(19)
(11)EP 2 980 538 B1

(12)EUROPEAN PATENT SPECIFICATION

(45)Mention of the grant of the patent:
29.07.2020 Bulletin 2020/31

(21)Application number: 15176759.7

(22)Date of filing:  15.07.2015
(51)International Patent Classification (IPC): 
G01F 23/26(2006.01)
B22D 11/20(2006.01)
B22D 11/18(2006.01)
G01F 25/00(2006.01)

(54)

MOLTEN METAL LEVEL MEASURING METHOD BY EDDY CURRENT AND RELATED MEASURING DEVICE

WIRBELSTROMFÜLLSTANDSMESSVERFAHREN FÜR METALLSCHMELZE UND ZUGEHÖRIGE FÜLLSTANDSMESSVORRICHTUNG

PROCÉDÉ DE MESURE DU NIVEAU DE MÉTAL FONDU PAR COURANTS DE FOUCAULT ET DISPOSITIF DE MESURE ASSOCIÉ


(84)Designated Contracting States:
AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TR

(30)Priority: 01.08.2014 WO PCT/JP2014/070340
12.02.2015 JP 2015024950

(43)Date of publication of application:
03.02.2016 Bulletin 2016/05

(73)Proprietor: Nireco Corporation
Tokyo 192-8522 (JP)

(72)Inventor:
  • KOYAMA, Fumio
    Tokyo, 192-8522 (JP)

(74)Representative: Viering, Jentschura & Partner mbB Patent- und Rechtsanwälte 
Am Brauhaus 8
01099 Dresden
01099 Dresden (DE)


(56)References cited: : 
JP-A- S56 129 819
JP-A- 2013 166 167
JP-A- S60 216 959
US-A- 4 647 854
  
      
    Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention).


    Description

    Technical Field



    [0001] The present invention relates to an eddy current mold level measuring device and a mold level measuring method, which are used in continuous casting equipment.

    Background Art



    [0002] In continuous casting equipment, molten metal is poured into a mold, cooled and solidified there, such that a desired shape of strand is produced. In continuous casting equipment, measurement and control of the level of a surface of molten metal in the mold, that is, the mold level are essential for improving quality of a produced strand.

    [0003] Eddy current mold level measuring devices measure a level of a surface of molten metal in the mold by the use of the phenomenon that magnitude of voltage which is induced in a detecting coil by eddy current generated around a surface of molten metal in the mold varies depending on a distance between the detecting coil and the surface of molten metal. Eddy current mold level measuring devices are excellent in responsivity and therefore appropriate for a high-accuracy control of a level of a surface of molten metal in the mold, but are susceptible to ambient temperature and electromagnetic field which surrounds the devices. Accordingly, calibration is essential to eddy current mold level measuring devices. As methods for calibrating conventional eddy current mold level measuring devices, a method which uses measurements obtained by visual observations of the operator (for example, JPS61239120(A)), a method which uses a thermocouple type mold level meter (for example, JPH02140621(A)), a method which uses an electrode type mold level meter (for example, JPH08233632(A)), and the like have been developed. However, any of the above-described methods are insufficient in accuracy and are not adaptable to dynamic disturbances such as a change in vertical position of a tundish and a change in width of a slab mold. In particular, in the case of small-section molds, such as those for bloom and billet, eddy current mold level measuring devices are significantly affected by a change in vertical position of the tundish, and the change in vertical position of the tundish has been an obstacle to improvement of accuracy of the measuring devices.

    [0004] Further, a method in which characteristics of a mold level measuring device are determined by the use of signals generated by mold oscillation has been proposed (JPS60216959(A)). However, the method disclosed in JPS60216959(A) has problems as described below. Since mold level measuring devices are used for control of pouring rate of molten metal into the mold, an error in measurement may lead to a serious accident. Accordingly, when calibration of a mold level measuring device is carried out for the duration of continuous casting process, safety and reliability of the calibration has to be ensured. However, JPS60216959(A) does not say anything about how calibration of a mold level measuring device should be carried out while safety and reliability of the calibration are ensured for the duration of continuous casting process. Accordingly, the method disclosed in JPS60216959(A) cannot be brought into practical use in calibration for the duration of continuous casting process. In the method disclosed in JPS60216959(A), feedforward-type correction is made by a correction circuit 12 as shown in Fig. 4, and a positive feedback ratio is not corrected unlike the present invention. The present invention will be described in detail later.

    [0005] Thus, any of the conventional methods cannot provide a sufficient degree of accuracy of measurement while adapting to changes in surrounding conditions in the continuous casting process.

    [0006] US 4 647 854 A discloses an apparatus for measuring the level of molten metal in the oscillating mold of a continuous casting machine in operation by an eddy-current type distance measuring device, wherein the oscillation signal component due to the oscillation of the mold is extracted from the output signal of the distance measuring device and the detection sensitivity of the distance measuring device is compensated and controlled so as to maintain the amplitude value of the oscillation signal component at a constant value thereby linearizing the molten metal surface level versus output characteristic. Deviation of measurement is reduced by controlling a positive feedback ratio of a feedback amplifier.

    [0007] 

    Patent document 1: JPS61239120(A)

    Patent document 2: JPH02140621(A)

    Patent document 3: JPH08233632(A)

    Patent document 4: JPS60216959(A)



    [0008] Accordingly, there is a need for an eddy current mold level measuring device and a mold level measuring method in which calibration of the mold level measuring device can be carried out while safety and reliability of the calibration are ensured for the duration of continuous casting process and a sufficient accuracy of measurement can be guaranteed.

    Summary of Invention



    [0009] The invention provides a mold level measuring method according to claim 1 and an eddy current mold level measuring device according to claim 5. Further embodiments of the invention are described in the dependent claims.

    Brief Description of Drawings



    [0010] 

    Figure 1 shows an arrangement of continuous casting equipment;

    Figure 2 is a block diagram of the eddy current mold level measuring device according to an embodiment of the present invention;

    Figure 3 illustrates a method of calibration of the eddy current mold level measuring device;

    Figure 4 shows a relationship between level of the surface in the mold (that is, mold level) and the output voltage of the feedback amplifier;

    Figure 5 shows a relationship between mold level and output of the mold level measuring device;

    Figure 6 illustrates the continuous casting process;

    Figure 7 is a flowchart for illustrating mold oscillation signal calibration (MOSC);

    Figure 8 illustrates how the theoretically corrected value of positive feedback ratio is determined in step S1030 of Fig. 7; and

    Figure 9 shows a relationship between a ratio of the difference ΔV in the output of the level measuring device, which corresponds to the known value of amplitude S0 to ΔV0 and a ratio of the theoretically corrected value of positive feedback ratio to the reference value of positive feedback ratio.


    Description of Embodiments



    [0011] Figure 1 shows an arrangement of continuous casting equipment.

    [0012] Molten metal such as molten steel stored in a tundish 210 is poured into a mold 310 through an immersion nozzle 220, made to solidify in the mold 310 and delivered from the mold 310 through the use of pinch rolls 330. Further, the mold 310 is equipped with a mold oscillation device 320. Mold oscillation will be described later.

    [0013] When in a certain time period an amount of molten metal which is poured into the mold 310 is greater than an amount of metal which is delivered from the mold 310, a level of a surface 400 of the molten metal in the mold 310 goes up. On the other hand, when in a certain time period an amount of metal which is delivered from the mold 310 is greater than an amount of molten metal which is poured into the mold 310, the level of the surface 400 of the molten metal in the mold 310 goes down. Under the above-described situation, an eddy current mold level measuring device according to an embodiment of the present invention measures the level of the surface 400 of the molten metal in the mold 310. The eddy current mold level measuring device according to the present embodiment includes a detecting section 105 and a signal processing section 110. The detecting section 105 and the signal processing section 110 are connected to each other by a cable 1055. Since the detecting section 105 is located near the molten metal, a joint section 1053 and air piping 1051 are provided to supply cooling air to the detecting section 105.

    [0014] Figure 2 is a block diagram of the eddy current mold level measuring device 100 according to an embodiment of the present invention. As described above, the eddy current mold level measuring device 100 includes the detecting section 105 and the signal processing section 110. The detecting section 105 includes an exciting coil 105C1, a detecting coil 105C2 and a reference coil 105C3. The signal processing section 110 includes a signal amplifying section 120, an output generating section 130, a mold oscillation signal calibration section 140, a pre-pouring calibration section 150, a display section 160 and an input section 170.

    [0015] In the detecting section 105, current of a predetermined frequency is send through the exciting coil 105C1, so that the exciting coil 105C1 generates an AC magnetic field. The AC magnetic field passes through the detecting coil 105C2 and the reference coil 105C3. Further, when the AC magnetic field interacts with molten metal existing within a predetermined distance, eddy current is generated in the molten metal, and in reaction to the generated eddy current impedance of the detecting coil 105C2 will change. An amount of the change in impedance of the detecting coil 105C2 varies depending on a distance between the detecting coil 105C2 and the surface of the molten metal. Accordingly, the distance between the detecting coil 105C2 and the surface of the molten metal can be measured by measuring the amount of the change in impedance of the detecting coil 105C2. Further, if an amount of a change in difference between impedance of the detecting coil 105C2 and impedance of the reference coil 105C3 is used instead of the amount of a change in impedance of the detecting coil 105C2, influence of temperature and electromagnetic field which surrounds the device can be reduced. In the present embodiment, the signal amplifying section 120 employs the amount of a change in difference between impedance of the detecting coil 105C2 and impedance of the reference coil 105C3. However, the present invention can be applied to any types of eddy current mold level measuring devices besides the difference type to which the present embodiment belongs.

    [0016] The signal amplifying section 120 includes a reference oscillator 1201 for generating AC voltage of a predetermined frequency and of a predetermined amplitude, a feedback amplifier 1203, a feedback impedance 1205 having a variable positive feedback ratio, a differential amplifier 1207 receiving a difference between impedance of the detecting coil 105C2 and impedance of the reference coil 105C3 as input, and an amplitude modulator 1209 for amplitude modulation of AC voltage of output of the feedback amplifier 1203. When output voltage of the reference oscillator 1201 is represented as Vin, output voltage of the feedback amplifier 1203 is represented as Vout, an amplification factor of the feedback amplifier 1203 is represented as G1, an amplification factor of the differential amplifier 1207 is represented as G2, the positive feedback ratio is represented as K, and a level of the molten metal is represented as h, the following equation holds.

    "f" represents a function of the level h of molten metal. When the level h of the molten metal rises and therefore a distance between the detecting coil 105C2 and the surface of the molten metal becomes smaller, "f" becomes greater in Equation (1). Accordingly, when the level h of the molten metal rises, an absolute value of output voltage |Vout| of the feedback amplifier 1203 becomes smaller according to Equation (1).

    [0017] Calibration of the eddy current mold level measuring device will be described below. As described above, the eddy current mold level measuring device detects a change in impedance of the detecting coil 105C2 caused by a change in the level h of the surface of the molten metal in the mold 310, using Equation (1). However, impedance of the detecting coil 105C2 will vary depending not only on the level of the surface of the molten metal, but also on temperature and electromagnetic field which surrounds the device. Accordingly, for measurement using the eddy current mold level measuring device, calibration of the eddy current mold level measuring device is required.

    [0018] Figure 3 illustrates a method of calibration of the eddy current mold level measuring device. Calibration of the eddy current mold level measuring device includes a calibration using a calibration plate and a calibration which is carried out before molten metal is poured into the mold.

    [0019] The calibration using a calibration plate is carried out independently of the continuous casting process. In the calibration using a calibration plate, the output voltage of the feedback amplifier 1203 is measured and recorded while distance between the detecting coil 105C2 and the calibration plate is changed. The horizontal axis of Fig. 3 represents level of the surface of the molten metal or of the calibration plate. The level is represented by distance between the detecting coil 105C2 and the surface of the molten metal or the calibration plate. Level of 0 means the state in which the distance between the detecting coil 105C2 and the surface of the molten metal or the calibration plate is 0. The vertical axis of Fig. 3 represents the output voltage of the feedback amplifier 1203. The solid line R1 in Fig. 3 shows the result of the calibration using a calibration plate. Assuming that the measuring range of the eddy current mold level measuring device is from 0 to 150 millimeters, values of the output voltage which correspond to values of distance between the detecting coil 105C2 and the calibration plate in the above described range are measured as shown in Fig. 3. Further, a value V01 of the output voltage is measured when the calibration plate is removed or in other words, the calibration plate is located at an infinite distance from the detecting coil 105C2. This value V01 of the output voltage is referred to as reference voltage. The positive feedback ratio K is adjusted when necessary such that a shape of the solid line R1 is appropriately determined.

    [0020] The calibration which is carried out before molten metal is poured into the mold is carried out for the duration of continuous casting process by the pre-pouring calibration section 150. The pre-pouring calibration section 150 records a value which is obtained by a linearizer which has processed the output voltage of the feedback amplifier 1203, before molten metal is poured into the mold. The calibration which is carried out before molten metal is poured into the mold may be carried out by the command of the operator. The state before molten metal is poured into the mold corresponds to the state in which the calibration plate is at an infinite distance from the detecting coil 105C2. Accordingly, a value of the output voltage of the feedback amplifier 1203, which is measured before molten metal is poured into the mold should be equal to the above-described reference voltage V01. In practice, however, temperature and electromagnetic field which surrounds the device vary according to casting conditions, and therefore the measured value of the output voltage is not necessarily equal to the reference voltage V01.In Fig. 3, the dot-dash line A1 shows the case in which a value of the output voltage V, which is measured before molten metal is poured into the mold is greater than V01, while the dot-dot-dash line B1 shows the case in which a value of the output voltage V, which is measured before molten metal is poured into the mold is smaller than V01.In these cases, the pre-pouring calibration section 150 adjusts the positive feedback ratio K in Equation (1) such that the output voltage V is equal to the reference voltage V01. More specifically, in the case of the dot-dash line A1, the positive feedback ratio K is increased such that the output V of the level measuring device is made smaller and in agreement with V01. In the case of dot-dot-dash line B1, the positive feedback ratio K is reduced such that the output V of the level measuring device is made greater and in agreement with V01. The calibration which is carried out before molten metal is poured into the mold is performed for each casting process.

    [0021] As described later, the value of positive feedback ratio, which has been determined by the pre-pouring calibration section 150 is used as the reference value.

    [0022] Figure 4 shows a relationship between level of the surface in the mold (that is, mold level) and the output voltage of the feedback amplifier 1203. The horizontal axis of Fig. 4 represents mold level while the vertical axis represents output voltage of the feedback amplifier 1203. The position of the detecting coil 105C2 is designated as 0 of mold level.

    [0023] The output generating section 130 shown in Fig. 2 includes a mold oscillation filter (MOF) 1301, an analog-to-digital converter 1303, and a linearizer 1305. The linearizer 1305 performs linearization between output of the level measuring device and mold level such that a ratio between an amount of change in output of the level measuring device and an amount of change in mold level is constant. Two types of signals, one of which has not passed through the mold oscillation filter 1301 and the other of which has passed through the mold oscillation filter 1301 are delivered to the analog-to-digital converter 1303 as inputs. Outputs of the analog-to-digital converter 1303, which correspond to the two types of signals are delivered to the linearizer 1305. Among the outputs of the analog-to-digital converter 1303, which correspond to the two types of signals, the output which corresponds to the signal which has passed through the mold oscillation filter 1301 serves as an output of the mold level measuring device. Among the outputs of the analog-to-digital converter 1303, which correspond to the two types of signals, the output which corresponds to the signal which has not passed through the mold oscillation filter 1301 is used by the mold oscillation signal calibration section 140. The mold oscillation signal calibration section 140 includes an amplitude determining section 1401, an operation section 1403 and a positive feedback ratio correcting section 1405. The function of the mold oscillation signal calibration section 140 will be described later.

    [0024] Figure 5 shows a relationship between mold level and output of the mold level measuring device. Output of the mold level measuring device is obtained by having the output of the feedback amplifier 1203 undergo linearization by the linearizer 1305. The horizontal axis of Fig. 5 represents mold level while the vertical axis of Fig. 5 represents output of the eddy current mold level measuring device.

    [0025] Figure 6 illustrates the continuous casting process. The continuous casting process includes a step of pouring in which molten metal is fed from the tundish 210 into the mold 310 until the surface 400 of molten metal in the mold 310 rises to a predetermined level and a step of withdrawal in which solidified metal is withdrawn from the mold 310 while molten metal is being fed from the tundish 210 into the mold. In the step of pouring, a dummy bar is installed in the mold such that it forms the bottom, molten metal is stored in a space surrounded by the mold 310 and the dummy bar while it is being solidified and the surface of the molten metal, that is, the mold level rises. When the mold level reaches a predetermined level, the process goes to the step of withdrawal. Withdrawal means withdrawing metal in the form of plates, bars and the like which has been solidified in the mold 310 from the mold 310 by the use of pinch rolls 330 installed in the lower part of the mold 310. Before the start of the withdrawal, powder which prevents oxidation of the surface of the molten metal and functions as a lubricant between the solidified metal and the mold is sprayed onto the surface of the molten metal. Further, by the mold oscillation device 320 shown in Fig. 1, mold oscillation (MO) in which the mold 310 is made to oscillate in the vertical direction is started. An amplitude (stroke) of mold oscillation is from 2 millimeters to 6 millimeters while a period of mold oscillation is from 30 cycles to 450 cycles per minute. The withdrawal is started after the mold oscillation has been started, and the mold oscillation continues while the withdrawal is being carried out.

    [0026] As shown in Fig. 6, before the start of pouring of molten metal into the mold 310, the above-described pre-pouring calibration of the mold level measuring device is carried out. After the start of pouring of molten metal into the mold 310, the level of the molten metal in the mold 310 rises. After the level of the molten metal in the mold 310 has reached the measuring range of the mold level measuring device 100, measurement of the mold level is carried out by the mold level measuring device 100. Further, for the duration of mold oscillation, mold oscillation signal calibration (MOSC) is carried out. The mold oscillation signal calibration will be described later.

    [0027] The output generating section 130 receives a signal which is proportional to the cyclic movement of mold oscillation from a controller which is not shown in Fig. 1, of the mold oscillation device 320 shown in Fig. 1. The output generating section 130 changes a rejection band of the mold oscillation filter 1301 according to the cycle (the frequency) of the above-described signal. The mold oscillation filter 1301 eliminates an influence of movement of the surface of the molten metal caused by the mold oscillation, on the output of the mold level measuring device by removing components of mold oscillation from the output of the signal amplifying section 120. Further, the influence of movement of the surface of the molten metal caused by the mold oscillation may be removed by an adder in place of the mold oscillation filter 1301. The adder is used to add a signal which is antiphase with the mold oscillation, and which has been received from the controller to the output of the signal amplifying section 120 such that components of mold oscillation therein can be eliminated. Thus, in place of the signal which is proportional to the cyclic movement of mold oscillation, which is used in the present embodiment, the signal which is antiphase with the mold oscillation can also be used in another embodiment.

    [0028] Figure 7 is a flowchart for illustrating mold oscillation signal calibration carried out by the mold oscillation signal calibration section 140 of the eddy current mold level measuring device 100.

    [0029] In step S1010 of Fig. 7, the mold oscillation signal calibration section 140 determines whether the MOSC is should be started or not. If the MOSC is started, the process goes to step S1020. If the MOSC is not started, the process goes on standby. The mold oscillation signal calibration section 140 determines the point in time when the MOSC is started, by the use of the above-described signal which is proportional to the cyclic movement of mold oscillation or the signal which is antiphase with the mold oscillation. In the steady state, the frequency of fluctuations of the molten metal surface (the surface of the molten metal) is from 0.1 Hz to 0.5 Hz. Accordingly, the mold oscillation signal calibration section 140 may be configured to start the MOSC when a high-pass filter or a band-pass filter which has a pass band corresponding to the frequency of mold oscillation detects components of mold oscillation and the amplitude of the components is greater than a predetermined value. Further, the output of the above-described filter may be used in the following steps.

    [0030] In step S1020 of Fig. 7, the amplitude determining section 1401 of the mold oscillation signal calibration section 140 obtains and updates the maximum value and the minimum value of the output of the mold level measuring device 100. For example, the amplitude determining section 1401 can be configured such that it obtains the maximum value and the minimum value in a time period from the point in time which precedes the point of updating by a predetermined period to the point of updating. The predetermined period is determined to be longer than the period of mold oscillation. The amplitude determining section 1401 configured as described above is able to successively obtain the maximum value and the minimum value even when at least one of the maximum value and the minimum value changes. The amplitude determining section 1401 obtains a difference between the maximum value and the minimum value as a measurement of amplitude.

    [0031] In step S1030 of Fig. 7, the operation section 1403 of the mold oscillation signal calibration section 140 obtains a deviation of measurement by the use of the above-described measurement of amplitude and the known value of amplitude of mold oscillation. As described above, the value of amplitude of mold oscillation varies in the range from 2 millimeters to 6 millimeters depending on an object to be casted. Accordingly, the operation section 1403 may obtain the value of amplitude of mold oscillation by the use of the signal which is proportional to the cyclic movement of mold oscillation or the signal which is antiphase with mold oscillation. Alternatively, the operation section 1403 may receive the value of amplitude of mold oscillation for each casting process from the outside, such as the controller and the operator. By way of example, the deviation of measurement can be represented by a ratio of a difference in the output of the measuring device, which corresponds to the above-described measurement of amplitude to a difference in the output of the measuring device, which corresponds to the known value of amplitude of mold oscillation and which is obtained in the case that the positive feedback ratio is the reference value. When the above-described ratio is 1, there is no deviation of measurement. In other words, the measuring device in which the positive feedback ratio is the reference value can be judged to be functioning appropriately. The greater a difference between the above-described ratio and 1, the greater the deviation of measurement is. A relationship between the above-described ratio and positive feedback ratio will be described later.

    [0032] The operation section 1403 may be configured such that it sends the above-described ratio which represents the deviation of measurement to the display section 160, by which the deviation of measurement is displayed to the operator.

    [0033] Further, the operation section 1403 obtains a theoretically corrected value of positive feedback ratio, which makes the above-described measurement of amplitude equal to the known value of amplitude of mold oscillation.

    [0034] Figure 8 illustrates how the theoretically corrected value of positive feedback ratio is determined in step S1030 of Fig. 7. The known value of amplitude is represented as S0. As to the solid line R2 in Fig. 8, a difference ΔV in the output of the level measuring device, which corresponds to the known value of amplitude S0 is equal to ΔV0. When the relationship represented by the solid line R2 shown in Fig. 8 holds, the positive feedback ratio is equal to the reference value. In other words, the difference ΔV0 corresponds to the case in which the positive feedback ratio is equal to the reference value. The difference ΔV0 is referred to as a standard value of difference in the output of the measuring device. As to the dot-dash line A2 in Fig. 8, the difference ΔV in the output of the level measuring device, which corresponds to the known value of amplitude S0 is greater than ΔV0. As to the dot-dot-dash line B2 in Fig. 8, the difference ΔV in the output of the level measuring device, which corresponds to the known value of amplitude S0 is smaller than ΔV0. In the case of the dot-dash line A2, the theoretically corrected value of positive feedback ratio is made greater than the reference value such that the difference ΔV in the output of the level measuring device is reduced and made equal to ΔV0. In the case of the dot-dot-dash line B2, the theoretically corrected value of positive feedback ratio is made smaller than the reference value such that the difference ΔV in the output of the level measuring device is increased and made equal to ΔV0.

    [0035] After that, the operation section 1403 obtains a ratio of the theoretically corrected value of positive feedback ratio described above to the reference value of positive feedback ratio. As described in connection with Fig. 8, in the case that the difference ΔV in the output of the level measuring device, which corresponds to the known value of amplitude S0 is greater than ΔV0, the above-described ratio of the theoretically corrected value of positive feedback ratio to the reference value of positive feedback ratio is greater than 1 (100%). In the case that the difference ΔV in the output of the level measuring device, which corresponds to the known value of amplitude S0 is smaller than ΔV0, the above-described ratio of the theoretically corrected value of positive feedback ratio to the reference value of positive feedback ratio is smaller than 1 (100%).

    [0036] Figure 9 shows a relationship between a ratio of the difference ΔV in the output of the level measuring device, which corresponds to the known value of amplitude S0 to ΔV0 and a ratio of the theoretically corrected value of positive feedback ratio to the reference value of positive feedback ratio. The horizontal axis of Fig. 9 represents a ratio of difference ΔV in the output of the level measuring device, which corresponds to the known value of amplitude S0 to ΔV0, while the vertical axis of Fig. 9 represents a ratio of theoretically corrected value of positive feedback ratio to the reference value of positive feedback ratio.

    [0037] In step S1040 of Fig. 7, the positive feedback ratio correcting section 1405 corrects the positive feedback ratio of the feedback impedance 1205. The positive feedback ratio correcting section 1405 is provided with a limiter for limiting a corrected value of positive feedback ratio within a predetermined range including the above-described reference value. LA1 and LB1 in Fig. 9 represent upper limits of the limiter, while LA2 and LB2 represent lower limits of the limiter. The positive feedback ratio correcting section 1405 may be configured to automatically change positive feedback ratio within the above-described predetermined range. Alternatively, the positive feedback ratio correcting section 1405 may be configured such that the operator who has recognized the deviation of measurement by the display section 160 can determine the necessity and an amount of correction and can send command for the correction through the input section 170.

    [0038] The positive feedback ratio correcting section 1405 may be configured to change the upper and lower limits of the limiter, that is, the above-described predetermined range depending on a period of mold oscillation. More specifically, in the case of a longer period, the above-described predetermined range is made narrower than in the case of a shorter period. In Fig. 9, an example of the upper and lower limits of positive feedback ratio in the case of a longer period is shown by LA1 and LA2, while an example of the upper and lower limits of positive feedback ratio in the case of a shorter period is shown by LB1 and LB2.

    [0039] The reason that in the case of a longer period, the predetermined range is made narrower than in the case of a shorter period is as below. In the case of a longer period (for example, in the case of 60 cycles per minute), frequency of the calibration is smaller and reliability of the calibration is lower than in the case of a shorter period (for example, in the case of 240 cycles per minute). Accordingly, the range of the limiter is made smaller in order to enhance stability and to promote safety.

    [0040] The positive feedback ratio correcting section 1405 may be configured such that it resets the value of positive feedback ratio to the reference value independently of the period of mold oscillation when a ratio of the difference ΔV in the output of the level measuring device, which corresponds to the known value of amplitude S0 to ΔV0 does not come within a predetermined range (for example, ±100%) within a predetermined period (for example, the period of calibration) after the positive feedback ratio correcting section 1405 has corrected the value of positive feedback ratio. According to an experiment which uses the present measuring device, when an amount of deviation (the above-described ratio of difference ΔV to ΔV0) exceeds ±100% after the value of positive feedback ratio has been corrected in calibration, calibration is completed in a shorter time in the case that at first the value of positive feedback ratio is reset to the reference value and then calibration is carried out than in the case that calibration is continued without resetting the value of positive feedback ratio.

    [0041] In step S1050 of Fig. 7, the mold oscillation signal calibration section 140 determines whether the MOSC is to be completed or not. The mold oscillation signal calibration section 140 determines a point in time when the MOSC is terminated by the use of the above-described signal which represents the cyclic movement of mold oscillation. If the MOSC is terminated, the process is terminated. If the MOSC is not terminated, the process goes to S1020.

    [0042] In the MOSC, a high accuracy is achieved because calibration is carried out using an amount of change in the output of the eddy current mold level measuring device and a known value of amplitude of mold oscillation. Further, according to the MOSC, calibration can be successively carried out during a time period of withdrawal as shown in Fig. 6. Accordingly, the MOSC is sufficiently adaptable to dynamic disturbances such as a change in vertical position of the tundish and a change in width of a slab mold. Thus, the MOSC makes it possible to realize a mold level measuring device which has a sufficiently high accuracy and is adaptable to the dynamic disturbances. Further, since the calibration is carried out by the use of output of the eddy current mold level measuring device itself, other types of level measuring devices such as a thermocouple type mold level meter and an electrode type mold level meter are not required.

    [0043] Further, since in a mold level measuring device and a mold level measuring method according to an embodiment of the present invention, a corrected value of positive feedback ratio is limited within a predetermined range including the reference value, calibration is prevented from making the operation unstable. Accordingly, calibration can be carried out with reliability and high accuracy can be maintained.


    Claims

    1. A mold level measuring method for measuring a level of molten metal in a mold by an eddy current mold level measuring device, the method comprising the steps of:

    determining a reference value of a positive feedback ratio (K) of an amplifying section (120) of the measuring device in environmental conditions before pouring of molten metal into the mold;

    detecting, by a coil (105C2) of the measuring device, a change in a value of impedance caused by a change in the level of molten metal in the mold;

    amplifying, by the amplifying section (120), a voltage corresponding to the change in the value of impedance;

    obtaining a standard value of difference in an output of the measuring device, the standard value of difference corresponding to a known value of amplitude of mold oscillation when the positive feedback ratio (K) is the reference value, obtaining a deviation of measurement based on a difference between the maximum value and the minimum value of the output of the measuring device during a time period of mold oscillation and the standard value of difference; and

    correcting the positive feedback ratio (K) so as to reduce the deviation of measurement while the positive feedback ratio (K) is maintained within a predetermined range having an upper limit (A1, B1) and a lower limit (A2, B2) and including the reference value.


     
    2. A mold level measuring method according to claim 1, wherein the predetermined range is changed depending on a period of mold oscillation.
     
    3. A mold level measuring method according to claim 1 or 2, wherein the positive feedback ratio (K) is reset to the reference value when the deviation of measurement does not come into a predetermined range within a predetermined time period after the positive feedback ratio has been corrected.
     
    4. A mold level measuring method according to any one of claims 1 to 3, wherein the difference between the maximum value and the minimum value of the output of the measuring device and the deviation of measurement are successively obtained during the time period of mold oscillation.
     
    5. An eddy current mold level measuring device for measuring a level of molten metal in a mold according to the method of claim 1, the device comprising:

    a detecting section (105) including the coil (105C2) and configured to perform the detecting step;

    an amplifying section (120) configured to perform the amplifying step;

    a pre-pouring calibration section (150) configured to perform the determining step; and

    a mold oscillation signal calibration section (140) configured to perform the steps of obtaining the deviation of measurement and of correcting the positive feedback ratio (K).


     
    6. An eddy current mold level measuring device according to claim 5, further comprising a display section (160) configured to display the deviation of measurement.
     
    7. An eddy current mold level measuring device according to claim 5 or 6, which is configured to change the predetermined range depending on a period of mold oscillation.
     
    8. An eddy current mold level measuring device according to any one of claims 5 to 7, wherein the mold oscillation signal calibration section (140) is further configured to reset the positive feedback ratio (K) to the reference value when the deviation of measurement does not come into a predetermined range within a predetermined time period after the mold oscillation signal calibration section (140) has corrected the positive feedback ratio.
     
    9. An eddy current mold level measuring device according to any one of claims 5 to 7, wherein the mold oscillation signal calibration section (140) is further configured to successively obtain the difference between the maximum value and the minimum value of the output of the measuring device and the deviation of measurement during the time period of mold oscillation.
     
    10. An eddy current mold level measuring device according to any one of claims 5 to 9, wherein the mold oscillation signal calibration section (140) is further configured to receive a signal which represents a cyclic movement of mold oscillation from the outside and to determine a time period in which calibration using mold oscillation is to be carried out, according to the signal which represents the cyclic movement of mold oscillation.
     
    11. An eddy current mold level measuring device according to any one of claims 5 to 10, wherein the mold oscillation signal calibration section (140) is further configured to determine the value of amplitude of mold oscillation based on an input from the outside.
     
    12. An eddy current mold level measuring device according to any one of claims 5 to 11, further comprising a filter (1301) configured to detect components of mold oscillation alone wherein the mold oscillation signal calibration section (140) is further configured to use signals which have passed through the filter when obtaining the difference between the maximum value and the minimum value of the output of the measuring device during the time period of mold oscillation.
     


    Ansprüche

    1. Ein Formfüllstandsmessverfahren zum Messen eines Füllstands von geschmolzenem Metall in einer Form mittels einer Wirbelstrom-Formfüllstandsmessvorrichtung, das Verfahren aufweisend die Schritte des:

    Ermittelns eines Referenzwerts eines positiven Rückkopplungsverhältnisses (K) eines Verstärkungsabschnitts (120) der Messvorrichtung bei Umgebungsbedingungen vor dem Eingießen des geschmolzenen Metalls in die Form,

    Erfassen, mittels einer Spule (105C2) der Messvorrichtung, einer Veränderung eines Impedanzwerts, welche durch eine Veränderung des Füllstands des geschmolzenen Metalls in der Form verursacht wird,

    Verstärken, durch den Verstärkungsabschnitt (120), einer Spannung, welche der Veränderung des Impedanzwerts entspricht,

    Erlangen eines Differenz-Standardwerts in einer Ausgabe der Messvorrichtung, wobei der Differenz-Standardwert einem bekannten Wert einer Formoszillationsamplitude, wenn das positive Rückkopplungsverhältnis (K) der Referenzwert ist, entspricht, Erlangen einer Messabweichung basierend auf einer Differenz zwischen dem Maximalwert und dem Minimalwert der Ausgabe der Messvorrichtung während einer Zeitdauer der Formoszillation und dem Differenz-Standardwert, und

    Korrigieren des positiven Rückkopplungsverhältnisses (K), um die Messabweichung zu verringern, während das positive Rückkopplungsverhältnis (K) innerhalb eines vorbestimmten Bereichs, welcher einen oberen Grenzwert (A1, B1) und einen unteren Grenzwert (A2, B2) aufweist und den Referenzwert enthält, gehalten wird.


     
    2. Ein Formfüllstandsmessverfahren gemäß Anspruch 1, wobei der vorbestimmte Bereich in Abhängigkeit von einer Formoszillationsperiode verändert wird.
     
    3. Ein Formfüllstandsmessverfahren gemäß Anspruch 1 oder 2, wobei das positive Rückkopplungsverhältnis (K) auf den Referenzwert zurückgesetzt wird, wenn die Messabweichung nicht in einen vorbestimmten Bereich innerhalb einer vorbestimmten Zeitdauer nach dem Korrigieren des positiven Rückkopplungsverhältnisses kommt.
     
    4. Ein Formfüllstandsmessverfahren gemäß irgendeinem der Ansprüche 1 bis 3, wobei die Differenz zwischen dem Maximalwert und dem Minimalwert der Ausgabe der Messvorrichtung und die Messabweichung nacheinander während der Formoszillation-Zeitdauer erlangt werden.
     
    5. Eine Wirbelstrom-Formfüllstandsmessvorrichtung zum Messen eines Füllstands von geschmolzenem Metall in einer Form nach dem Verfahren von Anspruch 1, die Vorrichtung aufweisend:

    einen Erfassungsabschnitt (105), welcher die Spule (105C2) aufweist und dazu eingerichtet ist, den Erfassen-Schritt durchzuführen,

    einen Verstärkungsabschnitt (120, welcher dazu eingerichtet ist, den Verstärken-Schritt durchzuführen,

    einen Vor-Einfüllung-Kalibrierabschnitt (150), welcher dazu eingerichtet ist, den Ermitteln-Schritt durchzuführen, und

    einen Formoszillationssignal-Kalibrierabschnitt (140), welcher dazu eingerichtet ist, die Schritte des Erlangens der Messabweichung und des Korrigierens des positiven Rückkopplungsverhältnisses (K) durchzuführen.


     
    6. Eine Wirbelstrom-Formfüllstandsmessvorrichtung nach Anspruch 5, ferner aufweisend einen Anzeigeabschnitt (160), welche dazu eingerichtet ist, die Messabweichung anzuzeigen.
     
    7. Eine Wirbelstrom-Formfüllstandsmessvorrichtung gemäß Anspruch 5 oder 6, welche dazu eingerichtet ist, den vorbestimmten Bereich in Abhängigkeit von einer Formoszillationsperiode zu verändern.
     
    8. Eine Wirbelstrom-Formfüllstandsmessvorrichtung gemäß irgendeinem von Ansprüchen 5 bis 7, wobei der Formoszillationssignal-Kalibrierabschnitt (140) ferner dazu eingerichtet ist, das positive Rückkopplungsverhältnis (K) auf den Referenzwert zurückzusetzen, wenn die Messabweichung nicht in einen vorbestimmten Bereich innerhalb einer vorbestimmten Zeitdauer, nachdem der Formoszillationssignal-Kalibrierabschnitt (140) das positive Rückkopplungsverhältnis korrigiert hat, kommt.
     
    9. Eine Wirbelstrom-Formfüllstandsmessvorrichtung gemäß irgendeinem von Ansprüchen 5 bis 7, wobei der Formoszillationssignal-Kalibrierabschnitt (140) ferner dazu eingerichtet ist, nacheinander die Differenz zwischen dem Maximalwert und dem Minimalwert der Ausgabe der Messvorrichtung und die Messabweichung während der Formoszillation-Zeitdauer zu erlangen.
     
    10. Eine Wirbelstrom-Formfüllstandsmessvorrichtung gemäß irgendeinem von Ansprüchen 5 bis 9, wobei der Formoszillationssignal-Kalibrierabschnitt (140) ferner dazu eingerichtet ist, ein Signal, welches eine zyklische Formoszillationsbewegung repräsentiert, von außerhalb her zu empfangen und eine Zeitdauer, während welcher die Kalibrierung unter Verwendung von Formoszillation auszuführen ist, gemäß dem Signal, welches die zyklische Formoszillationsbewegung repräsentiert, zu ermitteln.
     
    11. Eine Wirbelstrom-Formfüllstandsmessvorrichtung gemäß irgendeinem von Ansprüchen 5 bis 10, wobei der Formoszillationssignal-Kalibrierabschnitt (140) ferner dazu eingerichtet ist, den Wert der Formoszillationsamplitude basierend auf einer Eingabe von außerhalb zu ermitteln.
     
    12. Eine Wirbelstrom-Formfüllstandsmessvorrichtung gemäß irgendeinem von Ansprüchen 5 bis 11, ferner aufweisend ein Filter (1301), welches dazu eingerichtet ist, Formoszillationskomponenten alleine zu erfassen, wobei der Formoszillationssignal-Kalibrierabschnitt (140) ferner dazu eingerichtet ist, Signale, welche das Filter passiert haben, beim Erlangen der Differenz zwischen dem Maximalwert und dem Minimalwert der Ausgabe der Messvorrichtung während der Formoszillation-Zeitdauer zu benutzen.
     


    Revendications

    1. Procédé de mesure de niveau de moule pour mesurer un niveau de métal en fusion dans un moule par un dispositif de mesure de niveau de moule par courant de Foucault, le procédé comprenant les étapes :

    de détermination d'une valeur de référence d'un rapport de rétroaction positive (K) d'une section d'amplification (120) du dispositif de mesure dans des conditions environnementales avant le déversement de métal en fusion dans le moule ;

    de détection, par une bobine (105C2) du dispositif de mesure, d'un changement d'une valeur d'impédance provoqué par un changement du niveau de métal en fusion dans le moule ;

    d'amplification, par la section d'amplification (120), d'une tension correspondant au changement de la valeur d'impédance ;

    d'obtention d'une valeur standard de différence d'une sortie du dispositif de mesure, la valeur standard de différence correspondant à une valeur connue d'amplitude d'oscillation de moule lorsque le rapport de rétroaction positive (K) est la valeur de référence, en obtenant un écart de mesure sur la base d'une différence entre la valeur maximum et la valeur minimum de la sortie du dispositif de mesure pendant une période de temps d'oscillation de moule et la valeur standard de différence ; et

    de correction du rapport de rétroaction positive (K) de manière à réduire l'écart de mesure alors que le rapport de rétroaction positive (K) est maintenu dans une plage prédéterminée ayant une limite supérieure (A1, B1) et une limite inférieure (A2, B2) et comprenant la valeur de référence.


     
    2. Procédé de mesure de niveau de moule selon la revendication 1, dans lequel la plage prédéterminée est changée en fonction d'une période d'oscillation de moule.
     
    3. Procédé de mesure de niveau de moule selon la revendication 1 ou 2, dans lequel le rapport de rétroaction positive (K) est réinitialisé à la valeur de référence lorsque l'écart de mesure n'entre pas dans une plage prédéterminée dans les limites d'une période de temps prédéterminée après que le rapport de rétroaction positive a été corrigé.
     
    4. Procédé de mesure de niveau de moule selon l'une quelconque des revendications 1 à 3, dans lequel la différence entre la valeur maximum et la valeur minimum de la sortie du dispositif de mesure et l'écart de mesure sont obtenus successivement pendant la période de temps d'oscillation de moule.
     
    5. Dispositif de mesure de niveau de moule par courant de Foucault pour mesurer un niveau de métal en fusion dans un moule conformément au procédé de la revendication 1, le dispositif comprenant :

    une section de détection (105) comprenant la bobine (105C2) et configurée pour effectuer l'étape de détection ;

    une section d'amplification (120) configurée pour effectuer l'étape d'amplification ;

    une section d'étalonnage pré-déversement (150) configurée pour effectuer l'étape de détermination ; et

    une section d'étalonnage de signal d'oscillation de moule (140) configurée pour effectuer les étapes d'obtention de l'écart de mesure et de correction du rapport de rétroaction positive (K).


     
    6. Dispositif de mesure de niveau de moule par courant de Foucault selon la revendication 5, comprenant en outre une section d'affichage (160) configurée pour afficher l'écart de mesure.
     
    7. Dispositif de mesure de niveau de moule par courant de Foucault selon la revendication 5 ou 6, qui est configuré pour changer la plage prédéterminée en fonction d'une période d'oscillation de moule.
     
    8. Dispositif de mesure de niveau de moule par courant de Foucault selon l'une quelconque des revendications 5 à 7, dans lequel la section d'étalonnage de signal d'oscillation de moule (140) est en outre configurée pour réinitialiser le rapport de rétroaction positive (K) à la valeur de référence lorsque l'écart de mesure n'entre pas dans une plage prédéterminée dans les limites d'une période de temps prédéterminée après que la section d'étalonnage de signal d'oscillation de moule (140) a corrigé le rapport de rétroaction positive.
     
    9. Dispositif de mesure de niveau de moule par courant de Foucault selon l'une quelconque des revendications 5 à 7, dans lequel la section d'étalonnage de signal d'oscillation de moule (140) est en outre configurée pour obtenir successivement la différence entre la valeur maximum et la valeur minimum de la sortie du dispositif de mesure et l'écart de mesure pendant la période de temps d'oscillation de moule.
     
    10. Dispositif de mesure de niveau de moule par courant de Foucault selon l'une quelconque des revendications 5 à 9, dans lequel la section d'étalonnage de signal d'oscillation de moule (140) est en outre configurée pour recevoir un signal qui représente un mouvement cyclique d'oscillation de moule en provenance de l'extérieur et pour déterminer une période de temps pendant laquelle un étalonnage en utilisant l'oscillation de moule doit être effectué, conformément au signal qui représente le mouvement cyclique d'oscillation de moule.
     
    11. Dispositif de mesure de niveau de moule par courant de Foucault selon l'une quelconque des revendications 5 à 10, dans lequel la section d'étalonnage de signal d'oscillation de moule (140) est en outre configurée pour déterminer la valeur d'amplitude d'oscillation de moule sur la base d'une entrée appliquée de l'extérieur.
     
    12. Dispositif de mesure de niveau de moule par courant de Foucault selon l'une quelconque des revendications 5 à 11, comprenant en outre un filtre (1301) configuré pour détecter les composantes d'oscillation de moule seules, dans lequel la section d'étalonnage de signal d'oscillation de moule (140) est en outre configurée pour utiliser les signaux qui sont passés à travers le filtre lors de l'obtention de la différence entre la valeur maximum et la valeur minimum de la sortie du dispositif de mesure pendant la période de temps d'oscillation de moule.
     




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    Cited references

    REFERENCES CITED IN THE DESCRIPTION



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    Patent documents cited in the description