IN-PROCESS MEASUREMENT OF BENDING ANGLES BASED ON THE PRINCIPLE OF LIGHT SECTIONING
Dipl.-Phys. Werner Heckel
Scientific Assistant
University of Erlangen-Nürnberg, Chair of Manufacturing Technology
Univ.-Prof. Dr.-Ing. M. Geiger, Egerlandstr. 11, D-8520 Erlangen, Tel. +49/9131/85-7140
Germany
ISPRS Commission V
ABSTRACT:
This paper presents an industrial application of optical 3-D sensing based on the principle of light sectio-
ning. The presented project aims at an in-process measurement of the bending angle in a commercial bending
machine. Air bending is a valuable manufacturing method in a flexible and computer integrated manufacturing
+
system, because it allows forming of sheet metals with a great variety of bending angles. But achieving high
working accuracies requires adaptive control strategies
gle caused by material tolerances.
to overcome deviations from the desired bending an-
The investigations comprise fundamentals of system design for integrating such a measuring system in any
commercial bending machine. System design considerations about the choice of the components and synchroniza-
tion of illumination and detection system are described. Furthermore, the different steps for achieving 3-D
data are sketched. First step is evaluating the position of the profile on the CCD sensor with subpixel
accuracy due to interpolation algorithms. Next step is the calculation of 3-D coordinates which correspond
to these sensor positions.
The bending angle can be evaluated with statistical deviations of about 1 angular minute. But measuring
absolute values of bending angles presupposes knowledge of the correspondence between camera and workpiece
coordinates. The basic equations to describe this correspondence and the application of photogrammetric
calibration methods for a simple model of the detection system are presented. First experimental results
prove the derived equations and show promising results even for the simple model of the detection system.
KEY WORDS:
racy, Camera Calibration
INTRODUCTION
Modern manufacturing systems have to deal with the
requirements of 'just-in-time' manufacturing, i.e.
flexible manufacturing of a great variety of parts
with very small lot sizes. Air bending allows for-
ming of sheet metals with a great variety of bending
angles without tool changes and is therefore a valu-
able manufacturing method for flexible and computer
integrated manufacturing systems.
Profile
| | "in process" Measurement
Machine Control System of Bending Angles |
Process Mode
Adaptive control system for air bending
Fig. 1:
In spite of high precision control systems for mo-
dern bending machines, material tolerances (for ex-
ample sheet thickness, yielding point, anisotropy,
etc.) cause deviations from the desired bending an-
gle. Assuming the material tolerances as specified
by sheet metal manufacturers, these deviations can
amount up to 1,5°. This demands for adaptive control
strategies to achieve a sufficient working accuracy.
A simple adaptive control system for air bending is
depicted schematically in Fig. 1. The fundamental
component of this adaptive control system is an in-
process measuring system for the bending angle. But
knowing the in-process bending angle still requires
modelling of the bending process, because the ela-
stic resiliency of the workpiece has to be compensa-
ted.
Thus the presented project aims at the development
of an measuring system for in-process measurement of
bending angles. The primary investigations should
evaluate fundamentals of system design for the inte-
gration of this in-process measuring system in any
commercial bending machine. The selected 3D-sensing
system bases on the well-known triangulation prin-
ciple of light sectioning. This non-contact, simple,
fast and precise optical 3-D measuring method meets
3-D, Image Processing, Mechine Vision, System Design, Interpolation Algorithms, Subpixel Accu-
the requirements for an in-process measuring of ben-
ding angles.
THE PRINCIPLE OF LIGHT SECTIONING
In [1] the principle of a modified light sectio-
ning system with large depth and high resolution was
reported. There an axicon (2),[3] was used to
overcome the depth of focus problem. Scanning the
diffraction pattern of an axicon produces a thin,
deep light sheet, which results from the incoherent
superposition of axicon patterns along the scanner
path. This so-called 'light knife' allows sharp il-
lumination of an object within a longitudinal range
of typ. 1500 mm. With a detector design regarding to
the Scheimpflug condition (see for example 1[4])
sharp imaging of the object all over the range of
the 'light knife' can be achieved. Thus a 3-D sensor
with high resolution and nearly unlimited depth of
focus could be constructed.
The reported 3-D sensing method for measuring ben-
ding angles is based mainly on the same principle,
as it is shown in Fig. 2.
Galvano Laser + 250 _— Interpolation
Scanner Collimator
_ 150
5 5
Camera = —
1 253 255. 257 512
Line Number ——
/ Intensity of this column
/
Light PI
/ rene Profile
/
= 512
Profile PC-AT 4 1 Columns 512
Frame Grabber Computer Image Heckel
Fig. 2: Principle of Light Sectioning
The bending angle usually refers to the original
state of a plane sheet metal. Hence the bending an-
gle is 180°-complementary to the angle between the
two legs of the bent workpiece. These legs have only
very small curvature, which reduces the demands for
lateral resolution of the measuring system. Further-
more, evaluation of a profile includes interpolation
algorithms to calculate the center of the intensity
distribution of the profile. Therefore slightly de-
focused profiles are acceptable and laser beam fo-
cusing with a spherical lens is sufficient. This
avoids some drawbacks of axicons, like for example
409
