3. OPTICAL ROBOT CALIBRATION SYSTEM 
3.1 The Task 
Nowadays robots are programmed off line. Off line 
programming means a teaching of robots using a 
computer simulation program. Due to technical 
limits in the mechanical moving of a robot arm, 
there is a difference between the programmed 
positions of a robot arm and the real position. 
Due to this difference a correction of the robot 
movement has to be considered. This difference 
can be measured with the robot calibration system 
and correction polynoms for the different ankles 
can be derived. Using this correction for the 
simulation, the accuracy of the real robot 
movement can be increased (Peipe, 1989). 
3.2 Hardware Components 
3 or 4 CCD-cameras are mounted on the basic 
framework to observe the measuerment volume. The 
measurement volume can be flexible depending on 
the work space of the robot and the required 
accuracy. 
For the detection and measurement of the robots 
tool center point a specially designed star 
signalized with three retro reflecting targets is 
mounted to the tool center point. The position of 
the targets relative to the tools center point is 
calibrated. For calibration purposes a scale bar 
can be attached to the tool center point with two 
retro reflecting targets on both ends and a 
calibrated distance and position to the tools 
center point. 
  
Fig.3: Schematic system configuration 
The calibration system is designed as a subsystem 
of the robot control unit. The system is 
compatible to different robot control systems and 
can be implemented with some additional hardware 
for the image processing unit to the robot 
control systems. 
  
   
3.3 Calibration Process 
The calibration of a robot consists of two steps, 
the calibration and the actual measurement. 
In the first step transformation from the robot 
coordinate system to the world coordinate system 
is derived. Therefore the scale bar is attached 
to the tool center point of the robot and is 
moved in different positions covering the whole 
work space. In every position all cameras record 
an image of the scene and the image coordinates 
of the projected points will be measured. Every 
single image of one camera will finaly put 
together to only one image with all image points. 
The image coordinates are processed in a bundle 
adjustment with the image coordinates as 
observations and camera positions and world 
coordinates of the scale points as unknowns 
(Wester-Ebbinghaus, 1985). Using the robot 
coordinates of the scale points as approximate 
values in a free net adjustment the 
transformation of the robot coordinate system and 
the world coordinate system can be derived in the 
same process. In this case the datum of the world 
coordinate system is defined as the best fit 
model of the adjusted coordinates into the robot 
coordinate system. 
In the second step the actual calibration process 
is performed. The robot is now attached with the 
measuring star and moves special figures covering 
the whole work space. Every time the robot stops, 
the cameras capture an image and the image 
coordinates of the targets on the star are 
measured. The spatial coordinates of these points 
are simply calculated by a ray intersection in 
space. The coordinates of the three points 
deliver the coordiantes of the tool center point 
and the orientation of the tool center. 
In a third step these coordinates and the 
orientation of the tool center are compared with 
the robot coordinates and orientation in the 
robot coordinate system. The differences between 
these coordinates are used to derive the 
correction polynoms for the ankles. 
First experiments in the lab have shown that the 
achievable accuracy is about + 0.05 pixel in the 
image coordinates. The whole system is still in 
the prototype stage, but the first results show 
that a sufficient accuracy can be achieved. Major 
impact on the system have the vibrations of the 
robot. Because of the predection that the cameras 
are mounted stable on the frame and do not change 
their position relativly to the robot a loss in 
accuracy can occur, if these predection is not 
fulfilled. 
    
4. OP 
4.1 TI 
A cori 
as an 
to th 
corner 
precis 
Becaus 
the sa 
so th 
adapte 
measur 
spatia 
fittin 
measur 
manufa 
millin 
The ma 
handli 
proces 
work 
corner 
0.8 m 
mx 1. 
to ful 
4.2 Sy 
Due t 
record 
proces 
aircra 
camera 
synchr 
The pi 
networ! 
proces 
The su 
patter 
Every 
the ac 
circle 
inform 
about 
corner 
200 ta 
Additi 
scene 
coordii 
4.3 Me: 
The me 
the tai 
Then ti 
of th 
automat 
transpu 
process 
with t 
process 
monitor 
additic 
process