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

5.2 Rotation Time of Rotating Mirror of Laser Scanner 97Table 5.5 shows the influence of S on the center angle a. The important value is the difference Aa betweenthe 'desired' center angle a and the 'falsified' center angle ß. Equating the resultingdifference Aa to theprecision of the center angle aa and referring to Table 5.2, it can be seen that, except for the implausibledeviation S of 10 °, all resulting differences Aa causes an error in the rotation time T of less than 1 /xs.Table 5.5: Difference Aa between 'desired' center angle a and falsified' center angle ß caused by a deflection angle5 using autocollimation. The longer the radius r and the smaller the deviation 5, the smaller the resulting differenceAa.r = Imr = 2mr = 5mS[°] 105 1 105 1 105 1a[°]1.41850.70930.28371.41850.70930.28371.41850.70930.2837ßl°]1.39700.69850.27941.41310.70660.28261.41830.70920.2837Aa[°] 0.0215 0.0108 0.0043 0.0054 0.0027 0.0011 0.0002 0.0001 0.0000The result of this discussion is that the autocollimation is well-adapted.The autocollimation can be carriedout with high accuracy. Here, the accuracy of autocollimation is further influenced by the precision of theorientation of the mirror mounted on the plate, cf. Figure 5.5. Furthermore, a sufficient precision for thecenter angle a can be achieved so that a time resolution, i.e. precision, for the rotation time T of 1 /xs ispossible.As described above, the most crucial parameter for the center angle a is the precision of the base ab-In ad¬dition to the precision of the base, the position of the reflection point on the autocollimation mirror also hasto be in the middle, between the two photo diodes. To determine this center point with high accuracy, theautocollimation mirror is covered with an adhesive tape. Only a small gap with a width of approximately1 mm is left in the middle. This setup allows for both sufficient central positions of the reflection pointrequired for autocollimation and a reflected laser beam necessary to perform the autocollimation. Further¬more, a scale is installed on the rotating mirror of the laser scanner. This scale includes a hole throughwhich the laser beam is emitted. The reflected laser beam hits the scale, and based on the position on thescale, the autocollimation mirror can be aligned perpendicular to the laser beam by means of the adjustingdevice. The autocollimation mirror is perpendicular to the laser beam if the reflected laser beam is overlaidwith the emitted laser beam. In Figure 5.7, on the left side the plate including the two photo diodes, theautocollimation mirror covered with the adhesive tape as well as the left gap, defining the position for theautocollimation, can be seen. On the right side, the scale fixed on the rotating mirror is shown.rotating mirrorcoverage ofautocollimation mirrorposition forautocollimationlaser beamscale forautocollimationFigure 5.7: Left: Arrangement of the two photo diodes and the coverage of the autocollimation mirror. Right: Scaleattached on the deflecting mirror of the laser scanner for measuring the offset between the emitting and reflecting laserbeams.