Krzysztof Malowany

Application of 3D digital image correlation in maintenance and process control in industry

3D digital image correlation method is widely used for displacements measurements in laboratory conditions and for experimental applications in industries. In this paper we present enhancements of the standard method, which enable application of 3D DIC for in situ monitoring and process control in industries and out-door environment. Enhancements concern software modifications (new visualization methods and a method for automatic merging of data distributed in time) and hardware improvements (protecting equipment against hard environmental conditions). The modified 3D DIC system is applied in two interesting cases: measurements of steel struts at construction site and measurements of a pipeline in an intermediate pumping station. In both applications we additionally used an infrared camera in order to correlate deformations of measured objects with temperature changes.

Multi-camera DIC system with spatial data stitching procedure for measurements of engineering objects

The paper presents the way that colour can serve solving the problem of calibration points indexing in a camera geometrical calibration process. We propose a technique in which indexes of calibration points in a black-and-white chessboard are represented as sets of colour regions in the neighbourhood of calibration points. We provide some general rules for designing a colour calibration chessboard and provide a method of calibration image analysis. We show that this approach leads to obtaining better results than in the case of widely used methods employing information about already indexed points to compute indexes. We also report constraints concerning the technique. Nowadays we are witnessing an increasing need for camera geometrical calibration systems. They are vital for such applications as 3D modelling, 3D reconstruction, assembly control systems, etc. Wherever possible, calibration objects placed in the scene are used in a camera geometrical calibration process. This approach significantly increases accuracy of calibration results and makes the calibration data extraction process easier and universal. There are many geometrical camera calibration techniques for a known calibration scene [1]. A great number of them use as an input calibration points which are localised and indexed in the scene. In this paper we propose the technique of calibration points indexing which uses a colour chessboard. The presented technique was developed by solving problems we encountered during experiments with our earlier methods of camera calibration scene analysis [2]-[3]. In particular, the proposed technique increases the number of indexed points points in case of local lack of calibration points detection. At the beginning of the paper we present a way of designing a chessboard pattern. Then we describe a calibration point indexing method, and finally we show experimental results. A black-and-white chessboard is widely used in order to obtain sub-pixel accuracy of calibration points localisation [1]. Calibration points are defined as corners of chessboard squares. Assuming the availability of rough localisation of these points, the points can be indexed. Noting that differences in distances between neighbouring points in calibration scene images differ slightly, one of the local searching methods can be employed (e.g. [2]). Methods of this type search for a calibration point to be indexed, using a window of a certain size. The position of the window is determined by a vector representing the distance between two previously indexed points in the same row or column. However, experiments show that this approach has its disadvantages, as described below. * E-mail: A.Nowakowski@ire.pw.edu.pl Firstly, there is a danger of omitting some points during indexing in case of local lack of calibration points detection in a neighbourhood (e.g. caused by the presence of non-homogeneous light in the calibration scene). A particularly unfavourable situation is when the local lack of detection effects in the appearance of separated regions of detected calibration points. It is worth saying that such situations are likely to happen for calibration points situated near image borders. Such points are very important for the analysis of optical nonlinearities, and a lack of them can significantly influence the accuracy of distortion modelling. Secondly, such methods may give wrong results in the case of optical distortion with strong nonlinearities when getting information about the neighbouring index is not an easy task. Beside this, the methods are very sensitive to a single false localisation of a calibration point. Such a single false localisation can even result in false indexing of a big set of calibration points. To avoid the above-mentioned problems, we propose using a black-and-white chessboard which contains the coded index of a calibration point in the form of colour squares situated in the nearest neighbourhood of each point. The index of a certain calibration point is determined by colours of four nearest neighbouring squares (Fig.1). An order of squares in such foursome is important. Because the size of a colour square is determined only by the possibility of correct colour detection, the size of a colour square can be smaller than the size of a black or white square. The larger size of a black or white square is determined by the requirements of the exact localisation step which follows the indexing of calibration points [3]. In this step, edge information is extracted from a blackand-white chessboard. This edge information needs larger Artur Nowakowski, Wladyslaw Skarbek Institute of Radioelectronics, Warsaw University of Technology, Nowowiejska 15/19, 00-665 Warszawa, A.Nowakowski@ire.pw.edu.pl Received February 10, 2009; accepted March 27, 2009; published March 31, 2009 http://www.photonics.pl/PLP

Application of digital image correlation (DIC) for tracking deformations of paintings on canvas

A non-invasive and non-contact optical method for tracking overall and local deformations of canvas painting is presented. The technique was tested on a model painting on canvas with inhomogeneities introduced by infilling gaps, mending tears, and applying patches on the reverse of the canvas. The deformation of the sample was induced by changes of relative humidity. The feasibility of 3D Digital Image Correlation technique for evaluation of conservation methods is discussed.

Application of Image Based Methods for Monitoring and Measurements of Structures in Power Stations

The implementation of selected full-field optical methods for monitoring and measurements of displacements, strains and shape of structures in power plants are reported. Digital Image Correlation, Fringe Projection and integrated thermovision-DIC method have been utilized for monitoring and control of repair processes of selected elements during general overhauls in power plants, including control of welds annealing process in boiler drum and steam pipes and measurements of geometry changes of steam pipes in “hot” and “cold” states. The experience gathered during the measurement sessions in power plants has been used for enhancement and adaptation of typical architecture of measurement systems to demanding and difficult industrial environment conditions. The measurements had been carried out in different power plants located in Poland. The possible future application of full-field optical measurement methods as the alternative to standard techniques (ultrasound, X-ray, strain gauges) and their advantages and disadvantages are discussed.

Digital image correlation method: a versatile tool for engineering and art structures investigations

Optics as the enabling technology is applied in many applications of engineering, medicine, multimedia and conservation of cultural heritage. Most of these applications require close cooperation with the end user and often they enforce modification and enhancement of an optical tool. In the paper we show how optical metrology, specifically the application of digital image correlation method is implemented to two completely different tasks: performing preoperating tests of low cost building structures subjected to loading conditions which simulate the natural load e.g. introduced by the weight of snow and monitoring of canvas paintings for tracking humidity-induced deformations, which may appear in museum (or other location of a piece of art e.g. church). The presented examples are the background for a general discussion on different measurement scenarios with application of DIC method, as well as the required enhancements and modifications which have been introduced.

Application of 3D digital image correlation to track displacements and strains of canvas paintings exposed to relative humidity changes.

This paper introduces a methodology for tracking displacements in canvas paintings exposed to relative humidity changes. Displacements are measured by means of the 3D digital image correlation method that is followed by a postprocessing of displacement data, which allows the separation of local displacements from global displacement maps. The applicability of this methodology is tested on measurements of a model painting on canvas with introduced defects causing local inhomogeneity. The method allows the evaluation of conservation methods used for repairing canvas supports.

Diagnostic of structures in heat and power generating industries with utilization of 3D digital image correlation

In the paper we present implementation of 3D DIC method for in-situ diagnostic measurements of expansion bellows in heating chambers. The simultaneous measurements of a supply and a return pipeline were carried out in a heating chamber in Warsaw at the peak of the heating season in cooperation with Dalkia Warszawa. Results of the measurements enabled assessment of the risk of failure of expansion bellows. In-situ measurements were preceded by feasibility tests carried out in the Institute of Heat Engineering of Warsaw University of Technology. Potential implementations and a direction of future works are discussed in conclusions.

Measuring structural displacements with digital image correlation

Digital image correlation (DIC) methods enable accurate 2D and 3D measurements of changes in images.1 These methods use tracking and image registration techniques to make full-field non-contact measurements of displacements and strains in a wide variety of engineering applications. DIC techniques can also be used to measure changes in art objects. However, most of these applications require long-term monitoring. We have designed a new method that automatically merges temporally distributed DIC data.2, 3 The software and hardware of the basic DIC method have been enhanced. In addition, the results are combined with thermal camera data, so that the new procedure is suited to power plant and chemical industry process control applications, e.g., boiler drum annealing supervision, district heating pipeline diagnostic tests, and pipeline displacement measurements.4–6 The technique can also be used to monitor the health of art exhibits. In the DIC method we have designed, we use a flat calibration artifact (CA) as a reference for merging data acquired by two cameras placed at different positions. The CA remains permanently at the measurement site and is kept at a fixed position during consecutive measurements. For every data set, we transform the images obtained from the pair of cameras to the coordinate system of the reference CA. The transformation matrix is estimated by analyzing the 3D positions of markers, which is achieved through triangulation of the corresponding 2D positions in the stereo camera images. Standard 3D DIC analysis is subsequently performed on the images we obtain through our automatic data merging procedure (see Figure 1). Figure 1. Flow chart illustrating the automatic merging of temporally distributed data in the 3D digital image correlation (DIC) method. CAref 3D: Calibration artifact in reference data set. CAimg3D: Calibration artifact in data set to be merged. T3D: 3D transformation matrix.

Optical sensor based on combined GI/DSPI technique for strain monitoring in crucial points of big engineering structures

The data from a monitored structure/object should be easy acquired, processed and sent to the user, who can assess the health of a structure in short time and schedule necessary maintenance in order to prevent accidences. Systems which provide such information are fundamental for Structural Health Monitoring (SHM). In the paper novel optical sensor designed for in-plane displacement and strain monitoring in crucial points of a big engineering and civil structures is presented. It combines two techniques: Grating Interferometry (GI) and Digital Speckle Pattern Interferometry (DSPI). GI requires specimen grating attached to the surface of an object under test. It is the unique technique which may provide the information about fatigue process and increased residual stresses. DSPI works with a rough object surface but due to differential measurements cannot be simply used for long time monitoring but to explore the actual behavior of a structure. The sensor which combines these techniques provides user with wide possibilities concerning functionality, measuring range, object surface and environmental conditions. The crucial issue in implementation of this sensor is the choice of its location(s) at the investigated structure. Therefore it is proposed to be as one of the elements of hierarchical sensors net, which gives complete information about structure state. As the method for supporting the choice of GI/DSPI sensor location we proposed the system based on 3D digital correlation method. The paper presents mechanical and optical sensor design along with laboratory tests of main component such as sensor heads in form of monolithic (plastic) and cavity waveguides. Finally the possible application of proposed sensor in combination with 3D DIC system is presented.

Application of 3D digital image correlation for development and validation of FEM model of self-supporting metal plates structures

Many building structures due to complex geometry and nonlinear material properties are difficult to be analyzed with FEM methods. A good example is a self-supporting metal plates structure. Considering uncommon geometry and material characteristic of a metal plate (due to plastic deformations, cross section of a trough, a goffer pattern), the local loss of stability can occur in unexpected regions. Therefore, the hybrid experimental-numerical methodology of analysis and optimization of metal plates structures has been developed. The methodology is based on three steps of development and validation of a numerical model with utilization of Digital Image Correlation measurements. In each step, the measurements are performed in different environments, with different accuracies and different scales. In this paper, the results of analysis performed with Digital Image Correlation, that enabled development and validation of FEM model are presented. The performed modification of a measurement setup is also described.