EP1828715A1 - Sequential multi-probe method for measurement of the straightness of a straightedge - Google Patents
Sequential multi-probe method for measurement of the straightness of a straightedgeInfo
- Publication number
- EP1828715A1 EP1828715A1 EP05824115A EP05824115A EP1828715A1 EP 1828715 A1 EP1828715 A1 EP 1828715A1 EP 05824115 A EP05824115 A EP 05824115A EP 05824115 A EP05824115 A EP 05824115A EP 1828715 A1 EP1828715 A1 EP 1828715A1
- Authority
- EP
- European Patent Office
- Prior art keywords
- straightedge
- carriage
- measurement
- along
- straightness
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Withdrawn
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B21/00—Measuring arrangements or details thereof, where the measuring technique is not covered by the other groups of this subclass, unspecified or not relevant
- G01B21/20—Measuring arrangements or details thereof, where the measuring technique is not covered by the other groups of this subclass, unspecified or not relevant for measuring contours or curvatures, e.g. determining profile
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B5/00—Measuring arrangements characterised by the use of mechanical techniques
- G01B5/20—Measuring arrangements characterised by the use of mechanical techniques for measuring contours or curvatures
- G01B5/207—Measuring arrangements characterised by the use of mechanical techniques for measuring contours or curvatures using a plurality of fixed, simultaneously operating transducers
Definitions
- the invention relates to a sequential multi-probe method for measurement of straightness of a straightedge using a multi-probe device for sequential measurements along the straightedge using a carriage moving along a guide way.
- the invention also relates to an apparatus for measuring position errors in a machine having a movable element, and a straightedge and a measurement system for measurement of the straightness of the straightedge, said measurement system comprising a multi-probe device for sequentially measuring along the straightedge using a carriage moving along a guide way.
- the invention also relates to a measurement system for measurement of the straightness of the straightedge, said system comprising a multi-probe device for sequentially measuring along the straightedge using a carriage moving along a guide way.
- Machine tools and multi-axis machinery require a high standard precision in line with the development of high precision engineering. High precision in manufacturing can only be accomplished if it is possible to measure and calculate accurately the errors of machine components.
- Coordinate measurement machines are used for 1- 2 or 3 dimensional inspection of work pieces such as machine parts.
- a work piece is typically secured to a fixed table, and a measuring probe is used which is movable in one, two or three dimensions.
- the probe In brought into contact with the point or in other ways for instance capacitively a measurement is made, and measuring scales or other sensors on the machine are read.
- Probes may be of any type, the probes may contact probes which make contact with the straightedge, may be optical probes, or may be contactless probes based on Eddy currents or capacitance.
- the position of the point is typically expressed as X, Y and/or Z coordinates within a working volume of the machine.
- coordinate measuring machines typically have features such as high resolution measuring systems, electrical contact probes, motor drives, computer controlled drives and computer acquisition and processing of data.
- a moving bridge machine One type of coordinate measuring machine is known as a moving bridge machine.
- a bridge moves in the Y direction along guide ways on a table.
- a carriage moves in the X direction along guide ways on the bridge.
- Scales associated with each of the movable elements indicate the positions of the movable elements in three axial directions.
- the accuracy of a coordinate measuring machine is limited by inaccuracies in the scales or other measuring devices, and by faults in the guide ways or other elements which define machine motions.
- One approach to increasing accuracy is simply to improve the construction techniques and to reduce tolerances of the system so that errors are reduced.
- the reduction of errors becomes progressively more expensive as required accuracies increase and as the size of the work pieces increase.
- a precision straightedge is used as a means of calibration of linearity in the direction along the straightedge.
- the current solution to straightness calibration is to measure the 'individual machine fault' of the individual machine with (often) laser measurement tools and store it. Then all kind of efforts are made to keep conditions the same, so that the 'individual machine fault' stays the same. This requires a very good control over conditions such as temperature and humidity, the use of often (very) expensive materials such as Zerodur and invar to reduce as much as possible any deviation of the established 'individual machine fault'. Even then, the measurement procedure has to be regularly repeated after any service activity that could have affected the configuration.
- Li et al describe in SPIE vol. 2101 Measurement technology and intelligent Instruments (1993), page 483 describe a sequential-three-points method for measurement of the straightness of precision straightedges.
- Multi-probe' measurement means within the concept of the present invention, that at least three probes are used. In such measurement a carriage in which three probes are provided is moved along a guide way and sequentially sets of points (at least three) are measured on the straightedge. Using the sets of measured points, Li et al state that it is possible to calculate the straightness of the straightedge, independent of the errors in guide way or yaw error of the carriage.
- the method, apparatus and system is characterized in that the carriage is moved along one surface of the straightedge to take measurements, and is subsequently moved along an opposite surface of the straightedge to take measurements.
- the invention is based on the insights that: Ideally, the sequential multi-probe measurement should be free of systematically errors in guide way or carriage, however, the inventors have realized that a systematic error persists.
- a carriage is any means or device(s) by which the three or more probes are positioned in respect of each other, it need not be a single unity
- 'guide way' is any means or device(s) by which the carriage is moved along the straightedge and "rotating the carriage' comprises any method in which the position of the three or more probes is changed such that, thereafter, the position of the probes in respect to each other is changed in accordance with a rotation along one or more of said axes. It is not necessary for the method in accordance with the invention that the guide ways at both sides of the straightedge are identical, as will be explained below, although for reasons of manufacturing it is preferred that they are similar.
- the method in accordance with the invention allows a continuous measurement of straightness of the straightedge, reducing any error that may occur due to changes in conditions in between measurements of the straightness of the straightedge, and making in-line measurements possible.
- Fig. 1 is a schematic drawing of a x-y moving machine.
- Fig. 2 shows an example of a precision gauge straightedge on a machine as shown in Fig. 1.
- Figs. 3 and 4 illustrate the three probe sequential method.
- Fig. 5 illustrates a systematic error
- Fig. 6 illustrates that the probe probes at opposite surfaces of the straightedge.
- Figs. 7 and 8 illustrate two different arrangements for the probe at opposite surfaces of the straightedge.
- Figs. 9 to 12 illustrate measurements.
- FIG. 1 is a schematic drawing of a x-y moving machine 1.
- the machine has a moving part 2 for moving a part A over a table B in two perpendicular directions x and y.
- Such machine may be any kind of precision machinery.
- Machine tools and multi-axis machinery require a high standard precision in line with the development of high precision engineering. High precision in manufacturing can only be accomplished if it is possible to measure and calculate accurately the errors of machine components.
- the current solution to straightness calibration is to measure the 'individual machine fault' of the individual machine with (often) laser measurement tools and store it. Then all kind of efforts are made to keep conditions the same, so that the 'individual machine fault' stays the same. This requires a very good control over conditions such as temperature and humidity, the use of often (very) expensive materials such as Zerodur and invar to reduce as much as possible any deviation of the established 'individual machine fault'. Even then, the measurement procedure has to be repeated at regular intervals, e.g. bi-yearly, and after any service activity that could have affected the configuration. In order to be able to measure accurately a gauge must be available. However, this gauge has to be measured also. Ultimately, therefore, the accuracy of the gauge determines the accuracy of manufacturing.
- FIG. 2 shows schematically the position of such a straightedge 3.
- a measuring device 4 is used, in which three probes 4a, 4b and 4c are provided. The measuring device is moved along a guide way G(x) and at intervals the position S(x) is sequentially measured. Thus, sequentially measurements using three probes are taken, which is the reason that this method is called the sequential three probe method.
- a relatively large number of unknown parameters play a part. First of all the to be measured surface S(x) is a priori unknown. Secondly the guide way G(x) is unknown, thirdly, the yaw angle ⁇ of device 4 is unknown.
- probe 4a measures a distance a(n) between the guide way G(x) and the surface S(x) at position n.
- probe 4b measures a distance b(n) between the guide way and the surface at position n+1 and probe 4c measures a distance c(n).
- b(n) G(n)-S(n+1)+L ⁇ (n), where ⁇ (n) is the yaw error at position n.
- c(n) G(n)-S(n+2)+2L ⁇ (n).
- a(n)-2b(n)+c(n) -S(n)+2S(n+l)-S(n+2).
- a(n+l )-2b(n+l )+c(n+l ) -S(n+l )+2S(n+2)-S(n+3).
- a(n+2)-2b(n+2)+c(n+2) -S(n+2)+2S(n+3)-S(n+4).
- the sequential three probe measurement method is based on such calculations.
- the center probe 4b may be offset by an amount ⁇ in respect of a straight line through the outer probes 4a and 4c. This offset is a fixed offset, independent of the value n.
- a(n) G(n)-S(n).
- b(n) G(n)-S(n+l) - ⁇ +L ⁇ (n)
- c(n) G(n)-S(n+2)+2L ⁇ (n).
- a(n)-2b(n)+c(n) -S(n)+2S(n+l)+ ⁇ -S(n+2).
- a(n+l )-2b(n+l )+c(n+l ) -S(n+l )+2S(n+2)+ ⁇ -S(n+3).
- a(n+2)-2b(n+2)+c(n+2) -S(n+2)+2S(n+3)+ ⁇ -S(n+4).
- the unknown parameter ⁇ influences the outcome of the equations.
- an error in measurement occurs which is cumulative, i.e. whereas at the first point the error is small ( ⁇ ) at the n 411 point the error is approximately n(n-l) ⁇ .
- the systematic error ⁇ may be small, due to the cumulative influence, the error in measurement may be large.
- the size of the device has a tendency to increase and the number of measurement points also, so that this systematic error becomes appreciable.
- Figure 6 illustrates the method and device in accordance with the invention.
- the device 4 with the probes 4a, 4b and 4c is moved along one surface S (x) of the straightedge 3, and measurements are taken.
- the device is then brought to the opposite surface of the straightedge 3, to measure the opposite surface S'(x) of the straightedge 3.
- Figure 7 and 8 illustrates two different configurations.
- the device 4 may, at opposite surfaces of the straightedge be oriented, such that probe 4a faces probe 4a or faces probe 4c, i.e. the same probes face each other.
- Figure 7 the same probes face each other
- Figure 8 different probes face each other, i.e.
- Figure 9 illustrates measured values ( ⁇ +S(x)) in arbitrary units (a.u) as a function of x in arbitrary units (a.u) as determined by the common sequential three probe method.
- the thickness of the straightedge can be controlled to a very high accuracy, and is also hardly dependent on other parameters such as temperature and humidity, that may have an appreciable influence on the straightness of the straightedge. It is to be noted, that one could think that the scheme could only work if the two guide ways (G(x)) and the yaw error ( ⁇ (x)) are the same for the opposite surfaces of the straightedge. This would, if true, pose a serious limitation on the method, since it is hardly likely that this would be the case. However, this is not the case, G(x) and ⁇ (x) drop out of the equations at both sides of the straightedge 3, and independent of each other.
- said measurement system comprises a multi-probe (4a, 4b, 4c) device (4) for sequentially measuring along the straightedge (3) using a carriage (4) moving along a guide way (G(x)).
- the carriage (4) is moved along one surface (S(x)) of the straightedge (3) to take measurements, is then transferred to an opposite surface (S'(x)) of the straightedge (3) and moved along the opposite surface of the straightedge (3) to take measurements.
- a three-probe method is used, but more than three probes could be used. On the one hand an additional error would be introduced, however, also additional information would be available. In preferred embodiments a three probe method is used.
- a straightedge may be provided in one direction or two or more directions.
- the method of the invention works, since the deviations in thickness are much better controllable than the deviations in straightness.
- the dimensions of the straightedge are for instance typically 5 mm (thickness) by 2-3 meters (length).
- the thickness of the straightedge can during manufacturing be controlled to within micrometers.
- One type of systematic error would be a systematic change in thickness of the straightedge along its length.
- Such a systematic, known change in thickness along the length of the straightedge can be accounted for in the measurements by accurately measuring the variation of thickness of the straightedge along the straightedge before putting it in the apparatus or while in the apparatus, and accounting for such variation when comparing the measurements at opposite surfaces of the straightedge.
- a simple look-up table comprising such systematic deviation as a function of position along the straightedge would suffice.
- Deviations due to temporal influences on the thickness of the straightedge, such as temperature variations, would then not be accounted for, but such errors are orders of magnitude less than the deviations of straightness.
- the straightedge may have the form of a lath or a plate, or any suitable shape or form.
- the probes do not need to be all on one side of the straightedge. For instance, a number n (n ⁇ l) of probes could be probing a first surface of the straightedge, while simultaneously m (m ⁇ l), wherein n+m> 3, probes probe the opposite surface of the straightedge.
- the method would then comprise taking measurements along one direction, where after, the carriage is rotated so that the m probes probe the first surface, and the n probes probe the opposite surface, and the measurements are repeated.
- either the carriage may be transferred to an opposite side of the straightedge leaving the straight edge in position, or the carriage is left in position and the straight edge is turned around so that the opposite surface faces the carriage.
- the straight edge is convex or concave (i.e. having a form departing from a true straight edge) due to an intrinsic curvature in the straightedge a curvature in the straightedge due to the mounting (e.g. clamping or screwing) of the straightedge.
- the present method, apparatus and system is suitable for on-line measurement.
- An apparatus as described regularly travels along the straightedge, during such travels back and forth, the carriage, when it has reached a final position, is changed in position so that the probes which before such change were probing one surface, are thereafter probing the opposite surface.
- the temporal influence such as temperature and humidity do not change to much, the method allows on-line accurate calibration of straightness. It is emphasized that the change in position of the probes, i.e. from one surface of the straightedge to the opposite, makes it possible to perform such accurate and in-line measurement.
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Abstract
A system and method for measurement of the straightness of a straightedge, said measurement system comprising a multi-probe (4a, 4b, 4c) device (4) for sequentially measuring along the straightedge (3) using a carriage (4) moving along a guide way (G(x)) . The carriage (4) is moved along one surface (S (x) ) of the straightedge (3) to take measurements, is then transferred to an opposite surface (S ' (x) ) of the straightedge (3) and moved along the opposite surface of the straightedge (3) to take measurements. By adding and subtracting of the measurement points taken at the opposing surfaces of the straightedge, a systematic error due to the probe can be identified, whereby the measurement of the straightness of the straightedge is improved. Errors in manufacturing and measurements of work pieces and other parts may thereby be reduced. The method and apparatus can also be used for on-line calibrations of straightness .
Description
SEQUENTIAL MULTI-PROBE METHOD FOR MEASUREMENT OF THE STRAIGHTNESS OF A STRAIGHTEDGE
The invention relates to a sequential multi-probe method for measurement of straightness of a straightedge using a multi-probe device for sequential measurements along the straightedge using a carriage moving along a guide way.
The invention also relates to an apparatus for measuring position errors in a machine having a movable element, and a straightedge and a measurement system for measurement of the straightness of the straightedge, said measurement system comprising a multi-probe device for sequentially measuring along the straightedge using a carriage moving along a guide way.
The invention also relates to a measurement system for measurement of the straightness of the straightedge, said system comprising a multi-probe device for sequentially measuring along the straightedge using a carriage moving along a guide way.
Machine tools and multi-axis machinery require a high standard precision in line with the development of high precision engineering. High precision in manufacturing can only be accomplished if it is possible to measure and calculate accurately the errors of machine components.
Coordinate measurement machines are used for 1- 2 or 3 dimensional inspection of work pieces such as machine parts. A work piece is typically secured to a fixed table, and a measuring probe is used which is movable in one, two or three dimensions. To measure the position of a point on the work piece, the probe in brought into contact with the point or in other ways for instance capacitively a measurement is made, and measuring scales or other sensors on the machine are read. Probes may be of any type, the probes may contact probes which make contact with the straightedge, may be optical probes, or may be contactless probes based on Eddy currents or capacitance. The position of the point is typically expressed as X, Y and/or Z coordinates within a working volume of the machine. To measure a distance between two points, the points are measured successively, the coordinates of points are read, and distance is calculated from the coordinates. State of the art coordinate measuring machines typically have features such as high resolution measuring
systems, electrical contact probes, motor drives, computer controlled drives and computer acquisition and processing of data.
One type of coordinate measuring machine is known as a moving bridge machine. A bridge moves in the Y direction along guide ways on a table. A carriage moves in the X direction along guide ways on the bridge. Scales associated with each of the movable elements indicate the positions of the movable elements in three axial directions.
The accuracy of a coordinate measuring machine is limited by inaccuracies in the scales or other measuring devices, and by faults in the guide ways or other elements which define machine motions. One approach to increasing accuracy is simply to improve the construction techniques and to reduce tolerances of the system so that errors are reduced. However, the reduction of errors becomes progressively more expensive as required accuracies increase and as the size of the work pieces increase. A precision straightedge is used as a means of calibration of linearity in the direction along the straightedge.
The current solution to straightness calibration is to measure the 'individual machine fault' of the individual machine with (often) laser measurement tools and store it. Then all kind of efforts are made to keep conditions the same, so that the 'individual machine fault' stays the same. This requires a very good control over conditions such as temperature and humidity, the use of often (very) expensive materials such as Zerodur and invar to reduce as much as possible any deviation of the established 'individual machine fault'. Even then, the measurement procedure has to be regularly repeated after any service activity that could have affected the configuration.
Various techniques to measure and calculate such errors are known. In such techniques often a precision straightedge is used as a gauge to measure the error of machine components or machine movements. In order for the measurement to be accurate the straightness of the precision straightedge itself has to be precisely and accurately measured, i.e. the straightedge surface error has to be accurately measured.
Li et al describe in SPIE vol. 2101 Measurement technology and intelligent Instruments (1993), page 483 describe a sequential-three-points method for measurement of the straightness of precision straightedges. "Multi-probe' measurement means, within the concept of the present invention, that at least three probes are used. In such measurement a carriage in which three probes are provided is moved along a guide way and sequentially sets of points (at least three) are measured on the straightedge. Using the sets of measured points, Li et al state that it is possible to calculate the straightness of the straightedge, independent of the errors in guide way or yaw error of the carriage.
However, although the authors Li et al claim that it is possible to calculate the straightness of the straightedge, the inventors have realized that a systematic error persists. This systematic error in the measurements accumulates as the length of the straightedge increases. This systematic error cannot or at least not easily be distinguished from true deviations of the straightedge. Also it is difficult to provided on-line measurements.
In fact, all known prior art systems for calibrating coordinate measuring machines have been relatively complex and expensive. In addition, calibration procedures are lengthy, complex, expensive and subject to error.
It is an object of the invention to provide a method, apparatus and system as described in the 'field of the invention' paragraph with an increased accuracy and/or which is relatively simple and/or is better suited for on-line measurements.
To this end the method, apparatus and system is characterized in that the carriage is moved along one surface of the straightedge to take measurements, and is subsequently moved along an opposite surface of the straightedge to take measurements. The invention is based on the insights that: Ideally, the sequential multi-probe measurement should be free of systematically errors in guide way or carriage, however, the inventors have realized that a systematic error persists.
By moving the carriage along an opposite surface of the straightedge, at both sides of the straightedge the same systematically error is produced. Taking measurements at one surface of the straightedge does not enable to distinguish between a true deviation of the straightedge and the systematic error. However, when the opposite surface of the straightedge is also measured, any deviation in the straightedge itself changes of sign (convex becomes concave and vice versa) while the systematically error due to the measurement does not chance sign. By adding and subtraction of the measured points at the opposite surfaces of the straightedge the systematically error and true deviation of the straightedge from a perfect straight line can be established. Within the concept of the invention a carriage is any means or device(s) by which the three or more probes are positioned in respect of each other, it need not be a single unity, 'guide way' is any means or device(s) by which the carriage is moved along the straightedge and "rotating the carriage' comprises any method in which the position of the three or more probes is changed such that, thereafter, the position of the probes in respect to each other is changed in accordance with a rotation along one or more of said axes.
It is not necessary for the method in accordance with the invention that the guide ways at both sides of the straightedge are identical, as will be explained below, although for reasons of manufacturing it is preferred that they are similar.
By measuring at the opposite surface of the straightedge an additional error is introduced due to deviations in the thickness of the straightedge. However, the thickness of a straightedge can be much better controlled and is much less dependent on conditions than the straightness of the straightedge, the positive effect of the reduction in error (point 2) substantially outweighs the increment in error due to difference in thickness (point 3).
Furthermore, the method in accordance with the invention allows a continuous measurement of straightness of the straightedge, reducing any error that may occur due to changes in conditions in between measurements of the straightness of the straightedge, and making in-line measurements possible.
These and further aspects of the invention will be explained in greater detail by way of example and with reference to the accompanying drawings, in which: Fig. 1 is a schematic drawing of a x-y moving machine. Fig. 2 shows an example of a precision gauge straightedge on a machine as shown in Fig. 1. Figs. 3 and 4 illustrate the three probe sequential method.
Fig. 5 illustrates a systematic error.
Fig. 6 illustrates that the probe probes at opposite surfaces of the straightedge. Figs. 7 and 8 illustrate two different arrangements for the probe at opposite surfaces of the straightedge. Figs. 9 to 12 illustrate measurements.
The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the present invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiment set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.
Figure 1 is a schematic drawing of a x-y moving machine 1. The machine has a moving part 2 for moving a part A over a table B in two perpendicular directions x and y. Such machine may be any kind of precision machinery. Machine tools and multi-axis machinery require a high standard precision in line with the development of high precision engineering. High precision in manufacturing can only be accomplished if it is possible to measure and calculate accurately the errors of machine components.
The current solution to straightness calibration is to measure the 'individual machine fault' of the individual machine with (often) laser measurement tools and store it. Then all kind of efforts are made to keep conditions the same, so that the 'individual machine fault' stays the same. This requires a very good control over conditions such as temperature and humidity, the use of often (very) expensive materials such as Zerodur and invar to reduce as much as possible any deviation of the established 'individual machine fault'. Even then, the measurement procedure has to be repeated at regular intervals, e.g. bi-yearly, and after any service activity that could have affected the configuration. In order to be able to measure accurately a gauge must be available. However, this gauge has to be measured also. Ultimately, therefore, the accuracy of the gauge determines the accuracy of manufacturing.
Often a straightedge is used for straightness calibration. Figure 2 shows schematically the position of such a straightedge 3. To measure the straightness of the straightedge various methods are known, one of which is the so-called three probe method, illustrated in Figure 3. A measuring device 4 is used, in which three probes 4a, 4b and 4c are provided. The measuring device is moved along a guide way G(x) and at intervals the position S(x) is sequentially measured. Thus, sequentially measurements using three probes are taken, which is the reason that this method is called the sequential three probe method. A relatively large number of unknown parameters play a part. First of all the to be measured surface S(x) is a priori unknown. Secondly the guide way G(x) is unknown, thirdly, the yaw angle γ of device 4 is unknown.
At each point n, n+1, n+2, n+m etc. probe 4a measures a distance a(n) between the guide way G(x) and the surface S(x) at position n.
a(n)=G(n)-S(n).
Likewise probe 4b measures a distance b(n) between the guide way and the surface at position n+1 and probe 4c measures a distance c(n).
b(n)= G(n)-S(n+1)+Lγ(n), where γ(n) is the yaw error at position n. c(n)=G(n)-S(n+2)+2Lγ(n).
Adding and subtracting gives:
a(n)-2b(n)+c(n)=-S(n)+2S(n+l)-S(n+2). a(n+l )-2b(n+l )+c(n+l )=-S(n+l )+2S(n+2)-S(n+3). a(n+2)-2b(n+2)+c(n+2)=-S(n+2)+2S(n+3)-S(n+4).
Etc.
Through two points (for instance the starting points (S(I) and S(2)) a line can be drawn, and thus these points S(I) and S(2) can be set at zero. This will allow to establish all points S(n) using the above equations. It is remarked that the above equations are independent of the guide way G(x) and the yaw error γ(x). Thus, by measuring with three probes at n points it is possible to determine the values of S(x) independent of the guide way G(x) and the yaw error γ(x).
The sequential three probe measurement method is based on such calculations.
If G(x) and γ(x) would be only unknown quantities, the sequential three (or more) probe method would indeed allow to eliminate the unknown quantities G(x) and γ to establish S(x) and thereby measure the straightness of the straightedge, which can then be used as a calibration for other straightness measurements for instance of a work piece.
However, the inventors have realized that a small systematic error occurs, which is illustrated in Figure 5.
The center probe 4b may be offset by an amount δ in respect of a straight line through the outer probes 4a and 4c. This offset is a fixed offset, independent of the value n. The above mentioned equations now become.
a(n)=G(n)-S(n). b(n)=G(n)-S(n+l) - δ +Lγ(n), c(n)=G(n)-S(n+2)+2Lγ(n).
Adding and subtracting gives:
a(n)-2b(n)+c(n)=-S(n)+2S(n+l)+δ-S(n+2).
a(n+l )-2b(n+l )+c(n+l )=-S(n+l )+2S(n+2)+δ-S(n+3). a(n+2)-2b(n+2)+c(n+2)=-S(n+2)+2S(n+3)+δ-S(n+4). Etc.
The unknown parameter δ influences the outcome of the equations. In fact an error in measurement occurs which is cumulative, i.e. whereas at the first point the error is small (δ) at the n411 point the error is approximately n(n-l)δ. Even though the systematic error δ may be small, due to the cumulative influence, the error in measurement may be large. The size of the device has a tendency to increase and the number of measurement points also, so that this systematic error becomes appreciable.
Figure 6 illustrates the method and device in accordance with the invention. The device 4 with the probes 4a, 4b and 4c is moved along one surface S (x) of the straightedge 3, and measurements are taken. The device is then brought to the opposite surface of the straightedge 3, to measure the opposite surface S'(x) of the straightedge 3. Figure 7 and 8 illustrates two different configurations. The device 4 may, at opposite surfaces of the straightedge be oriented, such that probe 4a faces probe 4a or faces probe 4c, i.e. the same probes face each other. In Figure 7 the same probes face each other, in Figure 8 different probes face each other, i.e. the sequence of probes is reversed, "facing each other' means that when the center probe is positioned at the same coordinate along the straightedge, the coordinates of the 4a probes are substantially the same (Figure 7) or the coordinates of the 4a and 4c probes are substantially the same.
Figure 9 illustrates measured values (Δ+S(x)) in arbitrary units (a.u) as a function of x in arbitrary units (a.u) as determined by the common sequential three probe method. The measured values (Δ + S(x)), illustrated by the large triangles, actually comprise two components, namely the true values for S(x), illustrated by the small squares, i.e. the bends in the supposedly straight straightedge 3, and the systematic error Δ, illustrated by the small diamonds, which systematic error Δ grows more or less in a quadratic form as a function of the measurement points n. One could presume that, knowing that the systematic error grows in a particular manner as a function of the number of the measured point n, and thus as the distance x traveled along the straightedge one could isolate this systematic error Δ. However, bends in the straightedge 3 usually also follow a more or less quadratic curve, so that the two contributions Δ (systematic error) and S(x) (true deviation of a straight line) cannot be separated, at least not easily.
Figure 10 illustrates the same measurements but now taken at the opposite surface of the straightedge 3. The measured values (Δ+S'(x)) actually comprise two components, namely the true values for S'(x), i.e. the bends at the opposite surfaces, and the systematic error Δ. The systematic error is due to the error δ which has not changed. The values for S'(x) are of opposite sign for those of S(x), since what was convex at one surface of the straightedge, is concave and vice versa at the other side of the straightedge. Again, using only these measurements the two contributions cannot be separated. However, using both measurements, it is possible to separate the two contributions, i.e. the systematic error and the true curvature of the straightedge, to the measurements, as is illustrated in Figures 11 and 12. Adding and subtraction the measurements gives, on the one hand the value for the systematic error Δ and on the other hand the value for the curvature S(x). This scheme works, provided that the thickness th (Figure 7, 8) of the straightedge can be better controlled than the curvature (bends) of the straightedge. The latter is, however, almost always the case. The thickness of the straightedge can be controlled to a very high accuracy, and is also hardly dependent on other parameters such as temperature and humidity, that may have an appreciable influence on the straightness of the straightedge. It is to be noted, that one could think that the scheme could only work if the two guide ways (G(x)) and the yaw error (γ(x)) are the same for the opposite surfaces of the straightedge. This would, if true, pose a serious limitation on the method, since it is hardly likely that this would be the case. However, this is not the case, G(x) and γ(x) drop out of the equations at both sides of the straightedge 3, and independent of each other. It is essential, though, that the same probe is used at the opposite surfaces of the straightedge, and the position of the probes in respect to each other remains the same, so that the error δ remains the same. Using two different probes simultaneously at opposite surfaces does not fall under the scope of the invention, nor does using the same probe two at the same surface, but in different orientations.
In short the invention can be described as follows:
In an apparatus for measuring position errors in a machine having a movable element (2) and a straightedge (3) and a system for measurement of the straightness of the straightedge, said measurement system comprises a multi-probe (4a, 4b, 4c) device (4) for sequentially measuring along the straightedge (3) using a carriage (4) moving along a guide way (G(x)). The carriage (4) is moved along one surface (S(x)) of the straightedge (3) to take measurements, is then transferred to an opposite surface (S'(x)) of the straightedge (3) and moved along the opposite surface of the straightedge (3) to take measurements. By adding and subtracting of the measurement points taken at the opposing surfaces of the straightedge,
a systematic error due to the probe can be identified, whereby the measurement of the straightness of the straightedge is improved. Errors in manufacturing and measurements of work pieces and other parts may thereby be reduced. The method and apparatus can also be used for on-line calibrations of straightness. It will be clear that within the framework of the invention many variations are possible. It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. The invention resides in each and every novel characteristic feature and each and every combination of characteristic features. Reference numerals in the claims do not limit their protective scope. Use of the verb "to comprise" and its conjugations does not exclude the presence of elements other than those stated in the claims. Use of the article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.
For instance, in preferred embodiment a three-probe method is used, but more than three probes could be used. On the one hand an additional error would be introduced, however, also additional information would be available. In preferred embodiments a three probe method is used.
A straightedge may be provided in one direction or two or more directions.
The method of the invention works, since the deviations in thickness are much better controllable than the deviations in straightness. The dimensions of the straightedge are for instance typically 5 mm (thickness) by 2-3 meters (length). The thickness of the straightedge can during manufacturing be controlled to within micrometers.
One type of systematic error would be a systematic change in thickness of the straightedge along its length. Such a systematic, known change in thickness along the length of the straightedge can be accounted for in the measurements by accurately measuring the variation of thickness of the straightedge along the straightedge before putting it in the apparatus or while in the apparatus, and accounting for such variation when comparing the measurements at opposite surfaces of the straightedge. A simple look-up table comprising such systematic deviation as a function of position along the straightedge would suffice.
Deviations due to temporal influences on the thickness of the straightedge, such as temperature variations, would then not be accounted for, but such errors are orders of magnitude less than the deviations of straightness.
The straightedge may have the form of a lath or a plate, or any suitable shape or form.
The probes do not need to be all on one side of the straightedge.
For instance, a number n (n≥l) of probes could be probing a first surface of the straightedge, while simultaneously m (m≥l), wherein n+m> 3, probes probe the opposite surface of the straightedge. The method would then comprise taking measurements along one direction, where after, the carriage is rotated so that the m probes probe the first surface, and the n probes probe the opposite surface, and the measurements are repeated.
In principle, within the broadest concept of the invention, either the carriage may be transferred to an opposite side of the straightedge leaving the straight edge in position, or the carriage is left in position and the straight edge is turned around so that the opposite surface faces the carriage. The straight edge is convex or concave (i.e. having a form departing from a true straight edge) due to an intrinsic curvature in the straightedge a curvature in the straightedge due to the mounting (e.g. clamping or screwing) of the straightedge. Turning the straight edge around does not change the intrinsic curvature of the straightedge (and thus what was convex in respect of the carriage becomes concave and vice versa), however, the mounting of the straight edge may be changed due to the running around of the straightedge which may introduce a change in curvature and such change may lead to an error. Thus it is preferred that the straightedge is left unchanged and the carriage is transferred to the opposite surface of the straightedge.
The present method, apparatus and system is suitable for on-line measurement. An apparatus as described regularly travels along the straightedge, during such travels back and forth, the carriage, when it has reached a final position, is changed in position so that the probes which before such change were probing one surface, are thereafter probing the opposite surface. Provided that during such movement back and forth along the straightedge, the temporal influence, such as temperature and humidity do not change to much, the method allows on-line accurate calibration of straightness. It is emphasized that the change in position of the probes, i.e. from one surface of the straightedge to the opposite, makes it possible to perform such accurate and in-line measurement.
Claims
1. A sequential multi-probe method for measurement of straightness of a straightedge (3) using a multi-probe (4a, 4b, 4c) device (4) for sequential measurements along the straightedge (3) using a carriage (4) moving along a guide way (G(x)), wherein that the carriage is moved along one surface (S(x)) of the straightedge 3 to take measurements, and is subsequently moved along an opposite surface (S'(x)) of the straightedge to take measurements.
2. A sequential multi-probe method as claimed in claim 1, wherein at opposite surfaces (S(x), S'(x)) of the straightedge (3) the carriage (4) is so oriented that the same probes (4a, 4b, 4c) face each other.
3. A sequential multi-probe method as claimed in claim 1, wherein at opposite surfaces (S(x), S'(x)) of the straightedge (3) the carriage (4) is so oriented that the sequence of probes (4a, 4b, 4c) is reversed.
4. A sequential multi-probe method as claimed in claim 1, wherein three probes (4a, 4b, 4c) are used.
5. A method as claimed in claim 1, wherein the measurements taken for opposite surfaces (S(x), S'(x)) of the straightedge (3) are compared to provide a measurement of the straightness of the straightedge.
6. A method as claimed in claim 5, wherein, in the comparison, a systematic error in thickness (th) of the straightedge (3) is taken into account.
7. An apparatus for measuring position errors in a machine having a movable element (2) and a straightedge (3) and a system for measurement of the straightness of the straightedge, said measurement system comprising a multi-probe (4a, 4b, 4c) device (4) for sequentially measuring along the straightedge (3) using a carriage (4) moving along a guide way (G(x)), wherein the apparatus comprises means for moving the carriage (4) along one surface (S(x)) of the straightedge (3) to take measurements, transferring the carriage (4) to an opposite surface (S'(x)) of the straightedge (3) and moving the carriage along the opposite surface of the straightedge (3) to take measurements.
8. A measurement system for measurement of the straightness of the straightedge
(3), said system comprising a multi-probe (4a, 4b, 4c) device (4) for sequentially measuring along the straightedge (3) using a carriage (4) moving along a guide way (G(x)), wherein the measurement system comprises means for moving the carriage (4) along one surface (S(x)) of the straightedge (3) to take measurements, transferring the carriage (4) to an opposite surface (S '(x)) of the straightedge (3) and moving the carriage (4) along the opposite surface (S'(x)) of the straightedge (3) to take measurements.
Priority Applications (1)
Application Number | Priority Date | Filing Date | Title |
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EP05824115A EP1828715A1 (en) | 2004-12-16 | 2005-12-12 | Sequential multi-probe method for measurement of the straightness of a straightedge |
Applications Claiming Priority (3)
Application Number | Priority Date | Filing Date | Title |
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EP04106644 | 2004-12-16 | ||
EP05824115A EP1828715A1 (en) | 2004-12-16 | 2005-12-12 | Sequential multi-probe method for measurement of the straightness of a straightedge |
PCT/IB2005/054175 WO2006064445A1 (en) | 2004-12-16 | 2005-12-12 | Sequential multi-probe method for measurement of the straightness of a straightedge |
Publications (1)
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EP1828715A1 true EP1828715A1 (en) | 2007-09-05 |
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EP05824115A Withdrawn EP1828715A1 (en) | 2004-12-16 | 2005-12-12 | Sequential multi-probe method for measurement of the straightness of a straightedge |
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EP (1) | EP1828715A1 (en) |
JP (1) | JP2008524576A (en) |
CN (1) | CN101080609A (en) |
WO (1) | WO2006064445A1 (en) |
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JP4980817B2 (en) * | 2007-08-07 | 2012-07-18 | 株式会社ナガセインテグレックス | Multipoint probe zero error related value recording device |
JP4980818B2 (en) * | 2007-08-07 | 2012-07-18 | 株式会社ナガセインテグレックス | Variation detection method of zero error of multi-point probe |
JP5210911B2 (en) * | 2009-02-03 | 2013-06-12 | 株式会社ナガセインテグレックス | Shape measuring device |
CN102519408B (en) * | 2011-12-12 | 2013-09-11 | 陕西宝成航空仪表有限责任公司 | Method for measuring a plurality of parts at one time by three-coordinate measuring machine |
CN103673844A (en) * | 2013-12-03 | 2014-03-26 | 高玉树 | Pipe or bar straightness detecting ruler |
Family Cites Families (3)
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JPS5112154A (en) * | 1974-07-22 | 1976-01-30 | Nippon Kokan Kk | Hyomenheitandosokuteihoho |
US4084324A (en) * | 1975-04-23 | 1978-04-18 | The Rank Organisation Limited | Measuring instrument |
US5205046A (en) * | 1991-06-05 | 1993-04-27 | Ford Motor Company | Method for measuring surface waviness |
-
2005
- 2005-12-12 CN CNA200580043016XA patent/CN101080609A/en active Pending
- 2005-12-12 JP JP2007546258A patent/JP2008524576A/en active Pending
- 2005-12-12 WO PCT/IB2005/054175 patent/WO2006064445A1/en active Application Filing
- 2005-12-12 EP EP05824115A patent/EP1828715A1/en not_active Withdrawn
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WO2006064445A1 (en) | 2006-06-22 |
JP2008524576A (en) | 2008-07-10 |
CN101080609A (en) | 2007-11-28 |
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