rder to produce 
ence, indicating 
range measur- 
ield of view. 
the laser range 
iation of range 
ormal ambient 
r a heating up 
of the scanner 
outside heater, 
Was detected. 
th the scanner 
during an out- 
€). 
ner 
rer, or its re- 
and a series of 
tic point in the 
ition in each of 
of a small wall 
range data and 
“jump edges of 
e scanner is not 
o the scanning 
er profile, only 
about 8 mm/m, 
ection 
ns of the laser 
uated. Figure 6 
be measured in 
tunnel tube are 
system. Tunnel 
faces) is in the 
tunnel 
  
  
m 
00 3000 
  
4000 
3500 
3000-2] ct 
polar range [mm] 
2010 eMe MM 
  
2000 [- db ll 
  
  
i | i | | 
50 100 150 200 250 300 
scanning angle ß [deg] 
  
6b) Polar plot of reference tunnel profile 
Fig. 6: Reference tunnel profile 
Figure 7 shows the profile measured with the laser range scan- 
ner and compares it to the reference tunnel. 
  
4000 --—- 
3500 
w 
© 
co 
e 
T 
polar range [mm] 
210 EeÁJ 
  
  
  
| i | i | | 
50 100 150 200 250 300 
scanning angle ß [deg] 
  
7a) Measured profile (polar plot) 
  
© 
ce O1 
T 
log10 of range error [mm] 
e 
N 
     
  
  
   
  
| | ] | | | 
“50 100 150 200 250 300 
scanning angle b [deg] 
7b) Deviations with profile measurement 
Fig. 7: Tunnel profile measurement results 
Resulting deviations between reference and measured profile 
of several mm correspond to examinations of Chapter 3 and 4 
and fulfill all demands with tunnel surface inspection. Corres- 
ponding grey level profile is used for visual inspection, surface 
classification, and documentation purposes. 
5. CONCLUSIONS 
In this paper we have examined in detail the measurements 
supplied by an amplitude-modulated laser range sensor. We 
presented experimental performance data and discussed the ap- 
plication of the sensor hardware for tunnel inspection. The 
quantitative performance, in terms of accuracy and precision 
with different surface material types, fulfills the demands of 
tunnel surface inspection. Compared [20] to other laser range 
sensors, the laser range measuring system shows good perfor- 
mance data. Robustness of the laser range scanner even under 
varying temperature and lighting conditions is sufficient for 
application in industrial environments. Ambiguity interval of 
range measurement and range accuracy of the laser range scan- 
ner may be adapted to other applications by selecting other 
modulation frequencies. 
Parallel to "range" measurements reflectance of surface ma- 
terial "active grey level", is measured. Both, range and grey 
level data of a target point are registered at the same time and 
correspond to one single target point defined by the laser beam 
direction. The range profile of the laser range scanner reflects 
geometric dimensions of the tunnel tube whereas the grey 
level profile is used for visual inspection, surface classifica- 
tion, and documentation purposes. 
In a next step of our research we will test the laser range 
scanner in several railway tunnels and mount it onto a vehicle 
in order to achieve range and grey-level images of the com- 
plete tunnel tube. Effects of ambient temperature gradients in 
the tunnel as well as humidity in the tunnel to range accuracy 
and precision have to be studied in detail. 
ACKNOWLEDGEMENTS 
The work reported in this paper was supported by the 
Deutsche Forschungsgemeinschaft, as part of an interdiscipli- 
nary research project on "Information processing in Autono- 
mous Mobile Robots" (SFB 331) and by Spacetec GmbH, Frei- 
burg, Germany. The authors would thank Dipl. -Ing. (FH) R. 
Wigler (Spacetec GmbH) for the improvements of the receiver 
optics. 
REFERENCES 
[1] Schmidt, G.: "Towards Integration of Autonomous Sub- 
systems for Assembly and Mobility into Flexible Manu- 
facturing". Proc. of Internat. Workshop on Information 
Processing in Autonomous Mobile Robots, München, 
(1991), pp. 3 - 20 
[2] Besl, P.: "Active, optical range imaging sensors". Ma- 
chine Vision & Applications. Vol. 1, (1988), pp. 127-152 
[3] Kanade, T.: Three-Dimensional Machine Vision. Kluwer 
Academic Publishers, 1987 
477