US3519060A - Continuous casting apparatus with a molten metal level control - Google Patents

Continuous casting apparatus with a molten metal level control Download PDF

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US3519060A
US3519060A US703750A US3519060DA US3519060A US 3519060 A US3519060 A US 3519060A US 703750 A US703750 A US 703750A US 3519060D A US3519060D A US 3519060DA US 3519060 A US3519060 A US 3519060A
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coil
level
output
mold
coils
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George Vischulis
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Acme Steel Co
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Interlake Steel Corp
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    • 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
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/16—Controlling or regulating processes or operations
    • B22D11/18—Controlling or regulating processes or operations for pouring
    • B22D11/181—Controlling or regulating processes or operations for pouring responsive to molten metal level or slag level
    • B22D11/186—Controlling or regulating processes or operations for pouring responsive to molten metal level or slag level by using electric, magnetic, sonic or ultrasonic means
    • G—PHYSICS
    • G05—CONTROLLING; REGULATING
    • G05D—SYSTEMS FOR CONTROLLING OR REGULATING NON-ELECTRIC VARIABLES
    • G05D9/00—Level control, e.g. controlling quantity of material stored in vessel
    • G05D9/12—Level control, e.g. controlling quantity of material stored in vessel characterised by the use of electric means

Definitions

  • This invention relates to level detector apparatus for indicating and controlling the level of a conductor confined within a boundary.
  • the invention is particularly adapted for sensing the level of liquid or molten metal within a metal processing machine, and using the level indication to maintain the molten metal at a predetermined level.
  • molten metal is poured into an elongated mold through which it passes and cooled to produce a continuous cast output strip or billet.
  • the mold is formed from vibrating sections having both transverse and longitudinal reciprocating movement following a harmonic or elliptical path, such movement causing the mold sections to repeatedly contact and propagate the casting in a forward stroke.
  • An example of a continuous casting machine is shown in U.S. Pat. 3,075,264 to Wognum, to which reference should be made for background information. In order to control the casting process, it is necessary to maintain the molten metal at a predetermined level in the mold.
  • level detector Another important factor which limits the type of level detector usable with a continuous casting machine is the presence of large amounts of metal in the machine itself, which acts as a shorted secondary winding and desensitizes a coil, thus masking the change in inductance otherwise caused by the variation in metal adjacent the coil.
  • gamma ray detectors or thermal cables located at spaced locations within the mold wall of the casting machine to sense the level of molten metal within the mold. While gamma ray detectors are responsive to a percent change in transmittance of radiated gamma rays to determine the level of the metal, they are ineffective for certain metals, such as aluminum, which have a high gamma ray transmittance, and the signal re- 3,519,060 Patented July 7, 1970 spouse is low Where the cast metal scanned is small relative to the entire metal scanned. Gamma ray detectors also present a safety hazard in terms of radiation. Thermal cables mounted within the mold wall have problems in that the mold walls in a continuous casting machine must oscillate away from the cast metal for approximately one-half of the time. Both systems are of relatively slow response time, and limit the speed at which a continuous casting machine may be operated.
  • a satisfactory level detector system requires a minimum amount of equipment located at the continuous casting machine itself, and such equipment at the machine should be of a type which can be easily added to existing machinery without requiring substantial modifications. None of the prior art level detectors used with continuous casting machines meets all of these diverse requirements.
  • novel apparatus for determining the level of a conductor confined within a boundary, which apparatus is especially adapted for determining and controlling the level of molten metal in a continuous casting machine.
  • Plural coils having a particular type of change in an electrical characteristic for a change in metal level are sensed in a differential manner to produce a level signal independent of fluctuating temperatures, other extraneous influences affecting inductance and the presence of large bodies of metal in the vicinity of the sensing system.
  • One object of this invention is the provision of an improved level detector for indicating and/ or controlling the level of a conductor confined within a boundary.
  • Another object of this invention is the provision of a level detector and control for a continuous casting machine, which uses an inductive and an AC resistance effect to determine the level of molten metal within the mold of the casting machine.
  • One feature of this invention is the provision of a level detector for a confined conductor, having a plurality of winding means located adjacent the boundry of a conductor, and a differential circuit coupled to the plurality of winding means for sensing a level signal which is independent of a common mode change in inductance of the winding means caused by temperature variations or other factors not directly related to the actual level of the conductor.
  • Another feature of this invention is the provision of a level detector for a continuous casting machine, which connects a plurality of coils surrounding the machine mold to an oscillator to produce self-resonant oscillations having a frequency dependent upon the value of inductance and AC resistance of the coils.
  • Yet another feature of this invention is the provision of a level detector which alternately connects a pair of coils to an oscillator to produce trains of self-resonant oscillations, the phase difference between the trains being measured by a differential circuit which includes means for compensating for common mode changes in the frequency determining characteristics of both coils.
  • FIG. 1 is a top plan view of a continuous casting machine and a portion of the level detector apparatus
  • FIG. 2 shows a front elevation of a continuous casting machine as viewed along lines 2-2 of FIG. 1, and more particularly shows the top portion of a continuous casting machine and a portion of the level detector apparatus;
  • FIG. 3 is a block diagram of the level detector apparatus for detecting and controlling the level of metal in the continuous casting machine of FIGS. 1 and 2;
  • FIG. 4 is a curve of oscillatory output frequency versus metal level when the level determining coils of FIGS. 1
  • FIGS. 5A and 5B are a single schematic diagram of the circuit of FIG. 3, in which the broken lines across the right-hand side of FIG. 5A and the left-hand side of FIG. 5B are placed together to show the continuation of the lines across the drawings; and
  • FIGS. 6A6T are diagrams illustrating the time relationship between the signals generated by the circuit of FIG. 5.
  • FIGS. 1 and 2 a continuous casting machine 10 is generally illustrated for gradually solidifying a mass of molten metal 11 poured from a tundish 12 into a mold 14 for the machine.
  • Mold 14 is composed of four mold sections 16 which are generally rectangularly block-shaped, depending upon the shape desired for the cross-sectional area of the final cast product.
  • Each mold section 16 is held by a mold retainer 18 which is vibrated through an elliptical orbit by a conventional motor 19.
  • a stationary corner strip 20 is held in place by a corner strip retainer 21 which is fixedly attached with respect to the base of machine 10.
  • the stroke is ordinarily maintained through a fixed distance or path, while the frequency or rate of completing a single elliptical path is varied to change the rate of metal flow through the machine.
  • each of the mold sections may make from 1,000 to 2,500 revolutions per minute, while following the path of a 10 to 1 ellipse.
  • the orbital amplitude by which the mold sections withdraw from the metal strip is very small, and is typically on the order of a few thousandths of an inch.
  • mold retainers 18 and mold strips 16 are of hollow construction to permit water or other cooling fluid to flow up through the retainer 18 and downwardly along the inner surface of mold section 16 in order to cool the molten metal adjacent the surface of the mold.
  • a continuous aperture or opening 25 is formed through the elongated portions of the mold 14 in order to permit water flow 26 to pass therethrough and cool the machine.
  • LEVEL SENSING COILS In order to control the quality of the cast billet, and other factors including preventing the metal pour from overflowing from the top of mold 14, it is important to determine the point 30 at which the thin molten metal pour spout flows outwardly and first contacts the surface wall of the mold sections 16.
  • the level point 30 will be referred to hereinafter as the level of the metal within the mold, although it will of course be apparent that some metal, in the form of the thin pour spout from the tundish 12, extends above the so-called level of metal within the mold.
  • a level sensor, in the form of coil means 32 located adjacent mold 14, provides an electrical condition which changes in accordance with changes in the level 30' of metal 11 within the mold. This change in electrical condition is possible because mold 14 is discontinuous or segmented, as will be explained in detail later.
  • Means 32 comprises a first coil U and a second coil L which may be split into an upper section L located above coil U and a lower section I.” located below coil U, for reasons to be explained hereinafter.
  • the net inductive effect of the pair of split coil sections L and L" is effectively the same as a single coil L located slightly below coil U.
  • the coils U and L are wound on a coil form or bobbin 35 which encircles the mold 14, and which is sealed by an encircling insulated form 36.
  • a coil shield 3'8 surrounds coil forms 35, 36 and performs the dual function of electrically isolating the coils U and L from the elfects of extraneous metal bodies located near the coils, and forming a splash shield to prevent molten metal 11 from contacting the coils or coil forms.
  • Shield 38 preferably formed of conductive material such as copper or stainless steel, is especially effective as a barrier which isolates coils U and L from the effect of tundish 12 and the varying level of molten metal 11 within the tundish.
  • Coil means 32 is rigidly mounted by fastening means 40 to a base 41 which is fixed with respect to machine 10 and tundish 12.
  • discontinuous in this context refers to a nonhomogeneous barrier which substantially prevents an induced circulating current from flowing through the barrier.
  • Entirely separate mold sections 16, FIG. 1, are only exemplary of one manner of forming a boundary having a discontinuity to the flow of an induced current.
  • the formation of a metal oxide layer on each mold section is believed to aid to providing the discontinuity to the flow of circulating currents.
  • the circulating current losses in the molten conductor change with the level of the conductor, and thus provide a means of detecting the conductor level. Since the circulating or eddy currents losses are reflected back and change both the inductance and the AC resistance characteristics of the coils U and L, either or both characteristics can be monitored to indicate metal level.
  • the circuit of FIG. 3 uses these characteristics of the coils in order to provide an indication of the level 30 of the molten metal within mold 14.
  • each coil U and L is connectable with an oscillator 50 in order to produce natural oscillations having a frequency dependent upon the value of the inductance and the AC resistance of the coil.
  • the frequency of oscillation is inversely proportional to the inductance and directly proportional to the AC resistance of a coil.
  • the inductance of the illustrated coils decrease while the AC resistances of the coils increase. Since both electrical effects contribute to an increase in frequency of oscillations, the oscillatory frequency can be used as a direct indication of the metal level.
  • the preferred circuit of FIG. 3 is responsive to the frequency of oscillations, affected both by the inductance and the AC resistance characteristics of the coils, a circuit could of course be used which was responsive to one of these electrical characteristics by itself.
  • Servo motor 52 may form a part of any conventional servomechanism for controlling the level of molten metal.
  • motor 52 could be connected by conventional circuitry to control the revolutions per minute of motor 19 of FIG. 2, which drives the mold sections 16 in the continuous caster. Such operation, affecting the rate at which the mold sections 16 complete a single cycle of elliptical motion, varies the rate of travel of the molten metal.
  • servo motor 52 could be connected to a valve (not illustrated) on tundish 12 in order to control the amount of molten metal flowing into mold 14, and hence also controlling level 30.
  • Lower coil L is comprised of a greater number of turns than upper coil U, and hence has a greater range of frequency deviation, when connected to oscillator 50, for a given change in metal level.
  • the coils U and L are de-tuned or otherwise so chosen so that the frequency curve of one of them is shifted relative to the other in order to produce a cross-over point at which the inductance of each coil is equal.
  • a characteristic curve is produced which can uniquely determine metal level. More particularly, the magnitude of the difference in the frequency of oscillations occurring when the two coils are connected to an oscillator is directly proportional to the distance variation of the metal level from a reference value. Furthermore, the frequencies generated by a particular coil, in this case coil L, are of higher value for metal levels above the reference level, and of lower frequency relationship than the other coil for metal levels below the reference level, and thus provide a unique indication of the direction in which the level has varied.
  • cross-over point 55 be chosen to occur at the level at which the metal is to be maintained, so that a zero volt signal is produced when the desired level of metal is maintained.
  • a single time shared oscillator 50 is provided for both coils U and L. Such a circuit is preferred in order to eliminate different temperature drift effects which would occur with separate oscillators for each coil.
  • An electronic switch 60 alternately connects coil U and coil L to oscillator 50, in order to produce at an output line 61 a train of oscillatory signals with continuously alternate portions of the train being attributable to either coil U or coil L.
  • the train of oscillations is heterodyncd in a mixer-detector 63 with continuous oscillations from a beat frequency oscillator 64 in order to produce a different or beat frequency on an output line 65 of detector 63.
  • a central clock 70 continuously produces timing pulses which are used for reference purposes in the circuit.
  • Clock 70 drives a steering flip-flop (PF) 72, to alternately enable one of a pair of output lines connected with electronic switch 60. The enabled line triggers the electronic switch associated therewith to connect one of the coils with oscillator 50.
  • the steering flip-flop 72 also produces output signals which are connected with remaining portions of the circuitry in order to synchronize the operation of various components with the connection of coil U or L to the oscillator 50.
  • the train of oscillations from detector 63 are coupled in parallel to a sample gate 80, responsive to portions of the train of oscillations attributable to coil U, and a sample gate 81, responsive to the remaining portions of the train of oscillations attributable to coil L.
  • Steering flip-flop 72 actuates or enables gate at the same time that electronic switch 60 connects coil U to oscillator 50. Similarlly, when electronic switch 60 connects coil L to oscillator 50, sample gate 81 is enabled.
  • the sample gates 80 and 81 when enabled steering flip-flop 72, pass a portion of the third cycle of oscillation to pulse width detector 83 for that sample gate.
  • a timing circuit 85 coupled between clock 70 and the sample gates provides a plurality of timing pulses, to be described in detail later.
  • each detector 83 produces an analog output signal which is directly proportional to the width of the pulse coupled thereto.
  • the output of both of the pulse width detectors 83 is coupled to a differential amplifier 87 to produce a single output signal representative of the difference between the two input signals. If the outputs of each of the pulse width detectors 83 are equal, it indicates that both coil U and coil L are generating the same frequency of oscillation, which occurs only at cross-over point 55 of FIG. 4, and hence a zero voltage is produced by differential amplifier 87. Should the analog signal from one of the pulse width detectors 83 exceed the other, differential amplifier 87 will produce a corresponding output signal of one polarity, representative of the difference therebetween. Conversely, should the outputs from the pulse width detectors 83 vary in the opposite direction, an opposite polarity signal representative of the difference therebetween will be generated.
  • the output signal from differential amplifier 87 is directly proportional to the difference between the oscillatory frequencies for the L and U coils. More particularly, the polarity of the output signal indicates whether the level is rising or falling from the desired or reference level, whereas the magnitude of the output signal indicates the amount or distance the metal level has varied from the desired level.
  • line 51 is connected to a servomechanism which includes servo motor 52 for controlling either the rate of adding molten metal to the continuous casting machine, or the rate of withdrawing metal from the continuous casting machine, as previously described.
  • the servomechanism includes a summing output amplifier 90 coupled between line 51 and servo motor 52.
  • a feedback network or sensor 91 is connected in a conventional manner with summing output amplifier 90 in order to produce a servomechanism which seeks null.
  • servo motor 52 is driven until the feedback signal from network 91 exactly balances the signal on line 51.
  • the inductance of coils U and L can change over a substantial range.
  • the inductance changes cause a resulting change in frequency of oscillator 50, which causes the curves of FIG. 4 to shift upwardly or downwardly from their illustrated position.
  • the difference in the frequency of oscillation between the pair of coils remains in the same proportion as that illustrated in FIG. 4. Since the circuit of FIG. 3 is only responsive to the difference rather than the absolute value of the frequencies, common mode extraneous influences are eliminated.
  • a common mode influence is one which affects both of the coils U and L equally, the described differential type circuit therefore exhibits common mode rejection, in that it discriminates against changes in a fundamental quantity affecting both coils.
  • the circuit rejects common mode inductance and like effects, an inductance change could produce frequency changes of sufficient magnitude so as to exceed the range of the fixed timing signals generated by the circuit.
  • the outputs of both of the pulse width detectors 83 are coupled to an averaging network 95 which generates an output signal representative of the absolute sum of both pulse width detectors.
  • This signal drives a conventional automatic frequency control (AFC) amplifier 96 to control the frequency of beat oscillator 64 in a known manner, as by varying the capacitance in oscillator 64 in inverse proportion to the amplitude of the control signal from averaging network 95.
  • AFC automatic frequency control
  • the AFC circuit 96 is responsive to a lesser amount of control signal to increase the frequency of beat oscillator 64, returning the beat between the oscillators 50 and 64 closer to the original value, and thus insuring that the timing signals produced by the circuit have the proper time relationship to enable sample gates and 81 at the proper time.
  • the legends carried by the inputs to the flip-flops indicate the manner in which the flip-flop is triggered by an input signal.
  • An input labeled RUN indicates that an input pulse will change or flip the instantaneous output of the flip-flop, regardless of the particular polarity values on a single or pair of output lines.
  • An input labeled A means that an input pulse will produce a positive output pulse or waveform at the A output of the flip-flop.
  • an input labeled A means a positive pulse will produce a negative pulse or waveform (i.e. a zero signal) at the A output of the flip-flop. All flip-flops trigger on the positive going edge of an input waveform.
  • Coil oscillator 50 may be a Colpitts circuit having component values so that when either of the coils U or L is switched into the oscillator circuit, a natural frequency of oscillations on the order of 200 kilocycles (kc.) or so is generated.
  • the coil electrical characteristics, especially inductance and AC. resistane, are believed to be mainly affected by the periphery and surface effects of the molten metal, with little penetration of the oscillations into the actual center of the molten metal.
  • the nominal frequency of oscillation is chosen by considering several factors. The number of cycles change or shift due to a level change is greater for higher frequencies. However, lower frequencies provide better penetration of the mold and casting machine.
  • coil U was formed from 30 turns of wire; while coil L was wound in two sections, the upper section L of FIG. 2 being formed from 18 turns of wire, and the lower section L" being formed from 19 turns of wire.
  • the splitting of coil L is be lieved, for certain continuous casting machines, to aid in averaging out the thermal effects across the coils and in distributing the effect of the large masses of metal of the continuous casting machine, which acts as a shorted secondary and tends to desensitize the coils.
  • the volume within which the coils can be mounted is of certain practical limits, and it is desired that a minimum number of turns be used within this volume. Lesser turns allows a proportionate increase in wire size, lowering the resistance of the coil in order to incerase the Q of the circuit.
  • the level detector is generally insensitive to the type of metal being cast.
  • Beat oscillator 64 produces a continuous frequency output at 194.180 kc., which beats or heterodynes in detector 63 with the oscillations from oscillator 50 produce a difference or beat frequency of nominally 5.820 kc.
  • Wave shapers 105 and 106 are connected to the output of coil oscillator 50 and beat oscillator 64, respectively, to eliminate extraneous signals. Each wave shaper 105 and 106 may have a diode gate poled to pass only positive portions of the waveform to mixer-detector 63.
  • the resulting beat frequency signal at output 65 is coupled to a wave shaper 108, in the form of a Schmitt trigger, in order to produce a square wave output of nominally 5 .820 kc., as illustrated in FIG. 6T.
  • the sample rate at which coils U and L are alternately switched or connected with oscillator 50 is determined by the control requirements of the continuous casting process, and depends upon the casting rate for the machine and the response time of the system. For a particular machine and a casting rate on the order of 100 feet per minute, the sample rate was chosen to be approximately 400 cycles per second. For such a sampling rate, clock 70 has a repetition rate for its complete cycle of 1,250 microseconds. As seen in FIG. 5A, the output of clock 70 is nonsymmetrical, and consists of a first portion on line A having a duration of 250 microseconds, and a subsequent portion on line B having a duration of 1,000 microseconds.
  • FIG. 6 For simplification, only one complete cycle of clock 70 is shown in FIG. 6. During each 1,250 microseconds, only one of the coils is sampled, which for illustrative purposes is coil U (FIG. 6B). It should be understood that during the next complete cycle of clock 70 (not illustrated), the opposite coil L would be connected to coil oscillator 50, and would produce the same relative time relation between the signals as illustrated for coil U.
  • Sample gates 80 and 81 are NAND logic blocks, activated by the absence of all positive inputs, i.e., all inputs must be negative in order to produce a positive output.
  • the square wave output of wave shaper 108 is inverted by a detector gate 112, FIG. 5A, when actuated or enabled by the positive output of lock-out bi-stable switch 114.
  • the two inputs to switch 114 are coupled to the A and B outputs of clock 70 (and to other circuitry, described later, for blocking switch 114, causing switch 114 to have an output at A after the lapse of the first 250 microseconds from the start of each cycle output from clock 70, FIG. 6B.
  • This output opens detector gate 112, causing it to invert the output of wave shaper 108, FIG. 6T, and thus causing a series of three pulses, FIG. 6F, to be passed to the sample gate inputs.
  • FIG. 5B Another input to each sample gate 80 and 81, FIG. 5B, is coupled to a different A or B output of steering FF 72, FIG. 5A. Since steering FF 72 only triggers on a positive going waveform, it in effect sums the nonsymmetrical output of clock 70, producing an output waveform having a total cycle time of 1,250 microseconds.
  • the A output of steering PF 72, FIG. 6D enables switch coil 60 for coil U, thus causing oscillations 100, FIG. 5A, attributable to coil U, the same positive A output, coupled to sample gate 81, disables the L sample gate.
  • the B output of steering FF 72 which is negative, is coupled to sample gate 80, and thus tends to enable the sample gate for the U coil.
  • Timing circuit produces timing signals which allow only the latter portion of the third cycle of oscillation which exists beyond a fixed time period to pass to the pulse width detectors 83.
  • a small transition time occurs before the oscillatory output on 61 is actually representative of the natural frequency of oscillation of the circuit.
  • Sample R-S flip-flop 120 being triggered only by the positive going edges of the pulse outputs from detector gate 112, in effect divides by two, as can be seen in FIG. 6G.
  • the A output from FF 120 is connected to an input of each of the sample gates 80 and 81.
  • the B output of FF 120 triggers the RUN input of a sample auxiliary (AUX.) R-S flip-flop 122, FIG. 5B, which again divides by two, and couples its A output to an input of each of the sample gates 80 and 81.
  • the sample AUX. FF 122 was initially reset by the A output of clock 70 which is coupled to an A input of FF 122.
  • the B output of sample FF 120 is also coupled to a delay timer 124, FIG. 5B, which when actuated is delayed in resetting by a fixed amount of time, as 400 microseconds, FIG. 6].
  • the time interval is chosen depending upon the frequencies and time durations of the signals generated by the circuit, and as can be seen by referring to FIGS. 6] and 6T, the time interval is chosen to lapse at some point during the occurrence of the third pulse from wave shaper 108.
  • the A output from delay timer 124 is coupled to a reset timer 126, FIG. 5B, which resets for a fixed time period, as 10 microseconds, when the A input thereto goes positive, as can be seen by comparing FIGS. 6] and 6K.
  • the purpose of reset timer 126 is to produce a short timing pulse at output B which is used to discharge the pulse width detectors 83.
  • the A output of reset timer 126 is coupled to a gate bi-stable switch 128, FIG. 5B, producing an output at A which is also coupled to sample gates "80 and 81. As can be seen from FIG. 6L, the A output of switch 128 goes negative as the A output of reset timer 126 again goes positive.
  • sample gate 80 produces a positive output sample pulse having a duration equal to the remaining width of the pulse from wave shaper 108 (which is inverted by detector gate 112 to produce the signal seen in FIG. 6F).
  • the sample pulse output, FIG. 6M, from sample gate 80 has a width which is proportional to the duration of the portion of the third cycle oscillation which occurs after a predetermined point fixed with reference to time.
  • the square wave output of wave shaper 108, FIG. 6T drops to Zero, causing the inverted output from detector gate 112 to rise in a positive manner, as seen in FIG. 6F.
  • the positive output from detector gate 112 is also coupled to the RUN input of sample R-S PF 120, and the A input .of gate bi-stable switch 128. This causes PF 120 and switch 128 to reset to their positive conditions, as seen in FIGS. 6G and 6L, respectively.
  • the resulting A output of gate bi-stable switch 128 is coupled to the A input of lock-out bi-stable switch 114, thereby terminating the A output of switch 114, as seen in FIG. 6E.
  • Each of the pulse width detectors 83 preferably has a rapid rate of response, in order that the system may follow rapidly changing levels of molten metal within the mold. Any conventional circuit may be used for detectors 83 which produces an analog voltage output proportional to the width of a pulse input, and which can respond quickly to pulse inputs having varying widths.
  • An integration circuit by itself, does not have a sufficiently short time constant to meet these requirements.
  • detectors 83 may consist of a dischargeable integration circuit which charges during the occurrence of a sample pulse, and holds or maintains the voltage to which the integration circuit has been charged after the cessation of the sample pulse. The held or maintained charge may then be discharged just prior to the occurrence of another sample pulse by a reset circuit which shorts or jumpers the integration circuit.
  • a pair of reset switches 130 are associated with each of the pulse width detectors 83.
  • Each reset switch 130 is coupled to a different output from steering flip-flop 72, in order that only the reset switch 130 corresponding to the coil which is being read at that time will be enabled.
  • the other input to the reset switches is coupled to the B output of reset timer 126, and is fed the microsecond pulse, FIG. 6K, which occurs immediately prior to the time at which the sample pulse will be gated through the sample gate.
  • the reset switch 130 corresponding to the coil being switched to the coil oscillator generates a positive output which is used, by means of any conventional circuit, to discharge the pulse width detector associated therewith.
  • the pulse width detector 83 maintain an analog voltage output which exists throughout the reading cycle, except for the short 10 microsecond discharge time.
  • Pulse width detectors 83 are coupled to differential amplifier 87, which with summing output amplifier 90 form a conventional 3-term control. More particularly, a differential amplifier 87 is formed from an output differential amplifier 135, an integrate differential amplifier 136, and a derivative differential amplifier 137. The outputs from integrate differential amplifier 136 and derivative differential amplifier 137 do not cancel since the time of response of each is substantially different.
  • the derivative difi'erential amplifier 137 responds to the rate of change 12 of the analog signal, and has a time constant of about second, whereas the integrate differential amplifier 136 has a time constant in the range of 20 seconds, and responds roughly in proportion to the product of the input analog signal and time.
  • a DC level compensation amplifier 139 responds in inverse proportion to the input to output voltage swing of output differential amplifier 135, in order to maintain an average sum level to the remaining differential amplifiers.
  • the servomechanism may use any conventional circuit, and as illustrated uses a servopower amplifier 141 which is responsive to the output from summing output amplifier to drive servomotor 52 in the proper direction. The direction is determined by which signal at the two inputs to amplifier 141 exceeds the other signal.
  • the circuit of FIG. 5 has been described in detail, it will be apparent that other systems could be used in accordance with the teachings of this invention. With in the purview of this invention, it will be apparent that whether the conductor whose level is being sensed is of relatively good or bad electrical conductivity is unimportant. It is merely necessary that the mass of material whose level is being sensed exhibit sufiicient conductivity to allow circulating eddy currents to be induced therein, producing a loss which is reflected back to the coil generating the magnetic field which induced the eddy currents.
  • the term conductor is defined to be any ma terial capable of producing a change in an electrical characteristic of a coil located adjacent thereto.
  • a control for maintaining the molten metal at a predetermined level in said mold comprising:
  • coil means adjacent said mold and responsive to a change in level of molten metal within said mold for varying an electrical characteristic of said coil means
  • said coil means comprises a pair of coils located adjacent said mold and responsive to a change in level of molten metal for varying at least the inductance of each coil, and
  • said signal producing means is responsive to the difference in inductance between said pair of coils for producing said output signal, said output signal being independent of a common mode change in inductance of said pair of coils.
  • said signal producing means includes an oscillator having a self-resonant frequency dependent upon the value of inductance of the coil coupled thereto, and means for alternately coupling said pair of coils to said single oscillator.

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  • Fluid Mechanics (AREA)
  • Automation & Control Theory (AREA)
  • Mechanical Engineering (AREA)
  • Continuous Casting (AREA)

Description

CONTINUOUS CASTING APPARATUS WITH A MOLTEN METAL LEVEL CONTROL Filed Feb. '7, 1968 G. VISCHULIS July 7, 1970 5 Sheets-Sheet 1 find fnuenfvrxw QwUNVW .wAA/
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CONTINUOUS CASTING APPARATUS WITH A MOLTEN METAL LEVEL CONTROL July 7, 1970 5 SheetsSheet 5 Filed Feb. '7, 1968 m itqmL T mil wumSBmS T 4 HQ QFkDnZS 3 9m E :55
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CONTINUOUS CASTING APPARATUS WITH A MOLTEN METAL LEVEL CONTROL Filed Feb. '7, 1968 Sheets-Sheet 5 f SAMPLE PULS E GATE WIDTH u DETECTOR U L "RESET" swwcu /1/30 33 sgrzm E PULSE T WIDT L DETECTORL 81 T A "5555; AVERAGTNG A LE SWITCH 130 5 #3 139 A RESET OUTPUT D.C.LEVEL J26" U U U 0| FFER. COM PEN.
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United States Patent "ice 3,519,060 CONTINUOUS CASTING APPARATUS WITH A MOLTEN METAL LEVEL CONTROL George Vischulis, Hickory Hills, 11]., assignor to Interlake Steel Corporation, Chicago, [1]., a corporation of New York Filed Feb. 7, 1968, Ser. No. 703,750 Int. Cl. B22d 11/10; B22c 19/04 U.S. Cl. 164155 Claims ABSTRACT OF THE DISCLOSURE A pair of coils encircling the mold of a continuous casting machine are connected to an oscillator to produce oscillations having a frequency dependent on the level of molten metal. The oscillations are compared by a differential circuit which determines the difference in the oscillatory frequency of the coils, and eliminates changes caused by common mode temperature effects, to generate a level signal which controls the level of molten metal within the mold.
This invention relates to level detector apparatus for indicating and controlling the level of a conductor confined within a boundary.
The invention is particularly adapted for sensing the level of liquid or molten metal within a metal processing machine, and using the level indication to maintain the molten metal at a predetermined level. For example, in a continuous casting machine, molten metal is poured into an elongated mold through which it passes and cooled to produce a continuous cast output strip or billet. The mold is formed from vibrating sections having both transverse and longitudinal reciprocating movement following a harmonic or elliptical path, such movement causing the mold sections to repeatedly contact and propagate the casting in a forward stroke. An example of a continuous casting machine is shown in U.S. Pat. 3,075,264 to Wognum, to which reference should be made for background information. In order to control the casting process, it is necessary to maintain the molten metal at a predetermined level in the mold.
Attempts to use a change in molten metal level to produce an inductive or other related type change in a coil which can then be converted into a level signal have been unsuccessful due to a number of factors. An important limiting factor is the intense heat generated during the casting process, which is conveyed to the surrounding structure and causes extreme temperature ariations as the level of molten metal varies within the mold. Such temperature variations may produce a change of inductance in a coil which will exceed the inductance change caused by the variation in the level of molten metal adjacent the coil.
Another important factor which limits the type of level detector usable with a continuous casting machine is the presence of large amounts of metal in the machine itself, which acts as a shorted secondary winding and desensitizes a coil, thus masking the change in inductance otherwise caused by the variation in metal adjacent the coil.
For these reasons, continuous casting machines and the like have used gamma ray detectors or thermal cables located at spaced locations within the mold wall of the casting machine to sense the level of molten metal within the mold. While gamma ray detectors are responsive to a percent change in transmittance of radiated gamma rays to determine the level of the metal, they are ineffective for certain metals, such as aluminum, which have a high gamma ray transmittance, and the signal re- 3,519,060 Patented July 7, 1970 spouse is low Where the cast metal scanned is small relative to the entire metal scanned. Gamma ray detectors also present a safety hazard in terms of radiation. Thermal cables mounted within the mold wall have problems in that the mold walls in a continuous casting machine must oscillate away from the cast metal for approximately one-half of the time. Both systems are of relatively slow response time, and limit the speed at which a continuous casting machine may be operated.
A satisfactory level detector system requires a minimum amount of equipment located at the continuous casting machine itself, and such equipment at the machine should be of a type which can be easily added to existing machinery without requiring substantial modifications. None of the prior art level detectors used with continuous casting machines meets all of these diverse requirements.
In accordance with the present invention, novel apparatus is disclosed for determining the level of a conductor confined within a boundary, which apparatus is especially adapted for determining and controlling the level of molten metal in a continuous casting machine. Plural coils having a particular type of change in an electrical characteristic for a change in metal level are sensed in a differential manner to produce a level signal independent of fluctuating temperatures, other extraneous influences affecting inductance and the presence of large bodies of metal in the vicinity of the sensing system.
One object of this invention is the provision of an improved level detector for indicating and/ or controlling the level of a conductor confined within a boundary.
Another object of this invention is the provision of a level detector and control for a continuous casting machine, which uses an inductive and an AC resistance effect to determine the level of molten metal within the mold of the casting machine.
One feature of this invention is the provision of a level detector for a confined conductor, having a plurality of winding means located adjacent the boundry of a conductor, and a differential circuit coupled to the plurality of winding means for sensing a level signal which is independent of a common mode change in inductance of the winding means caused by temperature variations or other factors not directly related to the actual level of the conductor.
Another feature of this invention is the provision of a level detector for a continuous casting machine, which connects a plurality of coils surrounding the machine mold to an oscillator to produce self-resonant oscillations having a frequency dependent upon the value of inductance and AC resistance of the coils.
Yet another feature of this invention is the provision of a level detector which alternately connects a pair of coils to an oscillator to produce trains of self-resonant oscillations, the phase difference between the trains being measured by a differential circuit which includes means for compensating for common mode changes in the frequency determining characteristics of both coils.
Futher features and advantages of the invention will be apparent from the following description, and from the drawings, in which:
FIG. 1 is a top plan view of a continuous casting machine and a portion of the level detector apparatus;
FIG. 2 shows a front elevation of a continuous casting machine as viewed along lines 2-2 of FIG. 1, and more particularly shows the top portion of a continuous casting machine and a portion of the level detector apparatus;
FIG. 3 is a block diagram of the level detector apparatus for detecting and controlling the level of metal in the continuous casting machine of FIGS. 1 and 2;
FIG. 4 is a curve of oscillatory output frequency versus metal level when the level determining coils of FIGS. 1
3 and 2 are connected in circuit with the oscillator of FIG. 3;
FIGS. 5A and 5B are a single schematic diagram of the circuit of FIG. 3, in which the broken lines across the right-hand side of FIG. 5A and the left-hand side of FIG. 5B are placed together to show the continuation of the lines across the drawings; and
FIGS. 6A6T are diagrams illustrating the time relationship between the signals generated by the circuit of FIG. 5.
While an illustrative embodiment of the invention is shown in the drawings and will be described in detail herein, the invention is susceptible of embodiment in many different forms and it should be understood that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the embodiment illustrated. The scope of the invention will be pointed out in the appended claims.
GENERAL OPERATION OF CONTINUOUS CASTING MACHINE Turning now to FIGS. 1 and 2, a continuous casting machine 10 is generally illustrated for gradually solidifying a mass of molten metal 11 poured from a tundish 12 into a mold 14 for the machine. Mold 14 is composed of four mold sections 16 which are generally rectangularly block-shaped, depending upon the shape desired for the cross-sectional area of the final cast product. Each mold section 16 is held by a mold retainer 18 which is vibrated through an elliptical orbit by a conventional motor 19. To seal the line between adjacent mold sections 16, a stationary corner strip 20 is held in place by a corner strip retainer 21 which is fixedly attached with respect to the base of machine 10.
As is known in the art of continuous casting, as exemplified by U.S. Pat. 3,075,264, the particular type of orbital motion produced by motor 19 and associated linkages is such that opposite mold sections 16 are brought into contact with the metal 11 billet or strip being cast within machine 10 while those mold sections are moving toward each other and downwardly. Before the bottom of the elliptical stroke, the mold sections move away from each other and the metal billet, and thereafter rise upwardly on a return stroke in a direction opposite to the direction of travel of the metal through the mold. During this return stroke, the other opposite pair of mold sections 16 move toward each other and downwardly, continuing to urge the metal downwardly through mold 14.
The stroke is ordinarily maintained through a fixed distance or path, while the frequency or rate of completing a single elliptical path is varied to change the rate of metal flow through the machine. For example, each of the mold sections may make from 1,000 to 2,500 revolutions per minute, while following the path of a 10 to 1 ellipse. The orbital amplitude by which the mold sections withdraw from the metal strip is very small, and is typically on the order of a few thousandths of an inch. To progressively cool the molten metal 11 as it passes through mold 14, in order that it may gradually solidify and emerge continuously belowthe mold sections as a solidified cast billet or strip, mold retainers 18 and mold strips 16 are of hollow construction to permit water or other cooling fluid to flow up through the retainer 18 and downwardly along the inner surface of mold section 16 in order to cool the molten metal adjacent the surface of the mold. For this purpose, a continuous aperture or opening 25 is formed through the elongated portions of the mold 14 in order to permit water flow 26 to pass therethrough and cool the machine.
LEVEL SENSING COILS In order to control the quality of the cast billet, and other factors including preventing the metal pour from overflowing from the top of mold 14, it is important to determine the point 30 at which the thin molten metal pour spout flows outwardly and first contacts the surface wall of the mold sections 16. The level point 30 will be referred to hereinafter as the level of the metal within the mold, although it will of course be apparent that some metal, in the form of the thin pour spout from the tundish 12, extends above the so-called level of metal within the mold.
A level sensor, in the form of coil means 32 located adjacent mold 14, provides an electrical condition which changes in accordance with changes in the level 30' of metal 11 within the mold. This change in electrical condition is possible because mold 14 is discontinuous or segmented, as will be explained in detail later. Means 32 comprises a first coil U and a second coil L which may be split into an upper section L located above coil U and a lower section I." located below coil U, for reasons to be explained hereinafter. The net inductive effect of the pair of split coil sections L and L" is effectively the same as a single coil L located slightly below coil U. The coils U and L are wound on a coil form or bobbin 35 which encircles the mold 14, and which is sealed by an encircling insulated form 36.
A coil shield 3'8 surrounds coil forms 35, 36 and performs the dual function of electrically isolating the coils U and L from the elfects of extraneous metal bodies located near the coils, and forming a splash shield to prevent molten metal 11 from contacting the coils or coil forms. Shield 38, preferably formed of conductive material such as copper or stainless steel, is especially effective as a barrier which isolates coils U and L from the effect of tundish 12 and the varying level of molten metal 11 within the tundish. Coil means 32 is rigidly mounted by fastening means 40 to a base 41 which is fixed with respect to machine 10 and tundish 12.
As the level 30 of molten metal rises towards the top of mold 14, the volume of the slug of molten conductor within the encircling coils U and L increases. It has been found that this causes the inductance of each coil to decrease and the AC resistance of each coil to increase due to increased eddy current losses occurring within the center slug or mass of molten conductor. The eddy current losses within the molten conductor occur because mold 14 is not formed from a continuous annular part, but rather from segmented mold sections. More particularly, if mold 14, as viewed in FIG. 1, was formed from a continuous encircling or annular mold form (instead of separate mold sections 16), the magnetic flux generated when the coils U and L are connected to an oscillator would induce a current which would entirely encircle the molten conductor. This induced current would in turn generate a flux which would cancel the flux in the molten conductor, preventing circulating eddy currents from being induced in the molten conductor.
However, by providing the combination of a mold with a discontinuity to the flow of induced currents and a magnetic flux field generating means, a circulating current is induced which does not completely encircle the molten conductor. As a result, circulating or eddy currents are induced in the molten conductor, creating a power loss which is reflected back to the magnetic field generating means as a change in the inductance and the AJC resistance of the coils. By detecting either or both the inductance and AC resistance characteristic of the coils, the amount of eddy current loss, which is proportional to the level of metal, can be determined.
The term discontinuous in this context refers to a nonhomogeneous barrier which substantially prevents an induced circulating current from flowing through the barrier. Entirely separate mold sections 16, FIG. 1, are only exemplary of one manner of forming a boundary having a discontinuity to the flow of an induced current. In the illustrated embodiment, the formation of a metal oxide layer on each mold section is believed to aid to providing the discontinuity to the flow of circulating currents.
The circulating current losses in the molten conductor change with the level of the conductor, and thus provide a means of detecting the conductor level. Since the circulating or eddy currents losses are reflected back and change both the inductance and the AC resistance characteristics of the coils U and L, either or both characteristics can be monitored to indicate metal level. The circuit of FIG. 3 uses these characteristics of the coils in order to provide an indication of the level 30 of the molten metal within mold 14.
GENERAL OPERATION OF LEVEL DETECTOR AND CONTROL Turning now to FIG. 3, each coil U and L is connectable with an oscillator 50 in order to produce natural oscillations having a frequency dependent upon the value of the inductance and the AC resistance of the coil. The frequency of oscillation, as is well known, is inversely proportional to the inductance and directly proportional to the AC resistance of a coil. For an increase in metal level, the inductance of the illustrated coils decrease while the AC resistances of the coils increase. Since both electrical effects contribute to an increase in frequency of oscillations, the oscillatory frequency can be used as a direct indication of the metal level. While the preferred circuit of FIG. 3 is responsive to the frequency of oscillations, affected both by the inductance and the AC resistance characteristics of the coils, a circuit could of course be used which was responsive to one of these electrical characteristics by itself.
The relative difference in the frequency of oscillations produced by oscillator 50' when connected to coils U and L, being representative of the difference in inductance and AC resistance between the coils, is used to generate an output signal on line 5-1 which drives a servo motor 52 for returning the level of molten metal to a predetermined desired value. Servo motor 52 may form a part of any conventional servomechanism for controlling the level of molten metal. By way of example, motor 52 could be connected by conventional circuitry to control the revolutions per minute of motor 19 of FIG. 2, which drives the mold sections 16 in the continuous caster. Such operation, affecting the rate at which the mold sections 16 complete a single cycle of elliptical motion, varies the rate of travel of the molten metal. By thus controlling the rate at which metal is removed from mold 14, the level of the molten metal within the mold is controlled. Alternatively, servo motor 52 could be connected to a valve (not illustrated) on tundish 12 in order to control the amount of molten metal flowing into mold 14, and hence also controlling level 30.
As the level 30 of molten metal varies within mold 14, the resulting change in the electrical characteristics of the coils causes the frequency output from oscillator to vary, as can be seen with reference to FIG. 4. Lower coil L is comprised of a greater number of turns than upper coil U, and hence has a greater range of frequency deviation, when connected to oscillator 50, for a given change in metal level. The coils U and L are de-tuned or otherwise so chosen so that the frequency curve of one of them is shifted relative to the other in order to produce a cross-over point at which the inductance of each coil is equal. For levels of metal above cross-over point 55, it is noted that the inductance of one of the coils (in this case coil L) decreases at a greater rate than the other coil, hence producing a higher frequency of oscillations, while for metal levels below cross-over point 55, the curves change positions, and the one coil (namely coil L) now increases in inductance at a greater rate than the other coil, and hence causes a lower frequency of oscillations.
By de-tuning the pair of coils in this manner, a characteristic curve is produced which can uniquely determine metal level. More particularly, the magnitude of the difference in the frequency of oscillations occurring when the two coils are connected to an oscillator is directly proportional to the distance variation of the metal level from a reference value. Furthermore, the frequencies generated by a particular coil, in this case coil L, are of higher value for metal levels above the reference level, and of lower frequency relationship than the other coil for metal levels below the reference level, and thus provide a unique indication of the direction in which the level has varied. When connected as a servomechanism for returning metal level to a desired level, it is preferred that cross-over point 55 be chosen to occur at the level at which the metal is to be maintained, so that a zero volt signal is produced when the desired level of metal is maintained.
Returning to the circuit of FIG. 3, a single time shared oscillator 50 is provided for both coils U and L. Such a circuit is preferred in order to eliminate different temperature drift effects which would occur with separate oscillators for each coil. An electronic switch 60 alternately connects coil U and coil L to oscillator 50, in order to produce at an output line 61 a train of oscillatory signals with continuously alternate portions of the train being attributable to either coil U or coil L. The train of oscillations is heterodyncd in a mixer-detector 63 with continuous oscillations from a beat frequency oscillator 64 in order to produce a different or beat frequency on an output line 65 of detector 63.
Since oscillator 50 is alternately switched between the two coils, the particular portions of the beat frequency on line 65 which is attributable to each coil must be determined. A central clock 70 continuously produces timing pulses which are used for reference purposes in the circuit. Clock 70 drives a steering flip-flop (PF) 72, to alternately enable one of a pair of output lines connected with electronic switch 60. The enabled line triggers the electronic switch associated therewith to connect one of the coils with oscillator 50. The steering flip-flop 72 also produces output signals which are connected with remaining portions of the circuitry in order to synchronize the operation of various components with the connection of coil U or L to the oscillator 50.
The train of oscillations from detector 63 are coupled in parallel to a sample gate 80, responsive to portions of the train of oscillations attributable to coil U, and a sample gate 81, responsive to the remaining portions of the train of oscillations attributable to coil L. Steering flip-flop 72 actuates or enables gate at the same time that electronic switch 60 connects coil U to oscillator 50. Similarlly, when electronic switch 60 connects coil L to oscillator 50, sample gate 81 is enabled.
While the relative inductance and AC resistance difference between coils U and L could be determined in several manners, in the illustrated embodiment, the sample gates 80 and 81, when enabled steering flip-flop 72, pass a portion of the third cycle of oscillation to pulse width detector 83 for that sample gate. In order to open the sample gates 80 and 81 for the third cycle of oscillation, and thereafter close the sample gates prior to the fourth cycle of oscillation, a timing circuit 85 coupled between clock 70 and the sample gates provides a plurality of timing pulses, to be described in detail later.
As the frequency output from oscillator 50 increases or decreases from the nominal frequency output occurring at cross-over point 55 of FIG. 4, the train of oscillations appears to be expanded or compressed, thereby varying the width of the portion of the third cycle of oscillation which is passed by sample gates 80, 81 to detectors 83. Each detector 83 produces an analog output signal which is directly proportional to the width of the pulse coupled thereto.
The output of both of the pulse width detectors 83 is coupled to a differential amplifier 87 to produce a single output signal representative of the difference between the two input signals. If the outputs of each of the pulse width detectors 83 are equal, it indicates that both coil U and coil L are generating the same frequency of oscillation, which occurs only at cross-over point 55 of FIG. 4, and hence a zero voltage is produced by differential amplifier 87. Should the analog signal from one of the pulse width detectors 83 exceed the other, differential amplifier 87 will produce a corresponding output signal of one polarity, representative of the difference therebetween. Conversely, should the outputs from the pulse width detectors 83 vary in the opposite direction, an opposite polarity signal representative of the difference therebetween will be generated.
The output signal from differential amplifier 87 is directly proportional to the difference between the oscillatory frequencies for the L and U coils. More particularly, the polarity of the output signal indicates whether the level is rising or falling from the desired or reference level, whereas the magnitude of the output signal indicates the amount or distance the metal level has varied from the desired level.
To control the level of the molten metal, line 51 is connected to a servomechanism which includes servo motor 52 for controlling either the rate of adding molten metal to the continuous casting machine, or the rate of withdrawing metal from the continuous casting machine, as previously described. The servomechanism includes a summing output amplifier 90 coupled between line 51 and servo motor 52. A feedback network or sensor 91 is connected in a conventional manner with summing output amplifier 90 in order to produce a servomechanism which seeks null. As the signal on line 51 goes, for example, in a positive direction, servo motor 52 is driven until the feedback signal from network 91 exactly balances the signal on line 51.
As the temperature and other extraneous influences on the structure surrounding the mold 14 varies, the inductance of coils U and L can change over a substantial range. The inductance changes cause a resulting change in frequency of oscillator 50, which causes the curves of FIG. 4 to shift upwardly or downwardly from their illustrated position. However, the difference in the frequency of oscillation between the pair of coils remains in the same proportion as that illustrated in FIG. 4. Since the circuit of FIG. 3 is only responsive to the difference rather than the absolute value of the frequencies, common mode extraneous influences are eliminated. A common mode influence is one which affects both of the coils U and L equally, the described differential type circuit therefore exhibits common mode rejection, in that it discriminates against changes in a fundamental quantity affecting both coils.
Although the circuit rejects common mode inductance and like effects, an inductance change could produce frequency changes of sufficient magnitude so as to exceed the range of the fixed timing signals generated by the circuit. In order to obviate such a problem, the outputs of both of the pulse width detectors 83 are coupled to an averaging network 95 which generates an output signal representative of the absolute sum of both pulse width detectors. This signal drives a conventional automatic frequency control (AFC) amplifier 96 to control the frequency of beat oscillator 64 in a known manner, as by varying the capacitance in oscillator 64 in inverse proportion to the amplitude of the control signal from averaging network 95.
As the inductance of both coils decreases and the AC. resistance increases, for example, causing a resultant increase in the frequency output of oscillator 50, the output of the pulse width detectors 83 decreases. This produces a lesser amount of control signals from averaging network 95. The AFC circuit 96 is responsive to a lesser amount of control signal to increase the frequency of beat oscillator 64, returning the beat between the oscillators 50 and 64 closer to the original value, and thus insuring that the timing signals produced by the circuit have the proper time relationship to enable sample gates and 81 at the proper time.
DETAILED CIRCUIT In the following detailed description of the schematic diagram, the folowing conventions will be used. All flipflops (FF) have either a single output line, or a pair of output lines labeled A and B. The B output is at all times opposite the A output. All signals are either at a zero voltage level or at a positive voltage level with respect to zero volts.
The legends carried by the inputs to the flip-flops indicate the manner in which the flip-flop is triggered by an input signal. An input labeled RUN indicates that an input pulse will change or flip the instantaneous output of the flip-flop, regardless of the particular polarity values on a single or pair of output lines. An input labeled A means that an input pulse will produce a positive output pulse or waveform at the A output of the flip-flop. Conversely, an input labeled A means a positive pulse will produce a negative pulse or waveform (i.e. a zero signal) at the A output of the flip-flop. All flip-flops trigger on the positive going edge of an input waveform.
While separate blocks are illustrated for each function the circuit is to perform, it should be understood that, in some cases, more than one function may be performed by the same component, and that the operation of a particular function block may be performed by a single resistor or other passive or active electrical component combined with other components performing the func tions of other blocks in the circuit.
Turning now to the detailed drawings, the structure and operation of the circuit of FIG. 5 will be described in connection with the waveform diagrams illustrated in FIG. 6, which show the time relation between the various signals generated by the circuit. Throughout the specification, values will be given for certain frequencies and certain time relations between signals in order to disclose a complete, operative embodiment of the invention. However, it should be understood that such values are merely representative and are not critical unless specifically so stated.
Coil oscillator 50, FIG. 5A, may be a Colpitts circuit having component values so that when either of the coils U or L is switched into the oscillator circuit, a natural frequency of oscillations on the order of 200 kilocycles (kc.) or so is generated. The coil electrical characteristics, especially inductance and AC. resistane, are believed to be mainly affected by the periphery and surface effects of the molten metal, with little penetration of the oscillations into the actual center of the molten metal. The nominal frequency of oscillation is chosen by considering several factors. The number of cycles change or shift due to a level change is greater for higher frequencies. However, lower frequencies provide better penetration of the mold and casting machine.
For one particular embodiment, coil U was formed from 30 turns of wire; while coil L was wound in two sections, the upper section L of FIG. 2 being formed from 18 turns of wire, and the lower section L" being formed from 19 turns of wire. Although it was found advantageous to split coil L into two portions, such a construction is not essential. The splitting of coil L is be lieved, for certain continuous casting machines, to aid in averaging out the thermal effects across the coils and in distributing the effect of the large masses of metal of the continuous casting machine, which acts as a shorted secondary and tends to desensitize the coils.
For any given casting machine, the volume within which the coils can be mounted is of certain practical limits, and it is desired that a minimum number of turns be used within this volume. Lesser turns allows a proportionate increase in wire size, lowering the resistance of the coil in order to incerase the Q of the circuit.
Since molten metals tend to approach generally the same resistivity, although the resistivity of the metals in the solid state may be substantially different, the level detector is generally insensitive to the type of metal being cast.
As electronic switch 60 alternately connects coils U and L to oscillator 50, a train of output oscillations is generated on line 61, with a particular portion 100, FIG. A, of the oscillations being attributable to coil U, and the remaining portion 101 of the oscillations being attributable to coil L. This multiplex operation, in which each coil time shares a common oscillator, prevents the coils from interacting. As the metal level varies several inches from its desired value, the frequency of oscillations from oscillator 50 will vary about 1,000 c.p.s. or so from the nominal 200 kc. center frequency.
Beat oscillator 64 produces a continuous frequency output at 194.180 kc., which beats or heterodynes in detector 63 with the oscillations from oscillator 50 produce a difference or beat frequency of nominally 5.820 kc. Wave shapers 105 and 106 are connected to the output of coil oscillator 50 and beat oscillator 64, respectively, to eliminate extraneous signals. Each wave shaper 105 and 106 may have a diode gate poled to pass only positive portions of the waveform to mixer-detector 63. The resulting beat frequency signal at output 65 is coupled to a wave shaper 108, in the form of a Schmitt trigger, in order to produce a square wave output of nominally 5 .820 kc., as illustrated in FIG. 6T.
The sample rate at which coils U and L are alternately switched or connected with oscillator 50 is determined by the control requirements of the continuous casting process, and depends upon the casting rate for the machine and the response time of the system. For a particular machine and a casting rate on the order of 100 feet per minute, the sample rate was chosen to be approximately 400 cycles per second. For such a sampling rate, clock 70 has a repetition rate for its complete cycle of 1,250 microseconds. As seen in FIG. 5A, the output of clock 70 is nonsymmetrical, and consists of a first portion on line A having a duration of 250 microseconds, and a subsequent portion on line B having a duration of 1,000 microseconds.
For simplification, only one complete cycle of clock 70 is shown in FIG. 6. During each 1,250 microseconds, only one of the coils is sampled, which for illustrative purposes is coil U (FIG. 6B). It should be understood that during the next complete cycle of clock 70 (not illustrated), the opposite coil L would be connected to coil oscillator 50, and would produce the same relative time relation between the signals as illustrated for coil U.
Sample gates 80 and 81 are NAND logic blocks, activated by the absence of all positive inputs, i.e., all inputs must be negative in order to produce a positive output. The square wave output of wave shaper 108 is inverted by a detector gate 112, FIG. 5A, when actuated or enabled by the positive output of lock-out bi-stable switch 114. The two inputs to switch 114 are coupled to the A and B outputs of clock 70 (and to other circuitry, described later, for blocking switch 114, causing switch 114 to have an output at A after the lapse of the first 250 microseconds from the start of each cycle output from clock 70, FIG. 6B. This output opens detector gate 112, causing it to invert the output of wave shaper 108, FIG. 6T, and thus causing a series of three pulses, FIG. 6F, to be passed to the sample gate inputs.
Another input to each sample gate 80 and 81, FIG. 5B, is coupled to a different A or B output of steering FF 72, FIG. 5A. Since steering FF 72 only triggers on a positive going waveform, it in effect sums the nonsymmetrical output of clock 70, producing an output waveform having a total cycle time of 1,250 microseconds. When the A output of steering PF 72, FIG. 6D, enables switch coil 60 for coil U, thus causing oscillations 100, FIG. 5A, attributable to coil U, the same positive A output, coupled to sample gate 81, disables the L sample gate. The B output of steering FF 72, which is negative, is coupled to sample gate 80, and thus tends to enable the sample gate for the U coil.
Timing circuit produces timing signals which allow only the latter portion of the third cycle of oscillation which exists beyond a fixed time period to pass to the pulse width detectors 83. When each coil is first connected to coil oscillator 50, a small transition time occurs before the oscillatory output on 61 is actually representative of the natural frequency of oscillation of the circuit. The initial 250 microsecond output A from clock 70, coupled to lock-out bi-stable switch 114, prevents the initial oscillations from being counted or to otherwise determine the third pulse which will be passed by the sample gates.
When lock-out switch 114 enables detector gate 112, the first positive going output from the detector gate, FIG. 6F, is coupled to the RUN input of a sample R-S flip-flop 120, FIG. 5B, triggering the flip-flop out of its reset condition to initiate the start of the count which will cause only the back portion of the third pulse to be gated through sample gate 80. It should be noted that sample R-S flip-flop was previously triggered to a reset condition by input A, coupled to the A output of clock 70.
Sample R-S flip-flop 120, being triggered only by the positive going edges of the pulse outputs from detector gate 112, in effect divides by two, as can be seen in FIG. 6G. The A output from FF 120 is connected to an input of each of the sample gates 80 and 81. The B output of FF 120 triggers the RUN input of a sample auxiliary (AUX.) R-S flip-flop 122, FIG. 5B, which again divides by two, and couples its A output to an input of each of the sample gates 80 and 81. The sample AUX. FF 122 was initially reset by the A output of clock 70 which is coupled to an A input of FF 122.
The B output of sample FF 120 is also coupled to a delay timer 124, FIG. 5B, which when actuated is delayed in resetting by a fixed amount of time, as 400 microseconds, FIG. 6]. The time interval is chosen depending upon the frequencies and time durations of the signals generated by the circuit, and as can be seen by referring to FIGS. 6] and 6T, the time interval is chosen to lapse at some point during the occurrence of the third pulse from wave shaper 108.
The A output from delay timer 124 is coupled to a reset timer 126, FIG. 5B, which resets for a fixed time period, as 10 microseconds, when the A input thereto goes positive, as can be seen by comparing FIGS. 6] and 6K. The purpose of reset timer 126, as will appear hereinafter, is to produce a short timing pulse at output B which is used to discharge the pulse width detectors 83. The A output of reset timer 126 is coupled to a gate bi-stable switch 128, FIG. 5B, producing an output at A which is also coupled to sample gates "80 and 81. As can be seen from FIG. 6L, the A output of switch 128 goes negative as the A output of reset timer 126 again goes positive.
At this time, all inputs to sample gate 80 are negative, and, accordingly, sample gate 80 produces a positive output sample pulse having a duration equal to the remaining width of the pulse from wave shaper 108 (which is inverted by detector gate 112 to produce the signal seen in FIG. 6F). The sample pulse output, FIG. 6M, from sample gate 80 has a width which is proportional to the duration of the portion of the third cycle oscillation which occurs after a predetermined point fixed with reference to time. As the frequency of oscillation varies, caused by a At the end of the third cycle of oscillation, the square wave output of wave shaper 108, FIG. 6T, drops to Zero, causing the inverted output from detector gate 112 to rise in a positive manner, as seen in FIG. 6F. This positive output from detector gate 112, which is coupled to both sample gates, now blocks sample gate 80 and thus ter rninates the end of the reading of the sample.
The positive output from detector gate 112 is also coupled to the RUN input of sample R-S PF 120, and the A input .of gate bi-stable switch 128. This causes PF 120 and switch 128 to reset to their positive conditions, as seen in FIGS. 6G and 6L, respectively. The resulting A output of gate bi-stable switch 128 is coupled to the A input of lock-out bi-stable switch 114, thereby terminating the A output of switch 114, as seen in FIG. 6E. As the A output of switch 114 returns to the zero level, it blocks detector gate 112 from passing any further signals from wave shaper 108. This returns the various flipflops in timing circuit 85 to their rest or reset condition until the start of the next 1,250 microsecond output from clock 70, which now switches flip-flop 72 to initiate the above described series of operations all over again, but this time for coil L.
Each of the pulse width detectors 83 preferably has a rapid rate of response, in order that the system may follow rapidly changing levels of molten metal within the mold. Any conventional circuit may be used for detectors 83 which produces an analog voltage output proportional to the width of a pulse input, and which can respond quickly to pulse inputs having varying widths. An integration circuit, by itself, does not have a sufficiently short time constant to meet these requirements.
By way of example, detectors 83 may consist of a dischargeable integration circuit which charges during the occurrence of a sample pulse, and holds or maintains the voltage to which the integration circuit has been charged after the cessation of the sample pulse. The held or maintained charge may then be discharged just prior to the occurrence of another sample pulse by a reset circuit which shorts or jumpers the integration circuit.
For this purpose, a pair of reset switches 130 are associated with each of the pulse width detectors 83. Each reset switch 130 is coupled to a different output from steering flip-flop 72, in order that only the reset switch 130 corresponding to the coil which is being read at that time will be enabled. The other input to the reset switches is coupled to the B output of reset timer 126, and is fed the microsecond pulse, FIG. 6K, which occurs immediately prior to the time at which the sample pulse will be gated through the sample gate.
During the simultaneous occurrence of the positive discharge pulse from reset timer 126 and a positive output from steering flip-flop 72, the reset switch 130 corresponding to the coil being switched to the coil oscillator generates a positive output which is used, by means of any conventional circuit, to discharge the pulse width detector associated therewith. It should be noted that the inherent time delay necessary for the pulse from the A output of timer 126 to be coupled through gate bistable switch 128 and enable the sample gate, is sufficient to allow the pulse width detector 83 to fully discharge and reset prior to the occurrence of the next sample pulse. Thus, the pulse width detectors 83 maintain an analog voltage output which exists throughout the reading cycle, except for the short 10 microsecond discharge time.
Pulse width detectors 83 are coupled to differential amplifier 87, which with summing output amplifier 90 form a conventional 3-term control. More particularly, a differential amplifier 87 is formed from an output differential amplifier 135, an integrate differential amplifier 136, and a derivative differential amplifier 137. The outputs from integrate differential amplifier 136 and derivative differential amplifier 137 do not cancel since the time of response of each is substantially different. The derivative difi'erential amplifier 137 responds to the rate of change 12 of the analog signal, and has a time constant of about second, whereas the integrate differential amplifier 136 has a time constant in the range of 20 seconds, and responds roughly in proportion to the product of the input analog signal and time.
In order to smooth the fluctuations which would otherwise occur when the pulse width analog signal is terminated during the time that the pulse width detectors 83 are being discharged, a DC level compensation amplifier 139 responds in inverse proportion to the input to output voltage swing of output differential amplifier 135, in order to maintain an average sum level to the remaining differential amplifiers.
The servomechanism may use any conventional circuit, and as illustrated uses a servopower amplifier 141 which is responsive to the output from summing output amplifier to drive servomotor 52 in the proper direction. The direction is determined by which signal at the two inputs to amplifier 141 exceeds the other signal.
While the circuit of FIG. 5 has been described in detail, it will be apparent that other systems could be used in accordance with the teachings of this invention. With in the purview of this invention, it will be apparent that whether the conductor whose level is being sensed is of relatively good or bad electrical conductivity is unimportant. It is merely necessary that the mass of material whose level is being sensed exhibit sufiicient conductivity to allow circulating eddy currents to be induced therein, producing a loss which is reflected back to the coil generating the magnetic field which induced the eddy currents. Thus, as used in the context of a level determining apparatus, the term conductor is defined to be any ma terial capable of producing a change in an electrical characteristic of a coil located adjacent thereto. Rather than using a change in an electrical characteristic of the coils to produce a change in frequency which is measured by the phase shift between oscillatory signals, it will be apparent that other systems which accomplish the same purpose could be substituted therefor. Other changes and modifications apparent to those skilled in the art are intended to be within the scope of the invention, as defined in the following claims.
I claim:
1. In a continuous casting machine including a mold into which molten metal is poured and means for varying the level of the molten metal in said mold, a control for maintaining the molten metal at a predetermined level in said mold, comprising:
coil means adjacent said mold and responsive to a change in level of molten metal within said mold for varying an electrical characteristic of said coil means;
means coupled to said coil means for producing an output signal proportional to said electrical characteristic of said coil means; and
means for controlling said level varying means to cause the level of said molten metal to be proportional to said output signal.
2. The control of claim 1 wherein said coil means comprises a pair of coils located adjacent said mold and responsive to a change in level of molten metal for varying at least the inductance of each coil, and
said signal producing means is responsive to the difference in inductance between said pair of coils for producing said output signal, said output signal being independent of a common mode change in inductance of said pair of coils.
3. The control of claim 2 wherein said coils have the same inductance when said molten metal is at said predetermined level.
4. The control of claim 2 wherein said signal producing means includes an oscillator having a self-resonant frequency dependent upon the value of inductance of the coil coupled thereto, and means for alternately coupling said pair of coils to said single oscillator.
13 14 5. The control of claim 1 wherein said signal produc- FOREIGN PATENTS ing means includes oscillator means coupled to said coil 226 894 4/1963 Austria means for generating oscillations having a frequency de- 711,067 6/1965 Canada pendent on the electrical characteristic of said coil means, 1)029:O98 3/1953 France:
and means cou pled to said oscillator means for convert- 5 1,373,146 4/1964 France. ing said oscillations into said output signal. P
References Cited J. SPENCER OVERHOLSER, Primary Examiner UNITED STATES PATENTS R. S. ANNEAR, Assistant Examiner 2,905,989 9/1959 Black 164155 10 us. c1. X.R. 3,204,460 9/1965 Milnes. 324-34 3,456,715 7/1969 Freedman et a1. 164-155
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Cited By (13)

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Publication number Priority date Publication date Assignee Title
US3670801A (en) * 1970-01-16 1972-06-20 Borg Warner Continuous casting mold level control
US3677333A (en) * 1970-01-26 1972-07-18 Borg Warner Start-up system for continuous casting mold
US4014379A (en) * 1970-06-09 1977-03-29 Getselev Zinovy N Method of forming ingot in process of continuous and semi-continuous casting of metals
US4027233A (en) * 1973-07-23 1977-05-31 Eduard Ivanovich Shmakov Contactless inductance pickup for detecting the interface of two media
FR2369892A1 (en) * 1976-11-05 1978-06-02 Arbed INSTALLATION FOR MEASURING AND ADJUSTING THE LIQUID STEEL LEVEL IN A CONTINUOUS CASTING LINGOTIER
FR2403136A1 (en) * 1977-09-19 1979-04-13 Concast Ag METHOD AND DEVICE FOR REGULATING THE LEVEL OF THE BATH IN A CONTINUOUS CASTING LINGOTIER, USING AN ALTERNATIVE ELECTROMAGNETIC FIELD
US4175612A (en) * 1977-11-15 1979-11-27 Arbed Acieries Reunies De Burbach-Eich-Dudelange S.A. Apparatus for measuring and controlling the level of molten steel in a continuous-casting mold
EP0010539A1 (en) * 1978-10-25 1980-04-30 Arbed S.A. Method to measure the filling level of liquid metals in containers, especially in continuous-casting moulds
EP0060800A1 (en) * 1981-03-18 1982-09-22 Arbed S.A. Method and device for measuring the filling level in continuous-casting moulds
EP0077747A2 (en) * 1981-10-16 1983-04-27 Arbed S.A. Controlling process for a continuous casting mould during casting
US4446562A (en) * 1981-10-13 1984-05-01 Electric Power Rsearch Institute, Inc. Method and apparatus for measuring crucible level of molten metal
DE3432131A1 (en) * 1983-09-07 1985-03-21 USS Engineers and Consultants, Inc., Pittsburgh, Pa. METHOD AND DEVICE FOR REGULATING A LIQUID LEVEL
US20150000859A1 (en) * 2011-10-21 2015-01-01 Jp Steel Plantech Co. Method of and apparatus for controlling molten metal surface in mold of continuous-casting machine and continuous-casting machine including the apparatus

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US3204460A (en) * 1962-08-13 1965-09-07 United States Steel Corp System for indicating the liquid level in a continuous-casting mold or the like
US3456715A (en) * 1966-08-16 1969-07-22 Gen Dynamics Corp Apparatus for the measurement of levels of high temperature materials

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CA711067A (en) * 1965-06-08 Pototschnigg Gunter Metal flow regulation
FR1029098A (en) * 1949-12-05 1953-05-29 Boehler & Co Ag Geb Method and control device for bar casting of metals such as iron or steel
US2905989A (en) * 1956-05-04 1959-09-29 Koppers Co Inc Method and apparatus for continuous casting of metals
AT226894B (en) * 1959-06-03 1963-04-10 Schloemann Ag Method and device for regulating the flow of the melt in continuous casting plants
US3204460A (en) * 1962-08-13 1965-09-07 United States Steel Corp System for indicating the liquid level in a continuous-casting mold or the like
FR1373146A (en) * 1963-08-12 1964-09-25 Tsnii Tchornoy Metallurg I P B Automatic continuous casting plant for steel
US3456715A (en) * 1966-08-16 1969-07-22 Gen Dynamics Corp Apparatus for the measurement of levels of high temperature materials

Cited By (18)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3670801A (en) * 1970-01-16 1972-06-20 Borg Warner Continuous casting mold level control
US3677333A (en) * 1970-01-26 1972-07-18 Borg Warner Start-up system for continuous casting mold
US4014379A (en) * 1970-06-09 1977-03-29 Getselev Zinovy N Method of forming ingot in process of continuous and semi-continuous casting of metals
US4027233A (en) * 1973-07-23 1977-05-31 Eduard Ivanovich Shmakov Contactless inductance pickup for detecting the interface of two media
FR2369892A1 (en) * 1976-11-05 1978-06-02 Arbed INSTALLATION FOR MEASURING AND ADJUSTING THE LIQUID STEEL LEVEL IN A CONTINUOUS CASTING LINGOTIER
US4212342A (en) * 1977-09-19 1980-07-15 Concast Ag Method and apparatus for regulating the bath level of a continuous casting mold
FR2403136A1 (en) * 1977-09-19 1979-04-13 Concast Ag METHOD AND DEVICE FOR REGULATING THE LEVEL OF THE BATH IN A CONTINUOUS CASTING LINGOTIER, USING AN ALTERNATIVE ELECTROMAGNETIC FIELD
US4175612A (en) * 1977-11-15 1979-11-27 Arbed Acieries Reunies De Burbach-Eich-Dudelange S.A. Apparatus for measuring and controlling the level of molten steel in a continuous-casting mold
EP0010539A1 (en) * 1978-10-25 1980-04-30 Arbed S.A. Method to measure the filling level of liquid metals in containers, especially in continuous-casting moulds
US4279149A (en) * 1978-10-25 1981-07-21 Arbed Acieries Reunies De Burbach-Eich-Dudelange, Societe Anonyme Process for measuring the level of metal in vessels, especially in continuous casting molds
EP0060800A1 (en) * 1981-03-18 1982-09-22 Arbed S.A. Method and device for measuring the filling level in continuous-casting moulds
US4441541A (en) * 1981-03-18 1984-04-10 Arbed S.A. Method of and apparatus for determining the melt level in a continuous-casting mold
US4446562A (en) * 1981-10-13 1984-05-01 Electric Power Rsearch Institute, Inc. Method and apparatus for measuring crucible level of molten metal
EP0077747A2 (en) * 1981-10-16 1983-04-27 Arbed S.A. Controlling process for a continuous casting mould during casting
EP0077747A3 (en) * 1981-10-16 1983-11-16 Arbed S.A. Controlling process for a continuous casting mould during casting
DE3432131A1 (en) * 1983-09-07 1985-03-21 USS Engineers and Consultants, Inc., Pittsburgh, Pa. METHOD AND DEVICE FOR REGULATING A LIQUID LEVEL
US20150000859A1 (en) * 2011-10-21 2015-01-01 Jp Steel Plantech Co. Method of and apparatus for controlling molten metal surface in mold of continuous-casting machine and continuous-casting machine including the apparatus
US9174273B2 (en) * 2011-10-21 2015-11-03 Jp Steel Plantech Co. Method of and apparatus for controlling molten metal surface in mold of continuous-casting machine and continuous-casting machine including the apparatus

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