For obtaining extremely narrow thickness tolerances and good strip flatness, six-high roll mill is preferred because of the facility of positioning the intermediate rolls and also the facility for positive/negative bending of both the work and intermediate rolls. For automation of such a mill, the electrical and hydraulic screw down equipment has to be optimised.
Various controls necessary are:
(i) Strip thickness control
(ii) Mass flow control
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(iii) Flatness control with intermediate roll position as well as +/– bending
(iv) Automatic coil tracking.
In addition a computer model is needed for automatic presetting. Further the computer controlled management of the high bay is also essential. This would include facilities for acquisition of quality data, advanced communication system, and fast diagnostic functions.
The strip thickness at the exit is measured by X-ray equipment and is corrected by the feedback control, which adjusts the position of the rolls or the roll force accordingly. Because of the long dead time between the roll gap and the thickness gauge, this control loop is only able to compensate errors caused by elongated waves in the strip. Deviations from the nominal thickness on the entry—side are detected by the feed forward pre-controller, allowing the positioning set-point to be corrected via a shifting resister.
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The combined feed-back and feed-forward controls result in obtaining thicknesses with narrow tolerances for the main section of the strip. Small discrepancies are not obtained with this strategy but the discrepancies often double during the acceleration and deceleration phases.
Best tolerances can be achieved with control loops which measure the deviation in thickness practically without delay, i.e., in the roll gap itself. An additional mass flow controller is installed to meet this requirement.
It controls the exit thickness ha according to:
where ve = entry speed; va = exit speed and he = entry thickness.
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The control response is several times better than that of the feedback control, which is based on the signals from the thickness gauge at the exit. The discrepancy is reduced considerably, particularly during acceleration and deceleration, since there is no dead time involved in measuring the exit thickness.
Laser based measuring systems have proved very successful in the harsh rolling mill environment, and are capable of measuring entry and exit speeds with a resolution of 1 mm/s and an error of only 0.1 percent.
Not only the strip thickness tolerance but also the strip flatness is often decisive factor during production, failing which edge cracks can occur in the later industrial processing. The roll gap is set to correspond exactly to the gauge profile of the entering strip.
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To adjust the gap, the bending and inclination values are changed and the rolls systematically cooled by controlling the cooling on a zone-by-zone basis. Six- High stands have the added advantage that the intermediate rolls can be positioned to reduce edge drop.
To ensure a uniformly high quality for the wide range of rolled products the rolling conditions in the mill must be reproducible.
The supervisory computer performs this demanding task and has the following functions:
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(i) Material flow tracking
(ii) Modelling of rolling conditions
(iii) Order management
(iv) Data saving and reporting.
Material Flow Tracking keeps the Operator Well-Informed:
Sheet flow track is continuously tracked and data telegrams are sent to the supervisory computer to report the corrected data for each zone. A fully graphic mimic diagram, displayed on the monitor, informs the operator about the current mill status.
Where small batches have to be rolled, rolling schedule may involve very many changes in set-up. Computer-aided set-point generation, automatic coil transport, including coil supply and removal, are essential for reducing end scrap as well as for speeding up coil changing and increasing material throughput.
To solve the related problems and achieve a high, reproducible strip quality, a software has to be developed for modelling the rolling conditions. This model provides, among other things, the parameters for the roll gap, roll cooling, forward slip and rolling speed.
To improve and secure the quality of the information, the principle of redundancy is followed for the key parts, i.e., the results of a physical-theoretical model are compared with those of an empirical part featuring a knowledge database and statistical data compression.
Production planning is made easier and support is provided for the operators—especially in the communications area, where errors are avoided by using process computers to determine the sequence of passes (including the respective set-point data record) and to manage the high bay.
As soon as everything is ready for a new coil the computer requests, the next coil from the high bay computer and sends, in synchronism with the coil, the set-point data record to the group controller. After completion of rolling, this information and the corresponding data are signalled back to the computer and the coil is automatically transported either to the intermediate storage area to cool down or to the store for finished products.
The process computer calculates from the coil data, the time needed for cooling between passes and sends the information to the planning computer, which fixes the time for the next pass.
The management functions of the process computer cover the rolling mill from the entry to the exit. The high bay has its own computer.
The computers also perform important reporting functions. Reports act as verification of quality assurance for customers, while the statistically organized data provide a source for the recording of production experience in the form of a database.
Two types of reports provide key quality information:
(i) The shift report which contains the production data for every coil rolled in that shift,
(ii) The coil report.
The coil report provides the main production data and records of the measured strip thickness deviation, flatness tolerance and exit speed. To be able to compare the strip thickness and flatness tolerance for the individual passes better, the strips are divided into several sections. This allows comparison of the measurement curves for the separate passes, with identification of the causes of quality deviation during the previous passes.
The man-machine interface is designed to provide the following facilities:
(i) Display of process diagrams, trend curves and lists
(ii) User-friendly dialogue to help the operator during manual control
(iii) Display of disturbance message and events
(iv) Long-time storage of historical data for trend curves
(v) Indication of control system status
To ensure fast response to every operational situation, operator stations equipped with monitors are installed at the three key strategic points in the mill, namely the main control cabin, the switchgear room and the basement in which the hydraulic equipment is installed. In addition, there is an event recorder in the switchgear room. (Refer Fig. 44.2).
Access is via monitor pictures selected from hierarchically structured menus. Switching can be performed at any one of the operator stations. Interlocks prevent a function from being simultaneously accessed by more than one operator station at the same time.
The main process data are displayed to the operator on his monitor. User-friendly features are the ergonomic structuring of the data with the different screen areas and the use of different colours and sizes for the alphanumeric symbols.
For each step in the automatic sequences, there are different interlock and step enabling conditions that have to be fulfilled. To avoid the plant outages that are caused every now and then by the operators departing from the usual procedure or because they are unfamiliar with it due to a lack of working practice with the automatic systems, the user is shown the steps enabling the interlock conditions on sequenced screen displays on his monitor.
Any modifications to the control logic which involve the automatic steps are automatically integrated in the sequencing.
Events and alarms are displayed in chronological order with a resolution of the order of 1 ms. A wraparound event list is used, each new event being written over the oldest one of the list. Alarms are deleted from the alarm list just as soon as they have been acknowledged by the operator and on condition that the alarm criteria are no longer active.
The last unacknowledged alarm is shown on all the monitors, regardless of whichever process diagram has been selected. Colour coding of the alarm line allows the user to immediately identify the category, i.e., whether it is an alarm or disturbance.
In addition, the alarms are shown as red status information in displays with flashing picture elements which represent the equipment affected. These status displays flash until the alarm has been acknowledged, remaining as normal status displays until the conditions which led to the alarm no longer exist.
Signals and text used to describe the events and alarms are transmitted direct from the databases of the Master stations.
For every alarm and event, the user can define:
(i) Colour for the display
(ii) Text describing the type of event
(iii) Layout of the event and alarm test
The date and time an event occurs is shown in every case.