signal and image processing ,
optical detection theory ,
low signal-to-ratio clutter ratio processing ,
laser radar and optical systems ,
optical design and engineering
The objective of this case study is to characterise and to numerically assess the volume of impact damages in composite laminates with multiple non-destructive testing techniques. Reliable and robust assessments of the damage volume will be used as input and validation data for the existing and developing numerical models of composite materials.
In everyday work, researchers in the area of composite materials mostly use 2D assessment of the damaged area acquired with conventional inspection methods as ultrasonic c-scan. This practice is supported by the relative simplicity of the data analysis carried in 2D. It is proposed to assess the damage volume in 3D which may serve as a more reliable criterion to improve coupling with the numerical modelling.
In this paper micro X-ray computed tomography (CT) was used as the reference technique for the damage volume assessment and comparison with an industrial ultrasonic c-scan. In addition, for this case study, ultrasonic phased array technique with the full matrix capture algorithms was used for the inspection together with shearography (speckle pattern shearing interferometry). A comparison of all techniques referenced to the CT data formed the guidance for their applicability. In addition, an attempt to fuse the multiple assessments was made to support the comparison of 2D and 3D inspection techniques.
For the experimental part, carbon fiber reinforced polymer (CFRP) specimens of three layups were manufactured and damaged with 4 different impact energies in the range of 18-45 J. The layups were orthotropic, quasi-isotropic and a novel one adapted for the impact events. In total 12 specimens were inspected with multiple non-destructive testing techniques to numerically assess the volume of the impact damages. 3 additional specimens were used to assess the repeatability of the damage in the specimens with the same layup and impact energy. 3D CT reconstructions were made by a rotation of specimens using a cone-beam system with the voxel side size of 50 μm. A developed automatic segmentation algorithm was used for the damage volume estimation in all specimens. The phased array inspection was made with a full-matrix capture technique with the frequencies of 2.25 and 5 MHz. An available 3D shape shearography setup was used to characterise the damages both with the in- and out-of-plane surface strains with a thermal loading of the specimen.
Future developments include the comparison of the inspection results together with the numerical modelling predictions and with residual strength after the impact.
In this work it is shown that high-intensity microprismatic tapes have a potential to be used as a good substrate for bright and cheap fiducial marks in machine vision metrology applications. The drawback of the tapes is that they have technological netting pattern distributed across the surface. The proposed image processing technique allows good suppression of the parasitic technological netting pattern by a harmonic mean image filtering followed by circle shape recovering based on Fourier descriptors. It was also shown that the combination can provide good results in mark position estimations. In experiments it was shown that subpixel accuracy of position estimation can be achieved after applying proposed image processing, while without filtering the error can exceed 4 pixels in some cases.
Modern demands for railway track measurements require high accuracy (about 2-5 mm) of rails placement along the track to ensure smooth, safe and fast transportation. As a mean for railways geometry measurements we suggest a stereoscopic system which measures 3D position of fiducial marks arranged along the track by image processing algorithms. The system accuracy was verified during laboratory tests by comparison with precise laser tracker indications. The accuracy of ±1.5 mm within a measurement volume 150×400×5000 mm was achieved during the tests. This confirmed that the stereoscopic system demonstrates good measurement accuracy and can be potentially used as fully automated mean for railway track inspection.
3D position estimation of an object usually involve computer vision techniques, which require fiducial markers attached to the objects surface. Modern technology provides a high intensity retroreflective material in the form of a tape which is easy to mount to the object and can be used as a base for fiducial marks. But inevitable drawback of the tapes with the highest retroreflective intensity is the presence of technological pattern which affects spatial distribution of retroreflected light and deforms border of any print on tape's surface. In this work we compare various shapes of metrological pattern and examine Fourier descriptors based image processing to obtain estimation of accuracy of mark image position. To verify results we developed a setup consisting of a camera based on Sony ICX274 CCD, 25 mm lens, 800 nm LED lightning and high intensity microprismatic tape. The experiment showed that there is no significant difference between proposed mark shapes as well as between direct and indirect contrast when proposed image processing is used. The experiments confirmed that the image processing implemented without elimination of non-reflective netting pattern can only provide an accuracy of coordinates extraction close to 1 pix.
In this paper we introduce a system for deflection measurement of floating dry docks. The system contains two measurement channels observing opposite directions of the dock. It also includes set of reference marks, an industrial computer and a display. Each channel contains CMOS camera with long focal-length lens. Reference marks are implemented as IR LED arrays with 940 nm working wavelength for better performance within bad weather conditions (e.g. fog, rain, high humidity etc.). In the paper we demonstrate results of an analysis of different optical schemes for coupling the oppositely directed channels of the measurement unit and show that the scheme with two image sensors with separated lenses is an optimal option, because it allows usage of nonequidistant location of reference marks and demonstrates the least value of parasitic shift caused by rotations of the measuring unit. The developed system was tested both on specially-designed setup and in real infrastructure of a floating dry dock. The conducted tests proved that a measuring error of the system is smaller than ± 1.5 mm within the measurement range of ± 150 mm when deflection of 100 m dock is measured. Obtained results showed that the system demonstrates an ability to work in a harsh environment including poor weather conditions.
This work addresses accurate position measurement of reference marks made of retroreective microprismatic materials by image-based systems. High reflection microprismatic technology implies tiny hermetically sealed pockets, which improve material reflectivity, but result in non-reflective preprinted netting pattern. The mark pattern to be used for measuring can be simply printed on the reflective material as an opaque area with predefined shape. However, the non-reflecting pattern acts as a spatial filter that affects resultant spatial reflectivity of the mark. When an image of the mark is taken, the desired mark shape can be deformed by the netting pattern. This deformational may prevent accurate estimation of the mark position in the image. In this paper experimental comparison of three image filtering approaches (median filtering, morphological close and filtering in a frequency domain) in order to minimize the affection of the netting pattern is provided. These filtering approaches were experimentally evaluated by processing of the images of the mark that was translated in a camera field of view. For that a developed experimental setup including a camera with LED backlight and the mark placed on a translation stage was used. The experiment showed that median filtering provided better netting pattern elimination and higher accuracy of key features position estimation (approximately ±0.1 pix) in the condition of the experiment. The ways of future use of reference marks based on microprismatic material in image-based control applications are discussed.
In this work we present a pattern recognition method based on geometry analysis of a flat pattern. The method provides reliable detection of the pattern in the case when significant perspective deformation is present in the image. The method is based on the fact that collinearity of the lines remains unchanged under perspective transformation. So the recognition feature is the presence of two lines, containing four points each. Eight points form two squares for convenience of applying corner detection algorithms. The method is suitable for automatic pattern detection in a dense environment of false objects. In this work we test the proposed method for statistics of detection and algorithm's performance. For estimation of pattern detection quality we performed image simulation process with random size and spatial frequency of background clutter while both translational (range varied from 200 mm to 1500 mm) and rotational (up to 60°) deformations in given pattern position were added. Simulated measuring system included a camera (4000x4000 sensor with 25 mm lens) and a flat pattern. Tests showed that the proposed method demonstrates no more than 1% recognition error when number of false targets is up to 40.
This paper presents results of development of a hydroacoustic activity method based on use of vibrosensitive properties of optical fiber with fiber Bragg gratings. Analysis of changes for parameters of radiation in reflected signals, which take place due to microscopic hydroacoustic impacts on the optical fiber, allows to determine position of the source and to identify its class. The practical usage of the suggested approach has demonstrated its essential viability.
The paper deals with the creation of integrated monitoring systems. They combine fiber-optic classifiers and local sensor networks. These systems allow for the monitoring of complex industrial objects. Together with adjacent natural objects, they form the so-called geotechnical systems. An integrated monitoring system may include one or more spatially continuous fiber-optic classifiers based on optic fiber and one or more arrays of discrete measurement sensors, which are usually combined in sensor networks. Fiber-optic classifiers are already widely used for the control of hazardous extended objects (oil and gas pipelines, railways, high-rise buildings, etc.). To monitor local objects, discrete measurement sensors are generally used (temperature, pressure, inclinometers, strain gauges, accelerometers, sensors measuring the composition of impurities in the air, and many others). However, monitoring complex geotechnical systems require a simultaneous use of continuous spatially distributed sensors based on fiber-optic cable and connected local discrete sensors networks. In fact, we are talking about integration of the two monitoring methods. This combination provides an additional way to create intelligent monitoring systems. Modes of operation of intelligent systems can automatically adapt to changing environmental conditions. For this purpose, context data received from one sensor (e.g., optical channel) may be used to change modes of work of other sensors within the same monitoring system. This work also presents experimental results of the prototype of the integrated monitoring system.
LEDs have many advantages over traditional lighting, such as high brightness, small size, broad range of wavelengths
being emitted and ability to be placed with high density over flat or even-shaped surface. This offers promising choice
for many industrial and consumer applications and especially important for machine vision applications, where bright
and homogeneous illumination offers better visibility of features of interest. This can be obtained both with
multicomponent source configuration and analysis of distribution of optical energy density and color on an illuminated
surface. Required illuminating properties are produced by multicomponent source with certain structure and power
configuration. In this paper it is shown how to obtain required color and energy distribution on the surface of interest by
varying parameters of multicomponent source (matrix dimension, the distance between elements in the matrix, the
distance between the source and illuminated surface, etc.). Superposition of individual elements spectra is also taken into
account. This paper has proposed technique of the RGB multicomponent source simulation, which provides
homogeneous illumination on a flat surface of interest both in optical energy density and color. The ripple of luminance
on the surface shouldn't exceed the value of 2%.
KEYWORDS: Laser systems engineering, LIDAR, Signal processing, Signal detection, Energy efficiency, Signal to noise ratio, Target detection, Receivers, Interference (communication), Mathematical modeling
There are stringent false alarm probability demands on laser radar systems systems, although their operation is often
accompanied by a complex target environment, when the signal-to-noise ratio is low. New signal processing technique
for laser radar systems is suggested. The technique provides detection of a backscattered signal from a target during an
interval between a receiver noise bursts A pulse shoot is matched with a trailing edge of a noise burst, and the signal
presence decision is made according to leading edge of the next burst. There is a contradiction between the impulse
frequency and false alarm probability demands, that is why double-threshold processing is offered. The lower level
induces outpulsing while the higher one determines target detection performance. Since duration of such sophisticated
time intervals is random, statistic analysis was made via numerical model. The technique is aimed at providing low false
alarm probability and energy efficiency of the system at the same time.
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