By Robert S. Ampulski and Holger
Hollmark
The maximum web speed for serviette printing is currently
limited by printing technology. So to increase productivity,
startup, resetting and stoppage times must be reduced and
the spoilage ratio minimized. One way to do this is to make
full use of modular machines with single-axis drives, together
with complete motion control automation and system-integrated
drives.
Schnitt-Druck-Falz Spezialmaschinen (SDF) based in Monheim,
Germany, specializes in machines for tissue and non-woven
products, with special emphasis on varied-format serviette
printing and folding machines. The company regards itself
as market leader in Europe for high-quality machines in
applications such as serviettes and table sets. One reason
for its success was its early and unequivocal acceptance
of singleaxis drives: fewer equipment parts means greater
flexibility for the plant’s operators.
As well as supplying equipment, SDF frequently takes charge
of maintenance and retrofit jobs for machines from third-party
manufacturers. The company, founded in 1997 by Andreas Rother
and Peter Allard which started with two employees, has now
grown to employ 32 people, despite a deliberate intention
to keep the business small: the company limits its focus
to design, final assembly, commissioning and service (parts
production is assigned to reliable partners). Nowadays,
approximately 80 percent of all serviette-printing machines
built by SDF have eight- or nine-color printing stations,
whereas in the mid-1990s, four-color printing still dominated
even in the high-quality segment. In addition to the trend
towards the highest possible quality printing, SDF is seeing
a trend to produce ever-smaller lot sizes as product shelf
lives continue to shorten. At the same time, format ranges
are increasing, and there is a growing trend to increase
product value, even at the lower end of the market, through
the use of high-quality printing. Jobs of only 20,000 packages
are no longer unusual for many manufacturers. Whereas previously
the same job, or at least the same format, would be on the
run for days or even weeks, nowadays two or three resetting
actions are often required during a single workday.
For the first time, automation has made mechanical equipment
flexible. Because output is limited by printing technology
– depending on the paper quality, web speeds of between
300 m/min and 400 m/min are being achieved for high-quality
serviette printing – minimizing startup and resetting
times is becoming increasingly important for falling lot
sizes. Modular machines with single-axis drives provide
several advantages here.
Günther Kluge, electro-technical design manager at
SDF, had the idea of converting the automation of the serviette
printing machines to a Siemens system. Together with motion
control experts from the Ruhr marketing region and the application
center in Cologne, the group came up with an automation
concept which makes the production of high quality serviettes
simpler and more flexible. Special focus was placed on minimizing
unproductive downtimes.
Ten minutes won each day
The
latest SDF serviette machine with an eight-color print station
and variable-format folding is automated using a central
Simotion D controller and Sinamics S120 drives. Micromaster
MM440 converters are used for the actuating and auxiliary
drives. A Simotion CPU and Sinamics drives form one unit;
while the peripherals and Micromaster drives are all connected
to the CPU using PROFIBUS. This greatly reduces the number
of cables and plugs, and thus also fault probability. In
addition, commissioning and diagnosis are simplified because
of the clear cabling and the central calculation of the
position control for all axes means that it is possible
to trace several axes simultaneously. With the exception
of a few induction motors, all the machine’s motors
are equipped with electronic nameplates. The fetching of
the nameplates by the Sinamics drives permits the quick
semi-automated commissioning of the axis groups.
For operators of the serviette machine, stringent data storage
and communication have several advantages: the drives no
longer need to be referenced. This means that production
can be started every day approximately ten minutes earlier
than previously.
Some 29 servo drives ensure that mechanical settings can
be made quickly and are easily reproduced. The new automation
system stores all machine parameters in non-volatile memory,
through to the threshold value, for which an electronic
eye uses the print image to initiate the cutting equipment
– leaving no print marks on the tissue. This allows
all data and parameters required for processing a job to
be stored in recipes. If a re-run of a specific serviette
motif is needed, the operator only needs to insert the appropriate
printing rollers and fetch the "recipe" of the
original job – the machine is then ready for production.
In addition, the operator can create basic recipes for various
paper types and serviette formats giving the machine the
most important initial settings. This significantly reduces
the effort of setting up the serviette machine, even for
first jobs, for a serviette motif.
Even if a CPU, drive or motor is replaced, work can be continued
after just a short break. All machine parameters and user
programs are stored on a single CF (Compact Flash) card.
Consequently, a single action is all that’s needed
to transfer all data to the new hardware.
Less spoilage
Technologically
too, this new automation brings several improvements, as
CEO Andreas Rother explains: "The electronic eye can
reliably determine the exact cutting position only after
a certain minimum web speed is attained. This always results
in several spoiled packages during start up, for example,
after changing the paper roll. Improved synchronization
means that we can increase the web speed significantly,
with a lot significantly less spoilage than previously."
The individually-defined regulating quality for each drive
with Simotion also plays an important role. "Each axis
develops a certain degree of independence on such a high-performance
machine. For example, whereas smooth control is required
in the calendaring unit, a very hard synchronous operation
is required in the print towers to produce a clean print
image."
The new automation also significantly improves operating
performance. Because paper thickness for tissue papers often
varies greatly from roll to roll, and often even within
a roll, and the material also reacts very sensitively to
changes in air temperature and humidity, serviette printing
machines must be "driven" carefully. The operator
must monitor the print image, the web tension at various
points of the process, and the folding and cutting position,
and readjust these as necessary during the run. Experience
is still irreplaceable here, but the system can help by
displaying various differentiated and practice-related feedback
messages on an easy-to-understand 15-inch operator panel.
This simplifies the immediate interactive response for deviations
of paper quality.
A "clean" layout of serviette printing machines
is very important, especially for companies with limited
floor space and in this case, the particularly compact Sinamics
S120 double-axis modules are a good choice. All 29 axis
drives of the machine are only 950 mm wide when combined
and, together with the regulated power infeed, fit in a
single switchgear cabinet.
The active regulated power infeed, a Sinamics Active Line
Module, also ensures a DC link voltage that is largely unaffected
by phase effects, very constant and higher than non-regulated
power infeeds. This not only makes full use of the high
regulating quality of the Simotion/Sinamics combination,
but also increases the power of the motors that reach field
saturation later for higher DC link and motor voltage.
The Active Line Module returns any braking energy that cannot
be used to the DC link to the supply system. This eliminates
the need for braking resistors, which also saves space and
helps to reduce the thermal loading of small operating areas.
Although a quick response service goes without saying, it
can also mean stress for a relatively small staff with limited
time reserves. Consequently, remote maintenance is an important
factor. The Ethernet connections of the Simotion CPU not
only allow remote access to the controller, but also to
the Sinamics and Micromaster drives, including all attached
drive-related peripherals.
Face the future, gain perspectives
The flexibility gained from the new automation extends far
beyond the individual machine. For example, Simotion provides
a large degree of freedom for programmers and can also be
embedded seamlessly in other automation systems. In conjunction
with the very good hardware and software scalability of
the motion control system, the machine modules can also
be included more easily in third-party machines –
for example, other machine constructors frequently integrate
SDF calendaring units and folding stations in their production
lines. Because the system is also set up for integrating
state-of-theart technologies, Simotion-based automation
offers improved expandability. For example, the use of the
"PROFIBUS on Ethernet basis" – Profinet
– is now possible. The OPC and OPC XML functionality
available as system options make the vertical integration
of production machines for tissue and nonwovens very much
easier. A retrofitted Web server permits the ad-hoc diagnosis
using a standard browser should no laptop be available.
In a nutshell, according to the team at SDF: "The flexibility
of our machine is simply better supported by the Siemens
automation solution."
TW 
