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    On-line flatness measurementof large steel plates

    using moir topography

    Jussi Paakkari

    VTT Electronics

    Dissertation for the degree of Doctor of Technology to be presented,with the permission of the Department of Electrical Engineering

    of the University of Oulu, for public discussion in Auditorium L10, Linnanmaa,on June 5th, 1998, at 12 noon.


  • ISBN 9513852377 (nid.)ISSN 12350621 (nid.)

    ISBN 9513852385 (URL: http://www.inf.vtt.fi/pdf/)ISSN 14550849 (URL: http://www.inf.vtt.fi/pdf/)

    Copyright Valtion teknillinen tutkimuskeskus (VTT) 1998


    Valtion teknillinen tutkimuskeskus (VTT), Vuorimiehentie 5, PL 2000, 02044 VTTpuh. vaihde (09) 4561, faksi 456 4374

    Statens tekniska forskningscentral (VTT), Bergsmansvgen 5, PB 2000, 02044 VTTtel. vxel (09) 4561, fax 456 4374

    Technical Research Centre of Finland (VTT), Vuorimiehentie 5, P.O.Box 2000, FIN02044 VTT, Finlandphone internat. + 358 9 4561, fax + 358 9 456 4374

    VTT Elektroniikka, Optoelektroniikka, Kaitovyl 1, PL 1100, 90571 OULUpuh. vaihde (08) 551 2111, faksi (08) 551 2320

    VTT Elektronik, Optoelektronik, Kaitovyl 1, PB 1100, 90571 ULEBORGtel. vxel (08) 551 2111, fax (08) 551 2320

    VTT Electronics, Optoelectronics, Kaitovyl 1, P.O.Box 1100, FIN90571 OULU, Finlandphone internat. + 358 8 551 2111, fax + 358 8 551 2320

    Technical editing Leena Ukskoski


  • Dedicated to my wife Outi and daughters Marianna and Tiina

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    Paakkari, Jussi. On-line flatness measurement of large steel plates using moir topography.Espoo 1998, Technical Research Centre of Finland, VTT Publications 350. 88 p.

    Keywords machine vision, optical measurement, three-dimensional systems, steelplates, flatness, moir


    The quality demands for steel plate and strip products are increasing, andflatness and dimensional measurements performed on these products mustfulfill quality standards, and above all the needs of the clients. Many qualityassurance methods exist with regard to flatness and dimensionalmeasurements in the steel industry. Manual gauging methods are often stillused, but they are costly in terms of time and labour. Automated non-contact optical measurement systems are becoming more common due totheir benefits over traditional methods.

    In this thesis a projection moir topography is applied for automaticmeasurement of the flatness of large steel plates and a fully automatedsystem applicable to varying surface properties of large steel plates isimplemented in a hostile industrial environment. The specifications, designand performance of the system are presented. The measurement area of thesystem is 3.5 metres x 1.0 metre (width x length), and it achieves aspecified depth resolution of 0.3 mm. The system operates in real time onthe production line at a speed of 1 m/s.

    Experiences gained with the system and the different requirements applyingto another measurement site then led to the invention of a novelmultipurpose flatness measurement system, the design, laboratory prototypeand performance of which are also described in this thesis. The usabilityand competitiveness of the novel system have been increased by adding 2Ddimensional measurements such as edges, widths, lengths etc., and evensurface quality inspection, to the same unit. The depth resolution of thelaboratory system is better than 0.6 mm.

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    This thesis is based on the authors research work on moir topography and3D machine vision carried out at VTT Electronics during the years 1992 -1997.

    The research and development work was undertaken jointly by followingorganizations: VTT Electronics, Spectra-Physics VisionTech Oy, Rauta-ruukki Steel and the Industrial Technology Institute (ITI).

    I would like to thank Prof. Matti Pietikinen of the University of Oulu forhis guidance during my studies, and to express special thanks to Dr. HeikkiAilisto and Dr. Ilkka Moring who have both taught me and helped me in theresearch into 3D machine vision. Ilkka has shown excellent skills inmanaging and formulating difficult research problems, and we have hadmany technical discussions with Heikki on new ideas or on problemsencountered during the research projects. Thanks also go to Prof. HarriKopola who has supervised this thesis and improved its standard with hiscomments and his positive attitude.

    I would like to thank my co-workers at VTT Electronics and other researchinstitutes and companies who participated in the flatness measurementprojects involving the moir technique. Particular thanks are expressed toDr. Kevin Harding and Eric Sieczka for introducing me to the projectionmoir technique and fringe images. I must also thank Pekka Kantola, TimoPiironen, Jukka Kinnula and Seppo Lehtikangas for their support andpatience during the development of the measurement system. Thanks go toMasa Mkrinen and the company CIM-Tech Oy for participating in thedevelopment project and making the performance measurements.

    Special thanks should be expressed to Professor Henrik Haggrn and Dr.Kevin Harding for the time they have devoted to reviewing and commentingon this thesis. Ms. Tuija Soininen is acknowledged for finalising thedrawings, Ms. Irja Kontio for helping with the editing of the text and Mr.Malcolm Hicks for revising the English language.

    The Technology Development Centre of Finland (TEKES), VTT, and theabove mentioned companies are acknowledged for the support they havegiven to the research projects on which this dissertation is based. VTT,Tekniikan Edistmissti and Seppo Synjkankaan Tiedesti providedthe financial support that enabled me to finish this thesis.

    Oulu, April 1998

    Jussi Paakkari

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    1 INTRODUCTION 111.1 Motivation 111.2 Measurement of the flatness of steel plates 111.3 Contribution of this thesis 16

    2 PRINCIPLES AND STATUS OF THE MOIR SHAPEMEASUREMENT TECHNIQUE 182.1 Moir shape measurement principles 18

    2.1.1 Shadow moir 192.1.2 Projection moir 20

    2.2 Fringe pattern analysis 222.3 Current status of the moir shape measurement technique 28

    2.3.1 Status of optical moir shape measurement 292.3.2 Electronic moir contouring 352.3.3 Computer aided moir 362.3.4 Colour-encoded moir contouring 38

    2.4 Industrial applications and commercial products 39

    3 ON-LINE SYSTEM FOR MEASURING THE FLATNESS OFLARGE STEEL PLATES USING PROJECTION MOIR 423.1 Measurement specifications 423.2 Design of the flatness measurement system 43

    3.2.1 System overview 433.2.2 Optomechanical design of the measurement unit 453.2.3 Image analysis software 513.2.4 Calibration 53

    4 THE MULTIPURPOSE MOIR FLATNESS MEASUREMENTSYSTEM 554.1 Principle of flatness measurement in the multipurpose system 564.2 Integration of width measurement and surface quality inspection 594.3 Experimental system 61

    5 PERFORMANCE EVALUATION 655.1 Performance of the optical projection moir flatness measurement

    system 665.2 Performance of the multipurpose flatness measurement system 71

    5.2.1 Performance of the flatness measurement function 715.2.2 Performance of the width measurement function 745.2.3 Performance of the surface quality inspection function 75

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    7 SUMMARY 79


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    1D one-dimensional2D two-dimensional3D three-dimensional angle between the grid normal and the illumination angle between the grid normal and the observation phase of the moir signal noise standard deviationa background intensity of the moir imageASTM American Society for Testing and Materialsb local contrast value of the moir imageC temperature in degrees CelsiusCAD computer aided designCCD charge coupled deviceCMM co-ordinate measuring machined distance separating the light source and observerD/A digital to analog converterDSP digital signal processingEN European Standardf focal length of the projection and observation lensesG image intensityH heightI flatness indexl distance of the light source and observer from the grating plane,

    when lo = lslo distance of the observer from the grating planelp distance from the reference plane to the centre of the entrance

    pupils of the projection and observation opticsls distance of the light source from the grating planeL wave intervalLCD liquid crystal displayLP line pairsN contour orderP image of the projected gratingp grating pitchp grating pitch of the projected grating onto an objectPLL phase locked looprpm revolutions per minuteS steepness indexSCPS spatial-carrier phase shiftingSFS Finnish Standards AssociationSPS spatial phase shiftingTDI time delay and integrationTPS temporal phase shifting

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    TTL transistor-transistor logicV virtual binary grating in the computer memoryVTT Technical Research Centre of Finlandx Cartesian co-ordinatey Cartesian co-ordinatez Cartesian co-ordinatez distance between adjacent contour planeszN contour plane of order N

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    Flatness and dimensional criteria are included in the current qualitystandards for many sheet and plate products, e.g. steel plates, aluminiumplates, door leaves, insulating products for building applications and floorcoverings. It is evident that there are many reasons why one would want toknow and control the shapes and dimensions of such products. Theconsumers of steel plate and strip products are increasingly requiring tighterdimensional tolerances and better profiling and shaping. The followingmotivations for flatness measurement can be listed in the case of steelplates:

    Analysis of the flatn