EP0130231A1 - Circuit arrangement to regulate the tensile force transferred between the housings of a multiple housing mill train - Google Patents

Abstract

1. A circuit arrangement for the regulation of the tensile force transferred in the rolled stock between the stages (minimal tensile regulation) and the rolling speed of a mill train comprising m or 4 stages, each of which is equipped with driving means, a speed control unit for each driving means and a control circuit which comprises an actual value calculator (16) for determining the actual values of the torque, which is conditional upon the tension, from the respective values of driving, accelerating and deforming moments, and which comprise a theoretical value calculator (17) for calculating the theoretical values of the torques, which are conditional upon the tension, of each individual stage from the predetermined values of the (specific) tensile stress which is related to the cross-sectional unit of the rolled stock, the roller radius and the cross-section of the rolled stock for each stage, where a correction value for the theoretical value of the speed regulator is formed from the theoretical/actual value comparison of the torques which are conditional upon the tension, and which comprises a superimposed correction regulator which forms a correction value sigma k * which is fed to the theoretical value calculators for the superimposed correction of the theoretical values of the torques which are conditional upon the tension, characterised in that in order to stabilise the rolling speed of the mill train during the transit phase of the rolled stock the speed regulator (4.7) of the last stage (m or 4) is operated by means of a firmly predetermined, uncorrected theoretical value (n*) and the control circuit of the last stage (4.16; 4.17; 32) is designed to be such that on the basis of the theoretical/actual value comparison of the tension-dependent torque (DELTA Mz) of the stage it forms a correction value (sigma k *) for the superimposed control devices assigned to the speed regulators of all stages (1.7 to 4.7), which influences the theoretical values for the output specific tensile force (sigma A *) of the individual stages on the basis of the correction value (sigma k *) in such a manner that the deviation of the tensile force between the penultimate (m-1 or 3) and the ultimate stage (m or 4) is compensated by a predetermined theoretical value.

Description

The invention relates to a circuit arrangement of the type specified in the preamble of claim 1.

During continuous hot rolling of profile and bar steel, only small, defined longitudinal stresses may occur in the rolling stock between the individual stands coupled by the rolling stock if the dimensional accuracy of the end product is to be guaranteed. Longitudinal stress can be rolled if each drive only applies the torque required to deform the material. This is the case when the circumferential speed of the rolls and thus the speeds of the stand drives are correctly adapted to the speed of the rolling stock.

DE-A-25 41 071 describes a method and a device of the aforementioned type, with the aid of which it should be possible to set the speed ratios required for a defined tensile stress in the rolling stock in a short time and in a simple manner. For this purpose, the tension in front of and behind each stand, to which a tension difference control device is assigned, is determined from the drive torque, the acceleration torque and the deformation torque in the roll gap. One of the stands takes on the function of a guide stand, which essentially determines the rolling speed in the street. However, the use of this method does not rule out that deviations from the setpoint in front of and behind the control tower Tensile stresses occur that result from the sum of the moment errors of the other stands. These deviations become clearer the greater the number of stands of such a rolling mill.

In order to eliminate this unfavorable behavior of the known so-called minimum tension control, EP-PS 0 008 037 describes a device for regulating the tensile force transmitted in the rolling stock in a rolling mill containing m stands, in which speed controllers are also assigned to the stand drives, their setpoints using Tension difference control devices are corrected according to the parameters of a minimum tension control. The previous scaffolding should also be provided with such a control device. This initially avoids that the torque errors of the other stands on the guide stand add up and that there are changing tensions within the rolling stock. Without further measures, however, this could lead to the fact that the tension-dependent speed correction, which is now effective on all stands, in turn leads to a summation effect, which would result in a constant change in the speed of the road and thus in the rolling speed. In order to avoid this, a common correction controller is superimposed on the control device assigned to each stand, to which the output signal of one of the control devices for the correction of the speed setpoint value is fed as an input variable. The correction controller forms an output signal from this input variable, which, as a higher-level control variable, corrects the setpoints of the (specific) tensile stresses of the individual rolling stands related to the cross section of the rolling stock in such a way that any changes in speed of the reference stand are compensated for. By the action of corrective size of the correction controller to the values of the specific tensile stresses of the individual stands, a statistical distribution of the correction values proportional to the rolling stock cross section is achieved in the sense that the stands with the larger rolling stock cross sections are given a correspondingly greater weighting. In other words, the influence of correction decreases as the cross section of the rolling stock becomes smaller, so that thickness errors due to control effects in the sensitive last stands are avoided.

A further improvement in the control behavior while saving a separate control circuit can be achieved by the invention specified in claim 1. This ensures that a change in speed does not first have to occur in the reference stand in order to carry out a tension correction, but that the tension change in the last stand itself and directly forms the correction value for the other stands in the manner described. The rolling speed of the road remains absolutely constant because of the speed control of the last stand, which was uncorrected during the throughput phase.

The formation of the circuit arrangement according to claim 2 results in an overall smoother control process and thus a largely balanced behavior of the rolling mill.

• Fig. 1 das Prinzipschaltbild des Drehzahlregelkreises eines Gerüstantriebes mit Sollwertkorrektur,
• Fig. 2 das Prinzipschaltbild einer Regelschaltung zur Erzeugung eines Drehzahl-Korrekturwertes aufgrund verschiedener mechanischer und elektrischer Parameter sowie einstellbarer bzw. korrigierbarer Sollwerte,
• Fig. 3 das Prinzipschaltbild des Drehzahlreglers des Leitgerüstes bei dem in der Durchlaufphase keine Korrektur des Drehzahlsollwertes erfolgt,
• Fig. 4 die Regelschaltung des Leitgerüstes zur Erzeugung der Stellgröße für die Korrektur der spezifischen Zugspannungen der übrigen Gerüste,
• Fig. 5 das Prinzipschaltbild für eine vielgerüstige Walzenstraße mit den erfindungsgemäßen Regelkreisen und
• Fig. 6 bis 8 die Arbeitsweise der Walzstraße beim Einführen des Walzgutes.

The invention is explained in more detail below on the basis of exemplary embodiments. Show it:

• 1 shows the basic circuit diagram of the speed control loop of a scaffold drive with setpoint correction,
• Fig. 2 shows the basic circuit diagram of a control circuit for generating a speed correction value due to various mechanical and electrical para meters as well as adjustable or correctable setpoints,
• 3 shows the basic circuit diagram of the speed controller of the control tower, in which no correction of the speed setpoint takes place in the continuous phase,
• 4 shows the control circuit of the control tower for generating the manipulated variable for the correction of the specific tensile stresses of the other stands,
• Fig. 5 shows the basic circuit diagram for a multi-stand roller mill with the control loops according to the invention and
• Fig. 6 to 8, the operation of the rolling mill when inserting the rolling stock.

In Fig. 1, the work rolls 5 of a roll stand, not shown, including the associated DC drive motor 6 and a speed control arrangement 7 with the speed controller 8 and the actuator 9 are shown schematically. A tachometer generator 10 coupled to the drive motor 6 supplies an output signal which is proportional to the speed and which is compared with a setpoint value n. In order to achieve the correct speed relationships between the drives of several stands of a rolling mill, an additional speed setpoint Δ * is fed to the comparison point 11. Furthermore, a current transformer 12, which supplies _a voltage proportional to the armature current i _a , and a sensor 13 are provided, the output voltage of which is proportional to the flux 1 in the field winding 14 of the drive motor. F _E denotes the tensile force acting in the rolling stock 15 in the rolling direction on the entry side of the stand and F, the tensile force on the exit side.

In order to determine the correction value Δ n for the speed setpoint n, a control circuit shown in FIG. 2 is present. This consists of an actual value Computer 16, a setpoint computer 17, a comparator 18 and a correction controller 19 (minimum tension controller).

The drive torque Ma = F - i _a is composed of the acceleration torque Mb and the rolling torque Mw, which in turn is the sum of the deformation moments Mh and Mv of the horizontal and vertical rollers, and the torque Mz caused by the tensile stress.

Das Beschleunigungsmoment Mb läßt sich aus der Drehzahl n ableiten und die Verformungsmomente Mh und Mv ergeben sich aus den mittels Walzkraftaufnehmern gemessenen Walzkräften Fh und Fv durch Multiplikation mit einem dem Hebelarm der Walzkräfte entsprechenden Faktor Kh bzw. Kv. Der Faktor Kh läßt sich für Universalgerüste aufgrund der Walzspaltgeometrie bestimmen und wird von der Bedienungsperson vorgegeben. Für Duo-Gerüste wird der Hebelarmfaktor Kh in einer Abgleichschaltung berechnet. Der Faktor Kv ist von verschiedenen, theoretisch nicht erfaßbaren Einflußgrößen abhängig und wird daher ebenfalls mit Hilfe der Abgleichsschaltung ermittelt und anschließend gespeichert. Der Istwertrechner 16 erhält dementsprechend die Meßwerte i_a, ∳, n und Fh sowie gegebenenfalls die Größen Fv und Kh. Die dem Antriebsmoment Ma proportionale Ausgangsspannung eines Multiplizierers 20 ist entsprechend der für Mz aufgestellten Momentenbilanz einem Summierglied 21 additiv zugeführt, während die dem Beschleunigungsmoment Mb entsprechende Spannung, die mittels eines Differenziergliedes 22 gebildet ist, und die den Verformungsmomenten proportionalen Spannungen, die aus den Walzkraftsignalen Fh und Fv mittels eines Multiplizierers 23 bzw. eines Multiplizierers 24 gebildet sind, subtraktiv an dem Summierglied anstehen.Here is:

The acceleration torque Mb can be derived from the speed n and the deformation moments Mh and Mv result from the rolling forces Fh and Fv measured by means of rolling force transducers by multiplication by a factor Kh or Kv corresponding to the lever arm of the rolling forces. The factor Kh can be determined for universal stands based on the roll gap geometry and is specified by the operator. The lever arm factor Kh is calculated in a balancing circuit for duo scaffolds. The factor Kv is dependent on various influencing variables that cannot be determined theoretically and is therefore also determined with the aid of the adjustment circuit and then stored. The actual value computer 16 accordingly receives the measured values i _a , ∳, n and Fh and, if appropriate, the variables Fv and Kh. The output voltage of a multiplier 20, which is proportional to the drive torque Ma, is additively fed to a summing element 21 in accordance with the torque balance set up for Mz, while the acceleration torque Mb Corresponding stress, which is formed by means of a differentiating element 22, and the stresses proportional to the deformation moments, which result from the rolling force signals Fh and Fv by means of a multiplier 23 or a multiplier 24 are formed, are subtractively applied to the summing element.

To determine the factor Kh or Kv, the differential voltage at the output of the summing element 21, which is proportional to the moment Mz, is fed to a comparison point 25. If the scaffolding is a duo, switches 26 and 27 assume the position shown in dashed lines.

After the tapping in the first scaffold of the road, an integrator 28 connected downstream of the comparison point 25 changes the lever arm factor until the product Fh. Kh and the product φ · i _a minus the acceleration torque Mb are of the same magnitude. The lever arm factor Kh calculated in this way is saved for the rest of the stitch in the second stand before the tapping.

For a universal scaffold, the switches 26 and 27 assume the position shown for determining the lever arm factor Kv.

When calculating Kv, inaccuracies in the manually entered factor Kh are largely compensated for.

For all subsequent stands of the rolling mill, the automatic adjustment of the lever arm factor Kh for duo or Kv for universal stands with subsequent storage takes place in the same way, but with additional consideration of the train F _E on the entry side. The associated input-side train-related torque Mg supplied to the comparison point 25 in FIG. 2 corresponds to the output in the adjusted state Share of the tensile force setpoint, which is given to the preceding control device and thus the preceding stand, multiplied by the roller radius of the stand for which the lever arm factor is being calculated. In any case, the calculation runs so fast that it ends before the rolling stock enters the next stand.

* den Walzgutquerschnitten A, dem Walzendurchmesser dw und einer Korrekturgröße ermittelt. Der vom Minimalzugregler 19 gelieferte Drehzahlkorrektur-Sollwert Δn* wird während der Anstichphase jedes Gerüstes der Drehzahlregeleinrichtung des folgenden oder aller folgenden Antriebe - im dargestellten Beispiel als Δn*2' der Drehzahlregelung am Gerüst 2 - und nach dem Anstich im folgendem Gerüst über einen Umkehrverstärker 29 der Drehzahlregeleinrichtung des eigenen Antriebes - im Beispiel als Δn*1' der Drehzahlregeleinrichtung am Gerüst 1 - zugeführt. Zur Umschaltung ist ein Schalter 30 vorgesehen, der jeweils kurzzeitig die gezeichnete Zwischenstellung einnehmen muß, damit das Ausgangssignal des Minimalzugreglers auf Null zurückgestellt wird. Das Betätigungssignal für den Schalter 30 wird zweckmäßigerweise mit Hilfe eines bekannten und daher nicht dargestellten Walzgutwegnachbildung ermittelt.The difference between the output signal Mz of the summing element 21 and a torque setpoint Mz is fed to the minimum tension controller 19. The torque setpoint is determined by means of the setpoint calculator 17 from the specific train specified by the operator

* the rolling stock cross-sections A, the roll diameter dw and a correction value are determined. The speed correction setpoint value Δn * delivered by the minimum tension controller 19 is during the tapping phase of each stand of the speed control device of the following or all subsequent drives - in the example shown as Δn * 2 'of the speed control on stand 2 - and after tapping in the following stand via a reversing amplifier 29 the speed control device of the own drive - in the example as Δn * 1 'of the speed control device on stand 1 - supplied. A switch 30 is provided for switching, which must briefly assume the intermediate position shown so that the output signal of the minimum train controller is reset to zero. The actuation signal for the switch 30 is expediently determined with the aid of a known and therefore not shown rolling stock replica.

_K* für die spezifischen Zugsollwerte aller Gerüste welcher aus der Soll-Istwertabweichung der Zugspannung Mz des letzten Gerüstes gebildet wird.The control circuit shown and described in FIGS. 1 and 2 is assigned to each individual scaffold drive with the exception of the last one. The control circuit assigned to the last scaffold differs from that described above essentially in that the minimum tension controller 19 does not generate a speed correction variable, but rather a correction setpoint

_K * for the specific tensile setpoints of all scaffolds, which is formed from the target actual value deviation of the tensile stress Mz of the last scaffold.

This control circuit assigned to the control tower is shown in FIGS. 3 and 4. Identical function elements are provided with the same reference symbols.

The control element 7 shown in FIG. 3 differs from that according to FIG. 1 in that no correction value is supplied to its comparison point 31 during the through phase. The correction setpoint Δn * shown in dashed lines during the tapping phase remains unaffected. The speed control therefore works autonomously during the run-through phase and always keeps the drive at the speed value determined during the tapping phase.

_K* als Stellgröße erzeugt.The actual value calculator 16 shown in FIG. 4, like the target value calculator 17, is identical to that according to FIG. 2. The correction controller 32 differs from the minimum tension controller 19 of FIG. 2 in that it does not have a speed correction setpoint, but rather a correction setpoint formed from the tension deviation A Mz arising at the comparison point 18

_K * generated as manipulated variable.

*_K, die aber nicht zur Drehzahlkorrektur des Antriebes des letzten Gerüstes dient. Die Drehzahl des letzten Gerüstes wird vielmehr vom Drehzahl-Regelglied 4.7 auf einen konstanten, während der Anstichphase mit Hilfe von Δn*4' ermittelten Drehzahlwert n geregelt, sobald die Anstichphase beendet ist. Der vom Korrekturglied 32 erzeugte Stellwert

*_K ist eine aus der Abweichung des Zugmomentes Mz4 vom Sollwert Mz 4 gebildete Größe, welche den vorgegebenen Sollwerten der spezifischen Ausgangszüge

*_A1 bis

*_A3 an den entsprechenden Vergleichspunkten 1.33 bis 3.33 und dem Sollwertrechner 4.17 des letzten Gerüstes unmittelbar zugeführt ist. Durch diese Stellgröße *

_K erfahren die spezifischen Zugspannungs-Sollwerte eine der Abweichung des Zugmomentes Mz4 des letzten Gerüstes von seinem Sollwert Mz 4 einseitig entgegenwirkende Korrektur. Da es sich dabei um die Beeinflussung der spezifischen Ausgangs-Zugspannungen handelt, werden die einzelnen Walzgerüstantriebe an der Fehlerkompensation absolut proportional zum jeweiligen Walzgutquerschnitt beteiligt. Dies hat die gewünschte Folge, daß der Erfahrung Rechnung getragen wird, daß die Wahrscheinlichkeit der Fehlerentstehung dort am größten ist, wo auch die größten Verformungsmomente aufgebracht werden müssen. Außerdem wird die Fehlerkompensation so gewichtet, daß die Wirkung dort am größten ist, wo am wenigsten nachteilige Folgen zu befürchten sind, nämlich zu den großen Walzquerschnitten hin. Ferner bietet die Schaltungsanordnung nach der vorliegenden Erfindung den Vorteil, daß, wie aus einem Vergleich der Figuren 1 und 3 bzw. 2 und 4 hervorgeht, bei allen Gerüsten einschließlich dem Leitgerüst weitgehend baugleiche Regelungselemente verwendet werden können.In Fig. 5 a four-stand roller mill is shown schematically. The stands including the drive are indicated by the rollers 1 to 4. Each scaffold is a control circuit in the form of an actual value calculator 16, a setpoint calculator 17, a minimum Zugreglers 19 and a speed control element 7 assigned, which are designated in this figure according to the assignment to the respective scaffold with 1.7 to 4.7, with 1.16 to 4.16, with 1.17 to 4.17 and with 1.19 to 3.19. The correction controller 32 of the last stand generates the manipulated variable

* _K , which is not used to correct the speed of the drive of the last scaffolding. Rather, the speed of the last stand is regulated by the speed control element 4.7 to a constant speed value n determined during the tapping phase with the aid of Δn * 4 'as soon as the tapping phase has ended. The manipulated variable generated by the correction element 32

* _K is a variable formed from the deviation of the pulling torque Mz4 from the setpoint Mz 4, which corresponds to the specified setpoints of the specific output trains

* _A1 to

* _A3 is fed directly to the corresponding comparison points 1.33 to 3.33 and the setpoint calculator 4.17 of the last stand. With this manipulated variable *

_K , the specific tensile stress setpoints experience a correction that counteracts the deviation of the tensile torque Mz4 of the last stand from its setpoint Mz 4. Since this involves influencing the specific output tensile stresses, the individual mill stand drives are involved in error compensation in absolute proportion to the respective cross-section of the rolling stock. This has the desired consequence that experience is taken into account that the probability of the occurrence of errors is greatest where the greatest moments of deformation have to be applied. In addition, the error compensation is weighted so that the effect is greatest where the least disadvantageous consequences are to be feared, namely towards the large roll cross sections. Furthermore, the circuit arrangement according to the present invention has the advantage that, as can be seen from a comparison of FIGS. 1 and 3 or 2 and 4, largely identical control elements can be used in all scaffolds including the guide scaffold.

*_K nur jeweils einseitig wirken, d.h. nur die Austrittsmomente beeinflussen. Würde sie auch auf die Eintrittsmomente Einfluß nehmen, würden sich beide Einflüsse in Bezug auf die Fehlerkompensation in ihrer Wirkung aufheben.As already stated, the manipulated variable may

* _K only act on one side, ie only affect the exit moments. If it also had an influence on the moments of entry, both effects in terms of error compensation would cancel each other out.

Further details of the operation of the rolling mill result from FIGS. 6, 7 and 8. During the tapping phase in stand 1, ie during the time from tapping the beginning of the rolling stock in stand 1 to tapping in stand 2 (FIG. 6), this is from Train dependent moment of the scaffold 1 M _z1 - 0, because M _E1 = M _A1 = 0. During the piercing phase, the lever arm factor K of the rolling force for stand 1 is determined, as is explained in connection with FIGS. 2 and 4.

The determination of the lever arm factor is complete as soon as the tensile torque from the scaffold 1 M _{z1 of} the actual value calculator 1.16 has become equal to the associated setpoint (for scaffold 1 before the scaffold 2 is tapped) it has become zero. The lever arm factor is saved for the rest of the stitch. The determination of the lever arm factor must be completed before tapping the scaffold 2.

Folglich ist bei Gerüst 1 M_Z 1 = - M_A1, weil das eingangsseitige Moment M_E1 vor dem Gerüst 1 stets Null ist.With the tapping in stand 2 (FIG. 7), the minimum tension controller 1.19 becomes effective, at the actual value input of which the train-dependent torque M _{z1 of} the actual value calculator 1.16 is applied and whose setpoint input receives an initially constant setpoint M _z 1. The output voltage of the The minimum tension controller 1.19 is fed via a proportional element 1.33 with storage behavior to the speed controller of the drive of the second stand as an additional speed setpoint Δ n 2 '. If the rule adjustment of the minimum train controller 1.19 is reached, the relationship applies

Consequently, for scaffold 1, M _Z 1 = - M _A1 , because the input moment M _{E1 in} front of scaffold 1 is always zero.

During the piercing phase in the scaffold 2, the associated lever arm factor is determined and stored in the same way as described in connection with FIG. 6. The actual torque value M _z2 = M _E2 then arises at the output of the actual value calculator 2.16, since ^M _{A2 is} still = 0.

Just in time before the rolling stock 15 is pierced in stand 3, the proportional element 1.33 is switched to "save", the output signal of the minimum tension controller 1.19 is briefly set to zero and the output of the proportional element 2.33 (FIG. 8) is then via the reversing amplifier 29 (FIG. 2) switched as Δn * 1 to speed controller 1.7 on scaffold 1 (self-adjustment for scaffold 1). The additional speed setpoint .DELTA.n * 2 'is still given to the speed controller 2.7 on the frame 2 from the memory 1.33. This additional speed setpoint remains additively superimposed if, according to FIG. 8, shortly before the rolling stock in stand 4 is tapped, the correction controller 2.19 is switched to self-adjustment and specifies the additional speed setpoint Δn * 3 '. The additional speed setpoint .DELTA.n * 3 'of the correction controller 2.19 (FIG. 5), which acted on the speed controller 3.7 of the tower 3 until it tapped into the tower 4, is also further given to the tower 3 as a stored value .DELTA.n * 3, while rend the output of 2.19 is switched to the speed controller of stand 2 after the previous zero setting with the opposite sign as Δn * 2. This procedure guarantees a smooth transition to self-adjustment.

_K* als Korrekturwert für die austrittsseitig wirksamen spezifischen Zugspannungen

_A1*...

A*_(m-1) der übrigen Gerüste benutzt.that has already been described in the explanation of the switchover of the minimum draft controller 2.19 to self-adjustment, the processes described in connection with the scaffold 2 are repeated on all other scaffolds. The sequence on the scaffold 4 or on the last scaffold m differs from these processes in that this scaffold is not switched to self-adjustment, but is operated at constant speed after adjustment by the proportional member 3.33 (FIG. 5) and completed lever arm calculation by 4.16. In addition, the manipulated variable calculated by the correction controller (32)

_K * as a correction value for the specific tensile stresses on the outlet side

_A1 * ...

A * _{(m-1) of} the other scaffolds used.

To accelerate the setting of the final speed ratios, the additional speed setpoint is expediently not only fed to the speed controller of the following stand (shown in dashed lines), but to the speed controllers of all subordinate stands during the tapping phase. In this way, speed errors of a stand can be taken into account by presetting all the stands still directly involved in the rolling process, which leads to a significant shortening of the control process in the later tapping in these stands.

Claims (2)

1.Circuit arrangement for controlling the tensile forces transmitted in the rolling stock between the stands (minimum tension control) and the rolling speed of a rolling mill with m stands, each of which is equipped with drive means, a speed control for each drive means and a control circuit which consists of an actual value computer (16) to determine the actual values of the train-related torques from the respective values of drive acceleration and deformation torques, as well as a setpoint calculator (17) to calculate the setpoints of the train-related torques of each stand from the specified values of the (specific) tensile stress related to the cross-sectional unit of the rolling stock , the roll radius and the cross-section of the rolling stock for each stand, whereby a correction value for the setpoint of the speed controller is formed from the setpoint / actual value comparison of the train-related torques, and a higher-level correction controller which provides a correction value

_k *, which is fed to the setpoint computers for superordinate correction of the setpoints of the required torques, characterized in that, in order to stabilize the rolling speed of the rolling train during the running phase of the rolling stock, the speed controller (4.7) of the last stand (4) with a fixedly specified one , not corrected setpoint (s) is operated and the control circuit of the last scaffold (4.16; 4.17; 32) is designed such that it uses the setpoint-actual value comparison of the train-related torque (A Mz) of this scaffold to provide a correction value (

_k *) for the superimposed control devices assigned to the speed controllers of all scaffolds (1.7 to 4.7), which form the setpoints for the exit-specific train tensions (

_A *) of the individual frameworks based on the correction value (

_k *) influenced in such a way that the deviation of the tensile force between the penultimate and the last scaffolding is compensated for by a predetermined target value. 2. Circuit arrangement according to claim 1, characterized in that the control time constant of the correction controller (32) of the last stand is chosen to be greater than that of the control devices (1.19 to 3.19) of the other stands.

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