Internal porosity is an inherent phenomenon to many manufacturing processes, such as casting, additive manufacturing, and others. Since these defects cannot be completely avoided by improving production processes, it is important to have a reliable method to detect and evaluate them accurately. The accurate evaluation becomes even more important concerning current industrial trends to minimize size and weight of products on one side, and enhance their complexity and performance on the other. X-ray computed tomography (CT) has emerged as a promising instrument for holistic porosity measurements offering several advantages over equivalent methods already established in the detection of internal defects. The main shortcomings of the conventional techniques pertain to too general information about total porosity content (e.g. Archimedes method) or the destructive way of testing (e.g. microscopy of cross-sections). On the contrary, CT is a nondestructive technique providing complete information about size, shape and distribution of internal porosity. However, due to the lack of international standards and the fact that it is relatively a new measurement technique, CT as a measurement technology has not yet reached maturity. This study proposes a procedure for the establishment of measurement traceability in porosity measurements by CT including the necessary evaluation of measurement uncertainty. The traceability transfer is carried out through a novel reference standard calibrated by optical and tactile coordinate measuring systems. The measurement uncertainty is calculated following international standards and guidelines. In addition, the accuracy of porosity measurements by CT with the associated measurement uncertainty is evaluated using the reference standard.
Typical radio telescopes have the primary reflector surface which is composed of several single panels that have
dimensions of a meter a side. The manufacturing of these radio panels yield a micrometric precision over the volume on
the single panel, hence the surface roughness of the panels can be measured with very high accuracy by means of the low
coherence interferometry (LCI) technique which reaches micrometric spatial and depth resolution and has the advantage
of being contact-less.
We have developed a multi-channel partially coherent light interferometer to realize non contact 3D surface topography.
The technique is based on the LCI principle, for which a bi-dimensional sensor - a CMOS - has been developed to
directly acquire images. Tri-dimensional measures are recovered with a single scanning along the depth direction in a
millimetric range, and every single pixel of the bi-dimensional sensor measures a point on the object, this allows a fast
analysis in real time on square centimeter areas.
In this paper we show the results obtained by applying the LCI technique method to analyze the surface roughness of the
panels of a large radio antenna of 64 m of width and used for astronomical observations at 100 GHz; by measuring their
3D structure at micrometric resolution it is possible to verify their fabrication errors.
The main results of an industrial inter-laboratory comparison for CMMs equipped with optical sensors are presented in
this paper. The comparison, named VideoAUDIT, was organized and coordinated by the Laboratory of Industrial and
Geometrical Metrology - University of Padova - and carried out in Italy and Switzerland from August 2007 to June 2008.
A total of 16 CMMs from different companies participated in the Project, using different kinds of optical sensors. The
participants were asked to measure a set of calibrated artefacts, following detailed procedures. The Audit items have
been chosen with the following criteria: (1) objects that can be measured with different types of optical sensors and (2)
including both reference artefacts for performance verification and common industrial products. Special attention has
been paid to the design of the comparison in order to respect the proficiency testing rules; in particular, the long term
stability of the audit items was checked during the comparison as a main requirement. An important task of the
comparison was to test the ability of the participants to determine the uncertainty of their measurements.
Optical sensors are increasingly used for dimensional and geometrical metrology. However, the lack of international
standards for testing optical coordinate measuring systems is currently limiting the traceability of measurements and the
easy comparison of different optical systems. This paper presents an experimental investigation on artefacts and
procedures for testing coordinate measuring systems equipped with optical distance sensors. The work is aimed at
contributing to the standardization of testing methods. The VDI/VDE 2617-6.2:2005 guideline, which is probably the
most complete document available at the state of the art for testing systems with optical distance sensors, is examined
with specific experiments. Results from the experiments are discussed, with particular reference to the tests used for
determining the following characteristics: error of indication for size measurement, probing error and structural
resolution. Particular attention is given to the use of artefacts alternative to gauge blocks for determining the error of
indication for size measurement.
The aim of this study is to assess angle-dependent systematic errors and measurement uncertainties for a conoscopic
holography laser sensor mounted on a Coordinate Measuring Machine (CMM). The main contribution of our work is the
definition of a methodology for the derivation of point-sensitive systematic and random errors, which must be
determined in order to evaluate the accuracy of the measuring system. An ad hoc three dimensional artefact has been
built for the task. The experimental test has been designed so as to isolate the effects of angular variations from those of
other influence quantities that might affect the measurement result. We have found the best measurand to assess angle-dependent
errors, and found some preliminary results on the expression of the systematic error and measurement
uncertainty as a function of the zenith angle for the chosen measurement system and sample material.
Conference Committee Involvement (2)
3D Imaging Metrology
24 January 2011 | San Francisco Airport, California, United States
3D Imaging Metrology
19 January 2009 | San Jose, California, United States
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