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and hence provide information on the vibration
behaviour of the whole roll stand. The
manager of the rolling mill explains: „The
chocks have a natural frequency in the low
frequency range. Hence, when being excited,
they show a very slow vibration. When low
frequency vibrations become high frequency
vibrations, we can conclude that we are dealing
with external excitation.”
For being able to determine the external
vibrations, the staff needs to know exactly
the vibration frequencies of all elements in
the roll stand. The evaluation unit of the CM
system – the ibaPDA-V6 measurement value
acquisition system – acquires all sensors with a
sampling rate of up to 100 kHz time synchronously,
continuously and scaled to physical
units. ibaPDA-V6 is offered in several variants
regarding the number of signals. Licenses for
64 to 2,048 signals as well as for an unlimited
number of signals are available. For the
normal measurement data acquisition, there
are time bases of 1 to 1,000 ms available. For
higher speeds of signal changes, also shorter
acquisition rates of up to 10 μs can be realised.
In case a machine component is damaged, it
causes vibrations of a certain frequency band.
The additional vibrations that are acquired
by the evaluation unit, can be clearly assigned
– on the basis of these values – to certain
components. If e.g. a ball bearing is mechanically
defective, it causes additional high frequency
vibrations of 1,200 Hz. Within this
frequency band, the vibrations imply defective
bearings.
Online monitoring of
complex vibration behaviours
The machine vibrations can be assigned successfully,
if the Condition Monitoring data
and the process data can be related to each
other in real time. Due to ibalnSpectra it is
possible at the Novelis Nachterstedt plant to
establish online correlations between data
like speed, rolling force, torque, temperature,
rolling oil pressure on the one hand and vibration
data on the other. For being able to
exert a positive influence on the vibrations
and thus minimising them, Novelis can – using
ibalnSpectra – also adjust the influencing
variables on the vibration behaviour online
during the process. For that purpose, the
measurement data are visualised by means of
ibaQPanel in the control pulpit in real time.
Moreover, alarm messages can be generated
which either trigger an automatic adaptation
of the rolling speed or initiate a manual adjustment.
Before the implementation of the
chatter monitoring system, no exact conclusions
could be drawn concerning the causes
for the vibration conditions of the cold rolling
mill.
The analysis options of ibalnSpectra range
from simple parameter calculations from time
signals like minimum, maximum or RMS values
to complex evaluations in the frequency
range like amplitude spectra, envelope curve
spectra or Cepstrum analysis. Frequency spectra
can be calculated using the Fast Fourier
Transformation (FFT). Also statistical parameters
can be calculated. Using drag & drop,
the parameterised analysis object can be used
in the visualisation. Significant values of all
machine-, process-, material- and quality data
can be extracted to a database. Flexible and
efficient analysis tools allow for the early
detection of significant correlations between
these data. For the cross-plant analysis of the
measurement data it is an advantage that the
devices of the lnSpectra product lines can be
operated autonomously, but also that the data
can be transferred to a central evaluation unit
via diverse network interfaces.
Only by combining different measures in
an online based chatter monitoring system
that includes the process optimisation and
plant monitoring, the operators of a rolling
mill come as close as possible to their ideal
of reliable and fault-free products. Together,
iba AG and Novelis Nachterstedt have taken
a significant step forward in this matter.
Authors
Dr Ulrich Lettau is CEO of iba AG, which is located
in Fürth, Germany.
Dr David Skingley Wright is manager Technology
and Development, Cold Rolling at Novelis Inc.
Aluminium smelting industry
(USD782m) yuan. Chalco follows in Alcoa’s
and Rusal’s footsteps: both companies have
only recently announced production cuts.
Alcoa will close two Soderberg potlines at its
Baie-Comeau smelter in Québec representing
a production capacity of 105,000 tpy while
Rusal announced production cuts of around
300,000 tpy.
Trimet to acquire two
aluminium plants in France
© Dubal
Chalco cuts aluminium production
Aluminium Corp. of China (Chalco) has announced
it will temporarily close 380.000 tpy
of production capacity due to weak market
conditions. The curtailment corresponds to
9% of the company’s annual aluminium output.
Low aluminium prices and rising costs led
to a net annual loss of 8.2bn yuan (USD1.3bn)
in 2012 and an operating loss of 4.8bn
German aluminium producer Trimet has submitted
a binding offer to acquire two Rio Tinto
Alcan (RTA) plants in France: namely the aluminium
smelter in Saint-Jean-de-Maurienne
with a production capacity of 140,000 tpy
and the Castelsarrasin plant which produces
aluminium wire rod for electric cabling and
connecting elements for the automobile indus-
66 ALUMINIUM · 7-8/2013