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

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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
�� The flexibility <strong>of</strong> <strong>the</strong> iba visualization tool is its real strength. As simple example <strong>of</strong> <strong>the</strong> flexibility is depicted in Fig. 5 showing a coil report with <strong>the</strong> thickness data presented as a function <strong>of</strong> length, with tolerances and coil statistics. Easy to use features allow for time based, and length based data analysis. Built in ma<strong>the</strong>matical tools such FFT can point process engineers to mill components that might need maintenance. In this situation, mill downtime can be best managed, and unplanned downtime dramatically reduced. The data archiving feature can also be con- figured to accept and record data from o<strong>the</strong>r sensors with in <strong>the</strong> mill. Any data point that is available to <strong>the</strong> mill computer through an E<strong>the</strong>rnet connection can be collected by <strong>the</strong> system to allow for comprehensive data analysis. Thus permitting <strong>the</strong> pairing <strong>of</strong> thickness measurement output to mill tensions and speeds in such a way that complies with <strong>the</strong> IEC 61336 testing standards. The IEC 61336 Annex B defines appropriate test points for data collection and analysis. While traditional analog outputs are typically used for gauge validation, it is equally acceptable to use <strong>the</strong> data transferred via E<strong>the</strong>rnet, or o<strong>the</strong>r means to a data archive. Summary Advances in online measurement <strong>of</strong> flat sheet have culminated in a state <strong>of</strong> <strong>the</strong> art X-ray based sensor system that provides high speed/low noise measurements permitting <strong>aluminium</strong> producers to realize material savings and quality improvements not previously achievable. This sen- MEASURING & CONTROL sor incorporates alloy and clad compensation based on <strong>the</strong> fundamental principles <strong>of</strong> radiation physics. It is housed in a robust frame designed for <strong>the</strong> rigors <strong>of</strong> <strong>the</strong> <strong>aluminium</strong> rolling mill. The complete package is manufactured and tested following IEC 61336 definitions which assure process engineers receive clear thickness data with out ambiguity. Thus allowing for mill optimization to achieve world class quality and strip uniformity at <strong>the</strong> highest rolling speeds. This results in maximizing <strong>the</strong> return for <strong>the</strong> <strong>aluminium</strong> rolling mill owners and investors. Acknowledgement The author thanks <strong>the</strong> International Electrotechnical Commission (IEC) for permission to reproduce Information from its International Standard IEC 61336 ed.1.0 (1996). All such extracts are copyright <strong>of</strong> IEC, Geneva, Switzerland. All rights reserved. Fur<strong>the</strong>r information on <strong>the</strong> IEC is available from www.iec.ch. IEC has no responsibility for <strong>the</strong> placement and context in which <strong>the</strong> extracts and contents are reproduced by <strong>the</strong> author, nor is IEC in any way responsible for <strong>the</strong> o<strong>the</strong>r content or accuracy <strong>the</strong>rein. Fig. 5: Typical coil report using iba data archiving tools � ��
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