EP0227517A1 - Process and apparatus for controlling the amount of metal electrolytically deposited on a continuously moving strip - Google Patents

Abstract

Process for regulating the quantity of metal electrolytically deposited on a continuously travelling band to be coated in a coating plant comprising a plurality of tanks filled with electrolyte. The process comprises determining experimental curves of the yield as a function of the strength of the supply current of each bridge of the plant, collecting (32) indications relating to the bridges in operation or out of operation, establishing analog values of the strength for each bridge and of the maximum strength of the current for all of the bridges, measuring the velocity of the travel of the band (37), establishing set values (39) relating to the quantity of metal to be deposited, measuring the total quantity of metal deposited by means of a gauge employing a periodic scanning, determining the lower and upper means of the quantity of metal measured by the gauge in each scan, and establishing a regulation model from the aforementioned data.

Description

The present invention relates to the technique of depositing an electrolytic coating on a continuously moving metal strip and relates more particularly to the regulation of metal deposition using a microprocessor.

It is known to proceed with the tinning of a strip by passing this strip successively through several reservoirs filled with electrolyte.

The tin is supplied to the tinning installation in the form of bars placed on a copper support serving as an anode.

By way of example, mention may be made of a tinning installation comprising twelve successive tanks.

With two supports or bridges per side of metal in each bin, there are a total of twenty four bridges per side.

The number of tin bars on each support depends on the width of the strip to be tinned.

The tin bars which are in fact consumable electrodes are mounted on conductive slides, which makes it possible to replace them when they are worn, continuously and without stopping the line.

In each tray are placed a lower rubber roller and a chromed upper roller between which the band is stretched. Together, they form the cathode of the corresponding tank.

The bridges are supplied with a direct voltage of 24V and receive a current limited to 4500 A.

The amount of tin deposited is a function of the width of the strip, of the running speed of the latter and of the overall current which is distributed over the different bridges in service.

v = vitesse de ligne en m/mn
l = largeur de la bande en mètres
E = taux d'étain en g/m2
n = rendementThe intensity of the current is given by the following relation taken from Faraday's law.

v = line speed in m / min
l = width of the strip in meters
E = tin content in g / m2
n = yield

In known installations, the operator sets the target tin rate by intervening directly on the global current (1G). It must first display the width of the strip. The tinning rate is kept constant by regulating the current to a value proportional to the line speed. However, this regulation does not prevent under-tinning and over-tinning during intermediate states (speed change, rate change, cut or addition of a bridge).

Indeed, the quantity of tin deposited is equal to:

In the stable state all the speeds of passage under the bridges are identical, so we have:

With each transient, however, this relationship is no longer true, as all of the vi can be different, and therefore the quantity of tin can differ by more than 20% from the target value.

In the recent past, the measurement of the rate of deposited tin was carried out as follows.

The operators displayed using a table, a current reference according to the coating to be made. A measurement was made by destructive testing. The current was then readjusted. This measurement took between a few minutes and three quarters of an hour and it was necessary to repeat these operations several times before obtaining a satisfactory result.

Given the inertia of the system, on short programs, the setting was often obtained at the end of the operation. In addition, to avoid litigation, we aimed at a rate higher than the nominal rate from the start. Hence the excessive cost of the tinning operation.

More recently, a continuous measurement gauge has been installed. This allows the measurement to be transcribed in the form of a graph via a screen. The operator can therefore immediately correct the errors.

This gauge works as follows.

The measurement is based on the principle of X-ray fluorescence. The gauge uses two sources of curium 244 with a radioactive period of 17.6 years. The energy released by the source causes an emission of fluorescent rays from iron, part of which is absorbed by tin. It is by determining the quantity of radiation remaining that the tin deposited is calculated.

The signal processing is as follows.
- Conversion of the exponential signal sent by the cells into a linear signal proportional to the coating.
- Calculation of the difference between the measurement and the nominal rate targeted.
- Possible correction of the signal value of plus or minus 5% depending on the aging of the sources for example.
- Finally, a microcomputer records the signals and transmits them to a cathode-ray screen located on the tinning line.

The gauge scans approximately every 30 seconds. Simultaneously, the transverse profiles of the coating appear, the instantaneous average measured values and those of the last scan, and the minimum threshold authorized by the standards in force for tinning operations such as EURONORM. For comparison, the last saved profile remains on the screen.

With the known techniques mentioned above, we are faced with the problem of the variation of the tin rate at each speed transient.

The invention therefore aims to create a method and a device for regulating the electrolytic deposition of a metal coating on a continuously moving strip of metal enabling these drawbacks to be remedied by taking into account the quantities of metal deposited by each bridge and by adapting the settings on the deposit line according to these quantities.

It therefore relates to a method of regulating the quantity of a metal deposited electrolytically on a strip to be coated continuously scrolling in a deposition installation comprising several reservoirs filled with electrolyte, the strip passing over a conductive roller forming cathode associated with each tank and the coating metal being supplied by bars of said metal carried by conductive bridges forming anodes arranged in each tank on a part of the path of the strip in said tank, characterized in that it consists in calculating at each movement of the strip between two successive bridges, the deposition of metal on each bridge in function of the intensity of the supply current of this bridge, of the speed of the strip and of the efficiency of the bridge, to follow separately each length of strip equal to the distance between two successive bridges by cumulating the successive metal deposits, to be established the balance sheet of the deposit under the last bridge delivering current in order to determine the intensity necessary under this bridge in order to complete the metal deposit, to determine the overall intensity necessary to obtain the desired intensity under this last bridge, and at each acquisition of an average measurement over the entire width of the strip, to be calculated taking into account the transfer distance, the difference between this average value and a preset preset value in determining nt a correction coefficient for the theoretical yields of the metal deposit under each bridge.

According to a particular characteristic of the invention, the process defined above also comprises the phases consisting in determining experimental curves of the yield as a function of the intensity of the supply current of each bridge of the installation, in collecting indications relating to bridges in service or out of service, to establish the analog values of the intensity on each bridge and of the maximum current intensity on all the bridges, to measure the speed of travel of the strip, to establish, set values relating to the quantity of metal to be deposited, to measure the overall quantity of metal deposited using a dipstick periodic lay-up, determining the lower and upper averages of the quantity of metal measured by the gauge on each scan and establishing from the above data a regulation model.

The invention will be better understood with the aid of the description which follows, given solely by way of example and made with reference to the appended drawings, in which:
- Fig.1 is a schematic perspective view with partial cutaway of a tinning tank used in the construction of a tinning installation to which the invention is applied;
- Fig.2 is a schematic top view of the tray of Fig.1;
- Fig.3 is a schematic view of implantation of the tin level measurement gauges in an installation to which the invention is applied;
- Fig.4 is a block diagram of a circuit for processing the data relating to the coating applied to the sheet and for developing the correction coefficients;
- Fig.5 is a flowchart of data acquisition operations relating to the deposited tin rates;
- Fig.6 is a flow diagram of the fast loop for controlling the tin deposit calculation operations on each bridge;
- Fig.7 is a flowchart for controlling the return of the gauge; and
- Fig.8 is a set of yield curves of the installation bridges for various supply currents.

In Fig.1, there is shown a tinning tank used in the construction of an installation tinning to which the invention is applied.

It should however be noted that the invention also applies to installations for the electrolytic deposition of coatings of metals other than tin such as chromium, copper or the like.

The tank comprises a reservoir 1 containing electrolyte not shown.

In the bottom of the tank is rotatably mounted a roller 2 on which passes continuously a strip B to be coated with a layer of tin. The roller 2 is made for example of rubber. Above the reservoir 1 is disposed a second roller 3, for example chrome-plated, of conductive material which ensures the tension of the strip and its transfer into the reservoir 1 from an identical reservoir, not shown, which, with other reservoirs of the same type, arranged upstream and downstream of the tank 1, is part of the tinning installation.

The roller 3 acts as a cathode associated with the reservoir 1.

A wringing roller (not shown) presses the strip B against the roller 3 in order to avoid the formation of electric arcs.

The strip B passes into the tank 1 between two pairs of supports 4 and 5 (Fig. 2) constituted by copper bars on which are placed side by side vertical tin bars 6 whose feet are engaged in a guide 7 U-shaped.

The copper bars 4 and 5 form slides for the tin bars and are connected to a corresponding bar 7 for supplying current.

The strip B therefore passes through two passages formed by the tin bars 6 carried by their corresponding supports 4 and 5, formed respectively on its descent and ascent path in the reservoir 1 filled with electrolyte.

The supports or bridges 4,5 and the tin bars 6 act as the anode of the device.

The container thus formed is carried by a frame 10 which also supports the other containers of the installation (not shown).

A lining 11 made of insulating material is interposed between the frame and the connection 12 of the supports 4,5 to the bar 7 for supplying current.

Downstream of the last tank of the installation is installed, a gauge formed by two cells arranged as shown in Fig.3.

At the exit of the installation, the strip B on the two faces of which a coating of tin has just been deposited passes over a deflector roller 15 opposite which is disposed a first cell 16 intended to measure the coating of tin of a first face of the loop B. The cell 16 comprises a source 17 of curium 244 placed on a support 18 mounted oscillating on a base 19 and movable around its axis of oscillation 20 by a pneumatic cylinder 21.

The strip B then passes over a second deflector roller 22 opposite which is disposed a second cell 23 similar to the cell 16 and intended to measure the coating of tin on the opposite face of the strip B.

This cell also includes a source 24 of curium 244 placed on a support 25 mounted oscillating on a base 26 and actuated by a pneumatic cylinder 27.

The outputs (not shown from the two cells 16 and 23 of the gauge are connected to corresponding inputs of the processing circuit of the Fig.4 which will now be described.

This circuit includes an analog-digital and digital-analog converter 30, for example of the ADAC 735 type which comprises, for an installation with twelve tinning tanks forty eight analog inputs 31 relating to the intensities of the currents applied to the supports of all the tanks, such as the bridges 4,5 of the tank in Fig. 1 and 2.

The converter 30 also comprises two analog inputs 32 intended to receive information on the position of the cells 16, 23 from the gauges and two analog inputs 33 intended to receive information relating to the average values of the deposits of tin on the two faces of the strip. .

The converter 30 further comprises an analog input 34 intended to receive signals relating to the width of the band B processed, two analog inputs 35 relating to the maximum upper and lower intensities and two analog outputs relating to the overall lower and upper intensity to be distributed on the bridges of the installation.

The converter 30 is connected to a multi-conductor bus 36.

The circuit of Fig.4 further includes a counter 37 whose input is connected to the output of a band B pulse generator (not shown) and which is also connected to bus 36, a circuit interface 38 of the SBC 519 type manufactured and sold by Intel, with thirty two digital inputs 39 relating to the lower and upper set values of the tin rate to be obtained, thirty two digital inputs 40 relating to the commercial set values, one input 41 for validation of automatic / manual operation and an input 42 of setpoint validation. Circuit 38 is also connected to bus 36.

Finally, the circuit in Fig. 4 includes a microprocessor 43 of the Intel 8088 type for example, connected to bus 36 and intended to control changes in the level of tin to be deposited in the various tanks of the installation as a function of the information qu 'he receives.

The operation of the installation will now be described with reference to Fig. 4 and to the flowcharts in Figs. 5 to 7.

A first phase of operation of the installation is the phase of acquiring information relating to the operation in progress.

The converter 30 receives on its forty-eight inputs measurements of the intensities on the bridges 4.5 of the twelve tanks of the installation.

During phase 50 of the flow diagram of FIG. 5, the converter 30 reads the currents on each of the bridges. This intensity information is transmitted to the microprocessor 43 which, during phase 51, calculates the values of the tin deposits under each bridge, taking into account the information on the speed of travel of the strip which is supplied to it by the counter 37, of the efficiency of each bridge and of the position of the gauge materializing the width of the strip, these two pieces of information being delivered by the converter 30.

During phase 52, the microprocessor 43 cumulates information relating to the deposit in progress with the previous deposit.

Then, as shown in the flow diagram of Fig. 6, there is determination of the last bridge depositing the tin. This operation is carried out at during phase 53 of the "fast loop" flowchart in Fig.6.

The information relating to the last bridge depositing tin during a sweep of the gauge is received on the analog inputs 31 of the converter 30.

During phase 54, the quantity of tin to be deposited by the last bridge is calculated from the information on set point rates, lower and higher to be obtained, entered by the operator on the inputs 39 of the interface circuit 38. Then, during phase 55, the microprocessor 43 calculates the approximate intensity necessary as a function of the information on the quantity of tin to be deposited by the last bridge and the information on bandwidth, the value of the coating measured by the gauge and the speed of travel of the strip it receives by the bus 36, coming from the converter 30 and the counter 37.

During phase 56, the microprocessor 43 calculates the efficiency of the bridge from the intensity calculated during phase 55 and this from pre-established curves shown in FIG. 8.

Then, during phase 57, the microprocessor calculates the necessary intensity corresponding to the yield determined during phase 56, taking into account the value of the coating measured by the gauge and the speed of travel of the strip.

During phase 58, there is a question about the difference between the intensity required and the intensity actually applied to the last bridge.

If the difference is small, there is a sending during phase 59 of signals corresponding to the calculated overall intensity which appear on the ana outputs. logic 36 of converter 30, this intensity being distributed over the various bridges of the installation.

Finally, during phase 60, the strip is brought forward by one step.

If the answer to the interrogation of phase 58 is no, the calculations of phases 56 and 57 are repeated on the data relating to the deposit of tin by a bridge located downstream until the difference in intensity is low.

The flowchart in Fig.7 is a "slow loop" flowchart which controls drift corrections.

The acquisition of a measurement made during phase 61 is the reading of the average tin deposit value made by the converter 30 of FIG. 4 at each end of scanning of the gauge of FIG. 3.

This phase is followed by an interrogation phase 62 relating to the transition from installation to automatic.

If the answer is no, we pass to an interrogation phase 63 relating to the start of the line.

If the answer to this new interrogation is no, a third interrogation is carried out during phase 64 with regard to the change in tin rate.

In the event of a negative response, the microprocessor 43 proceeds during phase 65, to the calculation of a gauge yield, that is to say of the ratio between the deposit of tin measured by the gauge and the deposit to be obtained.

If the answers to the three preceding interrogations are yes, one lets pass a sweep of the gauge and one proceeds to new interrogations.

During this time, the affirmative answer to the interrogation relating to the passage to automatic causes the validation of the automatic operation.

The affirmative response to the query relating to line starting controls the pulse generator (not shown) which is associated with the counter 37 in FIG. 4.

The affirmative response to the interrogation of phase 64 causes the setpoint to be validated via the interface circuit 38.

The process which has just been described has the following advantages over known processes.

It makes it possible to take into account all the transients such as the variation of the speed of travel of the strip, the stops or the putting into service of the bridges.

It takes into account the efficiency of the electrolysis under each bridge, which allows great precision in the direct obtaining of good tinning at each set point change.

This is particularly important in the case of thin coatings or when the maximum intensity of the bridges is low, since there are then yields which can be very low on the first bridges.

Current corrections are also small in absolute values and operator intervention is more precise.

Finally, it makes it possible to obtain a small difference between the tin deposit obtained and the set value.

An example is given below of the sequence of operations for regulating the deposit of tin in a tinning installation with twelve tanks and twenty four bridges.

A) Data entry

- Line speed
- Band width
- Targeted tin rate
- Current charged by bridge

B) Calculation of the number of theoretical bridges

First of all, you should know that each time the program is finished, the tape has traveled approximately 4 meters. This corresponds to a program step and to the distance separating the N bridge from the N + 1 bridge.

During the first step N = 1 and at each step we will add 1 to N. We will therefore ask each step to put an additional bridge downstream, at the maximum possible intensity.

C) Calculation of the tin deposited by bridge

At each step, the theoretical tin rate deposited under each bridge will be calculated.

Configuration example

In order to simplify the example, it will be assumed here that a bridge theoretically deposits 0.5 g / m² of tin on the metal.

D) Test on the rate obtained under the last bridge

Once the calculation of the deposited tin has been carried out, we will look at the rate obtained under the last bridge set to maximum intensity. Two cases of possible treatment depending on whether the rate is higher or lower than what is targeted. In digital applications this maximum intensity is taken at 4500 Amps.

E) Regulation for a rate higher than the target rate, if not a bridge upgrade.

In the first case, a regulation current (IC) will be calculated which will be applied to the last bridge.

Using the previous example and assuming that the target rate (TV) is 1.8 g / m², it will be seen in step 4 that the calculated rate (TC = 2 g / m²) is higher than the target rate TV. We will then calculate the correction C.
C = TC - TV

We will deduct the IC current required on deck 4 to obtain 1.8 g / m².

In the second case, we will add additional bridges to arrive at the first case.

F) Editing of results

When the calculations are finished, we send the requested current.

Continuation of the previous example (TV = 1.8 g / m²)

G) Change of pitch

Once the current is sent, we go to the next step:
P + 1

H) New data

We take into account the new data.

I) Tin gauge measurement

Then comes the measurement of the rate actually deposited (MJ).

This will determine the new Yield Gauge (RJ) which will be used in the calculations at the next step.
RJ - 3/4 (1 - (MJ / TV))

(The coefficient 3/4 is there to cushion the correction of the yield).

The actual measurement of the deposited rate does not take place at each step but at each sweep of the gauge.

Claims (5)

1. Method for regulating the quantity of a metal deposited electrolytically on a strip to be coated continuously passing through a deposition installation comprising several reservoirs (1) filled with electrolyte, the strip (B) passing over a conductive roller (3) forming a cathode associated with each reservoir and the coating metal being supplied by bars (6) of said metal carried by conductive bridges (4,5) forming anodes disposed in each reservoir on part of the path of the strip in said tank, characterized in that it consists in calculating (51) at each movement of the strip between two successive bridges, the metal deposition on each bridge as a function of the intensity of the supply current of this bridge, of the speed of the strip and the yield of the bridge, to follow separately each length of strip (B) equal to the distance between two successive bridges, by cumulating the successive metal deposits (52), to establish the balance sheet of the deposit under the last flow bridge of current (53) in order to determine the necessary intensity (55) under this bridge in order to complete the metal deposition, to determine the overall intensity necessary to obtain the desired intensity under this last bridge (57), and at each acquisition (61) of an average measurement over the entire width of the strip, to be calculated (65) taking into account the transfer distance, the difference between this average value and a preset preset value by determining a coefficient correcting the yields theory of metal deposition under each bridge. 2. Method according to claim 1, characterized in that it also comprises the phases consisting in determining experimental curves of the efficiency as a function of the intensity of the supply current of each bridge in the installation, to collect (32) indications relating to bridges in service or out of service, to establish the analogical values of the intensity on each bridge and the maximum current intensity on all the bridges, to measure the running speed of the strip (37), to establish, set values (39) relating to the quantity of metal to be deposited, to measure the overall quantity of metal deposited using a gauge (16, 23) with periodic scanning, determining the lower and upper averages of the quantity of metal measured by the gauge (16,23) at each scanning and establishing from above data a regulatory model. 3. Method according to any one of claims 1 and 2, characterized in that the metal whose electrolytic deposition is controlled is tin, chromium or copper. 4. Method according to any one of claims 1 to 3, characterized in that the electroplating of the coating of the strip taking place on both sides of the latter, the regulation of the deposition is ensured from data delivered by a gauge formed by two cells (16,23) each arranged on one side of the strip (B) at the outlet of the electrolytic deposition installation. 5. Device for implementing the method according to any one of the preceding claims, characterized in that it comprises an analog-digital, digital-analog converter (30) intended to receive analog data relating to the intensity supply currents to the bridges of the installation, at the value of the metal deposit measured by the gauge (16,23), at the position of this the latter and to the width of the strip (B) to be coated as well as to the maximum lower and upper intensities of the supply currents of the bridges and to transmit this data in digital form to a microprocessor (43) to which is also connected a counter (37) for the speed of travel of the strip (B) in the installation as well as an interface circuit (38) for transmitting to said microprocessor data (39) relating to the instructions for lower metal content and greater than obtaining, on the validation of automatic-manual operation (41) and on the validation of the setpoints (42), said converter (30) further comprising analog outputs intended for transmission, for the installation of instructions relating to the intensity of the supply currents to be applied to the bridges of the installation produced by the microprocessor (43) as a function of the data received.

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