Calibration of a Terrestrial Laser Scanner - Institute of Geodesy and ...

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20 2. Components of Terrestrial Laser Scannerstrips of up to 200 is overlapped by the corresponding diametrically opposed pattern.Based on the Moirépattern, the resolution can be increased by measuring the phase. Therefore, the reading index has to bepositioned within the last period of the Moiré pattern. Further details can be found in [Ingensand, 1998]and [Schlemmer, 1996].2.2.2 Binary EncodingThe binary encoding method is also called the absolute method since the reference base is physicallymarked on the graduated glass circle. Light from a light source passes through the encoded glass cir¬cle, which is marked with the number of the graduation in a binary code. Each corresponding digitbinary code is represented by a transparent or an opaque element.of theThe glass circle contains either coded rings or is marked by an absolute code, which can be read by an arrayof CCD5 sensors. In Figure 2.15, a glass circle with coded rings (left) and an absolute code (right)seen. The required resolution can be achieved by interpolation algorithms.[Ingensand, 1998] and [Schlemmer, 1996].can beFurther details can be found inFigure 2.15: Binary encoding with absolute orientation according to [Ingensand, 1998]. Coded rings (left) andabsolute code (right).2.3 Deflection SystemScanning the environment requires the deflection of the laser beam in two directions, horizontally andvertically, respectively. Due to constant angle increments the result is a grid of points defining a pointcloud. The horizontal deflection step width is often forced to equal the vertical deflection steppoints are arranged in vertical profiles and shifted by a discrete horizontal angle increment.width. TheThe deflection can be separated into a primary deflection and in a secondarydeflection for two-mirrorsystems,cf. Section 2.3.1. Each mirror rotates about one specific axis to deflect the laser beam in a 2D plane.A common practice is to perform a primary deflection in the vertical direction to generate a vertical profile.The secondary deflection then shifts the laser beam horizontally to acquire the next vertical profile adjacentto the previous profile. In the case of a one-mirror system, the laser beam is deflected by a rotating mirror,which is moving about two perpendicular axes, cf. Section 2.3.2.The dimensions of the environment thatcan be scanned depends on the principle used for deflectingthe laser beam. This field of view is eitherlimited by a section, i.e. camera scanner, or covers a full view, i.e. panorama scanner, cf. Figure 2.16.There are different techniques for deflecting the laser beam. A detailed discussion is given in [Kern, 2003],where it is distinguished between a deflection using plane mirrors, polygon mirrors, and prisms. Anotherpossibility is the distinction regarding the motion of the mirrors. They can either oscillate or simply rotate.5CCD: Charged Coupled Device
2.3 Deflection System 21AFigure 2.16: Different types of terrestrial laser scanners classified by field of view according to [Runne et al, 2001]:panorama scanner (left) and camera scanner (right).In the following,deflection unit.a distinction is madebetween a rotating beam deflection unit and an oscillating beamThe deflection of the laser beam implementedin laser scanners isoften a combination of different de¬flection methods. The primary rotation is faster than the secondary rotation. Considering the panoramascanner, e.g. laser scanners of Callidus, Faro, Riegl, Zoller+Frohlich, the primary deflection is performed bya fast-rotating mirror. The secondary deflection involves a rotation of the completedeflection unit insteadof rotating only the mirror. Appendix A shows the imaging system of the laser scanner ImagerZoller+Frohlich.5003 of2.3.1 Oscillating MirrorThe primary rotation is described by a mirror that oscillates about an axis. The samplinginterval is definedby the minimum increment of the encoder unit which causes the mirror to oscillate to the next position.The encoder position and the distance correspondrotation, the synchronisation is not as crucial as it is for the rotatingwith each other. Since the oscillation is not as fast as themirror. The combination of two oscilla¬tion mirrors deflects the laser beam in the desired directions. The vertical and horizontal sampling intervalsare defined by the minimum increment of the encoder controllingdrawbacks of oscillating mirrors are the limited field of view and the performancethe oscillations of the mirrors. The mainof the deflection.Figure 2.17: Oscillating mirrors according to [Schlemmer, 2004].
Page 1: DISS. ETH NO. 17036Calibration of a
Page 5 and 6: ZusammenfassungSeit einigen Jahren
Page 7 and 8: ContentsAbstractiiiZusammenfassungv
Page 9 and 10: Contentsix6.1.3 Results Ill6.2 Stat
Page 11 and 12: 1Introduction1.1 Terrestrial Laser
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72 3. Calibration of Terrestrial La
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74 4. Static Laser Scanning4.1.2 Mi
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76 4. Static Laser ScanningOptical
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78 4. Static Laser Scanningmaximum
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80 4. Static Laser Scanningand gene
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82 4. Static Laser Scanning• Ther
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84 4. Static Laser Scanning4.3.3 NU
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88 5. Kinematic Laser Scanningmirro
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90 5. Kinematic Laser ScanningThe a
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92 5. Kinematic Laser Scanninghas a
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'94 5. Kinematic Laser Scanningbeam
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cf.aroundframe regardingsetupusinga
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112 6. Applications of Terrestrial
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132 7. Summaryrange, achievable by
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134 7. Summary
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136 A. Imaging System of Imager 500
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138 B. Technical Data of Imager 500
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u140 C. Adjustment of SphereA =dfi,
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142 C. Adjustment of Sphere
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144 D. Electronic Circuit for Deter
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-GenauigkeitsbetrachtungenInvestiga
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152 BibliographyWunderlich, T. A. [
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List of Figures3.22 Residuals of th
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156 List of Figures
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158 List of Tables
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200220072002Curriculum VitaePersona
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