WO2002014574A1 - A device and a method for controlling the thickness of a coating on a metallic object - Google Patents

A device and a method for controlling the thickness of a coating on a metallic object Download PDF

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Publication number
WO2002014574A1
WO2002014574A1 PCT/SE2001/001736 SE0101736W WO0214574A1 WO 2002014574 A1 WO2002014574 A1 WO 2002014574A1 SE 0101736 W SE0101736 W SE 0101736W WO 0214574 A1 WO0214574 A1 WO 0214574A1
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WO
WIPO (PCT)
Prior art keywords
stabilising
electromagnetic
plane
magnetic force
wiping
Prior art date
Application number
PCT/SE2001/001736
Other languages
French (fr)
Inventor
Bengt Rydholm
Sven Karlsson
Magnus HALLBÄCK
Göte Tallbäck
Per L. Eriksson
Original Assignee
Abb Ab
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority to SE0002889-4 priority Critical
Priority to SE0002889A priority patent/SE0002889L/en
Application filed by Abb Ab filed Critical Abb Ab
Publication of WO2002014574A1 publication Critical patent/WO2002014574A1/en

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Classifications

    • CCHEMISTRY; METALLURGY
    • C23COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23CCOATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/14Removing excess of molten coatings; Controlling or regulating the coating thickness
    • C23C2/24Removing excess of molten coatings; Controlling or regulating the coating thickness using magnetic or electric fields
    • CCHEMISTRY; METALLURGY
    • C23COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23CCOATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/003Apparatus, e.g. crucibles, heating devices

Abstract

The invention refers to a device and a method for controlling the thickness of a coating on a metallic object (2). The coating is applied by continuous transportation of the object from an arrangement (1) for depositing a molten metal onto the object. The metallic object (2) is intended to be transported from said arrangement in a transportation direction along a transportation path (4) including a plane (x). The device (8) includes an electromagnetic wiper device arranged to wipe off excessive molten metal from the object by applying a magnetic force onto the object (2). Moreover, the device (8) includes an electromagnetic stabilising device arranged to stabilise the position of the object (2) with respect to the plane (x).

Description

A device and a method for controlling the thickness of a coating on a metallic object
THE BACKGROUND OF THE INVENTION AND PRIOR ART
The present invention refers to a device for controlling the thickness of a coating on a running metallic object, said coating being applied by continuous transportation of the object through an arrangement for depositing a molten metal onto the object, the metallic object being intended to be transported from said arrangement in a transportation direction along a transportation path including a plane, wherein the device includes an electromagnetic wiper device arranged to wipe off excessive molten metal from the object. Moreover, the invention refers to a method for controlling the thickness of a coating on a metallic object, said coating being applied by continuous transportation of the object through an arrangement for depositing a molten metal onto the object.
Such a device is particularly advantageous in continuous galvanising of a steel strip. The present invention will hereinafter be described with reference to such an application. However, it is to be noted that the invention is also applicable to galvanising of other metal objects, such as wires, bars, pipes or any other elongated elements. Furthermore, the present invention is applicable to other coating processes, such as when applying somewhat thicker coatings by continuously casting a metallic coating onto an object, such as a metal strip, wire, bar, pipe or any other elongated element. During continuous galvanising of a steel strip, the steel strip is continuously passed through a bath containing molten metal, normally zinc. In the bath, the strip normally passes beneath a submerged roll and then moves upwardly through stabilising and correcting rolls. The strip emerges from the bath and is transported through a set of gas knives, which blow the excess zinc off the strip and back down to the bath, thus controlling the coating thickness. The gas ejected by the knives may be air, nitrogen, steam or an inert gas, but air and nitrogen are most commonly used. The strip is then transported unsupported until the coating has cooled down and is solidified. The coated steel strip is then guided or directed via a top roll to an arrangement for cutting the strip into separate strip elements or winding the strip onto a roll. Normally, the strip moves in a vertical direction from the submerged roll through the correcting and stabilising rolls and the gas knives to the top roll.
When galvanising steel strips, it is aimed at an even and thin coating thickness. The coating mass is normally measured after the strip has passed around the top roll, and this reading is used to control the gas knives and hence regulate the coating thickness. However, due to the geometry of the steel strip, the distance the strip has to run unsupported, its speed, and the blowing action of the gas knives, the steel strip will move or vibrate in a direction generally perpendicularly to its transportation direction. Certain measures, such as the use of the correcting and stabilising rolls, a precise control of the gas flow from the gas knives, and an adjustment of the steel strip speed and/or an adjustment of the distance over which the strip has to run unsupported, may be taken in order to reduce these transverse movements. If not reduced, these transverse movements will significantly disturb the precise wiping of the gas knives, resulting in an uneven coating thickness. A higher strip speed as well as an accurate and precise wiping of excessive coating material is required. However, the gas pressure from the gas knives has to be increased as the speed of the strip is increased. Such a pressure increase will result in sputtering of the molten coating and air turbulence problems at the edge of the strip. This means that there is a clear upper limit to the maximum strip speed when gas knives are used. Moreover, such gas knives are very noisy.
To solve these problems, it has been proposed to combine or replace the gas knives with a magnetic wiping device. The magnetic wiping device generates an alternating magnetic field which is used to wipe off excessive coating from the metal strip. Thereby, it may be dispensed with the gas knives or the gas pressure from the gas knives may be lowered while, at the same time, the same or even a better wiping effect is accomplished. The use of the magnetic wiping device helps to make it possible to increase the strip speed while maintaining an accurate and exact wiping off of excessive coating.
US-A-4 273 800 discloses such an electromagnetic wiper device for controlling the thickness of a deposited metal coating on a running metal strip. The excessive molten metal is wiped off from the strip by applying a pulsating or alternating magnetic flux to produce a flux in a looped path through the coating.
One problem connected with such magnetic wiper devices is that the magnetic forces acting on the metal strip, when the strip is not perfectly centred, tend to attract the magnetic strip towards the wiping coil. Consequently, the use of a magnetic wiper device may cause oscillation of the metal strip running from the bath to the top roll . US-A-4 655 166 discloses an arrangement for galvanising a metal strip, wherein air knives are provided for removing recesses molten metal from the strip. The arrangement includes an electromagnetic device for preventing oscillations of the running metal strip. The electromagnetic device includes permanent magnet units arranged in the vicinity of two opposite side edges on the running strip. Detectors are provided to detect the gap between the side edges and the respective magnet unit. In order to maintain the size of the gap at a determined level, control motors are provided to adjust the position of the magnet units in response to the detected gap.
SUMMARY OF THE INVENTION
The object of the present invention is to reduce oscillations of a running metal strip and to stabilise the position of the metal strip. More specifically, the purpose of the invention is to stabilise a metal strip in connection with the use of a magnetic wiper device for wiping off excessive molten metal from the strip.
This object is obtained by the device initially defined, which is characterised in that it includes an electromagnetic stabilising device arranged to stabilise the position of the object with respect to the plane. The inventors have found out that an electromagnetic stabilising device may be combined with an electromagnetic wiper device in order to reduce oscillations of the running metal object, which are at least partly caused by the alternating electromagnetic field generated by the electromagnetic wiper device. By this solution, the gas knives previously used may be dispensed with, and the electromagnetic wiper device may be operated to provide a highly efficient wiping off action, since the metal object is stabilised by the electromagnetic stabilising device.
According to an embodiment of the invention, the electromagnetic wiper device includes at least a first pair of wiping members, including one wiping member on each side of the plane at a determined distance from the plane. Preferably, each electromagnetic wiper member includes at least two wiping poles. By such an arrangement, a magnetic flux circuit may be provided to extend from one pole of one wiping member through the metal object to one pole of the other wiping member and through the other pole of the other wiping member through the metal object to the other pole of the one wiping member in a substantially closed loop.
According to a further embodiment of the invention, the electromagnetic stabilising device includes at least a first pair of electromagnetic stabilising members, including one stabilising member on each side of the plane at a determined distance from the plane. Preferably, each electromagnetic stabilising member includes at least two stabilising poles. In the same way as for the electromagnetic wiper device, a substantially closed flux loop may be provided for the electromagnetic stabilising device. Moreover, each electromagnetic stabilising member may form a substantially closed loop together with a portion of the metal object located between the two poles of the stabilising member.
According to a further embodiment of the invention, the wiping poles of the wiping member on one of said sides of the plane and the stabilising poles of the stabilising member on this side of the plane are arranged in such a way that at least one of said wiping poles is positioned between two stabilising poles. Consequently, after the stabilising pole on each side of the plane a wiping pole is provided in the transportation direction of the metal object, and after these wiping poles a stabilising pole is provided in such a manner that every second pole is a stabilising pole on each side of the plane. Preferably, the first pole seen in the transportation direction is a stabilising pole and the last pole seen in the transportation direction is a stabilising pole. By such an arrangement of stabilising poles and wiping poles, an efficient stabilising of the metal object is obtained. Especially it may be prevented that the metal object oscillates when leaving the electromagnetic wiper device.
According to a further embodiment of the invention, the electromagnetic wiper device and the electromagnetic stabilising device form an integrated unit. Although the electromagnetic wiping members and the electromagnetic stabilising members may be arranged as separate parts in the device according to the invention, such an integrated unit provides an elegant solution to the problem of providing an electromagnetic wiper ensuring a stable running of the metal object therethrough. Furthermore, by providing the wiper device and the stabilising device as an integrated unit it is possible to arrange the different poles relatively close to each other. It is advantageous to provide a stabilising pole close to the disturbance force due to mechanical reasons. The closer the stabilising pole is to the wiper pole the higher magnetic wiper force may be employed. Moreover, stabilising poles arranged close to each other prevent deflection of the object, such as a metal band or strip. Advantageously, the electromagnetic wiping members on one of said sides of the plane and the electromagnetic stabilising members on this side have a common magnetic flux carrying member, and thus form a common electromagnetic element. The electromagnetic element on one of said sides of the plane may be substantially identical to the electromagnetic element on the other side of the plane. Moreover, the electromagnetic element on one of said sides of the plane may be positioned essentially exactly opposite of the electromagnetic element on the other side of the plane.
According to a further embodiment of the invention, the device includes a sensor arranged to sense the value of a parameter depending on the position of the object with respect to the plane, wherein the electromagnetic stabilising device is arranged to apply a magnetic force to the object in response to the sensed value, the magnetic force including at least a force component directed transversely to the transportation direction and transversely to the plane. In such a way, it is possible to obtain a proper positioning of the object with respect to the plane so that the object runs along the plane in a stable manner.
According to a further embodiment of the invention, the device includes a control unit arranged to apply a voltage to the electromagnetic stabilising device in response to said sensed value, thereby generating said magnetic force. Preferably, the control unit may be arranged to increase the magnetic force of one of the electromagnetic stabilising members when the object moves in a transversal direction away from this electromagnetic stabilising member. Furthermore, the electromagnetic stabilising member may include at least one winding connected to said control unit, wherein the control unit is arranged to apply said voltage to the winding. Thereby, the sensor may be arranged to sense the level of the electric current through the winding. Such sensor does not need to contact the running metal object and the sensing function will not influence the position of the object. The control unit may be arranged to increase the magnetic force applied by the electromagnetic stabilising member when the level of the current through the corresponding winding increases . The current through the winding will increase when the magnetic reluctance of the magnetic flux circuit decreases, i.e. when the distance from the stabilising poles to the metal object increases.
According to a further embodiment of the invention the control unit is arranged to apply said voltage in form of voltage pulses having a substantially constant amplitude. Preferably said voltage pulses have a substantially square wave shape, which means that when a voltage is applied it is a DC voltage. Thereby, the control unit may be arranged to vary the magnetic force of the electromagnetic stabilising device by varying the length of the voltage pulses.
According to a further embodiment of the invention, the sensor includes a first sensor member arranged to sense the level of the current through the electromagnetic stabilising member on one of said sides of the plane and a second sensor member arranged to sense the level of the current through the electromagnetic stabilising member on the other side of the plane, wherein the control unit is arranged to compare the current levels of the electromagnetic stabilising members and to increase the length of the voltage pulses through the winding of the electromagnetic stabilising member having the highest current level. Preferably, the control unit includes a so-called DC-chopper.
The object of the invention is also obtained by the method initially defined and including the steps of: transporting the metallic object from said arrangement in a transportation direction along a transportation path including a plane, wiping off excessive molten metal from the object by applying a magnetic force to the object, and stabilising the position of the object with respect to the plane by applying a stabilising magnetic force to the object. Preferred embodiments of the method are defined in the dependent claims 22 to 28.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will now be explained in more detail with reference to different embodiments, disclosed by way of example only, and with reference to the drawings attached hereto.
Fig 1 discloses schematically an arrangement for applying a coating to a running metal strip.
Fig 2 discloses schematically a device for controlling the thickness of the coating on the metal strip according to the present invention.
Fig 3 discloses an electric circuit for an electromagnetic stabilising device of the device in Fig 2 according to one embodiment . Fig 4 discloses an electric circuit for an electromagnetic stabilising device of the device in Fig 2 according to another embodiment .
DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION
The arrangement disclosed in Fig 1 includes a container 1 arranged to contain a bath of molten metal to be deposited onto a metal strip 2 running through the bath in the container 1. The metal strip 2 is transported through the container 1 by means of a roll 3 provided in the container 1 to redirect the metal strip 2 from a downward direction to a substantially vertically upwardly directed transportation direction 4 along a transportation path between the roll 3 and an upper roll 5 arranged to redirect the metal strip 2. The transportation path between the roll 3, 5 includes a substantially vertical plane x. After the upper roll 5 the metal strip is wound onto a roll 6 or cut into strip elements . It is to be noted that further stabilising or correcting rolls may be provided to guide the metal strip 2. However, in order to make the embodiment more illustrative such possible rolls are not disclosed in the figures.
From the surface 7 of the metal bath in the container 1 the metal strip 2 runs generally unsupported all the way to the upper roll 5. As the metal strip 2 leaves the bath in the container 1 it is coated with molten metal from the bath. At a predetermined distance from the surface 7 a device 8 for controlling the thickness of the coating on the metal strip 2 is provided. After the device 8, seen in the transportation direction 4, there are provided elements 9 for cooling the coating before it reaches the upper roll 5. Thereby, the coating will be solidified before the metal strip 2 is redirected by the upper roll 5.
The device 8 for controlling the thickness of the coating is disclosed more closely in Fig 2 and includes an electromagnetic wiper device 11, 12, arranged to wipe off excessive molten metal from the metal strip 2 by applying a magnetic force onto the strip 2, and an electromagnetic stabilising device 13, 14, arranged to stabilise the position of the metal strip 2 with respect to the plane x. The electromagnetic wiper device includes a first pair of wiping members 11, 12 including a first wiping member 11 on one side of the plane x and a second wiping member 12 on the other side of the plane x. Each wiping member 11, 12 includes two wiping poles 11a, lib and 12a, 12b, respectively. The electromagnetic stabilising device includes a pair of electromagnetic stabilising members 13, 14, including a first stabilising member 13 on one side of the plane x and a second stabilising member 14 on the other side of the plane x. As appears from Fig 2, the wiping poles 11a and lib on one side of the plane x and the stabilising poles 13a, 13b and 13c on this side of the plane x arranged in an alternating order, i.e. in such a way that each wiping pole 11a and lib is positioned between two stabilising poles 13a and 13b, and 13b and 13c, respectively. The wiping poles 12a and 12b and the stabilising poles 14a, 14b, 14c on the other side of the plane x are arranged in a corresponding order. It is to be noted that the device 8 may include more or less stabilising poles and wiping poles than those disclosed in this embodiment. For instance, two wiping poles and two stabilising poles may be arranged in each side of the plane x.
By the device 8 the electromagnetic wiping device 11, 12 and the electromagnetic stabilising device 13, 14 are arranged to form an integrated unit. Thereby the electromagnetic wiping members 11 and the electromagnetic stabilising members 13 on one side of the plane x have a common magnetic flux carrying member 15, and thus form a common electromagnetic element 11, 13, 15. In the same way, the electromagnetic wiping member 12 and the electromagnetic stabilising member 14 on the other side of the plane x have a common magnetic flux carrying member 16, and thus form a further common electromagnetic element 12, 14, 16. Each of the magnetic flux carrying members 15, 16 includes an iron core of iron plates or iron powder and is designed to provide the desired number of poles, in the present case 5 poles. The poles are formed by portions of the respective member 15, 16 extending substantially perpendicular towards the plane x. It is to be noted that the stabilising poles 13a, 13b, 13c, 14a, 14b, 14c are positioned at a greater distance from the plane x than the wiping poles 11a, lib, 12a, 12b. Each such portion of the stabilising poles 13a, 13b, 13c, 14a, 14b, 14c is surrounded by a winding 17 arranged to induce a magnetic flux in the electromagnetic elements 11-16. In the same way each portion of the wiping poles 11a, lib, 12a, 12b is surrounded by a winding 18. The electromagnetic element 11, 13, 15 on one side of the plane x is substantially identical to the electromagnetic element 12, 14, 16 on the other side of the plane x. Moreover, the electromagnetic element 11, 13, 15 on one side of the plane x is positioned essentially exactly opposite to and at the same distance as the electromagnetic element 12, 14, 16 on the other side of the plane x.
The electromagnetic wiping device 11, 12 is arranged to provide an alternating magnetic flux circuit extending, in the embodiment disclosed, in a closed circuit from the pole 11a through the metal strip 2 to the pole 12a through the flux carrying member 16 to the pole 12a through the metal strip 2 to the pole lib and through the flux carrying member 15 back to the pole 11a. Preferably the alternating magnetic flux is provided by the application of an AC-voltage to the windings 18 at a frequency between 40-120 Hz, for instance 50, 60, 70, 80, 90, 100, 110 Hz. A too high frequency would heathen the metal strip 2 in an undesired manner. On the other hand, if the frequency is too low, the metal strip 2 would oscillate in an unacceptable manner. It is to be noted that the device for providing said AC-voltage is not disclosed in the figures.
Furthermore, the device 8 includes a sensor in the form of sensor members 21, 22, see Fig 3, arranged to sense the value of a parameter depending on the position of the metal strip 2 with respect to the plane x. In the embodiment disclosed, each sensor member 21, 22 is arranged to sense a current level through the respective winding 17 of the stabilising poles 13a, 13b, 13c, 14a, 14b 14c. The electromagnetic stabilising device 13, 14 is arranged to apply a magnetic force to the metal strip 2 in response to the sensed current level. The magnetic force includes at least a force component directed transversally to the transportation direction 4 and' the plane x. It is to be noted that Fig 3 merely discloses sensor members 21, 22 for two stabilising poles 13a and 14a, although also the other stabilising poles may be connected to similar sensor members. The magnetic force of the stabilising poles 13a, 14a are obtained by the application of a voltage to the winding 17 of the respective pole 13a, 14a. The voltage is applied by means of a control unit 23 connected to said windings 17 for applying said voltage. The control unit 23 is arranged to increase the magnetic force of one of the stabilising poles 13a, 14a when the metal strip 2 moves in a transversal direction away from the stabilising pole 13a, 14a. More specifically the control unit 23 is arranged to increase the magnetic force of one of the stabilising poles 13a, 14a when the level of the current through the corresponding winding 17 increases. The control unit 23 is arranged to apply said voltage in the form of voltage pulses, preferably square wave pulses having a substantially constant amplitude, and thereby vary the magnetic force of the stabilising pole 13a, 14a in question by varying the length of the voltage pulses.
Each stabilising pole 13a, 14a is arranged to induce a magnetic flux extending in a substantially closed loop. Thereby, one such loop extends mainly from the pole 13a to the metal strip 2 and in the metal strip along the plane x to the pole 13b and via the magnetic flux carrying member 15 back to the pole 13a. A corresponding loop extends on the other side of the plane x. The windings 17 of the poles 13a, 13b may be electrically connected to each other to form one winding 17. It is possible to dispense with the winding 17 of one of the poles 13a, 13b. In the same way a closed circuit may extend with respect to the poles 13b and 13c, and the poles 14b and 14c. For the latter magnetic flux loops a separate further control unit may be provided. The control unit 23 forms a so-called DC-chopper and includes a line rectifier 24 to be supplied with a three- phase AC-voltage. In parallel to the rectifier 24, a capacitance 25 is provided. The windings 17 of the stabilising poles 13a, 14a are also connected in parallel to the rectifier 24 via the respective sensor member 21, 22 and a respective switching device represented by an anti- parallel diode 26, 27 and a one-way current conducting switch 28, 29 (the filled arrows indicate the current direction) . Furthermore, the control unit 23 includes a processor 30, which is connected to the sensor members 21, 22 and the switches 28, 29.
The device 8 operates in the following manner. The sensor member 21 and the sensor member 22 are arranged to sense the current level through the respective winding 17. The processor 30 is arranged to receive the sensed level from the sensor members 21, 22, and to initiate switching of the switches 28 and 29 between an open state, see switch 28, and a closed state, see switch 29. By closing the switch 28, 29, a voltage will be applied to the respective winding 17. Consequently, the processor 30 will apply voltage pulses to the windings 17 of a certain length, which pulses induces a current in the respective winding 17. By means of the sensor members 21, 22 the current level through the windings 17 is measured. The current level depends on the magnetic reluctance of the respective magnetic flux circuit. If the distance between the metal strip 2 and one of the poles 13a, 14a increases, the magnetic reluctance of the magnetic flux circuit decreases, which means that the current through the winding 17 will increase since the magnetic reluctance of the magnetic flux circuit is reciprocally proportional to the current through the winding 17.
As appears from Fig 4, the device according to the invention may be provided with sensor members of another type than those disclosed in Fig 3, for instance position sensors mounted to the respective stabilising pole 13a, 14a. As in the previous embodiments the sensor members 21, 22 are connected to the processor 30. It is to be noted that any kind of proximity sensors or possibly mechanical sensors sensing the position of the strip 2 by contact may be employed in the device according to the invention.
The present invention is not limited to the embodiment disclosed but may be varied and modified within the scope of the following claims .
It is to be noted the more than one device 8 may be provided along the transportation path. Especially, when the strip 2 has a width exceeding a certain value, two devices 8 may be provided along the width of the strip 2.

Claims

Claims
1. A device for controlling the thickness of a coating on a metallic object (2), said coating being applied by continuous transportation of the object through an arrangement (1) for depositing a molten metal onto the object, the metallic object (2) being intended to be transported from said arrangement in a transportation direction (4) along a transportation path including a plane (x) , wherein the device (8) includes an electromagnetic wiper device (11; 12) arranged to wipe off excessive molten metal from the object (2) by applying a magnetic force onto the object, characterised in that the device (8) includes an electromagnetic stabilising device (13; 14) arranged to stabilise the position of the object with respect to the plane .
2. A device according to claim 1, characterised in that the electromagnetic wiper device (11; 12) includes at least a first pair of wiping members, including one wiping member
(11, 12) on each side of the plane (x) at a determined distance from the plane.
3. A device according to claim 2, characterised in that each electromagnetic wiping member (11; 12) includes at least two wiping poles (11a, lib; 12a, 12b) .
4. A device according to any one of claims 1 to 3, characterised in that the electromagnetic stabilising device includes at least a first pair of electromagnetic stabilising members (13; 14), including one stabilising member (13; 14) on each side of the plane at a determined distance from the plane (x) .
5. A device according to claim 4, characterised in that each electromagnetic stabilising member includes at least two stabilising poles (13a, 13b, 13c; 14a, 14b, 14c) .
6. A device according to claims 3 and 5, characterised in that the wiping poles (11a, lib; 12a, 12b) of the wiping member on one of said sides of the plane (x) and the stabilising poles (13a, 13b, 13c; 14a, 14b, 14c) of the stabilising member on this side of the plane are arranged in such a way that at least one of said wiping poles is positioned between two stabilising poles .
7. A device according to any one of the preceding claims, characterised in that the electromagnetic wiping device (11; 12) and the electromagnetic stabilising device (13; 14) form an integrated unit.
8. A device according to claims 2, 4 and 7, characterised in that the electromagnetic wiping members (11; 12) on one of said sides of the plane (x) and the electromagnetic stabilising members (13; 14) on this side have a common magnetic flux carrying member (15; 16) , and form a common electromagnetic element.
9. A device according to claim 8, characterised in that the electromagnetic element (11, 13, 15) on one of said sides of the plane (x) is substantially identical to the electromagnetic element (12, 14, 16) on the other side of the plane.
10. A device according to any one of claims 8 and 9, characterised in that the electromagnetic element (11, 13, 15) on one of said sides of the plane (x) is positioned essentially exactly opposite to the electromagnetic element (12, 14, 16) on the other side of the plane.
11. A device according to any one of the preceding claims, characterised in that the device (8) includes a sensor (21, 22) arranged to sense the value of a parameter depending on the position of the object (2) with respect to the plane (x) , wherein the electromagnetic stabilising device (13, 14) is arranged to apply a magnetic force to the object in response to the sensed value, the magnetic force including at least a force component directed transversely to the transportation direction (4) and transversely to the plane (x) .
12. A device according to claim 11, characterised in that the device (8) includes a control unit (30) arranged to apply a voltage to the electromagnetic stabilising device (13; 14) in response to said sensed value, thereby generating said magnetic force.
13. A device according to claims 4 and 12, characterised in that the control unit (23) is arranged to increase the magnetic force of one of the electromagnetic stabilising members (13; 14) when the object (2) moves in a transversal direction (4) away from this electromagnetic stabilising member (13; 14) .
14. A device according to claims 5 and 13, characterised in that electromagnetic stabilising member (13; 14) includes at least one winding (17) connected to said control unit (30) , wherein the control unit is arranged to apply said voltage to the winding.
15. A device according to claim 14, characterised in that the sensor (21, 22) is arranged to sense the level of the electric current through the winding (17) .
16. A device according to claim 15, characterised in that the control unit (23) is arranged to increase the magnetic force applied by the electromagnetic stabilising member (13; 14) when the level of the current through the corresponding winding increases .
17. A device according to any one of claims 12 to 16, characterised in that the control unit (23) is arranged to apply said voltage in the form of voltage pulses having a substantially constant amplitude.
18. A device according to claim 17, characterised in that the control unit (23) is arranged to vary the magnetic force of the electromagnetic stabilising device (13; 14) by varying the length of the voltage pulses.
19. A device according to any one of claims 11 - 18, characterised in that the sensor (21, 22) includes a first sensor member (21) arranged to sense the level of the current through the electromagnetic stabilising member (13a) on one of said sides of the plane (x) and a second sensor member (22) arranged to sense the level of the current through the electromagnetic stabilising member (14a) on the other side of the plane (x) , wherein the control unit (23) is arranged to compare the current levels of the electromagnetic stabilising members (13a; 14a) and to increase the length of the voltage pulses through the winding (17) of the electromagnetic stabilising member (13a; 14a) having the highest current level.
20. A device according to any one of claims 12 to 19, characterised in that said control (23) unit includes a so called DC-chopper.
21. A method for controlling the thickness of a coating on a metallic object, said coating being applied by continuous transportation of the object through an arrangement for depositing a molten metal onto the object, the method including the steps of: transporting the metallic object from said arrangement in a transportation direction along a transportation path including a plane, wiping off excessive molten metal from the object by applying a magnetic force to the object, and stabilising the position of the object with respect to the plane by applying a stabilising magnetic force to the object.
22. A method according to claim 21, including the steps of: sensing the value of parameter depending on the position of the object with respect to the plane, and applying said stabilising magnetic force to the object in response to the sensed value, wherein the magnetic force includes at least a force component directed transversely to the transportation direction and transversely to the plane.
23. A method according to claim 22, including the step of: applying a voltage to the electromagnetic stabilising device in response to said sensed value, thereby generating said magnetic force.
24. A method according to claim 23, wherein the stabilising electromagnetic force is applied by applying voltage pulses to a winding of a stabilising pole adjacent to the plane.
25. A method according to claim 24, wherein said sensing step includes sensing of the level of the electric current through the winding.
26. A method according to claim 25, wherein said application step includes increasing of the magnetic force when the level of the current through the corresponding winding increases.
27. A method according to claim 26, wherein said application step includes applying of said voltage in the form of voltage pulses having a substantially constant amplitude, and varying the magnetic force of the electromagnetic stabilising device by varying the length of the voltage pulses.
28. A method according to any one of claims 24 - 27, wherein said sensing step includes sensing the level of the current through the winding of a stabilising pole provided on one of said sides of the plane and sensing the level of the current through the winding of a stabilising pole provided on the other side of the plane, and wherein said current levels are compared and the length of the voltage pulses through the winding of the stabilising pole having the highest current level are increased.
PCT/SE2001/001736 2000-08-11 2001-08-10 A device and a method for controlling the thickness of a coating on a metallic object WO2002014574A1 (en)

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