WO2008078910A1

WO2008078910A1 - Method and apparatus for controllin coating weight in continuous galvanizing process

Info

Publication number: WO2008078910A1
Application number: PCT/KR2007/006707
Authority: WO
Inventors: Yeon Tae Kim; Gyu Sam Hwang; Gun Young Kim; Byung Hak Kim; Hae Doo Park
Original assignee: Posco
Priority date: 2006-12-22
Filing date: 2007-12-21
Publication date: 2008-07-03
Also published as: KR100815814B1

Abstract

There is provided a method and apparatus for controlling a coating weight in a continuous galvanizing process, the method and apparatus capable of effectively controlling a coating deviation of right and left of a steel strip, a coating weight deviation of front and rear surfaces of the steel strip, and a deviation from a target coating weight, caused by various disturbances, based on simplified coating weight estimation model. To control a coating weight by adjusting a gap between front and rear air knives and the steel strip and wiping pressure, a pressure setting value and an air knife gap setting value are adjusted to compensate occurrence of a coating weight deviation caused by occurrence of a skew and parameters are updated from actual measurement data to reduce an error of the coating weight estimation model.

Description

METHOD AND APPARATUS FOR CONTROLLIN COATING WEIGHT IN CONTINUOUS GALVANIZING PROCESS

Technical Field

[1] The present invention relates to a continuous hot-dip galvanizing process of coating a steel strip, and more particularly, to a method and apparatus for controlling a coating weight in a continuous galvanizing process, which are capable of reducing a deviation of a molten zinc coating weight on a steel strip by optimally adjusting an air injection pressure or a gap between an air knife and the steel strip according to a disturbance causing the deviation of the molten zinc coating weight, such as a change of a velocity of the steel strip or a change of a pass line, and a change of a target coating weight. Background Art

[2] A coating weight of a steel strip in a continuous hot-dip galvanizing process may be controlled by adjusting an ejection pressure of air ejected from nozzles of an air knife which is an air jet system installed vertical to the steel strip passing through a coating tub, or adjusting a gap between the nozzles of the air knife and the steel strip.

[3] There are provided many methods of controlling a coating weight on a steel strip in a continuous galvanizing process.

[4] Korean Patent Laid-Open Publication No. 2003-0052336 entitled "Adaptive

Coating Weight Controller in Continuous Steel Strip Galvanizing Process" discloses a coating weight estimation model for conducting the estimation and control of a coating weight required in changing a target coating weight or changing a velocity of a line of a steel strip. Also, parameters included in the coating weight estimation model are adapted to reduce a difference between a model calculation value and a measured coating weight by using a least square method on parameters caused by changes of working condition and others. In a method of updating parameters of a model by using the least square method, the parameters are adjusted to minimize the deviation between the model and a measured value. Though the difference between a coating weight calculated from the model and the measured value is minimized, when determining an air ejection pressure and a gap between the air wiping nozzles of the air knife and the metal strip by using the parameters, no optimal adjustment value for acquiring a target coating weight may be obtained. This is because there are a myriad of parameter sets of the estimation model for acquiring one coating weight. Accordingly, there is required a method of setting parameters to optimally calculate an accurately estimated coating weight and a control weight using the estimated coating weight.

[5] Japanese Patent Laid-Open Publication No. Hei 5-141918 discloses a method for automatically controlling a coating weight in which, a coating weight controllable range from the desired maximum to minimum coating weight is set, the range is divided into continuous n sections, the distance between the strip and nozzle is specified for each section, and an equation for calculating the air ejection pressure from the nozzle for each section and a correction factor for finely adjusting the ejection pressure are provided.

[6] Japanese Patent Laid-Open Publication No. Hei 9-184054 discloses a method of a method for controlling coating metal weight in hot-dip metal coating, in which a pressure of air knifes are controlled according to a following equation for a coating weight estimation model to acquire a target coating weight. However in this case, since a structure of an equation model is complicated and there are many setting coefficients, it is difficult to determine optimal coefficients.

[7]

C_^ = Ic₁P V Cm ₁+!? ) t T

[8] where kl to k6 are regression coefficients, ml and m2 are integers, P indicates pressure, V indicates line velocity, and B indicates a distance between nozzles and a steel strip.

[9] Japanese Patent Laid-Open Publication No. Hei 9-279323 discloses a method for controlling coating weight in transverse direction of hot-dip coated steel sheet and an apparatus therefore, in which the shape of the steel sheet to be plated is detected by using a laser just above a wiping nozzle. The warpage quantity is calculated from the detected value and a synchronizing roll is moved horizontally in accordance with the relation between the horizontal moving quantity of the synchronizing roll and the warpage quantity predetermined to minimize the warpage quantity, by which the variations in the coating in the transverse direction of the steel sheet are minimized.

[10] Japanese Patent Laid-Open Publication No. Hei 9-195022 discloses a method for controlling coating weight of hot-dip coating, in which air is discharged at a supersonic speed by using nozzles capable of obtaining supersonic air flux and a coating weight is controlled by using a distance between the nozzles and a steel sheet. Japanese Patent Laid-Open Publication No. Hei 7-243015 discloses a plating deposition control method of continuous type hot-dip metal coating line, in which, to prevent generation of splashing and scumming and to deal with a remarkable change of a set plating deposition even during operation, to set the height of the air-jetting nozzle from the surface of the molten metal surface is fed forward controlled according to the set nozzle jet pressure, the set plating deposition, the pass speed of the steel strip and the nozzle spacing. The nozzle jet pressure is corrected and set to eliminate the deviation between the set coating deposition and the actual coating deposition. Further, the nozzle height is controlled according to the corrected and set nozzle jet pressure and other elements. Japanese Patent Laid-Open Publication No. 1995-180019 discloses a method for controlling coating weight of hot-dip metal coating, in which a lower limit of a gap between nozzles is automatically set by considering tremor or shape of a steel sheet and a life of a coating apparatus is checked. A distance between the steel sheet and the nozzles is measured by using a range finder, the distance between the nozzles is automatically set and the life of the coating apparatus is determined analyzing a frequency of a measured value.

[11] However, in a continuous hot-dip galvanizing process, a velocity of a steel strip may be rapidly changed from a present velocity when a work condition is changed or a trouble occurs in facilities. When a velocity of line is greatly changed in a short time, particularly, when workers manually change the air ejection pressure or the gap between the air wiping nozzles of the air knife and the metal strip instead of automatically controlling, it is impossible to suitably cope with a velocity change and a coating defect may occur.

[12] Also, when a pass line is changed due to a change in a material or thickness of a steel strip while a coating weight measuring apparatus is considerably separated from air wiping nozzles, a deviation between coating weights of top and bottom occurs and a deviation as a length from measuring a coating weight to adjusting the coating weight occurs.

[13] Also, when there is formed a skew on a strip passing through a coating bath and air knives, a deviation between coating weights on right and left of the strip occurs. With respect to this, when there is no control model for automatically controlling a coating weight, generally, a driver controls a deviation by adjusting the gap between the air wiping nozzles of air knife and the metal strip between nozzles and the strip from a measured coating weight. A deviation may continuously occur since there is no adjustment though a control weight is not suitable or there is a deviation.

[14] However, it is impossible to effectively adjust a coating deviation caused by various disturbances by using conventional coating weight control technologies. Disclosure of Invention Technical Problem

[15] An aspect of the present invention provides a coating weight control method and apparatus in a continuous galvanizing process, the method and apparatus capable of effectively controlling a coating deviation between right and left of a steel strip, a coating weight deviation between front and rear surfaces of the steel strip, and a deviation from a target coating weight, which may occur due to various disturbances. Technical Solution [16] According to an aspect of the present invention, there is provided a method of controlling a coating weight in a continuous galvanizing process by adjusting an ejection pressure of air knife nozzles in front and rear surfaces of a steel strip and a gap between the air knite nozzles and the metal strip according to a preset pressure setting value and a preset gap setting value, the method including: checking whether a target coating weight is changed; calculating a deviation between a previous target coating weight and a present target coating weight when the target coating weight is changed; adjusting the gap setting value according to the target coating weight when the calculated deviation of the target coating weight is less than a first reference value; and adjusting the pressure setting value and the gap setting value according to the target coating weight at the same time when the calculated deviation of the target coating weight is the first reference value or more.

[17] According to another aspect of the present invention, there is provided an apparatus for controlling a coating weight in a continuous hot-dip galvanizing process, the apparatus including: front and rear air knives controlling a coating weight by wiping air to front and rear surfaces of a steel strip passing through a coating bath; a pressure control valve controlling pressure of the air provided to the front and rear air knives;

[18] a front and rear air knife gap detection unit detecting a gap between the steel strip and the front air knife and a gap between the steel strp and the rear air knife; a front and rear air knife gap detection unit detecting the gaps between the steel strip and the front and rear air knives; a pass line measuring unit measuring a change of a pass line of the steel strip; a strip velocity measuring sensor measuring a velocity of the steel strip; a front and rear coating weight measuring unit measuring front and rear coating weights of the steel strip passing through the front and rear air knives; an air knife setting unit checking continuous hot-dip galvanizing schedule information and measured values of the pass line and the velocity and setting the wiping pressure of the air knives and the gaps between the air knives and the steel strip to reduce coating weight deviations caused by a change of a target coating weight and a movement of the pass line and coating weight deviations caused by occurrence of a skew and an error of a coating weight estimation model; and an air knife control unit controlling the pressure control valve and the front and rear air knife gap control unit according to the pressure and the gap set by the air knife setting unit.

Advantageous Effects

[19] According to the described method and apparatus for controlling a coating weight, parameters of a coating weight estimation model from real-time work data are set according to a target coating weight range and a speed range to reduce a deviation between a target coating weight and an estimated coating weight and improve accuracy of the estimation model. Accordingly, a change of the target coating weight, which frequently occurs, may be quickly controlled. As a result thereof, a coating weight deviation at an end portion of the steel strip to be coated may be reduced. Also, pressure and a gap of air wiping nozzles are automatically changed by sensing a change of a line speed, thereby preventing occurrence of a coating weight deviation caused by the change of the line speed. A pass line change such as a change in thickness, material, the gap of the nozzles, and a gap of a correction roll is sensed by automatically measuring a pass line gap, thereby reducing a deviation between coating weights of front and rear surfaces of the steel strip. Though a coating weight deviation occurs due to an unexpected cause, the coating weight may be automatically and accurately controlled by feedback controlling using a deviation between a measured value measured by a coating weight measuring unit installed in the rear surfaces of the air knives and a target value, without a manual operation of a worker. Brief Description of the Drawings

[20] Fig. 1 is a block diagram illustrating an entire configuration of facilities for controlling a coating weight, to which the present invention is applied;

[21] Fig. 2 is a block diagram illustrating a detailed configuration of an air knife setting unit according to an exemplary embodiment of the present invention;

[22] Fig. 3 is a flowchart illustrating a method of controlling a coating weight, according to an exemplary embodiment of the present invention;

[23] Fig. 4 is a flowchart illustrating a process of setting a pressure and a gap of an air knife when a target coating weight is changed, in the controlling a coating weight according to an exemplary embodiment of the present invention;

[24] Fig. 5 is a flowchart illustrating a feed forward calculation process when a line speed is changed in the controlling a coating weight according to an exemplary embodiment of the present invention;

[25] Fig. 6 is a flowchart illustrating a feedback control weight calculation process in the controlling a coating weight according to an exemplary embodiment of the present invention;

[26] Fig. 7 is a mimetic diagram illustrating an occurrence of a difference between coating weights in front and rear according to a pass line gap change; and

[27] Fig. 8 is a mimetic diagram illustrating a state of controlling gaps between top and bottom and right and left nozzles when a skew of a steel strip occurs. Best Mode for Carrying Out the Invention

[28] Hereinafter, exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. Only, in describing operations of the exemplary embodiments in detail, when it is considered that a detailed de- scription on related well-known functions or constitutions unnecessarily may make essential points of the present invention be unclear, the detailed description will be omitted.

[29] Also, in the drawings, the same reference numerals are used throughout to designate the same or similar components.

[30] In addition, throughout the specification, when it is describe that a part is

"connected to" another part, this includes not only a case of "being directly connected to" but also a case of "being electrically connected to", interposing another device therebetween. Also, when it is described that an apparatus "includes" an element and there is no opposite description thereof, this does not designate that the apparatus excludes other elements but designates that the apparatus may further include other elements.

[31]

[32] Fig. 1 is a block diagram illustrating an apparatus for controlling a coating weight in a continuous galvanizing process according to an exemplary embodiment of the present invention.

[33] Referring to Fig. 1, the apparatus includes an air knife setting unit 2, an air knife control unit 3, pass line measuring sensor control unit 4, a pass line measuring sensor 5, a strip velocity measuring sensor 6, front and rear coating weight measuring units 7a and 7b, a pressure control valve 9, front and rear air knives 10a and 10b, front and rear air knife gap control units 11a and 1 Ib, front and rear air knife gap detection units 12a and 12b, and a pressure measuring sensor 13.

[34] The air knife setting unit 2 sets a wiping pressure of an air knife and a gap between the air knife and a steel strip 14 to reduce a change of a target coating weight, a coating weight deviation caused by a transfer of a pass line, a coating weight deviation caused by an error of a coating estimation model. The air knife control unit 3 controls the pressure control valve 9 and front and rear air knife gap controllers l la and 1 Ib according to the pressure and gap set by the air knife setting unit 2. The pass line measuring sensor control unit 4 and the pass line measuring sensor 5 are installed above the front and rear air knives 10a and 10b to measure a change of a pass line of the steel strip 14. The strip velocity measuring sensor 6 measures a velocity of the steel strip 14. The front and rear coating weight measuring units 7a and 7b measure coating weights on front and rear surfaces of the steel strip 14. The pressure control valve 9 controls a pressure of air provided from an air supply source 8 to the air knives 10a and 10b. The front and rear air knives 10a and 10b control a coating weight by wiping air provided via the pressure control valve 9 to the front and rear surfaces of the steel strip 14 passing through a coating bath 17. The front and rear air knife gap control units l la and 1 Ib control a gap of the front and rear air knives 10a and 10b. The front and rear air knife gap detection units 12a and 12b detect the gap between the front air knife 10a and the steel strip 14 and the gap between the rear air knife 10b and the steel strip 14. The pressure measuring sensor 13 measures a pressure of the air provided from the pressure control valve to the front and rear air knives 10a and 10b.

[35] In a continuous hot-dip galvanizing process, the steel strip 14 is heated in a heating furnace, passes through the coating bath containing molten zinc, and is transferred upward in a vertical direction.

[36] In this case, the front and rear air knives 10a and 10b wipe air at a certain pressure to the front and rear surfaces of the steel strip 14 leaving the coating bath 17, and control a coating weight on the steel strip 14. In this case, the coating weight is controlled by adjusting the pressure of the air provided to the front and rear air knives 10a and 10b and the gap between the front and rear air knives 10a and 10b and the steel strip 14 by using the air knife control unit 3 in such a way that the coating weight becomes the target coating weight.

[37] To control the wiping pressure, the air knife control unit 3 compares a measured pressure value of the pressure measuring sensor 13 measuring the pressure of the air provided to the front and rear air knives 10a and 10b with a pressure setting value and controls the pressure control valve 9 controlling the pressure of the air provided from the air supply source 8 to the front and rear air knives 10a and 10b.

[38] Also, to control the gap between the steel strip 14 and nozzles of the front and rear air knives 10a and 10b, a measured gap value detected by the front and rear air knife gap detection units 12a and 12b with a gap setting value and controls the front and rear air knife gap control units 1 Ia and 1 Ib moving the front and rear air knives 10a and 10b.

[39] When a coating weight lacks a desired coating weight, a defect occurs. When a coating weight is excessive, a large amount of zinc is consumed, thereby increasing manufacturing costs. Accordingly, there are an upper limit and a lower limit of a target coating weight. Also, in a continuous galvanizing process, a size of a steel strip may be changed or a target coating weight may be changed for each steel strip. When the target coating weight is greatly changed, it is required to change a pressure of an air knife, to change a gap between nozzles of the air knife, or change both of the pressure and gap to attain a changed target coating weight.

[40] In this case, when the pressure or the gap of the nozzles is not accurately controlled, the coating weight lacks the target coating weight or is excessive.

[41] Accordingly, the coating weight of the front and rear surfaces of the steel strip 14 is measured by the front and rear coating weight measuring units 7 a and 7b installed at a considerable distance from the front and rear air knives 10a and 10b and provided to the air knife control unit 3 to control the coating weight by changing the pressure or the distance of the nozzles when there is a difference between a measured coating value and a target coating weight.

[42] Also, though a setting value is accurate, when a speed of a line is changed, a pass line is moved, or there is present a curvature, there occurs a coating weight deviation.

[43] For this, a gap change of the pass line is detected via the pass line measuring sensor control unit 4 and the pass line measuring sensor 5, and is provided to the air knife setting unit 2 to control adjust the gap of the nozzles of the air knife and the pressure setting value. A strip speed is measured via the strip velocity measuring sensor 6 and is provided to the air knife control unit 3 to adjust the gap of the nozzles and the pressure.

[44] Generally, a change of a pass line may occur in a real process when a thickness or material of a steel strip is changed. As described above, when the pass line is changed, as shown in Fig. 7, a distance between the steel strip 14 and the nozzles of the air knives 10a and 10b is changed, thereby generating a deviation between coating weights of the front and the rear surfaces of the steel strip 14. Particularly, the coating weight deviation is serious at an end portion of the steel strip 14.

[45] Accordingly, to reduce the coating weight deviations of the front and rear surfaces and width wise direction of the end portion of the steel strip 14, the pass line measuring sensor 5 is located on a straight top of the air knives 10a and 10b and includes three laser range finders provided at a center and both ends of the steel strip 14 and a motor moving the laser range finders at the both ends according to a strip width. Also, the pass line measuring sensor control unit 4 receives a gap control signal and a laser range finder signal of the motor, checks and filters whether there is abnormality in received data, and transmits the filtered data to the air knife setting unit 2 via communication.

[46] In addition, the pass line measuring sensor 5 and the pass line measuring sensor 4 may include a housing for protecting the laser range finder and the motor from a high peripheral temperature, an air cooler and distributing pipes for dropping a temperature inside the housing to be a stable operation temperature of the laser range finder or less, and an air purging device wiping air to a window of the laser range finder in such a way that a coating powder is not attached to the window of the laser range finder.

[47] Since a surface of the steel strip 14 is smooth and the steel strip 14 is trembled, the pass line measuring sensor control unit 4 may miss a laser signal. The pass line measuring sensor control unit 4 receives data from a laser range finder signal measured at a high frequency of 1000 Hz for one second and calculates an average of data of detected signals, and provides a value obtained by the calculation, to the air knife setting unit 2.

[48] In the present invention, to automatically control a coating weight deviation capable of occurring due to a change of a target coating weight, a change of a pass line, a change of a velocity of a steel strip, and a skew of the steel strip, the air knife setting unit 2 adjusts pressure/gap setting values according to such changes. Hereinafter, a process of adjusting the pressure/gap setting values by the air knife setting unit 2 will be described in detail.

[49] The air knife setting unit 2 receives a target coating weight and a size of rolled material from a super ordinate computer 1 administrating a continuous hot-dip coating schedule, a measured pressure value from the pressure measuring unit 13, a measured strip transfer speed value from the strip velocity measuring sensor 6, a gap of the nozzles from the front and rear air knife gap detection units 12a and 12b, measured pass line values at work sides, a center, and a drive side of the steel strip 14 from the pass line measuring sensor 5. From the received values, the pressure and/or gap setting value of the front and rear air knives 10a and 10b to automatically control the coating weight.

[50] The air knife setting unit 2, as shown in Fig. 2, includes a pressure and gap adjustment value calculator 21, a feed forward adjustment value calculator 22, a feedback adjustment value calculator 23, a skew adjustment value calculator 24, a pass line adjustment value calculator 25, a coating weight estimation model parameter calculator 26, and a parameter updater 27.

[51] The pressure and gap adjustment value calculator 21 calculates setting values of the air knife pressure and the gap of the air wiping nozzles according to a change of the target coating weight. The feed forward adjustment value calculator 22 controls a coating weight deviation when a velocity of the steel strip 14 is changed. The feedback adjustment value calculator 23 calculates an adjustment value from a deviation between measured coating weight values measured by the front and rear coating weight measuring units 7a and 7b and a target coating weight. The skew adjustment value calculator 24 measures a skew of the steel strip 14 and controls a deviation between coating weights of right and left of the steel strip 14. The pass line adjustment value calculator 25 calculates an adjustment value of a deviation between coating weights on the front and rear surfaces of the steel strip 14, shown from a pass line change value measured by the pass line measuring sensor 5. The coating weight estimation model parameter calculator 26 reads a parameter of a coating weight estimation model from the target coating weight. The parameter updater update the parameter of the coating weight estimation model by using a coating weight actual measurement value.

[52] Fig. 3 is a flowchart illustrating a method of controlling a coating weight in the apparatus, according to an exemplary embodiment of the present invention.

[53] Referring to Fig. 3, the air knife setting unit 2 checks whether there is a change in a target coating weight, before a welding point reaches a location of the air knife (S301 and S302). As a result of the checking, when the target coating weight of a steel strip that will be coated next, setting values of wiping pressure and a nozzle gap of the front and rear air knives 10a and 10b are calculated by the pressure and gap adjustment value calculator 21 according to the change of the target coating weight (S303).

[54] The air knife setting unit 2 checks whether a velocity of the steel strip is changed from a measured velocity value of the strip velocity measuring sensor 6 (S304).

[55] As a result of the checking, when the velocity of the steel strip is changed, the feed forward adjustment value calculator 22 calculates a feed forward adjustment value compensating a velocity change value (S305).

[56] The air knife setting unit 2 checks whether the welding point passes through a coating weight measuring gage (S306). When the welding point passes through the coating weight measuring gage, the feedback adjustment value calculator 23 and the skew adjustment value calculator 24 calculate a feedback adjustment value and a skew adjustment value (S307).

[57] Also, a change of a gap of a pass line is checked from a measured value of the pass line measuring sensor 5 (S308). When there is shown the change of the gap, the pass line adjustment value calculator 25 calculates a pass line adjustment value (S309).

[58] Parameters of a coating weight estimation model used in the calculation are updated in such a way that an error between an estimated value and an actual measurement value (S310).

[59] Hereinafter, the respective calculation methods will be described in detail.

[60] The pressure and gap adjustment value calculator 21 calculates pressure and gap adjustment values according to the change of the target coating weight. Fig. 4 is a flowchart illustrating a process of the calculation of the pressure and gap adjustment value calculator 21.

[61] Referring to Fig. 4, when the target coating weigth is changed, the pressure and gap adjustment value calculator 21 reads target coating weights of a previous coil and a present coil, a velocity of a steel strip, pressure and a gap of air knives, and parameters of a coating weight estimation model (S401).

[62] A difference between the target coating weight of the present coil and the target coating weight of the previous coil is calculated (S402).

[63] When the calculated difference is 10 g/m or less, a change value of a gap setting value of air knife nozzles is calculated (S404). When the calculated difference is greater than 10 g/m , not only the change of the gap setting value but also a change value of a pressure setting value of the air knife nozzles are calculated (S405).

[64] In S404 and S405, the change value of the gap setting value of the air knife nozzles is calculated from the difference of the target as follows.

[65] G= exp(l/6 • (log(Tcw)-a • log(F))-c • log(P)))

[66] ... Equation 1

[67]

ΔG=β ^• (G-Gm)

... Equation 2

[68] In Equations 1 and 2, Tew indicates the target coating weight of the present coil, P indicates air wiping pressure, V indicates a velocity of a steel strip, and a, b, and c are parameters of a coating weight estimation model.

[69] Referring to Equations 1 and 2, to calculate the change value of the gap setting value of the air knife nozzles, a setting value of a gap between front and rear air knife nozzles according to a present target coating weight is calculated by using Equation 1, a difference between the setting value obtained using Equation 1 and a present gap value Gm is calculated by using Equation 2, and a gap change value is calculated by multiplying the difference by an application value β. An initial value of the application value β is differently set according to a range of ΔTcw when ΔTcw is 10 g/m or more. That is, when ΔTcw is within a range from 10 to 20 g/m , β becomes 0.7. When ΔTcw is within a range from 20 to 30 g/m , β becomes 0.6. When ΔTcw is within a range from 30 to 50 g/m , β becomes 0.5. When ΔTcw is 50 g/m or more, β becomes 0.4. When the gap value of the air knife nozzles is a preset limit or less, the application β is lowered than an initial setting value and adjusted until the gap value becomes the limit or more. Then β in the case of being greater than the limit is applied.

[70] By Equation 2 to which β is applied as described above, the change value ΔG of the gap setting value of the air knife nozzles is finally calculated. A final air knife nozzle gap is set as a value obtained by adding the change value ΔG to the present nozzle gap (Gset=Gm+ΔG).

[71] In S405, the pressure setting value Pset is calculated as following Equation 3,

[72]

[73] ... Equation 3

[74] That is, the target coating weight of the present coil, the velocity of the steel strip, and the calculated gap setting value Gset are applied to an equation model as Equation 3.

[75] As described above, the pressure and gap adjustment value calculator 21 may reduce a coating weight deviation occurrence length at an end portion caused by a delay time where pressure reaches a setting value, by simultaneously controlling the pressure and a nozzle gap and controlling a coating weight by using the nozzle gap when a change of a target coating weight is small. This is, since a response of nozzle gap control is quicker than a response of pressure control to control the coating weight, deviation occurrence at the end portion may be reduced by properly mixing the pressure control and the nozzle gap control. [76] Fig. 5 is a flowchart illustrating a feed forward adjustment value calculation process for controlling a coating weight deviation occurring when a velocity is changed.

Referring to Fig. 5, the feed forward adjustment value calculator 22 of Fig. 2 and S305 of Fig. 3 will be described in detail. [77] When a line speed is changed and a feed forward auto control starts, the feed forward adjustment value calculator 22 reads a speed, a gap and pressure of air knife nozzles, and parameters of an equation model (S501). [78] A velocity of a steel strip, read from the strip velocity measuring sensor 6, is stored as a reference speed at a point in time that the feed forward auto control is turned on

(S502). [79] A difference between the stored reference speed and a measured velocity value of the steel strip read for each preset feed forward auto control period is calculated

(S503). [80] A feed forward adjustment value is calculated based on the calculated difference. In this case, when the difference is 2 mpm or less, the feed forward control is not performed. When the difference is greater than 2 mpm, a pressure adjustment value according to the difference is calculated using following Equations 4 and 5 (S505 to

S507). [81]

P(£+ l) = exp(— ^• (log(VreJ{k)> ^ioB(Vm(k)))+log(Pm(k)))

[82] ... Equation 4

[83]

AP_sum(k+ l)=AP_sum(k)+APref(k+ l) APref(k+ l)=P(k+ \yPm(k)

[84] ... Equation 5

[85] In Equations 4 and 5, Vref(k) indicates a reference speed at kth control time, Vm(k) indicates a velocity of the steel strip presently passing through air knives, Pm(k) indicates a presently measured value of the air knives, and a, b, and c are parameters of a coating weight estimation equation model, which are read from an update table.

[86] That is, a pressure adjustment value that will be used next time is calculated in

Equation 4. A present velocity is substituted for the reference speed. A pressure change value ΔPref(k+l) is obtained from the difference between the pressure setting value calculated fro each control period and the present pressure setting value. The pressure change value is added to a pressure adjustment value accumulation value ΔP_sum(k) as Equation 5. [87] When the calculated final pressure setting value is smaller than a pressure lower limit PL (S508), the pressure setting value is set as the pressure lower limit PL (S509).

After that, when a speed is reduced, the coating weight is controlled by adjusting an air knife gap (S510 to S514). [88]

[89] ... Equation 6

[90]

AGFF(k+ l )=oFF - (Gm(k)-G(k+ l ))

[91] ... Equation 7

[92] where αFF indicates a control coefficient.

[93] That is, the pressure setting value is smaller than the pressure lower limit PL, a speed measured at that point in time is set as the reference value of the speed (Vref(k)=Vm) (S510 and S511). When a speed difference from the set reference value of the speed is 2 mpm or more (S512), as shown in Equation 6, a gap adjustment value of air knife wiping nozzles according to the speed change is calculated. An air knife gap change value ΔGFF(k+l) is calculated by using Equation 7. When the gap change value is calculated, a gap adjustment value accumulation value ΔGFF_SUM(k) calculated at a previous time is added to a gap adjustment value accumulation value ΔGFF_SUM(k+l) calculated at a present time. A gap setting value is calculated by adding an accumulation value to a reference gap (S513 and S514).

[94] After changing the present velocity of the steel strip is changed into the reference speed value, the operations are performed again from S501 (S515).

[95] Fig. 6 is a flowchart illustrating a method of calculating an adjustment value for a deviation between a measured coating weight value and a target coating weight in the feedback adjustment value calculator 23. An average measured coating weight value is calculated using measured coating weight values in a lateral direction of front and rear surfaces of the steel strip, measured by the front and rear coating weight measuring units 7a and 7b. When there is no abnormality, a deviation between the target coating weight and the measured coating weight is calculated and a gap change value of the nozzles of the front and rear air knives with respect to the deviation is calculated.

[96] Referring to Fig. 6, the feedback adjustment value calculator 23 reads a target coating weight of a present coil, a velocity of the steel strip, a gap and pressure of the air knives, and parameters of an equation model (S601).

[97] When a welding portion of the steel strip passes through a distance Lcg[m] from the front and rear air knives 10a and 10b to the front and rear coating weight measuring units 7 a and 7b (S 602 to S 604), measured values measured by the front and rear coating weight measuring units 7a and 7b are read to check whether there is abnormality and a deviation from the target coating weight is calculated (S605 and S606).

[98] Front and rear gap change values

AG _t t_oop_t}, AG bot are calculated. In this case, an initial adjustment value is calculated by using Equations 8 and 9. [99] α^* ΔT _cw{top) ΔCr t,_Λop =- ^■ n c b- \

V ' P ' b ^r G

... Equation 8 [100]

... Equation 9

[101] In Equations 8 and 9, indicates an adjustment coefficient, ΔTcw indicates the target coating weight-the measured coating weight, V indicates the velocity of the steel strip, P indicates the pressure of the air knives, G indicates the gap of the front and rear air knives, and a, b, and c are parameters of a present coating weight estimation model stored in a parameter update table.

[102] A gap setting value of the nozzles with respect to the gap change value of the front and rear air knives is calculated as shown in Equations 10 and 11 (S 607).

[103]

^ top set ^~ ^^Jtop ACr _fop

... Equation 10 [104]

G b_{ot set} ⁼ Gb_ot ⁺ΔG_boi ... Equation 11

[105] In Equations 10 and 11, Gtop and Gbot indicate front and rear air knife gaps measured at a calculation point in time, respectively.

[106] A next feedback control point in time Lc is updated (S608), which is updated as shown in Equation 12.

[ 107 ] L = current location of welding portion +Lcg

[108] ... Equation 12

[109] Next, a skew adjustment value calculation process performed by the skew adjustment value calculator 25 will be described. The skew adjustment value calculator 25 calculates average coating weights of a work side w/s, a center, and a drive side d/s laterally scanned by the front and rear coating weight measuring units 7a and 7b, respectively, makes a quadratic polynomial equation from the three measured values and the width of the steel strip, calculates a skew amount from a coefficient of a linear term and the width, and a gap setting value of air wiping nozzles to prevent occurrence of a skew is calculated from a coating weight estimation model.

[110] In the described above, when a linear coefficient of a quadratic polynomial equation in a front portion is b(top) and a linear coefficient of a quadratic polynomial equation in a rear portion is b(bot), the skew is calculated as shown in Equation 13.

[I l l]

[112] ... Equation 13

[113] The work side w/s, the drive side d/s, and the skew control amount are calculated by using Equation 14. [114]

(X ₁ • 0.5 • skew

^⁷ top_ws _τ/α ^ T s-ib- \ ' ^^Jtop_ds ^G ^' _top__ws,

^AGbot_d_S ^{= AG}top_ds> ^AGbot__WS ^{= AG}top_w_S

[115] ... Equation 14

[116] In Equation 14, V indicates a velocity of the steel strip, P indicates pressure of the air knives, G indicates a gap of the front and rear air knives, and a, b, and c are updated parameters. [117] Next, the coating weight estimation model parameter calculator 26 and the coating weight estimation model parameter updater 27 will be described. A coating weight estimation model estimates a coating weight by using Equation 15. [118] Cw- V¹ - G ^b - P^c

... Equation 15

[119] In Equation 15, a, b, and c are parameters of an equation model and are updated by the coating weight estimation model parameter updater 27. As shown in Table 1, the parameters a, b, and c are divided according to a target coating weight and a velocity of the steel strip and initial values thereof are stored. The initial values of the respective parameters are experiment data of an actual working line. An optimal value is set, in which a coating weight deviation does not occur when applying an error of a coating weight estimation model of Equation 15 an adjustment value calculated therefrom.

[120] Table 1

[121] The coating weight estimation model parameter updater 27 reads parameter setting values from Table 1 by suing the velocity of the steel strip, the pressure of the air knife nozzles, and the nozzle gap, estimates a coating weight by using a coating weight estimation model of Equation 15, calculates a difference between the calculated coating weight estimation value and a measured coating weight value measured by the coating weight measuring units 7a and 7b, changes the parameters until the difference becomes within a predetermined range, and stores the finally changed parameters in an update table such as Table 1.

[122] Since there is a distance between the air knives and the coating measuring units, it is required that the coating weight estimated by using the equation model is identical to the measured coating weight in an air knife gap.

[123] A method of changing the parameters of the model from a deviation between the coating weight estimated by using the equation model and the measured coating weight is as follows.

[124] A coating weight error is calculated as shown in Equation 16.

[125] Coating weight estimation error

[126] = measured coating weight - estimated coating weight

[127] ... Equation 16

[128] When the calculated coating weight error is smaller than 0, the parameters a, b, and c are calculated using following Equation 17. When the calculated coating weight is greater than 0, the parameters a, b, and c are calculated using following Equation 18 [129] a(k+l) = a(k) - α b(k+l) = b(k) - β c(k+l) = c(k) + Y

... Equation 17 [130] a(k+l ) = a(k) + α b(k+l) = b(k) + β c(k+l ) = c(k) - Y

... Equation 18

[131] In Equation 18, α, β and γ have the relation of α < β < γ and they are set as constant values smaller than 0.01.

[132] Using the calculated parameters a(k+l), b(k+l), and c(k+l) and Equation 15, the coating weight estimation value is calculated. The coating weight estimation error is obtained as shown in Equation 16.

[133] Until the coating weight estimation error becomes smaller than a predetermined value, the calculation is repeated.

[134] As described above, when the coating weight estimation error becomes smaller than a setting value, values of corresponding parameters a(k+l), b(k+l), and c(k+l) are stored in Table 1 using the target coating weight of the present coil and the velocity of the steel strip.

[135] As described above, a process of updating the parameters of the coating weight estimation model has a simpler calculation than that of a conventional least square method and has no danger caused by a parameter. Also, not only the error of the model but also an error of an adjustment value may be more accurately calculated than conventional methods by considering a main cause of an error of a model, which is not reflected in the model.

[136] Next, in the present invention, when a skew occurs in the steel strip or a pass line is changed, the occurrence of the skew and the change of the pass line are automatically measured and reflected in the control to prevent front and rear coating weight deviations, which will be described later.

[137] The skew adjustment value calculator 24 and the pass line adjustment value calculator 25 of the air knife setting unit 2 determine three measurement points as xl=width/2, x2=0, and x3=width/2 from a center, both edges, and a width of the steel strip. When pass line measurement information of the three measurement points transmitted from the pass line measurement sensor control unit 4 are yl, y2, and y3, a and b are obtained from xl, x2, x3, yl, y2, and y3 by using Equation 19. An average pass line transfer value is calculated from yl, y2, and y3 as shown in Equation 20. A skew value and a curvature value are calculated from the calculated a and b as shown in Equations 21 and 22.

[138]

( (yl -y2) ^■ (χ2-χ3)-(y2-y3 ) ^■ (χl -χ2)) a ( (xl ^• xl -x2 ^• χ2) ^• (χ2-χ3)-(χ2 ^• χ2-χ3 ^• χ3) ^• (χl -χ2)) ( (y2-y3 )-a ^• (χ2 ^• χ2-χ3 ^• χ3 )) (χ2-χ3)

[139] ... Equation 19

[140]

... Equation 20 [141] skew ⁼ b ' width

... Equation 21 [142] curve = a * (width /2) * (width /2)

... Equation 22

[143] A front and rear coating weight deviation adjustment value is calculated from a deviation between a pass line measurement value PLm calculated using Equation 20 and a pass line reference point PLr. The pass line reference point PLr is determined as the pass line measurement value PLm calculated using Equation 20 at a point in time a difference between a front measured coating weight and a rear measured coating weight, which is received from the coating weight measuring units 7a and 7b, is a predetermined weight or less. A front and rear air knife gap moving value according to a pass line change by the pass line adjustment value calculator 25 is calculated using Equation 23. [144]

APL=PLm-PLr G_PL_TOP=a_PL - APL/2 G_PL_BOT=-a_PL • APL/2

... Equation 23

[145] According to the calculated gap moving value, the air knife control unit 3 controls a distance between the air knives and the steel strip. For example, in the case of the pass line measuring sensor 5 installed in front of the steel strip, when a pass line moves to a front surface, a coating weight of the front surface is smaller than that of a rear surface. Accordingly, a gap from the front surface is adjusted in such a way that the distance between the air knives and the steel strip is more increased than now by G_PL_TOP. Also, a gap from the rear surface is adjusted in such a way that the distance between the air knives and the steel strip is more decreased than now by G_PL_TOP.

[146] Similarly, the skew adjustment value calculator 24 calculates a skew adjustment value to prevent occurrence of a skew by using calculated skew information as shown in Equation 24.

[147]

G TOP_WS_REF^~ & τopws^+asκEw ^* ΔG_SKEWi2

G TOP_DS_REF^~^TOPDS^{~ 0L}SKEW ' ^G_SKEWi2

G BOi -_ws_REF^{~ ~} G Topws' '^asκ∑w ' ΔG_SKEWI2

G BOT_DS_REF^~G TOPDS^{+ a}SKEW ' ^G_SKEWi2

[148] ... Equation 24

[149] According to the calculated skew adjustment value, the air knife control unit 3 controls the gap of the nozzles. As shown in Fig. 8, in a skew, seeing in a width direction of the steel strip, a gap of D/S is deviated from W/S. For example, when a sign of a skew is positive, the gap of the D/S of the air knife in the rear surface of the steel strip is set to be greater than ΔG /2 and the gap of the W/S is set to be smaller r^& SKEW ^{& r} than ΔG SKEW /2. Also, on the contrary ^J , in the case of the front surface,' the gtoarp of the D/

S is set to be smaller than ΔG /2 and the gap of the W/S is set to be greater than ΔG

SKEW

SKEW 12.

[150] While the present invention has been shown and described in connection with the exemplary embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

[1] A method of controlling a coating weight in a continuous galvanizing process by adjusting ejection pressures of air knife nozzles of front and rear surfaces of a steel strip and a gap between the air knite nozzles and the metal strip according to a preset pressure setting value and a preset gap setting value, the method comprising: checking whether a target coating weight is changed; calculating a deviation between a previous target coating weight and a present target coating weight when the target coating weight is changed; adjusting the gap setting value according to the target coating weight when the calculated deviation of the target coating weight is less than a first reference value; and adjusting the pressure setting value and the gap setting value according to the target coating weight at the same time when the calculated deviation of the target coating weight is the first reference value or more.

[2] The method of claim 1, wherein the first reference value is 10 g/m .

[3] The method of claim 2, wherein a gap calculation value G is calculated by using

G= exp(l/6 • (\og(Tcw)-a • \og(V))-c • log(P))) where Tew indicates the present target coating weight, P indicates an air ejection pressure, V indicates a velocity of the steel strip, and a, b, and c are parameters of a coating weight estimation model, a fluctuation value ΔG is calculated by using

ΔG= β ^• (G- Gm) where Gm indicates a present air knife gap value and β indicates a variable differently set according to the target coating weight, and the gap setting value G is calculated by using set

G_set=Gm-AG

[4] The method of claim 3, wherein the pressure setting value is calculated by using

Pset=exp(l/c ^• (log(Tcw)-a ^• \og(V)-b ^• \og(Gset)))

[5] The method of claim 4, further comprising dividing a range of the target coating weight and a range of the velocity of the steel strip in a parameter setting table by n number, setting the parameters a, b, and c of the coating weight estimation model for each portions obtained by the division, estimating the coating weight by using the parameters, comparing the estimated coating weight with a value obtained by measuring a real coating weight, and adjusting the parameters in such a way that a deviation between the estimated coating weight and the measured coating weight becomes 0.

[6] The method of claim 5, wherein the estimated coating weight Cw is calculated coating weight by using

Cw = V^{^} ' G^b - P^c

[7] The method of claim 5, further comprising calculating a deviation between coating weights of the front and the rear surfaces caused by pass line transfer and a coating weight deviation in a lateral direction caused by a skew or curvature of the steel strip, and calculating pass line and skew adjustment value in such a way that the coating weight deviation in the lateral direction becomes 0 to control the gap of the air knife nozzle.

[8] The method of claim 7, further comprising adjusting the pressure setting value and the air knife nozzle gap setting value according to a change of the velocity when the change of the velocity is a second reference value or more as a result of checking the change of the velocity of the steel strip.

[9] The method of claim 8, wherein the pressure setting value is obtained by: calculating a pressure adjustment value P(k+1) by using

P(k+l) = exp(j ^• (log(VreJ{k))- log(Vm(k)))+\og(Pm(k))) where Vref(k) indicates a reference velocity of a kth control time, Vm(k) indicates a velocity of the steel strip passing through a present air knife, Pm(k) indicates a pressure measure of the present air knife, Gm(k) indicates a gap value of the present air knife, and a, b, and c are parameters for estimating the coating weight; obtaining a pressure change value from a difference between the pressure adjustment value and the present pressure setting value; and adding the pressure change value to a previous pressure adjustment value.

[10] The method of claim 9, wherein the gap setting value is obtained by: calculating a nozzle gap adjustment value according to a velocity change by using

G(£+l)=exp(^ • (log(Fre/(£))-log(Fm(£)))+log(Gm(£))) b where Vref(k) indicates a reference velocity of a kth control time, Vm(k) indicates a velocity of the steel strip passing through a present air knife, Pm(k) indicates a pressure measure of the present air knife, Gm(k) indicates a gap value of the present air knife, and a, b, and c are parameters for estimating the coating weight; calculating a gap change value by using

ΔGFF(Ar+ I )=CtFF ^• (Gm(k)-G(k+ l )) where αFF is an adjustment coefficient; calculating a change value for each time; accumulating the change value for each time; and calculating a final air knife gap adjustment value by adding a value obtained by accumulating, to a reference gap value.

[11] The method of claim 8, further comprising changing the gap of the air knife by calculating a deviation between the target coating weight and the measured coating weight when welding points of two steel strips pass through a coating weight measurement unit.

[12] The method of claim 11, wherein an adjustment value of the gap of front and rear air knives according to the deviation from the target coating weight is calculated using following equations,

and

₌ ^α ' ^cwjbot)

where α indicates an adjustment coefficient, ΔTcw indicates the target coating weight-the measured coating weight, V indicates the velocity of the steel strip, P indicates the pressure of the air knives, G indicates the gap of the front and rear air knives, and a, b, and c are parameters of a present coating weight estimation model stored in a parameter update table.

[13] An apparatus for controlling a coating weight in a continuous hot-dip galvanizing process, the apparatus comprising: front and rear air knives controlling a coating weight by wiping air to front and rear surfaces of a steel strip passing through a coating bath; a pressure control valve controlling pressure of the air provided to the front and rear air knives; a front and rear air knife gap control unit controlling gaps between the steel strip and the front and rear air knives; a front and rear air knife gap detection unit detecting a gap between the steel strip and the front air knife and a gap between the steel strp and the rear air knife; a pass line measuring unit measuring a change of a pass line of the steel strip; a strip velocity measuring sensor measuring a velocity of the steel strip; a front and rear coating weight measuring unit measuring front and rear coating weights of the steel strip passing through the front and rear air knives; an air knife setting unit checking continuous hot-dip galvanizing schedule information and measured values of the pass line and the velocity and setting the wiping pressure of the air knives and the gaps between the air knives and the steel strip to reduce coating weight deviations caused by a change of a target coating weight and a movement of the pass line and coating weight deviations caused by occurrence of a skew and an error of a coating weight estimation model; and an air knife control unit controlling the pressure control valve and the front and rear air knife gap control unit according to the pressure and the gap set by the air knife setting unit.

[14] The apparatus of claim 13, wherein the pass line measuring unit comprises: a pass line measuring sensor having three laser range finders located above the front and rear air knives and installed in a center and both edges to face one another, and motors moving the laser range finders in the both edges according to a strip width; and a pass line measuring sensor controller controlling the motors according to a strip width of the steel strip, receiving signals from the three laser range finders, checking whether the received signal is abnormal, filtering the signal, and providing processed data to the air knife setting unit.

[15] The apparatus of claim 13, wherein the air knife setting unit comprises: a pressure and gap adjustment value calculator calculating setting values of the pressure of the air knives and the gap of air wiping nozzles according to the change of the target coating weight; a feed forward adjustment value calculator for controlling a coating weight deviation when the velocity of the steel strip is changed; a feedback adjustment value calculator calculating an adjustment value from a deviation between a coating weight measurement value measured by the front and rear coating weight measuring unit and the target coating weight; a skew adjustment value calculator measuring a skew of the steel strip and controlling a deviation between right and left coating weights of the steel strip; a pass line adjustment value calculator calculating an adjustment value of a deviation between coating weights of the front and rear surfaces of the steel strip, which is shown as a pass line change value; a parameter calculator reading parameters of a coating weight estimation model from the target coating weight; and a parameter updater comparing a coating weight estimation value calculated using the parameters read by the parameter calculator with an actually measured coating weight and updating the parameters of the coating weight estimation model in such a way that a deviation between the estimation value and the actual measured value becomes 0.