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'üsseldorf. 
REAL-TIME PHOTOGRAMMETRIC ALGORITHMS FOR ROBOT CALIBRATION 
J. Hefele 
ifp, Institute for Photogrammetry, Geschwister-Scholl-Str. 24 70174 Stuttgart, Germany 
juergen.hefele@ifp.uni-stuttgart.de 
Commission V, WG V/1 
KEY WORDS: Industry, Robotics, Calibration, Measurement, Close Range 
ABSTRACT 
The aim of this project is to investigate a new photogrammetric approach to determine the pose of the robot endeffector in real-time 
for updating the robot model. Specifically, the two fundamental photogrammetric algorithms are investigated for that purpose: 
intersection and resection. In both cases, cameras are mounted on the moving robot observing targets fixed on the floor. In the first 
approach the camera pose (exterior orientation) with respect to the target co-ordinate system can be measured directly by using the 
collinearity equation. In the second approach, the stereo-camera measures the position of the observed targets with respect to the 
camera co-ordinate system. If the target co-ordinates are available the camera position with respect to the target co-ordinate system 
can be determined. With a standard algorithm for hand-eye calibration the misalignment in-between camera and end-effector is 
computed. Different set-ups with respect to ease of implementation, accuracy, and workspace size are compared. These setups were 
simulated and verified in several investigations. In our test environment we an industrial robot KUKA KR15/2 and digital CCD 
cameras with near infrared illumination were used. 
1. INTRODUCTION 
By ISO 8373 (International Standard Organization) industrial 
robots are defined as freely programmable appliances with a 
series of rigid components connected by joints. One end of the 
component chain is fixed while the other end (end-effector) can 
be moved by computer control. If there are for example six or 
more revolute joints, the industrial robot can reach every point 
of its working cell with every orientation. State-of-the-art 
industrial robots are able to move objects with weights up to 
500 kg and with a repeatability of 0.3 mm or better. In most 
cases the joints are powered by electric motors whereas very 
heavy robots are powered hydraulically. Because of the 
maximum power at a relatively high speed of the electro 
motors, the speed must be geared down. Forward kinematic 
describes the relation between the motion of each joint and the 
motion of the end-effector and thereby the position and 
orientation (pose) of the end-effector in arbitrary coordinate 
systems can be computed. Therefore robot model parameters 
such as length of segments, distance between two adjacent 
segments and rotation angle for revolute joint between two 
segments have to be known. The rotation angle can be 
measured exactly by a position encoder between motor and gear 
unit. Other parameters are defined by the design plan of the 
robot. But due to manufacturing and other environmental 
influence they do not operate accurately. 
The overall errors can be subdivided in geometric errors such as 
etolerance of the segment length, 
eangle error, 
and non-geometric such as 
e gear elasticity 
esegment elasticity 
etemperature influence. 
By a robot-calibration the influence of several errors can be 
eliminated (Whitney 1986, Heisel 1998, Wiest 2001), but there 
are still remaining time dependent errors such as temperature 
influence, tear and wear. These errors can only be reduced by 
the design of the robot in order to get it in accordance with the 
mathematical model. Still, the absolute accuracy of a robot is 
much lower composed to the repeatability and can be as large as 
several millimeters. The disadvantage of the approach of 
absolute calibration is obvious since replacement of robot 
components requires a complete recalibration. In addition, the 
main disadvantage is that off-line programming is not possible, 
as the required accuracy cannot be reached. To remove the 
disadvantages an external measurement system for the direct 
measurement of the robots pose is required. There are several 
photogrammetric and  non-photogrammetric measurement 
systems on the market (Wiest 2001). Also, industry acceptance 
of camera-based systems has increased in the last few years. 
The main disadvantage of these systems is that they cannot be 
used during production. Furthermore, the costs of these systems 
are very high, sometimes higher than the cost of a robot. On the 
other hand photogrammetry is certainly able to determine the 
robots pose very accurately by using industrial standard 
cameras at low cost (Maas 1997). In the ideal case the 
frequency of the direct measurement of the robot pose and the 
frequency of the robot control loop are the same. For modern 
industrial robots the frequency of the control loop is between 
200 and 1000 Hz. With inexpensive standard industrial camera 
this frequency is not possible, because of the high data rate. 
However, in our special case, only those parts of an image 
containing targets are necessary for measurement. In the near 
future CMOS-Cameras with direct access to the pixel will be 
available. With these cameras a measurement rate of 200 Hz 
and more is possible. 
Nevertheless the problem of shadowing effects remains. For 
example, a robot moving into a car body to fix a new element 
does not have a direct view to targets fixed outside the car body. 
In this case, the robot pose cannot be determined. Therefore, it 
is easier to use direct pose measurement for updating the robot 
model than to correct the control loop directly. 
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