Zero-Defect Strategy Optical Character Recognition
for Tracking & Tracing and
Quality Control
Schott Solar Thin Film GmbH
In the series production of thin-film solar power modules, code
reading systems with OCR (optical character recognition) are
employed for tracking & tracing the individual processing steps.
The continuous monitoring of all parameters ensures a reproducible
quality of the photovoltaic products.
In the fall of 2007, the Schott Solar Thin
Film GmbH began with the production of
thin-film solar power modules based on
amorphous silicon. Nowadays, an average
of 1000 thin-film modules exit the plant in
Jena (Germany) per day, which means a
production capacity of over 33 MW per
year. With an experienced staff of about
180 and state-of-the-art production facilities operating 24/7, the company is wellpositioned in the market for photovoltaic
products. A major success factor is its high
level of technological competence: The
thin-film technology is seen as having the
greatest potential, since the lower material
requirements during manufacturing –
compared to conventional silicon technology – offer enormous savings
opportunities.
SIMATIC Ident
Thin-film solar modules are formed by
applying a film of conductive and semiconductive materials only a few microns
thin onto a pane of glass. As first layer
(under the covering glass), a thin, electrically conductive layer is applied – the socalled TCO layer (transparent conductive
oxide). Through it, the light reaches a second layer consisting of a gaseous silicon
compound, which is roughly a hundred
times thinner than found in conventional
crystalline technology. A laser divides conductive and semi-conductive layers into
strips for the later connection in series;
these strips are considered individual solar
cells. The back electrodes, too, consist of
metallically conductive layers. The modules, after being encapsulated, are completed with connection boxes, cables and
connectors.
Zero-Defect Strategy
Avoidance of incorrect Entries and
Tracking & Tracing Errors
When the Schott Solar Thin Film GmbH
started with the production of thin-film
modules, the now usual marking with twodimensional data matrix codes (DMCs) –
which allows more information to be
stored on the same area – was not yet so
well known and spread. In addition, the
Schott production was not set up for the
product tracking & tracing with DMC. At
the time the facility was planned, the
tracking & tracing of the module manufacturing by means of temporary labels
with barcode identification was the best
available technology. The use of barcodes,
however, had its disadvantages: The response speed of the devices is relatively
slow and the reading distance between
scanner and barcode is short. Furthermore, the use of labels is relatively expensive, since they need to be thermally
resistant, and – after exiting the module
manufacturing – be disposed of.
A conveyor belt moves the glass pane over the
OCR code reading system, which ensures the
continuous acquisition of the serial numbers.
.
“In the raw module area, we are working
with a virtual tracking & tracing. Only in
the finished module area does the real
tracking begin,” explains Kay-Uwe Jahn,
project manager for service and maintenance at the Schott Solar Thin Film GmbH.
“Incorrect assignments of the two data
records proceeded through the entire
production line and were in some cases
only discovered at the end.” Manually
entered serial numbers were sometimes
not identical with those of the modules,
which led to a number mismatch and
considerable extra work for the error
correction. It happened that numbers
appeared twice and their assignment in
the manufacturing execution system
(MES) thus failed. Jahn continues: “Since
we wanted to rule out incorrect entries as
well as tracking & tracing errors in the
future, we decided to replace the barcode
solution with an OCR code reading system.
With it, we can interpret the plain text
serial numbers – applied directly onto the
module at the very beginning of the production process – and require no additionnal markings.”
Number Mismatch led to extra Work
For the Schott project team, the initial
challenge was to select code reading
systems for the optical character recognition that fit the production processes. The
project management called in Siemens
consultants, who in cooperation with experts from the iiM AG developed a custom
solution. The company – located in Meiningen about 140 km southwest of Jena –
specializes in solutions for optical measurement and test engineering and also
manufactures LED lighting systems for
industrial machine vision. “It was important for us to receive a complete and wellfunctioning solution from a single source,”
emphasizes the project manager.
Jahn further: “Since our experience with
SIMATIC had been very positive over the
years, a SIMATIC system was preferred in
this case as well.” Regarding controllers,
Siemens – with its SIMATIC S7 PLCs – has
been a key partner for solar module manufacturers for years. “We chose an OCR
solution that made possible production
control and batch tracking & tracing, and
at the same time, increased the quality
and data integrity.” Taking into consideration all options, the Schott Solar Thin Film
GmbH picked devices of the stationary
1D/2D code reading system SIMATIC
MV440, because it not only supports a fast
and uncomplicated use, but also is part of
Totally Integrated Automation (TIA) –
enabling a seamless and effortless integration into the existing automation solution
(see below).
By means of the SIMATIC License Manager,
any model of SIMATIC MV440 code reader
can be augmented by the functionality for
text recognition. The text recognition is
self-adapting over a wide range of character sizes and – after activation – provides
maximum reading reliability without additional teach-in. The task of the commissionning engineer is reduced to making
sure that the characters to be read are of
sufficient contrast and completely within
the image frame. Necessary presettings
can be carried out without special expertise in just a few minutes, e.g., the definition of the reading area within the
image.
Zero-Defect Strategy
The LED ring flash concentrates the light in order to
evenly and precisely illuminate the reflective glass
and metal surfaces so that the font is displayed rich
in contrast and read error-free.
The connection of the reader to the components of the automation solution is also
optimally supported according to the TIA
concept. For example, the transfer of the
product identification read to the SIMATIC
S7 PLC takes place by calling a function
block, which is included in the delivery
scope. The integration of the reader into
the visualization of the plant can be performed by calling the ready-made user
interface from the web server of the reader. Except for an HTML browser, such as
Internet Explorer with Java Runtime Environment, no other software is required.
The user interface visualizes the product
identification read; furthermore, statistical
data from preceding readings and their
associated image information are available. The latter can be used to assess the
stability of the marking process.
Less than one Misreading per Day
The serial numbers of the solar thin-film
modules are lasered onto the layers of metallic
back contacts. Despite reflections, a reading
accuracy of 99.9 percent is achieved.
“In the spring of 2010, we started testing
various fonts, optics and lights,” explains
Axel Müller, managing director of the iiM
AG, the procedure. For the system selection, the Schott project team provided
samples of solar modules with different
OCR codes to the iiM system house, which
then ran reading trials. “The tests showed
which fonts patterns were most suitable.
The goal was to achieve an accuracy of at
least 99.9 percent, which equals less than
one misreading per day,” adds Müller.
Initial attempts with code reading systems
capturing the fonts from the top, however,
only yielded blurry font outlines due to the
strong reflections from the metallic surfaces.
In the end, the code reading system was
installed to read the OCR marking from
below. The serial numbers – lasered onto
the layers of metallic back contacts – are
unsharp system-inherently, which make
them difficult to focus and read for the
reader. On top of that, the metallic surfaces are highly reflective when illumineted and the characters are low in contrast
– a challenge for any code reading system.
“We solved these issues with a special
arrangement of the optics and an optimized lighting,” stresses iiM managing
director Müller. To boost the built-in ligh-
ting of the SIMATIC MV440, the iiM AG
recommended the use of an additional
external lighting system to the Schott
project team.
The most effective and economical solution – according to the iiM managing
director – is a combination of suitable
optics and powerful lighting. A supplemental LED ring light was tested with
different operating distances and orientations. “The ring flash chosen concentrates
the light so that the reflective glass and
metal surfaces are evenly and precisely
illuminated, and the font is displayed rich
in contrast and read error-free,” describes
Müller the result. A small controller synchronizes the flash with the code reading
system and also allows an individual adjustment of the brightness. At the time of
the reading operation, the system emits an
impulse, whereupon the ring flash supplies its maximum light intensity within
three microseconds. “Influences by extraneous light – resulting from sunlight,
ceiling spotlights, reflective materials, etc.
– can thus be completely ruled out,” states
Müller.
Expectations fully met
A conveyor belt moves the glass pane over
the MV440 reading system. “As soon as
the writing passes into the field of view,
the reader reads the lasered serial number
within a fraction of a second,” explains
Carsten Oder, project engineer for service
and maintenance at the Schott Solar Thin
Film GmbH. In the past, the conveyors had
to stop for the barcode acquisition, since
the scanners could only read at a standstill.
Thanks to the short exposure times and
the enormous light intensity, the code
reading systems now read at the full operating speed. The new solution also supports the preventive maintenance of the
lasers. Project engineer Oder on this: “We
recognize early on whether a laser system
is aging. From experience, an indication
for this is a bad lettering quality, which in
turn leads to read errors.”
According to Carsten Oder, the MV440
systems are not “only the replacement” for
the previous barcode solution. They also
enable a hundred percent tracking &
tracing during the manufacturing so that
faulty modules can be stopped prior to
delivery. After being read, the data reaches
the MES via the SIMATIC PLC. The direct
PROFINET connection takes place via a
SIMATIC function block (FB79) – making
an additional interface module unnecessary and ensuring a reliable transfer of the
data read. “The measurement results together with timestamps and images are
stored in the MES database, and available
to reconstruct sequences in the case of
misreadings,” adds Oder. By March 2011,
the six code reading systems planned are
to be fully integrated into the production
control. After assuming live operation,
they will replace the barcode scanners
used so far.
Conclusion
“The new solution for the optical character
recognition meets our high quality standards. With iiM, we have a competent
partner on-site, who possesses extensive
know-how with regard to code reading
systems, optics and lighting. In tandem
with the project experience, this contributed greatly to the successful implementation,” summarizes the Schott project
manager. “During the various processing
steps, we can now immediately influence
the product quality through a series of
factors.” It is therefore important to capture data from each production process in
real-time. Possible error sources can then
be precisely determined and corrected as
soon as possible. “Should we at a later
time decide to employ data matrix codes
after all, the SIMATIC solution can continue to be used without modification. We
then only have to apply DMC instead of
the OCR markings,” concludes Jahn.
Siemens AG
Industry Sector
Sensors and Communication
P.O. Box 4848
90026 NUREMBERG
GERMANY
www.siemens.com/ident
As soon as the writing passes into the field of view of the code reading system, the SIMATIC MV440
reads the lasered serial number within a fraction of a second.
German version published in:
Automationspraxis 06/2011
© Siemens AG 2011
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