Computer Controlled Continuous Tandem Cold Rolling Mill

Computer Controlled Continuous Tandem Cold Rolling Mill

Copyright c' IFAC Automation in Mining , Mineral and Metal Processing, Helsinki, Fin la nd , 1983 COMPUTER CONTROLLED CONTINUOUS TANDEM COLD ROLLING ...

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Copyright c' IFAC Automation in Mining , Mineral and Metal Processing, Helsinki, Fin la nd , 1983

COMPUTER CONTROLLED CONTINUOUS TANDEM COLD ROLLING MILL M. Riess Berez'ch GrundstoJJindustrie, Prozef3technik und Entwicklung Siemens AG, Erlangen, Federal Republic of Germany

Abstract. The capacity of a cold-rolling tandem mill can be substantially increased using fully continuous operation. As a result, the idle times caused by threading the individual coils in and out and the off-size lengths disappear. Extended mechanical mill components, e.g. welding machine, coil looper, have been combined to form a novel overall concept. It is realized with the aid of highly dynamic drive systems, automatic sequence controls, two closed-loop control computers and an optimizing computer. A mathematical model is used to calculate the optimum setpoints for mill setting. The setpoint output to the mill is done depending on the weldseam position in the mill. Thus even large dimensional differences at the weldseam between the strips can be handled by the computer control in short transition ranges. Keywords. Rolling mills; Process control; Computer applications

DESIGN AND PLANNING OF THE PLANT

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to a strip accumulator (looper). The rolling mill proper - in this case a 5-stand mill - is supplied with material from the looper, so that rolling is possible even during welding. As a result time consuming operation phases for threading and tailingout, which also lead to a reduction in quality, are obviated. During passage of the welding seam through the stands the rolling program is automatically altered as a result of commands generated by the automatic control system. The strip is cut following the last stand by means of rotatory shears.

The productivity of tandem cold rolling mills has risen dramatically in recent years due to a number of developments. Refinement of the automatic control systems has played a mayor role in addition to improvements made in the design of mechanical and electrical equipment. As a result of employment of modern automation equipment it has been possible to remove limitations on production capacity due to inadequate human response.

Two coilers are installed in the delivery section. The head of the new strip is attached to the free coiler which has been prepared for take-up. The coiled strip is transported away and the vacated coiler is prepared for take-up of the next length of strip.

The result of these developments will be demonstrated taking automation of a continuous tandem cold rolling mill as an example. The schematic design of a continuous tandem cold rolling mill is shown by Fig. 1. Continuous strip rolling is achieved by welding together the strips to be rolled. This method involves employment of a special entry section comprising two decoilers and a welding machine. The strips which have been welded together are initially fed

The strip can also be directly fed to first stand of the rolling mill from a 3rd decoiler. In this case operation is identical to that of a conventional rolling mill.

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The rolling mill described has the following data: - 5 stands - Strip width maximum 1850 mm - Entry thickness 1.8 to 4.5 mm - Finished strip thickness 0.35 to 2mm - Exit speed maximum 1900 mpm - Take-up speed of the looper 780 npn - Effective looper capacity 800 m DESIGN OF THE AUTOMATIC CONTROL SYSTEM

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As a result of the short coil cycle time and of the thereby resultant large number of coils, which can be simultaneously in the vicinity of the adaptive rolling program employed manual operation of the rolling plant is excluded for all practical purposes.

Fig.1: Plant Schematic of a Continuous Tandem Cold Rolling Mill

Planning Objectives In order to make possible fully automatic operation the following design criteria were defined for the automatic control system: - Improved depiction of operating sequences by employment of suitable communication equipment and of logs - Optimum process control by exact simulation of the rolling mill process employing mathematical models - Automation of all rapid partial processes for quality assurance and improvement - Reduction of the work load on the operating staff with regard to manual control action. Implementation of these far reaching aims is facilitated by division of the tasks into automatic control levels. - Production control level - Process control level - Control interface level. The tasks to be performed by the production control level extend over the complete cold roll shop. The most important tasks are material tracking, output of optimized production plans to the individual process controll systems and assistance in preparation for work by means of a comprehensive information system. The aim of the process control system is to achieve optimum operation of the rolling mill both with regard to the control strategy and to technological requirements based on the given material data.

The computers of the control system perform superordinated technological control and output of the reference values calculated by the process control system to the process. The comprehensive interlocking and sequence controls are implemented in programmable controllers. In Fig. 2 the tasks assigned to the process control system and the control interface level are depicted in a functional sequence. Tasks of the Process Control System Data input. The data of the coils to be rolled are received from the automation system of the up-line pickling line via a data link. Comparison of the inputted data with the actually measured data is effected at a terminal in the vicinity of the decoilers. At a later date the input of the coil data together with the planned rolling sequence will be input by a production control system. In this case the direkt data link from the pickling-line will no longer be required. Material tracking. That is a central function to coordinate all the technological tasks by means of coil data aquisition within the complete automation zone between the decoilers and the weighing equipment in the exit section of the rolling mill. Pass schedule calculation based on mathematical modules for calculation of the reference values for preliminary setting and adaptive rolling program modification.

Computer Controlled Continuous Tand em Cold Rolling Mill

Data Link f rom Pi c kling- L.

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Data Link t o e . g . CGL, CAPL SIEMENS Process Co ntro l Sy stem Material

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Fig. 2 : Functional Diagram

of the Automation

Measured value acquisition and statistical securing of measured values as a basis for adaptive control of model equations and reference value checking. Communication system for information exchange between the operating personnel or technologists and the computer system employing video display units and logging devices. Monitoring of welds, that is checking of the welding procedure. Tasks of the Control Interface Level The control interface level is responsible for output of the reference values calculated by the process system to the process at the correct time. As a result of the large number of functions involved in a continuous rolling mill the tasks allotted to this automation level are divided among two process computers and numerous programmable controllers. These are connected to the process control computer by means of highspeed data links. The two computers perform the following functions: AM-V

Control of strip thickness, effective for stands 1 and 2 and stand 5 Position control of the screw-down including the functions necessary f o r operation of the stand including automatic calibration even under loaded stands. Measured value acquisition. In add ition to the measured value required for the control loops of the computer the values required for the process control system are also acquired and transmitted to the process control system v ia a data link. Drive control of the plant with generation of reference v alues, setting of the speed reference v alue and switch-ov er of analog controllers in the various operating phases of the rolling mill. Strip tracking, that is tracking of the head end and tail end or of a strip segment along the stands. Tracking of welding seams from the welding machine through the looper and the mill up to the coilers.

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Slow automatic for braking of the decoilers at the tail of the strip, at a weld, when the looper is empty or when the coiler is full. Monitoring of the looper including measuring of the current strip length in the looper and output of the permissible speeds to the entry and exit sections of the mill. Position control for positioning of the side guides, gap positioning for the levelling rolls and for transport of the coils to the decoiler including automatic measurement of coil width and coil diameter . Monitoring of strip thickness at the coil preparation section between decoiler and welding machine. Strip shape control in conjunction with tension me asurement in individual strip width zones acting on the roll cooling and on the roll deflection

To complete the picture the functions of the programmable controllers are given. The following mainly tasks are performed by the programmable controllers: - Control of cqil supply to the decoilers including coil preparation - Control of the mechanical equipment of the rolling mill including strip guide tables, tension measuring rollers, strip presses for threading and tailing-out - Control of automatic roll changing - Control of coil transport from the coiler to the weighing machine - Control of the hydraulic and emulsion system

teristics of the rolling process. In order to achieve quasi-continuous control action cycle times of down to 10 ms are required for certain functions whereby sometimes a number of control loops must be processed within the cycle. Main memory processors are employed in conjunction with the software package specifically designed to meet these requirements. The software package corrprise& in addition to the programs for the functions mentioned, programs for testing, operator control functions and for monitoring input and output data to the process in particular for monitoring high-speed binary and analog control functions. A comprehensive disturbance signal acquisition system informs the operating staff of faults and their causes for all areas of the extensive mechanical, hydraulic and electrical equipment.

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Hardware Design Organization of the automatic control system into a number of levels is ruso reflected in the hardware design as by Fig. 3 depicted. The process control system comprises a process computer with external memory units for process data and updating of the program system in addition to a process input/output interfac e and standard interfaces for control functions (data links, terminals, video display units, printers). Most of the programs are written in Fortran but parts employ assembler language where this is appropriate. Implementation of the tasks performed by the control interface level necessitates employment of a high-speed computer to match the dynamic charac-

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Fig. 3: Hardware Configuration

SPECIAL TASKS FOR THE CONTINUOUS ROLLING A number of the functions mentioned above are also required for automation of conventional rolling mills and are therefore generally known. Those functions, which are uniquely required for continuous rolling operation and which had to be newly developed, will now be described in more detail.

Computer Controlled Continuous Tandem Cold Rolling Mill

Material Tracking In order to automate control of a continuous rolling mill the automatic control system must at every instant know where a specific strip is l ocated within the rolling mill. The material tracking function performs central coordination of production scheduling. This comprises: Making available of material data for the relevant plant section Initiation of the pass schedule calcu lati on Allocation of the production data and measured values to the coils Output of information to the oper ating personnel on the position and condition of the materia l within the section of the production plant. Therefor the operating conditions on changeover from continuous rolling to conventional rolling and vice versa have to be taken into account in add~ tion to requirements affecting reduction or increase in the coils size . The data of the coils to be rolled are displayed in the desired rolling sequence by means of a terminal located near by the decoilers. Modification of the rolling sequence, e.g. necessitated by a fault in the rolling mil l, can be entered to the pro cess control system by means of a second interactive terminal. The coil data displayed must be acknowledged by the operator. At this pOint a pre liminary pass scheduling calculation is performed in order to determine whether the material whose data has been entered can be continuously rolled in the desired sequence. Further steps in the automatic pro duction sequence are released only when the result of this check is positive. Material tracking relates the data accumulated during the production sequence to the material. After completion of rolling the coil is auto matically wighed and marked with a coil number, a production log is output and the data of the processed coil are transmitted to the next processing line in accordance with the processing schedule. In this example a continuous annealing plant and the galvanizing plant are involved . Provision has been made for future transmission of the data to the production control system.

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Continuous rolling operation further permits processing of coils which have been optimized with regard to strip length, coil weight or coil diameter whereby various sheet thicknesses may be employed within a single coil. Tracking of Welding Seams All control signals required for material tracking are taken either directly from the plant or from a weldseam tracking unit which is of particular importance in a continuous rolling process for precise control of the adaptive rolling program. Weld seam tracking commences at the welding machine or at the decoilers in the case of welds produced in the pickling line. Tracking is effected employing strip length measurement ahead of and following the looper at the rolling stands. Tracking is synchronized by means of weld seam de tectors. Deviations measured at synchroniza tion are employed for correction or the length measurements. The number of welds in the looper, the distance between the nearest weld seam and the first rolling stand and the position of a weld seam in the rolling mill are displayed to the operating staff by means of numerical displays and video display units . If aftermlling the strip is not cut at the weld seam the position of the weld seam within the coil is registered and incorporated in the data set associated with the coil. Dynamic Rolling Program Change Primarily the strip thickness must be varied . It must further be taken into account that even where the thickness is identical, the material strength, the strip width and the thickness step sequence may vary between two strips. Resetting of the rolling mill is effected at a rolling speed of approx . 300 m/min referred to the outlet. The basic of all these requirements is a very exactly calculation of the necessary setpoint values. This is on ly possible by a pass schedule calculation based on adaptive mathematical models. As said, a precalculation of the setpoints is already effected when the operator accapts the coil data at the entry station . Final calculation of the reference values for the dynamic rolling program change is effec ted approx.50 m before the weld seam reaches the first rolling stand.This

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calculation is started automatically by the material tracking funktion.The following data are required for the calculation: - Input material data - Data of the desired final product - Technological rolling regulations, stored material specific in the computer - Data concerning the material strength, available in thickness classes and material types, calculated from the self-learn-system - Adaption factors for the mathematical model, available also in thickness classes and material types, calculated from the adaption function - Roll data - Data concerning the mechanical and electrical equipment of the mill.

tracking function to track the weld seam and the start of the wedge through all stands. Calculation is based on the condition that the length of the wedge is less than the distance between the stands, i.e. the maximum length of the wedge between the last two stands is approximately 4 m. The distance between the start of the wedge and the weld seam is calculated so that optimum conditions are obtained for the thickness step at the weld seam, i.e. the rolling forces neither exceed nor fall below the permissible values.

As a result of pass schedule calculation all reference values required for the rolling mill are obtained:

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Thickness references values for the individual stands Speeds for all stands including the maximum speed Screwdown position for threading Rolling forces Strip tension Material and stand parameters

and specific for continuous rolling - Parameters for dynamic rolling program change - Parameters for the strip looper - Parameters for the welding machine - Positions for the strip leveller and side guides in the exit section. All reference values are checked with respect to technological and plant specific limit values and are passed on to the subordinated control computer. What about the parameters for the dynamic rolling program change itself? As depicted in Fig. 4 resetting to the new reference values is not effected in a single step at the weld seam but transition from one strip thickness to another is effected in the form of a wedge. It is thus ensured that all the control loops involved in resetting can follow the resetting procedure. The task of pass schedule calculation is determination of the exact point for reference value resetting, i.e. the length of the wedge and the position of the weld seam within the wedge. This calculation is performed for all stands. As a result it is possible for the weld seam

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Fig.4: Strip Transition zone with Program Adaption Formation of the wedge is initially performed by a step-by-step changing of the thickness setpoint value for the load gap control of the thickness control loop of stand 1.This control loop is fully operative during the complete resetting procedure. Re-adjustment of the stands to the new reference values is effected in every case proportionally to the length of the wedge employing length measurement. As a result it is ensured that an acceleration or shutdown of the rolling mill during resetting, the reference values are always modified by the correct amount at the correct point. After generation of the thickness wedge in stand 1 modification of thickness in the following stands is effected solely by alteration of the speed relationship. This is,possible, since thickness control lS based on the principle of constant mass flow. V, *H , =V, l

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Comput er Controlled Continuous Tandem Cold Rolling Mill

This assumes that constant speed controls are employed in conjunction with tension control acting on the rolling force of the following stand and thus causing modification of the rolling gap of stands 2 - 5. In addition to the setting of stand 1 the rolling speed is the most important correcting variable for resetting. The principle of using the speed change for the dynamic rolling program change is depicted in Fig.5. The speeds V. of the individual stands depicted in the diagramm correspond to the relevant changes in thickness H. in accordance with the principle of constant mass flow. The rolling speed V. is thus the product of the relati v e1speed VR. and the reference speed v alue VL7 which is common to all stands (see Fig. 5, Eg. (1)). After formation of the wedge in stand 1 the relative speed value VRB1 obtained on calculation of the new pass schedule is employed for this stand. The reference speed value VLA for this stand is modified by a factor VLF1 to the reference speed value VLB, so that the actual strip speed V1 is maintained at this point in time (see Fig. 5, Eg. ( 3 )).

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On entry of the wedge into stand 2 the reference speed value VLB of the preceding stands is modified as a function of distance so that after the wedge has passed the stand 2 the speed relation between stand 1 and stand 2 is in accordance to the desired thickness reduction (see Fig.5, Eg. (4)) . Subsequently the new relative speed VRB2 is employed for stand 2 and the reference speed value is altered to the reference speed VLB of the preceding stands (see Fig.5, Eg. (5)). Finally at stand 5 the rolling speed of approximately 300 m/ min is obtained, however, in conjunction with new relative speeds VR1 through VR5. Rolling program change is now complete. It should further be mentioned that the reference values for tension control loops are also v aried as a function of the position of the resetting point (propo rtional to the wedge). The individual thickness control loops functi o n during resetting and are switched off momentarily only when the weld s e a m ent e ~ the control area of a thickness c o ntrol l o op. Switching of the thickness r e f e ren ce v alue is carri e d out

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Fig.5: Speed adjustment during adaption of rolling program

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after the wedge has left the area covered by the relevant measuring equipment. By means of the procedure described it is possible to cope with variations in thickness of up to 0.6 mm at the entry section to the rolling mill. Also it is possible to change the desired exit thickness from i.e. 1.0 mm up to 1.5 mm and vice versa independing of changes of the strip width and the material strength. It should be noted that the maximum permissible variation in thickness is dependent not only on the characteristics of the rolling mill but it is also influenced for example by the characteristics of the welding machine. It is worthy of note that so-called "stepped strip" can be rolled employing the procedure described. In this a strip is produced having a number of thickness steps from an input strip of constant thickness whereby the out-of-tolerance lengths are minimized. This is of particular importance for better utilization of conventional rolling trains. Model Adaption During rolling measured values are acquired for individual strip segments. Measured value acquisition is performed by the control computer approximately every 25 ms, since the measured values are required for the control alogarithm modules. Approximately every 200 ms the values are transmitted to the process control computer where they are sUbjected to a statistical measured value evaluation. Adaptive control is based on these measured values. The coefficients of the individual mathematical modules are adapted depending of thickness classes and material types. On the basis of newly calculated coefficients a pass schedule calculation is performed so that the operating parameters of the rolling mill are matched more closely to operating requirements and to ensure that all subordinated control loops can operate in a stable manner. The major parameters involved are those for thickness variation and to maintain the preselected rolling forces. The sequence / measured value evaluation / adapt ion / calculation / resetting / is performed cyclically approximately every 3 s. After a number of adaptive steps the material strength is calculated from the individual coefficients and the strain hardening for the decrease in

thickness is calculated. These values are stored materialwise and are available on future rolling of the identical material. As a result of these measures the process control system has a selflearning capability which allows it to slowly learn the rolling characreristics of individual materials and of the rolling mill. The values obtained are naturally available for use by the technologists. For this purpose a listing of the actual reference values, measured values and adaptive factors can be either printed out as a log or displayed on a video display unit. Weld Monitoring An important factor in operation of continuous rolling mill is the quality of the weld seam. Strip rupture, particularly in the looper, leads to considerable disruption of the rolling operation. For this reason measured values for welding current, welding pressure and material loss are acquired by the computer during automatic welding. Comparison with material specific values stored in the computer allows conclusions to be drawn regarding the quality of the weld. However the procedure is not without problems in view of the large number of material types employed and on account of measuring problems. RESULTS, FUTURE DEVELOPMENTS The marked increase of the production capaci ty of a continuous rolling plant as compared to a conventional rolling plant can be fully utilized economically, only when all rolling programs can be operated in an optimum manner. The automation concept described plays a major role in satisfying this requirement. The success of this concept may be attributed to employment of autoadaptive calculation models, which precisely describe the rolling process, for obtaining a wide range of rolling programms, and to the improved monitoring and understanding of rolling operations. The high reliability of modern computer technology employed for the scope of automation described has resulted in a considerable increase in the total availability of the production plant. A number of the functions specifically designed for continuous rolling operation, e.g. rolling of strips with thickness steps, will be employed in future conventional rolling plants. It may, however, be no-

Comp ut e r Controlled Continuous Ta ndem Co ld Ro llin g Mill

ted that solution of a number of general problemes associated with cold rolling, e. g. strip shape control and monitoring of surface quality, will have to be incorporated in the automation of such high-speed plants. In conclusion, it remains to be said that realization of the scope of automation described was made possible only as a result of close cooperation between all specialists associated with the project in the field of mechanical, electrical and automation engineering on the one hand, and with the purchaser on the other hand.

REFERENCES 1) Bald, W.: Stahl u. Eisen 102 (1982),No. 1,pp. 11/12. 2) Bald, W.: MPT, MetallurgicalPlant and TeChnOl~, 4 (1981), No.2,pp.4 13. 3) Aniol,H.: Siemens-Zeitschrift, October 1973 4) Fleischer,R.; Seyfried, H.: Siemens-Zeitschrift, October 1973

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