JP2001041711A - Method and apparatus for correcting table deflection of image measuring machine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method and apparatus for correcting table deflection of an image measuring machine capable of measuring accurately regardless of the position of a table surface of the image measuring machine. SOLUTION: The X-, Y- and Z-axial positions of arbitrary four points on the fringe part of a table surface are detected respectively so that a reference face position of the table can be calculated, errors ΔZn, n to the reference face position of Z-axial position Zn, n on each lattice point (Xn, Yn) of the table surface are calculated respectively, and the calculated errors ΔZn, n are stored in a work memory corresponding to each lattice point. At the time of work measuring, X-, Y- and Z-axial positions on a work measuring point put on the table are entrapped (Step S21), Z-axial position errors ΔZ of the coordinate (X, Y) on the basis of ΔZn, n value corresponding to the lattice point according to the coordinate (X, Y) are calculated by using the liner interpolation or the like if necessary (Step S24, S27), and the table deflection correction is conducted by subtracting ΔZ value from the Z-axial position (Step S25).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for correcting a table deflection of an image measuring machine, and more particularly to a non-contact type for acquiring necessary measurement information by imaging an object to be measured by an imaging device such as a CCD camera. The present invention relates to a method and an apparatus for correcting a table deflection of an image measuring machine.

[0002]

2. Description of the Related Art In an image measuring device for measuring the size and shape of a work (object to be measured) placed on a table, an image pickup device such as a CCD camera is used to measure the position of the work placed on the table where the measurement is desired. The work is measured by adjusting the focus of the image pickup apparatus so that the objective lens of the image pickup apparatus always focuses on the measurement surface of the work, and then picking up an image of the work (Japanese Patent Laid-Open No. 8-247719). No.).
Further, as one of the methods for performing the focus adjustment, there is a contrast method (Japanese Patent Application Laid-Open No. 9-304684, etc.), and in this method, the image data of the image data obtained by imaging the work with the imaging device is used. The contrast of the contour is obtained, and the position of the imaging device with respect to the work is determined so that the contrast is maximized.

After the focus adjustment is completed so that the upper surface of the work is in focus by the above-described contrast method, so-called transmission illumination is performed by irradiating light from below the table to the objective lens of the imaging apparatus via the table. In addition, the contrast of the contour of the workpiece can be increased, and the edge detection can be performed with high accuracy, so that highly accurate dimension measurement can be performed. For this reason, glass is generally used for the table.

[0004] When measuring the size and shape of a work with an image measuring machine, the table surface on which the work is placed is bent because the imaging device is in focus at any measurement position of the work. It is desirable that there be no plane and the plane perpendicular to the optical axis of the imaging device.

[0005]

However, in recent years,
As this type of image measuring device, the demand for an image measuring device for measuring an object having a measurement range of 800 to 1500 mm, such as a large printed wiring board, a ceramic substrate, a plasma display, and a precision resin molded product, has been increasing. Yes,
The size of the glass table is also increasing, and the table surface is apt to bend.

As a result, the peripheral portion of the table supported by the gantry does not bend, but the center is bent by the weight of the table or the like. There is a problem that an unfocused portion occurs, which makes it difficult to measure a workpiece and easily causes an error.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method and an apparatus for correcting a table deflection of an image measuring device, which enable accurate measurement regardless of the position on the table surface of the image measuring device.

[0008]

According to a first aspect of the present invention, there is provided a method for correcting a bending of a table of an image measuring machine, the method comprising: measuring an image of a size and a shape of an object to be measured placed on the table; A table deflection correction method for a measuring machine, wherein a Z-axis direction position of a peripheral portion of a table surface is measured as a reference surface position, and a Z-axis direction position at a plurality of XY coordinate positions other than the peripheral portion of the table surface is measured. Measure, calculate the error between the reference plane position and the measured Z-axis position at the plurality of XY coordinate positions, create an error file, and measure the measurement points of the DUT placed on the table. Is measured, and the measured Z-axis position is corrected using the error file.

According to the first aspect of the present invention, the position of the periphery of the table surface in the Z-axis direction is measured as a reference surface position, and the reference surface position and a plurality of XY are measured.
An error file is created by calculating an error between the coordinate position and the Z-axis direction position, an Z-axis direction position at a measurement point of the DUT placed on the table is measured, and the measured Z-axis direction position is determined. Since the correction is performed using the error file, it is possible to easily correct the error in the Z-axis direction position due to the bending of the table, to improve the measurement accuracy in the Z-axis direction position, and to perform the X, Y-axis pitching error and the like. Can be corrected, and the measurement accuracy of the image measuring device can be improved.

According to a second aspect of the present invention, in the method of correcting a table deflection of the image measuring device according to the first aspect, the error file is stored in a form of a look-up table. .

According to a third aspect of the present invention, in the method of correcting a table deflection of the image measuring device according to the first aspect, the error file is stored in the form of a calculation formula.

According to a fourth aspect of the present invention, in the method of correcting a table deflection of the image measuring device according to any one of the first to third aspects, the reference plane position is set to the position of the table surface. The measurement is performed based on the Z-axis direction reference positions of a plurality of points on the peripheral edge.

According to the fourth aspect of the present invention, the reference plane position of the table is measured based on the Z-axis reference positions of a plurality of points on the periphery of the table surface. Measurement accuracy of the reference plane position can be improved.

According to a fifth aspect of the present invention, there is provided a method of correcting a table deflection of an image measuring machine according to any one of the first to fourth aspects, wherein the measurement of the reference plane position and the plurality of the plurality of times are performed. The measurement of the position in the Z-axis direction at the XY coordinate position is performed by a laser displacement meter.

According to a fifth aspect of the present invention, the measurement of the reference plane position of the table and the measurement of the Z-axis position at a plurality of XY coordinate positions of the table surface are performed by the laser displacement meter. Since the measurement is performed, the measurement accuracy of the bending of the table can be improved.

According to a sixth aspect of the present invention, there is provided a table deflection correcting apparatus for an image measuring device for measuring a size and a shape of an object placed on a table. A first device for measuring a Z-axis direction position of a peripheral portion of a table surface as a reference surface position and measuring a Z-axis direction position at a plurality of XY coordinate positions other than the peripheral portion of the table surface. Measurement means; error file creation means for calculating an error between the reference plane position and the measured Z-axis positions at the plurality of XY coordinate positions to create an error file; and storage means for storing the error file Second measuring means for measuring a Z-axis position at a measurement point of the object placed on the table; and correcting the measured Z-axis position using the error file. Characterized in that it comprises a stage.

According to a sixth aspect of the present invention, an error between the reference plane position and a plurality of X-Y coordinate positions in the Z-axis direction is calculated and stored as an error file. The Z-axis position at the measurement point of the object placed on the measuring object is measured, and the measured Z-axis position is corrected using the error file. The correction can be easily performed, and the measurement accuracy of the position error in the Z-axis direction can be improved, and the pitching error in the X- and Y-axis directions can be corrected, so that the measurement accuracy of the image measuring device can be improved. be able to.

According to a seventh aspect of the present invention, in the table deflection correcting apparatus for an image measuring machine according to the sixth aspect, the storage means stores the error file in the form of a look-up table. It is characterized by the following.

According to a eighth aspect of the present invention, in the table bending correction apparatus for an image measuring machine according to the sixth aspect, the storage means stores the error file in the form of a calculation formula. It is characterized by.

In a ninth aspect of the present invention, there is provided a table deflection correcting apparatus for an image measuring device according to any one of the sixth to eighth aspects, wherein the first measuring means is adapted to control the reference deflection. The surface position is measured based on the Z-axis direction reference positions of a plurality of points on the periphery of the table surface.

According to the ninth aspect of the present invention, the first measuring means determines the reference surface position of the table based on a plurality of Z-axis direction reference positions on the peripheral portion of the table surface. Since the measurement is performed, the measurement accuracy of the reference plane position of the table can be improved.

According to a tenth aspect of the present invention, there is provided a table deflection correcting apparatus for an image measuring machine according to any one of the sixth to ninth aspects, wherein the first measuring means is a laser displacement meter. Characterized by comprising:

According to the tenth aspect of the present invention, since the first measuring means comprises a laser displacement meter, the accuracy of measuring the deflection of the table can be improved.

[0024]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a perspective view showing a configuration of an image forming system to which a method for correcting a table deflection of an image measuring device according to an embodiment of the present invention is applied.

In FIG. 1, the image measurement system includes a non-contact type image measurement device 1, a computer system 2 which controls the driving of the image measurement device 1 and executes necessary data processing, and prints the measurement results. Out printer 3.

The image measuring machine 1 has a gantry 11, and a table 13 on which a work (measured object) 12 is mounted is mounted on the gantry 11. This table 13 is, for example, 1
It is made of a glass flat plate having a size of 500 mm × 1500 mm and is driven in the Y-axis direction by a Y-axis driving mechanism (not shown). Support arms 14 and 15 extending upward are fixed to the center of both sides of the gantry 11.
An X-axis guide 16 is fixed so as to connect both upper ends of the X-ray guides 4 and 15.

The X-axis guide 16 has an imaging unit 1
7 are supported. The imaging unit 17 is driven on the X-axis guide 16 in the X-axis direction by an X-axis driving mechanism (not shown). At the lower end of the imaging unit 17, a CCD camera 18 is mounted so as to face the table 13 and have its optical axis perpendicular to the surface of the table 13.
A focusing mechanism, a Z-axis drive mechanism for moving the CCD camera 18 in the Z-axis direction, and the like (both not shown) are built in the interior of the camera 7. Further, a laser displacement meter or a contact type position sensor (not shown) can be attached to the imaging unit 17 instead of the CCD camera 18.

The CCD camera 18 determines the position of the measurement point of the work 12 in the Z-axis direction based on the determination of the focal point based on the contrast of the screen. The in-focus determination by the CCD camera 18 is difficult in imaging in a very small area, and requires imaging in a relatively large area. Therefore, a laser displacement meter is used for measuring the position in the Z-axis direction in a minute range. In addition, the CCD camera 18 has a depth of focus of, for example, 1 to several μm. If the depth of focus is less than this range, the focus is always determined, so that a measurement error occurs. On the other hand, a laser displacement meter uses a laser holo scale. For example, the position can be measured with a resolution of about 0.01 μm in the Z-axis direction.

The X-axis guide 1 is provided on the support arms 14 and 15.
A cover 19 for protecting the imaging unit 6 and the imaging unit 17 is attached.

Further, a transmission illumination device (not shown) is provided below the table 13. This device irradiates light from below the measurement table 13 toward the objective lens of the CCD camera 18 in order to increase the contrast of the outline of image data obtained by imaging the work 12 with the imaging unit 17. .

In addition, in order to measure the relative position of the CCD camera 18 with respect to the work 12, the X-axis driving mechanism, the Y-axis driving mechanism, and the Z driving mechanism are respectively provided with the X-axis of the CCD camera 18 or the laser displacement meter. It incorporates an X-axis encoder 40 for detecting the directional position, a Y-axis encoder 41 for detecting the Y-axis position of the table 13, and a Z-axis encoder 42 for detecting the Z-axis position of the CCD camera 18 (FIG. 2).

The computer system 2 includes a computer main body 21, a keyboard 22, a joystick 23, a mouse 24, and a CRT display 25.

FIG. 2 is a block diagram showing a configuration of the computer main body 21 in the image measurement system of FIG.

In FIG. 2, the computer main body 21 is C
The CPU 31 includes a multi-valued image memory 32
And a display control unit 33 are connected. The multi-valued image memory 32 is connected to the display control unit 33, and the multi-valued image memory 32 is connected to the CCD camera 18 via an interface (hereinafter referred to as “I / F”) 34. Is connected to a CRT display 25. The CPU 31 is connected to a laser displacement meter via an I / F 35, a joystick 23 and a mouse 24 via an I / F 36, and further includes a program memory 37, a work memory 38, and an X-axis encoder 40. , Y-axis encoder 41 and Z-axis encoder 42 are connected.

With such a configuration, the CCD camera 18
Is input to the multi-valued image memory 32 via the I / F 34, and the multi-valued image information stored in the multi-valued image memory 32 is displayed on the CRT display 25 via the display control unit 33. Is done. On the other hand, X and Y axis direction position information input from the joystick 23 or the mouse 24 is input to the CPU 31 via the I / F 36.

Further, the X-axis encoder 40, the Y-axis encoder 41 and the Z-axis encoder 42
Alternatively, information on the X-axis position of the laser displacement meter, the Y-axis position of the table 13, and the Z-axis position of the CCD camera 18 are input to the CPU 31. The position information in the Z-axis direction input from the laser displacement meter is sent to the CPU 31 via the I / F 35.
Is input to

The CPU 31 displays a pointer at a position on the CRT display 25 designated by the joystick 23 or the mouse 24 in accordance with a program stored in the program memory 37, and generates an error described later based on click information from the mouse 24. It executes file creation processing, table deflection correction processing, and the like.

The work memory 38 stores the X and Y axis position data of the table 13 specified by the mouse 24,
The measurement result by the image measuring device 1, the processing result by the CPU 31, and the like are stored.

Next, a method of correcting the table deflection of the image measuring device 1 in the image measuring system thus configured will be described with reference to FIGS.

First, the grid points (Xn, Xn,
Yn) (where n is a positive integer, the same applies hereinafter) (error)
An error file creation process for creating an error file (lookup table) is performed.

FIG. 3 is a flowchart showing an error file creating procedure.

First, a laser displacement meter is mounted on the imaging unit 17 of the image measuring device 1 (step S11). This is for accurately setting the position of the surface of the table 13 in the Z-axis direction.

Next, the coordinate system of the table 13 is set by the keyboard 22 and the mouse 24 based on the operator's command (step S12), and any four points on the surface periphery of the table 13, specifically, the four corners are set. The positions of the four points in the X, Y, and Z-axis directions are detected (step S13).

Further, the reference plane position Z0 of the table 13 is calculated from the data of the four sets of X, Y, and Z-axis positions detected in step S13 (step S14). Specifically, one reference plane position Z0 is specified by calculating an average value of these four sets of Z-axis positions from the data of the four sets of X, Y, and Z-axis directions by the least square method or the like. . In order to calculate the reference plane position Z0, data of three sets of positions in the X, Y, and Z-axis directions, that is, data at three points among four points at four corners is sufficient, and three or more sets of data are used. Accordingly, highly accurate reference plane position information can be obtained. At this time, the gantry 11 supporting the peripheral portion of the table 13
Is a rigid body, the reference plane (reference plane position Z0) defined by the four points on the peripheral edge of the table 13 is
7 is a camera vertical ideal plane which is perpendicular to the optical axis and is a plane.

Next, a plurality of predetermined XY coordinate positions (hereinafter, referred to as "lattice points") (Xn, Yn) other than the four corners on the table surface using a laser displacement meter are used as the Z-axis position Zn, n.
Is measured (step S15).

Next, from the measured Z-axis position Zn, n and the reference plane position Z0 of the table 13, the table 13
Calculates the Z-axis direction position error ΔZn, n (ΔZn, n = Zn, n−Z0) at each grid point (Xn, Yn) on the surface (step S16), and based on this calculation result, is shown in Table 1. Such an error file is created as a lookup table and stored in the work memory 38 (step S17).

[0048]

[Table 1]

Instead of this lookup table, an error file based on the error ΔZn, n is created in the form of a calculation formula such as a binary cubic formula f (X, Y), and the work memory 38
May be stored. An error file created using a non-volatile memory as the work memory 38 may be used for the next and subsequent measurements.

Further, the calculation of the reference plane position Z0 of the table 13 in the procedure of steps S11 to S14 may be performed by using the CCD camera 18 instead of the laser displacement meter.

Next, with reference to FIG. 4, a description will be given of the bending correction processing of the table 13 of the image measuring device 1. FIG. 4 is a flowchart illustrating a deflection correction processing procedure of the table 13 of the image measuring device 1.

First, the CCD camera 18 is mounted on the imaging unit 17 of the image measuring device 1 (step S20), and the X, Y, and Z of the measurement point of the work 12 placed on the table 13 are measured.
The axial position (coordinates (X, Y, Z)) is captured using the image measuring device 1 (step S21).

Next, in step S22, it is determined whether or not the fetched coordinates (X, Y) match the lattice point (Xn, Yn). From the error file stored in the memory 38, Z at the grid point (Xn, Yn) corresponding to the coordinates (X, Y)
The axial position error ΔZn, n is read (step S23),
From the read ΔZn, n value, the error Δ of the coordinates (X, Y)
Z is obtained (step S24). Specifically, since the coordinates (X, Y) coincide with the grid point (Xn, Yn), ΔZ
The n, n values are directly used as the error ΔZ.

Finally, the deflection of the table 13 is corrected by subtracting the ΔZ value from the position of the measurement point of the work 12 in the Z-axis direction (step S25), and this processing ends.

As a result of the determination in step S22, the coordinates (X,
Y) does not coincide with the grid point (Xn, Yn), each Δ of four grid points (Xn, Yn) adjacent to the coordinates (X, Y)
The value of Zn, n is read (step S26), and the coordinates (X,
Y), the four grid points (Xn, Yn), and the respective ΔZn, n values corresponding thereto are used to determine an error ΔZ by a linear interpolation method or the like (step S27). Thereafter, the process proceeds to step S25, in which the deflection of the table 13 is corrected by subtracting the ΔZ value from the position of the measurement point of the work 12 in the Z-axis direction (step S25), and the process ends.

According to the present embodiment, X, X, and X at four arbitrary points on the peripheral portion of the surface of the table 13 are described.
The Y- and Z-axis positions are respectively detected to calculate the reference plane position Z0 of the table 13 (steps S13 and S14), and the Z-axis position Zn, n at each grid point (Xn, Yn) on the surface of the table 13 is calculated. An error ΔZn, n with respect to the reference plane position Z0 is calculated (step S16), and the calculated error ΔZn, n is stored in the work memory 38 as an error file (Table 1) corresponding to each grid point (step S17). ), X-, Y-, and Z-axis positions (coordinates (X, Y, Z)) at measurement points of the work 12 placed on the table 13 during work measurement.
(Step S21), and based on ΔZn, n values corresponding to the grid points corresponding to the coordinates (X, Y), if necessary, the position error Δ in the Z-axis direction of the coordinates (X, Y) using linear interpolation or the like.
Z is obtained (steps S24 and S27), and the deflection of the table 13 is corrected by subtracting the ΔZ value from the position in the Z-axis direction (step S25). Therefore, it is easy to correct the error in the Z-axis position due to the bending of the table 13. Measurement accuracy in the Z-axis direction is improved, and X, Y
It is also possible to correct an axial pitching error and the like, thereby improving the measurement accuracy of the entire image measuring device. At the same time, for example, the Z-axis position error ΔZ due to the bending of the support of the imaging unit 17 can be corrected, and the measurement accuracy of the entire image measuring device can be improved.

Note that instead of taking in the error ΔZn, n from the error file in steps S23 and S25, the coordinates (X,
The position in the Z-axis direction from which the error ΔZ has been removed from Y) may be directly calculated.

When the CCD camera 18 moves from the above-mentioned measuring point of the work 12 to another measuring point, the CCD camera 18 moves on the basis of the change ΔX value corresponding to the change in the X and Y-axis positions and the change ΔZ value corresponding to the change ΔY value. By correcting the Z value of another measurement point
The measurement of the image measuring device 1 can be performed while the CCD camera 18 is always kept in focus.

[0059]

As described above in detail, according to the method of correcting a table deflection of the image measuring device according to the first aspect, the position of the periphery of the table surface in the Z-axis direction is measured as the reference surface position. An error file is created by calculating an error between the position and the Z-axis position at a plurality of XY coordinate positions, and the Z-axis position at a measurement point of the object placed on the table is measured. Since the measured Z-axis position is corrected using the error file, the Z-axis position error due to the bending of the table can be easily corrected. , The pitching error in the Y-axis direction and the like can be corrected, so that the measurement accuracy of the image measuring device can be improved.

According to the fourth aspect of the present invention, the reference plane position of the table is measured based on the Z-axis reference positions of a plurality of points on the periphery of the table surface. Measurement accuracy of the reference plane position can be improved.

According to the fifth aspect of the present invention, the measurement of the reference plane position of the table and the measurement of the Z-axis direction position at a plurality of XY coordinate positions of the table surface are performed by the laser displacement meter. Since the measurement is performed, the measurement accuracy of the bending of the table can be improved.

According to a sixth aspect of the present invention, an error between the reference plane position and the Z-axis position at a plurality of XY coordinate positions is calculated and stored as an error file. The Z-axis position at the measurement point of the object placed on the measuring object is measured, and the measured Z-axis position is corrected using the error file. The correction can be easily performed, and the measurement accuracy of the position error in the Z-axis direction can be improved, and the pitching error in the X- and Y-axis directions can be corrected, so that the measurement accuracy of the image measuring device can be improved. be able to.

According to the ninth aspect of the present invention, the first measuring means determines the reference plane position of the table on the basis of a plurality of Z-axis reference positions on the periphery of the table surface. Therefore, the measurement accuracy of the reference plane position of the table can be improved.

According to the image measuring system of the present invention, since the first measuring means comprises a laser displacement meter, the accuracy of measuring the bending of the table can be improved.

[Brief description of the drawings]

FIG. 1 is a perspective view illustrating a configuration of an image measurement system to which a table deflection correction method of an image measurement device according to an embodiment of the present invention is applied.

FIG. 2 is a block diagram showing a configuration of a computer main body 21 in the image measurement system of FIG.

FIG. 3 is a flowchart illustrating an error file creation processing procedure.

FIG. 4 is a flowchart showing a deflection correction processing procedure of a table 13 of the image measuring device 1;

[Explanation of symbols]

 Reference Signs List 1 image measuring machine 2 computer system 3 printer 11 gantry 12 work 13 table 14, 15 support arm 16 X-axis guide 17 imaging unit 18 CCD camera 21 computer body 22 keyboard 23 joystick 24 mouse 25 CRT display

Continued on front page F-term (reference) 2F065 AA02 AA04 AA06 AA45 AA51 AA65 BB01 CC00 EE00 FF11 FF67 GG04 JJ03 JJ09 JJ26 PP03 PP12 PP22 QQ00 QQ03 QQ18 QQ23 QQ24 QQ25 QQ42 SS13 2F069 AAGG GG07 HH09 MM24 MM32 MM34 NN08 NN17 NN21 NN26 QQ10 5B057 CD12 DA07 DB03 DC03 DC09

Claims (10)

[Claims] 1. A method for correcting a deflection of a table of an image measuring device for measuring a size and a shape of an object to be measured placed on a table, wherein a position of a peripheral portion of the table surface in a Z-axis direction is measured as a reference surface position. Then, measuring the Z-axis position at a plurality of XY coordinate positions other than the periphery of the table surface, and calculating the error between the reference surface position and the measured Z-axis position at the plurality of XY coordinate positions. Calculating and creating an error file, measuring a Z-axis position at a measurement point of the DUT placed on the table, and correcting the measured Z-axis position using the error file. A table bending correction method for an image measuring device, characterized by comprising: 2. The method according to claim 1, wherein the error file is stored in the form of a look-up table. 3. The method according to claim 1, wherein the error file is stored in the form of a calculation formula. 4. The image according to claim 1, wherein said reference plane position is measured based on a plurality of Z-axis direction reference positions on a peripheral portion of said table surface. Table deflection correction method for measuring machine. 5. The measurement of the reference plane position and the plurality of Xs
The method according to any one of claims 1 to 4, wherein the measurement of the position in the Z-axis direction at the Y coordinate position is performed by a laser displacement meter. 6. A table deflection correction device for an image measuring device for measuring the size and shape of an object to be measured placed on a table, wherein the position of the periphery of the table surface in the Z-axis direction is measured as a reference surface position. First measuring means for measuring a Z-axis direction position at a plurality of XY coordinate positions other than the peripheral portion of the table surface; and Z at the reference surface position and the measured XY coordinate positions. Error file creation means for calculating an error with respect to the axial position to create an error file; storage means for storing the error file; and a Z-axis position at a measurement point of the DUT placed on the table. And a correcting means for correcting the measured position in the Z-axis direction using the error file. 7. The apparatus according to claim 6, wherein the storage unit stores the error file in the form of a look-up table. 8. The apparatus according to claim 6, wherein the storage unit stores the error file in the form of a calculation formula. 9. The apparatus according to claim 6, wherein said first measuring means measures said reference plane position based on Z-axis reference positions of a plurality of points on a peripheral portion of said table surface.
7. The table deflection correcting device for an image measuring machine according to claim 1. 10. The apparatus according to claim 6, wherein said first measuring means comprises a laser displacement meter.