International Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 5. Hakodate 1998 
AN AUTOMATIC SYSTEM FOR THE MEASUREMENT OF FLAT WORKPIECES 
Thomas Luhmann, Holger Broers 
Institute for Applied Photogrammetry and Geoinformatics 
University of Applied Sciences Oldenburg 
Ofener Str. 16, D-26121 Oldenburg 
E-mail: lunmann@fh-oldenburg.de 
GERMANY 
Commission V, Working Group V/2 
KEY WORDS: single-image photogrammetry, video imaging, quality assurance, automatic production control 
ABSTRACT 
This paper reports on the development of an automatic photogrammetric system which is used for the dimensional 
control of large plain workpieces in industrial environments. The system consists of one or more standard video 
cameras, a video frame grabber and a NT workstation computer. À diffuse light source is used to illuminate the object 
space. 
Plain objects may be measured by single image techniques such as projective rectification. In this case at least 4 
control points are given in object space and are used to determine the perspective transformation between cameras and 
object space. Due to the selected cameras significant variations of principle point coordinates and distortion values have 
to be considered. Using self-calibrating bundle adjustment the parameter of interior orientation are calculated iteratively 
in order to get distortion-free image for the perspective transformation. 
The measurement of the parts is performed by automatic line following using digital image processing. Approximate 
values of edge positions are calculated from existing CAD data of the object. 
The system has been implemented in a production environment for concrete ceilings. These parts have to be controlled 
with an accuracy of about 5-10mm. First results have shown that a system accuracy of less than 3mm is given under 
industrial conditions. The time for a complete measurement of object edges is less than 1 second. 
1 INTRODUCTION accuracy of about 1:2.000 yields to an image accuracy of 
about 3/10 of the pixel size. 
1.2 Specifications 
: : Due to the fast production rate the object surface can not 
This report presents a real-time photogrammetric system be marked by artificial targets. Thus, the natural edges of 
which is used for the geometric shape control of large flat the workpieces have to be located and measured 
workpieces. As a first application the measurement and automatically. Manual interaction is not permitted. After 
control of precast concrete ceilings was performed. The transformation of image measurement into equivalent 
concrete parts are manufactured almost automatically in ^ values in object space a quality check is performed. 
a computer-controled production environment. 
The objective was to develop a system with the following 
specifications: 
2.5m 
e High production rates (measurement time « 1sec) 
difficult environmental conditions (e.g. dust, vibrations, 
changing light conditions) 
processing of natural, non-targeted object structures 
high accuracy requirements for video images 
low-cost system 
interface to existing production control system 
  
Figure 1: Variety of workpiece shapes 
The spatial dimension of the concrete objects is up to 
approx. 10.0m x 2.5m. The required accuracy is specified 
to +5mm in object space. Using a one-camera setup and 
standard video resolution (750 x 580 pixel) the relative 
The workfloor control system delivers individual 2-D CAD 
data of the parts. Each precast concrete part may have 
another shape, i.e. outer dimensions as well as additional 
features such as holes, windows etc (Figure 1). On the 
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