Master Thesis - Fachbereich Informatik

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3.2. CAMERA SETUP 41 further distance can be imaged over the whole image size with such lenses. However, the minimum object distance is larger for long focal length lenses. For two-dimensional measuring tasks most accurate and precise results can be achieved with telecentric lenses (see Figure 3.5). These special lenses are designed to map objects of the same size in the world to the same image size, even if the object to lens distance differs. It is important to note that the maximum object size can not be larger than the diameter of the lens. This makes telecentric lenses useful only in connection with relatively small objects. In addition, such lenses reach a size of over 0.5m for objects of about 100mm and a mass of approximated 4kg [18]. Finally, telecentric lenses are very expensive. Although a telecentric lens would be advantageous in the imaging properties, a less expensive solution had to be found for the prototype development in this application. The optical system must be able to map objects between 20 and 100mm to an 1/2” CDD sensor at a relative short camera-object distance, and which is expected not to be affected too much by aberrations and radial distortion. However, this is an optimization problem that has no universal solution for all tube lengths. Different tube lengths need different magnification factors and field of views if the maximum possible resolution should be exploited to reach the highest accuracy. Changing the magnification factor means changing either the focal length of the optical system or the distance between object and camera, or both. If moving the camera toward the object, the minimum object distance of the lens has to be considered to be able to yield sharp images. Zoom lenses could be used to change the focal length without changing the whole optical system. However, zoom lenses should be avoided in machine vision applications [40], since they have to make larger compromises than fix-focal lenses and usually have a minimum working distance of one meter and more. Hence, if using a fix-focal lens, this implies changing the camera-object distance to adapt to different tube lengths, or to physically exchange the lens when a new length is cut by the machine which can not be covert by the current lens. Several commercial lenses designed for machine vision applications have been compared to find the lens that is best suited to inspect different tube sizes (see Table 3.1). Figure 3.6 gives an overview on the parameters that influence a camera’s field of view. The angle of view θ is specified by the lens manufacturer, and is depending on the focal length and the camera sensor size. All values in the following are oriented at an 1/2” CCD sensor, since this is the sensor size of the Marlin F-033C and F-046B. The working distance d is here defined as the distance between lens and conveyor. O represents the object size, and L indicates the size of the measuring area with respect to a certain tube size. L canbeapproximatedastwicetheobjectsizeO. The goal is to find a combination of a lens with a working distance that yields a visual field so that the size V of the imaged region of the conveyor equals the measuring area L. Note, in this context size can be replaced by length in horizontal, i.e. in the moving direction of the conveyor, since this is the measuring direction in this constraint application. Thus, in the following only this direction is considered. The geometry in Figure 3.6 leads to the following relationship between θ, d and V : � � θrad V =2dtan (3.5) 2
42 CHAPTER 3. HARDWARE CONFIGURATION Figure 3.6: Parameters that influence the field of view (FoV) of a camera. θ indicates the angle of view of the optical system, d the distance between lens and conveyor, O the object size, V is the size of the region on the conveyor that is imaged, and L representing the size of the measuring area depending on the current tube size. The goal is to find a lens that yields afieldofviewsuchasV ≈ L at short distance. Model f θ dmin Pentax H1214-M 12mm 28.91 250mm Pentax C1614-M 16mm 22.72 250mm Pentax C2514-M 25mm 14.60 250mm Pentax C3516-M 35mm 10.76 400mm Pentax C5028-M 50mm 7.32 900mm Table 3.1: Different commercial machine vision lenses and there specifications including focal length f, horizontal angle of view θ (in degrees) with respect to an 1/2” sensor, and minimum object distance dmin respectively. where θrad represents the angle of view θ in radians. Using this equation one can compute the length of the conveyor that is imaged in horizontal direction at the minimum object distance of a lens. The results can be found in Table 3.2. This shows, none of the compared lenses is able to image small objects (< 30mm) in focus onto the camera sensor in a way that the object covers about half the full image width. Thus, the minimum tube size that can be inspected at full resolution under this assumption is 30mm. However, if one shrinks the image width manually (for example using the AOI function of the camera), the constraints can be reached even for tubes below 30mm. The real world representation s of one pixel in the image plane can be approximated as follows: s = V Wimg (3.6) where Wimg represents the image width in pixels. For example, for a 16mm focal length lens and a working distance of 250mm, one pixel represents about 0.12mm at this distance in the real world if the image resolution is 780 in horizontal direction. At the same distance, a 25mm focal length lens yields a pixel representation of about 0.08mm at the
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Fachbereich Informatik Department o
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Acknowledgments First of all, I wou
Page 8 and 9: Contents Acknowledgments iii Abstra
Page 10 and 11: Contents ix 5.3.7. TubeLength .....
Page 12 and 13: List of Tables 1.1. Rangeoftubetype
Page 14 and 15: List of Figures 2.1. AccuracyandPre
Page 16 and 17: 1. Introduction Heat shrinkable tub
Page 18 and 19: 1.2. PROBLEM STATEMENT 3 hazards, r
Page 20 and 21: 1.4. RELATED WORK 5 Length [mm] Tol
Page 22 and 23: 1.5. THESIS OUTLINE 7 The vision pa
Page 24 and 25: 2. Technical Background 2.1. Visual
Page 26 and 27: 2.1. VISUAL MEASUREMENTS 11 (a) Fig
Page 28 and 29: 2.1. VISUAL MEASUREMENTS 13 Figure
Page 30 and 31: 2.1. VISUAL MEASUREMENTS 15 � xd
Page 32 and 33: 2.2. ILLUMINATION 17 tubes alternat
Page 34 and 35: 2.2. ILLUMINATION 19 effect of comp
Page 36 and 37: 2.3. EDGE DETECTION 21 i 2 i 1 0 (a
Page 38 and 39: 2.3. EDGE DETECTION 23 N × 1 kerne
Page 40 and 41: 2.3. EDGE DETECTION 25 These operat
Page 42 and 43: 2.3. EDGE DETECTION 27 (a) 0 ◦ (b
Page 44 and 45: 2.4. TEMPLATE MATCHING 29 accuracy
Page 46 and 47: 3. Hardware Configuration This chap
Page 48 and 49: 3.2. CAMERA SETUP 33 3.2. Camera se
Page 50 and 51: 3.2. CAMERA SETUP 35 eachcolorchann
Page 52 and 53: 3.2. CAMERA SETUP 37 Figure 3.4: Co
Page 54 and 55: 3.2. CAMERA SETUP 39 adjusting scre
Page 58 and 59: 3.3. ILLUMINATION 43 f V 12mm 129mm
Page 60 and 61: 3.3. ILLUMINATION 45 (a) (b) (c) Fi
Page 62 and 63: 3.3. ILLUMINATION 47 (a) (b) Figure
Page 64 and 65: 3.4. BLOW OUT MECHANISM 49 A light
Page 66 and 67: 4. Length Measurement Approach Whil
Page 68 and 69: 4.2. MODEL KNOWLEDGE AND ASSUMPTION
Page 74 and 75: 4.3. CAMERA CALIBRATION 59 4.2.7. B
Page 76 and 77: 4.3. CAMERA CALIBRATION 61 Figure 4
Page 78 and 79: 4.3. CAMERA CALIBRATION 63 is compu
Page 80 and 81: 4.4. TUBE LOCALIZATION 65 (a) (b) F
Page 82 and 83: 4.4. TUBE LOCALIZATION 67 � 1 Psm
Page 84 and 85: 4.4. TUBE LOCALIZATION 69 Global Th
Page 86 and 87: 4.4. TUBE LOCALIZATION 71 but also
Page 88 and 89: 4.4. TUBE LOCALIZATION 73 Global Re
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Page 104 and 105: 4.6. MEASURING 89 side of the tube,
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4.6. MEASURING 91 fcor(x) =−(c1x
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4.7. TEACH-IN 93 inthemeasuringplan
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4.7. TEACH-IN 95 The pair of a pixe
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5. Results and Evaluation 5.1. Expe
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5.1. EXPERIMENTAL DESIGN 99 between
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5.1. EXPERIMENTAL DESIGN 101 where
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5.1. EXPERIMENTAL DESIGN 103 Ground
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5.2. TEST SCENARIOS 105 enough, the
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5.3. EXPERIMENTAL RESULTS 107 The t
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5.3. EXPERIMENTAL RESULTS 109 Detec
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5.3. EXPERIMENTAL RESULTS 111 Lengt
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5.3. EXPERIMENTAL RESULTS 113 GTD [
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5.3. EXPERIMENTAL RESULTS 115 gray
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5.3. EXPERIMENTAL RESULTS 119 (a) (
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5.3. EXPERIMENTAL RESULTS 121 (a) (
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5.3. EXPERIMENTAL RESULTS 125 Total
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5.3. EXPERIMENTAL RESULTS 127 Color
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5.3. EXPERIMENTAL RESULTS 129 Task
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5.4. DISCUSSION AND FUTURE WORK 131
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6. Conclusion In this thesis a func
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Appendix 135
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138 APPENDIX A. PROFILE ANALYSIS IM
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140 APPENDIX A. PROFILE ANALYSIS IM
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142 APPENDIX B. HARDWARE COMPONENTS
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144 APPENDIX B. HARDWARE COMPONENTS
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146 Bibliography [18] C. Demant, B.
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148 Bibliography [51] K.K. Pingle.
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Master Thesis - Fachbereich Informatik

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