CT INSPECTION OF CASTINGS WITH IMPROVED CALIBRATION

Transcription

1 Copyright 2004 SAE International O5M-498 CT INSPECTION OF CASTINGS WITH IMPROVED CALIBRATION Charles R. Smith, Kyung S. Han BIR Inc Uwe Bischoff, Bernd Georgi, Ferdinand Hansen, Frank Jeltsch Volkswagen Commercial Vehicles ABSTRACT Maintaining the high quality level of today's cars requires inspection by a variety of non-destructive testing methods. Specialized techniques are needed for highly stressed parts such as cast aluminum cylinder heads. Computerized Tomography (CT) is one of the best testing methods for checking complex areas such as combustion chambers, inlet/exhaust runners, and coolant passages. CT inspection simultaneously detects in-homogeneities, pores, shrinkages, and cracks while acquiring complete 3D dimension information on all internal and external surfaces. This adaptation of wellproven medical technology is now maturing to applications in the industrial environment and achieving harmonization with other production tools such as coordinate measuring machines and CAD software packages. An important goal in today's fast paced design environment is rapid feedback to engineers from first article inspection and process control activities. CT provides accurate repeatable external and internal measurements or complete geometry capture without physical sectioning. However, the reliability of CT systems' results must be verifiable against acceptable well defined standards and practices. Many factors in the use of industrial CT systems affect measurement accuracy without being well understood. One well-known phenomenon is the circular artifact problem caused by variations in the response of the system's detector array according to time, temperature, material, path length, or x-ray energy. A well-designed system mitigates all of these factors under control of the designer. The one variable that is always an unknown quantity by definition is the path length through the part being inspected. In this paper, we present the results of a new calibration technique that pre-measures and calibrates the detector response for path length variations prior to inspection of all series and prototype parts (cylinder heads, manifolds) in the Technology Centre of Volkswagen Foundry at Hannover. The level of ring artifacts is significantly reduced in images produced with the new calibration technique. As a result, flaw detection, measurement accuracy and repeatability are improved. INTRODUCTION In today s world, automobiles are one of the most developed and inspected mass-production high technology products. The need to deliver high levels of safety and functionality in these products requires a qualified and permanent quality inspection process, subject to its own continuous improvement cycle. Heavily stressed components like cylinder heads require complete non-destructive inspection in the design cycle and periodically during production. Comprehensive inspection must include information on conditions such as porosity, gas inclusions, shrinkage, dimension measurement of functional areas, and wall thickness checks. Defect detection and dimension measurements can both be captured in a complete CT examination of the part. No other non-destructive testing method has the same combination of comparable results for complete three-dimensional inspection of a part. Figure 1 shows the process of CT inspection of a cylinder head. The cast part is scanned in the CT system to produce a stack of grey level cross-sectional images at uniform intervals along its length. Surface information is extracted from the grey level images to create an STL model of the part with full internal and external details of the part. The STL model of the scanned part is then automatically registered to the CAD model of the part and compared dimensionally to the design to produce a color-coded variance map. Deviations of the real CT system behavior from the idealized mathematical model and potential instabilities in the CT imaging process may introduce unwanted variations of the grey levels in the images comprising the 3D stack. If left uncorrected, such grey level variations

2 lead to measurement inaccuracy, non-repeatable results, and unreliable defect characterization. Our results show that a 5% local variation of grey level can lead to measurement errors of 0.1 mm or greater per single dimension. measurement problem areas in cylinder heads are the oil circulation and water-cooling passages. These critical regions on the interior of the part can only be accurately measured non-destructively by CT and the simple linear thresholding technique requires that the grey level images being provided as input to the process must be of the highest quality level possible. This is the genesis of our improved calibration and correction method. CT SYSTEM DESCRIPTION The BIR ACTIS 600/450 was installed at the Volkswagen Technology Centre in October of The system can inspect parts weighing up to 100 kg with a maximum diameter of 600 mm and maximum height of 1200 mm. The radiographic equipment is a 450 kv x-ray source and a line array detector with 1024 channels pitched at mm. The typical inspection of a part produces a stack of grey level cross-section images in units of approximate material density. These images provide complete internal defect information of the part as well as very accurate geometry for all external and internal surfaces. The level of accuracy and detail acquired is a function of the system s ultimate spatial resolution capability, the scanning parameters selected by the operator, and the amount of time devoted to the inspection. The CT system is shown in Figures 2 and 3. Figure 1. The CT inspection process is illustrated. The part is scanned to produce a stack of images. An STL model is built from the image stack and then compared to the CAD model. REQUIREMENTS FOR CYLINDER HEAD DIMENSION MEASUREMENTS The irregular structure of a cylinder head presents a significant challenge to a CT system. There are continuous material path length changes from slice to slice between thick and thin walled sections. Defect detection and dimensional measurements are both done by grey level analysis of the CT image stack. Third party software programs use a simple linear thresholding of the image grey level to create an STL model of the part. As a result, any deviations of the image grey levels produced by the CT system that diverge from the true representation of the part will result in loss of measurement accuracy and repeatability as well as false defect reporting. Such deviations are known as artifacts and two main ones are considered here, centered rings and beam hardening streaks. The causes of both are discussed in detail below. A typical problem is that circular rings introduce structured non-repeatable measurement errors on surfaces and features. Another problem is that a beam-hardening streak from a long path length of material through a thick section of the part can intersect a thin walled area and erroneously reduce the thin wall below its specification. Examples of CT Figure 2. An overview of the CT system is shown with the control console at the left, the door control pedestal at the center, and the entrance to the radiation protection cabin at the right. A cylinder head is mounted on the turntable ready for inspection.

3 DAILY CALIBRATION Offset Figure 3. A cylinder head is mounted on the turntable for CT inspection. The 450 kv x-ray source and collimation system is at the left. The 1024 channel line array detector system is at the right. In the case of defect analysis, third party software is available that will automatically locate and analyze porosity to produce a report with defects graded by size, located by X,Y,Z coordinate, correlated with crosssectional or 3D view of the part, etc. In the case of geometry information, the grey level CT images are normally converted to an STL model for analysis. Examples are shown below in the section RESULTS OF NLG CALIBRATION FOR CYLINDER HEADS. In the example presented here, the cylinder head was inspected using scan parameters of 445 kv, 5 ma, 80 seconds/slice 1024x1024 matrix, mm pixel size 0.75 mm slice pitch, 1.0 mm slice width 578 slices CALIBRATION PROCESSES After installation and commissioning, the CT system requires periodic calibration to maintain peak performance. Certain calibrations should be done on a daily basis while others are only necessary over the span of several weeks or months. The CT system employs an array of 1024 individual x-ray detectors to measure the attenuation of the beam through the part. Ideally, the response through a given path length of material should be identical from each channel. The first stage of the detection process is an x-ray to light conversion scintillator, which in turn is coupled to a photodiode. The photodiode converts the light signal to a current, which is amplified, conditioned and eventually converted to a digital number. This silicon photodiode is the device that leads to the need for daily calibration. The detector s silicon photodiode is used in the reverse bias photovoltaic mode where the offset voltage is susceptible to a significant fluctuation with respect to temperature. The detector array housing is equipped with a servo controlled thermoelectric cooler to maintain the interior temperature within ±0.1ºC of a set point usually chosen to be slightly below the normal ambient temperature of the scanner room. At the beginning of each shift, output readings are collected from the detector array with the x-ray source off and saved for future reference while scanning. This Offset calibration compensates for any short-term drift of the silicon photodiode offset voltage. In general, the only parameter that effects the Offset calibration is the integration time setting. Gain There are many silicon solid state devices associated with amplification and signal conditioning of each detector channel that are also located in the temperature controlled enclosure. These devices have an effect on the detector s transfer function from input to output. The transfer function is characterized by collecting detector array readings through air. This Gain calibration is typically taken as near as possible to saturation or the maximum signal level. The Gain calibration is also effected by variables such as integration time, slice thickness, x-ray technique settings, and other factors that would change the detector signal level. When these factors are changed, the Gain calibration must be redone. MECHANICAL ALIGNMENT AND CALIBRATION Mechanical alignment and calibration procedures are used to check relative positioning and accuracy of the system s linear and rotary axes. These processes are required only when mechanical adjustments have been made to the system or for periodic verification of mechanical performance. Measuring the rotation center of the part holding turntable with respect to the coordinate frame of the x-ray imaging system is the most common alignment calibration. The operator typically does the turntable axis calibration weekly for alignment confirmation. Precision laser alignment of the entire mechanical system is done at six-month intervals as a service function to insure orthogonality of axes and parallelism of the x-ray beam plane to the turntable rotation surface. This mechanical alignment is necessary to guarantee dimensional accuracy of the CT system.

4 NON-LINEAR GAIN CALIBRATION Beam Hardening The type of x-ray sources used in commercial imaging systems produce a spectrum of x-rays from approximately 40 kv to the peak energy used to excite the target (450 kv in this case). Wide spectrum sources lead to an inherent non-linear behavior of the x-ray attenuation process causing problems in all radiographic imaging applications including CT. The resulting beam hardening is a non-linear effect caused by the preferential absorption of low energy x-ray photons such that as a beam passes farther and farther through a path of material, the average energy increases and the apparent penetrating power of the beam increases (or hardens) [1]. This hardened beam penetrates the material more readily than expected and produces an artifact making the part appear less dense than it actually is. Thus a solid cylinder of aluminum would be progressively darker grey near the center or dished in appearance [2]. Beam hardening creates more complicated artifacts in more complicated shapes but is always quite easy to recognize since it is directly related to material path length. A solid square section of aluminum will show an X shaped darker grey cross from corner to corner. Non-Uniformity of Scintillator Response The scintillator material used in this CT system is cadmium tungstate (CdWO 4 ), the preferred type for medium to high-energy industrial CT systems. To complicate matters, the manufacturer s uniformity of response specification for CdWO 4 is only ± 10%. This means that two neighbor channels of the array can have significantly different response to identically the same input signal even though the scintillator crystals are cut from exactly the same region of the original boule. Such a large variation in channel-to-channel response is a serious problem for CT systems since an individual channel is responsible for reconstructing a particular radius of the part. If a given channel has a higher or lower response than its neighbors by as little as 0.1% that cannot be corrected perfectly, it will create a circular artifact or ring in the image at that radius with a brighter or darker grey level. Since STL model generation and all dimensional measurements are based upon grey level analysis, systematic errors affecting image grey levels will definitely have an impact upon measurement accuracy and STL model fidelity. An Alternative Solution via Non-Linear Gain Calibration For many years, the standard Gain calibration worked well for routine inspection jobs involving only defect detection and relatively simple parts. As more complex parts with a mixture of thin walls and long path lengths of material became critical inspection requirements for CT systems, the effects of beam hardening and nonuniformity of crystal response became more noticeable obstacles to obtaining the best possible results for both defect detection and measurement accuracy. Therefore, the non-linear gain calibration technique was developed and implemented as a convenient and economical method of coping with these deviations of the physical measurement system from the ideal case. Methodology The methodology of the non-linear gain calibration is quite simple in concept and works especially well for the case of casting inspection where the object is composed of one homogeneous material. The objective is to measure the response of each individual detector channel through increasingly thick paths of the material comprising the parts to be inspected. A set of aluminum bars, shown schematically in Figure 4, are used to collect Non-Linear Gain data through aluminum paths of 0 to 250 mm in twenty uniform steps. The longest calibration path should exceed the longest path length anticipated in the parts to be inspected. The software is fully parameterized so any number of steps or value of maximum thickness may be used. The process is simply one of collecting Gain data through air for the 0 mm path length, one bar for the first step, two bars for the second, step and so forth until the maximum thickness has been collected. In the semiconductor industry, this type of problem would be solved by cutting many thousands of crystals; measuring their responses; grading them into bins; and then building arrays from sets of crystals with matched responses. This approach is not economically feasible due to the high cost of the material and the relatively low volume of production currently achieved with industrial CT.

5 X-RAY TUBE FOCUS SPOT X-RAY FAN BEAM TURNTABLE CALIBRATION PLATES where I o is the x-ray flux entering the part, I is the x-ray flux exiting the part, μ is the attenuation coefficient of the material (in units of cm -1 ), and x is the path length through the part. The degree of the polynomials was selected empirically based on the effectiveness of artifact reduction on real parts. Three detector channel polynomials are illustrated in Figure 5 below. The upper and lower plots are the extremes for this particular array while the center plot is representative of the majority of the 1024 channels in the array. NLG Polynomial DETECTOR DETECTOR Figure 4. The NLG calibration aluminum test bars are shown in place on the turntable between the x-ray tube at top and detector array at bottom. The setups for three bars and thirteen bars are shown. Corrected Ouput The function of the calibration bars is to model the scattering and non-linear beam hardening characteristics of a typical part. Various other experimental approaches have been tried for collection of this data including wedding cakes, step wedges, solid cylinders arranged horizontally, and others. In security inspection systems, the step wedge approach is used since sufficient space is available in these larger systems to permanently deploy the calibration piece and automate the data collection process [3]. The current configuration of individual bars has been found to be flexible, practical, and effective for industrial CT. In order to best mimic the part and its maximum path length, the first bar is placed at the center of the turntable and the path lengths are built up from the center outwards. The bars are made from forged tool plate to eliminate any significant porosity or inclusion defects. A closely packed stack of bars looks like a solid piece to the detector array in the orientation of Figure 4. Scatter generated in the bars tends to be reabsorbed so that the significant scatter reaching the detector is generated from the surface of the bar closest to the detector array itself. The non-linear gain calibration data serves as input to a curve fitting process that uses linear regression analysis to generate coefficients for fourth order polynomials for all channels of the detector array. The resulting Non- Linear Gain (NLG) polynomials map the incoming raw data signal from each channel to an equivalent material path length according to the basic law of radiation transmittance equation: I = I o e -μx Raw Input Figure 5. Plot of NLG Polynomial for three detector channels. The NLG polynomial coefficient data for individual channels are averaged together to form a global Beam Hardening Correction (BHC) polynomial that is also applied to the raw data. Together, the NLG and BHC corrections are effective to remove artifacts such as streaks, shading, and severe rings. There are frequently vestigial arc segment artifacts remaining in the images so a standard Ring Free Correction (RFC) algorithm is applied to remove them [4]. The improved correction sequence demonstrated here is the individual channel NLG [5], the global BHC, and the image based RFC. The NLG correction and NLG based BHC are the improvements to this process over previous versions of software. The NLG calibration and correction is sensitive to scan parameters that cause significant changes to the detector signal level through an inspected part. These include x-ray technique, detector collimator opening (slice thickness), integration time, and beam filtration. In practice, these parameters are normally fixed for routine operations and the NLG calibration data is stable for many weeks. USAGE OF NLG CALIBRATION The effectiveness of the NLG calibration is illustrated using scans of a real part. A Volkswagen cylinder head (type 03G) was scanned and images were reconstructed

6 from one set of x-ray data in order to compare the effects of the NLG correction. In Figure 6, only the standard Gain calibration is done on a thick walled section of the part where long path lengths of the material cause both types of artifacts. A noticeable ring artifact appears near the center of the part and beam hardening streaks coincide with long path lengths through the material. To demonstrate potential problems of artifacts, a dimension cursor was intentionally placed on a ring artifact at the center of the part and erroneously measures the slot width at mm. The subtraction of Figure 6 from Figure 7 is shown in Figure 8. The features in Figure 8 represent all the artifacts in Figure 6 that have been removed by NLG corrections in the image of Figure 7. This difference image between the original image and corrected image actually shows a significant outline of the part itself in addition to the artifacts. This shadow portion of the part in the subtraction is the contribution of the BHC correction. Careful examination of the subtraction image also shows subtle streaks between neighboring holes and other features that have been removed by the BHC portion of the correction. Additional streaks in the air surrounding the part are corrections attributed to BHC. Figure 6. A cylinder head image is shown through a thick walled section with standard Gain calibration only and no Beam Hardening correction. Ring artifacts and dark beam hardening streaks are apparent. The slot at the center measures larger than normal at mm mainly due to ring and shading artifacts as seen in the inset at the upper right. The NLG correction sequence was applied to the same x-ray data to produce the image in Figure 7. The resulting image is free of ring artifacts and the dark streaks are greatly reduced. The slot at the center now measures mm due to the successful correction of artifacts. The measurement error due to artifacts was significant at mm in this case, but corrected by the improved calibration process. Figure 8. The effect of NLG correction is shown in a difference image. Figure 6 is subtracted from Figure 7 to show the artifacts removed by NLG correction. The NLG calibration is also effective in thin walled areas of the cylinder head as illustrated in the next series of images. The same technique was used again. One set of x-ray data was collected and then reconstructed with and without the NLG corrections for comparison. In Figure 9, only the standard Gain calibration is done on a thin walled section of the part. No ring artifacts or beam hardening streaks are visible in the image at this setting of contrast and grey level. Figure 7. Cylinder head image with NLG corrections applied. Ring artifacts are removed and the dark streaks are significantly reduced. The measurement of the slot is now mm due to correction of ring, streak, and shading artifacts. Figure 9. Cylinder head image though a thin walled section with standard Gain calibration only and no Beam Hardening correction. No obvious artifacts are visible. The slot at the center measures larger than normal at 7.70 mm due to ring artifacts. An enlarged view of the measurement cursor is shown in the insert at lower left.

7 The NLG corrections were applied to the same x-ray data to produce the image in Figure 10. The slot at the center now measures 7.57 mm mainly due to correction of the ring, streak, and shading artifacts. The improved calibration process corrected the mm measurement error in this thin walled section of the part but there were no obvious artifacts in the original image of Figure 9 to indicate any potential measurement problems. matched to its neighbor for a well-balanced response to a given range of material path lengths. RESULTS OF NLG CALIBRATION FOR CYLINDER HEADS The software package VGStudioMAX1.2 from Volume Graphics of Heidelberg was used to make an STL model from the stack of NLG corrected CT images of a cylinder head. A section of this model is shown in Figure 12. No noise reduction techniques, filtering, or other post processing of the CT images were employed prior to selection of the STL threshold to improve the appearance of the model. The STL generator of VGStudioMax1.2 also has options for smoothing large regular areas to reduce the number of triangles in the model. This can sometimes artificially improve the appearance of a model made from poor quality data. No smoothing or decimation options were used to build the model shown below. Figure 10. Cylinder head image with NLG correction applied. Appearance of the image is relatively unchanged from Figure 9 but the measurement of the slot is now 7.57 mm due to correction of subtle ring, streak, and shading artifacts. An enlarged view of the measurement cursor is shown in the insert at lower left. While the images of Figures 9 and 10 are very similar in appearance, their subtraction image in Figure 11 shows all the defects in Figure 9 that have been removed by the improved NLG calibration/correction process. Figure 12. A section of the CT based STL model is shown. This portion was made from 122 slices at 1 mm spacing using VGStudioMax1.2 by VolumeGrapics of Heidelberg. Figure 11. The effect of NLG correction is shown in a difference image. Figure 9 is subtracted from Figure 10 to show the artifacts removed by the corrections. Even in the thin walled sections of the part there are still significant ring artifacts and beam hardening artifacts removed by the improved calibration process. The subtraction images of Figures 8 and 11 demonstrate that the NLG calibration process and image correction successfully improves the modeling of the CT image reconstruction mathematics to closely match the physical measurement situation of the system. The output of each individual detector channel in the array is The software package PolyWorks from InnovMetrics Software of Quebec was used to compare the STL model with the CAD design model. There is no need to fixture the part in the CT system since PolyWorks can automatically register the CAD and CT models if they are within relatively the same orientation. Alternatively, a registration can be performed quickly by defining common planes, lines, and points on both models. After registration, the color-coded variance map displays the difference between the real part and the design. In the color bar to the right, red and purple are ±0.7 mm respectively.

8 accuracy. This is a major improvement in the early development stages of industrial CT. Many others will follow as we stand at the threshold of finding new synergies between non-destructive testing and reverse engineering techniques. CONTACT Figure 13. The Variance map produced by Polyworks from InnovMetrics Software of Quebec is shown. The CAD and CT data sets are usually represented as STL models. PolyWorks automatically registers th. e CT and CAD models together. It then displays the difference between the two models as a color-coded variance map. The maximum variance on the color bar is ±0.7 mm. CONCLUSION Producing the highest possible quality of CT images at the first step of the process is essential since all of the following stages of analysis are dependent upon the simple thresholded STL model. Small 5% errors in CT grey levels can lead to dimensional errors of ±0.1 mm or larger. Since these errors are caused by wellunderstood physical phenomena, they can be measured and corrected by properly designed calibration techniques. The NLG calibration and correction process counteracts problems caused by non-linearities introduced by material properties, radiation physics, and the practical considerations of fielding an economical system. The NLG calibration and correction process has proven to be reliable and repeatable, while providing excellent, high quality CT image stacks as input to third party software tools responsible for building STL models and variance maps. CT image defects such as ring artifacts, shading, and streaks often cause measurement errors, false indications of defects, or other problems. Even when such defects are not obvious to the observer, they may cause significant measurement errors. The NLG calibration and correction technique has been developed to reduce the level of image artifacts while increasing measurement accuracy and repeatability. Our new process for calibration and correction has achieved success in a production environment. The method has now been applied to aluminum cylinder heads and most recently to sand cores. The NLG method is an easy and fast calibration step for improvement of CT image quality and measurement Smith, Charles R. BIR Inc. 425 Barclay Boulevard Lincolnshire IL USA Tel Fax csmith@birinc.com DEFINITIONS, ACRONYMS, ABBREVIATIONS ACTIS: Advanced Computed Tomography Imaging System BHC: Beam Hardening Calibration / Correction CT: Computed Tomography NLG: Non-Linear Gain RFC: Ring Free Calibration / Correction Scintillator: X-ray to light conversion material REFERENCES 1. J. T. Bushberg, et al. The Essential Physics of Medical Imaging (Philadelphia: Lippincott Williams & Wilkins, 2001), pg F. Hopkins, et al. Analytical Corrections for Beam Hardening and Object Scatter in Volumetric Computed Tomography Systems, General Electric Global Research, Niskayuna, New York, USA, pg S. Ogorodnikov and V. Petrunin, Processing of Interlaced Images in 4-10 MeV Dual Energy Customs System for Material Recognition, Physical Review Special Topics Accelerators and Beams, Volume 5, (2002), pg R. Ketcham, et al. High Resolution X-ray CT Facility, The University of Texas at Austin, Department of Geological Sciences, CT/Ring artifacts. 5. M. D. Silver, Specification for NLG Calibration, BIR Inc. Document Number 8006A-001-SRS Rev. B, 20 Nov pgs