CA1198293A - Method of detecting the level of a melt in a continuous-casting mold - Google Patents
Method of detecting the level of a melt in a continuous-casting moldInfo
- Publication number
- CA1198293A CA1198293A CA000420648A CA420648A CA1198293A CA 1198293 A CA1198293 A CA 1198293A CA 000420648 A CA000420648 A CA 000420648A CA 420648 A CA420648 A CA 420648A CA 1198293 A CA1198293 A CA 1198293A
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- Canada
- Prior art keywords
- melt
- level
- mold
- voltages
- melt level
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
- G01F23/00—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm
- G01F23/22—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water
- G01F23/26—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water by measuring variations of capacity or inductance of capacitors or inductors arising from the presence of liquid or fluent solid material in the electric or electromagnetic fields
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- Physics & Mathematics (AREA)
- Engineering & Computer Science (AREA)
- Electromagnetism (AREA)
- Power Engineering (AREA)
- Thermal Sciences (AREA)
- Fluid Mechanics (AREA)
- General Physics & Mathematics (AREA)
- Measurement Of Levels Of Liquids Or Fluent Solid Materials (AREA)
- Continuous Casting (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
The invention is concerned with a method of detecting the level of a melt in a continuous-casting mold by means of a primary coil and at least three vertically spaced secondary coils surrounding the mold. According to the invention, a primary magnetic field is formed with the primary coil, which extends horizontally through the mold into the melt generally at the melt level. This primary field generates eddy currents in the melt that in turn generate secondary fields that induce in the secondary coils voltages that peak as the melt level comes level with the secondary coils. Each of the induced voltages is combined with each of the other induced voltages to provide a plurality of output voltage curves each correspond-ing to the combined outputs of the respective two induced voltages and each peaking when the melt level lies at a respective location intermediate the respective induced voltages. The melt level position relative to the coils is derived from these output voltage curves.
The invention is concerned with a method of detecting the level of a melt in a continuous-casting mold by means of a primary coil and at least three vertically spaced secondary coils surrounding the mold. According to the invention, a primary magnetic field is formed with the primary coil, which extends horizontally through the mold into the melt generally at the melt level. This primary field generates eddy currents in the melt that in turn generate secondary fields that induce in the secondary coils voltages that peak as the melt level comes level with the secondary coils. Each of the induced voltages is combined with each of the other induced voltages to provide a plurality of output voltage curves each correspond-ing to the combined outputs of the respective two induced voltages and each peaking when the melt level lies at a respective location intermediate the respective induced voltages. The melt level position relative to the coils is derived from these output voltage curves.
Description
The present invention relates to a method for determining the level of a melt in a continuous-casting moldO More particularly, the invention is concerned with a measurement procedure for measuring exactly the position of the melt level in the mold.
In the continuous casting of steel to produce in a single operation shapes equivalent in section to conventional semifinished shapes, molten steel is poured from a tundish or ladle into the top of a cooled mold having a throughgoing passage. The steel cools and at least its surface hardens in the mold, so that a cast strand can be pulled continu-ously by pinch rollers from the bottom of the mold. The mold is frequently reciprocated continuously vertically in order to prevent the casting from adhering to it, as such sticking could cause a brea~out of molten steel in or below the mold.
One of the parameters which is absolutely critical is the melt level, or the vertical level of the upper surface of the liquid steel in the mold. This level must be maintained above a lower limit so that the casting is sufficiently strong as it exits from the mold to not break out, and below an upper limit which would cause the mold to overflow or the casting to be so rigid as it exits the mold that, for example, it could not be bent, as the strand invariably exits down from the rnold and usually must be bent to a horizontal position for ~urther treatment. The range hetween these two limits is quite small. The melt level can be varied either by with-drawing the casting at a greater speed to lower it or more 510wly to lift it, or by reducing the fill rate to lower it or increasing the fill rate to lift it.
In French Patent No~ 2,251,811, measurements are made of the impedance of a coil, in the moving alternating current field of which a conductor, e.g~ a melt, is moved.
This coil is juxtaposed with the top of the melt, which obviously is of conductive material while whatever above it is not, and can therefore roughly detect the melt level.
Two U-shaped inductor cores are ernployed which are parallel to each other but whose coils are oppositely connected. Such inductors must be aligned with an opening or nonmagnetic window in a mold at the melt level since a standard conductive mold would effectively shield the material it holds and make this type of measuring device useless. It is unfortunately impossible to provide such a window in a steel-casting mold which is normally made of thick water cooled copper in which eddy currents would form that would create a powerful secondary field completely masking the secondary field created by the eddy currents forrned in the melt inside it. Arranging the system to hang down inside the mold has not worked out, as the level meter gets in the way and is quickly destroyed by the heat and corrosive chemicals generated by the processA
U~S. Patent No. 4,279,149 describes a system which eliminates the influence of the mold~ According to this patent, an alternating current field-forming primary coil and two oppositely connected similar secondary coils as well as the liquid metal form a system in which the position of the melt level relative to the coils produces an induced voltage.
The primary and secorldary coils engage without contact coaxially around the mold without touching it. The position of the melt level determines the voltages induced in the secondary coils as well as the electrical conductivity of the melt. The detected voltage must be corrected for the particular conductivity to determine the melt level. Such an arrangement can only be used for small tubular molds due to the necessity of providing annular coils surrounding the mold and the difficulty of providing such arrangements on a large scale.
European Patent Application No. 82O630~020 published on Septer~er 22, 19~32 under Serial No. 0060800 describes a sys-tem used with a vertlcally reciprocating continuous-casting mold. A nonhomogeneous steady magnetic field is formed ex-tending horizontally through the mold into the melt generally at the melt level. This field is vertically reciprocated jointly and synchronously with the mold and its field strength is detected at a sensing location after the field passes through the mold. This location is also vertically recipro-cated jointly and synchronously with the mold. The melt level is derived from the detected field strength, normally taking into account the melt conductivity and relative displacement rate of the mold and melt. Obviously, the readings taken will vary regularly with the reciprocation of the mold, but such variation can easily be compensated out by standard electronlc techniq~tes.
Such use of a steady field, that is, a ~ield produced by a permanent magnet or one produced by an electro-magnet energized with a direct current or alternating current of at most 10 Hz cmd preferably no more than 5 Hz, auto-matically eliminates the eEfect of the mold. Since the field cloes not move appreciably relative to the mold, it generates virtually no eddy currents in it that would generate secondary fields. Thus, the field simply passes through the normally nonrnagnetic copper mold. The field will be affected by the ferrous melt however and this effect can be measured and from it can be derived the melt level~ Several field sources can be used and when these are electromagnetic they can be connected in series. The magnetic permeability of the liquid metal of the melt is normally similar to that of vacuum, air, or a protective gas. Thus, the magnetic permeability of the melt is largely irrelevant.
Such arrangements are adequate for measuring the melt level within a limited range. Determining the melt level on system startup, however, requires that several such sets of coils be provided along the mold.
Another disadvantage of the known systems is that they are relatively inaccurate when the melt level lies close to the desired location. The induced voltages form a bell curve (voltage, normally in mV, plotted against melt height, normally in mm) which peaks at the middle of the range, making accurate measurement of it difficult or impossible~
The field density is greatest at the top of the melt so that as it comes level with a coil, the induced voltage in this coil reaches a maximum, then drops off as the top of the melt moves past the coil, forming a perfectly symmetrical bell curve if the relative displacement speed is constant. The relationship of induced voltage to mold position, which is fairly uniform in khe two flanks of this bell curve, changes drastically at the peak thereof such that the accuracy of measurements suf~ers greatly in this region.
It i8 therefore an object of the present invention to provicle an improved method for measuring the melt level in a continuous-casting mold, which overcomes the aforesaid drawbacks.
~:~9~
It is another object of the invention to provide a method which gives accurate measurements along its entire measurement range and which can readily be adapted to a very wide range, so it can also be used at startup when the mold is filled.
In accordance with the present invention, there is thus provided a method of detecting the level of a melt in a continuous-casting mold by means of a primary coil and at least three vertically spaced secondary coils surrounding the mold. According to the invention, a primary magnetic field is formed with the primary coil, which extends horizontally through the mold into the melt generally at the melt level.
This primary field generates eddy currents in the melt that in turn generate secondary fields that induce in the secondary coils voltages that peak as the melt level comes level with the secondary coils. Each of the induced voltages is combined with each of the other induced voltages to provide a plurality of output voltage curves each corresponding to the combined outputs of the respective two induced voltages and each peaking when the melt level lies at a respective location intermediate the respective induced voltages. The melt level position relative to the coils is derived from these output voltage curves.
The invention is based on the principle that the individual voltage curves of the secondary coils that are connected opposite to one another have a shape that is either generally linear or made of straight portions. rrhus, with one primary and t:hree secondary coils, there are according to the invention thr.ee curves, that of the first coil minus the second, the first minus the third, and the second minus :~19~3293 the third. of course, with a larger number n of coils a number of output voltages will be derived equal to the sum of all the whole numbers between 1 and (n-l). All of these curves are employed and the linear or straight portions are compared with the nonlinear or nonstraight portions and the dif-ferences are recorded. It is a relatively easy procedure to ignore readings corresponding to the inaccurate peaks of the curves, which pea~s are according to this invention positioned level with straight portions of the other curves so accurate readings can be made along the entire measurement range. In other words each of the curves has at least one fairly straiyht portions and the secondary coils are spaced such that the straight portions cover substantially the entire vertical measurement range.
In practice, the voltages in the three curves are followed and compared almost simultaneously with a linear curve derived from the immediate surroundings to correct for local magnetism. With this system, it is therefore possible to determine the melt`level within about 1 mm. Such accuracy has been hitherto unattainable.
Accordiny to a preferred embodiment of the invention, the output voltages are compared to derive the melt level.
Preferably the melt level position derived principally by analysis of the output voltage corresponding to the location to which the level is closest. The other output voltages can be compared with respective set points to determine the approximate position of the melt level.
When the position of the melt level is derived by analysis of the output voltage corresponding to the location to which the level is closest, the direction of change of the other output voltages may be detected to determine the direction of displacement of the melt level.
It is also possible to detect the position of the melt level by comparing the change of at least one of the output voltages with respect to time.
In addition, the displacement of the melt may be varied in response to the position o-f the melt level. Means of doing so are described in detail in the afore-mentioned European patent application ~o. 82.630.020.
Further features and advantages of the invention will become more readily apparent from the following description of preferred er~bodiments, reference being rnade to the accompanying drawings in which:
Fig. 1 is a diagrammatic representation of a system for carrying out a method according to the invention;
Fig. 2 shows another system using two coil sets each having one primary and three secondary coils; and Fig. 3 shows an arrangement using one secondary and three unevenly spaced secondary coils.
As shown in Fig. 1, a coil arrangement SPl coaxially surrounds a rnold K in which a ferrous melt M centered on a vertical axis A has a li~uid level L. The arrangement SPl includes a single primary coil Pl and n secondary coils Sl, S2, S3 .~. Sn 1 and Sn in which respective voltages Vl, V2, V3 ... Vn 1 and Vn are induced from the eddy currents induced in the melt M by the field of the primary coil. As is well known in the art, the melt is continuously fa]ling and is replenished frorn a ladle or tundish from above through a valve V (Fig. 3) so that it is moving relative to the steady primary magnetic field of the coil Pl. It is this relative ~8~
rnotion that generates eddy currents in the melt M that in turn general secondary magnetic fields.
When the mold K is empty, the total voltage induced in the secondary coils Sl -~ Sn is equal to zero~ As the level L rises, at first the induced voltage in the lowest coil Sn decreases so that the level of the resultant voltages /~Vn 1) - Vn ~ or (Vl - Vn) increases. At maximum asymmetry, that is when level L lies equidistant between the two like secondary coils whose induced voltages are combined for a given output voltage, this output voltage is at a maximum.
As the level L moves up past this middle point, asymmetry decreases along with this combined output voltage. Two induced voltages which each increase and decrease when the melt level comes up to and passes the respective secondary coils are combined to form an output voltage which peaks instead when the level reaches a point midway between the two secondary coils. This generally straight flanks of this output voltage can be interleaved with those of other secondary coil pairs so that the entire vertical measurement range is covered.
Fig. 2 shows an arrangement having an upper coil set SPl' and a lower coil set SP2' having respective primary coils Pl' and P2' and secondary coils Sl 1' Sl 2' Sl 3, and S2 1~ S2 2~ S2 3. The number of coil sets needed can be increased to follow the melt level during mo]d filling. In any case, there are at least three secondary coils for each primary coilO
In Fig~ 3, the system is identical to that of Fig.
1, but the coils Sl and S2 are spaced more than the coils S2 and S3. This offset subdivides the entire measurement range into three nearly equal zones.
The individual curves shown to the right in Fign 3 resemble Gaussian curves, none of which can be used to determine the melt level L. The vertical line from +Z -to -Z
represents the position of the level L, normally represented in millimeters, and the horizontal line the voltage, normally in millivolts. The curve Vl - V3 only is generally straight between the points 1 and 2 and between the points 3 and 4, but is curved between the points 2 and 3. More particularly, there is a~airly linear relationship in these straight portions between the distance from the vertical coordinate line and the vertical position of the level L along this line. In the curved peak of the output-voltage curve, this relationship does not hold. The same is true for the curves Vl - V2 and V2 ~ V3.
When the level L is in the indicated position, it would be impossible to accurately determine its position with the curve Vl - V3, at the point X3~ as one would be using the curved central portion where there is little relationship between voltage and actual position. In fact, if the level L
rises to the center of this curve there is virtually no voltage change corresponding to a change of position. It is however possible to determine at the points X1 and X2 on the other two output-voltage curves V1 - V3 and V2 - V3 the exact position of the level L, since these points lie on approximately straight portions of these curves.
Obviously the coils are arranged so that any given melt height in the desired measurement range lies on only a single straight stretch of the curve of a single output voltage. Nonetheless, the use of two overlapping stretches as sh~wn in Fig. 3 adds to the accuracy of the measurement.
_ g _ Taking two such measurements allows outside factors to be canceled out, here by determining the difference between the points Xl and X2~
Determining that a given value of Vl - V2 is greater than the value at the points 2 and 3 establlshes that the level L is in a given region. This allows the readings V2 - V3 to be pinpointed.
Fig. 3 also shows how a controller C is connected to a valve V that controls the filling rate for the mold K, and through an unillustrated connection to a traction roller R
that pulls the strand out of the mold ~. The coils Sl, S2, and S3 are connected to three combiners or adders Al, A2, and A3 to produce the output signals shown to the right in FigO 3.
The controller C is also connected to a transponder T
constituted by a coil that generates an output signal that is subtracted from all the signals to eliminate ambient magnetismO
The controller C is an electronic microprocessor which also compares the incoming signals to set points to determine the approximate position of the melt level L so that the signal from the curve that would give the most accurate reading for that portion of the range is employed. This is most easily done by establishing set points that represent crossovers between deriving the melt level from one output voltage and to a mode deriving it from another. Thus when, for instance, the level is being derived from the curve (Vl - V3) between the levels equal to points 3 and ~, a set point at the point 3 triggers the contro~ler C to take over the reading from the curve (Vl - V2) which is more linear above this level. In addition this controller C, in accordance with standard electronic practice, monitors the direction of change of any of the incoming signals to determine whether the level L is rising or falling.
The method according to this invention allows one to continuously monitor the system in that according to Fig. 3 an increase in the value Xl must be accompanied by a decrease in the value X2 and must indicate a ~all in the level L. In general, not only the individual values but also their changes with time can be used for monitoring the system and determining melt level. Since one normally only measures the effective value of the differential voltages, negative differences are not shown in the bottom half, but instead are flipped up over the abscissa. This creates as a rule bends in the curves whose tips lie on the abscissa. Angles to both side of the point touching the abscissa are identical. These inter-sections with the abscissa only occur at the outer ends of the bell curve and can be avoided by increasing or decreasing the measurement voltage.
Of course the various measurements are handled by a microprocessor which in turn controls the continuous-casting operation. When the voltages on the upper spools Sl and S2 are affected the drawing-off speed is increased and when the lower spools S3 or Sn are affected the speed is decreased by means of the roller R. Instead of changing the speed, it is also possible to vary the input rate for metal into the top of the mold K by adjusting the valve V.
,~
In the continuous casting of steel to produce in a single operation shapes equivalent in section to conventional semifinished shapes, molten steel is poured from a tundish or ladle into the top of a cooled mold having a throughgoing passage. The steel cools and at least its surface hardens in the mold, so that a cast strand can be pulled continu-ously by pinch rollers from the bottom of the mold. The mold is frequently reciprocated continuously vertically in order to prevent the casting from adhering to it, as such sticking could cause a brea~out of molten steel in or below the mold.
One of the parameters which is absolutely critical is the melt level, or the vertical level of the upper surface of the liquid steel in the mold. This level must be maintained above a lower limit so that the casting is sufficiently strong as it exits from the mold to not break out, and below an upper limit which would cause the mold to overflow or the casting to be so rigid as it exits the mold that, for example, it could not be bent, as the strand invariably exits down from the rnold and usually must be bent to a horizontal position for ~urther treatment. The range hetween these two limits is quite small. The melt level can be varied either by with-drawing the casting at a greater speed to lower it or more 510wly to lift it, or by reducing the fill rate to lower it or increasing the fill rate to lift it.
In French Patent No~ 2,251,811, measurements are made of the impedance of a coil, in the moving alternating current field of which a conductor, e.g~ a melt, is moved.
This coil is juxtaposed with the top of the melt, which obviously is of conductive material while whatever above it is not, and can therefore roughly detect the melt level.
Two U-shaped inductor cores are ernployed which are parallel to each other but whose coils are oppositely connected. Such inductors must be aligned with an opening or nonmagnetic window in a mold at the melt level since a standard conductive mold would effectively shield the material it holds and make this type of measuring device useless. It is unfortunately impossible to provide such a window in a steel-casting mold which is normally made of thick water cooled copper in which eddy currents would form that would create a powerful secondary field completely masking the secondary field created by the eddy currents forrned in the melt inside it. Arranging the system to hang down inside the mold has not worked out, as the level meter gets in the way and is quickly destroyed by the heat and corrosive chemicals generated by the processA
U~S. Patent No. 4,279,149 describes a system which eliminates the influence of the mold~ According to this patent, an alternating current field-forming primary coil and two oppositely connected similar secondary coils as well as the liquid metal form a system in which the position of the melt level relative to the coils produces an induced voltage.
The primary and secorldary coils engage without contact coaxially around the mold without touching it. The position of the melt level determines the voltages induced in the secondary coils as well as the electrical conductivity of the melt. The detected voltage must be corrected for the particular conductivity to determine the melt level. Such an arrangement can only be used for small tubular molds due to the necessity of providing annular coils surrounding the mold and the difficulty of providing such arrangements on a large scale.
European Patent Application No. 82O630~020 published on Septer~er 22, 19~32 under Serial No. 0060800 describes a sys-tem used with a vertlcally reciprocating continuous-casting mold. A nonhomogeneous steady magnetic field is formed ex-tending horizontally through the mold into the melt generally at the melt level. This field is vertically reciprocated jointly and synchronously with the mold and its field strength is detected at a sensing location after the field passes through the mold. This location is also vertically recipro-cated jointly and synchronously with the mold. The melt level is derived from the detected field strength, normally taking into account the melt conductivity and relative displacement rate of the mold and melt. Obviously, the readings taken will vary regularly with the reciprocation of the mold, but such variation can easily be compensated out by standard electronlc techniq~tes.
Such use of a steady field, that is, a ~ield produced by a permanent magnet or one produced by an electro-magnet energized with a direct current or alternating current of at most 10 Hz cmd preferably no more than 5 Hz, auto-matically eliminates the eEfect of the mold. Since the field cloes not move appreciably relative to the mold, it generates virtually no eddy currents in it that would generate secondary fields. Thus, the field simply passes through the normally nonrnagnetic copper mold. The field will be affected by the ferrous melt however and this effect can be measured and from it can be derived the melt level~ Several field sources can be used and when these are electromagnetic they can be connected in series. The magnetic permeability of the liquid metal of the melt is normally similar to that of vacuum, air, or a protective gas. Thus, the magnetic permeability of the melt is largely irrelevant.
Such arrangements are adequate for measuring the melt level within a limited range. Determining the melt level on system startup, however, requires that several such sets of coils be provided along the mold.
Another disadvantage of the known systems is that they are relatively inaccurate when the melt level lies close to the desired location. The induced voltages form a bell curve (voltage, normally in mV, plotted against melt height, normally in mm) which peaks at the middle of the range, making accurate measurement of it difficult or impossible~
The field density is greatest at the top of the melt so that as it comes level with a coil, the induced voltage in this coil reaches a maximum, then drops off as the top of the melt moves past the coil, forming a perfectly symmetrical bell curve if the relative displacement speed is constant. The relationship of induced voltage to mold position, which is fairly uniform in khe two flanks of this bell curve, changes drastically at the peak thereof such that the accuracy of measurements suf~ers greatly in this region.
It i8 therefore an object of the present invention to provicle an improved method for measuring the melt level in a continuous-casting mold, which overcomes the aforesaid drawbacks.
~:~9~
It is another object of the invention to provide a method which gives accurate measurements along its entire measurement range and which can readily be adapted to a very wide range, so it can also be used at startup when the mold is filled.
In accordance with the present invention, there is thus provided a method of detecting the level of a melt in a continuous-casting mold by means of a primary coil and at least three vertically spaced secondary coils surrounding the mold. According to the invention, a primary magnetic field is formed with the primary coil, which extends horizontally through the mold into the melt generally at the melt level.
This primary field generates eddy currents in the melt that in turn generate secondary fields that induce in the secondary coils voltages that peak as the melt level comes level with the secondary coils. Each of the induced voltages is combined with each of the other induced voltages to provide a plurality of output voltage curves each corresponding to the combined outputs of the respective two induced voltages and each peaking when the melt level lies at a respective location intermediate the respective induced voltages. The melt level position relative to the coils is derived from these output voltage curves.
The invention is based on the principle that the individual voltage curves of the secondary coils that are connected opposite to one another have a shape that is either generally linear or made of straight portions. rrhus, with one primary and t:hree secondary coils, there are according to the invention thr.ee curves, that of the first coil minus the second, the first minus the third, and the second minus :~19~3293 the third. of course, with a larger number n of coils a number of output voltages will be derived equal to the sum of all the whole numbers between 1 and (n-l). All of these curves are employed and the linear or straight portions are compared with the nonlinear or nonstraight portions and the dif-ferences are recorded. It is a relatively easy procedure to ignore readings corresponding to the inaccurate peaks of the curves, which pea~s are according to this invention positioned level with straight portions of the other curves so accurate readings can be made along the entire measurement range. In other words each of the curves has at least one fairly straiyht portions and the secondary coils are spaced such that the straight portions cover substantially the entire vertical measurement range.
In practice, the voltages in the three curves are followed and compared almost simultaneously with a linear curve derived from the immediate surroundings to correct for local magnetism. With this system, it is therefore possible to determine the melt`level within about 1 mm. Such accuracy has been hitherto unattainable.
Accordiny to a preferred embodiment of the invention, the output voltages are compared to derive the melt level.
Preferably the melt level position derived principally by analysis of the output voltage corresponding to the location to which the level is closest. The other output voltages can be compared with respective set points to determine the approximate position of the melt level.
When the position of the melt level is derived by analysis of the output voltage corresponding to the location to which the level is closest, the direction of change of the other output voltages may be detected to determine the direction of displacement of the melt level.
It is also possible to detect the position of the melt level by comparing the change of at least one of the output voltages with respect to time.
In addition, the displacement of the melt may be varied in response to the position o-f the melt level. Means of doing so are described in detail in the afore-mentioned European patent application ~o. 82.630.020.
Further features and advantages of the invention will become more readily apparent from the following description of preferred er~bodiments, reference being rnade to the accompanying drawings in which:
Fig. 1 is a diagrammatic representation of a system for carrying out a method according to the invention;
Fig. 2 shows another system using two coil sets each having one primary and three secondary coils; and Fig. 3 shows an arrangement using one secondary and three unevenly spaced secondary coils.
As shown in Fig. 1, a coil arrangement SPl coaxially surrounds a rnold K in which a ferrous melt M centered on a vertical axis A has a li~uid level L. The arrangement SPl includes a single primary coil Pl and n secondary coils Sl, S2, S3 .~. Sn 1 and Sn in which respective voltages Vl, V2, V3 ... Vn 1 and Vn are induced from the eddy currents induced in the melt M by the field of the primary coil. As is well known in the art, the melt is continuously fa]ling and is replenished frorn a ladle or tundish from above through a valve V (Fig. 3) so that it is moving relative to the steady primary magnetic field of the coil Pl. It is this relative ~8~
rnotion that generates eddy currents in the melt M that in turn general secondary magnetic fields.
When the mold K is empty, the total voltage induced in the secondary coils Sl -~ Sn is equal to zero~ As the level L rises, at first the induced voltage in the lowest coil Sn decreases so that the level of the resultant voltages /~Vn 1) - Vn ~ or (Vl - Vn) increases. At maximum asymmetry, that is when level L lies equidistant between the two like secondary coils whose induced voltages are combined for a given output voltage, this output voltage is at a maximum.
As the level L moves up past this middle point, asymmetry decreases along with this combined output voltage. Two induced voltages which each increase and decrease when the melt level comes up to and passes the respective secondary coils are combined to form an output voltage which peaks instead when the level reaches a point midway between the two secondary coils. This generally straight flanks of this output voltage can be interleaved with those of other secondary coil pairs so that the entire vertical measurement range is covered.
Fig. 2 shows an arrangement having an upper coil set SPl' and a lower coil set SP2' having respective primary coils Pl' and P2' and secondary coils Sl 1' Sl 2' Sl 3, and S2 1~ S2 2~ S2 3. The number of coil sets needed can be increased to follow the melt level during mo]d filling. In any case, there are at least three secondary coils for each primary coilO
In Fig~ 3, the system is identical to that of Fig.
1, but the coils Sl and S2 are spaced more than the coils S2 and S3. This offset subdivides the entire measurement range into three nearly equal zones.
The individual curves shown to the right in Fign 3 resemble Gaussian curves, none of which can be used to determine the melt level L. The vertical line from +Z -to -Z
represents the position of the level L, normally represented in millimeters, and the horizontal line the voltage, normally in millivolts. The curve Vl - V3 only is generally straight between the points 1 and 2 and between the points 3 and 4, but is curved between the points 2 and 3. More particularly, there is a~airly linear relationship in these straight portions between the distance from the vertical coordinate line and the vertical position of the level L along this line. In the curved peak of the output-voltage curve, this relationship does not hold. The same is true for the curves Vl - V2 and V2 ~ V3.
When the level L is in the indicated position, it would be impossible to accurately determine its position with the curve Vl - V3, at the point X3~ as one would be using the curved central portion where there is little relationship between voltage and actual position. In fact, if the level L
rises to the center of this curve there is virtually no voltage change corresponding to a change of position. It is however possible to determine at the points X1 and X2 on the other two output-voltage curves V1 - V3 and V2 - V3 the exact position of the level L, since these points lie on approximately straight portions of these curves.
Obviously the coils are arranged so that any given melt height in the desired measurement range lies on only a single straight stretch of the curve of a single output voltage. Nonetheless, the use of two overlapping stretches as sh~wn in Fig. 3 adds to the accuracy of the measurement.
_ g _ Taking two such measurements allows outside factors to be canceled out, here by determining the difference between the points Xl and X2~
Determining that a given value of Vl - V2 is greater than the value at the points 2 and 3 establlshes that the level L is in a given region. This allows the readings V2 - V3 to be pinpointed.
Fig. 3 also shows how a controller C is connected to a valve V that controls the filling rate for the mold K, and through an unillustrated connection to a traction roller R
that pulls the strand out of the mold ~. The coils Sl, S2, and S3 are connected to three combiners or adders Al, A2, and A3 to produce the output signals shown to the right in FigO 3.
The controller C is also connected to a transponder T
constituted by a coil that generates an output signal that is subtracted from all the signals to eliminate ambient magnetismO
The controller C is an electronic microprocessor which also compares the incoming signals to set points to determine the approximate position of the melt level L so that the signal from the curve that would give the most accurate reading for that portion of the range is employed. This is most easily done by establishing set points that represent crossovers between deriving the melt level from one output voltage and to a mode deriving it from another. Thus when, for instance, the level is being derived from the curve (Vl - V3) between the levels equal to points 3 and ~, a set point at the point 3 triggers the contro~ler C to take over the reading from the curve (Vl - V2) which is more linear above this level. In addition this controller C, in accordance with standard electronic practice, monitors the direction of change of any of the incoming signals to determine whether the level L is rising or falling.
The method according to this invention allows one to continuously monitor the system in that according to Fig. 3 an increase in the value Xl must be accompanied by a decrease in the value X2 and must indicate a ~all in the level L. In general, not only the individual values but also their changes with time can be used for monitoring the system and determining melt level. Since one normally only measures the effective value of the differential voltages, negative differences are not shown in the bottom half, but instead are flipped up over the abscissa. This creates as a rule bends in the curves whose tips lie on the abscissa. Angles to both side of the point touching the abscissa are identical. These inter-sections with the abscissa only occur at the outer ends of the bell curve and can be avoided by increasing or decreasing the measurement voltage.
Of course the various measurements are handled by a microprocessor which in turn controls the continuous-casting operation. When the voltages on the upper spools Sl and S2 are affected the drawing-off speed is increased and when the lower spools S3 or Sn are affected the speed is decreased by means of the roller R. Instead of changing the speed, it is also possible to vary the input rate for metal into the top of the mold K by adjusting the valve V.
,~
Claims (8)
1. A method of detecting the level of a melt in a continuous-casting mold by means of a primary coil and at least three vertically spaced secondary coils surrounding the mold, said method comprising the steps of.
a) forming with the primary coil a primary magnetic field extending horizontally through the mold into the melt generally at the melt level, whereby the primary field generates eddy currents in the melt that in turn generate secondary fields that induce in the secondary coils voltages that peak as the melt level comes level with the secondary coils, b) combining each of the induced voltages with each of the other induced voltages to provide a plurality of output voltage curves each corresponding to the combined outputs of the respective two induced voltages and each peaking when the melt level lies at a respective location intermediate the respective induced voltages, and c) deriving the position of the melt level relative to the coils from the output voltage curves.
a) forming with the primary coil a primary magnetic field extending horizontally through the mold into the melt generally at the melt level, whereby the primary field generates eddy currents in the melt that in turn generate secondary fields that induce in the secondary coils voltages that peak as the melt level comes level with the secondary coils, b) combining each of the induced voltages with each of the other induced voltages to provide a plurality of output voltage curves each corresponding to the combined outputs of the respective two induced voltages and each peaking when the melt level lies at a respective location intermediate the respective induced voltages, and c) deriving the position of the melt level relative to the coils from the output voltage curves.
2. A method as defined in claim 1, wherein the output voltages are compared to derive the position of the melt level.
3. A method as defined in claim 1, wherein the position of the melt level is derived principally by analysis of the output voltage corresponding to the location to which the level is closest, and wherein the method further includes the step of comparing the other output voltages with respective set points to determine the approximate position of the melt level.
4. A method as defined in claim 1, wherein each of the output voltage curves has at least one generally straight portions, the secondary coils being spaced such that the straight portions cover substantially the entire vertical measurement range.
5. A method as defined in claim 1, wherein the position of the melt level is derived principally by analysis of the output voltage corresponding to the location to which the level is closest, and wherein the method further includes the step of detecting the direction of change of the other output voltages to determine the direction of displacement of the melt level.
6. A method as defined in claim 1, wherein the position of the melt level is derived by comparing the change of at least one of the output voltages with respect to time.
7. A method as defined in claim 1, further including the step of varying the displacement of the melt in response to the position of the melt level.
8. A method as defined in claim 1, further including the step of nonuniformly spacing the secondary coils.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| LU83969A LU83969A1 (en) | 1982-02-23 | 1982-02-23 | METHOD FOR MEASURING THE LEVEL OF LIQUID METALS IN CONTINUOUS CASTING PLANTS |
| LU83969 | 1982-02-23 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1198293A true CA1198293A (en) | 1985-12-24 |
Family
ID=19729823
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000420648A Expired CA1198293A (en) | 1982-02-23 | 1983-02-01 | Method of detecting the level of a melt in a continuous-casting mold |
Country Status (4)
| Country | Link |
|---|---|
| EP (1) | EP0087382A1 (en) |
| JP (1) | JPS58198727A (en) |
| CA (1) | CA1198293A (en) |
| LU (1) | LU83969A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5103893A (en) * | 1989-09-19 | 1992-04-14 | Nippon Steel Corporation | Method and apparatus for detecting level of molten metal |
Families Citing this family (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| LU85754A1 (en) * | 1985-02-01 | 1986-09-02 | Arbed | DEVICE FOR MEASURING THE FILLING LEVEL OF LIQUID METALS IN CONTAINERS |
| FR2580801B1 (en) * | 1985-04-23 | 1987-07-10 | Novatome | DEVICE FOR MEASURING THE LEVEL OF AN ELECTRICALLY CONDUCTIVE MEDIUM CONTAINED IN AN ENCLOSURE AND ITS APPLICATIONS |
| DE3541967A1 (en) * | 1985-11-27 | 1987-06-04 | Interatom | FLAT INDUCTIVE LEVEL SENSOR FOR LIQUID METALS |
| IT1222337B (en) * | 1987-10-21 | 1990-09-05 | Ceda Costruzioni Elettromeccan | DEVICE FOR MEASURING THE LEVEL OF LIQUID METAL IN A CRYSTALLIZER FOR CONTINUOUS CASTING TIN |
| US6577118B2 (en) | 2001-02-02 | 2003-06-10 | B.D.H. Industries Inc. | System and method for measuring liquid metal levels or the like |
| EP1229313A3 (en) * | 2001-02-02 | 2003-09-03 | Bdh Industries Inc. | System and method for measuring liquid metal level |
| ITPN20060005A1 (en) | 2006-01-27 | 2007-07-28 | Ergoline S Lab S R L | PROCEDURE AND DEVICE FOR THE MEASUREMENT AND CONTROL OF THE HEIGHT OF LIQUID METAL IN A CRYSTALLIZER. |
| CN109186687B (en) * | 2018-10-30 | 2021-04-27 | 锦州艾德自动控制系统有限公司 | Salt pond concentration and liquid level detection device suit |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CH551821A (en) * | 1972-10-17 | 1974-07-31 | Concast Ag | PROCESS AND EQUIPMENT FOR REGULATING THE LEVEL OF THE MELT IN MOLDS OF CONTINUOUS CASTING PLANTS. |
| AT342325B (en) * | 1973-11-21 | 1978-03-28 | Inst Elektroswarki Patona | CONTACTLESS INDUCTION SENSOR FOR DETERMINING THE SEPARATION LIMIT BETWEEN TWO MEDIA |
| CH624323A5 (en) * | 1977-09-19 | 1981-07-31 | Atomenergi Ab | |
| LU80410A1 (en) * | 1978-10-25 | 1980-05-07 | Arbed | METHOD FOR MEASURING THE LEVEL OF METALS IN CONTAINERS, ESPECIALLY IN CONTINUOUS CASTING CHILLERS |
-
1982
- 1982-02-23 LU LU83969A patent/LU83969A1/en unknown
-
1983
- 1983-02-01 CA CA000420648A patent/CA1198293A/en not_active Expired
- 1983-02-15 EP EP83630023A patent/EP0087382A1/en not_active Withdrawn
- 1983-02-23 JP JP2784483A patent/JPS58198727A/en active Pending
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5103893A (en) * | 1989-09-19 | 1992-04-14 | Nippon Steel Corporation | Method and apparatus for detecting level of molten metal |
Also Published As
| Publication number | Publication date |
|---|---|
| JPS58198727A (en) | 1983-11-18 |
| LU83969A1 (en) | 1983-09-02 |
| EP0087382A1 (en) | 1983-08-31 |
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