In 2005 South America Aluminium producer Companhia Brasileira de Aluminio (CBA), located in Aluminio, Sao Paulo, put in train plans to extend its aluminium foil production capacity. The seven foil lines already operating were to be joined by Foil Mill No 8 and, when that was producing successfully, Mill No 9 would be established using the same design and technologies. Today both mills are in operation and capable of producing foils down to a 5.5 µm thick – around one-twentieth of the thickness of a human hair – at production speeds of 2,000m/min, or equivalent to 120km/h. Rolling material Hair’s-Breadth Precision Tension Control for Ultra-Thin Aluminium Strip Jörg Knauth and Steffen Roskowetz, ABB Automation 1 like this places enormous demands on controls in terms of tension and speed, as well as precise fine-tuning of automation and drive systems. When Mill No 8 went into operation in April 2006 it was the fastest running AC-driven foil mill in the world. In July 2008 Mill No 9 began production. (See table 1). In reducing mills, position control of the hydraulic screw is employed to increase the roll force to reduce material thickness to 0.1mm (100 µm). For foil rolling, however, especially below < 40 µm, increasing the roll force would not effect any change in material thickness Figure 1: System configuration and technologies SUB HEADING Sub heading Maximum strip width 2,150mm Max/min strip input thickness 0.6/2 x 0.01mm Minimum strip output thickness 2 x 5.5µm (1µm = 1 x 10 -6 metres) Maximum coil diameter 2,000mm Min/max strip entry tensions 0.6/18kN Min/max strip exit tensions 0.4/12kN Maximum strip speed 2,000m/min Table 1: Foil mills 8 and 9 data ABB Mining&Metal:AL Edit+Articles Master 14/8/09 11:17 Page 1
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In 2005 South AmericaAluminium producer CompanhiaBrasileira de Aluminio (CBA),
located in Aluminio, Sao Paulo, put intrain plans to extend its aluminium foilproduction capacity. The seven foil linesalready operating were to be joined byFoil Mill No 8 and, when that wasproducing successfully, Mill No 9 wouldbe established using the same designand technologies.
Today both mills are in operation andcapable of producing foils down to a 5.5 µm thick – around one-twentieth ofthe thickness of a human hair – atproduction speeds of 2,000m/min, orequivalent to 120km/h. Rolling material
Hair’s-Breadth Precision Tension Control for Ultra-Thin Aluminium StripJörg Knauth and Steffen Roskowetz, ABB Automation
1
like this places enormous demands oncontrols in terms of tension and speed, aswell as precise fine-tuning of automationand drive systems. When Mill No 8 wentinto operation in April 2006 it was the
fastest running AC-driven foil mill in theworld. In July 2008 Mill No 9 beganproduction. (See table 1).
In reducing mills, position control of thehydraulic screw is employed to increasethe roll force to reduce material thicknessto 0.1mm (100 µm). For foil rolling,however, especially below < 40 µm,increasing the roll force would not effectany change in material thickness
Figure 1: System configuration and technologies
SUB HEADINGSub heading
Maximum strip width 2,150mmMax/min strip input thickness 0.6/2 x 0.01mmMinimum strip output thickness 2 x 5.5µm (1µm = 1 x 10-6 metres)Maximum coil diameter 2,000mmMin/max strip entry tensions 0.6/18kNMin/max strip exit tensions 0.4/12kNMaximum strip speed 2,000m/min
values by the integral automation system.The latter employs sophisticated softwareto fine tune process parameters likefriction and loss compensation.
State of the art low voltage AC drives
While in the past foil mill drive systemshave been based on DC technologybecause they lent themselves morereadily to accurate speed and tensioncontrol, more recently there has been amove towards AC, as used in CBA Mills 1to 7. AC technology has continued toevolve and ABB’s design for the new foilmills is based on state of the art lowvoltage AC drives and direct torquecontrol (DTC). This offers equal or bettercontrol performance than DC technology,while reducing the maintenance burden.(See Table 3.)
AC-drive main motors combineACS800 technology with IGBTs (insulatedgate bipolar transistors) in a multi-driveconfiguration and are all on the same DCbus. This saves not only power on theincoming unit but also requires a smallerpower feeding transformer. By usingIGBTs in the incoming supply it is possibleto run the system in a regenerative modeso that braking energy becomes availableto the network. In the case of a networkpower failure emergency resistor-brakingis possible to slow down the drives andavoid tons of material being wasted. (Seetable 3.)
The ACS800 technology solutionincludes the most modern direct torquecontrolled AC drive and uses actual
tension is not lost. The automatic-gauge control’s refer-
ence values are almost constant and thedrive has to react to all changes in theprocess with its own control algorithm.This is more demanding during the accel-eration phase. To achieve highly sensitivetension control, and thus consistent thick-ness, it is necessary to deal in micronsand in fractions of kN, not seen in reduc-tion mills. (See Table 2 comparison.)
The ratio of losses to minimum tensionsshows that precise tension control isessential. This must be complemented bya sensitive and fast-reacting drivesystem, as well as a precise computing of
because under high pressure the workrolls in relation to the thickness to berolled are almost “flat”. Even when rolling5.5 µm-thick material, it is necessary todouble it (2 x 5.5 µm) to achieve sufficientthickness and tension values. Usually rollforce is kept constant and by varyingrolling speed and strip tension parame-ters alone is it possible to achieveadditional reduction to the material(speed-tension-control).
In order to achieve high productionrates, as foil coils have long rolling times,it is essential to have a maximal rollingspeed applying the lowest tensionpossible. This is achieved by means ofspeed-tension optimisation.
Using automatic tension control
Depending on material and processparameters like rolling oil consistency andquantity, roll surface, etc, strip reductionoccurs at the moment rolling speedreaches around 100-200m/min. It is vitallyimportant at that point that speed andtension are controlled to ensure strip
so-called filling factor, the addition to thediameter of a number of layers (factor <1). A further fine tuning was necessary forthe coil diameter calculation; at a certainspeed diameter calculation is switchedover from layer counter to speed compar-ison. It is especially important to checkboth values for plausibility during acceler-ation. To ensure reliable diameter calcula-tion for the auto slow down function,additional diameter measurementperformed by a laser is taken intoaccount.
A chart recorder display showing a partof the acceleration phase from 20-1,200m/min with reduction of foil materialfrom 2 x 18 to 2 x 9 micron gives someidea of the system’s precision. Evenduring mill acceleration, as soon as thematerial begins to reduce, the coiler hasto speed up to maintain strip tension. Theratio of the losses plus torque dv/dtcompared to tension torque demand isonly 1:1.27, ie, almost same compensa-tion as tension torque has to be applied.
To maintain an efficiently optimisedsystem it is desirable for friction curves,inertias and parameters to be checkedperiodically so that process deviationsover time can be incorporated. The userfriendly interactive graphic windows in theprogram and application-related HSIscreens support the operator in the taskof maintaining a fine adjustment of theplant. ■
The next step was to calculate exactlythe compensation needed to counteractstatic mechanical losses, ie friction,caused by the entire drive train over thetotal speed range. (The is also affectedby the rolling oil being used.) Drive trainfriction losses and motor torque/speedcurve parameters reside in the programdatabase to enable computing time to beoptimised. Various motor configurationshave been considered – for example, aone or two motor arrangement, or wide ornarrow spools – as they result in differentloss measurements. Exact measurementof the total inertia of a mechanicalarrangement to determine additionaltorque during acceleration phases wasperformed prior to first strip being rolled.
Adjustments were carried out to estab-lish the correct value for the appropriate
motor values, based on a very fast motorsoftware model. It calculates new driveset-ups every 25 micro-seconds andreacts immediately to process demands.A high end type AC800PEC controller isused for the computing of the set pointvalues.
The real-time software runs theprogramming languages IEC61131.Software performs all the compensationand calculations necessary for theprecise control of tension and speed, andthis is communicated to the drive usingfibre-optics.
Prior to the measurement of losses allmotors are given an identification run toenable the software motor model to befine tuned. Optimisation of the driveparameters is achieved using the motorsfast reversing modes to determine thespecial amplifying factors that are vitalwhen starting the mill up and duringacceleration phases.
SUB HEADINGSub heading
Figure 3: Measurement of losses
Uncoiler 2 x 250 1,212 Clutch between motorsMill stand 2 x 1,000 1,318 Fixed couplingUpcoiler 2 x 250 1,212 Clutch between motors
