US3893502A

US3893502A - Method and mechanism for indicating mold friction in a continuous-casting machine

Info

Publication number: US3893502A
Application number: US475082A
Authority: US
Inventors: Frank Slamar
Original assignee: United States Steel Corp
Current assignee: AG Industries Inc Pennsylvania
Priority date: 1974-05-31
Filing date: 1974-05-31
Publication date: 1975-07-08
Anticipated expiration: 1992-07-08

Abstract

A method and mechanism for determining frictional forces between the mold of a continuous-casting machine and the casting itself. During a casting operation the mold oscillates up and down, being driven by a motor and cam mechanism. The invention provides a method and means for determining the mold friction by measuring the armature current through the cam drive motor, but compensating for other factors which contribute to the load on the motor, whereby the average magnitude of the armature current through the oscillation cycle is indicative of mold friction only.

Description

United States Patent Slamar METHOD AND MECHANISM FOR INDICATING MOLD FRICTION IN A CONTINUOUS-CASTING MACHINE July s, 1975 3,478,572 11/1969 McRae et a1. 73/9 Primary ExaminerJerry W. Myracle Attorney, Agent, or FirmWalter P. Wood ABSTRACT A method and mechanism for determining frictional forces between the mold of a continuous-casting machine and the casting itself. During a casting operation the mold oscillates up and down, being driven by a motor and cam mechanism. The invention provides a method and means for determining the mold friction by measuring the armature current through the cam drive motor, but compensating for other factors which contribute to the load on the motor, whereby the average magnitude of the armature current through the scillation cycle is indicative of mold friction only.

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EI'I 1 METHOD AND MECHANISM FOR INDICATING MOLD FRICTION IN A CONTINUOUS-CASTING MACHINE This invention relates to an improved method and mechanism for determining frictional forces between the mold of a continuous-casting machine and the casting itself.

In a conventional continuous-casting operation. liquid metal is introduced continuously to the top of an open-ended, water-cooled, vertically oscillating mold, and a partially solidified casting of indefinite length emerges from the bottom. As the casting travels through the mold, frictional forces oppose its movement. Excessive mold friction may lead to defects in the product. or even may cause break-outs of liquid metal from the solidified shell of the casting. Hence there is a need to monitor the magnitude of the mold friction and to make certain it does not exceed a predetermined limit. Heretofore the magnitude of the mold friction has been determined with load cells placed under the mold, or by measuring the load on the drive motor for the withdrawal rolls which act on the casting below the mold, as shown in Barnard et al US. Pat. No. 3,047,915 and in Osborn US. Pat. No. 2,824,346 respectively. Both these arrangements have disadvantages. It is costly to install load cells under a mold, and not readily accomplished if the mold support was not designed originally to accommodate them. The second method does not give an accurate determination, since a casting travels through a roll-rack, which absorbs an indeterminate amount of the mold frictional forces. Hence the change in load on the drive motor is proportional only to the frictional forces not absorbed by the roll rack.

Conventional practice is to employ a cam mechanism driven by a variable speed constant-field d-c motor for oscillating the mold, one example of which is shown in Bode Reissue US. Pat. No. 27,469 of common ownership. During its downward travel in the oscillation cycle the mold moves at a speed about to percent greater than the casting speed. Usually the casting speed is adjusted so that the level of liquid metal in the mold remains substantially constant. The speed of the cam drive motor commonly is adjusted with adjustments in the casting speed to maintain the speed at which the mold moves downwardly at a predetermined ratio to the casting speed.

A characteristic of a d-c motor of the type used for driving the cams is that the magnitude of the armature current is proportional to the load on the motor at any instant. The load of course varies with the varying magnitude of torque which the motor develops through the oscillation cycle. Mold friction is only one of several factors which contribute to the load on the cam drive motor. Other factors include motor losses, mechanical losses in the gearing and other mechanisms, the varying force required to lift and lower the mold through different portions of its oscillation cycle, and accelerating forces brought about by speed changes. Since mold friction is a small factor compared with the others, and the lifting and accelerating forces continually vary through the oscillation cycle, direct measurement of the armature current through the cam drive motor does not itself afford a meaningful determination of mold friction.

An object of my invention is to provide an improved method and mechanism for determining the magnitude of the mold friction, which method and mechanism not only are readily applied to existing continuous-casting machines, but also afford accurate determinations.

A further object is to provide an improved method and mechanism for determining the magnitude of the mold friction in a continuous-casting machine in which I utilize a measurement of the armature current through the cam drive motor, but in which I compensate for the other factors contributing to the load on this motor.

A further object is to provide an improved method and mechanism for accomplishing the foregoing object in which I utilize an integrating amplifier or a digital computer to measure the average armature current to the cam drive motor through a representative number of oscillation cycles, but calibrated to compensate for the effects of factors other than mold friction.

In the drawings:

FIG. I is a line diagram of the circuit utilized in one embodiment of my invention utilizing an integrating amplifier;

FIG. 2 is a graph illustrating the manner in which the armature current to the cam drive motor varies through an oscillation cycle with the motor driving an empty mold and during a casting operation; and

FIG. 3 is a view similar to FIG. 1, but showing a modified embodiment utilizing a digital computer.

CONVENTIONAL OPERATION FIG. 1 shows schematically a continuous-casting mold 10 from the bottom of which a casting ll emerges. The mold oscillates vertically, commonly at a rate on the order of about one cycle per second, driven by a variable speed d-c motor 12 acting through a cam mechanism 13. A master reference signal automatically controls the casting speed so that a substantially constant level of liquid metal is maintained in mold 10. During normal operation the master reference signal serves also to adjust the speed of motor 12 so that the speed at which mold 10 moves downwardly is maintained at a constant ratio to the casting speed.

MASTER REFERENCE SIGNAL A conventional arrangement of components for producing a master reference signal is shown schematically within a block 14.'These components include a liquidlevel indicator 15, a liquid-level set-point potentiometer 16, a speed-adjusting potentiometer l7 and first and second operational amplifiers l8 and 19. The liquid-level indicator l5 and the set-point potentiometer l6 transmit input voltage signals to the first operational amplifier 18 representative respectively by the actual level in the mold 10 and the desired level. The first operational amplifier transmits an output voltage signal proportional to the algebraic sum of its two input signals. The second operational amplifier 19 receives input voltage signals from the speed-adjusting potentiometer 16 andfrom the first operational amplifier 18 representative respectively of the desired casting speed and the error in the liquid level. The second operational amplifier transmits the master reference voltage signal, likewise proportional to the algebraic sum of its two input signals. The master reference signal goes to a third operational amplifier 20 used in controlling the speed of motor 12, and to other

operational amplifiers

21, 22, etc. used in controlling other drives of the continuous-casting machine, not shown since they are not 3 involved in the present invention. Reference can be made to Milnes US. Pat. No. 3,204,460 of common ownership for a more detailed showing of a suitable liquid level indicator and an explanation of how a signal representing the liquid level in a mold may be used in controlling casting speed.

During normal operation the master reference signal is one of the input voltage signals to the third operational amplifier 20. The other input voltage signal to the latter is a feedback signal from a tachometer 23 which is operatively connected with motor 12. The output voltage signal from the third operational amplifier passes through a shunt 24 to a control (not shown) for the armature current of motor 12. The armature current and the load on the motor are directly proportional to the voltage at the shunt 24.

DETERMINATION OF MOLD FRICTION The present invention provides means for determining the magnitude of the mold friction through a representative number of oscillation cycles, for example ten to twenty, at any time during a casting operation. This determination is based on a measurement of the voltage at the shunt 24. The components of this means are shown schematically within a block 25 and include a conventional integrating amplifier 26, a calibration potentiometer 27, and a voltmeter 28 calibrated to read mold friction directly. The present invention makes this possible by compensating for the other factors which contribute to the load on motor 12, as hereinafter explained.

COMPENSATION FOR VARYING LIFTING AND LOWERING FORCES FIG. 2 shows the approximate way in which the armature current to motor 12 varies during the oscillation cycle. Curve A is representative of the magnitude of the armature current when the motor is driving an empty mold up and down. Curve B is representative of the magnitude of the armature current during a casting operation. In each instance the magnitude follows approximately sine wave, reaching maximum values as the mold travels downwardly. Curve B lies approximately a uniform distance above curve A and this distance represents the portion of the current utilized in overcoming mold friction. The integrating amplifier 26 automatically averages the measured magnitude of the armature current and thus compensates the reading on the voltmeter 28 for the varying force required to lift and lower the mold through different portions of each oscillation cycle.

COMPENSATION FOR MOTOR LOSSES AND MECHANICAL LOSSES I compensate the reading on voltmeter 28 for motor losses and mechanical losses in the gearing and other mechanisms by adjusting the calibration potentiometer 27. I operate the motor 12 through at least a representative number of oscillation cycles with the mold empty at a predetermined speed approximately equal to the speed of the motor during a casting operation. I adjust the potentiometer 27 so that the voltmeter 28 reads zero when the motor operates with the mold empty, whereby subsequent voltmeter readings during a casting operation exclude the portion of the armature current utilized in overcoming the foregoing losses. I have observed these losses vary during a casting operation.

When the casting machine is started after several hours of standing idle, the losses are at their maximum. As a casting operation progresses, the losses diminish and ultimately reach a steady magnitude. For this reason I prefer to adjust the calibration potentiometer 27 imme diately after completion of a casting operation. The adjustment remains valid for the next casting operation, provided conditions are similar.

COMPENSATION FOR ACCELERATING FORCES Preferably I also compensate the reading on voltmeter 28 for accelerating forces brought about by speed changes. The voltage of the master reference signal continuously varies to maintain a constant level of liquid metal in the mold. Each change in the master reference signal of course changes the voltage at shunt 24 and the armature current to motor 12. The change in armature current produces an acceleration or deceleration of the mold. Without compensation, such changes would be read on the voltmeter 28 as erratic variations in the mold friction. I compensate for accelerating forces by utilizing a steady reference signal to control the speed of motor 12 in place of the master reference signal throughout the oscillation cycles during which I determine mold friction. The magnitude of the steady reference signal approximately equals the magnitude of the master reference signal at the beginning of this determination. The casting operation is not appreciably upset if the rate at which the mold oscillates remains constant through a representative number of cycles needed to obtain a determination of mold friction.

The components which produce the steady reference signal are shown schematically within a block 29 and include a blocking rectifier 30, a potentiometer 31 and a differential voltmeter 32. The blocking rectifier is connected between the second and third operational amplifiers l9 and 20 and between the potentiometer 31 and the third operational amplifier. The voltmeter 32 has a center zero point and is connected between the lines which are at the master reference voltage and the voltage transmitted by the potentiometer 31 to measure the difference between these voltages. During normal operation of the casting machine, the potentiometer 31 is adjusted so that the master reference voltage exceeds the voltage transmitted by the potentiometer, the voltmeter 32 shows a positive voltage, and the blocking rectifier 30 prevents any voltage from the potentiometer from reaching the third operational amplifier 20. Whenever I wish to determine mold friction, I adjust the potentiometer 31 so that its voltage slightly exceeds the master reference voltage whereby the potentiometer transmits a steady reference signal which is approximately equal to the master reference signal at the moment. The blocking rectifier now prevents the master reference signal from reaching the third operational amplifier 20, and the steady reference signal takes over control of the armature current through motor 12. After I observe mold friction through a representative number of oscillation cycles, I return the potentiometer 31 to its original setting and resume normal operation.

MODIFICATION USING DIGITAL COMPUTER FIG. 3 shows a modification useful for casting installations in where there is a digital computer available. Some casting installations employ a computer for other purposes. such as controlling the volume of water applied to the casting to cool it and promote solidification. In such installations the computer conveniently can be used to obtain a print-out of the mold friction in place of the integrating amplifier shown in FIG. 1. If the installation does not already include a digital computer, the simpler embodiment of FIG. 1 of course is preferred. Except as specifically described, the embodiment shown in FIG. 3 is similar to that shown in FIG. 1, and the description is not repeated.

FIG. 3 shows a digital computer 40 to which are transmitted a speed signal, an armature current signal, and a cycle indicator signal. The speed signal is either the master reference signal or the steady reference signal, either of which is obtained as already described. The armature current signal again is the voltage at shunt 24. Alimit switch 42 is operatively associated with the mold to be opened and closed with each oscillation of 'the mold. I set the computer to determine the average magnitude of the armature current during a representative member of mold oscillation cycles. Operation of the limit switch signals the computer as to the number of cycles which take place. When the selected number of oscillations have taken place, I termi nate the observation. I operate the motor 12 with the mold empty as already described for calibration purposes, thus enabling the computer tosubtract the portion of the armature current signal utilized in overcoming motor losses and mechanical losses. The computer has a calibration switch 43, a read switch 44, and a mold friction print out 45.

OPERATION In both embodiments of the invention, the voltage at the shunt 2 4 is representative of the load on the motor 10. The factors which contribute to this load include the varying forces to lift and lower the mold during different portions of its oscillation cycle, motor losses, mechanical losses in the gearing and other mechanisms, and accelerating forces, beside mold frictions. I measure the voltage at the shunt 24, but I obtain a direct reading of mold friction in this measurement since I compensate for these other factors. I compensate for the varying lifting force by averaging the voltage through a representative number of cycles, either with an integrating amplifier as shown in FIG. 1 or with a digital computer as shown in FIG. 3. I compensate for motor losses and mechanical losses by calibrating the voltmeter 28 while driving the mold up and down empty. I compensate for the accelerating forces by temporarily utilizing a steady reference signal to control the speed of motor 12 in place of the varying master reference signal.

From the foregoing description, it is seen that my invention affords a simple method and mechanism for determining the magnitude of mold friction, and can be applied readily to existing casting machines without disturbing their structure, as is necessary if load cells are placed under a mold. The determination is accurate, since compensation is made for all other factors which would produce an erroneous determination. The mold friction in force units is readily determined from the motor load measurements when a proportionality factor is determined from motor characteristics, gear ratios, and cam designs.

I claim:

1. In a continuous-casting operation in which liquid metal is introduced continuously to the top of an openended mold, a partially solidified casting of indefinite length emerges from the bottom of the mold, and the mold is oscillated vertically, the movement of said casting through said mold being opposed by friction, and in which the drive means for oscillating the mold includes a motor operatively connected therewith, the magnitude of the armature current to said motor being proportional to the load on the motor, an improved torsfor which compensation is made include the varying force required to lift and lower the mold through different portions of its oscillation cycle, motor losses, and mechanical losses in the gearing and other mechanisms.

3.A method as defined in claim 2 in which the varying force required to lift and lower the mold is compensated for by averaging the current measurement through said cycles.

4. A method as defined in claim 2 in which the mold losses and mechanical losses are compensated for by running said motor with the mold empty and calibrating the measuring means to read zero with the motor driving the empty mold.

5. A method as defined in claim 2 in which an additional factor 'for which compensation is made is the accelerating force on the motor brought about by changes in the speed of mold oscillation, and the accelerating force is compensated for by operating the motor at a constant speed during said cycles.

6. In a continuous-casting operation in which liquid metal is introduced continuously to the top of an openended mold, a partially solidified casting of indefinite length emerges from the bottom of the mold, and the mold is oscillated vertically, the movement of said casting through said mold being opposed by friction and in which the drive means for oscillating the mold includes a variable speed d-c motor operatively connected therewith, and means for producing a master reference signal for controlling the casting speed and maintaining the speed at which the mold oscillatesat a predetermined ratio to the casting speed, the magnitude of the armature current being proportional to the load on the motor, an improved method of determining the magnitude of the mold friction, said method comprising measuring the average magnitude of the armature current to said motor with the motor oscillating the mold through a representative number of cycles during a casting operation, calibrating the measuring means to compensate for motor losses and mechanical losses by running said motor with the mold empty and setting the measuring means to zero, and replacing said master reference signal with a steady reference signal while the mold friction determination is made to compensate for accelerating forces.

7. A method as defined in claim 6 in which said steady reference signal approximately equals the magnitude of said master reference signal at the beginning of the determination.

8. A method as defined in claim 6 in which the average magnitude of the armature current is obtained by applying said steady reference signal to an integrating amplifier.

9. A method as defined in claim 6 in which the average magnitude of the armature current is obtained by applying said steady reference signal to a digital computer through a predetermined number of oscillation cycles.

10. A method as defined in claim 6 in which the mechanical losses are at a maximum at the beginning of a casting operation, but diminish and reach a steady magnitude as casting progresses, and said calibrating step is conducted immediately after the completion of a casting operation.

II. In a continuous-casting machine which comprises an open-ended mold for receiving liquid metal and from the bottom of which a partially solidified casting emerges, a motor operatively connected with said mold for oscillating it vertically, the magnitude of the armature current to said motor being proportional to the load on the motor, one of the factors contributing to said load being the frictional force between the mold and the casting, the combination therewith of a mechanism for determining the magnitude of the mold friction, said mechanism comprising; means for measuring the armature current, and means for compensating for other factors contributing to said load, whereby the armature current measurement affords a direct determination of mold friction.

12. A continuous-casting machine as defined in claim 11 in which another factor contributing to said load is the varying force required to lift and lower the mold through different portions of its oscillation cycle, and the means for compensating for variations in the lifting and lowering force includes means for averaging the armature current through a representative number of oscillation cycles.

13. A machine as defined in claim 12 in which said averaging means is a digital computer.

14. A machine as defined in claim 11 in which said averaging means is an integrating amplifier.

15. A continuous-casting machine as defined in claim 11 in which other factors contributing to said load are motor losses and mechanical losses, and the means for compensating for said losses include calibration means which are set to show a zero with said motor driving an empty mold.

16. A continuous-casting machine as defined in claim 11 in which another factor contributing to said load is acceleration force brought about by changes in the speed at which said mold oscillates. and the means for compensating for acceleration force includes means for producing a steady reference signal for controlling the armature current.

l7. In a continuous-casting machine which comprises an open-ended mold for receiving liquid metal and from the bottom of which a partially solidified casting emerges, a motor operatively connected with said mold for oscillating it vertically, and means for producing a varying master reference signal which controls the speed of said motor, the magnitude of the armature current to said motor being proportional to the load on said motor, the factors contributing to said load including mold friction, the varying force required to lift and lower the mold through different portions of its oscillation cycle, motor losses, mechanical losses, and accelerating forces brought about by speed changes, the combination therewith of a mechanism for determining the magnitude of the mold friction, said mechanism comprising means for measuring the armature current averaged over a representative number of oscillation cycles to compensate for the varying force required to lift and lower the mold, means connected with said measuring means for compensating for the effect of said losses, and means for replacing said master reference signal with a steady reference signal while the mold friction determination is made to compensate for said accelerating forces.

18. A mechanism as defined in claim 17 in which the means for measurng the armature current includes a voltmeter and an integrating amplifier operatively connected with said voltmeter and receiving said steady reference signal.

19. A mechanism as defined in claim 17 in which the means for measuring the armature current includes a digital computer and a print-out operatively connected with said computer, said computer receiving said steady reference signal.

20. A mechanism as defined in claim 17 in which the means for replacing said master signal with a steady reference signal includes a blocking rectifier and a potentiometer connected to the circuit which carries said signals, said blocking rectifier normally permitting said master reference signal to pass, but permitting said steady reference signal to pass when its magnitude is greater than said master reference signal.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 893 502 Dated uly 8 1975 Frank Slamar Invent0r(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 51 after "voltage" insert a comma.

Column 5, line 29, "read" should read "read" Column 7, line 45, after "zero" insert reading Signed and Scaled this sixteenth D21) Of Septemberl975 [SEAL] Attest.

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner of Pare/us and Trademarks

Claims

1. In a continuous-casting operation in which liquid metal is introduced continuously to the top of an open-ended mold, a partially solidified casting of indefinite length emerges from the bottom of the mold, and the mold is oscillated vertically, the movement of said casting through said mold being opposed by friction, and in which the drive means for oscillating the mold includes a motor operatively connected therewith, the magnitude of the armature current to said motor being proportional to the load on the motor, an improved method of determining the magnitude of the mold friction, said method comprising measuring the magnitude of the armature current to said motor with the motor oscillating the mold through a representative number of cycles during a casting operation, and compensating for factors which contribute to the load on the motor other than mold friction.

2. A method as defined in claim 1 in which the factors for which compensation is made include the varying force required to lift and lower the mold through different portions of its oscillation cycle, motor losses, and mechanical losses in the gearing and other mechanisms.

3. A method as defined in claim 2 in which the varying force required to lift and lower the mold is compensated for by averaging the current measurement through said cycles.

5. A method as defined in claim 2 in which an additional factor for which compensation is made is the accelerating force on the motor brought about by changes in the speed of mold oscillation, and the accelerating force is compensated for by operating the motor at a constant speed during said cycles.

6. In a continuous-casting operation in which liquid metal is introduced continuously to the top of an open-ended mold, a partially solidified casting of indefinite length emerges from the bottom of the mold, and the mold is oscillated vertically, the movement of said casting through said mold being opposed by friction and in which the drive means for oscillating the mold includes a variable speed d-c motor operatively connected therewith, and means for producing a master reference sIgnal for controlling the casting speed and maintaining the speed at which the mold oscillates at a predetermined ratio to the casting speed, the magnitude of the armature current being proportional to the load on the motor, an improved method of determining the magnitude of the mold friction, said method comprising measuring the average magnitude of the armature current to said motor with the motor oscillating the mold through a representative number of cycles during a casting operation, calibrating the measuring means to compensate for motor losses and mechanical losses by running said motor with the mold empty and setting the measuring means to zero, and replacing said master reference signal with a steady reference signal while the mold friction determination is made to compensate for accelerating forces.

11. In a continuous-casting machine which comprises an open-ended mold for receiving liquid metal and from the bottom of which a partially solidified casting emerges, a motor operatively connected with said mold for oscillating it vertically, the magnitude of the armature current to said motor being proportional to the load on the motor, one of the factors contributing to said load being the frictional force between the mold and the casting, the combination therewith of a mechanism for determining the magnitude of the mold friction, said mechanism comprising; means for measuring the armature current, and means for compensating for other factors contributing to said load, whereby the armature current measurement affords a direct determination of mold friction.

16. A continuous-casting machine as defined in claim 11 in which another factor contributing to said load is acceleration force brought about by changes in the speed at which said mold oscillates, and the means for compensating for acceleration force includes means for producing a steady reference signal for controlling the armature current.

17. In a continuous-casting machine which comprises an open-ended mold for receiving liquid metal and from the bottom of which a partially solidified casting emerges, a motor operatively connected with said mold for oscillating it vertically, and means for producing a varying master reference signal which controls the speed of said motor, the magnitude oF the armature current to said motor being proportional to the load on said motor, the factors contributing to said load including mold friction, the varying force required to lift and lower the mold through different portions of its oscillation cycle, motor losses, mechanical losses, and accelerating forces brought about by speed changes, the combination therewith of a mechanism for determining the magnitude of the mold friction, said mechanism comprising means for measuring the armature current averaged over a representative number of oscillation cycles to compensate for the varying force required to lift and lower the mold, means connected with said measuring means for compensating for the effect of said losses, and means for replacing said master reference signal with a steady reference signal while the mold friction determination is made to compensate for said accelerating forces.