1,720,968 research outputs found
Industrial X-ray computed tomography: accuracy enhancement and errors evaluation towards traceability of coordinate measurements
Full text linkIn the recent years, X-ray computed tomography (CT) has been increasingly used in the field of coordinate metrology for performing dimensional verifications on industrial parts.
X-ray CT provides significant advantages compared to traditional coordinate measuring machines, which have made this technology one of the most promising and innovative non-contact measuring techniques in the field of dimensional metrology. With a CT scan
of the component to be inspected, it is possible to obtain, in a relatively short time and in a non-contact way, its volumetric model fully describing its internal and external geometries. On this three-dimensional model a multitude of measuring tasks can be performed, which often are not possible with conventional measuring technologies; these include the measurement of inner and non-accessible complex geometrical features without any need to cut or destroy the part.
CT, therefore, has acquired a primary role in the field of dimensional metrology, enabling also the inspection and analysis of new and innovative products. However, despite the advantages that X-ray CT provides there are some current challenges that need to be
addressed in order for CT systems to be widely accepted in the field of coordinate metrology. As for all coordinate measuring systems, CT measurements must be traceable to the unit of length, the meter, and CT measurement uncertainty must be adequate for the measuring tasks performed. The establishment of measurements traceability and the study and enhancement of CT measurement accuracy however are complex tasks, mainly due to the several influence factors that affect CT measurement chain. At the state of the art, no internationally accepted standards are available for testing CT metrological performances and for determining the uncertainty of CT measurements – which is a fundamental requirement for establishing metrological traceability.
This PhD thesis contributes with research on these topics. The effects of relevant influence factors affecting CT measurements are studied and modelled with the aims of characterizing the measurement errors produced by each influence factor and to determine the sensitivity of measurement results to these error sources. This is a primary
fundamental step for the establishment of measurement traceability and for the enhancement of CT measurement accuracy. The effects of CT system geometrical errors are studied and quantitatively determined. Different experimental campaigns were designed in order to determine the measurement errors caused by geometric errors of the X-ray detector and of the rotary table. The sensitivity of the measurement results to each of the investigated geometrical errors is provided, moreover the influence of the measurement direction and object positioning in the CT volume is determined.
The analysis and mapping of geometrical errors is fundamental in coordinate metrology, and the system geometry influences the whole measurement chain. A thorough experimental description of the effects produced by CT system geometrical errors still
lacks in literature.
Measurement errors due to the cone-beam artifact – which is an inherent influence factor in circular cone-beam trajectories – are studied and mapped by means of experimental investigations. An advanced solution in the field of CT metrology, the use of helical
scanning trajectories, is studied in order to exploit all the benefits that CT can offer in terms of measurement accuracy, scan resolution and image quality. CT helical scanning metrological performances are investigated, as well as the effects of the main helical scanning parameters on the measurement accuracy.
Being a multi-purpose measuring technique, X-ray CT offers the possibility to scan a wide variety of industrial parts, which in some cases are characterized by high surface roughness. It is thus of significant importance to quantify the effects of the workpiece
surface roughness on CT dimensional measurements. For this purpose, different reference objects were designed in order to experimentally map the effects of surface roughness on CT dimensional measurements performed on periodic roughness profiles. Numerical simulation campaigns were also designed and performed in order to analyze difficult to produce profiles. The measurement errors caused by surface roughness with respect to conventional tactile coordinate measurements are reported and a model for roughness error correction is provided. Measurement uncertainty is calculated according to the substitution method and it is demonstrated how the correction of roughness effects is fundamental for a proper determination of CT measurement uncertainty and for a
significant enhancement of measurement accuracy
Investigation on metrological performances in CT helical scanning for dimensional quality control
No full textFor many industrial applications, workpieces characterized by high surface roughness are scanned; this is the case for example of parts produced by additive manufacturing. Surface roughness has a strong influence on CT dimensional measurements, causing relevant measurement errors with respect to reference tactile measurements, especially for parts characterized by high surface roughness. It comes that surface roughness effects on CT dimensional measurements must be quantified. In the present work, the influence of surface roughness on CT dimensional measurements, and the relation between tactile CMM and CT measurements are studied. Effects of larger as well as smaller surface roughness are taken into account. Experimental results prove the presence of a systematic error between tactile and CT measurements, due to surface roughness. The possibility to correct this systematic error to enhance the accuracy of CT measurements is outlined
Influence of surface roughness on X-ray computed tomography dimensional measurements of additive manufactured parts
Full text linkIn many industrial applications, components characterized by high surface roughness are measured by X-ray computed tomography (CT). This is the case, for example, of additive manufactured parts. Surface roughness has a strong influence on CT dimensional measurements, causing relevant measurement deviations with respect to tactile reference measurements by coordinate measuring machines (CMMs), especially for parts characterized by high surface roughness. It comes that roughness effects on CT dimensional measurements must be quantified.In this work, the influence of surface roughness on CT dimensional measurements, and the relation between tactile CMM and CT measurements are studied. Effects of larger as well as smaller surface roughness are taken into account, by means of three different additive manufactured samples characterized by different roughness. Experimental results prove the presence of a systematic error between tactile and CT measurements; the relation between this error and the Rz roughness parameter of the surface is analyzed
Effect of surface roughness on uncertainty of X-ray CT dimensional measurements of additive manufactured parts
No full textDetermination of measurement uncertainty for CT dimensional measurements is a difficult task due to the amount of complex error sources affecting the entire process chain. One of the critical aspects to deal with is the presence of error components that must be taken into account additionally. Surface roughness has a strong influence, causing a considerable increase of uncertainty especially for parts characterized by high surface roughness, such as additive manufactured parts. The paper shows the influence of surface roughness on measurement uncertainty calculated for repeated CT measurements and investigates the relation between tactile CMM and CT measurements. The possibility to substantially decrease measurement uncertainty by correcting roughness systematic effects is outlined
Investigation on the effects of X-ray CT system geometrical misalignments on dimensional measurement errors
No full textX-ray CT measurements are affected by a multitude of influence factors, among which the correct estimation of the CT system geometry is of major importance. The presence of geometrical misalignment and/or the wrong estimation of system geometry, indeed, leads to artifacts and distortions in the measured volume, and to measurement errors when performing metrological tasks. That is why it is important to quantify the effects of geometrical misalignments or misestimation on CT measurement accuracy. In this work, the effects of detector misalignment are experimentally investigated on a metrology CT system. Physical misalignments were purposefully induced on a flat-panel detector for investigating the influence of a detector out of plane rotation on measurement results. The experimental results show the effects of the induced detector misalignment on center-to-center measurements, diameter and form measurements on a calibrated ball bar. The effects of the amplitude of the angular misalignment induced are also shown
Computed tomography helical scanning for dimensional metrology: Evaluation of measurement errors
No full textIn industrial X-ray computed tomography, it is frequently necessary to scan long objects that exceed the dimensions of the detector, while meeting dimensional accuracy requirements (for example, in automotive and aerospace fields). Conventional scans using circular trajectories face limits when dealing with internal and complex features on elongated work pieces. To this extent helical scanning is a valuable solution that also enables obtaining a strong improvement on image quality, eliminating the cone beam artifacts that characterize conventional scans. To use helical scanning for dimensional quality control, the assessment of metrological performances is needed. In this work, metrological performances for helical scanning are investigated and compared with those of conventional circular scans. Repeated helical scans are performed to investigate the influence of main helical scans parameters when performing dimensional measurements. Experimental results show that helical scanning, with appropriate scan parameters, performs better than traditional circular scans with a strong improvement on image quality
Investigation on scan parameters for accurate dimensional measurements by CT helical scanning
No full textIn industrial X-ray computed tomography (CT), conventional scans using circular trajectories face limits when dealing with internal and complex features on elongated workpieces, mainly due to the dimensions of the detector. To this extent, helical scanning is a powerful solution that enhances flexibility of the system and, when using suitable parameters, enables obtaining a strong improvement on image quality, eliminating the cone-beam artifacts that characterize conventional scans. In this work, metrological performances in CT helical scanning are experimentally investigated and compared with those of conventional circular scans. The influence of main helical scan parameters are studied to enhance accuracy of dimensional measurements by CT helical scanning
Experimental investigation on the influence of detector misalignment on X-ray CT measurement accuracy
No full textIn X-ray computed tomography (CT), measurement accuracy and image quality are strongly affected by the presence of geometrical
misalignments and/or the wrong estimation of CT system geometry . In this work, the effects of detector misalignment are
experimentally investigated. CT measurements of a calibrated ball bar were acquired first with the system aligned according to the
manufacturer’s guidelines, and then after that the flat-panel detector was physically misaligned. The experimental results were
compared to show the effects of detector misalignment on CT measurement errors
Evaluating the use of industrial X-ray CT to the reverse engineering of bowed stringed instruments
No full textFor centuries, simple contact measuring instruments (e.g. callipers, profile and thickness gauges) have been used by violin makers for recording bi-dimensional information about their creations. Since its invention, film and digital photography have also been used to document shapes and colours. Traditionally, gypsum castings and RTM replicas are used to store information about the 3D shapes of back, bellies and scrolls. During the last 30 years the applications of non-contact systems such as X-ray computed tomography (CT), and laser and structured light scanners (LS) have opened new horizons to the bowed stringed instruments metrology. This work compares two state-of-the-art non-contact systems: an industrial X-ray computed tomography system and a Structured Light 3D scanner. Their results in terms of accuracy, repeatability and uncertainties are assessed and compared to reference tactile Coordinate Measuring Machine (CMM) measurements. Experimental results prove that, with the considered experimental set-up, CT provides better results than LS in terms of deviation from CMM reference measurements, and uncertainty
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