US2829231A - Device for automatic control of the temperature of an electric furnace - Google Patents

Description

P 1, 1958 w. ROOST DEVICE F OR AUTOMATIC CONTROL OF THE TEMPERATURE OF AN ELECTRIC FURNACE Filed Feb. 8, 1955 ,1 3 3* E C I H. F. ;HF FILTER QT c emsamon 5 I? I we 2% f k 9 SOURCE f3 INVERTER E 5 51 MOTOR m DC SOURCE 37 I CONSTANT REFERE'A/Cf VOLTAGE GENERATOR INVENTOR WILLEM TROOST AGENT 2,82%,23l Patented Apr. 1, 1958 DEVICE FUR AUTOMATIC CONTROL OF THE TEMPERATURE @F AN ELECTRIC FURNACE Willem Troost, Emmasiugel, Eindhoven, Netherlands, assigner, by mesne assignments, to North American Philips Company, Inc, New York, N. L, a corporation of Delaware Application February 8, 1955, Serial No. 486,965 Claims priority, application Netherlands February 9, 1954 3 Claims. (Cl. 219-20) The invention relates to a device for automatic control of the temperature of an electric furnace, which is fed from an A. C. source or a diiferent source of energy. Use is made herein of a suitable pick-up, for example a therrno-element or a resistance thermometer, which is arranged in or near the furnace and with the aid of which an electric voltage is obtained, which varies with the furnace temperature. This voltage may be used as a control-voltage, by means of which, if necessary via an amplifier, a control-member is controlled; this controlmember increases the supply of energy to the furnace if the temperature is too low and decreases the supply, if the temperature is too high.

Such arrangements are used with high-frequency furnaces which are fed by a current of high frequency produced by a high-frequency generator. The controlmember may, in this case, be constructed in the form of a slidable core of an inductor connected in series with the winding of the furnace. The high-frequency generator may be fed from the alternating-voltage source having a frequency of 50 cycles per second. Various kinds of control may be carried out. The invention may be used in devices in which a so-called proportional control is carried out; with this control the displacement of the control-member is substantially proportional to the extent to which the real temperature deviates from the desired temperature value. This applies to a given temperature range, the so-called proportionality range. A proportionul control-device responds to the value and the sense of the divergence of the magnitude to be controlled within a predetermined range. The proportional control may be combined with an integrating control or a differentiating control, in which the control-member is additionally accelerated proportionally to the divergence or additionally displaced proportionally to the velocity of the variation in the magnitude, or it may be combined with an integrating and a differentiating control.

If the load of the furnace increases, so that the temperature drops, the said control-voltage occurs, the control-member being thus actuated in a sense such that it increases the supply of energy to the furnace. After a certain amount of time, which depends mainly upon the heat inertia of the furnace, a stationary condition is reached, in which the initial temperature is substantially restored. In the case of a voltage variation of the supply source or a variation in condition of the source of energy, which implies a corresponding variation in the supply of energy to the furnace, the same accurs. The controlvoltage does not occur until due to the initial variation in the supply of energy to the furnace, the temperature has varied to a sufiicient extent. The resultant time lag is in many cases a disadvantage. Stabilization of the supply voltage can, in general, not be carried out,-since the power absorbed by the furnace is comparatively high.

The present invention has for its object the provision of a device of the aforesaid kind, in which this disadvantage is reduced to a great extent. The device of the present invention provides not only a more rapid but also a more accurate control in a sense such that the temperature variations due to variations in operational conditions are reduced.

in the device of the present invention, the control member is also subjected to a control-voltage, derived from the supply source voltage or from a condition of the source of energy, and is operative in a sense such that the effect of supply source voltage variations or condition variations of the source of energy on the temperature of the furnace is at least partly compensated substantially without delay.

The control-voltage derived from the temperature of the furnace is operative across a control-circuit, which, in general, is connected to the input circuit of an amplifier, the output circuit of which governs the controlmember. This is obtained, in general, by means of a motor, or by means of a pneumatic device actuated by a motor. The control-voltage derived from the supply voltage may govern an additional control-member. However, it is more advantageous to introduce this controlvoltage directly into the control-circuit, since the need for an additional control-circuit with governing members is thus avoided.

The control-circuit may, furthermore, comprise an impedance, across which occurs a voltage varying with the displacement of the control-member from the initial. position; this voltage counteracts the control-voltage derived from the temperature of the furnace. Thus the displacement of the control-member, in the case of a particular deviation of the furnace temperature from the desired value or in the case of supply voltage variation or condition variation of the source of energy, may be such that it is substantially proportional to the said deviation or variation respectively. Thus a proportional control is obtained.

The device is preferably such that the second controlvoltage is substantially equal to zero in the case of a normal supply source voltage of, for example, 220 volts, or a normal condition of the source of energy; the value of said second control voltage being substantially proportional to the supply source voltage variation or condition variation and being operative in the same sense as such variation.

In order that the invention may be readily carried into effect, it will now be described more fully with reference to the accompanying drawing, in which the figure is a schematic diagram of an embodiment of the device of the present invention.

In the figure, reference numeral 1 designates a highfrequency generator, which is fed from an alternatingvoltage supply source. The alternating high-frequency voltage produced serves to heat an element 2 by inductance; this element may, for example, be a graphite crucible, in which the material to be heated and to be kept at a constant temperature may be housed. The heating current is controlled by means of a series inductor 3, the inductance value of which may be adjusted by means of a slidable core 4 of magnetic material. The core 4 constitutes the control-member proper and is driven by a motor 19 in a manner to be described hereinafter.

In or near the element 2 provision is made of a pick-up, for example a thermo-element 2.9. The element 29 is connected through a high-frequency filter 5, which is intended to keep the measuring apparatus free from interfering oscillations, to an indicating device or recording device 6.

The device 6 may comprise a known self-compensating circuit. The voltage produced by the thermo-element 29, or the first'voltage, is, in this case, compensated in a compensation circuit by a known variable voltage, or the second voltage, which may for example be derived from a potentiometer. The residual voltage, which is the first residual voltage, acts, through an amplifier, upon a motor which drives a pointer 7. The pointer 7 is coupled with the slidable contact of the said potentiometer, so that the motor, and hence also said pointer, stand still as soon as the total voltage appearing across the compensation circuit is equal to zero; hence the voltage produced by the thermo-element 29 is equal to the voltage derived from the said potentiometer. The position of the pointer 7 thus indicates the value of the voltage supplied by the thermo-element 29, or the first voltage, which is at the same time a measure of the temperature of the element 2. At the pointer '7 provision may be made of a dial or scale calibrated in degrees. Such a self-compensating circuit has the advantage that a strong source of energy, i. e. a motor, is available for driving the pointer, so that the assembly may be of a rugged construction.

The pointer 7 is coupled mechanically with a sliding contact 8 of a potentiometer 9, which is connected to a stabilized source of direct voltage. The potentiometer 9 comprises a manually adjustable sliding contact 19, which determines the desired temperature or the second voltage. As will be evident hereinafter, the position of the contact 10 determines the temperature to which the furnace is adjusted or the desired temperature. This is the socalled adjusting point. If the furnace has exactly the desired temperature, the sliding contacts 8 and M are substantially at the same position. The voltage prevailing between the contact positions, or the first residual voltage, is a measure for the temperature deviation.

A resistor 11 is connected between the contacts 8 and 10. A voltage, which is the first residual voltage and which governs the control member, is derived from the resistor 11 by means of a sliding contact 12.

It may be temporarily assumed that the elements 23 to 23, of the figure, are not provided. The contact It? is then connected directly to a device 17. The contact 12 is connected through a parallel combination of two potentiometer resistors and 16, which are connected to a stabilized direct-voltage source and along which sliding contacts 13 and 14 are displaceable, to the input circuit of the device 17. The contacts 13 and 14 determine the component and third voltages, respectively. The device 17 is an oscillation converter or inverter, which converts the direct voltage operating across the input circuit into an alternating voltage. This alternating voltage is amplified in an amplifier 18, the output circuit of which controls the motor 19. The motor 19 and a generator 20 coupled mechanically thereto constitute a motor-generator aggregate known with such control and measuring arrangements. The motor and generator are fed from the same alternatingvoltage source (not shown in the figure) and the alternoting voltage produced by said generator is fed back to the amplifier iii. The generator 20 provides a damping control of the movement of the control-member 4, which is driven by the motor 19 (as is indicated by the broken line 21). By control of the feedback voltage supplied to the motor, the adjustment of the controlmember 4- may be oscillating, critically damped or overdamped.

The device of the present invention so far described operates as follows:

Starting from a stationary condition, in which the temperature of the furnace element 2 has substantially the desired value, the temperature drops at a variation of the load, for example an increase in load. The pointer 7 moves and the contact 8 moves and a voltage occurs across the resistor 11. The voltage across the resistor 11 is the first residual voltage. The part of this voltage appearing between the contacts 10 and 12 is converted in the device 17 into an alternating voltage having an amplitude proportional thereto; said alternating voltage governing the position of the control-member 4 through 4 the amplifier 18 and the motor 19. The control member 4- is thus displaced in such a sense that the inductance of the inductor 3 decreases and the current through the heating winding or element 2 of the furnace increases.

The parallel combination of the resistors 15 and 16, which may be combined to form a single potentiometer having the sliding contacts 13 and 14, insures that the displacement of the core 4 is proportional to the initial temperature variation. The sliding contact 13 occupies a fixed position, whereas the sliding contact 14 is coupled mechanically with the core 4. Thus, upon a movement of the core 4, a voltage, which is the third voltage, is introduced into the control-circuit; this voltage counteracts the voltage obtained from the resistor 11 to provide the second residual voltage. The motor 19 and the control-member 4 stand still as soon as the two voltages operating across the control-circuit are equal to one another. Thus a so-called proportional control is obtained, the Width of the proportionality range being adjustable by means of the contact 12. The contact 13 serves as a manual after-control of the adjusting point. Provisions may thus be made that the contact 8 coincides in position with the contact 10 in each stationary condition.

According to the invention, provision is made of means for obtaining a second proportional control in accordance with supply source voltage fluctuations. To this end provision is made of a transformer 23, having a primary winding connected to the alternating-voltage supply and a secondary winding connected to a rectifier 24. The output of the rectifier 24 comprises a smoothing element 25. The rectified supply voltage, which is the fourth voltage, is compared with a very constant reference voltage, which is the fifth voltage, derived from a constant reference voltage device 27, which is connected in series with an output potentiometer 26. A voltage which is the third residual voltage and which is substantially proportional to the difference between the output voltage of the rectifier and said constant reference voltage, occurs across the potentiometer 26. The adjustment is preferably such that said rectifier output and reference voltages are substantially equal to one another at the normal supply source voltage. In this case no voltage is operative across the potentiometer 26 and the controldevice operates in the manner described above. The constant reference voltage device 27 may comprise any suitable source of constant potential.

At a variation in supply source voltage a voltage occurs across the lower part of the potentiometer 26; this voltage being also operative in the control-circuit. In consequence thereof the motor 19 is actuated substantially without delay, so that the control-member 4 is displaced. At the same time, the sliding contact 14 is displaced in such a sense that in the control-circuit the voltage across the lower part of the potentiometer 26 is counteracted. The motor 19 stands still when the latter voltage is equal to the voltage prevailing between the points 13 and 14. Thus a proportional control is obtained, this is, however, a so-called anticipating control. This control may also be combined with a differentiating control. The control proper on a constant temperature is carried out by the part of the arrangement described above. The width of the proportionality range of the second control may be adjusted by means of a sliding contact 28.

By the combination of the present invention, as described, of two control-effects, a large part of the socalled dead time, i. e. the time required after a disturbance of the stationary condition to cause the control-member to become operative, is obviated as far as supply voltage variations are concerned, so that the control is not only more rapid but also more accurate.

As an alternative the second control-voltage may be derived not only directly from the supply source voltage but also from the output voltage of the high-frequency generator 1. As a further alternative, said second control voltage may be derived from a direct voltage operative in the generator 1; for example, from the anode supply voltage of the tube or tubes of said generator. In the latter case the transformer 23, the rectifier 24, and in many cases also the smoothing element 25, may be dispensed with.

The heat energy may be obtained not only from a high-frequency generator but also from the supply source. Thus, for example, the furnace may be heated by a heating medium, for example a liquid or vapor, which is heated by energy derived from said supply source. The second control-voltage may be derived from the temperature of the heating medium and hence indirectly from the supply voltage.

As an alternative, the heating energy may be supplied by a source different from the supply source such as, for example, by a gas container, the pressure of which varies, or by a vapor duct, the temperature'of which vapor exhibits variations.

With the arrangement of the present invention as described above, a furnace temperature of about 1000 degrees C. may be kept constant within a tolerance of about half a degree C.

While the invention has been described by means of a specific example and in a specific embodiment, I do not wish to be limited thereto, for obvious modifications will occur to those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. A device for automatic control of the temperature of an electric furnace comprising a heating element, means for applying electrical energy to said heating element, regulating means connected between said heating element and said energy applying means for varying the magnitude of energy applied to said element thereby varying the temperature of said furnace, means for varying the regulation of said regulating means positioned in operative relation therewith, means for producing a first residual voltage in accordance with the difference between a voltage dependent upon the magnitude of the temperature of said furnace an a voltage dependent upon a predetermined magnitude of the said temperature, means for producing a third voltage, means for varying the amplitude of said third voltage in accordance with the regulation of said regulating means, means for deriving a fourth voltage dependent upon the magnitude of the voltage of said energy applying means, means for produc-,

ing a fifth voltage, means for combining said first residual voltage with said third voltage thereby to produce a second residual voltage, means for combining said second residual voltage with said fourth voltage and said fifth voltage thereby to produce a control voltage, an electrically energized adjusting member coupled to the said regulation varying means, and means for applying said control voltage to said adjusting member to vary the magnitude of the temperature of said furnace whereby variations in the magnitude of the said voltage of said energy applying means are substantially compensated for substantially without time lag.

2. A device for automatic control of the temperature of an electric furnace comprising a heating element, means for applying electrical energy to said heating element, regulating means connected between said heating element and said energy applying means for varying the magnitude of energy applied to said element thereby varying the temperature of said furnace, means for varying the regulation of said regulating means positioned in operative relation therewith, means for deriving a first voltage dependent upon the magnitude of the temperature of said furnace, means for producing a second voltage dependent upon a predetermined magnitude of the said temperature, means for combining the said first voltage with the said second voltage thereby to produce a first residual voltage, means for producing a third voltage, means for varying the amplitude of said third voltage in accordance with the regulation of said regulating means, means for combining said first residual voltage with said third voltage thereby to produce a second residual voltage, means for deriving a fourth voltage dependent upon the magnitude of the voltage of said energy applying means, means for producing a fifth voltage, means for combining said fourth voltage with said fifth voltage thereby to produce a third residual voltage, means for combining said second residual voltage with said third residual voltage thereby to produce a control voltage, an electrically energized adjusting member coupled to the said regulation varying means. and means for applying said control voltage to said adjusting member to vary the magnitude of the temperature of said furnace whereby variations in the magnitude of the said voltage of said energy applying means are substantially compensated for substantially Without time lag.

3. A device for automatic control'of the temperature of an electric furnace comprising a heating element, means for applying electrical energy to said heating element, regulating means connected between said heating element and said energy applying means for varying the magnitude of energy applied to said element thereby varying the temperature of said furnace, means for varying the regulation of said regulating means comprising a control member positioned in operative relation there with, means for deriving a first voltage dependent upon the magnitude of the temperature of said furnace, means for producing a second voltage dependent upon a predetermined magnitude of the said temperature, means for combining the said first voltage with the said second voltage thereby to produce a first residual voltage, means for producing a component voltage, means for varying the amplitude of said component voltage comprising a first potentiometer having a control member connected to said first residual voltage producing means, means for producing a third voltage, means for varying the amplitude of said third voltage comprising a second potentiometer connected in parallel with said first potentiometer and having a control member coupled to the said control member of said regulation varying means, means for combining said first residual voltage with the said variable component voltage and the said variable third voltage thereby to produce a second residual voltage, means for deriving a fourth voltage dependent upon the magnitude of the voltage of said energy applying means, means for producing a fifth voltage, means for combining said fourth voltage with said fifth voltage thereby to produce a third residual voltage, said last-mentioned means comprising a third potentiometer having a control member connected to the control member of said second potentiometer, means for combining said second residual voltage with said third residual voltage thereby to produce a control voltage, an electrically energized adjusting member coupled to the said control member of said regulation varying means, and means for applying said control voltage to said adjusting member to vary the magnitude of the temperature of said furnace whereby variations in the magnitude of the said voltage of said energy applying means are substantially compensated for substantially without time lag.

References Cited in the file of this patent UNITED STATES PATENTS 2,474,441 Sparrow June 28, 1945 2,499,964 McRae Mar. 7, 1950 2,651,704 Prior Sept. 8, 1953 OTHER REFERENCES Eubank: Precision Thermostat for High Temperatures; The Review of Scientific Instruments; vol. 21, No. 10, October 1950.

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