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

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92 5. Kinematic Laser Scanninghas a negligibly small eccentricity of the scan center, cf. Section 3.4.1, and since the prism attached to thetop of the laser scanner has a negligibly small offset of Ay, the coordinates of the pillaron which the laserscanner is set up and the coordinates of the prism can be compared. Therefore, the coordinate differences inx- and y-direction define the component Ax of the translation vector between the origin of the laser scannersystem and the prism attached to the laser scanner. The resulting value of the component Ax has an offsetof 0.158 m. The accuracy is sufficient with a value of less than one millimeter.The component Az can be obtained by using the same procedure applied for deriving the componentAx. The only difference is the resulting coordinate difference between the z-values of the prismobservation pillar on which the laser scanner is mounted also contains the height of the originand theof the laserscannersystem. Thus, the height of the instrument has to be considered and subtracted. The resultingvalue of the component Az is an offset of 0.159 m. The accuracy is sufficiently highone millimeter.with a value of less thanTable 5.1 summarizes the translation vector between the prism on top of the laser scanner and the origin ofthe local scannersystem.Table 5.1: 3D translation vector between the prism atop the laser scanner and the origin of the local scanner system.Ax [m] Ay [m] Az [m]0.158 0.000 0.1595.2 Rotation Time of Rotating Mirror of Laser ScannerThe laser scanner isreference is required to synchronizeoperated in the profile mode. For each point measured by the laser scanner, a timethe laser scanner data with data from additional instruments. Themirror of the laser scanner rotates quickly about its horizontal axis and the data acquisitionTherefore, the generation of a time tag for each single laser pointrate is fast.is not recommended. It seems morereasonable to validate if the rotation of the mirror is constant and is, therefore, appropriatefor the use astime base. If there is a constant periodic time for one full rotation about 360 °, the rotation time T can bederived byT=^-. (5.2)The rotation time T then serves as time information.By knowingthis rotation time and the number ofmeasured points for one vertical profile, a time tag can be calculated for each single point measured by thelaser scanner. If a common time base is available, data from different instruments can be synchronized. Forexample, one typical time tag is the PPS signal provided by GPS signals with an accuracy of approximately1 ms. The following sections deal with the determination of the rotation time T. The aim is to specify therotation time with a precision of
5.2 Rotation Time of Rotating Mirror of Laser Scanner 93with the total number of rotations of the mirror during a measurement and derives the rotation time Tindirectly.5.2.1 Direct MethodIn the direct method, the principle of deriving the rotation time is based on measuring a specific time delayAt.This time delay is defined by two adjacent photo diodes, which detect the rotating laser beam of thescanner.By knowing both the time delay At and the corresponding center angle a, defined between thecenter point of the rotation mirror and the two diodes, the rotation time T can then be calculated.implemented diodes1 are especially suitable for wavelengthsbetween 460 nm and 1060 nm.TheThe carrierwave of the laser scanner shows a wavelength of « 700 nm and therefore, fits into the sensitivity interval.The maximum sensitivity is at 850 nm. Since the high frequency signal 'hfs' and the low frequency signalTfs' have wavelengths of Ahfs « 6.7 m, Aifs « 54 m (close) and Aifs « 108 m (far), respectively,cf. Section3.2, they fit not into the sensitivity interval of the diodes. Thus, the modulation frequenciesthe determination of the rotation time T usingthese diodes.cannot disturbrotating mirror(laser beam)diode(stop) "autocollimationmirrorFigure 5.4: Experimental setup for deriving the rotation time T directly. Two photo diodes have to be aligned bymeans of autocollimation.diodes and the center angle a.The rotation time T can be calculated by knowing the time delay detected by the photoAn illustration of the principle is shown in Figure5.4. At a certain distance, two diodes mounted on aplate were set up. One diode starts and the other diode stops the time measurement. A closer look toFigure 5.4 shows that the start-diode is below the stop-diode.This means that the derived time At doesnot correspond to the center angle a but rather to the opposite adjacent angle a. since in this setup theorientation of the rotation of the laser beam is clockwise. Consequently,the rotation time T results fromT2ttAt2tt2ttAt. (5.4)Before explaining more in detail the experimental setup, the principle of deriving the time delaycenter angle a, some notes arenecessary regarding the use of two photoAt and thediodes. The ideal case is to mountonly one photo diode on a plate and to detect the rotating laser beam hitting this diode. After initializing,the first detection of the rotating laser beam defines the start time and the second detection of the rotatinglaser beam defines the stop time. The theory seems to be simple, but in practice,it is more difficult. The laser1 Mini-silicon NPN phototransistor SFH 305 of Siemens, Germany
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DISS. ETH NO. 17036Calibration of a
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ZusammenfassungSeit einigen Jahren
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ContentsAbstractiiiZusammenfassungv
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Contentsix6.1.3 Results Ill6.2 Stat
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1Introduction1.1 Terrestrial Laser
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1.2 Motivation 3Precision [m10~1 "1
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6 1. Introduction
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8 2. Components of Terrestrial Lase
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10 2. Components of Terrestrial Las
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24 3. Calibration of Terrestrial La
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'28 3. Calibration of Terrestrial L
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"[m] of system 90%) 3540 distance b
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!36 3. Calibration of Terrestrial L
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138 3. Calibration of Terrestrial L
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Page 52 and 53: 42 3. Calibration of Terrestrial La
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Page 84 and 85: 74 4. Static Laser Scanning4.1.2 Mi
Page 86 and 87: 76 4. Static Laser ScanningOptical
Page 88 and 89: 78 4. Static Laser Scanningmaximum
Page 90 and 91: 80 4. Static Laser Scanningand gene
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Page 94 and 95: 84 4. Static Laser Scanning4.3.3 NU
Page 96 and 97: 86 4. Static Laser Scanning
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Page 142 and 143: 132 7. Summaryrange, achievable by
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Page 146 and 147: 136 A. Imaging System of Imager 500
Page 148 and 149: 138 B. Technical Data of Imager 500
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142 C. Adjustment of Sphere
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144 D. Electronic Circuit for Deter
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aMulti-SensorPlatform,PhDGeodesy Sw
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Accuracy-AeinTPS1200 AG, -Version 1
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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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