EoW January 2013

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News
Technology
January 2013 35
To compete in today’s markets, it is
necessary to reduce operating costs. But,
cutting costs cannot mean sacrificing
a high quality standard, and an in-line
measurement and control system
incorporated on an extrusion line can
reduce material usage.
In-line measurements are a requirement
for many extrusion lines. The objectives
of such measurement systems vary
from a simple quality control tool to a
completely integrated closed-loop control
system. There are different measurement
technologies that can be used in the wire
and cable industry, and the implementation
of such systems can result in significant
material and scrap savings.
These technologies include:
• Laser and infrared technologies suitable for
all applications
• Ultrasonics for jacketing lines (cable, fibre,
etc), insulating lines and multi-layer
applications
• Inductance for primary lines and insulating
lines
• X-ray for CV lines
Once the method of measurement is
established, the system’s processor can be
configured to perform closed-loop control.
Using the example of a line running 75m/
min and a measurement point location
of 2ft from the cross head. In this case, the
delay time is dramatically improved (2ft
divided by 75m/min equals 0.5 seconds).
Although the short-term variation has
been significantly improved, the issue of
shrinkage must still be considered. An
offset can be effectively used, but changes
due to such variables as material, cooling,
tension, etc, have to be considered. In most
applications, these variables are enough to
warrant a second loop to the controller. This
is called a dual loop system.
In a dual loop system, both the “hot
end” measurement point and “cold end”
measurement point are used. The system
takes the readings from the hot end and
constantly compares them to the readings
at the cold end to establish an accurate
offset. These values are shifted to ensure
“same spot”comparison.
This dual loop (DLP) control strategy
can be taken a step further for jacketing
applications. To realise the highest potential
savings, the desired controlled parameter is
the minimumwall. This minimumwall value
can be established using a combination of
ultrasonics and laser.
Zumbach’s “DIACAL” technology is used
to calibrate the ultrasonics based on the
cold values in the same way the dual loop
controller does.
With any controller, not only is establishing
the correct control strategy important, but
setting up the controller properly can make
all the difference. A controller can be set
up to be as aggressive as the process can
tolerate. The more the process is statistically
“in control”, the better a controller can
perform.
To account for unexpected events in a
process, a controller can utilise statistics
to tune it. Once accurate measurement
has been established, and the controller
strategy has been optimised, yet another
function can be utilised to improve material
savings.
Since a product’s key characteristic has
a tolerance and a nominal, the typical
controller would be set to tolerate the
nominal as the controller’s set point.
Most likely, these nominal and tolerance
values were established based on the line’s
performance with the lower tolerance in
mind. At one point, someone established
the set point with the logic that this is where
the operator needs to target to play it safe
to ensure the lower tolerance is not violated.
It is assumed these parameters were
established with a statistical study of some
sort. In the real world, these nominals are
rarely adjusted. The results are parameters
that are padded unnecessarily.
By enabling statistics to determine the
correct nominal, all factors can be taken
into consideration in real time. Zumbach
Electronic uses a CPK Pilot lot to perform
this function. By entering the desired CPK
level, the controller can determine the
capabilities of the line. Once this is known,
the controller will adjust the nominal value
as low as possible, maintaining the desired
CPK level.
Zumbach Electronic AG – Switzerland
Website:
Materials savings from Zumbach
▲ ▲ Reducing costs but keeping a high quality standard