Process Control
Fully Integrated Process Control
On-line quality inspection system for laser materials processing
Christoph Franz and Michael Ungers
Laser beam brazing and welding are
well established joining techniques in
the automotive industry; in particular for the body in white production.
These processes are mainly used to
join two-piece trunk lids as well as to
connect the roof and sidewall with a
visible seam or to weld doors in steel or
aluminum. Hence, the requirements
upon the optical appearance of the
brazed seams are very high and quality
monitoring is essential. In cooperation
of Scansonic MI GmbH and Fraunhofer ILT, a first prototype of a fully
integrated on-line process control system has been developed – SCeye.
The system is an add-on for laser welding and brazing processes in the automotive production of bodies in white.
It consists of an uncompared innovative and powerful illumination module
combined with a high-speed camera;
all fully integrated into the ALO3 brazing and welding head, as can be seen
in Fig. 1. The illumination and camera
module are aligned coaxially, which is
a key technology for integrated process
monitoring and which has been invented by Fraunhofer ILT many years
ago.
Without affecting the accessibility
of the brazing optics, the system is capable to acquire high resolution images
during mass production, which can be
used for process and quality control
purposes. It allows a full documentation
of the process behaviour and the product quality.
This article describes features of the
SCeye product, which has been released
at the EALA congress in January of 2015
and shows furthermore the newest scientific achievements in controlled laser
brazing and pore detection. The investigated control algorithm is based on
Fig. 1 Technology transfer – left: Scientific setup of ILT; right: SCeye product development
of Scansonic. Both setups consist of a high dynamic camera and an innovative illumination module.
velocity measurement and on direct
control of the feeding wire velocity in
real-time. The quality inspection algorithm is demonstrated as an on-line
pore detection solution.
The SCeye system
SCeye is an add-on for laser welding
and brazing processes and will first be
available for the Scansonic ALO3 head,
of which actually more than 1000 heads
are in use worldwide. While other inspection systems often require additional control cabinets, SCeye is fully
integrated into the Scansonic processing head.
The system consists of a CMOS imaging camera with high dynamic range,
an illumination module for bright laser
illumination on the work piece and an
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
advanced image and data processing
unit attached to the camera directly at
the processing head. The data processing unit processes the acquired images
in real-time, compresses the data, sends
the images via broadcast to the webbased interface (Fig. 2) and additionally
stores the raw data on its built-in storage.
SCeye for ALO3 assists the use of ALO3
heads in three scenarios:
Szenario 1: Teach-in process
In teach mode the process field is illuminated by eye safe LEDs. The camera
system broadcasts a live video to any
network connected client. While programming the robot trajectory, the user
may use the live feed to position the
processing head relatively to the work
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area homogeneously and ensure crisp
images even in bright processes. Whilst
still broadcasting compressed live video
to the network, SCeye records raw image data by fieldbus command. Each
video will be saved on the implemented
file system (“first in first out” continuously) while part numbers provided via
the fieldbus interface may be used to
assign videos to the actually produced
part. The system logs the signals of the
fieldbus interface and the additional
analogue values of the ALO3 head synchronized to the taken process video.
Fig. 2 Browser based user interface of SCeye.
Szenario 3: Inspection process
piece very accurately. At the same time,
the system provides continuously actual
values of the ALO3 system such as the
swivel axis position and the measured
forces applied to the feeding wire. Furthermore, the system logs all fieldbus
signals coming from the robot and
thereby facilitates the programming
process at the robot.
Szenario 2: Welding/Brazing process
During the welding or brazing process, the SCeye system may switch its
illumination module from LED illumination to laser VCSEL illumination.
Up to 40 W of optical power can then
be provided to illuminate the process
Company
Scansonic MI GmbH
Berlin, Germany
Scansonic develops and manufactures systems
for automated joining in modern production
processes. With new methods and products
Scansonic expands the limits of what is technically feasible. For its customers, Scansonic
opens up the advantages of laser and arc technology – creating higher efficiency, precision
and quality in production. In the field of seam
tracked brazing and welding Scansonic has
reliable and effective solutions. Scansonic also
offers the capabilities of a laser application center, where a vast variety of robots, laser sources
and different welding and brazing heads can
easily be tested and new applications are developed. Since its founding in 2000, Scansonic
has gained an established position with its sales
and service partners in the international automotive industry. Many parts of the body in white
of most OEMs are joined with equipment from
Scansonic.
While still producing and recording,
users are empowered to review the recorded videos and fieldbus data of past
processes. SCeye logs up to eight hours
of video and data. Thereby, an inspection of the process state can be performed after a faulty process has been
detected in a further processing step.
Users may also decide to copy and document all acquired data for example after each part or after each shift to their
own network attached storage or server
in their network.
Scientific setup for system development
As SCeye is not yet capable of process
control, its real time engine, combined
with a powerful FPGA, allows the application of advanced algorithms for further monitoring and control in the future as scientific research of ILT shows.
To develop, test and evaluate image
processing algorithms for monitoring
and control purposes, ILT has built a
scientific setup, see Fig. 1 left side. It uses
the same camera chip and the same illumination principle as realised in the
SCeye system. As illumination source
the VCSEL technology has been shown
to be ideally suitable to ensure homogeneous and directional independent
illuminated images [1]. The resulting
images are shown in Fig. 3. The entire
process zone is visualised; the incoming
brazing wire, the liquid melt pool and
the solidified seam are visible.
Unlike in the SCeye system, the raw
images are transferred to an industrial
PC unit via CameraLink standard in the
scientific setup. They are acquired and
processed with FPGA technology making the algorithms suitable for real-time
applications such as control purposes
and quality inspections during the manufacturing process. The full brazing
process is documented in an on-line
configuration; machine parameters are
monitored as well as the product quality
by applying dedicated image processing
algorithms to the captured images. In
the following two examples of such image analysis are demonstrated.
■ Monitoring of seam imperfections
like pores and documentation of the
product quality
■ Measurement of the actual movement of the handling system at the
tool-centre-point
Documentation of product quality
Laser brazed seams are frequently used
as stylistic elements in the body in
white production. After painting they
Fig. 4 Documentation of the product quality – pores are marked in a panorama image of
the entire brazed seam.
www.scansonic.de
Fig. 3 Visualization of the process zone.
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© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Process Control
part where seam is already solidified.
This opens up the possibility of saving
an image of each joint during mass production and will massively reduce the
amount of data that needs to be saved
for quality assurance purposes over the
long-term view.
Application for process control –
controlled laser brazing
Fig. 5 Controlled laser brazing – laser power PL as well as wire feed rate vW are controlled
based on the actual velocity measurement.
are direct visible to the end user, making pores in the surface of the seam an
unacceptable seam defect. Hence, it is
important to detect those seam defects.
As pores are mostly open to the surface
they remain as dark spots in the bright
illuminated seam. It seems to be obvious
to detect such dark spots via blob detection for example. But this approach is
non-satisfying as it depends on thresholds and the actual illumination situation. One approach that is more promising is to use a classification-based
detection of pores, as demonstrated in
[2]. Dedicated image features are used
to classify the solidified seam into defective or non-defective parts. Mean
value and standard deviation of the intensity and the gradient image are used
for classification. Trained values for
defective and non-defective parts are
compared on FPGA technology making it possible to judge the joint quality
in a real-time configuration. Pores in a
range of diameter from a few hundred
microns up to a few millimetres can be
detected with this approach. The results
are visualised in panorama images as
shown in Fig. 4; pores are marked in red
whereas non-defective parts of the seam
are marked in green.
This classification algorithm benefits
from the power-full and homogeneous
VCSEL illumination, developed for
the SCeye system. Moreover, the high
image quality makes it possible to put
together the entire seam out of small
image patches taken form the image
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Despite form quality inspections imaging based process control is also demonstrated successfully to measure machine
parameters. Beside joint tracking and
detection of the laser spot position relative to the joint the measurement of the
process, velocity is one main parameter
that can be measured on basis of the acquired images [3]. Due to the homogeneous and directional independent illumination image processing algorithms
can be applied to the acquired images
in order to track a specific pattern in
two sequenced images. A displacement
vector can be determined and in combination with the acquisition rate the
resulting velocity is calculated. In this
case a block-matching algorithm [4] is
used as it can be easily implemented on
FPGA technology. This approach has
not only real-time capabilities but also
suits as input single for velocity based
controlling strategies, e. g. to ensure a
constant energy per unit length.
In the case of laser brazing laser
power PL as well as the velocity of the
filler wire vW needs to be adjusted pro-
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portionally to the measured velocity [5].
The results of controlled laser brazing
are shown in Fig. 5. The system has been
tested under process conditions for
brazing a flanged joint configuration. In
the experiments the velocity was varied
in a range from 3 down to 0.72 m/min.
Despite this wide variation the controlled brazing process remains stable
and fluctuations in the velocity of the
guiding robot system are compensated.
Furthermore, a smooth and nearly homogeneous optical appearance of the
seam surface is achieved.
A relevant application one can think
of is the joining of the boot lid where the
laser head needs to be realigned during
the brazing process due to the geometry
of the work piece and the brazing velocity drops down to low values while the
optics is turned around the edge of the
numberplate’s salient.
Conclusion and Outlook
An on-line quality inspection as well as
closed loop controlled laser brazing has
been demonstrated successfully. Online inspection of the seam quality will
open up the possibility to disclaim post
process inspections and reduce effort
for quality assurance. Whereas controlled laser processes will enhance the
process stability and benefits the product quality especially when process windows are small. In particular teach-in
procedures will benefit from the measurement of machine parameters.
Future work will not only concentrate on further enhancement of algorithms for velocity measurement but
also think of new monitoring and controlling tasks, e. g. the detection of brazing wire tip or position of the laser spot
relative to the joint geometry. Since the
actual SCeye system is not yet capable
of automated process inspection, some
of the features shown with the scientific
setup of Fraunhofer ILT may find their
use in future production enhancements.
In the long-term view, the goal is to
integrate full process control, i. e. control and monitoring algorithms and
connect more data sources and provide
automated parameterisation, to reduce
the complexity in the use of welding and
brazing heads for their users. As algorithms are suitable for the implementa-
tion on FPGA technology, no additional
control cabinet or any external processing unit will be needed to monitor and
control the laser joining process in networks of sensors and actors.
DOI:10.1002/latj.201500013
[1] U. Thombansen, M. Ungers: Illumination
for Process Observation in Laser Material
Processing, Physics Procedia 56 (2014)
1286; DOI: 10.1016/j.phpro.2014.08.053
[2] M. Ungers et al.: FPGA-Programmed Detection of Seam Defects for the Application of Laser Brazing. In: Proceedings of
LAMP2013. The 6th International Congress on Laser Advanced Materials Processing, (2013) 23–26.07.2013. Toki Messe,
Niigata, Japan.
[3] S. Kaierle et al.: Understanding the Laser
Process, Laser Technik Journal 7 (2010) 49;
DOI: 10.1002/latj.201090027
[4] D. Liu, W. Sun: Block-Based Fast Motion
Estimation Algorithms in Video Compression, Department of Electrical Engineering
and Computer Engineering, Stevens Institute of Technology, Hoboken, NJ 07030,
(1998)
[5] M. Ungers et al.: Hardware based Analysis
and Process Control for Laser Brazing Applications, Physics Procedia 41 (2013) 517;
DOI: 10.1016/j.phpro.2013.03.111.
Authors
Christoph Franz
Institute
Fraunhofer-Institute for Laser Technology ILT
Aachen, Germany
With more than 250 employees and 10,000 m²
of usable floor space the Fraunhofer-Institute
for Laser Technology ILT is world-wide one
of the most important development and contract research institutes of its specific field.
The activities cover a wide range of areas
such as the development of new laser beam
sources and components, the use of modern
laser measurement and testing technology and
laser-supported manufacturing. This includes for
example laser cutting, caving, drilling, welding,
soldering and brazing as well as surface treatment, micro-processing and rapid prototyping.
Furthermore, the Fraunhofer Institute for Laser
Technology is engaged in laser plant technology
and process control as well as the entire system
technology.
26 studied mechanical
engineering in Aachen
and received his
Diploma in production
technologies at the
University of Applied
Sciences Aachen. In a
total of seven years of
experience in process
monitoring and control of laser processes at
Fraunhofer ILT, Aachen and at Coopération
Laser Franco-Allemande in Evry, France,
he worked as project manager for scientific
and industrial projects and solutions for process recording, monitoring and control of a
vast variety of laser processes. Since 2012
Christoph Franz works as product manager
at Scansonic MI GmbH for process monitoring products as well as for laser edge
welding heads..
Michael Ungers
studied physics at
RWTH-University
Aachen and at HeriotWatt University of
Edinburgh, Scotland,
and received the
diploma in physics
as well as the Master
of Physics MPhys.
Since six years he is engaged as scientific
staff in the Process Control and System
Technology Group at Fraunhofer ILT. As
project manager for scientific and industrial projects he is involved in research
and development of process observation
systems, process monitoring and process
control. During the last years he works
especially in the field of process monitoring, on-line quality control and controlled
laser processes for the application of laser
brazing.
www.ilt.fraunhofer.de
C. Franz, Scansonic, Rudolf-Baschant-Straße 2, 13086 Berlin, E-mail: christoph.franz@scansonic.de, M. Ung­
ers, Fraunhofer-Institut für Lasertechnik ILT, Steinbachstraße 15, 52074 Aachen, E-mail: michael.ungers@
ilt.fraunhofer.de
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