A. Cobo

Laser-Induced Breakdown Spectroscopy: Fundamentals, Applications, and Challenges

Laser-induced breakdown spectroscopy (LIBS) is a technique that provides an accurate in situ quantitative chemical analysis and, thanks to the developments in new spectral processing algorithms in the last decade, has achieved a promising performance as a quantitative chemical analyzer at the atomic level. These possibilities along with the fact that little or no sample preparation is necessary have expanded the application fields of LIBS. In this paper, we review the state of the art of this technique, its fundamentals, algorithms for quantitative analysis or sample classification, future challenges, and new application fields where LIBS can solve real problems.

Spectroscopic Sensor System for Quality Assurance of the Tube-To-Tubesheet Welding Process in Nuclear Steam Generators

In a previous paper a new technique was proposed to allow real-time, online operation for arc-welding quality assurance based on plasma spectroscopy. In this paper, the proposed system has been used to determine the appearance of weld defects in the arc-welding nuclear steam generator tube-to-tubesheet process. The system was implemented in the facilities of Equipos Nucleares S.A. (ENSA), where several welding tests were performed on weld test coupons. Results will show the feasibility of the proposed system to be used in a real industrial scenario, presenting successful examples of weld defect detections.

Process control of laser keyhole welding

Online monitoring and control of laser keyhole welding is highly desirable, especially in safety-critical aeronautic applications, in order to detect and prevent possible defects in the weld. In this article we describe a closed-loop control system which ensures full penetration in keyhole welding by controlling the focus position and power of a 4 kW Nd:YAG laser. We present a laser power control system which determines the degree of penetration by analysis of an image of the welding keyhole. The focus position control system, meanwhile, is based on a previously reported system, exploiting the chromatic aberration of the focusing optics. We demonstrate performance in bead-on-plate ‘welding’ of Inconel 718 and Titanium. The focus control system maintained focal position on tilted and non-flat workpieces, and the penetration monitoring technique successfully controlled the laser power to maintain the full penetration regime in the presence of linear and step changes of thickness. Finally we discuss the performances and the limits of the systems when applied to a realistic complex aerospace component.Online monitoring and control of laser keyhole welding is highly desirable, especially in safety-critical aeronautic applications, in order to detect and prevent possible defects in the weld. In this article we describe a closed-loop control system which ensures full penetration in keyhole welding by controlling the focus position and power of a 4 kW Nd:YAG laser. We present a laser power control system which determines the degree of penetration by analysis of an image of the welding keyhole. The focus position control system, meanwhile, is based on a previously reported system, exploiting the chromatic aberration of the focusing optics. We demonstrate performance in bead-on-plate ‘welding’ of Inconel 718 and Titanium. The focus control system maintained focal position on tilted and non-flat workpieces, and the penetration monitoring technique successfully controlled the laser power to maintain the full penetration regime in the presence of linear and step changes of thickness. Finally we discuss the perf...

Optical fiber and integrated optics accelerometers for real-time vibration monitoring in harsh environments: in-lab and in-field characterization

The reliability of rotating machines such as electrical generator is critical to the overall reliability and operation of electrical power plants. The very high cost of these machines, makes it necessary to improve the lifetime of a wide set of currently installed hydrogenerators around the world and avoid costly maintenance. Vibration monitoring is essential in maintenance and protection programs. However, due to the harsh environment and the very low frequency spectrum of the vibrations, the above mentioned machines cannot be appropriately monitored by piezoelectric sensors. Several combination of optical fiber with silicon microstructures or in integrated optics, but because of technical or economic reasons, up to now, these sensors are not fully suited for the above-mentioned applications. In this paper both optical fiber and integrated optics accelerometers developed in order to satisfy specifications required for these applications are presented. Their developments were a part of a European project dedicated to real time defect detection and predict forthcoming failures of a generator group in an electric power plant in Spain. In-laboratory and in-field test are reported.

New low-cost fiber optic accelerometer system for stator winding monitoring of hydroelectric generating machines

A new fiber optic accelerometer system for stator winding simultaneous multi-point vibration monitoring of hydroelectric generating sets is presented. It is composed of as many sensor heads as points to be monitored, an optical channel and a processing unit distant from the sensing area, which is exposed to strong electromagnetic interference. The evaluation scheme is based on an intensity modulation and differential detection technique so that the measure is not influenced by external perturbations. The system is able to measure accelerations ranging from 30 to 450 Hz, with a maximum amplitude of 200 m/s2 RMS. Experimental results for the previous prototypes developed are showed. This sensor is intended to be integrated in a new complex system aimed to reach the goals of instantaneous protection, real time detection and evaluation of defects, failures and malfunctions in the above mentioned machines.

Raw Material Classification by Means of Hyperspectral Imaging and Hierarchical Temporal Memories

The recently proposed hierarchical temporal memory (HTM) paradigm of soft computing is applied to the detection and classification of foreign materials in a conveyor belt carrying tobacco leaves in a cigarette manufacturing industry. The HTM has been exposed to hyperspectral imaging data from 10 types of unwanted materials intermingled with tobacco leaves. The impact of the HTM architecture and the configuration of internal parameters on its classification performance have been explored. Classification results match or surpass those attained with other methods, such as Artificial Neural Networks (ANNs), with the advantage that HTM are able to handle raw spectral data and no preprocessing, spectral compression, or reflectance correction is required. It is also demonstrated that an optimized configuration of the HTM architecture and internal values can be derived from the statistical properties of the hyperspectral data, allowing the extension of the approach to other classification problems.

Efficient temperature and strain discrimination with a single type I fiber Bragg grating transducer

Writing a single fibre Bragg grating (FBG) with a proper non-sinusoidal axial in-core index increment profile, the highly desirable simultaneous measurement of temperature and strain using an FBG transducer can be efficiently reached. The optimised UV energy that equalize the first and second harmonics on a hydrogenated standard telecommunication fiber is demonstrated. Using this approach, the number of in-line multiplexed transducers for typical optical source bandwidths is practically doubled.

Pulse shape effects on the measurement of temperature using a Brillouin-based optical fiber sensor

Distributed fiber sensing based on Brillouin gain scattering (BGS) principle is a useful way to develop devices capable to measure temperature or/and strain in optical fibers. New effects or technologies that could achieve a larger distance and/or a better spatial resolution are a topic of special interest in this fiber sensing area. The influence of the probe-pulse shape in the interaction between the pulsed light and the continuous wave laser in a pump-probe system is presented. The purpose of this study is to improve the spatial resolution of the measurement without losing stability in the BGS. Also it is showed how the backscattering Brillouin gain is affected by inducing variations on the final value of the BGS intensity; this effect is illustrated by using an experimental set up based on the Brillouin optical time-domain analysis (BOTDA). Theoretical analysis of the probe pulse in the Brillouin shift and intensity value using triangular, sinc and saw tooth shapes around the medium phonon life time (~10ns) are presented; as well as the experimental results and possible applications are explained.

Evaluation of coaxial process control systems for Nd: YAG laser welding in aeronautics application

A sensor system for process monitoring of laser welding has been developed, based on co-axial detection of light produced in the welding keyhole. Two detection techniques are used in parallel, the first uses an optical fibre to collect light and guide it to discrete photodiodes, whilst the second uses a high speed camera to image the welding keyhole. This system has been developed for Nd:YAG welding of aeroengine components. Detection of focal errors and state of penetration are demonstrated for Inconel 718 and Titanium Ti-Al-6V.A sensor system for process monitoring of laser welding has been developed, based on co-axial detection of light produced in the welding keyhole. Two detection techniques are used in parallel, the first uses an optical fibre to collect light and guide it to discrete photodiodes, whilst the second uses a high speed camera to image the welding keyhole. This system has been developed for Nd:YAG welding of aeroengine components. Detection of focal errors and state of penetration are demonstrated for Inconel 718 and Titanium Ti-Al-6V.

Welding Diagnostics by Means of Particle Swarm Optimization and Feature Selection

In a previous contribution, a welding diagnostics approach based on plasma optical spectroscopy was presented. It consisted of the employment of optimization algorithms and synthetic spectra to obtain the participation profiles of the species participating in the plasma. A modification of the model is discussed here: on the one hand the controlled random search algorithm has been substituted by a particle swarm optimization implementation. On the other hand a feature selection stage has been included to determine those spectral windows where the optimization process will take place. Both experimental and field tests will be shown to illustrate the performance of the solution that improves the results of the previous work.