non-magnetic material. 17. The linear motion device with a displacement scale according to claim 16, wherein the wear resistant material is coated on the non-magnetic material coated surface. 18. The linear motion device with a displacement scale according to claim 15, wherein the coating surface is a smooth surface. 19. The portion where the easiness of entering a magnetic flux is different,
The linear motion device with a displacement scale according to claim 14, wherein the metal structure of the same base material is partially modified to have a different structure. 20. The linear motion device with a displacement scale according to claim 16, wherein the metal structure of the base material is austenite, and the pattern is made of martheinsite having different magnetic properties by quenching with laser light. 21. The linear motion device with a displacement scale according to claim 12, wherein the physical property is inductance. 22. The linear motion device with a displacement case according to claim 12, wherein the physical property is an optical property. 23. The linear motion device with a displacement scale according to claim 22, wherein the pattern constituting the displacement scale utilizing optical properties is a moire fringe.