fuzzy multiple stand tension control of a roughing rolling mill

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					                 Fuzzy Multiple Stand Tension Control of a Roughing Rolling Mill

                                           F. Janabi-Sharifi           G. Li∗
                              Department of Mechanical, Aerospace & Industrial Engineering
                                               Ryerson University, Canada
                                         fsharifi@acs.ryerson.ca                    guominli@hotmail.com

                               Abstract                                           product or/and damage the machinery. Therefore, it
    In steel rolling industry, control of hot metal rolling                       is very necessary to control the interstand tension
process plays an important role in assuring high product                          to achieve a safe, stable and high quality rolling
quality and safe process operation. Although many                                 process [1].
advanced looper control technologies for finishing rolling                            In most finishing mills, a looper [4] is usually
process have emerged, looperless interstand tension                               formed intentionally between each two adjacent roll
control of roughing rolling mills remains a challenging                           stands to buffer the speed mismatch between
problem. This paper proposes a multistand fuzzy tension                           stands. The tension control is converted into
control system for a roughing rolling mill. Combined with                         maintaining a suitable looper height. With
a novel decoupling strategy, the proposed scheme makes                            successful looper control, the interstand tension is
it possible to realize intelligent tension-free control of                        then often negligible due to the formation of the
multiple roughing rolling stands. The results from a                              looper and the lightweight of the bent thin strip.
virtual rolling test demonstrated the applicability and                               Many advanced control techniques have recently
effectiveness of the proposed technique.                                          been developed for looper control, such as non-
    Keywords: Fuzzy Control, Hot Metal Rolling, Complex                           interference control (NIC) [5], Optimal looper control [6],
Systems, Process Control, Virtual Reality                                         H-inf control [5, 7], and intelligent looper control [8, 9].
                                                                                  However, looper scheme cannot be employed in roughing
                                                                                  and intermediate rolling mills. Looperless interstand
1. Introduction                                                                   tension control remains a challenging problem [2]. The
                                                                                  top two obstacles are the unavailability of suitable process
   Hot metal rolling mill is a key steel processing facility                      modeling and the strong interstand interactions between
in steel plants. A typical hot rolling mill usually consists                      adjacent rolling stands. Current practice is usually to rely
of a roughing rolling sub-mill, an intermediate rolling                           on a human operator who manually controls the tension
sub-mill and a finishing sub-mill, each of which contains                         occurring at each interstand zone by a watch-and-correct
various number of rolling stands. Different long products                         procedure. Due to high skills required and possible
with various cross-sectional profiles, such as strips or                          inconsistencies, high product quality cannot be achieved
bars, are produced on the principle of multistage shaping.                        in this way.
That is, the cross-section of a billet or slab is reduced step                        This paper reports a multistand fuzzy tension control
by step under high pressure while it is passing through                           system for a roughing rolling mill. Combined with a novel
each mill stand such that the final product would meet                            decoupling strategy [3], the proposed scheme makes it
dimensional and mechanical specifications.                                        possible to separate interstand interactions and realize
   If the strip entry speed at a roll stand differs                               intelligent tension-free control of multiple roughing
from its exit speed from the previous stand, a                                    rolling stands. To verify the developed control strategy, a
longitudinal force will result inside the rolled billet                           virtual reality based system test was conducted. Test
between these two stands. This force, generally                                   results are given to show the effectiveness and
called interstand tension, will cause a push or pull                              applicability of the proposed scheme.
action of the rolled steel, introducing variations in
gauge (thickness and/or width) of the product and
thus deteriorating the product quality. In an
extreme case, the excessive tension may break the

    Previous address: Department of Automation, Tianjin University, P. R. China
2. Rolling mill and virtual reality                            obtaining a suitable model for tension control system
                                                               design is usually very difficult.
   A typical hot rolling mill usually consists of a number        A rolling mill is usually continuously operated on 24-
of roll stands. All roll stands are arranged in alignment      hours basis. Any unscheduled or prolonged process-down
and they are often categorized into roughing, intermediate     could result in a significant loss of plant revenue.
and finishing stands. Fig. 1 shows a general composition       Furthermore, any inappropriate operation of the mill may
of a rolling mill containing 8 roll stands.                    cause serious personal injury and/or property damages.
   A roll stand is typically composed of a housing             These features of rolling mills prevent the development
structure, a pair of work rolls, a pair of back rolls, a       and utilization of new control technologies. Therefore,
gearbox, a motor, a motor drive and a closed loop              virtual reality is then a good solution to control system
automatic speed regulator (ASR) for the drive.                 development. A virtual mill [10] can be employed as a
   The roll drive system of a stand is able to fully control   testbed for both control system design and evaluation. It
the roll speed of the stand but unable to fully govern the     can also serve as a tool to train operators without
speed of the strip through that stand. The dynamical           requirement of exposure to the real plant, which avoids
relationship between the motor speed and the mass entry        possible harm to involved human beings and damage to
and exit speeds is highly nonlinear and dependent on the       mill equipment resulting from any inappropriate
complicated deformation process. It is for this reason that    operation.

                    Fig. 1    Overview of a typical hot metal rolling mill containing 8 stands

   Shown in Fig. 2 is a schematic of the virtual               simulator only through a different interface. MCS
rolling mill employed in this study. It is primarily           main functions therefore would not distinguish
composed of a virtual mill simulator (VMS), a mill             between real mill and the VMS.
master control system (MCS) and a human machine                   HMI is an interface between human operator and
interface (HMI) facility.                                      the process. The operator monitors and controls the
                                                               process through various functional screens
                                                               provided by HMI, e.g. manually correcting the
 Real Mill                   MB+                               stand speeds by observing each motor current.
                                          LABVIEW VIs
                                                                  VMS is to simulate the complex dynamics of a
                                                               real mill consisting of 5 roughing rolling stands, 3
                                               VMS             intermediate rolling stands and 8 finishing rolling
                                                               stands. Although highly ordered, nonlinear, coupled
                                                               and imprecise process models are not suitable for
             PLCs                                              control system design, they are very useful in
                                                               process simulation and numerical analysis.
         MCS                                OPC Server            The information exchange in the virtual rolling
                                                               mill is completed through a MB Plus network. The
                          MB+            Control and Display   MCS and HMI can be switched to serve the real
                                                               rolling mill through a soft switching between
                                               HMI             aforementioned two signal interfaces of MCS to the
                                                               real mill or the VMS. It is for this reason that the
   Fig. 2 Schematic of a virtual rolling mill                  virtual reality technology becomes increasingly
   MCS is the core of process            control system
including all logic and continuos        process control
functions. Mainly composed of two        Modicon PLCs,
it is connected to the real mill         or virtual mill
3. Multiple             interstand            fuzzy       tension                  Interstand tension affects the dynamics of both
control                                                                         upstream and downstream stands. This makes the
                                                                                multiple interstand tension control a strongly
   For 5 roughing rolling stands, there are 5                                   coupled and highly ordered problem. The more the
interstand zones appealing for tension control.                                 rolling stands, the more complex the control
Since the tension results equivalently from the                                 problem. Apparently, each interstand zone cannot
speed mismatch of the upstream and downstream                                   be individually controlled well without a suitable
stand driving motors, it is possible to control                                 decoupling strategy.
(reduce or eliminate) the interstand tension by                                    The rolling dynamics of each stand is highly
correcting the motor speed of either upstream or                                nonlinear and difficult to model due to the complex
downstream stand.                                                               metal deformation process. Intelligent fuzzy control
                                                                                is then a good candidate to deal with such a
                                                                                nonlinear and imprecise problem.

                            IC                                      IC                                                 IC

                            FTC                                   FTC                                              FTC

                        +                                         +                                                +
                                  +                                         +                                               +
                                           CC                                      CC                                              CC
                  +                                           +                                              +
                             +                                          +                                          +
                                        Scheduled RPM 1                           Scheduled RPM 2                               Scheduled RPM 5

Reference RPM 1                             Reference RPM 2                                Reference RPM 5

                      ASR                                     ASR                                            ASR


                  Stand 1                                     Stand 2                                        Stand 5

         (FTC: Fuzzy tension controller; CC: Cascade controller; IC: Interval controller; ASR: Automatic speed regulator)

                                      Fig. 3 Multiple interstand fuzzy tension control system

   Shown in Fig. 3 is a schematic of the proposed                               3.1. Cascade controller (CC) and interval
fuzzy multiple interstand tension control system.                               controller (IC)
To ensure a constant product speed, the upstream
speed correction scheme is used. For each stand,                                   The combination of cascade controller and
there are 3 controllers of different type. They are                             interval controller is the means to physically
cascade controller (CC), interval controller (IC)                               decouple the multiple-stand interactions caused by
and fuzzy tension controller (FTC). All these                                   the interstand tension.
controllers are implemented in the PLC based MCS.                                  The cascade controller is to apply proportionally
Instead, the automatic speed regulator (ASR) is                                 a correction to the speed drive of its upstream stand
built in the speed drive system of each stand. The                              while the speed of this stand is being corrected.
process control is through the correction of the                                The purpose is to eliminate the interaction between
reference speed applied to each ASR.                                            the regulated stand and its adjacent upstream stand
                                                                                by means of avoiding the introduction of an extra
change to the backward tension of the regulated                                      k e,i , k ∆e,i , k v ,i ------ Current error, change of error
   The interval controller is to periodically control                             and speed correction scaling factors
the switch-in and switch-out of the fuzzy tension                                    µ i, j , µ i∆e
                                                                                                 ,j   ------- Input membership mappings
controller so as to remove the interaction caused by
the forward tension between the stand being                                          w j --------- Output fuzzy singletons
regulated and its adjacent downstream stand.
                                                                                     Five triangular membership functions are selected for
3.2. Fuzzy tension controller                                                     the current difference (Error), three membership functions
                                                                                  for the change of difference (Change of Error) and seven
   Interstand tension can be either sensed through                                symmetrical fuzzy singletons are chosen for the output
installation of load cells or derived from other detected                         speed correction (Output). These membership functions
physical variables [11]. Therefore, tension control can be                        are defined as follows. Error: NB [-1, -1, -0.6, -0.3], NS [-
classified into direct and indirect schemes.                                      0.6, -0.3, 0], Z [-0.3, 0, 0.3], PS [0, 0.3, 0.6], PB [0.3, 0.6,
   Due to physical limitations in most rolling mills, the                         1, 1]; Change of Error: N [-1, -1, -0.5, 0], Z [-0.5, 0, 0.5],
interstand tension in the proposed system is indirectly                           P [0, 0.5, 1, 1]; Output: NB [-0.75], NM [-0.5], NS [-
inferred through detecting the motor armature current.                            0.25], Z [0], PS [0.25], PM [0.5], PB [0.75]. The rule base
That is, the indirect current comparison method [11] is                           contains fifteen rules as shown in Table 1. The scaling
employed for achieving tension control. The difference                            factors are selected as k e,i =0.4, k ∆e,i =1.5, and
of the armature current between before and after
the billet hits on the next downstream stand                                      k v ,i =0.1.
indicates the occurrence of a forward interstand                                    The control algorithms of Eq. (1) are digitally
tension. If the armature current is controlled to                                 implemented in Modicon PLC programs.
trace its value before the billet hits on the
downstream stand, the forward tension will be                                                         Table 1 Rule base
brought to zero.                                                                                                       Error
   Fuzzy tension controller takes both the                                              Output         NB      NS       Z       PS       PB
difference and the change in difference of the
                                                                                                  N    NB      NM       NS      PS       PB
armature current between before and after the billet                                 Change
hits on the downstream stand as inputs. It emulates                                    Of         Z    NB      NM       Z       PM       PB
human operator and bases on a Mamdani and                                             Error       P    NB      NS       PS      PM       PB
Sugeno-type fuzzy reasoning to generate a
correction signal for the motor speed of the                                      4. Virtual rolling test
upstream stand.
   Let ei stand for the current difference between the                                An initial mill setup prescribing the reference speed of
forward-tension-free current and the actual armature                              each roll stand is usually designed to establish a
                                                                                  theoretically “perfect” speed-matching condition based on
current of the ι-th stand, ∆ei the change in the current                          the Conservation of Mass principle and stand parameters
difference and           ∆vi the correction to the reference speed                for each production schedule. Such a mill setup is a
                                                                                  necessity for the process startup although it cannot realize
of the ASR of the ι-th stand. The controller output for
                                                                                  perfect speed matching in reality due to complicated
each stand can be expressed as
                                                                                  deformation dynamics and other external disturbances.
                                                                                      Interstand tension results from speed mismatch. To
        k v ,i ∑ w j ⋅ µ ie, j (k e ,i ⋅ ei ) ⋅ µ i∆e (k ∆e ,i ⋅ ∆ei )
                                                   ,j                             evaluate on the virtual rolling mill the multiple-stand
                 j                                                                fuzzy tension control system, a speed mismatch is
∆vi =
                 ∑µ  j
                           i, j   (k e ,i ⋅ ei ) ⋅ µ i∆e (k ∆e ,i ⋅ ∆ei )
                                                                                  therefore generated for each interstand zone before
                                                                                  rolling. This was done by bringing the motor speed of the
                                                                                  upstream stand away from the selected initial mill setup.
                                                                                      The selected production schedule has a finishing speed
i = 1, ..... 5                                                              (1)   of 800FPM (feet per minute). The initial billet size is 5.5
                                                                                  in2 and the product size is 0.75 in2. Table 2 shows the
where                                                                             intentionally disturbed startup speed of each roughing
                                                                                  stand and the tension at each interstand zone after the first
                                                                                  billet has been rolled.
     Table 2: Initial and final motor speed and                 Acknowledgement
                 interstand tension
 Stand/Zone         1        2         3       4         5         This work was supported by the Natural Sciences and
 Stand Speed      130       109      220      300       295     Engineering Research Council of Canada (NSERC)
    (RPM)        (125)     (115)    (226)    (292)     (302)    through Collaborative Research and Development Grant
  Interstand      40.2    -177.6     -250    239.6    -205.4    CRDPJ 234028-99, and by Materials and Manufacturing
Tension (PSI)      (0)      (0)       (0)     (0)       (0)
                                                                Ontario Collaborative Research Grant IC403. The authors
                                                                would like also to thank the assistance of Quad
    The tension control system coordinately controls            Engineering Inc. in providing test facilities and for useful
the interstand tension of each interstand zone in a             comments.
repetitive and interval mode. After 5 billets, all
interstand tensions are brought to very insignificant           References
values. After 18 billets, all interstand zones are              [1] R. Takahashi, “State of the art in hot rolling process
brought to a tension-free status. Table 2 also shows                 control”, Control Engineering Practice, 9 (2001), pp.
the final speeds of roughing stands and the final                    987-993, 2001.
values of interstand tensions (data in the                      [2] H. Katori, R. Hirayama, T. Ueyama and K. Furuta, “On
parentheses).                                                        the possibility of looperless rolling on hot rolling
    To evaluate the transient performance of the control             process”, Proc. 1999 IEEE International Conference on
system, Figs. 4 and 5 show the overall armature current              Control Applications, vol. 1, pp. 18-22, 1999.
and motor speed responses of the selected stands 4 and 5.       [3] G. Li, “Decoupled intelligent tension control”, Technical
The response curves are for 20 rolled billets. Fig. 6 is an          Report, ITC.3 (internal), Ryerson University, Jan. 2002.
enlarged view of partial responses of Stand 5 for the last 5    [4] T. Hesketh, Y. A. Jiang, D. J. Clements, D. H. Butler and
billets. It can be seen that the armature current of each            R. van der Laan, "Controller Design for hot strip
                                                                     finishing mills", IEEE Trans. Control System
stand traces well its reference value after a few billets.
                                                                     Technology, vol. 6, no. 2, pp. 208-219, 1998.
Note that the current impacts while the billet is hitting on    [5] H. Imanari, Y. Morimatsu and K. Sekiguchi,
or leaving the controlled stand or hitting on its                    “Looper H-infinity Control for Hot-strip Mills”
downstream stand are inevitable in the rolling process and           IEEE Trans. Ind. Appl., vol. 33, pp. 790-796,
not handled by the control system.                                   May/June 1997.
    The current reference of a stand represents the             [6] Y. Seki, K. Sekiguchi, Y. Anbe, K. Fukushima, Y.
armature current of that stand in the forward-tension-free           Tsuji and S. Ueno, “Optimal multivariable looper
status. It is sampled for every billet and may include a             control for hot strip finishing mill,” IEEE Trans.
backward tension. Hence, the current reference is not                Industrial Applications, vol. 27, no. 1, pp. 124-
fixed until all interstand zones become tension-free as              130, Jan.-Feb., 1991.
seen in Figs. 4 and 5.                                          [7] M. Shioya, N. Yoshitani and T. Ueyanma,
                                                                     “Noninteracting        control      with     disturbance
    From the speed responses, one can see that the stand
                                                                     compensation and its application to tension-looper
reference speed is automatically adjusted to make its own            control for hot strip mill", Proc. 1995 IEEE 21st
match the speed of the downstream stand. The                         International       Conference         on      Industrial
instantaneous drop and rise of actual speed occurring                Electronics, Control and Instrumentation, IECON
while the strip is hitting on or leaving the controlled stand        1995, vol. 1, pp. 229-234, 1995.
are unavoidable and not controlled by the tension control       [8] F. Janabi-Sharifi and J. Fan, "Self-tuning fuzzy looper
system. There is a leading speed compensation designed               control for rolling mills", Proc. 39th IEEE
to reduce the speed drop from stand impact to an                     Conference on Decision and Control, vol. 1, pp.
acceptable extent. One can clearly see the speed auto-               376-381, 2000.
adjustment process from the first half of the enlarged          [9] F. Janabi-Sharifi, "Neuro-fuzzy looper control with T-
partial speed response of Fig. 6.                                    operator and rule tuning for rolling mills: theory and
                                                                     comparative study", Proc. 27th IEEE Annual Conference
                                                                     Industrial Electronics, IECON 2001, Nov. 2001, Denver,
5. Conclusion                                                        Co., pp. 58-63, 2001.
                                                                [10] L. Winitsky and R. Li, “Virtual rolling mill for real time
   Looperless interstand tension control of roughing                 control system tuning, operator training and rolling mill
rolling mills remains a hard-to-resolve problem. This                simulation”, Association of Iron and Steel Engineer,
paper proposes a successful multistand fuzzy tension                 1999.
control system for a roughing rolling mill. Test results on     [11] I. Shpancer and W. Kinsner, "A study of a tension control
a virtual rolling mill show that the proposed technique has          system architecture for Manitoba rolling mills", Research
                                                                     Project Proposal 0037-002 (internal), 1983.
made it possible to realize a satisfactory multiple stand
tension control of roughing rolling process.
Current (A)                                                                   Speed (rpm)
    120                                                                          240

                            Solid -- Actual
                            Dashed --- Reference






                                                                                                               Solid -- Actual
                                                                                                               Dashed --- Reference

    -20                                                                          210
          0   500    1000      1500           2000   2500       3000   3500            0    500        1000       1500              2000   2500   3000   3500

                                   Time (s)                                                                              Time (s)

                                       Fig. 4 Overall current and speed responses of Stand 4
Current (A)                                                                   Speed (rpm)
   140                                                                           315

                                                                                                               Solid -- Actual
   120                                                                                                         Dashed --- Reference



     60                                                                          300




    -20                      Solid -- Actual
                             Dashed --- Reference

    -40                                                                          280
          0   500    1000     1500            2000   2500       3000   3500            0    500        1000       1500              2000   2500   3000   3500

                                  Time (s)                                                                            Time (s)

                                       Fig. 5 Overall current and speed responses of Stand 5
Current (A)                                                                   Speed (rpm)
                                                                                                               Solid -- Actual
                             Solid -- Actual                                      297
                                                                                                               Dashed --- Reference
                             Dashed --- Reference
                                                                                            Leading speed
   130                                                                            296


   120                                                                            293



   110                                                                            290


     2600     2700   2800      2900           3000   3100       3200   3300         2600    2700        2800      2900              3000   3100   3200   3300

                                  Time (s)                                                                               Time (s)

                              Fig. 6 Enlarged partial current and speed responses of Stand 5

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Description: fuzzy multiple stand tension control of a roughing rolling mill