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alized by ultrasonic sounding. Due to its long 
wavelength only a poor geometrical resolution can 
be achieved with todays ultrasonic technology 
which is not applicable in molding industry. 
Regarding the state of the art in digitizing free- 
form surfaces the tactile sensors are the most ad- 
vanced and mostly used measurement systems. The 
very severe disadvantages are that the measurement 
result is distorted by friction and drifting for- 
ces effects and that only low sampling rates can 
be realized. Due to the relative long measuring 
time the digitization of large objects is very 
time consuming and therefore in many cases imprac- 
tical in industrial measurement applications. In 
addition processing problems of the digitized data 
rise if the actual tool has different size and 
geometry with respect to the sensing stylus. Most 
tactile sensors available on the market have got 
their own control, can be easily implemented in 
production systems and have standard interfaces 
(e.g. DEMIS, Verein Deutscher Automobilhersteller 
Freiformflächen-Schnittstelle  (VDAFS) etc.) to 
higher level CAD/CAM systems. 
In the following a new non-contacting optical four 
dimensional measurement system will be described. 
It is a so called 4D-Laser-Mapper (4D-LM) and en- 
sures short measurement times and versatility 
which is especially required in molding industry. 
Its measurement data is processed by a new soft- 
ware module. This module is optimized with respect 
to 4D-LM. Using 4D-LM and the advanced postproces- 
sing module the process from carrying out the 
object lasts only minutes. Finally it will be 
shown that the manufacturing loop, which comprises 
survey of objects, the generation of valid CAD/CAM 
data and at last CNC data for manufacturing the 
workpiece, can be closed effectively. 
2. 4D-LASER-MAPPER 
The 4D-Laser-Mapper (4D-LM) was developed at the 
Institute of Navigation at the University in 
Stuttgart (INS). It digitizes contours of objects 
by a scanning remote sensing laser beam and its 
design was optimized to the requirements of the 
machine-tool and molding industry. It achieves 
high sampling rates with accuracies in the sub- 
millimeter range and can be easily coupled with 
CNC-machines and handling devices (s. fig. 2-1). 
Therefore it is possible to control and syn- 
chronize on-line the 4D-LM with the control of the 
production machine. 
The 4D-LM belongs to the family of the flyingspot 
measurement systems, because the illumination spot 
and the instantaneous receiving field of view is 
synchronously moved across the object's surface by 
an opto-mechanical deflection device which is 
ealized by two galvanometer scanners (s. fig. 2- 
2). The object's surface is three dimensionally 
digitized by measuring in each illumination point 
- in the following also called picture element or 
pixel - the slant range R between the 4D-LM aper- 
ture and the illuminated point on the target's 
surface. As the deflection angles a, and a, of the 
two galvanometer scanners are known, Cartesian 
coordinates can be calculated straight forward by 
using a special raytracing algorithm which regards 
improper adjustments in the opto-mechanical setup 
of 4D-LM. In addition to the slant range R, the 
intensity I of the reflected laser light upon the 
object's surface is sampled. Therefore a four di- 
mensional vector (4D) with coordinates (x, y, 2, 
I) is stored for each pixel on a hard disc. 
The slant range R is obtained by modulating the 
optical power of the laser with a high frequency 
signal of 313 MHz. Using a semiconductor laser 
this can easily be achieved by modulating the 
drive current of the laser diode. This is also 
known as direct modulation. The modulated laser 
light is collimated so that a spot with a diameter 
of about 2 mm is illuminated upon the target's 
surface. This light spot is located within the 
instantaneous field of view of the receiving optic 
which imaged the laser spot on an optical detec- 
tor. The optical detector should have a high res- 
  
    
   
  
    
  
  
  
  
  
   
   
  
  
   
  
  
    
  
   
    
   
   
   
    
   
   
  
  
  
  
   
  
   
   
   
   
    
   
   
   
   
   
  
  
   
   
   
   
   
  
   
   
  
    
   
   
  
  
   
  
   
  
   
  
  
   
  
   
   
  
  
   
  
  
   
   
   
    
     
  
      
ponsivity and should respond very fast. Therefore 
an avalanche photodiode (APD) with large bandwidth 
was selected. The APD reproduces directly the 
313 MHz signal out of the received optical signal. 
As the received signal travels two times R (s. 
fig. 2-3), it is received delayed by a delay time 
tL: 
  
ty = (1) 
if c is the speed of light. This delay causes that 
the transmitted and received signal have a phase 
difference ¢ of 
2mefet, - ó (2) 
if f is the frequency of the modulation signal. In 
our case is f - 313 MHz. This phase difference is 
electronically measured (s. fig. 2-1) using ana- 
logue technique. This analogue phase value is 
converted into digital format by an analogue to 
digital converter, so that it can be transmitted 
via the PC-bus to a microprocessor which calc- 
ulates the slant range R by using equations (1) 
and (2): 
X 
R = azz' © (3) 
As the 4D-LM should be used in industry without 
special laser safty precautions the transmitted 
optical power must be reduced to a minimum regard- 
ing the high collimation of the laser beam. How- 
ever, a limited laser power means also limitation 
for the achievable accuracy of the range measure- 
ment cg, which is a function of the signal-to-noise 
ratio SNR and the frequency of the measuring tone: 
  
c i 
o0 7 277 * (4) 
4n SNR 
The SNR is determined by the magnitude of the 
received optical power and the bandwidth of the 
measurement system. Large bandwidth means a short 
measurement time. 
As on one hand the optical power must be limited 
and on the other hand the accuracy should be as 
high as possible a trade off in the 4D-LM's design 
must be carried. A design study showed that a 4D- 
LM with the technical specifications listed in 
table 2-1 is a relative optimum for the applica- 
tion in machine-tool and molding industry. 
table 2-1: Technical Data of 4D-LM 
  
  
  
  
  
  
Laser Power 0.5 mW 
Wavelength 670 nm 
Inst. Field of View 0.19 
(IFOV) (1 mm transmitting 
aperture) 
Total Field of View max. 30°x30° 
(FOV), variable 
Receiving Apertur 24 mm 
Scanning Method 2-dim. Linescan 
  
Number of Pixels 400x400 or 800x800 
  
Range max. 2m 
  
0.1 mm (diffuse 
reflecting target 
with 60% reflec- 
tivity, distance 1 m 
Ranging Accuracy 
  
  
  
Ranging Tones 10 MHz, 313 MHz 
Measurement Rate 2 kHz (using one 
tone), 
800 Hz (using two 
tones) 
Scanning Time for 20 sec 
200x200 pixels