Method for tension control

Abstract

A method for tension control in a band-shaped material between two tension points, in particular between two adjacent roll stands, wherein at least one of the tension points has a rotary drive as an actuator. In order to make known tension controls of this type more effective and faster the controller output signal is varied in connection with the conversion thereof into the actuating signal for the rotary drive, at least temporarily, in dependence on a variable representing the band-shaped material.

Claims

1. A method for tension control in a band-shaped material between two clamping points, wherein the two clamping points are two neighboring rolling stands of a rolling mill, wherein at least one of the rolling stands comprises a rotary drive for driving rotation of one roll of the roll stand or wherein one of the two clamping points is a pair of rolls and the other of the two clamping points is a coiling device downstream in a rolling direction, wherein the pair of rolls comprises the rotary drive for driving the rotation of a least one of the rolls and/or the coiling device comprises the rotary drive for driving rotation of a coil, the method comprising the steps of: determining actual tension between the two clamping points; comparing the actual tension and a given desired tension in a comparator to determine a control error e(t) as a difference between the actual tension and a given desired tension; entering the control error e(t) on a controller to generate a controller output signal R(t); converting the controller output signal R(t) into an actuating signal S(t) with a conversion device; varying speed of the rotary drive as an actuating element in accordance with the actuating signal S(t) to regulate the actual tension to the desired tension of the band-shaped material in the rolling mill; and varying the controller output signal R(t) in connection with the conversion into the actuating signal S(t) by the conversion device at least temporarily in dependence on a variable g(t) representing speed of the band-shaped material, the method including, in a first variant: forming with the conversion device a gain factor V(t): $\begin{array}{cc}V\left(t\right)=\frac{g\left(t\right)}{g\left(t_0\right)},& \left(1\right)\end{array}$ which represents a curve of the variable g(t) representing the speed of the band-shaped material plotted against time, normalized to a given constant g(t.sub.0); and forming the actuating signal S(t) by the following formula: $\begin{array}{cc}S\left(t\right)=R\left(t\right)-\underset{t_i\le t}{.Math.}\left(A_1\left(t_i\right)\right)+\frac{g\left(t\right)}{g\left(t_0\right)}*\underset{t_i\le t}{.Math.}\left(\frac{A_1\left(t_i\right)}{V\left(t_i\right)}\right)& \left(2\right)\end{array}$ with: A1(t0) given Z(t0) given t.sub.i: learning times t.sub.0: first learning time.

2. The method according to claim 1, wherein times at which the variable g(t) representing the speed of the band-shaped material each reach a given threshold value g.sub.LPi, or at which the variable g(t) representing the speed of the band-shaped material is no longer constant, but begins to change so that dg(t)/dt≠0 or at which magnitude of Al(t)—during an acceleration phase of the band-shaped material—goes beyond a given threshold value A.sub.1max, are set respectively as the learning times t.sub.i.

3. The method according to claim 1, wherein the actuating signal (S(t)) is computed by formula (2) in the conversion device, when the variable g(t) representing the speed of the band-shaped material falls below a given upper threshold value g.sub.max and goes beyond a given lower threshold value g.sub.min.

4. The method according to claim 1, including operating the tension control in a second variant so that a gain factor V(t) is formed by the conversion device as: $\begin{array}{cc}V\left(t\right)=\frac{g\left(t\right)}{g\left(t_0\right)},& \left(1\right)\end{array}$ which represents the curve of the variable g(t) representing the speed of the band-shaped material plotted against time, normalized to a given constant g(t0); and the actuating signal S(t) is formed by the following formula:
S(t)=R(t)*V(t),  (3) with R(t): controller output signal.

5. The method according to claim 4, wherein the actuating signal S(t) is computed in the conversion device by formula (3), when the variable g(t) representing the speed of the band-shaped material falls below a given upper threshold value g.sub.max2 and goes beyond a given lower threshold value g.sub.min2; or when the gain factor V(t) is supposed to have a greater influence on the dynamics of the tension control than in formula (2); or before the tension control is in a steady state, in which case then: V(t)=1.

6. The method according to claim 4, wherein the tension control is switched from the second variant to the first variant as soon as and for as long as:
g(t)>g.sub.min1>g.sub.max2  (4).

7. The method according to claim 4, wherein the gain factor V(t) is confined to a constant value if the variable g(t) representing the speed of the band-shaped material goes beyond a given threshold value g.sub.maxi.

8. The method according to claim 7, wherein in the case of formula 2: g.sub.min1<g.sub.maxi<g.sub.max1; (5) or in the case of formula 3: g.sub.min2<g.sub.maxi<g.sub.max2 (6).

9. The method according to claim 4, wherein the actuating signal S(t) is computed as in the second variant if the tension control is in a steady state.

10. The method according to claim 1, wherein the actuating signal S(t) is computed as in the first variant if the tension control is in a steady state.

11. The method according to claim 1, wherein the actuating variable S(t) is limited in the conversion device in dependence on the variable g(t) representing the speed g(t) of the band-shaped material (200), as follows:
S.sub.min(g(t))<S(t)<S.sub.max(g(t))  (7).

12. The method according to claim 1, wherein the actuating signal S(t) is computed by factoring in a forward slip of the band-shaped material.

13. The method according to claim 12, wherein the actuating signal S(t) is computed by multiplication with a function f(k), where k is the forward slip.

14. The method according to claim 12, wherein the forward slip k(g(t)) in turn is computed in dependence on the variable g(t) representing the speed g(t) of the band-shaped material.

15. The method according to claim 12, wherein the forward slip is given as a constant.

16. The method according to claim 1, wherein alternatively or additionally to the actuating signal S(t), a derivative signal of form dS(t)/dt, representing a correction of acceleration of the rotary drive, is also provided as an input signal for the rotary drive.

17. The method according to claim 1, wherein the controller output signal R(t) represents a change in the rotary speed for the rotary drive.

18. The method according to claim 1, wherein a thickness control is done at a first of the rolling stands in a rolling direction; and at a following second of the rolling stands in the rolling direction the rotary drive is present and actuated for at least one of the rolls of the second rolling stand, and wherein the tension of the band-shaped material clamped between the first and the second rolling stand is controlled by the rotary drive of the second rolling stand being actuated by the actuating signal S(t).

19. The method according to claim 18, wherein the controller output signal R(t) represents a change in a thickness decrease of the band-shaped material at the first rolling stand as a clamping point and functions as the actuating signal for the thickness decrease at the first rolling stand; and the controller output signal R(t) is converted as recited in the first or second variant into the actuating signal for the rotary drive, wherein the conversion also involves a conversion of the change in the thickness decrease into a change in the rotary speed for the rotary drive.

20. The method according to claim 1, wherein the pair of rolls is a pair of drive rolls or a pair of working rolls in a rolling stand.

21. A method for tension control in a band-shaped material between two clamping points, wherein the two clamping points are two neighboring rolling stands of a rolling mill, wherein at least one of the rolling stands comprises a rotary drive for driving rotation of one roll of the roll stand or wherein one of the two clamping points is a pair of rolls and the other of the two clamping points is a coiling device downstream in a rolling direction, wherein the pair of rolls comprises the rotary drive for driving the rotation of a least one of the rolls and/or the coiling device comprises the rotary drive for driving rotation of a coil, the method comprising the steps of: determining actual tension between the two clamping points; comparing the actual tension and a given desired tension in a comparator to determine a control error e(t) as a difference between the actual tension and a given desired tension; entering the control error e(t) on a controller to generate a controller output signal R(t); converting the controller output signal R(t) into an actuating signal S(t) with a conversion device; varying speed of the rotary drive as an actuating element in accordance with the actuating signal S(t) to regulate the actual tension to the desired tension of the band-shaped material in the rolling mill; and varying the controller output signal R(t) in connection with the conversion into the actuating signal S(t) by the conversion device at least temporarily in dependence on a variable g(t) representing speed of the band-shaped material, the method further including forming with the conversion device a gain factor V(t) as: $\begin{array}{cc}V\left(t\right)=\frac{g\left(t\right)}{g\left(t_0\right)},& \left(1\right)\end{array}$ which represents the curve of the variable g(t) representing the speed of the band-shaped material plotted against time, normalized to a given constant g(t0); and forming the actuating signal S(t) the following formula:
S(t)=R(t)*V(t),  (3) with R(t): controller output signal.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) Four figures are included with the description, in which

(2) FIG. 1 shows a diagram of a tension control according to the invention;

(3) FIG. 2 shows a diagram on the conversion of a controller output signal R(t) into an actuating signal S(t) according to the invention;

(4) FIG. 3 shows exemplary signal plots for a first variant of the method according to the invention; and

(5) FIG. 4 shows exemplary signal plots for a second variant of the method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(6) The invention shall be described below in detail with reference to the mentioned figures in the form of exemplary embodiments.

(7) FIG. 1 shows a diagram 100 of a tension control according to the present invention. The foundation of the invention is a feedback circuit for a tension control, as shown generally in FIG. 1. The feedback circuit calls for measuring or otherwise ascertaining the actual tension of a metal band with the aid of a determination device 160 when the metal band is clamped between two clamping points under tension or when it runs through these clamping points under tension. The term tension is synonymous here with tensile stress. The actual tension so determined is compared in a desired/actual value comparator 110 to a given desired tension for the metal band, and the result of this comparison, which typically involves the formation of a difference, is put out as a control error e(t) to a controller 120. The controller generates at its output a controller output signal R(t).

(8) This controller output signal R(t) typically represents a rotary speed change for a rotary drive. According to the invention, however, the controller output signal R(t) does not serve directly as an actuating signal for the actuating of an actuating element 140 in the form of a rotary drive, but instead the present invention calls for the controller output signal at first being transformed in a conversion device 130 in suitable manner, as will be described below, into an actuating signal S(t). Then only the actuating element S(t) will in fact serve for actuating the rotary drive 140. The rotary drive 140 is actuated in such a way that the tension of the metal band 200 is adjusted to the given desired value when the metal band runs through the control system 150, which substantially consists of two clamping points. The described control process preferably works continuously in time, so that the aforementioned determination of the actual tension of the metal band occurs continuously within the control system and the ascertained actual tension is adjusted continuously to the given desired tension.

(9) FIG. 2 shows the functional layout of the conversion device 130 shown in FIG. 3, in detail.

(10) First of all, it will be recognized that the conversion device 130 receives the controller output signal R(t) as an input variable and puts out said actuating signal S(t) as its output variable to the rotary drive 140 as the actuating element. Besides the controller output signal (R(t), the conversion device 130 furthermore receives a variable g(t) representing the speed of the metal band 200. This may be the particular speed of the metal band itself; but it may also be any other physical variable allowing an indication of the variable of the speed of the metal band between the two clamping points.

(11) Besides the actuating signal S(t), it may be advisable to also put out its time derivative dS(t)/dt=a(t) as an output signal a(t) to the rotary drive 140. The derivative signal a(t) then enables an acceleration correction for the rotary drive.

(12) The tension control according to the invention and especially the conversion device 130 may be operated in a first variant or alternatively in a second variant; depending on the variant, the functional blocks F1 and F2 within the conversion device 130 will be operated and configured differently. The respective different configuration and functioning of the conversion device 130 shall now be described primarily in mathematical form for both variants.

(13) For both variants, the block F2 within the conversion device 130 provides for the generating of a gain factor V(t), in which the received input signal g(t) is preferably normalized to a given constant g(to). Therefore, for V(t):

(14) $\begin{array}{cc}V\left(t\right)=\frac{g\left(t\right)}{g\left(t_0\right)}& \left(1\right)\end{array}$

(15) I. Description of the First Variant

(16) For the first variant of the tension control according to the invention, the conversion device 130 per FIG. 2 computes the actuating signal S(t) as follows:

(17) $\begin{array}{cc}\begin{array}{cc}S\left(t\right)=A_1\left(t\right)+A_2\left(t\right)& \\ A_1\left(t\right)=R\left(t\right)-\left(A_1\left(t_0\right)+A_1\left(t_1\right)+A_1\left(t_2\right)+.Math.+A_1\left(t_n\right)\right)& \mathrm{with}{t}_n\le t\\ A_2\left(t\right)=V\left(t\right)*\left(Z\left(t_0\right)+Z\left(t_1\right)+Z\left(t_2\right)+.Math.+Z\left(t_n\right)\right)& \mathrm{with}{t}_n\le t\\ Z\left(t_i\right)=\frac{A_1\left(t_i\right)}{v\left(t_i\right)}& \end{array}& \left(2.1\right)\end{array}$$\begin{array}{cc}V\left(t\right)=\frac{g\left(t\right)}{g\left(t_0\right)}& \left(1\right)\end{array}$

(18) Hence:

(19) $\begin{array}{cc}S\left(t\right)=R\left(t\right)-\underset{t_i\le t}{.Math.}\left(A_1\left(t_i\right)\right)+\frac{g\left(t\right)}{g\left(t_0\right)}*\underset{t_i\le t}{.Math.}\left(\frac{A_1\left(t_i\right)}{V\left(t_i\right)}\right)& \left(2\right)\end{array}$
with
t.sub.i: time of a learning point
t.sub.0: time of the first learning point

(20) FIG. 3 illustrates the generating of the actuating signal S(t) as the output signal of the conversion device 130 according to the first variant with the aid of specific examples for the input signals g(t) and R(t). In the example in FIG. 3, the gain factor V(t) is identical in its time plot to the input signal g(t), i.e., the normalization factor g(to) was set here at 1, for example. Besides the gain factor V(t), various other intermediate signals A1(t), Z(t) and A2(t) are generated within the conversion device 130, from which the actuating signal S(t) is ultimately computed. The computation of the intermediate signals is mathematically represented above and, as mentioned, is explained by an example in FIG. 3.

(21) One special feature in the context of the tension control by the first variant is that times t.sub.i at which special events occur are defined as so-called learning times. In the following, several examples of such events will be given, at which a learning time is set or triggered: g(t)=g.sub.LPi **If the current speed g(t) reaches a given or parametrized speed g.sub.LP, a learning point will be thus triggered g.sub.LP [m/s]: learning point speed with: g.sub.LPi: speed at which a learning point should be set; or

(22) $\frac{dg\left(t\right)}{dt}\ne 0$
**Preferably the reference acceleration will be analyzed. If the mill begins a positive or negative acceleration phase with

(23) $\frac{dg\left(t\right)}{dt}\ne 0,$
a learning point will thus be set at this time;
or

(24) $\frac{dg\left(t\right)}{\mathrm{dt}}\ne 0.Math..Math.A_1\left(t\right).Math.\ge A_{1\mathrm{Max}}$
**If during an acceleration phase

(25) 0$\frac{dg\left(t\right)}{dt}\ne 0$
the magnitude of A.sub.1(t) exceeds a certain value A.sub.1Max, a learning point will be triggered.

(26) Two of the just described events for the triggering of learning points are illustrated in FIG. 3. Thus, one will recognize in FIG. 3 that the learning time 1 is then or therefore set at time to because the mill at time to is starting an acceleration phase; in FIG. 3 this can be recognized in that the variable g(t) representing the speed of the metal band changes at this time. Specifically, the variable g(t) increases at this time, starting from a previously constant quantity, i.e., it starts a positive acceleration phase at time to. The second learning time in FIG. 3 is triggered because the left-side limit value of A.sub.1(t) reaches a given value A.sub.1max. or falls to this value during the then prevailing negative acceleration phase, i.e., during the prevailing deceleration phase. The setting of the learning points in each case has the effect that the function A.sub.1(t) has a step at the learning times, because it is then computed by formula 2.1 from the controller output signal R(t) minus a particular magnitude.

(27) Thanks to the set learning points, the pilot control is adapted at once and exactly to the current circumstances, in particular to speed-related changes in the mass flow. Thanks to the setting of the learning points, the future controller output signal R(t), i.e., the controller output signal after the particular set learning time, will be copied in the form of the signal Z(t) to the pilot control branch; see FIG. 2, so that the actuating signal S(t) overall does not change by the setting of the learning times. Otherwise, if a change occurs in the mill speed, the newly learned mass flow disturbance will be automatically precontrolled by the conversion device 130, in that the mass flow control is once more changed in linear manner to the mill speed by the actuating signal S(t). Ideally—if the actuating signal (St) has previously been ideally adapted to the change in the mill speed—the controller output signal R(t) must then perform little or no corrections when the mill changes its speed, i.e., when a change occurs in g(t).

(28) FIG. 3 shows as examples signal plots for the input signals R(t) and g(t) and the actuating signal S(t) computed from them by formula 2 in the conversion device 130. A comparison of the controller output signal R(t), which typically serves in the prior art directly as the actuating signal for a downstream rotary drive, with the actuating signal S(t) computed according to the invention reveals, especially between the times t.sub.0 and t.sub.2, that the controller output signal R(t) has been weighted or varied with the variable g(t) representing the speed of the metal band or the gain factor V(t) in order to compute the actuating signal S(t).

(29) II: Description of the Second Variant

(30) According to FIG. 2, the actuating signal S(t) in the second variant is computed in dependence on the controller output signal R(t) as follows:
S(t)=A1(t)+A2(t)
A.sub.1(t)=0
A.sub.2(t)=V(t)×Z(t) with Z(t)=R(t)
Hence:
S(t)=V(t)×R(t)  (3) with

(31) $\begin{array}{cc}V\left(t\right)=\frac{g\left(t\right)}{g\left(t_0\right)}& \left(1\right)\end{array}$

(32) One example for such a calculation of the actuating signal S(t) according to the second variant is represented in FIG. 4. Also in FIG. 4 a comparison of the controller output signal R(t) with the actuating signal S(t) shows that the controller output signal is weighted or varied according to the invention in dependence on the gain factor V(t) or in dependence on the variable g(t) representing the speed of the metal band. By contrast with the weighting per the first variant, the weighting in the second variant is implemented much more immediately, this is shown by the actually proportionate gain in the local maxima and minima, especially in the region Δt. In the first variant, this is not amplified, or only in weakened manner, as can be seen from the signal profile S(t) in FIG. 3.

(33) The second variant can be used not only when the tension control is in a steady state, but also even before reaching the steady state, e.g., when a metal band is being threaded into a mill, especially between the two clamping points, or during a tension build-up sequence, etc. Then, for variant 2, the following mathematical relation applies, for example:
V(t)=1
Hence
S(t)=R(t)

(34) This then corresponds to a direct switch-through/use of the controller output signal R(t) as the actuating signal S(t) for the rotary drive. In that case, the conversion of R(t) into S(t) according to the invention will not occur, or is reduced to a short circuit.

(35) III. Statements Holding for Both the First and the Second Variant

(36) If the tension control is in a steady state, it may be operated according to the invention either by the first or the second variant. In FIGS. 3 and 4 this steady state begins each at time to with the speed g(to). A switching between the first and the second variant can also be done in the steady state.

(37) A switching to the second variant may be done if a more favorable control behavior can be achieved due to a speed change in the mill, since the dynamics of the tension controller is likewise changed by virtue of the speed change. In the second variant, an adapting of the dynamics will occur automatically, at least in part, by the conversion of the variable R(t) into S(t) according to the invention.

(38) The direct amplification of the controller signal R(t) during the conversion into the actuating signal S(t) per the second variant has the advantage that the controller can be set up more quickly, since the dependency of the control dynamics on the speed is at least partly solved by the conversion of R(t) to S(t) according to the invention. The resulting continuous adapting of the dynamics of the controller to the requirements during and after a speed change can also be more precise as compared to the traditional adjustment for different working points.

(39) In certain situations, it may be advantageous not to further increase the gain of the controller output R(t) during the conversion into S(t). If this is the case, a switching from variant two to variant one may be done. This switching from variant two to variant one as well as the switching back from variant one to variant two preferably occurs by an additional logic, which prevents the actuating signal S(t) from changing on account of the switchover. For example, a switching from variant two to variant one will occur when the dynamics of the drive is the limiting variable of the tension controller dynamics.

(40) For both the first and the second variant there again exists the possibility of positively limiting the speed factor

(41) $V\left(t\right)=\frac{g\left(t\right)}{g\left(t_0\right)},$
for example. One example of the limiting is:
V(t)=g.sub.max/g(t0) , if g(t)≥g.sub.max;
otherwise:

(42) $\begin{array}{cc}V\left(t\right)=\frac{g\left(t\right)}{g\left(t_0\right)}& \left(1\right)\end{array}$

(43) Thus, V(t) is constant at speeds ≥g.sub.max. This makes it possible, at high speeds, to hold the correction of the tension controller absolute and the gain of the controller constant.

LIST OF REFERENCE NUMBERS

(44) 100 Tension control 110 Desired/actual value comparator 120 Controller 130 Conversion device 140 Actuating element, especially a rotary drive 150 Control system with two clamping points 160 Determination device for the actual tension 200 Band-shaped material, especially metal band e(t) (Tension) control error R(t) Controller output signal S(t) Actuating signal for rotary drive V(t) Gain factor a(t) Derivative signal g(t) Variable representing the speed of the metal band ti Time