WO2007131723A1 - Method for measuring a solid body - Google Patents

Abstract

The invention relates to a method for measuring a solid body (11), using a first measuring method (computed tomography method) X-raying the solid body, combined with a second measuring method for measuring at least one surface of the solid body (11). The aim of the invention is to increase utilization of the two measuring methods and to optimize the calculation of the results. According to the invention, measured values obtained by the second method are used to associate points on the surface of the solid body with respective local segmentation threshold values for the evaluation of the data obtained by the first measuring method. These threshold values can be used to determine additional local segmentation threshold values in the vicinity of the points.

Description

 Method for measuring a solid

The invention is in the field of surveying methods, by means of which the shape of solids can be detected multi-dimensionally.

One field of application of such a method is the industrial quality control of test pieces, for example in the manufacture of cast parts, in which the shape and dimensional accuracy of castings must be checked by three-dimensional measurement and compared with specifications. In this step, in addition to the external shape accuracy and the presence of voids or material distortions is checked. Overall, the actual geometry data is with

To compare target data, for example, from a CAD system.

Particularly in the case that internal structures of the bodies to be tested are also to be recorded. It is also necessary to use a radiographic method, for example an X-ray computer tomographic method. In such a method, a solid to be measured is successively irradiated from several different directions, wherein usually the x-ray source is fixed and the solid body is rotated about a fixed axis. When viewed from the X-ray source behind the solid is a detector, for example as a two-dimensionally resolving detector in the form of a CCD matrix, which detects the X-rays after passing through the solid. Depending on the position of the solid, there are different gray values, which are calculated in a known manner, taking into account the between the individual

Measurements completed angle of rotation can be further processed.

As a result, individual volume elements into which the solid is subdivided for calculation can be assigned so-called norms, intensity values which correspond to the degree of absorption of the X-radiation in the respective voxel.

From the degree of intensity can be concluded that the consistency of the solid in the respective volume unit. As a result, consistency limits of the solid, so for example, material or density limits or outer contours and boundaries of cavities can be determined.

Experience has shown that such measurements using X-ray computer tomography do not yet achieve the desired accuracy. On the one hand, this is due to the fact that the x-ray tubes used emit polychromatic radiation, but also The fact that the interaction of the X-ray radiation when passing through the matter depends not only on the absorption properties of the respective matter, but also on the wavelength of the X-ray radiation. In addition, scattered radiation effects, interference due to beam hardening and nonlinearities of the detector result, which cause artifacts, distortions and so-called cupping effects, which impair the image quality.

The present invention has the object to increase the reliability and accuracy of the measurement of solids by combining different surveying. In this case, the invention relates to a method for measuring a solid using a first surveying method (computed tomography method) radiating through the solid in conjunction with a second measuring method for measuring at least one point of at least one surface or at least one surface of the solid.

Such a method using the combination of two surveying methods is basically already known from the prior art. Such is already in the patent Abstract of Japan, Pub. No. 2002071345 A describes a method in which so-called surveying paths are determined by means of a first measuring method for a solid, by means of which and along which the measurement takes place by means of a second measuring method.

It is known from DE 103 31 419 A1 to measure a solid by means of two measuring methods such that the position of the solid is first detected by the first measuring method, and that it is moved by means of a manipulation device in the core detection area of a measuring system for the application of a second surveying method.

In the surveying methods mentioned, there is no real coupling in the evaluation of the data collected by the various surveying methods.

DE 10 2004 026 357 A1 discloses a device which is used for measuring an object and has an X-ray sensor system and another tactile or optical sensor system, wherein both sensor systems deliver evaluatable data in a common coordinate system. The X-ray sensor system is positioned according to the detection by the other sensor. Excellent points of the object to be measured are measured by means of the further sensor system and from this geometry features are determined which are used to calibrate the X-ray sensor system.

From the reprint "Quality Engineering 6," June 2005 (www.qe-online.de), it is also known to combine several surveying methods, in particular to compensate for inaccuracies in a X-ray computed tomographic measurement Measurement results supplied matched measurement results and the results of the inaccurate sensor with

Help of the more detailed information corrected. First, a so-called CT point cloud is generated and these data are corrected with the data acquired in another way. The present invention, however, has the object to improve the interaction of such different surveying methods in terms of the result and a facilitated and accelerated implementation.

According to the invention, the object is achieved by assigning a local segmentation threshold value for the evaluation of the data obtained by the first measurement method to points of the surface of the solid body by means of the measured values obtained by the second method. Advantageously, it can additionally be provided that these threshold values are each used as a basis for the determination of further local segmentation threshold values of the surroundings of the points.

The result of a computed tomography is normally in the form that intensity units or gray values are assigned to specific volume units (voxels) and that edges, surfaces or material boundaries of a solid body are represented by jumps in the gray value between the voxels. However, such jumps are not ideal and there are often blurred or gray scale transitions in real measurement results. According to the prior art, such a computed tomography image is evaluated by defining a threshold value of the gray values representing the boundary surfaces of the solid.

In accordance with the present invention, the concept of a global threshold is generally abandoned in favor of local thresholds. These are determined by local comparison of the data determined by the second measuring method with the data determined by the first measuring method. sets. That is, by means of the second assessment method, for example, an interface of the solid is detected at one point and it is determined where the local threshold value would have to lie so that the evaluation of the measured values determined by the first method would yield the correct result. This threshold value is defined as a local threshold value and also serves as a threshold value in the immediate vicinity of said location, or at least the threshold value prevailing in the immediate vicinity is determined from the threshold value defined as described. This can also be done, for example, by interpolation of the threshold values at different locations. For this purpose, measurements are carried out by means of the second measurement method at two spaced-apart locations, from which threshold values corresponding to threshold values determined by comparison with the first measuring method are determined and defined as local threshold values. Thereupon, at the positions lying between the two measuring points, further local threshold values are defined by interpolation, which serve for the further evaluation of the data obtained by the first measuring method.

Threshold values can also be determined, for example, in the interior region of the solid to be measured, where no measurement data can be acquired by the second measurement method, insofar as this is a surface measurement method, for example a strip projection method.

For storing the intensity matrix, a first memory device is advantageously used in a device according to the invention for carrying out the method according to the invention, a second memory being used for storing the local threshold values. chereinrichtung according to the invention.

In order to be able to use the data obtained from the second surveying procedure to process the data obtained from the first method, accurate matching, i. a conversion to a common coordinate system, necessary. This is advantageously realized by first measuring a known body by means of the first and second surveying methods for calibrating the surveying method and then determining a coordination transformation for converting the data obtained by the second surveying method into the coordinate system used in the first surveying method or vice versa ,

Thereafter, with known coordination transformation, the combined evaluation of the data according to the invention is possible without any problems, as long as the surveying devices for the first and the second surveying methods remain in a defined position relative to one another or are moved against each other in a defined manner.

The method according to the invention is particularly advantageous if the first measuring method is an X-ray computer tomography method by means of which an intensity value is assigned to each of the individual volume units.

However, the method can be advantageously applied to other types of computed tomography, such as neutron beam computer tomography.

In each case, a consistency limit in the evaluation of the intensity values by determining the Determines where the intensity value assigned by the first surveying method exceeds the predetermined local threshold.

In this case, as described above, the local threshold is partly known by matching the measurements from the second surveying method. However, the same threshold value can also be used in areas in the immediate vicinity by means of the measuring point measured in the second measuring method. The measuring points which are measured by means of the second measuring method can thus be spatially less densely distributed than the measuring points of the first measuring method.

It can also be provided that local threshold values are determined by interpolation between known local threshold values.

Determination of consistency limits of the solid can be done with an accuracy that exceeds the resolution given by volume units (voxels). For this purpose it is provided that, using the intensity values, a plurality of adjacent volume units (voxels), at least one of which has an intensity above the local threshold and at least one intensity below the local threshold value, the point at which the threshold value is exceeded is set approximately to a range smaller than one unit volume (subvoxel accurate).

Since the local threshold is initially often set only voxel exactly, a higher accuracy can be achieved by using an iterative method. For this purpose, for example, according to the subvoxelge- nauen Determination of the point at which a body boundary is again redefined by interpolation at exactly this point the threshold locally. After the renewed determination of the threshold value, in a further iteration step, the

Where the newly submitted threshold is exceeded, be redetermined.

With each iteration step results in this method, a greater accuracy.

For the automated performance of the evaluation, it is advantageous if the local threshold values for the volume units are determined individually and stored in a matrix. In this case, this matrix corresponds in size to the matrix with the voxel data, in which an intensity value is assigned to each individual voxel.

A further advantageous embodiment of the method provides that the local threshold values determined in each case for the same volume units are subtracted from the intensity values determined for the individual volume units (voxels) and that thereafter the segmentation is carried out by means of a global threshold value.

Thus, the last process step to perform the determination of the body boundaries by means of a global threshold corresponds to the previously common method. The advantage of the invention is shown in the intermediate step, in which local, different threshold values are used. This improves the resolution and accuracy and reduces the susceptibility to errors. A device according to the invention for carrying out the method according to the invention advantageously has a subtraction device which, after determining the local threshold values, reduces the individual elements of the matrix of intensity values from the first memory device by the local threshold values stored in the second memory device , The results are stored in a third memory device. The data stored in the third memory device can now be used for global evaluation, ie for the entire solids constant, threshold value. The nonuniformities and nonlinearities which are unavoidable when using the first measuring device are optimally calculated out in the data thus cleaned up in the third memory device. The evaluation of these data thus results in an adjusted three-dimensional image of the solid to be measured.

The method according to the invention can be used particularly advantageously if the second measuring method is a surface-resolution method, in particular a strip projection method. In a streak projection method, a known pattern, for example, a straight, parallel stripe pattern or grid lines, is projected onto the surface of the solid to be measured by means of an optical image and the image formed on the surface of the solid is taken from an angle that is typically different from the angle of incidence , From the distortions of the ideally straight stripe pattern can be closed by triangulation on the shape of the surface. Such a stiffener projection method is the ideal supplement to a transmission method, for example a computed tomography X-ray method, in order to achieve overall accurate and reliable data not only for the externally visible surface of a solid to be measured.

The expenditure for the mechanical construction of a corresponding measuring device and the accuracy and reliability of the measured data is optimized if the same manipulation device is used to move the solid body for the first and second measuring methods. Typically, during a stepwise rotation of the solid to be measured, both a number of

Transmitted radiographs as well as a corresponding number of surface survey records.

In the following, the invention will be shown with reference to an embodiment in Figures 1 to 3 and described below.

It shows

FIG. 1 shows a flow chart which represents the method according to the invention,

FIG. 2 shows a combined measuring device,

FIG. 3 shows a two-dimensional arrangement of voxel data and the corresponding processes during the evaluation of the data.

FIG. 1 shows by way of example and schematically simplified three two-dimensional fluoroscopy images 1, 2, 3, which are recorded and stored on a radiographic screen by means of an X-ray tomography method at different transillumination angles with respect to the transilluminated object as transillumination images. By means of the known computed tomography calculation method, these transillumination images, which represent the solid to be measured from different angles, can be converted into a three-dimensional image of the object. It is of course possible to use far more than three images for the calculation.

The three-dimensional image is in the form of gray values or intensity values in a three-dimensional voxel matrix 4. Each subvolume cube of this matrix 4 is assigned a separate intensity value.

On the right-hand side of FIG. 1, an illustration of a pyramid 5 is shown by way of example, which has been produced using a surface-measuring method, the second surveying method. For example, this measurement can contain fewer measurement points than the measurement obtained by the first measurement method, but the individual measurement points are more accurate than the computed tomography method.

The measurement results 5 of the image obtained by the second measurement method are included in the matrix by setting threshold values for the evaluation of the computed tomography data at the points at which they are determined on the basis of the body boundaries defined by the second measurement method, namely as local, if appropriate locally different thresholds.

Where appropriate information is not available by means of the second surveying method, the local threshold values are determined by interpolation of the remaining, present threshold values.

Thus, for the entire matrix 4 with the same density as the intensity values, there are also corresponding local threshold values, which are represented in the form of the matrix 6.

By applying the threshold values stored in the matrix 6 to the intensity values of computed tomography stored in the matrix 4, the body boundaries of the solid to be measured can be calculated precisely. The corresponding calculation result is shown symbolically in the matrix 7 and more clearly in the pyramid 8. The advantage of the final measurement result 8 lies in the fact that on the one hand partially measured values are available with the high measurement accuracy of the second measurement method, but on the other hand also where these Measurements are not present, can be obtained by the first surveying reliable values whose accuracy has been increased by using the second measurement method. As a result, cavities in the solid body, for example, which are not detected by means of the second measuring method, insofar as it detects a surface measurement, can be detected precisely by means of a transmission method. This is especially important when used in the quality assurance process in the inspection of castings or the like. FIG. 2 shows by way of example a device for carrying out the method according to the invention, the individual elements being shown only schematically. On a base plate 9, a movement device 10 is arranged for the solid 11 to be measured, with a drivable shaft 12, a motor 13 and a gear 14 and a turntable 15. The solid is firmly positioned on the turntable 15 and can be driven by the drive about the vertical axis 16 in small steps, for example, of 0.9 °, rotated and stopped therebetween.

In addition, two beam sources are fixedly mounted on the base plate 9, an X-ray source 17, for example in the form of a microfocus X-ray tube, which radiates the solid 16 to an X-ray screen 18, and a strip projection device 19 which radiates a geometric pattern onto the solid 11 and the distortions , which result from the shaping of the solid 11, detected.

Instead of the stiffener projection device 19, it is also possible to provide another surface imaging device, for example a touch device or a distance measuring device, using running time measurements and an interferometric measuring device.

It is important that the two devices 17, 19 are fixed to each other or, if they are moved against each other, this movement is precisely defined in the common coordinate system.

The data captured by the X-ray screen 18 is passed to a computing device 20 which also receives data from the drive means 12, 13, 14 receives about the angular position of the solid 11. In addition, the computing device 20 also receives the measurement results of the stiffener projection device 19. Thus, the computing device can perform the calculations shown in FIG. 1 and store and process the abovementioned three-dimensional matrices.

As a result, the computing device 20 determines a representation of the solid 11 in three dimensions, which is for example in different views on the screen 21 d3.rstellba.r.

With reference to FIG. 3, a simplified schematic two-dimensional example is used to illustrate how the results of the two surveying methods are computationally combined.

First, in the matrix 22, a set of pixels is shown, to which intensity values in the form of numbers are assigned. These represent a measurement result of computed tomography.

In addition, measurements of the body boundary of the solid to be measured are drawn in two places, in the form of lines 23, 24. These data were obtained by the fringe projection method.

If the result of the second surveying method was not taken into account in the evaluation of the tomography data, a global threshold value for determining the shapes of the solid body between the intensity 1 and the intensity 2 could be determined, for example. Correspondingly, it would be mathematically checked at which points in the two-dimensional matrix (in reality in a three-dimensional matrix). len matrix) this threshold is exceeded. This would result in a solid boundary, which is shown by dashed lines in the right-hand part in the matrix 25 in FIG. 3 and which runs between the pixels representing the intensity 1 and the respective adjacent pixels representing the intensity 2.

If the method according to the invention is used, local threshold values are determined instead of a single global threshold value, in that all present measurement results of the second surveying method, represented by the two lines 23, 24 in the matrix 22, are taken into account. This leads to the result that in the left part of the matrix the local threshold lies between 1 and 2, while in the right part of the matrix, i. in another area of the solid, lies between 2 and 3. It is now approximately this local threshold extended to neighboring areas and only then determined by means of the now known for the entire matrix thresholds the corresponding solid state limit. This results in the matrix 26 as a result. This results clearly in a difference from the result from the matrix 25, which was created using a single global threshold value.

Thus, with the aid of the method according to the invention, the results of both surveying methods can be combined with one another in an optimized manner

Measurement errors in the computed tomography measurement can be compensated.

Claims

.Patentansprüche 1. A method for measuring a solid body (11) using a first surveying method radiating through the solid in conjunction with a second measuring method for measuring at least one point of at least one surface of the solid, characterized in that by means of the measured values (23, 24) points of the surface of the solid (11) in each case a local Segmentierungsschwellwert for the evaluation of the data obtained by the first surveying method is assigned. 2. The method according to claim 1, characterized in that the associated threshold values are each used as a basis for the determination of further local segmentation threshold values of the surroundings of the points. 3. Method according to claim 1, characterized in that segmentation threshold values for the evaluation of the data obtained by the first measuring method pass through between different points of the solid body for which measured values (23, 24) respectively obtained by the second measuring method are present Be obtained interpolation. 4. The method according to any one of claims 1 to 3, characterized in that for calibration the surveying method is first measured a known body by means of the first or second surveying method and that thereon a coordination transformation for converting the data obtained by the second surveying method is determined in the used in the first surveying coordinate system or vice versa. 5. Method according to one of the preceding claims, characterized in that the first measuring method is a X-ray computed tomographic method, by means of which individual volume units (voxels) are each assigned an intensity value. 6. The method according to any one of the preceding claims, characterized in that the first surveying method is a computer tomographic method that uses the neutron radiation. 7. Method according to claim 5, characterized in that consistency limits of the solid are determined during the evaluation of the intensity values by determining the locations at which the intensity values assigned by means of the first measuring method exceed the predetermined local threshold values. 8. The method according to claim 7, characterized in that, taking into account the intensity values of a plurality of adjacent volume units (voxels), of which at least one has an intensity above the local threshold and at least one below the local threshold intensity, the location at which the threshold is exceeded is set approximately to a range that is smaller than a volume unit (subvoxelgenau). 9. The method according to claim 7 or 8, characterized in that after fixing the point again by interpolation at exactly this point, the threshold is set. 10. The method according to claim 9, characterized in that, after the renewed determination of the threshold value in a further iterating step, the point at which the newly defined threshold value is exceeded is newly determined. 11. The method according to any one of the preceding claims, characterized in that local threshold values for each volume unit (voxels) are determined and stored in a matrix. 12. The method according to any one of the preceding claims, characterized in that subtracted from the intensity values determined for the volume units, the local threshold values respectively determined for the same volume units and that thereafter the segmentation is performed by means of a global threshold value. 13. The method according to any one of the preceding claims, characterized in that the second measurement is a stiffener projection method. 14. The method according to any one of the preceding claims, characterized in that for the first and the second surveying the same manipulation device (12, 13, 14) for moving the solid body (11) is used. 15. The method according to any one of the preceding claims, characterized in that for carrying out the first or second Vermessungsverfahrens the solid state (11) is rotated in predetermined angular increments about at least one axis.