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Element technology, Analysis technology, Production technology

Measuring Methods for Product Development Using X-Ray CT Scanner

Toru Okidate* Takahiro Hotta* Yuki Kuno*

Abstract

Utilization of X-ray CT scanners has been expanding to wide industrial fields along with recent technological advancement. In our product development, this technology will particularly play important roles for identifying causality of changes inside products, especially geometric changes during manufacturing processes and temporal changes in endurance tests.For effective utilization of the X-ray CT scanner in product development phases, it is essential to develop efficient measurement technology and technique for each product with “high quality picture capturing”, “precise 3D imaging from the picture”, and “efficient data processing”.This report introduces the developed measurement technology and technique of the X-ray CT scanner with its application case in product development.

Key Words : Measurement, Heat exchanger , Electric motor / X-ray CT

1. Introduction Calsonic Kansei is committed to the achievement of “pre-vention of return in the development phase” and “zero defects” under the motto of “Dantotsu Monozukuri (Ulti-mate excellence in product development and manufactur-ing)” as one of its mid-term targets (Compass 2021). Under this framework, the company is striving to clarify the mechanisms between product design specifications and product performance, durability, productivity and yield, etc. For this purpose, measurement has a significant role, and further expansion of the measurable range is being requested in addition to demands for higher precision and less man-hours. Among others, significant un-met needs are observed in the measurement of dimensions and geometry of internal structures. Particularly in the case of heat exchanger products with complicated internal structures and inabil-ity to disassemble products after completion, it is difficult to capture the phenomena occurring in all parts despite the high importance. As most measurements are current-ly done by cutting the products, enormous workload, time and the number of parts are necessary (Fig. 1). In addi-tion, this approach makes it impossible to check detailed phenomenal changes before and after the process / tests using the same product. This issue poses a serious barrier in the clarification of mechanisms. On the other hand, the X-ray CT conventionally used in non-destructive internal observations and inspections has

started to be used in high precision geometry measure-ments supported by progress of X-ray imaging technology, computer performance and data processing technology. It is considered that we can solve the above-stated challenge using X-ray CT measurement equipment, if we can mea-sure internal dimensions of various parts of our products that our users request us to measure all at once using non-destructive methods.

Fig.1Conventionalinternalverificationmethod.

2. Principle of X-ray CT and problems with its application

We will briefly cover the measurement principle of X-ray CT below. X-ray refers to electromagnetic waves with a wave length of 1pm-10nm. While X-rays pass over the tar-get object, they are absorbed depending on the material, thickness and so forth of the object. The different level of this X-ray “permeability” of the target object is captured as transmitted X-ray intensity by an X-ray detector and

* Global Technology Division. Test Fundamental Technology Group

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Measuring Methods for Product Development Using X-Ray CT Scanner

turns it into a gray scale image （Material with low X-ray intensity appears black ⇔ Material with high X-ray inten-sity appears white) (X-ray transmission method). By using this X-ray transmission method, the X-ray CT obtains the image of target object from 360° as image information. Based on the image information, the X-ray intensity information obtained through imaging direc-tion and each sensor of the detector is reconstructed by computer, which then turns the internal and external structures of the target object into a 3D model. CT is an abbreviation for Computed Tomography, which is also called an “X-ray CT scan” as it scans objects (Fig.2)(1).

Fig. 2 Image of X-ray CT scan.

To maximize efficient use of the X-ray CT, we have intro-duced new equipment (Fig. 3) and also started to develop the following measurement technology to solve challenges that emerged in applying the equipment in practice.

1) High image quality imaging method suitable for products and efficient measurement methods

2) Multi-material measurement method3) Development of processing technology for X-ray

CT images In this paper, we will introduce a case study where the technology is applied in vehicle component development and describe details of the above-stated measurement technology development.

Fig. 3 Installed equipment.

3. Case studies for approaches to measure-ment-related issues3.1.… …High…image…quality…imaging…method…suitable…for…

products…and…efficient…measurement…method In the case of heat exchanger products such as built-in oil coolers, the geometry of fins and plates in each internal layer as well as their adhesion state influence product performance. Therefore, it is necessary to identify the mechanisms before and after each process, to realize secure adhesion and precise dimension of water passages at all layers. However, if conventional dimension measurements based on cutting were used, it is difficult to determine all the internal dimensions in addition to the enormous workload and time required. Because of this, it posed a barrier in fulfilling the improvement of development quality and reduction of development periods. To address this issue, we considered a procedure to measure internal dimensions with high precision using X-ray CT in all processes, from product components to the finished products, including interim processes (Fig. 4) . To realize above mentioned procedure, high image quali-ty imaging technology to realize high precision measure-ments and prompt detection of deviations from design specifications in addition to all the dimension changes in every component and every process were needed.

Fig.4Measurementflowineachprocess.

The key point for realizing high image quality is the reduction of unnecessary wavelength and energy that will cause noise in detecting the X-ray. For this purpose, we used appropriate X-ray radiation for specific X-ray intensi-ty depending on the material and thickness of the target object. Further, we reduced noise by using physical filters, etc. As a result, we could successfully obtain CT data that allows high precision measurement of joints between com-ponents, that were difficult to measure using conventional approaches (Fig. 5).

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Fig.5Highdefinitionbyconditionchange.

One of the challenges in detecting dimensions in a short period is the reduction of measurement frequency, since the frequency of measurement tends to increase with the measurement of every component at every process. To address this issue, multiple components were measured at once to reduce measurement frequency. To measure multiple components all at once, we created a jig made of low X-ray intensity that did not affect CT constitution. The dimensions of this jig was designed in accordance with the X-ray detector so as to achieve the precision necessary for the measurement. Further, the jig incorporates a function to maintain the measurement ob-ject in the direction adjusted to the orientation when the component is assembled in a finished product by giving consideration to the purpose of the geometry comparison from the component to the finished product (Fig. 6). Further, the data obtained by X-ray CT does not have a reference axis. However, to compare and validate dimensions before and after each process or when com-paring these with design specifications, setting of axis is necessary for each measurement. To address this issue, we used a marker for obtaining axis information during the measurement. The same material as the target object was used for the marker, because the marker should be constituted in the CT. Further, to facilitate comparison with the design specifi-cations, the marker was incorporated also in CAD data. By using these approaches, it has become possible to measure all dimension changes at every process within a short period.

Fig. 6 Measuring jig assuming product state.

3.2.… Multi-material…measurement…method In the evaluation of durability for motors adopted in air conditioning systems and heat exchangers, it is necessary to check aging-related changes of the brush and commu-tator integrated in the motor. In conventional approaches, measurement by disassembling was carried out after certain durability test cycles. For correct evaluation, many measurement samples and a high workload associ-ated with reassembling of disassembled components were needed to eliminate influence of product variation. While this issue can be addressed by adopting non-de-structive measurement based on X-ray CT (Fig. 7), we will introduce the multi-material measurement method (measurement of brush abrasion) cited in the case study.

Fig. 7 Durability evaluation process.

To measure brush abrasion, it is necessary to measure the motor housing (steel) and internal brush (synthetic resin) made from different materials at the same time. In X-ray CT measurement, threshold processing is con-ducted to X-ray intensity difference. Then, the borders are generated as isosurfaces, and the dimensions between the isosurfaces are measured. While steel motor housing has high X-ray intensity, X-ray intensity of the brush is low because of its synthetic resin material. As the brush cannot make a clear difference from the intensity of air, it is diffi-cult to fit the isosurface to the brush. To solve this issue, output of X-ray was enhanced to properly permeate the steel material. Further to enable capturing of resin material contour, noise was eliminated using a filter. Through these measures, it was possible to obtain brush geometry and to check the presence of abnormalities, such as defects or cracks, etc. (Fig.8). Then, contact points of the brush and spring were confirmed so that the measurement of the brush length could be possible based on the positions of the spring and commutator. As a result, high accuracy evaluation has become possi-

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Measuring Methods for Product Development Using X-Ray CT Scanner

ble, including the product variation and workload has been reduced by 90%.

Fig. 8 Observation section.

3.3.… …Development… of… processing… technology… for…X-ray…CT…images

Adhesion status between components is an important evaluation item for heat exchanger products. The use of X-ray CT has made possible to capture the adhesion status (number of points where normal adhesion status is observed) up to the internal parts of the products. On the other hand, it is difficult to analyze all the points manually because of the huge number of adhesion points. Further, the strong dependency of evaluation results on the skill of the evaluator is another challenge. To address these challenges, we conducted two types of technical development for efficiently screening and evaluating nec-essary data from X-ray CT data. First of all, the sectional images of each layer that pass through the adhesion section were selected from the imag-es of heat exchanger products with a multi-layer structure (Fig. 9). The template matching approach was adopted for this. Upon creating a template for sectional image of adhesion sections, level of similarity with the template was calculated for all the sectional images. The proximity of extreme value for the similarity level represents the sectional image that pass through the adhesion section. In some cases, section and adhesion face are not parallel. In such cases, the adhesion section is not reflected in a single sectional image. But, this issue could be solved by including more than one sectional image in proximity of extreme value into the scope of evaluation (Fig. 10).

Fig. 9 Finding adhesion sites.

Fig. 10 Complementing adhesions from several images.

Then, to improve the impact of image brightness caused by the difference of X-ray intensity associated with the product structure (Fig. 11), the sectional image was di-vided into multiple domains like a grid. For each domain, several approaches [such as Otsu’s method(2) etc.] were used to determine the threshold for binarization. After completing these two types of image processing, appropriate expansion and contraction were conducted (Fig. 12) before counting the number of adhesion sections. As a result, accuracy level equivalent to the level of an expert evaluator was achieved (Fig. 13) and also, the workload for evaluation of adhesion status of all points could be reduced by Approx. 90% compared to the level of visual checks.

Fig. 11 The variation of brightness.

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Fig. 12 Expansion processing.

Fig.13Thedifferenceoftwoevaluationmethods.

4. Summary As demonstrated in the above case, we could confirm that the X-ray CT is a useful technology for improving the quality of product development. We also perceived that the development of measurement methods and data processing technology that address peo-ple’s needs are necessary to realize efficient and effective use of X-ray CT in a wider area. Further, sophistication of X-ray CT performance is expected and therefore, the expansion of its usage from the perspective of product quality improvement and enhancement of development efficiency is also expected in the near future. We are also committed to the realization of competitive products in the future by developing and offering innova-tive measurement technologies to enhance efficiency of product development and to improve product quality. To conclude this paper, we would like to express our grat-itude to all the companies and agencies who supported our X-ray measurement.

References(1) GE Sensing & Inspection Technologies Co., Ltd. :phoe-

nix¦Introduction of x-ray products，RST11012，(2011)(2) Nobuyuki Otsu：An Automatic Thresholding Selection

Method based on discriminant and least squares stan-dards，Journal of Institute of Electronics and Commu-nication Engineers of Japan，vol.J63-D，No. ４，p.349-356 (1980)

Toru…Okidate Takahiro…Hotta Yuki…Kuno