Cesar Fraga

Conoscopic holography-based long-standoff profilometer for surface inspection in adverse environment

One of the more challenging applications of optical metrology is real-time dimensional control and surface inspection in industrial ap- plications, where strong requirements of cost, setup, and applicability in adverse environments greatly limit the number of applicable technolo- gies. This work shows an optic profilometer developed specifically for this purpose, the algorithms developed for efficient and accurate dis- tance 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 dis- tance 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 in- spection 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.

On-line submicron profile measurements from safe distances with conoscopic holography: feasibility and potential problems

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).

Thickness quality control

This article describes a series of methods to detect thickness defects in DWI tinplate. These methods provide information to identify the origin of the defects, allowing fast corrections and, therefore, improving mill performance. In this approach, an adapted thickness signal is spatially sampled from the input stages of the mills last X-ray gauge. The system is also able to detect other problems related to thickness quality such as third-octave chatter using only the thickness measures from an X-ray gauge. The usage of this system resulted in an improvement in the factory performance and considerable money savings.

Toward online non-contact roughness profile measurements with a sensor based on conoscopic holography: current developments

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 objects surface in a single shot with high precision from a relatively long standoff (several cm).

Conoscopic holography based profilometers for defect inspection : improvements in speed, resolution and noise reduction

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.

On-line non-contact measuring of synchronizer hubs

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.

Camber measurement system in a hot rolling mill

Camber is one of the most significant defects in the first stages of hot rolling of steel plates. This kind of defect may cause the clogging of finishing mills, but it is also the visible effect of alterations in the process. In this paper we describe the design and implementation of a computer vision system for real-time measurement of camber in a hot rolling mill. Our goal is to provide valuable feedback information to improve AGC operation. As ground truth values are almost impossible to obtain, we have analyzed the relationship among measured camber and other process variables in order to validate our results. The system has proved to be robust, and at the same time there is a strong relationship between known problems in the mill and system readings.

Improving signal processing performance on a conoscopic holography-based profilometer with phase-shifting interferometry algorithms

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.

A long standoff profilometer for surface inspection in adverse environments based on conoscopic holography

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.

Thickness quality control and diagnosis system for tinplate rolling mills

The draw wall ironed (DWI) tinplate market clearly trends to demand a reduction in thickness, which implies stricter tolerance levels as small variations could result in problems in the manufacturing process and therefore customers dissatisfaction in a highly competitive market. This paper describes a series of methods to detect thickness defects in DWI tinplate. In addition, these methods provide information to identify the origin of the defects, allowing fast corrections and therefore improving mill performance. In our approach an adapted thickness signal is spatially sampled from the input stages of the mills last X-ray gauge.