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

''iI98 5. Kinematic Laser ScanningAfter discussing the influencing parameters in detail,some notes regarding the principle of measuringthe time delay At are necessary. For this, a special electronic device was developed,which is controlled viaparallel port (LPT). After initializing the electronic circuit, the time passed between the detection of the laserbeam on the start diode and on the stop diode is stored. Therefore, the clock generator is based on oscillatorswith different frequencies, i.e.2 MHz, 4 MHz, 8 MHz, and 16 MHz. The derived time can be read from theelectronic circuit and again a new measurement can be initialized. This procedure is repeated within aloop.The total number of time measurements as well as the time gap between each measurement can bedefined manually. The stability of the oscillators is given by 50ppm. Since 1 Hz equals 1 /xs, an instabilityin the frequency of 50 ppm leads to time variations of a single time measurement between 25 /xsfor theoscillator frequency of 2 MHz up to 3/xs for the oscillator frequency of 16 MHz. Performing repeated timemeasurements, the precision of the mean value can be increased significantly up to the desired precisionfor the rotation time T of 1 /xs. Further details regarding the electronic circuit are described in Appendix D.ResultsIn general, two independent setups, including several data series, were performed. For each setup, theautocollimation had to be carried out and the center anglea had to be derived. First, three data seriesare presented. All data series include 1000 measurements with a time interval of 1 s. The frequencyoscillator used was 2 MHz. In Figure 5.8, the detected time delay At correspondingof theto the three differentrotation velocities, i.e.12 5 rps, 25 rps, and 33 rps, are plotted. As seen, the raw data contains some blunders.The noise can be reduced by applyinga median filter.The result is a smoothed data series from which therotation time T can easily be derived. In the data series, another fact is also visible. The noise present at12 5 rps is much higher than the noise present in 25 rps and 33 rps.The reason is in mass inertia. Massinertia leads to higher noise if the rotation is slower. On the contrary, the noise decreases if the rotation isfaster.To exclude systematic effects, the Fourier analysis is appliedsignificant frequencies were present and all amplitudes were within 2 /xs. Naturally,eliminated before.on the data series. The results were noall blunders have to be0 0804300 0804200 080410*teffi^raw datamed an filter0 0804000 080390400 500 000# measurements0 0402300 040220raw datamed an filter0 0402100 040200^A/^^r-m. *#*#f#&P^W*.tW^Ce*VW#*"*%ip^jBMt« H^ifÄ**! ï*V*iV#*'«*%TkfÇ#%*500# measurementsraw datamed an filter0 030460*f**v!k*%Jjy 0^k0^. »*v%#Hi*•mWtftf***f***m#^ n»**i$j*$f#k*^ ^•f***" &+4fo+