ROBUST BAND TENSION CONTROL

Abstract

A metal band (1) is first rolled in a front (upstream) and then in a rear (downstream) roll stand (2a, 2b) of a multi-stand rolling train. A looper (3) applied on the metal band (1) between the roll stands (2a, 2b) detects a band tension (Z) present in the metal band (1). The band tension (Z) is supplied to a first and a second tension controller (8, 9), which determine an application additional target value (δs*, δF*) and a speed additional target value (δv*). The second tension controller (9) only determines a value less than or greater than 0, as the speed additional target value (δv*), if the band tension (Z) is above or below an upper or lower band tension limit (Z1, Z2). Otherwise, same returns the speed additional target value (δv*) to the value 0. The first tension controller (8) is also supplied with a target tension (Z*) that falls between the band tension limits (Z1, Z2). The first tension controller (8) determines the application additional target value (δs*, δF*) using a determining standard based on the deviation of the band tension (Z) from the target tension (Z*). The determining standard also permits a value different to 0 as the application additional target value (δs*, δF*) if the band tension (Z) falls between the band tension limits (Z1, Z2). The application additional target value (δs*, δF*) acts on the rear roll stand (2b). The speed additional target value (δv*) acts on the front roll stand (2a) with a positive indicator, or on the rear roll stand (2b) with a negative indicator.

Description

SUMMARY OF THE INVENTION

The object of the present invention is to provide possibilities, by means of which a control concept for the basic automation is implemented, and therefore in spite of the existing interfering influences, the required thickness tolerances can be maintained well and the rolling process remains stable at the same time.

The object is achieved by a tension control method according to the invention.

According to the invention, a tension control method of the type mentioned at the outset is embodied in that,

• • a setpoint tension, which lies between the lower and the upper strip tension limit, is additionally supplied to the first tension controller,
  • the first tension controller determines the adjustment additional setpoint value using a determination rule on the basis of the deviation of the strip tension from the setpoint tension, and
  • the determination rule also permits a value not equal to 0 as the adjustment additional setpoint value if the strip tension is between the lower and the upper strip tension limits.

According to the invention, the strip tension is thus primarily and predominantly controlled by means of the adjustment additional setpoint value. However, if the strip tension leaves the permissible interval defined by the lower and the upper strip tension limit in spite of this control, influence is additionally exerted on the velocity at which the front or the rear roll stand is operated. The first and the second tension controller can be designed as needed. They are preferably controllers having integral behavior, for example, (solely) I controllers, PI controllers, or PID controllers.

It is possible that the adjustment additional setpoint value is a rolling force additional setpoint value. In this case, the rear roll stand is operated in a manner controlling the rolling force. Alternatively, it is possible that the adjustment additional setpoint value is a roll gap additional setpoint value. In this case, the rear roll stand is operated in a manner controlling the roll gap. Both embodiments lead to good results.

In the case of the roll gap control, it is preferably provided that a lower and an upper adjustment limit value are supplied to the first tension controller, and the first tension controller limits the output adjustment additional setpoint value at the bottom to the lower adjustment limit value and at the top to the upper adjustment limit value. Furthermore, in this case, the upper and the lower adjustment limit values are dynamically determined by a lower and an upper limit value determining unit as a function of a rolling force, with which the metal strip is rolled in the rear roll stand, and the adjustment additional setpoint value, and specified to the first tension controller. A dynamic adaptation is thus possible in dependence on the operating state of the rear roll stand.

In particular, it is possible that the lower limit value determining unit raises the lower adjustment limit value, as long as the rolling force, with which the metal strip is rolled in the rear roll stand, exceeds an upper rolling force limit value, and otherwise keeps the lower adjustment limit value at a predetermined distance from the adjustment additional setpoint value, and the upper limit value determining unit lowers the upper adjustment limit value, as long as the rolling force, with which the metal strip is rolled in the rear roll stand, falls below a lower rolling force limit value, and otherwise keeps the upper adjustment limit value at a predetermined distance from the adjustment additional setpoint value. The rear roll stand can thus always be operated within a permissible rolling force range. The two limit value determining units therefore preferably have an integral behavior.

If the second tension controller determines a velocity additional setpoint value not equal to 0, i.e., if the strip tension falls below the lower strip tension limit or exceeds the upper strip tension limit, the second tension controller preferably defines the velocity additional setpoint value such that the strip tension is set to the lower or upper strip tension limit, respectively.

It is preferably provided that the looper is held at a defined position by means of a position controller. Variations of the strip tension thus do not have an effect on the position of the looper. A negative influence on the stability of the rolling process is thus avoided.

It is possible that the metal strip is cold rolled in the front roll stand and the rear roll stand. However, the metal strip is preferably hot rolled in the front roll stand and the rear roll stand.

The object is furthermore achieved by a computer program according to the invention. According to the invention, a computer program of the type mentioned at the outset is embodied such that the processing of the machine code by the control unit causes the first tension controller to determine the adjustment additional setpoint value using a determination rule on the basis of the deviation of the strip tension from a setpoint tension, which lies between the lower and the upper strip tension limits, and causes the determination rule also to permit a value not equal to 0 as the adjustment additional setpoint value if the strip tension is between the lower and the upper strip tension limits.

Advantageous embodiments of the computer program correspond to those of the tension control method.

The object is furthermore achieved by a control unit having the features disclosed herein. According to the invention, the control unit is embodied such that it is programmed with a computer program according to the invention.

The object is furthermore achieved by a multi-stand rolling train for rolling a metal strip having the features disclosed herein in a multi-stand rolling train of the type mentioned at the outset.

Further advantages and details result from the following description of exemplary embodiments in conjunction with the drawings. In the schematic Figures:

FIG. 1 shows a multi-stand rolling train,

FIG. 2 shows a front and a rear roll stand, a looper arranged between these two roll stands and also shows a control unit,

FIG. 3 shows control variables as a function of the strip tension, and

FIG. 4 shows an embodiment of a part of the control unit of FIG. 3.

DESCRIPTION OF EMBODIMENT

According to FIG. 1, a metal strip 1 is to be rolled by means of a rolling train. The metal strip 1 can consist, for example, of steel or aluminum alternatively of another metal. The rolling train has multiple roll stands 2 for rolling the metal strip 1. The first roll stand is the front roll stand in the path of the strip. The second or later roll stand is a rear roll stand. In general, the number of roll stands 2 is between three and eight, in particular between four and seven, for example, five or six. The roll stands 2 generally have working rolls and support rolls, i.e., they are designed as quarto stands. In some cases, the roll stands 2 also have intermediate rolls in addition to the working rolls and the support rolls and are thus designed as sexto stands. Only the working rolls are shown in FIG. 1 and in FIG. 2.

The metal strip 1 passes through the roll stands 2 of the rolling train sequentially one after another in a transportation direction x. The metal strip 1 is rolled in the roll stands 2. The thickness of the metal strip 1 is thus gradually reduced in the transportation direction. A looper 3, which is applied to the metal strip 1, is respectively arranged between each two successive roll stands 2. The metal strip 1 may enter the first roll stand 2 of the rolling train, for example, at a temperature T which is between 850° C. and 1100° C. In this case, the metal strip 1 is hot rolled in the roll stands 2. In principle, however, it is also possible that the metal strip 1 is cold rolled in the roll stands 2.

The rolling train is controlled by a control unit 4. The control unit 4 is programmed using a computer program 5. The code is stored on a non-transitory recording medium. The computer program 5 comprises machine code 6. The machine code 6 is processable by the control unit 4. As a result of the processing, the control unit 4 executes a tension control method, which explained in greater detail hereafter.

The tension control method relates in each case to a portion of the metal strip 1, which is located between two directly successive roll stands 2. FIG. 2 shows such a portion of the metal strip 1, the two participating roll stands 2, and the looper 3 between these two roll stands 2. The present invention is explained hereafter in conjunction with these two roll stands 2 and the looper 3 between these two roll stands 2. The roll stand 2 through which the metal strip 1 first passes is referred to hereafter as the front roll stand. The roll stand 2b through which the metal strip 1 passes thereafter is referred to hereafter as the rear roll stand. The looper 3 is simply referred to as the looper 3, which means the looper 3 between the front roll stand 2a and the rear roll stand 2b.

The looper 3 is applied to the metal strip 1. For example, the control unit 4 can implement a position controller 7 to apply the looper 3 to the metal strip 1, as a result of the processing of the machine code 6. In this case, a corresponding position setpoint value p* is supplied to the position controller 7. The position setpoint value p* is generally constant. The position setpoint value p* can be generated, for example, inside the control unit 4. Alternatively, it can be externally specified to the control unit 4.

Furthermore, a corresponding position actual value p is supplied to the position controller 7. The position controller 7 then determines a control signal S for a positioning element 3′ (for example, a hydraulic cylinder unit), by means of which the position of the looper 3 is tracked if necessary, depending on the control deviation, i.e., the difference of position setpoint value p* and position actual value p. As a result, the looper 3 is therefore kept at a defined position by means of the position controller 7, namely the position setpoint value p*. The position controller 7 can be designed as needed. The position controller 7 is preferably designed as a controller having an integral component, for example, as a PI controller.

Furthermore, a strip tension Z, which prevails in the metal strip 1 between the front roll stand 2a and the rear roll stand 2b, is detected by the looper 3. For example, a torque exerted by the positioning element 3′ on the looper 3 or a corresponding force can be detected and the strip tension Z can be determined therefrom in conjunction with the position actual value p and geometric relationships of the roll stands 2a, 2b and the looper 3 in relation to one another. However, the looper 3 preferably has a load cell, by means of which the force with which the looper roll is pressed against the looper 3 is detected directly. A more accurate determination of the strip tension Z is thus possible.

The detected strip tension Z is supplied to the control unit 4 and accepted by the control unit 4. The control unit 4 implements a first tension controller 8 and a second tension controller 9 by processing the machine code 6. The strip tension Z is supplied to the first tension controller 8 and the second tension controller 9.

The first tension controller 8 determines an adjustment additional setpoint value δs* using a determination rule. The adjustment additional setpoint value δs* can be in particular a roll gap additional setpoint value δs*. The adjustment additional setpoint value δs* is connected in this case to an adjustment setpoint value s* given as a roll gap setpoint value s*.

The second tension controller 9 determines a velocity additional setpoint value δv*. The velocity additional setpoint value δv* is connected to a velocity setpoint value v*. The adjustment additional setpoint value δs* acts on the rear roll stand 2b. In particular, the adjustment additional setpoint value δs* acts on the adjustment of the rear roll stand 2b. The velocity additional setpoint value δv* can act on drives by means of which the rolls of the rear roll stand 2b are rotated. In this case, the velocity additional setpoint value δv* also acts on the rear roll stand 2b, corresponding to the illustration in FIG. 2. Alternatively, the velocity additional setpoint value δv* could act on the front roll stand 2a.

In addition to the strip tension Z, a lower strip tension limit Z1 and an upper strip tension limit Z2 are supplied to the second tension controller 9. The upper strip tension limit Z2 is greater than the lower strip tension limit Z1. If and as long as the strip tension Z lies between the lower and upper strip tension limits Z1, Z2, the velocity additional setpoint value δv* determined by the second tension controller 9 has the value 0, corresponding to the illustration in FIG. 3. If and as long as the strip tension Z lies above the upper strip tension limit Z2, in contrast, the second tension controller 9 determines a value greater than 0 as the velocity additional setpoint value δv*. Vice versa, if and as long as the strip tension Z lies below the lower strip tension limit Z1, the second tension controller 9 determines a value less than 0 as the velocity additional setpoint value δv*. The second tension controller 9 can determine the velocity additional setpoint value δv* in particular in such a way that the strip tension Z is set to the lower strip tension limit Z1 in the case that it falls below the lower strip tension limit Z1 and, vice versa, it is set to the upper strip tension limit Z2 in the case that it exceeds the upper strip tension limit Z2. If the strip tension Z again assumes a value between the lower and the upper strip tension limits Z1, Z2 after exceeding the upper strip tension limit Z2 or after falling below the lower strip tension limit Z1, the second tension controller 9 returns the velocity additional setpoint value δv* back to the value 0. The second tension controller 9 is preferably formed as a controller having an integral component, for example, as a PI controller.

If the velocity additional setpoint value δv* determined by the second tension controller 9 acts on the rear roll stand 2b, the velocity additional setpoint value δv* is added with a negative sign to a velocity setpoint value v* for the rear roll stand 2b, corresponding to the illustration in FIG. 2. Otherwise, if the velocity additional setpoint value δv* acts on the front roll stand 2a, the velocity additional setpoint value δv* is added with a positive sign to a velocity setpoint value for the front roll stand 2a.

In addition to the strip tension Z, a setpoint tension Z* is supplied to the first tension controller 8. The setpoint tension Z* lies between the lower and upper strip tension limits Z1, Z2. In particular, the setpoint tension Z* can lie approximately or even exactly in the middle between the lower and the upper strip tension limits Z1, Z2. In general, the equation Z*=kZ1+(1−k) Z2 applies, wherein the factor k is generally between 0.4 and 0.6, preferably even between 0.45 and 0.55. The first tension controller 8 determines the adjustment additional setpoint value δs* on the basis of the deviation of the strip tension Z from the setpoint tension Z. In contrast to the determination rule of the second tension controller 9, the determination rule for the first tension controller 8 also permits a value not equal to 0 as the adjustment additional setpoint value δs* if the strip tension Z lies between the lower and the upper strip tension limits Z1, Z2. The respective instantaneously determined adjustment additional setpoint value δs* can temporarily have the value 0 in the specific case. However, this is caused in this case by the specific values for the strip tension Z and the setpoint tension Z* and possibly the prior value curves thereof, but not by the fact that the strip tension Z lies between the lower and the upper strip tension limits Z1, Z2.

The determination rule can be, for example, such that the first tension controller 8 is designed as a controller having an integral component, for example, as a PI controller. If the instantaneous integral component is positive in such a case and the instantaneous proportional component is negative, the integral component and the proportional component can mutually compensate one another for a brief moment. If the deviation of the strip tension Z from the setpoint tension Z* is not equal to 0 for a longer time, however, necessarily at some point in time, the determined adjustment additional setpoint value δs* has to assume a value not equal to 0. This also applies if the strip tension Z only moves between the lower and the upper strip tension limits Z1, Z2 during the entire period of time. Similar circumstances result with other embodiments of the first tension controller 8, for example, as a PID controller or as an I controller and also in an embodiment as solely a P controller.

As explained up to this point, the adjustment additional setpoint value δs* is a roll gap additional setpoint value. In this case, the adjustment additional setpoint value δs* acts directly and immediately on the adjustment of the rear roll stand 2b. Alternatively, however, it is possible that the adjustment additional setpoint value δF* is a rolling force additional setpoint value δF*. In this case, the adjustment additional setpoint value δF* is connected to an adjustment setpoint value F* provided as a setpoint rolling force F* and acts indirectly, specifically via the rolling force F—on the adjustment of the rear roll stand 2b. This embodiment is shown by dashed lines in FIG. 2. The first tension controller 8 is also preferably designed as a controller having an integral component in this case, for example, as a PI controller. The other statements on the functionality of the first tension controller 8 also apply for this case.

It is even possible, according to the illustration in FIG. 2, that the first tension controller 8 is provided twice, namely once as the first tension controller 8 for determining the roll gap additional setpoint value δs* and once as the first tension controller 8 for determining the rolling force additional setpoint value δF*. In this case, it is decided by a selection signal A whether the one or the other first tension controller 8 is active. This is also shown by dashed lines in FIG. 2. The selection signal A can be specified to the control unit 4, for example, in the scope of a parameterization before startup. It is even possible to switch over the selection signal A during the operation of the rolling train. It is thus possible to operate the roll stand 2b shown in FIG. 2 sometimes in a manner controlling the roll gap and sometimes in a manner controlling the rolling force, and to determine the corresponding adjustment additional setpoint value δs*, δF* depending on the instantaneous operating mode, and connect it to the corresponding adjustment setpoint value s*, F*.

FIG. 4 shows a possible modification of the first tension controller 8. The statements on FIG. 4 refer in this case to the case in which the first tension controller 8 is designed to determine the roll gap additional setpoint value δs*.

According to FIG. 4, a lower and an upper adjustment limit value δs1*, δs2* are supplied to the first tension controller 8. In this case, the first tension controller 8 limits the output adjustment additional setpoint value δs* at the bottom to the lower and at the top to the upper adjustment limit value δs1*, δs2*. The lower and the upper adjustment limit values δs1*, δs2* can be dynamically determined, for example, corresponding to the illustration in FIG. 4, by a lower and an upper limit value determining unit 10, 11 as a function of a rolling force F, with which the metal strip 1 is rolled in the rear roll stand 2b, and the adjustment additional setpoint value δs*. The adjustment limit values δs1*, δs2* are specified to the first tension controller 8 by the two limit value determining units 10, 11.

In particular, it is possible, corresponding to the illustration in FIG. 4, that the upper limit value determining unit 11 checks whether the rolling force F, with which the metal strip 1 is rolled in the rear roll stand 2b, falls below a lower rolling force limit value F1. If this is the case, the upper limit value determining unit 11, proceeding from the last valid value for the upper adjustment limit value δs2*, reduces the upper adjustment limit value δs2* by a defined absolute value Δ2. The absolute value Δ2 can alternatively be constant or can depend on the amount by which the rolling force F falls below the lower rolling force limit value F1. Otherwise, the upper limit value determining unit 11 establishes the upper adjustment limit value δs2* such that it has a predetermined distance Δ2′ from the presently valid value of the adjustment additional setpoint value δs*.

In a similar manner, it is possible, corresponding to the illustration in FIG. 4, that the lower limit value determining unit 10 checks whether the rolling force F, with which the metal strip 1 is rolled in the rear roll stand 2b, exceeds an upper rolling force limit value F2. If this is the case, proceeding from the last valid value for the lower adjustment limit value δs1*, the lower limit value determining unit 10 elevates the lower adjustment limit value δs1* by a defined absolute value Δ1. The absolute value Δ1 can alternatively be constant or can depend on the amount by which the rolling force F exceeds the upper rolling force limit value F2. Otherwise, the lower limit value determining unit 10 establishes the lower adjustment limit value δs1* such that it has a predetermined distance Δ1′ from the presently valid value of the adjustment additional setpoint value δs*. The distance Δ1′ can be, but does not have to be the same distance Δ2′, which is set by the upper limit value determining unit 11 if the rolling force F does not fall below the lower rolling force limit value F1.

The reduction of the upper adjustment limit value δs2* can go so far that the upper adjustment limit value δs2* is less than the (actual) adjustment additional setpoint value δs*. In this case, the limiting by the upper adjustment limit value δs2* acts. The first tension controller 8 is therefore no longer capable of compensating for the deviation of the strip tension Z from the setpoint tension Z*. This has the result that the deviation of the strip tension Z from the setpoint tension Z* becomes greater until one of the strip tension limits Z1, Z2 is infringed. In this case, the second tension controller 9 engages in a corrective manner. Similar statements apply for the case in which the lower adjustment limit value δs1* is elevated further.

The present invention has many advantages. The rolling force and strip tension limits are thus reliably maintained even under unfavorable conditions (for example, overload or underload of the rear roll stand 2b). The rolling process is stabilized. This applies in particular in comparison to an ITC. By means of the tension control method according to the invention, for example, even a metal strip 1 having a thickness of 1 mm or less may be rolled stably and reliably in the scope of an endless casting-rolling method. This also applies to a conventional finishing train (HSM=hot strip mill). Furthermore, the hydraulic drive of the looper 3 can be simplified. This results in a cost reduction.

A further advantage is that neither an AGC nor a loop controller are required. It is merely required that the looper 3 does not move during the tension control. However, this can be readily ensured by the position controller 7. A superordinate thickness controller is required to compensate for thickness deviations at the exit of the rolling train. However, the thickness controller is also required in the prior art and also corresponds to the embodiment of the prior art.

Furthermore, the problems which occur in the case of an AGC are avoided by the control according to the invention of the strip tension Z. This is because in the case of control using AGC, the stand deflection has to be known very accurately, in order to achieve good results. It is problematic in this case that due to inadequate modeling of the stand deflection, the AGC is overcompensated and this results in an unstable rolling process. In the present invention, in contrast, the AGC is neither required nor used, and the stand deflection is also not required for a good compensation.

A further advantage is that a complex decoupling of a strip tension controller and loop controller is not required, since the strip tension controller has a different positioning element than is typical in the prior art and the loop controller is not required.

The above description serves exclusively to explain the present invention. The scope of protection of the present invention is exclusively to be defined by the appended claims, in contrast.

LIST OF REFERENCE SIGNS

• 1 metal strip
• 2, 2a, 2b roll stands
• 3 looper
• 3′ positioning element
• 4 control unit
• 5 computer program
• 6 machine code
• 7 position controller
• 8, 9 tension controller
• 10, 11 limit value determining unit
• A selection signal
• F rolling force
• F*, s* adjustment setpoint values
• F1, F2 rolling force limit values
• k factor
• p, p* position values
• S control signal
• T temperature
• v* velocity setpoint value
• x transportation direction
• Z strip tension
• Z1, Z2 strip tension limits
• Z* setpoint tension
• δ change value
• δ1, δ2 barriers
• δs*, δF* adjustment additional setpoint value
• δs1*, δs2* adjustment limit values
• δv* velocity additional setpoint value
• Δ1, Δ2 absolute values
• Δ1′, Δ2′ distances

Claims

1. A tension control method for a metal strip, wherein the metal strip is firstly rolled in a front roll stand of a multi-stand rolling train and then is rolled in a rear roll stand of the multi-stand rolling train, the method comprising: detecting a strip tension (Z), which prevails in the metal strip between the front roll stand and the rear roll stand, by means of a looper applied to the metal strip between the front roll stand and the rear roll stand;supplying the detected strip tension (Z) to a first tension controller, which determines an adjustment additional setpoint value (δs*, δF*);further supplying the strip tension (Z) to a second tension controller, which determines a velocity additional setpoint value (δv*),wherein the second tension controller determines a value greater than 0 as the velocity additional setpoint value (δv*) if the strip tension (Z) is above an upper strip tension limit (Z2), determines a value less than 0 as the velocity additional setpoint value (δv*) if the strip tension (Z) is below a lower strip tension limit (Z1), and returns the velocity additional setpoint value (δv*) to the value 0 if the strip tension (Z) is between the lower and the upper strip tension limits (Z1, Z2);wherein the adjustment additional setpoint value (δs*, δF*) acts on the rear roll stand, and the velocity additional setpoint value (δv*) acts with positive sign on the front roll stand or acts with negative sign on the rear roll stand;supplying a setpoint tension (Z*) to the first tension controller wherein the supplied tension lies between the lower and the upper strip tension limits (Z1, Z2);determining the adjustment additional setpoint value (δs*, δF*) by the first tension controller using a determination rule on the basis of the deviation of the strip tension (Z) from the setpoint tension (Z*); andusing the determination rule which also permits a value not equal to 0 as the adjustment additional setpoint value (δs*, δF*) if the strip tension (Z) is between the lower and the upper strip tension limits (Z1, Z2).

2. The tension control method as claimed in claim 1, wherein the adjustment additional setpoint value (δF*) is a rolling force additional setpoint value (δF*).

3. The tension control method as claimed in claim 1, wherein the adjustment additional setpoint value (δs*) is a roll gap additional setpoint value (δs*).

4. The tension control method as claimed in claim 3, further comprising supplying a lower and an upper adjustment limit value (δs1*, δs2*) to the first tension controller, wherein the first tension controller performs limiting the output adjustment additional setpoint value (δs*) at the bottom to the lower and at the top to the upper adjustment limit value (δs1*, δs2*), and dynamically determining the lower and the upper adjustment limit values (δs1*, δs2*) by a lower and an upper limit value determining unit as a function of a rolling force (F), with which the metal strip is rolled in the rear roll stand, and the adjustment additional setpoint value (δs*), and specified to the first tension controller.

5. The tension control method as claimed in claim 4, further comprising raising the lower adjustment limit value (δs1*) by the lower limit value determining unit if the rolling force (F), with which the metal strip is rolled in the rear roll stand, exceeds an upper rolling force limit value (F2), and otherwise sets a distance of the lower adjustment limit value (δs1*) from the adjustment additional setpoint value (δs*) to a predetermined value (Δ1′), and the upper limit value determining unit lowers the upper adjustment limit value (δs2*) if the rolling force (F), with which the metal strip is rolled in the rear roll stand (2b), falls below a lower rolling force limit value (F1), and otherwise sets a distance of the upper adjustment limit value (δs2*) from the adjustment additional setpoint value (δs*) to a predetermined value (Δ2′).

6. The tension control method as claimed in claim 1, further comprising if the strip tension (Z) falls below the lower strip tension limit (Z1) or exceeds the upper strip tension limit (Z2), the second tension controller defines the velocity additional setpoint value (δv*) such that the strip tension (Z) is set to the lower or upper strip tension limit (Z1, Z2), respectively.

7. The tension control method as claimed in claim 1, further comprising holding the looper at a defined position (p*) by means of a position controller.

8. The tension control method as claimed in claim 1, further comprising hot rolling the metal strip in the front roll stand and in the rear roll stand.

9. A computer program, which comprises machine code recorded on a non-transitory recording medium, and the machine code is processable by a control unit for a rolling train; wherein a metal strip is firstly rolled in a front roll stand of a multi-stand rolling train and then in a rear roll stand of the multi-stand rolling train;wherein a strip tension (Z), which prevails in the metal strip between the front roll stand and the rear roll stand, is detected by means of a looper applied to the metal strip between the front roll stand and the rear roll stand;wherein the processing of the machine code by the control unit causes;the control unit to accept the detected strip tension (Z);the control unit to implement a first tension controller, to which the strip tension (Z) is supplied and which determines an adjustment additional setpoint value (δs*, δF*);the control unit furthermore to implement a second tension controller, to which the strip tension (Z) is supplied and which determines a velocity additional setpoint value (δv*);the control unit to implement the second tension controller such that the second tension controller (9) determines a value less than 0 as the velocity additional setpoint value (δv*) if the strip tension (Z) lies below a lower strip tension limit (Z1), determines a value greater than 0 as the velocity additional setpoint value (δv*) if the strip tension (Z) lies above an upper strip tension limit (Z2), and returns the velocity additional setpoint value (δv*) to the value 0 if the strip tension (Z) lies between the lower and the upper strip tension limits (Z1, Z2);the adjustment additional setpoint value (δs*, δF*) to act on the rear roll stand, and the velocity additional setpoint value (δv*) to act with positive sign on the front roll stand or to act with negative sign on the rear roll stand; andwherein the processing of the machine code by the control unit causes the first tension controller to determine the adjustment additional setpoint value (δs*, δF*) using a determination rule on the basis of the deviation of the strip tension (Z) from a setpoint tension (Z′), which lies between the lower and the upper strip tension limits (Z1, Z2), and the determination rule also to permit a value not equal to 0 as the adjustment additional setpoint value (δs*, δF*) if the strip tension (Z) is between the lower and the upper strip tension limits (Z1, Z2).

10. The computer program as claimed in claim 9, wherein the processing of the machine code by the control unit causes the adjustment additional setpoint value (δF*) to be a rolling force additional setpoint value (δF*).

11. The computer program as claimed in claim 9, wherein the processing of the machine code by the control unit causes the adjustment additional setpoint value (δs*) to be a roll gap additional setpoint value (δs*).

12. The computer program as claimed in claim 11, wherein the processing of the machine code by the control unit causes a lower and an upper adjustment limit value (δs1*, δs2*) to be supplied to the first tension controller, the first tension controller to limit the output adjustment additional setpoint value (δs*) at the bottom to the lower and at the top to the upper adjustment limit value (δs1*, δs2*), and causes the control unit to implement a lower and an upper limit value determining unit by which the lower and the upper adjustment limit value (δs1*, δs2*) dynamically determined as a function of a rolling force (F), with which the metal strip is rolled in the rear roll stand, and the adjustment additional setpoint value (δs*), and specified to the first tension controller (8).

13. The computer program as claimed in claim 12, wherein the processing of the machine code by the control unit causes the lower limit value determining unit to raise the lower adjustment limit value (δs1*) if the rolling force (F), with which the metal strip is rolled in the rear roll stand, exceeds an upper rolling force limit value (F2), and otherwise to set a distance of the lower adjustment limit value (δs1*) from the adjustment additional setpoint value (δs*) to a predetermined value (Δ1′), and the upper limit determining unit to lower the upper adjustment limit value (δs2*) if the rolling force (F), with which the metal strip is rolled in the rear roll stand, falls below a lower rolling force limit value (F1), and otherwise to set a distance of the upper adjustment limit value (δs2*) from the adjustment additional setpoint value (δs*) to a predetermined value (Δ2′).

14. The computer program as claimed in claim 9, further comprising if the strip tension (Z) falls below the lower strip tension limit (Z1) or exceeds the upper strip tension limit (Z2), the processing of the machine code by the control unit causes the second tension controller to define the velocity additional setpoint value (δv*) such that the strip tension (Z) is set to the lower or upper strip tension limit (Z1, Z2), respectively.

15. The computer program as claimed in claim 9, further comprising the processing of the machine code by the control unit causes the control unit to implement a position controller, by means of which the looper is held at a defined position (p*).

16. The computer program as claimed in claim 9, further comprising the processing of the machine code by the control unit causes the metal strip to be hot rolled in the front roll stand and in the rear roll stand.

17. A control unit for a multi-stand rolling train for rolling a metal strip, wherein the control unit is programmed with a computer program as claimed in claim 9.

18. A multi-stand rolling train for rolling a metal strip, wherein the rolling train has a front and a rear roll stand, in which the metal strip is rolled;a looper arranged in the rolling train between the front roll stand and the rear roll stand, and the looper is applied to the metal strip and the looper is configured to detect a strip tension (Z), which prevails in the metal strip between the front roll stand and the rear roll stand; anda control unit for the rolling train as claimed in claim 17, to which the strip tension (Z) is supplied and which acts on the rear roll stand.