Influence of the natural aluminium oxide layer on ... - ALU-WEB.DE

MEASURING & CONTROL
Fig. 3: Sensor response as a function <strong>of</strong> <strong>aluminium</strong> thickness for different photon energies
for some producers so <strong>the</strong>y must resort to <strong>the</strong>
‘stop-and-measure’ technique. A single correction
factor is calculated by comparing <strong>the</strong>
uncorrected gauge measurement to a physical
contact measurement made while <strong>the</strong> strip is
stopped. While simple, it limits <strong>the</strong> accuracy
<strong>of</strong> <strong>the</strong> gauge to <strong>the</strong> accuracy <strong>of</strong> <strong>the</strong> contact
measurements which not only slow production
down, but are known to be operator dependent.
The challenges <strong>of</strong> alloy compensation are
fur<strong>the</strong>r complicated when <strong>the</strong> <strong>aluminium</strong> producer
rolls clad products. The alloy <strong>layer</strong>s have
different equivalent absorptions which by
means <strong>of</strong> proprietary s<strong>of</strong>tware can be converted
to a single correction factor for that product.
Without an alloy/clad compensation algorithm,
<strong>the</strong> compensation approach based on sample
sets for each alloy becomes a logistical headache
for storage and quality assurance checks.
International standards
International organizations like <strong>the</strong> Aluminum
Association, ASTM, Japanese Standards
Association and o<strong>the</strong>rs provide guidance on
not only alloy chemistry tolerances, but sheet
dimensional tolerances as well. For instrument
suppliers, <strong>the</strong> International Electrotechnical
Commission (IEC) has produced standards to
provide guidance and definition for specific
terms and tests used. The standards act as a
consistent scale to compare one instrument to
ano<strong>the</strong>r without confusing nomenclature obscuring
<strong>the</strong> true performance <strong>of</strong> each.
The Nuclear Instrumentation Technical
Committee (IEC Technical Committee 45)
produced IEC 61336 ‘Thickness measurement
systems utilizing ionizing radiation – Definitions
and test methods’. The first committee
release was in 1983, and an update was
drafted in 1996. The document is available
for purchase from <strong>the</strong> IEC at http://webstore.
iec.ch/webstore/webstore.nsf/ArtNum_PK/
21703?OpenDocument, as such we cannot
reproduce it here. The standard first defines
common terms in order to clarify what specific
words mean. For example <strong>the</strong> ‘mean response
time’ may be called <strong>the</strong> ‘first time constant’ by
some, and something else by o<strong>the</strong>rs. (Fig. 4)
The fixed definitions avoid any ambiguity in
interpretation. They additionally provide guidance
in setting up and carrying out <strong>the</strong> tests
defined in <strong>the</strong> second part <strong>of</strong> <strong>the</strong> standard.
The standard dedicates no less than nine
terms to clarify time based parameters. Some
are related to defining <strong>the</strong> time associated with
<strong>the</strong> time taken to respond to a change, while
o<strong>the</strong>rs are related to <strong>the</strong> digital processing <strong>of</strong>
signals. This is particularly critical with <strong>the</strong>
advent <strong>of</strong> high speed data processors. Incoming
data can be manipulated and processed by
advanced filters to mask or hide true statistical
variations. As depicted in Fig. 2 above, radiation
measurements are statistical by <strong>the</strong>ir very
nature. All noise figures should be quoted with
a reference to <strong>the</strong> number <strong>of</strong> Sigmas, or confidence
levels (CL). Most gauge manufactures
present 2 sigma (95% CL) noise figures, but
not all. Straightforward data processing provides
for predictable results and a better representation
<strong>of</strong> <strong>the</strong> process dynamics. If a change
occurs in <strong>the</strong> process, advanced filtering may
portray a portion <strong>of</strong> <strong>the</strong> change, but not <strong>the</strong>
full change. Process engineers and <strong>the</strong>ir AGC
algorithms may over react, or under correct
thanks to <strong>the</strong> manipulated
data.
In order
to ethically
improve <strong>the</strong>
speed <strong>of</strong> <strong>the</strong> sensor response
to change, without
increasing <strong>the</strong> statistical
noise on <strong>the</strong> measurement,
<strong>the</strong> physical characteristics
<strong>of</strong> <strong>the</strong> sensor
need to be optimized for
<strong>the</strong> application. The raw
signal; must be shielded to remove as much
electrical noise as possible. The ideal approach
for this is represented in <strong>the</strong> Thermo Scientific
RM 210 AS X-ray thickness gauge. In this versatile
instrument, <strong>the</strong> radiation detector output
is digitized right away. The analog detector signal
is converted to a digital number within a
few millimeters <strong>of</strong> its origin. This practically
eliminates <strong>the</strong> possibility <strong>of</strong> electrical noise
impinging on <strong>the</strong> signal. In comparison tests,
following <strong>the</strong> IEC 61336 guide, <strong>the</strong> noise on
<strong>the</strong> new designed detector improved by a factor
<strong>of</strong> 30%. Additionally, users <strong>of</strong> this system
can benefit fur<strong>the</strong>r by taking advantage <strong>of</strong> <strong>the</strong>
detector’s ability to operate at a 1 ms mean
response time. At this speed, and with <strong>the</strong> reduced
noise, process engineers have <strong>the</strong> tools
to analyze data at high speeds, revealing mill
chatter and o<strong>the</strong>r higher frequency anomalies.
In a typical rolling mill that produces can
stock <strong>aluminium</strong> at 250 um, <strong>the</strong> noise at a 10
ms mean response time might be on <strong>the</strong> order
<strong>of</strong> ±0.20% (2 sigma). With <strong>the</strong> improved
signal processing <strong>of</strong> this system <strong>the</strong> noise will
drop to ±0.15% (2-sigma). For a mill that produces
200,000 tonnes a year, that translates
to a savings <strong>of</strong> almost $200,000 in <strong>aluminium</strong>
material alone (using <strong>aluminium</strong> price at $1
per pound).
An additional benefit to digitizing <strong>the</strong> signal
so early in its journey to <strong>the</strong> AGC system,
is speed. Once digitized, <strong>the</strong> data can be processed
with out <strong>the</strong> time consuming ADC/DAC
conversions. The reduction <strong>of</strong> a few milliseconds
<strong>of</strong> process delay time can assure <strong>the</strong> AGC
has time to fully correct any strip thickness
deviations. This can result in higher quality
product.
Data archiving
A final benefit is realized in <strong>the</strong> powerful tool
<strong>of</strong> data archiving. This ideal system is available
with a s<strong>of</strong>tware feature that stores any
gauge data stream in <strong>the</strong> iba ‘.dat’ format. This
format is gaining popularity as <strong>the</strong> iba PDA
data analysis tool also grows in popularity.
Fig. 4: Graphical representation <strong>of</strong> gauge response
to an instantaneous thickness change
24 ALUMINIUM · EAC CONGRESS 2011