FIELD

The present invention relates to a utensil for evaluating a length measurement error in an X-ray CT device for three-dimensional shape measurement designed to measure the dimensions of a test object.

BACKGROUND

In recent years, the measurement of dimensions including those of the internal shape of a test object has been performed using an X-ray CT device developed as a device for observing the internal structure of a test object. Methods for evaluating the measurement accuracy of an X-ray CT device designed to measure a three-dimensional shape have been discussed in order to develop international standards; however, regarding current devices, for example, on the basis of measurement accuracy calculated in accordance with the Germany's domestic guideline VDI/VDE2630-1.3 (guideline on dimensional measurement by X-ray CT), accuracy assurance is provided for the devices. In addition, as a utensil for evaluating a length measurement error in an X-ray CT device for three-dimensional shape measurement (hereinafter referred to as a utensil) corresponding to VDI/VDE2630-1.3, a utensil made by Cael Zeiss Inc. is known (see Non-Patent Literature 1).

The utensil described in Non-Patent Literature 1 is one of a type referred to as a forest gauge. In the utensil referred to as the forest gauge, spheres are arranged in a space by installing upright support rods supporting the spheres on a stepwise base. One having 27 spheres and one having 22 spheres as the number of spheres are known.

Also, Patent Literature 1 proposes a calibrator of an X-ray CT device, which intends to accurately calibrate the dimensions of shapes including the internal shape of a test object from projection images obtained by the X-ray CT device. In a utensil described in Patent Literature 1, spheres are arranged in a space by firmly fixing the spheres on the outer circumference of a cylindrical body.

Before X-ray CT imaging of such a utensil, the coordinates of the respective spheres are measured by CMM (coordinate measuring machine: contact type three-dimensional coordinate measuring machine) or the like. In addition, from the difference between the value of an actual inter-sphere distance obtained by a coordinate measurement result and the value of an inter-sphere distance in an imaging space measured at the time of the X-ray CT imaging, a length measurement error by X-ray CT is evaluated.

CITATION LIST

Patent Literature

Patent Literature 1

Japanese Unexamined Patent Publication JP-A2014-190933

Non-Patent Literature

Non-Patent Literature 1

D. Weiss, R. Lonardoni, A. Deffner, C. Kuhn, Geometric image distortion in flat-panel X-ray detectors and its influence on the accuracy of CT-based dimensional measurements, 4th Conference on Industrial Computed Tomography (iCT), 19-21 Sep. 2012, Wels, Austria (iCT 2012)

SUMMARY

The calibrator described in Patent Literature 1 has the problem of being able to evaluate only a cylindrical region in an imaging space because no sphere can be arranged in the cavity inside the cylindrical body. On the other hand, in the utensil referred to as the forest gauge, it is possible to arrange spheres on the central axis; however, there is the following problem.

FIG. 15is a schematic diagram illustrating an evaluation range in an imaging visual space when performing X-ray CT imaging on a conventional forest gauge. In addition,FIG. 15illustrates the case where the 27-sphere forest gauge made by Carl Zeiss, Inc. is targeted for the X-ray CT imaging. InFIG. 15(a), the imaging visual space of a cylindrical shape is indicated by virtual lines, and inFIG. 15(b), an evaluation range at one time of X-ray CT imaging is illustrated. Also,FIG. 15(c)schematically illustrates evaluation ranges and unevaluable ranges in the imaging visual space when the X-ray CT imaging was performed three times at switched Z positions, andFIG. 15(d)is a diagram illustrating the mutual positional relationship among three conical spaces in the imaging visual space when the X-ray CT imaging was performed three times at the switched Z positions.

In a measurement space (X, Y, Z) specific to an X-ray CT device, the following distortion occurs. Firstly, a length reference in the X axial direction and a length reference in the Y axial direction are different, and for example, a perfect circle is deformed into an ellipse on a cross-sectional image orthogonal to the Z axis. Secondly, the length reference in the X axial direction and the length reference in the Y axial direction are gradually changed depending on the position of the Z axis, and thereby for example, a cylindrical shape is deformed into a truncated conical shape. Thirdly, local deformation occurs around a specific point. Fourthly, depending on the intersecting position of the X-Y plane with the Z axis, the X-Y plane is gradually rotated and twist-like deformation occurs in the space.

In a utensil like the forest gauge, spheres are arranged on a conical surface with the central sphere as the apex, and therefore a conical space as illustrated inFIG. 15(b)serves as an evaluation range where comparison with the value of an inter-sphere distance obtained from a coordinate measurement result by CMM is possible. For this reason, among the above-described first to fourth types of spatial distortion specific to an X-ray CT device, the second to fourth types of spatial distortion are difficult to capture.

Also, in the 27-sphere forest gauge made by Carl Zeiss Inc., the support rods supporting the spheres are installed upright on the base provided with steps, and a range where the shadow of the base part is imaged at the time of X-ray CT imaging is widened. In addition, only a space above the top step serves as an evaluation range (seeFIG. 15(b)). For this reason, as compared with an evaluation range on the X-Y plane, an evaluation range in the Z axial direction in measurement by one time of X-ray CT imaging is narrowed. For example, in three times of X-ray CT imaging, as indicated by hatching inFIG. 15(c), the unevaluable ranges remain between the conical spaces. In order to eliminate such unevaluable ranges, it is necessary to measure an inter-sphere distance by repeatedly performing X-ray CT imaging while finely changing the position of a utensil in the Z axial direction many times within a height range corresponding to the longitudinal direction of the light receiving area of an X-ray detector. That is, it takes time to evaluate the measurement accuracy of an X-ray CT device.

Further, in the 27-sphere forest gauge made by Carl Zeiss Inc., since the spheres are arranged on a conical surface, the Z axis is aligned only with the sphere at the apex of the conical space, and therefore the mutual positional relationship among conical spaces each serving as an evaluation target at one time of X-ray CT imaging among multiple times of repeated X-ray CT imaging cannot be evaluated. Accordingly, as illustrated inFIG. 15(d), the deformation of the conical spaces can be captured, but the mutual positional relationship of a conical space at each time of X-ray CT imaging cannot be captured.

The present invention has been made in order to solve the above-described problems, and a first object is to provide a utensil for evaluating a length measurement error in an X-ray CT device for three-dimensional shape measurement, which makes it possible to sufficiently capture spatial distortion specific to the X-ray CT device and evaluate the three-dimensional shape measurement accuracy of the X-ray CT device.

FIG. 16is a schematic diagram illustrating conventional methods for fixing a support rod supporting a sphere to a base. In a conventional forest gauge, when arranging spheres in a space by installing upright support rods supporting the spheres to a base, as a method for fixing the support rods, a method referred to as so-called split clamping has been employed. InFIG. 16(a), a fixing member140formed with a hole insertable with a support rod136is attached to a base131by screwing, and by operating a clamping screw145to narrow a clearance144in a split part, clamping force is given to the support rod136to fix the support rod136to the base131. Also, inFIG. 16(b), a support rod136is fixed to a base231by using a fixing member150provided with a hole to be inserted with the support rod136and a male screw part153, and screwing the male screw part153into a screw hole (female screw) formed in a base231in a state where the support rod136is bonded and fixed into the hole.

In a utensil where a sphere35is arranged by a conventional fixing method illustrated inFIG. 16, when bringing a probe of CMM into contact with the sphere for coordinate measurement, the position of the sphere35has been slightly changed in some cases because of the insufficiency of the rigidity and strength of the fixing member140,150. Also, when tilting the utensil or turning it upside down at the time of moving the utensil, the position of the sphere35has been slightly changed in some cases because of the insufficiency of clamping force by split clamping inFIG. 16(a), because of the engagement backlash of the screw inFIG. 16(b), or the like. The utensil for evaluating a length measurement error in the X-ray CT device for three-dimensional shape measurement is one for evaluating a length measurement error by X-ray CT from the difference between the value of an actual inter-sphere distance obtained from a coordinate measurement result and the value of an inter-sphere distance in an imaging space measured at the time of X-ray CT imaging, and therefore a change in sphere position after measuring the positions of respective spheres in the utensil by CMM is not allowed even if it is slight.

The present invention has been made in order to solve the above-described problem, and a second object is to provide a utensil for evaluating a length measurement error in an X-ray CT device for three-dimensional shape measurement, which prevents a sphere position from being changed even when a significant amount of external force is applied to a sphere or a support rod by contact of a probe or the like, or even when tilted or turned upside down at the time of movement.

FIG. 17is a schematic cross-sectional view illustrating conventional bonding structures of a sphere at the tip of a support rod.

In a conventional forest gauge, when arranging spheres in a space, the support of spheres by the support rods is achieved by bonding ruby spheres or sapphire spheres commercially available as probes for CMM to the tips of the support rods. Bonding structures of a sphere to a support rod include one in which as illustrated inFIG. 17(a), the upper surface of a support rod136side is processed in a concave shape so as to fit to a spherical shape, and a sphere35is bonded in such a manner as to place the sphere35on the concave part. Also, as illustrated inFIG. 17(b), there is one in which a perforated sphere235applied with hole drilling is prepared, the tip of a support rod is provided with a thin shaft239corresponding to the hole, and the thin shaft239on the support rod236side is press-fitted into the hole of the perforated sphere235and then bonded and fixed. It is considered ideal that the clearance between the processed surface of the concave part and the lower surface of the sphere35illustrated inFIG. 17(a), or between the side surface of the thin shaft239and the inner wall surface of the hole of the perforated sphere235illustrated inFIG. 17(b)is uniform, and the clearance is filled with bond for fixation.

The bonding structure inFIG. 17(a)has the problem that it is difficult to smoothly and accurately process the processed surface of the concave part on the support rod136side so as to fit to the curved surface of the sphere35, and the mutual positional relationship between the sphere35and the contact surface of the support rod136is unstable because of the unevenness of the processed surface of the concave part. Also, when the curvature radius of the processed surface of the concave part on the support rod136side is larger than the radius of the sphere, the sphere35rolls in the concave part of the support rod136, and contrary when the curvature radius of the processed surface of the concave part is smaller than the radius of the sphere35, the sphere35contacts with the support rod136at multiple partial points, such as contacting with parts inside the concave part and the edge of the concave part. For this reason, even when the same length of support rod136is made to support the sphere, the height position of the sphere is slightly different.

Further, in the bonding structure inFIG. 17(b), the thin shaft239made of a different material from the perforated sphere235is present inside the sphere. For this reason, there is a problem that a transmission image obtained by X-ray irradiation is disturbed. That is, non-negligible deviations occur between the central position of the X-ray CT image of the sphere itself and the shape of the sphere supposed to be detected, and the central position of the X-ray CT image of the sphere itself and the shape of the sphere obtained by actual X-ray irradiation.

The present invention has been made in order to solve the above-described problems, and a third object is to provide a utensil for evaluating a length measurement error in an X-ray CT device for three-dimensional shape measurement, which enables the positional relationships between respective multiple support rods and spheres supported by them to be made uniform.

Solution to Problem

A first aspect of the present invention includes: a base; multiple spheres arranged in an XYZ space on the base; and multiple support rods supporting each of the multiple spheres and installed upright on the base, in which in the XYZ space on the base, multiple spheres having different Z positions are arranged, and among the multiple spheres, one or more spheres are arranged in a vicinity of a Z axis, and in each of multiple X-Y planes having different Z positions, multiple outer circumference side spheres are arranged along one outer circumference.

A second aspect of the present invention is such that in the utensil for evaluating a length measurement error in an X-ray CT device for three-dimensional shape measurement according to the first aspect, outer circumference side spheres arranged in each of the multiple X-Y planes are cylindrically arranged.

A third aspect of the present invention is such that in the utensil for evaluating a length measurement error in an X-ray CT device for three-dimensional shape measurement according to the first or second aspect, the spheres arranged in the vicinity of the Z axis are multiple inner circumference side spheres.

A fourth aspect of the present invention is such that in the utensil for evaluating a length measurement error in an X-ray CT device for three-dimensional shape measurement according to any of the first to third aspects, in each of the multiple X-Y planes, multiple outer circumference side spheres and one or more inner circumference side spheres are arranged.

A fifth aspect of the present invention is such that in the utensil for evaluating a length measurement error in an X-ray CT device for three-dimensional shape measurement according to the third or fourth aspect, a distance between the inner circumference side spheres and the Z axis is set to be 20% or less of a distance between the outer circumference side spheres and the Z axis.

A sixth aspect of the present invention is such that in the utensil for evaluating a length measurement error in an X-ray CT device for three-dimensional shape measurement according to any of the first to fifth aspects, an arrangement range of the multiple spheres is a range where a distance in a Z direction between an X-Y plane having a lowest Z position and an X-Y plane having a highest Z position among the multiple X-Y planes is larger than a distance from the Z axis in XY directions.

A seventh aspect of the present invention is such that in the utensil for evaluating a length measurement error in an X-ray CT device for three-dimensional shape measurement according to any of the first to sixth aspects, the outer circumference side spheres are arranged in such a manner that an average value of the distances between respective outer circumference side sphere in each of the X-Y planes and the Z axis is equal amount the multiple X-Y planes.

An eighth aspect of the present invention is such that in the utensil for evaluating a length measurement error in an X-ray CT device for three-dimensional shape measurement according to any of the first to seventh aspects, the multiple X-Y planes are at least three X-Y planes whose mutual distances in a Z direction are equally separated.

A ninth aspect of the present invention is such that in the utensil for evaluating a length measurement error in an X-ray CT device for three-dimensional shape measurement according to any of the first to eighth aspects, as the arrangement of the outer circumference side spheres, multiple sets of two spheres separated from the Z axis by a predetermined distance and oppositely arranged with respect to the Z axis are arranged in each of the multiple X-Y planes having different Z positions, and thereby at least four are arranged in each of the X-Y planes.

A tenth aspect of the present invention is such that in the utensil for evaluating a length measurement error in an X-ray CT device for three-dimensional shape measurement according to the ninth aspect, the multiple X-Y planes are at least three X-Y planes whose mutual distances in a Z direction are equally separated, and among the multiple spheres, multiple sets of two spheres oppositely arranged with respect to the Z axis are arranged, and thereby spheres arranged in each of the three X-Y planes are equally arranged at intervals of substantially 30 degrees on a circle around the Z axis in a plan view where the three X-Y planes are superposed.

An eleventh aspect of the present invention is such that in the utensil for evaluating a length measurement error in an X-ray CT device for three-dimensional shape measurement according to the ninth or tenth aspect, in each of the multiple X-Y planes, one or more inner circumference side spheres are also arranged, and the one or more inner circumference side spheres are arranged on lines connecting the outer circumference side spheres that are oppositely arranged.

A twelfth aspect of the present invention is such that in the utensil for evaluating a length measurement error in an X-ray CT device for three-dimensional shape measurement according to any of the first to eleventh aspects, the outer circumference side spheres are arranged at regular intervals on a circle around the Z axis in a plan view of one X-Y plane.

A thirteenth aspect of the present invention is such that in the utensil for evaluating a length measurement error in an X-ray CT device for three-dimensional shape measurement according to any of the first to twelfth aspects, the base has a flat surface on a top thereof, and the lengths of the multiple support rods are different each other so that the Z positions of the arranged multiple spheres are different each other.

A fourteenth aspect of the present invention is such that in the utensil for evaluating a length measurement error in an X-ray CT device for three-dimensional shape measurement according to any of the first to thirteenth aspects, the multiple support rods are respectively provided with support rod holding mechanisms inserted into holes that are formed in the base so as to correspond to the number of the multiple support rods, and each of the support rod holding mechanisms includes: a bottomed cylindrical member that is inserted into one of the holes of the base, whose side surface is provided with multiple through-holes, and that is provided with a space into which a one of the support rods is inserted; a loading bolt whose central part is provided with a hole through which the support rod is made to penetrate and whose outer circumferential part is provided with a male screw part screwed into a female screw formed on the inner wall of an opening of the cylindrical member; fixing block inserted into one of the through-holes of the cylindrical member and having a convex part that abuts on the side surface of the support rod whose end part on a side opposite to a side where a sphere is supported abuts on the bottom of the cylindrical member, and a load transmitting block that is inserted into one of the through-holes of the cylindrical member and in two directions, transmits force generated when the loading bolt is fastened to the cylindrical member by screwing the female screw and the male screw part together, the two directions being a direction to constrain the degree of freedom of the support rod in a translation direction in an X-Y plane and a direction to constrain the degree of freedom of the support rod in a translation direction on the Z axis.

A fifteenth aspect of the present invention is such that in the utensil for evaluating a length measurement error in an X-ray CT device for three-dimensional shape measurement according to the fourteenth aspect, the through-holes of the cylindrical member are provided in three positions of the side surface of the cylindrical member at regular intervals around a cylinder axis, and in two of the through-holes, the fixing blocks are arranged, and in one, the load transmitting block is arranged.

A sixteenth aspect of the present invention is such that in the utensil for evaluating a length measurement error in an X-ray CT device for three-dimensional shape measurement according to the fourteenth or fifteenth aspect, the load transmitting block includes three wedge-shaped members mutually joined via a tilt surface.

A seventeenth aspect of the present invention is such that in the utensil for evaluating a length measurement error in an X-ray CT device for three-dimensional shape measurement according to any of the first to sixteenth aspects, one end of each of the support rods is provided with a conical concave part, and a sphere is supported in contact with the conical tilt surface of the conical concave part.

An eighteenth aspect of the present invention is such that in the utensil for evaluating a length measurement error in an X-ray CT device for three-dimensional shape measurement according to the seventeenth aspect, a bottom of the conical concave part of the support rod is provided with a through-hole.

Advantageous Effects of Invention

According to the first to eighteenth aspects of the present invention, since in the XYZ space on the base, one or more spheres are arranged in the vicinity of the Z axis, and in each of the multiple X-Y planes having different Z positions, multiple outer circumference side spheres are arranged along one outer circumference, not only from a region where the multiple outer circumference side spheres are arranged in the space, but also to a region where one or more spheres are arranged in the vicinity of the Z axis on the inner side of the region where the outer circumference side spheres are arranged, an imaging space can be evaluated. Also, since in each of the multiple X-Y planes having different Z positions, multiple outer circumference side spheres are arranged along one outer circumference, the problem that an unevaluable region remains between conical spaces as in a conventional forest gauge where spheres are arranged on conical surfaces can be solved.

According to the fourth aspect of the present invention, one or more inner circumference side spheres are arranged in each of the multiple X-Y planes, and thereby multiple spheres having different Z positions are arranged in the vicinity of the Z axis, so that multiple measurement points can be given in the Z direction at the center of an evaluation range. In doing so, the mutual positional relationship among cylindrical spaces each serving as an evaluation target at each time of X-ray CT imaging among multiple times of repeated X-ray CT imaging can be evaluated.

According to the sixth aspect of the present invention, since the arrangement range of the multiple spheres is set as a range where the distance in a Z direction between an X-Y plane having the lowest Z position and an X-Y plane having the highest Z position among the multiple X-Y planes is larger than a distance from the Z axis in XY directions, a cylindrical imaging visual space can be isotropically evaluated by a smaller number of times of X-ray CT imaging than before. The number of times of performing repeated measurement with a Z axis position changed can be reduced than before, and therefore imaging time for evaluating the measurement accuracy of an X-ray CT device can be shortened.

According to the ninth aspect of the present invention, since as the arrangement of the outer circumference side spheres, at least four spheres are arranged separated from the Z axis by a predetermined distance in each of the multiple X-Y planes having different Z positions, a conventional structure where spheres are arranged on conical surfaces is not formed, and therefore spatial distortion specific to an X-ray CT device can be sufficiently captured. Accordingly, the three-dimensional shape measurement accuracy of the X-ray CT device can be evaluated.

According to the thirteenth aspect of the present invention, since by forming the top of the base as a flat surface and differentiating the lengths of the multiple support rods, the arrangement of the spheres having different Z positions is achieved, the shadow of a step appearing when a base is formed stepwise as conventional is not imaged, and a range evaluable at one time of imaging is widened, thus making it possible to reduce the number of times of performing repeated measurement with a Z axis position changed than before.

According to the fourteenth to eighteenth aspects of the present invention, since each support rod can be constrained in the three-dimensional space by the support rod holding mechanism, a utensil that even when a significant amount of stress is applied to a sphere or a support rod, or even when turned upside-down during transportation or installation, prevents a change in sphere position can be fabricated.

According to the seventeenth and eighteenth aspects of the present invention, since in a support rod, the tip supporting a sphere is shaped as the conical concave part, the sphere comes into line contact with the conical tilt surface of the conical concave part, and individual differences among spheres can be made less influential to uniform the positional relationships between spheres and corresponding support rods.

According to the eighteenth aspect of the present invention, by providing the bottom of the conical concave part of the support rod with the through-hole, bond can be allowed to escape, and force for further pulling the sphere toward the support rod side is generated when the bond cures, making it possible to increase force for holding the sphere.

