/ave laser signal 
/) with two fre- 
from a target is 
iplitudes Pp, are 
received sine- 
' their reference 
¢, are propor- 
quencies. Since 
modulation fre- 
sufficient mea- 
olution. A low- 
ees a coarse but 
n), whereas the 
Hz) provides a 
mation over $,. 
| 9» of both fre- 
> measurements 
#0 
B, 0 
T 
Target 
ER 
consists of two 
1 and the beam 
independently 
itrol and moni- 
    
  
receiver 
optics 
-— d 
safe operation 
on mode of the 
atically. High- 
speed data transfer between the laser scanner and a sensor data 
computer is achieved by a transputer link (20 Mbit/s). 
3.1 Range measuring system 
The range measuring system is the major component of the 
laser range scanner. It consists of the laser head, high frequen- 
cy unit and the signal processing unit for range data preproces- 
sing. 
Laser Head: 
The laser head emits a continuous wave, intensity modulated 
infrared (IR) laser beam and detects the laser light back-scat- 
tered from a target. It consists of several electro-optical and 
micro-mechanical components. 
The laser beam is generated by a semiconductor laser diode 
that emits 20 mW of laser power at 810nm (near infrared). 
Varying the drive current to the diode (+15 mW, pp, am-cw) 
modulates the amplitude of the laser light. Eyesafety (DIN EN 
60825) is achieved by reducing emitted laser power by means 
of a grey-glass filter to Pp = 4.5mW (laser class 3A). Optical 
power reduction minimizes nonlinear effects of the laser diode 
as well as temperature and noise effects. A set of microlenses 
forms the laser cone from the diode into a coaxial path of rays 
with small divergence (8 = 0.01 mrad). The laser light (Pp) 
reflected from an environmental object returns on the same 
optical path to the receiver where receiver optics (J = 60 mm, 
f= 50 mm) focus it onto the detector. Using an avalanche pho- 
todiode as a detector and an IR filter for elimination of spec- 
tral noise in laser light guarantees high dynamic range with the 
reflectance (Py/Py — 296 .. 99%) of environmental objects. 
High frequency unit: 
The high frequency unit generates a modulation signal for the 
laser diode and consists of two quadrature receivers for fre- 
quency-selective demodulation of the back-scattered laser 
light. 
For modulation of the emitted laser light two sine signals with 
the frequencies o, = 10 MHz and ®, = 80 MHz are necessary. 
In order to achieve in-phase correlation of the two signals, a 
single 80 MHz oscillator in combination with a divider gener- 
ates modulation frequencies. The resulting signal for drive 
current modulation (Fig. 1) of the laser diode is generated by 
eliminating harmonic signals in the square-wave signals of the 
oscillator and adding sine signals. 
The back-scattered laser light is amplified, and band-pass fil- 
tered in order to seperate LFS (10 MHz) and HFS (80 MHz) 
channels, and then downconverted. Using homodyne quadra- 
ture mixers for distortion-free demodulation guarantees high 
dynamic range with detected signal levels (S/N = 90 dB). The 
quadrature mixers evaluate the respective in- phase (P) and 
quadrature (Q;) signal of each channel. 
In-phase (P) and quadrature signal (Q2 of each channel form a 
complex measurement vector. Phase €; of the vector is propor- 
tional to measured "range d." in the channel's specific ambi- 
guity interval, whereas magnitude my; of the vector directly in- 
dicates intensity "active grey level 8; Of back scattered light. 
Signal processing unit: 
After amplification and Bessel filtering in the analog unit, 
followed by simultaneous sample, hold and digitizing (f, 2 500 
kHz), the signals P, and Q; of each channel are subsequently 
processed in the didital unit. Bessel low-pass filtering guaran- 
tees constant group delay (no additional phase-shifts) and a 
minimum of overshooting in pulse-function response due to 
range-jumps. In order to eliminate range errors resulting from 
filtering a filter cut-off frequency of f, = 250 kHz ( f, = £72) 
has to be selected theoretically. Using this cut-off frequency, 
aliasing errors are dominant. As a good compromise between 
both effects, filter cut-off frequency has been selected at f. 
130 kHz. The resulting errors of range measurement due to the 
adaption of filter cut-off frequency are in the order of a few 
mm. Only range-jumps of several meters (contact wires, ed- 
ges, etc.) from one pixel to the other cause range errors at the 
respective jump-edges of several cm (Fig. 3). Hybrid 14-bit 
A/D converters are used to digitize filtered signals P; and Q; of 
each channel in order to fulfill the demands of high dynamic 
range (S/N = 56 dB) with reflectance of target surfaces and the 
high accuracy (S/N min = 25 dB) of range measurements. 
  
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473