limited diffraction efficiency, disturbance of the 
profiles caused by diffraction rings and difficult 
and expensive manufacturing of an axicon. 
For that reasons a commercial available laser diode 
with collimator optics is used for illumination. 
The focused laser beam is scanned over the object 
with a two-axis galvanometric scanner, which spans 
up a light plane. The intersection of light plane 
and workpiece surface, the so-called 'profile', is 
detected with a CCD TV-camera under the triangula- 
tion angle O. All components are connected to a PC- 
AT computer, which controls the whole system and 
evaluates the bending angle from the camera picture 
stored in a frame grabber. 
The 3-D coordinates of the workpiece surface are 
coded in the position of the image of 'profile' on 
the CCD sensor, respectively in the frame grabber 
coordinates of the profile. In the dimension across 
the profile these coordinates can be evaluated with 
very high precision by means of interpolation algo- 
rithms. Looking at the intensity distribution in 
Fig. 2 one can imagine the interpolation procedure: 
First the computer searches in each vertical column 
(resp. horizontal line) of the frame grabber for 
the pixel with maximum intensity, assuming it to be 
an approximate position of the profile. The inter- 
polation function (for example a Gaussian distribu- 
tion curve) is fitted through the intensity distri- 
bution of the highest pixel and its neighbourhood. 
The calculated center position of the fitted curve 
is interpreted as subpixel position of the profile 
in this column (resp. line) of the computer image. 
Hence evalution of one picture of the CCD TV-camera 
delivers 3-D coordinates of typically 500 spots on 
the workpiece surface along the scanner path. Each 
of the legs of the workpiece is represented by a 
straight line of measuring spots and the bending 
angle is complementary to the angle between these 
two lines. Linear regression analysis allows calcu- 
lation of this angle with very high precision and 
simultaneously statistical predictions about the 
performance of the measuring system. 
SYSTEM DESIGN 
The investigations of system design fundamentals 
for integration in commercial bending machines are 
undertaken with an off-line measuring system to 
provide necessary flexibility. Fig. 3 schematically 
shows the experimental setup for this off-line mea- 
suring system. 
  
  
  
  
  
  
  
  
  
  
  
  
Fig. 3: Experimental Setup 
All components are fixed to a three dimensional 
'optical bench'. The optical bench consists of a 
plane table for the positioning of the workpiece 
and a height-adjustable cross-head onto which the 
components of the measuring system are fixed. Spe- 
410 
cial mechanical adapters allow simple but reprodu- 
cible shifting of laser, scanner and camera along a 
common optical axis. Furthermore, the adapter of 
the camera allows turning the camera around the 
principal point of its objective. The host computer 
(PC-AT) controls and synchronizes all components of 
the measuring system, evaluates 3-D coordinates of 
the 'profile' and calculates the bending angle from 
this information. First some system design consi- 
derations about the choice of the main components 
of the system are discussed. Synchronization of the 
components and evaluation of 3-D data will be dis- 
cussed in consecutive chapters. 
Illumination of the object 
The illumination system consists of a laser diode 
(à = 789 nm, P, = 30 mW) which is focused on the 
workpiece surface by a collimator, consisting of a 
lens doublet (f = 90 mm, n.A. = 0,17), which is 
corrected for aperture aberrations. This collimator 
was primary designed for homogenous illumination of 
a circular field of 30 mm in diameter. The great 
astigmatism of laser diodes is diminished by inter- 
nal diaphragms, which cause loss of effective out- 
put power. Here the collimator is adjusted to a fo- 
cus distance of 1100 mm (instead of a parallel be- 
am) to avoid an additional focusing lens. This ad- 
justment increases distance from laser diode to 
collimator lens and causes additional internal ab- 
sorption. Hence the effective output power of the 
collimated laser beam decreases to about 9 mW. 
The generation of a light plane or 'light knife' 
from the focused laser beam is done by a two-axis 
galvanometric scanner. This scanner allows beam de- 
flections up to £20? (optical) with frequencies up 
to 80 Hz. Each dimension of the scanner field is 
divided into 65535 possible scanner addresses (16 
bit). Scanner electronics are programmed via a par- 
allel (Centronics) interface and include its own 
microprocessor to calculate flat-field corrected 
'micro-steps' of the scanner motion. Scanner elec- 
tronics independently switch laser power on and off 
via a TTL interface to the laser power supply. 
Detection of the profile 
The detection of the profile is done with a CCD TV 
camera. Reasons for the choice of this type of de- 
tector are the well-known advantages of CCD sensors 
like, for example, precise and stable sensor geome- 
try [5], high resolution and high light sensiti- 
vity. The aim of integrating the measuring system 
in industrial bending machines furthermore emphasi- 
zes the features of light weight, resistance to me- 
chanical shocks or magnetic fields and low costs. 
These considerations also suggest the use of a ca- 
mera with separate sensor head (size 44 x 31 x 25 
mm?, weight 80 g). To eliminate ambient light in an 
industrial environment a band-pass interference 
filter is built into the camera. 
SYNCHRONIZATION OF COMPONENTS 
The laser spot is moved over the workpiece surface 
during the comparatively short integration period 
of the detector. Hence a line of light is detected, 
the so-called 'profile', which, mathematically, is 
the intersection of the workpiece surface with the 
"light knife'. For perfect exposure one scan of the 
laser beam must be detected by the CCD sensor in 
one frame. 
Therefore illumination system (i.e. laser and scan- 
ner), CCD camera and image acquisition system have 
to be synchronized. The frame grabber automatically 
synchronizes to the camera's video cycle and the 
host computer's program execution is synchronized 
to the frame grabber. Hence the camera's video cy- 
cle is essential for synchronization of the whole 
measuring system. But integration timing of CCD 
cameras differs significantly with the sensor con- 
cept and a basic understanding of CCD sensor timing 
is necessary. 
Synchronization of frame-transfer CCD cameras 
Most wide-spread sensor concepts are frame-trans- 
fer-concept (FT) and interline-transfer-concept 
(IL). FT sensors have two sections of equal size, 
an imaging area and a read-out area. In the imaging