CROSS-REFERENCE TO RELATED APPLICATION

The disclosure of Japanese Patent Application No. 2019-163422 filed on Sep. 6, 2019 including specifications, drawings and claims is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a measurement X-ray CT apparatus for industrial products. In particular, the present invention relates to a measurement X-ray CT apparatus that can measure dimensions of an entire object to be measured, including its internal structure, with high precision even if a geometrical positional relationship between the focus of an X-ray source, an X-ray detector, and the like changes from when calibrated.

BACKGROUND ART

Measurement X-ray CT apparatuses have heretofore been used to observe and inspect voids in casting parts, welding defects in welded parts, and defects in electronic parts, which are difficult to identify from external appearance. Meanwhile, the prevalence of 3D printers in recent years is boosting demand for 3D dimension measurement inside workpieces and enhancement of its precision. To meet such a demand, measurement X-ray CT apparatuses taking length traceability into account have been developed. A variety of inventive contrivances inspired by and following such a trend are also being made to meet the demand for dimension measurement with higher precision.

FIG. 1(plan view) andFIG. 2(side view) show a configuration example of a conventional measurement X-ray CT apparatus. The measurement X-ray CT apparatus1includes an X-ray source12, an X-ray detector14, a rotating table16, a scanning mechanism18, and an arithmetic control unit20(seeFIG. 2) as main components. The X-ray source12emits X-rays. An example of the X-ray source12is an X-ray tube. The X-ray detector14detects an X-ray beam13that is emitted from the X-ray source12and transmitted through the periphery and interior of a work10that is an object to be measured, and renders a projection of the work10into a projection image. An example of the X-ray detector14is a flat panel display. The rotating table16is located between the X-ray source12and the X-ray detector14, and the work10is placed on the rotating table16. The scanning mechanism18moves the rotating table16to a given position in a measurement space region.

InFIG. 1, the horizontal direction from the X-ray source12toward the X-ray detector14is referred to as an X-axis, and a direction perpendicular to the X-axis is referred to as a Y-axis. A direction perpendicular to the XY plane is referred to as a Z-axis.

The rotating table16can be moved by the scanning mechanism18in the X-, Y-, and Z-axis directions, with the work10placed thereon. The rotating table16can rotate the work10about a 0-axis. By such adjustments, the projection image of the work10obtained by the X-ray detector14can be adjusted in position and magnification.

To obtain a three-dimensional image or volume data (CT image) of the work10, which is the ultimate goal of the measurement X-ray CT apparatus1, a CT scan is performed on the work10.

As shown inFIG. 3, a CT scan includes two processes, namely, acquisition of projection images of the work10and CT reconstruction. In the projection image acquisition process, the rotating table16with the work10placed thereon is continuously rotated at a constant speed or intermittently rotated in constant step widths during X-ray irradiation, whereby projection images of the work10in all directions around (constant intervals) are obtained. The volume data of the work10is obtained by performing CT reconstruction on the obtained projection images in all directions around (constant intervals) by using a CT reconstruction algorithm such as back projection and iterative reconstruction.

The obtained volume data can be used to perform various measurements such as dimension measurement and defect analysis.

In order for the foregoing measurement X-ray CT apparatus to perform dimension measurement with higher precision, it is important to perform various types of calibration specific to the apparatus before start of measurement. For example, Japanese Patent Application Laid-Open No. 2012-189517 describes a calibration method and an evaluation method of an X-ray CT apparatus using a standard gauge for calibration and evaluation.

A geometrical positional relationship between the focus of the X-ray source12, the X-ray detector14, and the rotation center of the rotating table16is calibrated in advance. The measurement X-ray CT apparatus1then obtains projection images by irradiating the work10with the X-ray beam13to perform a CT scan, and generates volume data of the work10by CT reconstruction of the projection images.

SUMMARY OF INVENTION

Technical Problem

The geometrical positional relationship between the focus of the X-ray source12, the X-ray detector14, and the like is known to change slightly from when calibrated, because of the effect of heat generation by various devices and variations in the surrounding environment. Such a change adversely affects the quality (image quality and geometrical precision) of the volume data.

Japanese Patent Application Laid-Open No. 2012-112790 describes capturing an image of markers attached to a rotating table with an optical camera, and correcting deviations of projection data in position and angle due to rotation accuracy. However, such a technique can only correct deviations due to the rotation accuracy, and has had a problem of failing to provide a sufficient effect since no reference frame is used.

The present invention is achieved in view of the foregoing problem, and an object thereof is to enable high-precision X-ray CT measurement of the dimensions of an entire object to be measured, including its internal structure, even if the geometrical positional relationship between the focus of the X-ray source, the X-ray detector, and the like changes from when calibrated.

Solution to Problem

The present invention solves the foregoing problem by the provision of a measurement X-ray CT apparatus including a rotating table for an object to be measured to be placed on and a scanning mechanism that is capable of moving the rotating table to a given position in a measurement space region. The rotating table and the scanning mechanism are located between an X-ray source and an X-ray detector. The measurement X-ray CT apparatus is configured to calibrate a geometrical positional relationship between a focus of the X-ray source, the X-ray detector, and a rotation center of the rotating table in advance, obtain projection images by irradiating the object to be measured with X-rays to perform a CT scan, and generate a three-dimensional image of the object to be measured by CT reconstruction of the projection images. The measurement X-ray CT apparatus further includes a reference frame that is made of a material and has a structure less susceptible to an environmental change, and a sensor that is located on the reference frame and intended to successively obtain calibration values of the geometrical positional relationship between the focus of the X-ray source and the X-ray detector during the CT scan. The calibration values are used as parameters of the CT reconstruction.

The reference frame may be disposed at positions of the X-ray source and the X-ray detector with the rotating table interposed therebetween.

The reference frame may be a metrology frame of rectangular solid shape.

The reference frame may be disposed at positions of the rotating table and the X-ray source, and a marker disposed on the X-ray detector may be detected by a camera disposed on the reference frame.

The reference frame may be a metrology frame of cubic shape.

The marker may be a light-emitting diode.

The reference frame may be disposed at the position of the X-ray source, and a corner cube disposed on the X-ray detector may be detected by a laser interferometer disposed on the reference frame.

An eccentricity measuring device may be disposed on the rotating table, and a sensor configured to detect a position of the eccentricity measuring device may be disposed on the reference frame.

A reference surface for eccentricity measurement may be formed on an outer periphery of the rotating table.

A sensor configured to detect a position of the rotating table may be located on the reference frame.

The sensor may be a laser tracker configured to detect a cat eye reflector disposed on a measurement object fixing jig on the rotating table.

Advantageous Effects of Invention

According to the present invention, high-precision X-ray CT measurement of the dimensions of an entire object to be measured, including its internal structure, can be performed even if the geometrical positional relationship between the focus of the X-ray source, the X-ray detector, and the like changes from when calibrated. This enables high-precision analysis because of improved quality of volume data, and can enhance measurement precision by the application of X-ray CT.

These and other novel features and advantages of the present invention will become apparent from the following detailed description of preferred embodiments.

