EP0621353A1 - Process for controlling the electroplating of a metal strip - Google Patents

Abstract

One of the faces of a metal strip forming a cathode runs past continuously at a specified speed in an electrolyte in front of the anodes (741 to 74N; 751 to 75N) which are powered by controllable rectifiers (R1 to RN) respectively. The strip is divided into length and width increments and predetermined coating ratios. A total current is determined beforehand to ensure the coatings of the increments. The control process then includes the determination of a number of rectifiers to be set in operation, the determination of an estimated current for each rectifier by dividing the total current equally among the said rectifiers to be set in operation, and the determination of current set points to be applied to the rectifiers to be set in operation. The process is cyclic for each of the faces of the strip (1). <IMAGE>

Description

The present invention relates generally to the electro-deposition of metal on a continuously moving metal strip.

It relates more particularly to a method for regulating the electrolytic deposition of a metal coating on a metal strip forming a cathode and continuously scrolling at a determined speed in an electrolyte, in front of anodes arranged periodically. The anodes are supplied with direct current respectively by controllable rectifiers, and the coating deposited on the strip depends on the current delivered by each rectifier.

The technique of metal electrodeposition is used for example for the tinning of a metal strip, as described in patent FR-A-2 590 278 which relates more particularly to the regulation of the deposition of metal at the using a microprocessor. This document describes a process for regulating the quantity of a metal deposited electrolytically on a strip to be coated which runs continuously in a deposition installation comprising several reservoirs filled with electrolyte. The strip passes over a conductive roller forming a cathode associated with each reservoir and the coating metal is supplied by bars of said metal carried by conductive bridges, forming anodes, disposed in each reservoir over part of the path of the strip in said reservoir.

The method consists in calculating at each movement of the strip between two successive bridges, the metal deposition under each bridge as a function of the intensity of the supply current of this bridge, of the speed of the strip and of the efficiency of the bridge, at separately follow each strip length equal to the distance between two successive bridges by accumulating successive metal deposits, to establish the balance sheet of the deposit under the last bridge delivering current in order to determine the intensity necessary under this bridge to complete the deposit of metal, to determine the overall current intensity necessary to obtain the desired current intensity under this last bridge, and each time an average measurement is obtained over the entire width of the strip, to be calculated taking into account the distance from transfer, the difference between this average value and a preset preset value by determining a coefficient correcting the theoretical yields of the metal deposit under each bridge. The method includes in particular the measurement of the metal deposited on each face using a periodic scanning gauge, disposed at the outlet of the installation, the deposit being regulated from data delivered by the gauge.

This process reduces the sensitivity of the installation to speed transients, compared to manual installations. However, such a method does not perfectly solve the problem of coating transients.

The present invention aims to provide a method of electrolytic coating regulation which quickly adapts to coating transients, so that a small strip length is lost in the event of coating transients. Furthermore, the present invention aims to provide a coating regulation method insensitive to speed transients.

To this end, a method of regulating the electrolytic deposition of a metal coating on one of the sides of a metal strip, said metal strip forming a cathode and running continuously at a determined running speed in an electrolyte in front of anodes, the anodes being supplied respectively by controllable rectifiers having respective lower and upper current limits, the strip being divided into increments having a fixed length, each increment having a predetermined width and coating rate, the method starting with the determination of a total current necessary for coating the increments between the anodes as a function of said rates covering, widths and said running speed, is characterized by
the determination of a number of rectifiers to be put into operation as a function of the total current required and of the lower current limits of the rectifiers,
determining a forecast current for each rectifier by distributing the total current required among said rectifiers to be operated between said respective lower and upper current limits, and
the determination of current setpoints to be applied to the rectifiers to be put into operation, a current setpoint for a given rectifier supplying a respective anode depending on the bandwidth and coating rate of a band increment located in front of said respective anode and of the setpoints current of the rectifiers preceding the given rectifier and being less than the forecast current of said given rectifier. The process is carried out cyclically for each of the strip faces to be coated.

Advantageously, the total current required is the current required to coat one of the increments having the product width by the highest coating rate. Thus, an over-coating is imposed with respect to an under-coating in the vicinity of a weld between two successive strip portions having to support different thicknesses of coating.

According to another characteristic of the invention, the number of rectifiers to be put into operation depends on a current density desired for the coating, and a minimum current density below which it is not desired to descend for the coating. The current density influences the appearance of the coating.

In another aspect of the invention, the determination of a forecast current for each rectifier depends on a state in service or out of service of said each rectifier and of states consigned or not consigned of the rectifiers.

The cyclic determination of a forecast current for each rectifier does not depend on the set state of the rectifiers in the case where the equal distribution of the total current required is not possible only in rectifiers in the non-set state. In all cases, the determination of the predicted currents of the rectifiers results in a set of non-zero forecast currents associated with the rectifiers in operation and a set of zero predicted currents.

According to another characteristic, the cyclic determination of the current setpoints of the rectifiers results in a set of non-zero current setpoints associated with the rectifiers in operation and a set of zero current setpoints. A forecast current of a zero rectifier, respectively non-zero, results in a current setpoint for the rectifier zero, respectively non-zero.

According to yet another characteristic, the method comprises, for each rectifier, the calculation of the ratio of the current setpoint of said each rectifier and the speed of travel of the strip, and the assignment of this ratio to the increment of strip present in front of the anodes supplied by this rectifier.

The method comprises, for the increment of the band passing in front of the respective anode supplied by the given rectifier, the calculation of an experience which is the sum of the ratios of the current setpoints and of the running speed. The ratios are calculated for the rectifiers preceding the given rectifier and assigned to the band increment passing in front of the respective anode supplied by the given rectifier.

Advantageously, for each rectifier, the calculation of the current setpoint takes into account the experience of the band increment present in front of the anodes supplied by the rectifier.

Preferably, at least one rectifier is always in operation, and this rectifier is the last rectifier which supplies an anode after which the metal strip face is covered with the intended metal coating.

Advantageously, each rectifier in operation receives a current setpoint greater than or equal to a predetermined minimum value.

In another aspect of the invention, the first rectifier which feeds an anode before which the metal strip face is not yet subjected to electrolytic deposition is still in operation and it receives a current setpoint equal to a predetermined value.

• la figure 1 est un diagramme schématique d'une ligne d'étamage électrolytique ;
• la figure 2 est une vue en coupe longitudinale schématique d'une unité d'étamage incluse dans la ligne de la figure 1 ;
• la figure 3 est un bloc-diagramme du circuit de commande des redresseurs de l'unité d'étamage ;
• la figure 4 est un algorithme de détermination du courant total, du nombre de redresseurs à mettre en fonctionnement et du courant moyen par redresseur en fonctionnement, pour étamer une bande de métal;
• la figure 5 est un algorithme de détermination des courants prévisionnels des premier et dernier redresseurs de l'unité d'étamage;
• la figure 6 est un algorithme de détermination des courants prévisionnels des redresseurs autres que les premier et dernier redresseurs de l'unité d'étamage;
• la figure 7 est un algorithme de correction des courants prévisionnels des redresseurs; et
• la figure 8 est un algorithme de détermination des consignes de courant appliquées aux redresseurs.

Other characteristics and advantages of the present invention will appear more clearly on reading the following description of several preferred embodiments of the invention with reference to the corresponding appended drawings in which:

• Figure 1 is a schematic diagram of an electrolytic tinning line;
• Figure 2 is a schematic longitudinal sectional view of a tinning unit included in the line of Figure 1;
• FIG. 3 is a block diagram of the control circuit of the rectifiers of the tinning unit;
• FIG. 4 is an algorithm for determining the total current, the number of rectifiers to be put into operation and the average current per rectifier in operation, for tinning a metal strip;
• FIG. 5 is an algorithm for determining the forecast currents of the first and last rectifiers of the tinning unit;
• FIG. 6 is an algorithm for determining the forecast currents of rectifiers other than the first and last rectifiers of the tinning unit;
• FIG. 7 is an algorithm for correcting the predicted currents of the rectifiers; and
• FIG. 8 is an algorithm for determining the current setpoints applied to the rectifiers.

With reference to FIG. 1, a steel tinning line upstream comprises a unwinding device 2, which unwinds coils of steel strip to be tin plated, a priori having different widths and lengths. A welding device 3 splits the strips of metal from a coil at the end of unwinding and from a coil at the beginning of unwinding, in order to constitute a continuous steel strip 1. The strip 1 passes through a loop tower 4 which comprises upper rollers 41 and lower rollers 42.

The strip 1 passes alternately over the upper and lower rollers, so that the loop tower acts as a strip accumulator. During the welding of the end of the strip of a reel with the start of the strip of the next reel, the running of the strip is interrupted at the level of the welding device 3. The lower rollers 42 of the loop tower 4 rise progressively towards the upper rollers 41, so as to reduce the amount of tape 1 accumulated and thus feed without interruption the part of the line downstream of the loop tower 4. When the welding operation is finished, the running of the tape from the new reel to the loop tower 4 resumes, and the loop tower again accumulates a quantity of tape by gradually lowering the lower rollers. Thus, the coil change and welding operations do not take place on the running of the strip downstream of the loop tower 4, which running remains continuous.

Following the loop tower, the strip 1 passes through a degreasing unit 5 and a stripping unit 6, in order to prepare the surface of the strip to be tin plated. The strip 1 then passes through a tinning unit proper 7, which will be described in detail with reference to FIG. 2. After the tinning, the strip enters a reflow device 8, in order to melt the tin to improve the bonding of tin and corrosion resistance. The strip 1 then passes through a chemical treatment device 9 to passivate the tinned surface, for example by chromate. Passivation also improves resistance to corrosion and in particular the adhesion of varnish to the tinned surface.

The strip 1 then reaches a second loop tower 10 and then to a shearing and winding device 11, which winds the strip into coils. The loop tower 10 acts as an accumulator opposite to that of the loop tower 4. When the shearing and winding device 11 finishes a reel, and before starting the next reel, the loop tower accumulates the tape to be wound until the winding device is ready to wind the next reel.

The tinned steel coils are then cut, shaped and assembled, for example to form food packaging.

With reference to FIG. 2, the tinning unit 7 comprises N identical and successive tanks 71₁ to 71 _N containing electrolyte, not shown, each tank constituting an elementary period of the unit 7. N is a positive integer , equal for example to ten or eleven. In order not to overload Figure 2, only the first tray 71₁, any tray along 71 _n and the last tray 71 _N have been completely represented, n being an integer index between 1 and N. In the bottom of each tray is rotatably mounted a deflector roller 72₁ to 72 _N under which continuously passes the strip 1 to be tin plated. The rollers 72₁ to 72 _N are made of non-conductive material. Above the transverse walls of the tanks, second rollers 73₁ to 73 _{N + 1} of conductive material stretch the strip and transfer it successively to all the tanks.

In each of the N trays 71₁ to 71 _N , the strip passes between two pairs of vertical anodes 74₁ and 75₁ to 74 _N and 75 _N , in the form of tin bars juxtaposed vertically in a support. Each anode faces a length portion of one of the upper and lower faces of the strip which constitutes the cathode of the electrolytic reaction. The rollers 73₁ to 73 _{N + 1} and therefore the strip 1 are referenced to the same cathode potential.

For the first tank 71₁, a rectifier R₁ supplies direct current through a current divider 78, the pair of anodes 74₁ facing a portion of the underside of the strip, and the pair of anodes 75₁ facing d 'an upper face portion of the strip. In any one of the following N-1 bins 71 _n , a controllable rectifier R _n supplies the two anodes facing the underside of the strip and an analog rectifier Re _n supplies the two anodes opposite the top face of the bandaged. According to an alternative embodiment not shown, each of the two pairs of anodes in the first tank is supplied by a respective rectifier, in a similar manner to the following N-1 tanks. According to another variant, in one or more tanks containing two pairs of anodes, each anode is supplied individually by a rectifier.

A device for measuring the running speed of the strip, not shown, for example in the form of a running pulse generator, is arranged in the tinning unit 7.

A given face, upper or lower of the strip 1, is tinned by electrolytic reaction between the cathode, that is to say the strip, and the succession of anodes facing the face. Each of the anodes being supplied with direct current by a rectifier, the electrolytic reaction between the strip face and the anode depends on the current supplied by the rectifier. Tinning on one side is independent of tinning on the other side, except for leakage currents.

At a given instant, the strip length developed in the tinning unit 7 between the upper end rollers 73₁ and 73 _{N + 1} is broken down into a series of N strip increments of fixed length. The length of an increment is for example equal to the length of strip included between the horizontal upper tangents of two successive upper rollers 73 _n and 73 _{n + 1} . A strip increment is characterized by its width and by the coating thicknesses specified on the faces of the increment. This increment length is independent of the location of the anode pairs: for example, a pair of anodes for coating the underside of the strip is located opposite the bottom roller 72; or the pairs of anodes or the anodes extend horizontally in a single large tank and are distributed between rollers between which the metal strip travels horizontally.

dans lequel :

• . INTENSITE est l'intensité de courant "reçue" par la face de l'incrément ;
• . VITESSE est la vitesse de défilement de la bande ;
• . k est une constante dépendant du métal déposé et des unités ;
• . LARGEUR est la largeur de l'incrément ;
• . REVETEMENT est la densité surfacique, dit également taux d'étamage, du revêtement souhaité de la face de l'incrément, exprimée en g/m² ; et
• . ρ est le rendement cathodique de l'électrolyse.

A band increment passes successively through all the electrolysis tanks, according to the embodiment illustrated in FIG. 2, that is to say that it receives on each face deposits of tin resulting from the electrolysis determined by l intensity of rectifiers. For the coating specified on one side of the strip increment to be achieved, the ratio must be checked

in which :

• . INTENSITY is the current intensity "received" by the face of the increment;
• . SPEED is the tape speed;
• . k is a constant depending on the deposited metal and the units;
• . WIDTH is the width of the increment;
• . COATING is the surface density, also called tinning rate, of the desired coating of the face of the increment, expressed in g / m²; and
• . ρ is the cathodic efficiency of electrolysis.

With reference to FIG. 3, a control circuit for controlling the rectifiers of the tinning unit 7 comprises a control unit 30, connected to the tinning unit 7, in order to receive measurement information for purposes displays and transmit commands, including regulating rectifiers. The control unit is also connected to a programmable memory 31 in which the regulation algorithm and the values calculated by this program are stored. Finally, the control unit is connected to an operator interface 32, comprising for example a screen, an alphanumeric keyboard and a printer, so that an operator enters data relating to the strip to be tinned and reads the various parameters in real time. tinning and data concerning the tape transmitted to memory 31 by computer means external to the control circuit.

With reference to FIGS. 4, 5, 6, 7 and 8, the determination of the current setpoints of the rectifiers of the tinning unit 7 is explained below. in accordance with the invention, which is composed of operating steps of an algorithm programmed in the memory 31. This algorithm comprises a first preset part E4O to E76, which determines the total current necessary to be applied to a tape face by l '' set of rectifiers, the number of rectifiers to be put into operation and the estimated current intensity delivered by each rectifier. The algorithm includes a second calculation part E8O to E87 during which the setpoint of each rectifier is calculated as a function of the total current to be applied to the face and the current already applied by the preceding rectifiers.

The algorithm is run cyclically, according to a predetermined period, for example 500 ms, for each of the faces to be tinned. The algorithm running period is fixed and is in particular independent of the fixed length of the strip increments or of the variable speed of travel of the strip in the tinning unit 7.

In the following description, only one side to be tin plated is considered, the rectifiers associated with this face, whether it is lower or higher, are denoted R₁ to R _N.

Referring more particularly to FIG. 4, the algorithm for determining the total current to be applied, the number of rectifiers to be put into operation and the average current per rectifier in operation, for tinning a strip face comprises the following steps E40 to E43 .

• de la largeur de chaque incrément de bande : LARGEUR(n) avec 1≦n≦N, en considérant les N incréments de bande présents à un instant donné dans une unité d'étamage 7 à N bacs 71₁ à 71_N,
• du taux de revêtement spécifié de chaque incrément, en g/m² : RSPECIFIE(n), avec 1≦n≦N,
• de l'intensité de courant maximale débitée par chaque redresseur, en ampères : IBUTEE(n), avec 1≦n≦N,
• de la vitesse actuelle de défilement de la bande : VACTU,
• de la table de rendement cathodique en fonction du nombre de redresseurs en fonctionnement et de l'intensité de courant moyenne débitée par ces redresseurs : TAB(ρ), et
• de la densité de courant en A/dm², à laquelle l'opérateur souhaite travailler et de la densité de courant minimale en A/dm², en deçà de laquelle l'opérateur ne souhaite pas travailler : DENS et DENSMIN, respectivement.

Step E40 is reading from memory:

• the width of each strip increment: WIDTH (n) with 1 ≦ n ≦ N, considering the N strip increments present at a given time in a tinning unit 7 to N trays 71₁ to 71 _N ,
• the specified coating rate of each increment, in g / m²: RSPECIFIE (n), with 1 ≦ n ≦ N,
• the maximum current intensity delivered by each rectifier, in amperes: IBUTEE (n), with 1 ≦ n ≦ N,
• the current tape speed: VACTU,
• the cathodic efficiency table as a function of the number of rectifiers in operation and the average current intensity drawn by these rectifiers: TAB (ρ), and
• the current density in A / dm², at which the operator wishes to work and the minimum current density in A / dm², below which the operator does not wish to work: DENS and DENSMIN, respectively.

The values of WIDTH (n) and RSPECIFIE (n) depend on the strip to be tinned and the desired coating on the strip face. The values of IBUTEE (n) are defined by the operator for each rectifier and represent a current limitation (clamping) of the rectifiers. The current tape speed VACTU is taken cyclically from the tape speed measuring device to be stored. The cathodic yield table is stored in memory.

The DENS current density and the minimum DENSMIN current density are defined by the operator. These last two parameters are considered to improve the appearance of the tinned strip. In fact, phenomena called "white edges", for example, which are to be avoided, are linked to the current density received by the strip, independently of the total intensity received, therefore of the coating thickness.

Step E41 includes the calculation of the following products relating to the N increments of band 1 present in unit 7: WIDTH (n) x RSPECIFIED (n) for 1 ≦ n ≦ N, and the determination of the largest of these products , noted L x R. The search for this larger product is justified by imposing an overcoat compared to an undercoat in the vicinity of a weld between two successive strips having to withstand different coating thicknesses.

   dans lequel RSPECIFIE AUTRE FACE(n) désigne le taux de revêtement spécifié pour l'autre face de l'incrément de bande et A, B, C sont des constantes. La constante B dépend du courant de fuite qui existe entre les deux faces et du taux de revêtement spécifié sur la face considérée.As a variant, RSPECIFIE (n) is replaced by a coating rate RVISE (n), previously calculated as follows:

$\text{RVISE (n) = A x RSPECIFIE (n) + B x RSPECIFIE OTHER FACE (n) + C,}$

where RSPECIFIES OTHER SIDE (n) denotes the coating rate specified for the other side of the band increment and A, B, C are constants. The constant B depends on the leakage current that exists between the two faces and on the coating rate specified on the face considered.

In step E41 are also calculated current intensity values IDENS and IDENSMIN corresponding to the current densities DENS and DENSMIN from the width corresponding to the product L x R determined from RSPECIFIE (n) or as a variant RVISE ( not).

Finally the cathodic yield ρ _o is initially fixed at 1.

Step E42 includes the calculation of the total current which is necessary to carry out the coating of the most demanding increment, that is to say that corresponding to the product L x R:

In step E42 is also calculated a number NBR of rectifiers to be put into operation, in addition to the first rectifier R₁, in dependence on the total current calculated ITOTAL; the first rectifier must be put into operation and delivers the IBUTEE constant current intensity (1), and each following rectifier put into operation delivers the IDENS current intensity:

In this relation, ENT designates the whole part of the quotient of the division (ITOTAL - IBUTEE (1)) / IDENS. The decimal part of the previous division is designated by DEC.

alors le nombre NBR est majoré de 1.If the decimal part DEC is equal to or greater than the ratio

then the NBR number is increased by 1.

Step E43 includes the calculation of the average current intensity IMOY delivered by each of the rectifier NBRs put into operation, the first rectifier delivering IBUTEE (1):

In step E43, the convergence of the calculations is also verified, by determining the cathode efficiency ρ₁ as a function of the current intensity IMOY and the number of rectifiers in NBR operation, with the cathode efficiency table TAB (ρ).

If the difference (ρ _O - ρ₁) is greater in absolute value than a predetermined threshold, the calculations are restarted, starting from the calculation of ITOTAL (step E42), with the new value of cathodic efficiency ρ₁. The values of ITOTAL, NBR and IMOY are recalculated and a new cathodic yield is determined from the table TAB (ρ); the calculations are restarted as many times as necessary to ensure convergence. In practice, with a threshold set at 0.05, two or three iterations are sufficient. As a variant, the calculation of ITOTAL, NBR and IMOY is carried out only once, with a fixed cathodic efficiency, for example equal to 1, and the convergence of the calculations is not verified. This variant, although less precise, does not require the table TAB (ρ).

After determining the ITOTAL, NBR and IMOY values, the algorithm proceeds to the part for calculating the predicted current intensities of the rectifiers. This part is composed of steps E5O to E54 to determine the forecast current intensities of the first and last rectifiers R₁ and R _N , of steps E6O to E64 to determine the forecast current intensities of the other rectifiers R₂ to R _N-1 , and from steps E7O to E76 to correct the forecast current intensities.

• de l'état en service ou hors service de chaque redresseur : ETATREDESHS(n), avec 1≦n≦N, qui est une valeur logique valant "1" si le redresseur R_n est en service, c'est-à-dire susceptible de délivrer un courant, ou "O" sinon,
• de l'état consigné ou non consigné de chaque redresseur : ETATCONSIGNE(n), avec 1≦n≦N, qui est une valeur logique fixée par l'opérateur, valant "0" si le redresseur est consigné, c'est-à-dire si l'opérateur ne souhaite l'utiliser qu'en cas de nécessité absolue, ou "1" si le redresseur n'est pas consigné.

With reference to FIG. 5, step E5O consists of reading from memory:

• the state in service or out of service of each rectifier: ETATREDESHS (n), with 1 ≦ n ≦ N, which is a logical value equal to "1" if the rectifier R _n is in service, that is to say likely to deliver a current, or "O" otherwise,
• the logged or unregistered state of each rectifier: ETATCONSIGNE (n), with 1 ≦ n ≦ N, which is a logical value set by the operator, equal to "0" if the rectifier is logged, ie - say if the operator wishes to use it only in case of absolute necessity, or "1" if the rectifier is not locked out.

In order to better fix the ideas, any rectifier R _n with 1 ≦ n ≦ N can be in the following states.

It may be out of service, for example due to maintenance operations; it is not able to deliver a current and will not be put into operation.

It can also be in service, it is then able to deliver current.

If the rectifier R _n is in service, it can be not logged, it can then be selected without restriction by the algorithm to be put into operation, that is to say effectively deliver current.

If the rectifier R _n is in service, it can be logged, it will then be chosen by the algorithm to be put into operation only in case of absolute necessity, as will be explained below.

Working variables are initialized in step E51, as well as the values of the forecast current intensities delivered by the rectifiers R₁ to R _N : IPREV (n), with 1 ≦ n ≦ N.

• INTAREPARTIR : intensité de courant à répartir, initialisée à la valeur ITOTAL calculée à l'étape E42;
• NB : nombre de redresseurs à utiliser, en plus du premier, initialisé à la valeur NBR calculée à l'étape E42;
• CUMULAREPARTIR : cumul des intensités de courant qu'il est impossible de faire débiter aux redresseurs, en raison de leur limitation en courant, initialisé à la valeur "0"; et
• CUMULDISPO : cumul des intensités de courant que peuvent encore débiter les redresseurs qui ne sont pas en butée, c'est-à-dire qui n'ont pas encore atteint leurs limitations en courant, initialisé à la valeur "O".

The working variables are:

• INTAREPARTIR: current intensity to be distributed, initialized to the ITOTAL value calculated in step E42;
• NB: number of rectifiers to be used, in addition to the first, initialized to the NBR value calculated in step E42;
• CUMULAREPARTIR: cumulation of current intensities which it is impossible to charge the rectifiers, due to their current limitation, initialized to the value "0"; and
• CUMULDISPO: cumulation of the current intensities which the rectifiers which are not yet able to stop, that is to say which have not yet reached their current limits, initialized to the value "O".

Step E52 relates to the calculation of the forecast current IPREV (1) delivered by the first rectifier R₁ of the tinning unit 7. As already said, the first rectifier is always in operation, and delivers the constant current IBUTEE (1) defined by the operator and is therefore not regulated. As a variant, the first rectifier delivers the current IMOY, if the value IMOY is less than the value IBUTEE (1), or IBUTEE (1) otherwise. In both cases, the working variable INTAREPARTIR is updated.

The last rectifier R _N is used in priority, after the first rectifier R₁, and before all the other rectifiers R₂ to R _N-1. Step E53 determines the forecast current intensity IPREV (N) supplied by the last rectifier of the tinning unit. The current intensity IPREV (N) is equal to IMOY, if the value IMOY is less than the value IBUTEE (N), or equal to IBUTEE (N) otherwise. In the first case, the working variable CUMULDISPO is updated, taking into account the fact that a current intensity equal to IBUTEE (N) -IMOY can still be assigned to the rectifier R _N if necessary, as it will be exposed in the continuation. In the second case, the working variable CUMULAREPARTIR is updated, taking into account that the current intensity equal to IMOY-IBUTEE (N) cannot be assigned to the rectifier R _N , and remains to be distributed to the other rectifiers.

In step E54, the working variables INTAREPARTIR, representing the current intensity remaining to be distributed, and NB, representing the number of rectifiers remaining to be tested, are updated.

The algorithm then proceeds to steps E6O to E64 to calculate the forecast current intensities of the other rectifiers R₂ to R _N-1 .

Referring to Figure 6, the rest of the current to be distributed, equal to ITOTAL-IPREV (1) -IPREV (N) is distributed over (NBR-1) rectifiers among rectifiers R₂ to R _N-1 , which are in service and not logged.

In step E60, the parameter n is initialized to 2, corresponding to the rectifier R₂, then in step E61 the variable INTAREPARTIR is compared to zero to find out whether current remains to be distributed. If the result is "no", the flow of the algorithm goes directly to step E80 which is described below. If the result is "yes", the on / off state and the set state or not of the rectifier R _n are tested. If the rectifier R _n is either out of service, or logged, then the algorithm goes to step E64. If the rectifier R _n is in service and not logged, then the forecast current intensity IPREV (n) of the rectifier R _n is calculated in step E62. The variables CUMULAREPARTIR or CUMULDISPO are calculated and updated in the same way as in step E53 relating to the rectifier R _N. Then in step E63 the current remaining to be distributed is updated, the number NB of rectifiers remaining to be tested is updated and tested.

If the number NB is zero, NBR rectifiers other than the rectifier R₁ have been tested, and the algorithm proceeds to step E70 which is described below.

Step E64 increments the parameter n, and if n is less than or equal to the value N-1, that is to say if at least one rectifier remains to be tested, the algorithm is looped back to step E61 previously described. Otherwise, the algorithm goes to step E75 which is described below.

Step E62 for determining the forecast current intensities IPREV (n) is carried out as long as there is current to be distributed, according to step E61, as long as the number NBR of rectifiers to be put into operation is not reached according to step E63, and as long as all the rectifiers R₂ to R _N-1 have not been tested according to step E64.

If all of the current intensity to be distributed ITOTAL is distributed over at most NBR + 1 rectifiers, comprising rectifier R₁, rectifier R _N and NBR-1 rectifiers among rectifiers R₂ to R _N-1 , the estimated current intensities calculated IPREV (n) with 1 ≦ n ≦ N are stored. Certain IPREV (n) current intensity may be zero, for example when a rectifier is out of service, or when NBR + 1 is strictly less than N.

If all the current intensity to be distributed could not be distributed, the distribution is restarted, several cases are then possible.

With reference to FIG. 7, steps E70, E71, E72, E73 and E74 are traversed when the number of rectifiers to be put into operation is reached and the current to be distributed could not be fully distributed on the rectifiers in operation, i.e. when CUMULAREPARTIR remains strictly positive, and when the rectifiers already in operation are still available to deliver the current of current remaining to be distributed, that is to say that the value of CUMULDISPO is greater than or equal to CUMULAREPARTIR. The current intensity distribution then consists in saturating rectifiers in operation at their respective value IBUTEE (n) from the rectifier R₂. When these conditions are verified in step E70, the variable n is initialized to 2 in step E71 to perform all of the steps E72, E73 and E74, rectifier by rectifier, from rectifier R₂.

In step E72, the calculated predicted current intensity IPREV (n) is tested; if it is zero, the rectifier R _n is not in operation and n is incremented by 1 to go to the next rectifier. If the current IPREV (n) is not zero, step E73 checks whether the current intensity remaining to be distributed CUMULAREPARTIR is greater than or equal to what it is still possible to assign to the rectifier R _n , that is ie IBUTEE (n) -IPREV (n). Depending on the result of step E73, the forecast current intensity IPREV (n) is recalculated in step E74.

If the current intensity remaining to be distributed CUMULAREPARTIR is no longer less than the value IBUTEE (n) -IPREV (n), then the current intensity IPREV (n) is increased by the value CUMULAREPARTIR. Thus, there is no longer any current intensity to distribute and the calculation of the forecast current intensities IPREV (n) is finished. The algorithm goes to step E80, which is described below.

If the current intensity remaining to be distributed CUMULAREPARTIR is greater than or equal to the current value IBUTEE (n) -IPREV (n), then IPREV (n) takes the value IBUTEE (n) and CUMULAREPARTIR is reduced by IBUTEE (n) - IPREV (n). As in this case there is still current intensity to be distributed CUMULAREPARTIR, the calculation is repeated for the following rectifier from step E72, and continues until the rectifier R _N-1 at most.

When the conditions of step E70 are not verified, that is to say the value of CUMULAREPARTIR is greater than the value of CUMULDISPO, or else when it is not possible to make the value of CUMULAREPARTIR zero saturating rectifiers in operation in step E74, or again when in step E64 all the rectifiers from R₂ to R _N-1 have been tested and the number of rectifiers to be put into operation has not been reached, because that there are too many registered rectifiers, step E75 checks whether rectifiers are registered. If the result is positive, step E76 deconsigns all the recorded rectifiers from R₂ to R _N-1 . If the result is negative, the number NBR of rectifiers to be put into operation is increased by 1, insofar as there are still available, the average current intensity IMOY is recalculated with this new value of NBR. In both cases, after step E76, the calculation is resumed in step E51, either with all the rectifiers disconnected, or with new values of NBR and IMOY.

With reference to FIG. 8, the steps E8O to E87 are described for determining the current intensity setpoints to be applied to each rectifier.

du premier redresseur est calculé puis est mémorisé incrément de bande par incrément de bande au cours du déroulement de la bande; en d'autres termes ce rapport est appliqué à l'incrément de bande présent devant les anodes alimentées par le redresseur R₁.In step E80, the current intensity setpoint IREF (1) of the first rectifier R₁ is equal to IBUTEE (1) since, as already explained according to this preferred embodiment, no regulation is provided for this rectifier which always operates in fixed current. The intensity / speed ratio $\text{[I / V] REF (1) = IBUTEE (1) / VACTU}$

of the first rectifier is calculated then is memorized band increment by band increment during the unwinding of the band; in other words this ratio is applied to the band increment present in front of the anodes supplied by the rectifier R₁.

The "lived" deposition ratio [I / V] VECU (n) is associated with each band increment, which is representative of the previous successive deposits on the increment and which is equal to the sum of the ratios [I / V] REF (i), with 1 ≦ i ≦ n-1, that the increment opposite the anodes 74 _n or 75 _n supplied by the rectifier R _n has "received" from the previous rectifiers. For a given band increment, the ratio [I / V] VECU (n) can be the sum of the means of each term [I / V] REF (i), with 1 ≦ i ≦ n-1.

The current intensity setpoints IREF (n) for 2 ≦ n ≦ N-1 are calculated from step E81. All the following calculations are carried out successively from rectifier R₂ to rectifier R _N-1 . As a variant, the order of the calculations can be reversed. Step E81 initializes the calculation for the rectifier R₂. Then step E82 tests whether the predicted current intensity IPREV (n) of the rectifier R _n , with 2 ≦ n ≦ N-1, calculated and finally memorized in step E62 or E74, is positive.

If the value IPREV (n) is zero, then the rectifier R _n is not in operation, and the values IREF (n) and [I / V] REF (n) are zero (step E83). The calculation for the rectifier R _n is finished, and n is incremented by 1 (step E86) and if n is not equal to N, the calculation is restarted from step E82.

, où la valeur de R(n) vaut RSPECIFIE(n) ou RVISE(n) selon les variantes. Le terme IPREV(N) résulte du fait que le redresseur R_N est toujours en fonctionnement.If the value IPREV (n) is not zero, then step E83 determines the value of [I / V] VECU (n) which is the sum of the terms [I / V] REF (i), 1 ≦ i ≦ n-1, that the increment opposite the anodes 74 _n or 75 _n supplied by the rectifier R _n has "received" from the previous rectifiers R₁ to R _n-1 . Then, the current intensity setpoint IREF (n) is then equal to the product of the difference between [I / V] VISE (n) and [I / V] VECU (n) by the measured running speed VACTU, reduced by IPREV (N). In this calculation, [I / V] VISE (n) is equal to $\text{(kx WIDTH (n) x R (n)) / ρ}$

, where the value of R (n) is worth RSPECIFIE (n) or RVISE (n) according to the variants. The term IPREV (N) results from the fact that the rectifier R _N is always in operation.

The current intensity setpoint IREF (n), if it is not zero, is bounded by IDENSMIN and IPREV (n). If IREF (n) leaves this range by lower value, respectively higher, IREF (n) is worth IDENSMIN, respectively IPREV (n). As a variant, the IREF (n) setpoint can be limited to an IMTMAX (n) value, which designates the maximum current flowable by the rectifier R _n , which can be saturated in voltage.

, comme exposé précédemment. A l'étape E86, le paramètre n est incrémenté de 1 pour passer au redresseur suivant, et si n n'est pas égal à N, le calcul pour le redresseur suivant commence à partir de l'étape E82.The value [I / V] REF (n) is calculated in step E85 and equals IREF (n) / VACTU. The value [I / V] REF (n) is memorized band increment by band increment, that is to say it is assigned to the band increment present in front of the anodes supplied by the rectifier R _n . The value [I / V] REF (n) is used to calculate the value of $\text{[I / V] LIVE (n + 1)}$

, as previously discussed. In step E86, the parameter n is incremented by 1 to go to the next rectifier, and if n is not equal to N, the calculation for the next rectifier starts from step E82.

When all the setpoints IREF (2) to IREF (N-1) for the rectifiers R₂ to R _N-1 have been calculated, the current intensity setpoint IREF (N) for the rectifier R _N is calculated at next step E87.

. Le terme $\text{[I/V]VISE(N) vaut (k x LARGEUR(N) x R(N))/ρ}$

, où R(N) est RSPECIFIE(N) ou RVISE(N), suivant les variantes.Step E87 determines the "lived" [I / V] VECU (N) which is the sum of the terms [I / V] REF (i), with 1 ≦ i ≦ N-1, that the increment opposite the 74 _N or 75 _N anodes supplied by the rectifier R _N received previous rectifiers. Then the current intensity setpoint IREF (N) of the last rectifier R _N in unit 7 is calculated. The setpoint IREF (N) is equal to $\text{([I / V] VISE (N) - [I / V] VECU (N)) x VACTU}$

. The term $\text{[I / V] VISE (N) is (kx WIDTH (N) x R (N)) / ρ}$

, where R (N) is RSPECIFIE (N) or RVISE (N), according to the variants.

The IREF (N) setpoint is never zero, and must be between IDEMSMIN, minimum intensity determined in step E41, and IBUTEE (N). If IREF (N) leaves this range by lower value, respectively higher, then the value IDENSMIN, respectively IBUTEE (N), is assigned to it. After calculating IREF (N), the term [I / V] REF (N) is determined. When all the current intensity setpoints have been calculated, the setpoints are applied to the rectifiers.

Then another cycle, typically 5OO ms, is again implemented from step E4O.

As already said, the regulation method according to the invention is twofold when coatings, different or not, are to be deposited on the two faces of the metal strip. Each of the two processes takes place in parallel with the other and is associated with respective initial parameters, such as RSPECIFIE, IBUTEE, DENS, DENSMIN, etc., relating to the coating itself and to the rectifiers connected to the anodes opposite the associated strip face.

The regulation method according to the invention has been described with reference to a tinning line, but it applies to any type of electrolytic deposition, such as electrogalvanizing.

Claims (12)

Method for regulating the electrolytic deposition of a metal coating on one of the faces of a metal strip, said metal strip forming a cathode and running continuously at a determined running speed (VACTU) in an electrolyte in front of anodes (74₁ to 74 _N ; 75₁ to 75 _N ), the anodes being supplied respectively by controllable rectifiers (R₁ to R _N ) having respective lower and upper current limits (IBUTEE (1) to IBUTEE (N); IDENS), the strip being divided into increments having a fixed length, each increment having a predetermined width and coating rate (WIDTH (n), R (n)), the method starting with the determination (E42) of a total current ( ITOTAL) necessary to ensure the coatings of the increments between the anodes as a function of said coating rates, widths and running speed,
characterized by
the determination (E41) of a number (NBR) of rectifiers to be put into operation as a function of the total current required (ITOTAL) and of lower current limits (IDENS, IDENSMIN) of the rectifiers,
determining (E52 to E76) a forecast current (IPREV (1) to IPREV (N)) for each rectifier by distributing the total current required (ITOTAL) among said rectifiers to be operated between said lower and upper current limits respective, and
the determination (E80 to E87) of current setpoints (IREF (1) to IREF (N)) to be applied to the rectifiers to be put into operation, a current setpoint for a given rectifier (R _n ) supplying a respective anode (74 _n ; 75 _n ) dependent on the bandwidth and coating rate ([I / V] VISE (n)) of a band increment passing in front of said respective anode and of the current setpoints (IREF (1 ) to IREF (n-1)) rectifiers (R₁ to R _n-1 ) preceding the given rectifier and being less than the forecast current (IPREV (n)) of said given rectifier. Method according to claim 1, characterized in that it is carried out cyclically for each of the faces of the strip (1). Method according to claim 1 or 2, characterized in that the total current required (ITOTAL) is the current necessary to coat one of the strip increments having the product width by highest coating rate. Method according to any one of Claims 1 to 3, characterized in that the number of rectifiers to be put into operation (NBR) depends on a current density (DENS) desired for the coating, and on a current density minimum (DENSMIN). Method according to any one of Claims 1 to 4, characterized in that said determination (E61) of a forecast current for each rectifier (IPREV (1) to IPREV (N)) depends on a state in service or out of service service of each rectifier (ETATREDESHS (1) to ETATREDESHS (N)) and of consigned or not consigned states of rectifiers (ETATCONSIGNE (1) to ETATCONSIGNE (N)). Method according to claim 5, characterized in that said determination (E76) of a forecast current for each rectifier (IPREV (1) to IPREV (N)) does not depend on the recorded state (ETATCONSIGNE (1) to ETATCONSIGNE (N)) of the rectifiers in the case where the equal distribution of the total current required (ITOTAL ) is not possible only in rectifiers in an unrecorded state. Method according to any one of Claims 1 to 6, characterized in that a forecast current (IPREV (n)) of a zero rectifier (R _n , 2 ≦ n ≦ N-1), respectively non-zero, results in a current setpoint (IREF (n)) for the rectifier (R _n ) zero, respectively non-zero. Method according to any one of Claims 1 to 7, characterized in that the determination of the current setpoint for each rectifier (R _n ) comprises the calculation (E8O, E85, E87) of a ratio of the current setpoint (IREF (n)) of said each rectifier (R _n ) and of the running speed (VACTU) of the strip (1), and the assignment of this ratio to the strip increment present in front of the anodes supplied by this rectifier (R _n ). Method according to any one of Claims 1 to 8, characterized in that a current setpoint for said band increment (1) passing in front of the respective anode supplied by said given rectifier (R _n ) is determined by calculating ( E83, E87) an experience ([I / V] VECU (n)) which is the sum of ratios of the current setpoints (IREF (i), 1 ≦ i ≦ n-1) and of the frame rate (VACTU) , said ratios being calculated for the rectifiers (R (i), 1 ≦ i ≦ n-1) preceding the given rectifier and assigned to said band increment. Method according to any one of Claims 1 to 9, characterized in that at least the last rectifier (R _N ) is always in operation. Method according to any one of Claims 1 to 10, characterized in that each rectifier in operation (R _n ) receives (E84, E87) a current setpoint (IREF (n)) greater than or equal to a predetermined minimum value ( IDENSMIN). Method according to any one of Claims 1 to 11, characterized in that the first rectifier (R₁) is always in operation and receives (E8O) a current setpoint (IREF (1)) equal to a predetermined value (IBUTEE ( 1)).

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