We describe the automated application of an area based registration method to the surface inspection of steel industry products as a tool to solve an intermediate mosaicing problem. The main problem of area based methods is that there is high probability that the results of a matching process will be incorrect if a region of interest without any relevant detail is used. The selection of a region of interest with relevant content continues to be a problem nowadays. We propose a method to select a salient area when using a zero mean normalised cross correlation metric and a block as a region of interest. The selection of the size and the position of the block is focused on ensuring a smooth unimodal similarity surface around the maximum similitude point. Experiments show a correlation between the surface kurtosis of the block autocovariance and the same coefficient measured over the correlation surface around the maximum similitude point for the three different steel products analysed. We check that the maximum correlation value is reached abruptly, in a small range of pixels around the maximum similitude point, in correlation surfaces obtained from blocks containing non-relevant information. On the other hand, salient blocks usually lead to unimodal smooth similarity surfaces with small sensitivity to noise in contrast with the ones obtained from non-remarkable blocks. Also, the method proposed allows the application of fast search algorithms based on the unimodality of the correlation surface, obtaining high computational time reduction in comparison with full search strategies using fast normalised cross correlation algorithms.
A common-path interferometric profilometer using a Savart plate as a lateral shearer has been successfully tested
under harsh environmental conditions to measure the shape of a surface, detecting defects and characterizing
surface properties. The whole profile is obtained from a single image and its depth sensitivity is easily scalable,
making this technique suitable for many different applications. Although this system has been successfully used
for surface inspection and defect detection, some behaviors cannot be explained by the usual simple model for
fringe formation, which, amongst other things, considers normal incidence of the incoming rays into the Savart
plate. These deviations from the ideal case are more noticeable for high resolutions from short distances. This
paper studies the formation of the fringe pattern, which is crucial for understanding the behavior of the system
and proper calibration.
We propose a common-path two-wavelength interferometric system based on a single optical element, a Savart plate, that is able to obtain single-shot profile measurements with submicron precision from safe working distances (beyond 100 mm). These characteristics make this sensor ideal for surface inspection in on-line applications. For the illumination branch, two lasers with close wavelengths are combined and then passed through a rotating holographic diffuser for drastic speckle reduction. In the acquisition branch, the interferometric signals of both wavelengths are captured simultaneously by a camera, and their phase signals are combined to extend the measurement range.
Early surface defects inspection in hot steel products is a difficult task, but can help to reduce significantly production
costs. This is the case of steel slabs when they are produced in the continuous casting line. Conoscopic holography
phase-based long stand-off profilometers have shown to be a great tool for this kind of inspection, and a breakthrough
system based on them is being used for more than 2 years in production conditions with high reliability and economical
impact. This paper presents the results of this system and the challenges it has overcome: hot material up to 900°C, dust,
scale over the inspected surface.
We propose a common-path two-wavelength intereferometric system based on a single optical element, a Savart
Plate, able to obtain profile measurements at frame rate. To improve precision up to the sub-micron levels from
safe working distances (beyond 100 mm), we use a speckle reduction system based on a rotating holographic
diffuser. The interferometric signals of the two wavelengths are obtained simultaneously and their phase signals
are combined to extend the measurement range.
The system's common-path interferometry nature, and the possibility of acquiring a distance profile in a
single frame, make it ideal for surface inspection in industrial environments.
Conoscopic Holography proved to be a very adequate solution for in-situ optical measurement in industrial
inspection and quality control systems, offering high-precision with a wide range of standoff distances, while
being quite insensitive to the harsh environmental conditions often encountered in industry, as it is a common-path
technique.
With the aim of extending their applicability, we have already addressed, with good results, several issues
that improve sensors based on this technology which include: the use of phase information to obtain one-shot
profile measurements at frame rate with higher precision; new signal processing techniques; and speckle reduction
to diminish measurement errors. However, the undesirable effect of using the phase information is that it reduces
the maximum steep that can be measured without ambiguity, which becomes an issue when working with high
precisions.
In this article we present our ongoing work towards using the concepts of multiple-wavelength interferometry
to extend the measurement range, something that, to our knowledge, has not been done for this technology
before.
On-line noncontact surface inspection with high precision is still an open problem. Usual methods are, in general, not applicable in hostile environments or not adequate for on-line measurement, as they are either slow in nature or need to work from very short, unsafe distances, providing small depths of field and apertures. The ongoing work toward the development of a noncontact optical profile measuring sensor that could be used for submicron measurements in on-line applications is presented here. Our approach is based on conoscopic holography and triangulation, and uses a very simple method for removing speckle noise, which is key for obtaining high precisions from safe distances (several centimeters).
One of the more challenging applications of optical metrology is real-time dimensional control and surface inspection in industrial applications, where strong requirements of cost, speed of operation, ease of setup and applicability in adverse environments greatly limit the number of applicable technologies. The design and uses of an optic profilometer, based on conoscopic holography, have been reported previously, but there are still some drawbacks that should be addressed. One of the most important is signal processing, which is a relatively expensive process and limits the acquisition rate at no more than 70 profiles per s. We present a new approach to the signal processing problem, deriving that the phase information contained in one fringe pattern, which corresponds to one profile, can be considered as a combination of multiple one-dimensional (1-D) patterns that carry the same information but for a phase difference in the carrier wave, making it possible to apply efficient phase-shifting interferometry (PSI) techniques and resulting in a reduction of more than two orders of magnitude in computational needs.
On-line non-contact roughness metrology is still an open problem. Usual methods involve either contact (stylus-type devices) or perform indirect evaluations of some roughness parameters, such as Ra, with light scattering techniques or speckle measurement (among the most common optical techniques), inductance (only for magnetic materials) or ultrasound methods. However, a generic method able to obtain every roughness parameter (what means recording the real distance profile), able to work with a variety of surface types, and able to be installed in production lines is still to be developed. In this article, the ongoing work towards the construction of a non-contact optical profile measuring sensor that could be used for roughness measurements is presented. Our approach is based on Conoscopic holography, a common-path interferometric technique which is a good candidate for industrial applications. Current research effort is focused in enhancing accuracy in these systems, by both reducing the coherence of the illuminating source (laser) and changing the hardware and software setup, with the aim of building a sensor able to capture a profile of an object's surface in a single shot with high precision from a relatively long standoff (several cm).
One of the more challenging applications of optical metrology is real-time dimensional control and surface inspection in industrial applications, where strong requirements of cost, setup, and applicability in adverse environments greatly limit the number of applicable technologies. This work shows an optic profilometer developed specifically for this purpose, the algorithms developed for efficient and accurate distance map calculation, and an example of its use in a harsh steelmaking environment. This device, based on conoscopic holography, is able to obtain a distance profile of a target in a single shot, works at long distance standoff (700 to 1200 mm), and still keeps good resolution (under 0.2 mm) with a very easy and reliable setup. The second part of the work focuses on a real example of this technology applied in an on-line inspection system in steel continuous casting funded by the European Coal and Steel Community, and which is currently working in the Aceralia LDA steelmaking factory in Asturias (Spain). The system is placed in the process line and performs on-line detection of surface defects over hot steel slabs (above 800°C in the inspection point) from a distance of 1200 mm. 100% of the production can be inspected without interfering with the process and without adding any delay.
One of the more challenging applications of optical metrology is real-time dimensional control and surface inspection in industrial applications, where strong requirements of cost, speed of operation, ease of setup and applicability in adverse environments, greatly limit the number of applicable technologies. An optic profilometer, based on Conoscopic holography, has been designed specifically for this purpose and used in several on-line inspection systems. This device is able to obtain a distance profile of a target in a single shot; works at long distance standoff (700-1200mm) and still keeps good resolution (under 0.2mm) with a very easy and reliable setup. However, there are still some drawbacks that should be addressed. The first one is the signal processing, which is a relatively expensive process and limits the acquisition rate at no more than 70 profiles per second. The second one is speckle noise, which is an inherent problem in systems that use coherent-light illumination and triangulation, and therefor could be extrapolated to many other optical inspection systems. This paper shows the current lines of research to solve both problems and presents some initial, yet very interesting, results. These improvements can be applicable to other ranges of devices using this technology in adverse environments, for roughness and vibration measurement or surface defects detection.
In the industry of sintered automobile synchronizer hubs, as in
many others, a fast on-line measurement system of the production
is a very important tool for production and quality control,
enabling among others: extended quality control to all the
production, lower start-up times, improved knowledge of the
process and influence of its parameters and database management of
the information. In this paper we describe an on-line optical
measuring system that is able to inspect 100% of the production,
making special emphasis on how the resolution and accuracy of the
measurement could be compromised by factors that are not related
to the sensor itself, as well as how to correct them. The paper
also describes how to validate such a system according to the ISO
5725 standard.
One of the more challenging applications of optical metrology is
real-time dimensional control and surface inspection in industrial
applications, where strong requirements of cost, setup and
applicability in adverse environments, greatly limit the number of
applicable technologies. This paper shows an optic profilometer
developed specifically for this purpose. This device, based on
Conoscopic holography, is able to obtain a distance profile of a
target in a single-axis scan; works from long distances and still
keeps good resolution with a very easy and reliable setup. The
first part of the paper introduces the working principles of
Conoscopic holography and shows the sensor set-up. Necessary
algorithms for obtaining the distance information are presented
and the whole process is illustrated with real captures of test
objects. The second part focuses on a real example of this
technology applied in an on-line inspection system in steel
continuous casting funded by the European Committee for Steel and
Carbon, and which is currently working in Aceralia LDA steelmaking
factory in Asturias (Spain). The system is placed in the process
line and performs on-line detection of surface defects over hot
steel slabs from a distance of 1200 mm. 100% of the production
can be inspected without interfering with the process and without
adding any delay.
One of the most important problems to address when applying interferometric techniques to industrial applications is the prevalence of noise, which results in poor fringe patterns and thus poor measurements. One of the techniques that suffers most from this problem is conoscopic holography. Though this interferometric technique is ideal for industrial inspection, the poor quality of fringe patterns obtained in adverse environments may make measurement impossible. Classical filtering techniques based on one-dimensional filters or general speckle removal filters, such as the Frost or the gamma, may not suffice in adverse conditions. Therefore, a new approach, based on the nature of the fringe pattern information itself, is worth looking into. In this paper we propose the use of orientational filters to develop a filtering method that not only removes noise of any nature, but also enhances the fringe-pattern information. Several approaches to these algorithms are implemented and evaluated using synthetic conoscopic fringe patterns under different noise conditions, showing how they clearly outperform classical filters with negligible distortion even in the worst conditions. Examples with real data acquired with the latest prototype of a conoscopic long-standoff profilometer are also provided.
One of the most important problems to address when applying
interferometric techniques to industrial applications is the high
presence of noise which results in poor fringe patterns and thus
poor measurements. One of the techniques that suffers most with
this problem is conoscopic holography. Even if this
interferometric technique is ideal for industrial inspection, the
poor quality of fringe patterns obtained in adverse environments
may even make measurement impossible. Classical filtering
techniques based on one-dimensional filters or general speckle
removal filters such as Frost or Gamma may not suffice in adverse
conditions, therefore a new approach based on the nature of the
fringe pattern information itself must be looked into. In this
paper we propose the use of orientational filters to develop a
filtering method, that not only removes noise of any nature, but
also enhances the fringe pattern information. Several approaches
to these algorithms are implemented and evaluated using synthetic
conoscopic fringe patterns under different noise conditions,
showing how they clearly outperform classical filters with a
negligible distortion even in the worst conditions. Examples with
real data acquired with the latest prototype of conoscopic
long-standoff profilometer are also provided.
The OMEGA project (Brite-EuRam 5470) intends to solve the problems arising from the lack of real time dimensional control on steel structural mill facility products. The main objective of the project is to assess the feasibility of an on-line, advanced passive non-contact measuring system, controlling complex geometry parts in adverse environment, featuring a high level of reliability and accuracy on measurements, with low cost. Such a measuring system will be based on Conoscopic Holography as the basic technology to be applied. Conoscopic Holography avoids some of the problems of conventional holography, such as the necessity of ultra high stability, coherent (but monochromatic) illumination and ultra high resolution sampling techniques. Thus, it is possible to produce holograms using incoherent light with a typical fringe period compatible with the resolution of normal electronic imaging devices, such as CCD cameras. In the application developed under this project the conoscopic head will have to meet requirements never tried before regarding the adverse working conditions. The paper will summarize the main objectives of the OMEGA project, now in progress, a description of the technology applied and the results available from the lab testing work when the paper is submitted.
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