X-ray computed tomography (CT) is increasingly used for dimensional metrology, allowing the inspection of both interior and exterior features impossible to observe using traditional optical and tactile measurement techniques. X-ray CT offers many benefits over traditional instruments as a visual inspection tool, however, extracting dimensional information from the reconstructed data-sets must be approached with caution due to error sources that can propagate through the image reconstruction processes. One error source originates from values of the source-to-object and source-to-detector distances; these are critical inputs as they define the voxel size, a global scalar directly influencing all dimensions extracted from the data. To reduce voxel size errors a reference workpiece can be scanned using the same measurement settings as the actual workpiece. By reconstructing the reference workpiece a reference dimension can be evaluated and this then used to adjust the voxel size of the actual workpiece. This reference dimension must be threshold independent, namely it is determined without the influence of edge detection thresholds. This paper offers a reference workpiece designed for measurement in an X-ray CT system, a coordinate measuring machine (CMM), and an optical profiler. Repeated measurements are made of the reference workpiece using all three instruments and
High-resolution X-ray is now an essential tool for internal defect and structure inspection in electronics and advanced
materials industry. However, it is always a challenge to use it for accurate dimensional measurement due to the nature of
the fan-beam X-ray source, particularly for cylindrical objects. This paper presents a novel hybrid X-ray and microscopy
method for the profile measurement of the internal hole of a
cylinder-shaped steel component. The part to be measured
has a beer bottle shape but is open at the bottom side. The objective is to measure the diametrical profile of the internal
hole with an accuracy of about 10μm. Traditionally this is measured with using a microscope after cutting and polishing
the specimen. This is not only a tedious work, but is also inaccurate due to the uncertainty in cutting and polishing. This
report demonstrates that the two edge-profiles of the internal hole can be obtained with X-ray inspection by sequentially
placing each of them at the central of the X-ray beam so that the
fan-beam effect can be minimized. The resolution of the
X-ray inspection is about 6µm under a 20x magnification. Subsequently, the diameter of the hole is measured at two
positions through the open end using a microscope with a 20x and a 10x objectives respectively. The results obtained
with the two methods are then combined to generate the whole diametrical profile of the internal hole.
All computed-tomography (CT) reconstruction algorithms used today require knowledge of the central ray, which is the projection of the center of rotation (COR) on the detector. The current common practice is to determine it using a wire phantom made of a dense metal material before the CT scan of the object to be inspected. This work presents four methods that can be used for this purpose, the center-of-sinogram, the opposite-angle-interpolation, curve-fitting, and geometrical methods. To our best knowledge, the last two approaches have not been reported before. The performance of the four methods is evaluated under four different situations. The comparison study shows that for all situations, the curve-fitting method and the interpolation method have consistent performance, and that only when the wire-to-COR is small, can all four methods generate close results. When the wire-to-COR distance becomes large, the center-of-sinogram method is generally not reliable. Although in principle the geometrical method is able to provide an exact solution, in practice its accuracy is limited by the finite size of the detector pixel and can be improved if the detector pixel size becomes smaller.
A vision system capable of extracting features from a semi-structured environment for vehicle guidance is described in this paper. The system is primarily used for road following via the detection of mud tracks in a tropical environment. The scene captured by a CCD colour camera is digitised into 24-bit colour images with a resolution of 320x240 pixels. Partitioning of the scene into road and non-road areas is based on the results of a colour image segmentation algorithm applied to these images. The RGB colour images from the camera are converted to HSI format. Training samples of road and non-road features of the terrain to be explored, stored in a database, are used to classify blocks of pixels using only the hue information content of the images. A Bayesian classifier in conjunction with a smooth thresholding function is used for the segmentation algorithm on a per block basis. This approach results in the recognition of traversable areas, particularly non-metalled roads. Experimental results have showed that the algorithm is invariant to shadow conditions, i.e. roads were detected under varying light conditions. Due to the soil conditions of the test sites, small puddles of water on the mud tracks are also classified as driveable areas. The system outputs a one bit 2-D map of the image every 200ms. Field results of the proposed approach have shown favourable responses for real-time implementation on an autonomous ground vehicle.
The purpose of the system described in this paper is to equip an off-road vehicle with a robust and reliable passive ranging system capable of providing information about the local terrain. This information should be sufficient to allow a separate navigation system to make appropriate path-planning decisions for autonomous travel between specified way-points.
The use of Fourier fringe analysis as a tool for surface characterization has been well documented. However, there remain several grey areas in the implementation of this technique as a practical tool. Principal among these is the catalog of errors associated with the backbone of the technique--the Fast Fourier Transform. These errors are well known, but their nature and form in this context are not fully understood. This paper gives a detailed review of these problems together with their impact on quantitative measurements and presents possible routes to their elimination and solution.
Surface parameterisation using phase measurement of
interferometric fringe patterns is supported by a strong theoretical
basis. However, little work has been published with regard to the
practical implementation of the available methods as tools for
industrial inspection. This paper attempts to justify the choice of
Fourier Fringe Analysis for this purpose and addresses some aspects
of its implementation.
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