CA1053338A - Two-level temperature control for induction heating apparatus - Google Patents
Two-level temperature control for induction heating apparatusInfo
- Publication number
- CA1053338A CA1053338A CA248,296A CA248296A CA1053338A CA 1053338 A CA1053338 A CA 1053338A CA 248296 A CA248296 A CA 248296A CA 1053338 A CA1053338 A CA 1053338A
- Authority
- CA
- Canada
- Prior art keywords
- temperature
- signal
- temperature level
- level
- voltage
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
- H05B6/00—Heating by electric, magnetic or electromagnetic fields
- H05B6/02—Induction heating
- H05B6/10—Induction heating apparatus, other than furnaces, for specific applications
- H05B6/105—Induction heating apparatus, other than furnaces, for specific applications using a susceptor
-
- G—PHYSICS
- G05—CONTROLLING; REGULATING
- G05D—SYSTEMS FOR CONTROLLING OR REGULATING NON-ELECTRIC VARIABLES
- G05D23/00—Control of temperature
- G05D23/19—Control of temperature characterised by the use of electric means
- G05D23/27—Control of temperature characterised by the use of electric means with sensing element responsive to radiation
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/42—Conversion of dc power input into ac power output without possibility of reversal
- H02M7/44—Conversion of dc power input into ac power output without possibility of reversal by static converters
- H02M7/48—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/505—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a thyratron or thyristor type requiring extinguishing means
- H02M7/515—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a thyratron or thyristor type requiring extinguishing means using semiconductor devices only
- H02M7/5152—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a thyratron or thyristor type requiring extinguishing means using semiconductor devices only with separate extinguishing means
- H02M7/5155—Conversion of dc power input into ac power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a thyratron or thyristor type requiring extinguishing means using semiconductor devices only with separate extinguishing means wherein each commutation element has its own extinguishing means
-
- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
- H05B6/00—Heating by electric, magnetic or electromagnetic fields
- H05B6/02—Induction heating
- H05B6/06—Control, e.g. of temperature, of power
Abstract
Abstract of the Disclosure An induction heating apparatus and related temperature control circuitry and method of controlled heating of a workpiece at two temperature levels. The apparatus and method utilizes a temperature sensing device, adjustable reference temperature circuitry utilizing a voltage dividing circuit triggered by a timing circuit to heat the workpiece to a first predetermined temperature, dwell at the predetermined temperature for an amount of time governed by the timing device and continue heating the workpiece to a second predetermined desired temperature.
The method and circuitry being particularly useful in heating a laminar structure where a ferromagnetic material is coated with a surface of nonconducting, nonferromagnetic material, such as paint.
The method and circuitry being particularly useful in heating a laminar structure where a ferromagnetic material is coated with a surface of nonconducting, nonferromagnetic material, such as paint.
Description
~S33313 TWO-LEVEL TEMPERATURE CONTROL FOR INDUCTION ~IEAT~ING APPAR~TUS :
Background of the Invention Many attempts have been made recently to develop a new technology of securing various mechanical elements to base structures by using adhesive rather than mechanical fasteners.
Two general classes of adhesive have been utilized in this technology, namely pressure sensitive and hea-t activated -~
adhesive. In the case of heat activated adhesive, the use of induction heating techniques were the logical choice as the energy source for activating the adhesive when at least one of the elements to be bonded had ferromagnetic charac-teristics. The main advantage of induction heating techniques over more customary heating methods is the ability to raise the temperature of metallic surfaces to high levels in a relatively short time. Due to the fact that heat is cJen-erated in the body to be heated and does not rely on heat transfer from the heat source to the body, the process is not associated with a general time lag characterizing other methods. Because of the short heating times involved, it becomes of utmost importance to devise techniques which would permit the exact control of the final temperature, either for the p~lrpose of terminating the heating process at the instant when the final temperature was reached or to be able to main-tain a desired final temperature at a constant level for an arbitrarily selected time period.
A special problem in this regard arises in the application where the ferromagnetic material to which the paxts must be bonded is covered with a nonconductive, non-ferromagnetic layer, such as for instance a layer of paint or other coating. All such coatings will have an upper temperature limit which cannot be exceeded without severely 5333~
. . .
damaging the coating. At the same time, it is desirable to maintain the temperature o~ the coating at a temperature su~ficient to activate the adhesi~e for a certain period of time in order to obtain satisfactory bonding. However, only the surface temperature of the coating and not the temperature at the interface between the coating and the sheet of ferro- ;~
magnetic material is accessible for purposes of measurement and control. Previous methods utilized to control the final temperature of a surface incorporated what is generally referred to in control engineering as "proportionate control methods" i.e. reducing the power input to the induction heating generator as a function of the rate of temperature rise or using the two position method, particularly in con-nection with high power vacuum tube generators, using magnetic amplifiers or similar techniques to disengage the high voltage rectifier system supplying power to the oscillator tube. There ;
are also several methods developed in industry where solid state generators were used and control was exercised by the '-use of power input to the workpiece, using pulse width modu-lation techniques or similar methods. In one example of prior art methodsj control is exercised by changing the repetition rate of pulses having e~ual energy content.
The major problem associated with previous methods is ~ `
the difficulty in avoiding overshooting the desired temperature levels or the complexity of circuitry to accomplish such a .~ ;
goal. In either event it is apparently necessary to establish control settings by rather complex experimental methods.
Summary of the Invention In accordance with one aspect of the invention, a U-3~ shaped inductance core is integrated as a unit with a sensing element and this core unit is then operatively associated with an inverter either of the parallel or series type. ---~, 2 ,',,,~ .
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In one broad aspect the invention comprehends an induction heating and temperature responsi~e control appara-tus including an inverter circuit. An induction core unit including a generally U-shaped ferromagnetic core with an exiter winding wrapped thereabout is in operative connection with the inverter circuit and means integrated with the core monitors the surface temperature of a workpiece as it is heated by virtue of the magnetic circuit formed with the workpiece by the core. Means are provided which are responsive to the temperature, and monitoring means automatically con-trols~ the output of the inverter so as to stop the supply of temperature inducing current to the core at a predetermined surface temperature of the workpiece. The means controlling ;~
the inverter oubput includes means for automatically and successively controlling the inverter to maintain a first predetermined temperature for a predetermined time period followed by a subsequent period of operation to increase the temperature to a second temperature level and maintain the second temperature level.
Another aspect of the invention pertains to a method of heating, by induction, a sheet of ferromagnetic base material which is coated with a nonconducting, non-ferromagnetic surface material, such as paint. The steps include heating the base material and simultaneously monitoring the temperature of the surface material, auto-matically stopping the heat input to the base material when temperature of the surface material reaches a first, pre-determined temperature level which is lower than a second, predetermined desired temperature level, allowing the tem-perature of the base material to equalize with the tem-perature of the surface material at the first temperature level, automatically resuming heat input to the base material until the surface temperature reaches the second predetermined ~3~
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temperature level, retaining the second temperature level for a predetermined time interval and therea~ter deactivating the induction heating apparatus.
More particularly the object is to heat by induction without exceeding a critical temperature related to the , deterioration of the coated paint layer. Such heating is accomplished by considering the different rates of heat relatable to both the paint la~er and the ~erromagnetic material, monitoring the surface temperature of the paint layer, turning the power off to eliminate the energy input to ;, the system when the temperature sensed at the paint layer is lower than the desired temperature and also at a temperature which reflects an interface temperature below the maximum tem-perature. The first predetermined temperature is maintained at a constant level for a fixed period of time after which the power is activated to further increase the temperature of both the paint and the ferromagnetic material in such a manner as to provide a temperature differential between the two which is very small, thus serving to eliminate the possibilities of overshooting.
The circuitry for accomplishing this heating method basically includesan infrared sensing detector, an operational amplifier comparator which accepts the output from the infrared .; .
detector and also accepts output from an adjustable reference ~
.. ... .
source which is calibrated in terms of temperature. The adjustable reference source is essentially a voltage dividing circuitry supplied from a constant voltage source which is responsive to a preset timing circuit. The operation of the ;; .
timing circuit in conjunction with the reference source is ~-such as to turn off the power at a predetermined tempera-ture for a given time span after which the voltage input to the com-parator from the reference source changes and is directly related to a slightly higher temperature level which now is the i 7 ~ 3 -4-3~
desired temperature level. Such a circuit thus allows a dwell period which will thereafter permit the surface layer to be heated to its desired temperature in such a manner as to greatly reduce the temperature differential between the paint layer and the ferromagnetic material. `
Other objects and features, as well as a full under-standing of the invention, will appear by reference to the ;
following detailed description, drawings and claims. `
~rief Description of the Drawings Fig. 1 is a partial perspective view of the core integrated with fiber optic sensing element and in position to heat a workpiece which has been coated with a nonconductive material.
Fig. 2 is a greatly enlarged cross-sectional view of a workpiece coated with a paint-type material.
Fig. 3 is a simplified circuit representing the electrical analogue having characteristics similar to the heat-ing characteristics of the workpiece section.
Fig. 4 is a curve representing the behavior of the voltage relative to time elapse of the electrical analogue.
Fig. 5 is a graphical representation of the time/tem-perature relationship occurring during heating by the method of the present invention.
Fig. 6 is a circuit schematic in block diagram form of the two-level heating system of the present invention.
Fig. 7 is a detailed schematic circuit diagram of a preferred embodiment of the two-level temperature control of the present invention.
Fig. 8 is a detailed schematic circuit of an alternate embodiment of the adjustable reference temperature portion of the instant invention.
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Description of the Preferred Embodirnent .... .
Turning now to Fig. 1, there is shown a preferred embodiment of the heating core unit 10 which includes a U-shaped core element 12 with an exciter coil 14 wound about, inducing a current therein. This coil 14 could, for example, be connected directly to the secondary winding of a transformer of a parallel inverter, e. g. an inverter of the MacMurray type. An example of such a core and power supply appropriate for purposes of this invention may be seen in U. S. Patent 3, 816, 690 issued on June 11, 1974. It will be seen that the core unit 10 will have integrated therein a fiber optic element extending through the bridge of the core, positioned by a holding structure 18 so that .
the receiving extremity of the fiber optic element looks at the workpiece to be heated at a posi-tlon Intermediate the legs oE the core in order -to obtain correct and valid tempera-ture measurements simultaneous with the heating of the workpiece. The entire unit thus described may there-.,~
after be potted in a conventional manner.
. . .
.`
The workpiece to be hea-ted 20 comprises a sheet oE ferromagnetic material 24 covered by a surface layer of paint or other nonconducting, nonferromagnetic material 22. It is this particular situation that the present invention is primarily concerned with. The fastening oE some device to the workpiece, and more specifically to the surface of the paint layer 22, through a bonding layer of heat actlvatable adhesive must be done cognizant of certain problems. All coa$ings, such as layer 22, have an upper temperature limit which cannot be exceeded without severely damaging the coating. All heat activatable adhesives have a certain minimum temperature which must be obtained in order ' ~, -6- i ~5333~ `
to effec-t bonding. In the e:xample described herein, the heat for activating the adhesive is obtained through contact wi-th the adhesive .
on the upper surface of the paint layer 22. Therefore, this upper .
surface 22 must be raised to a certain minimum temperature. It is clear that the temperature which the layer 22 is subjected to may necessarily be controlled within a narrow limit.
:;:
To reach the necessary activation temperature in the example -.
described, the sheet material 24 is heated by induction heating techniques using a core such as unit lO. With :~eference to Fig. 2, the thickness of the sheet 24 is generally small enough to justify the assumption that the temperature on the lowermost surface Sl and the interface surface S2 are equal and represented by temperature Tl when applying energ~ to it from an induction heating source. As heat is generated in the metallic sheet 24, it transfers this heat by heat conduction through the paint layer 22 to the heat activatable adhesive , layer. Because the paint layer does have a certain heat transfer coefficientJ the temperature Tl on the interface surface S2 w.lll be higher than the temperature T2 at the upper surface of the paint.
a temperature sensing device such as one utlli.zing the fiber 20 optic element 16, obviously, has only access to the outer surface S3 o:E the paint, IJnder circumstances where it is essential not to exceed a certain temperature in order not to damage the paint, it is evident that controlling to this temperature limit by sensing the -temperature on the outer surface of the paint is insufficient because the interface .
25 temperature Tl may be considerably higher. As an exa~ple of such ~.
, -7- ::
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condition, the activation temperature of an adhesive may be 285E
and the damaging temperature of the paint be 3002F. Controlling the outer paint surface by means of a temperature sensor set not to exceed 30ûF would be useless in as much as the actual interface tempera-ture Tl would probably be far in excess of 300F. Actual measurements of the heat transfer coeEficient of certain thermo~
plastic paints indicate that the temperature gradient between the interface of the paint and metal layer and the outer surface of the -paint might be as~ high as 15-20F.
,' ': ' `; ' ~ The temperature sensing problems described above may be represented by an electr~cal analogue circu~t shown in Fig. 3 which describes the charging of a capacitor C through a reslstance R from ~ .
a constant voltage source El. If -the initial charge on the capacitor C
corresponds to a voltage Eo, then the behavior Oe the voltage E across ;~
the capacitor is given by the standard equation (1~ with Figure 4 representing its characteristic curve.
:E = E~ e /RC ) + E e ~ t /RC
Using this équation and substituting temperatures for voltage T ~ Tl (1 - e t/RC) ~ To e - t/RC
Equation lA will accurately describe the transEer situation, where RC
this time denotes the thermal time constant of the nonconduct~ng, non-ferrorrlagnetic layer 22. HoweYer, equation lA does not apply to the actual heating situation described above inasmuch as the temperature at the interface Tl Itself increases as a function of time when power is absorbed in a continuous manner at a constant rate. We might, however, assume that this increase takes place as a linear function .
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of time. Tl, thus, will rise linearly with time from an ini-tial value To. If a thermal equilibrium existed at the start o~ the heat-ing process, -the surface temperature T2 of the coating 22 will rise from the same initial value but in a nonlinear manner, due to the finite value o~ the time constant RC. Obviously, Tl will rise at a more rapid rate than T21 and it is this behaviour of the surface temperature T2 which is the source of the problem to be solved by this invention.
It can be shown -that the temperature differential~ T exist-10 ing at any time t, measured in seconds, is described by the following equation.
~ ~ ( -t/RC ) (2) RC again denotes the thermal time constant of the coating 22 and o~
denotes the rate of temperature rise in degrees per second for a given constant value of absorbed power at the interface. Signific-antly, this equation No. 2 does not contain the term T of the initial temperature T , meaning that the temperature gradient between the interface surface S2 and the outer surface S3 of the coating is, a-t any given instant, independent of the value of the initial temper- t 20 ature existing at the instant when high frequency energy is again applied. It is this relationship which leads to the introduction of the two-level temperature control oE the present invention.
Reference to Figure 5 will facilitate the description of the two-level temperature control method of this invention. ~t time to both the interface temperature Tl and temperature T2 at outer surface S3 of the paint layer are substantially equal and identified as To. Upon application of heat energy by induction means to the sheet material 2~, the two temperatures Tl and T2 will rise as a function of time shown in this Figure 5. A predetermined temperature -~
30 level T2(1) of the paint layer 22 is monitored by means S~lCh as a infrared sensitive detector including a fiber optic elemellt. This predetermined temperature T2(1) will be lower than the final desired .~. . .
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temperature T2(2) which will be the activation temperature of the adhesive and will be determined utilizing the relationship of . "
equation 2 taking into account Tl(maX ) as the temperature not to be exceeded at the interface and the unavoidable temperature differential ~ T. When the temperature T2(1) is reached, T
will be higher than T2 but less than Tl(max )~ a control system .
maintains this temperature T2(1) substantially constant for a predetermined length of time, for example (t2 - tl), to permit equalization between the interface temperature Tl and the tem- .
perature T2. Once equalization of T2 and Tl has been reached, power is applied again at the same rate and continued until the .
surface temperature T2 has reached the desired final level T2(2) and is there monitored automatically for a second predetermined . .
additional length of time, such as (t4 - t3). During the time period (t2 - tl), power may be alternately turned off and on while the interface temperature Tl drops as a result of cooling ;. .
by radiation and conduction to surrounding areas, including to the nonmetallic coating 22. During this time period, when the power is basically off, heat flowing from the higher temperature interface surface S2 compensates for the reduction of the surface temperature T2 due to cooling thereof. In actual practice, the time interval (t2 - tl) will be relatively short in comparison with the final dwell time (t4 - t3), the last-mentioned time interval being determined by the chemical characterist:ics of -the heat activated adhesive. For most such adhesives, the dwell time necessary to obtain a reliable and satisfactory bond is in the order of magnitude of 5 to 15 seconds, a time which is long in comparison to that needed for the equalization of temperatures ~.
on the surface and the interface at the lower temperature level.
At timP t2, the surface and interface temperature are .. :~
equalized and power is applied once more allowing the interface temperature Tl to rise linearly with the same slope as occurring ~
in the interval (tl - to).
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: . . . . . . . .
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~ owever, during the interval (t3 - t2)~ the automatic -temperature monitor control has been changed to permit the surface temperature to ri~e -to its selected final level T2(2). It is important to note that at the end of the time period t4, ~T(2) will be far smaller than the ~T(l) existing before at T2(1~, thus enhancing a smooth control without overshoot.
- Figure 6 shows a simplified block diagram of the equipment use'd to implement the above described two control method. The entire unit may be operated from the 60 Hz alternating current line. A full wave rectifier 79 supplies the inverter 90, through an on-off SCR 86 and a ~.
filter cirouit 88 with D. C. current. The inverter 90 may be of the parallel inverter type driven at its gate by a 10 kHz trigger oscillator 91. A horseshoe type inductor coil 12 is coupled to the inverter and applied directly to the workpiece 20. The surface temperature of the ~' workpiece is .monitored by a fiber optic ele:ment 16. The infrared : ~: 15 radlation collected by the fiber optic element is supplied -tc~ an infrared sensltive detector 92 which may include a photo sensitive device capable ~
of being activated by infrared radiation, a chopper amplifier, operating , .
at a frequency of 400 cycles or thereabouts ancl an A. C. to D. C. converter to whlch a linearizing ampllfier is added to make the outpu-t of the infrared detector directly proportional to the temperature ~ensed by the fiber ~;
optic element 16. Appropriate analogue to digital converters might be added to supply a digi-tal read-out 104 for monitoring the temperature.
The analogue output of the infrared detector 92 is supplied to one input :
of a comparator circuit 94, which is usually one of a standard operational .
amplifier or any one of comme:: cially available, high sensit;vity voltage comparators. The other inpu-t of the comparator is connected to the ' ~:
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output of an adjustable reference -temperature source 96, which is calibrated in units of the temperature monitored by the fiber optic ~-element. As was explained above, it is desirable to be able to adjust the reference voltage to two different levels. The first temperature level, such as T2(l)~ is selected to be somewhat bélow the final desired surface temperature T2(2). To obtain thls two-level operation, the reference source is adjustable to two output voltage ranges in such a rnanner that the adjustable voltage correeponding to a given positon of a potentiometer slider con-tact is always a certain predetermined , :
-10 fraction when operating on the first range, corresponding to the lower .
- temperature level T2(l), then the same slider position corresponding to a higher temperature level corresponding to the desirecl final temperature T2(2). A switching device 98 permits the change in the - reference voltage. This is accomplished by an output pulse derived .
from the output terminal of the comparator 94 when the first temperature level is obtained. When the infrared detector senses the first temper-j , .
ature level, the output pulse of the comparator 94 actuates the switching device 98 and a similar pulse is applied to a timing circuit 100 whlch permits the temperature to remain at the first level, such as T2(1~
for a length of time selected by the adjustment of the timing circult, such as (t2 - tl). Once the switching device is positioned so that the output voltage of the reference temperature source 96 corresponds to a final higher temperature level, the comparator will once again generate a pulse when the final temperature is reached. During desired dwell times, such as (t2 - tl) and (t~ - t3), the temperature of the workpiece is kept constant by an on-off regulation of the inverter power, by means , 33~3 of supplying a proper signal to the on-off SCR 86. This is accomplished by a relay circuit 66 energized by the impulses of ` an AND gate 84. The AND gate 84 operating in such a manner that the relay circuit 66 can be operated only durin~ the duration of the time cycles of either timing circuits 100 or 102 and also upon receipt of a proper pulse from the comparator 94. The relay circuit will be closed, energizing a D.C~ source included in the relay circuit and supplying a D~C. triggering gate signal to the gate of the on-off SCR 86 which operates in the zero switching manner. Each time the temperature of the workpiece 20 is such that it supplies an output signal to the infrared detector which is below the set output voltage of the refere~ce source, the relay circuit 66 will opexate to apply a positive D.C. gate volt-age to SCR 86 by supplying power to the filter 88 and inverter 90.
As soon as the predetermined temperature is obtained, the output signal of comparator 94 will invert and relay circuit 66 will operate in such a manner as to remove the gating signal from the on-off SCR 86, thereby ceasing to supply power to the inverter.
The entire operation is started by depressing a start push button 81, thereby discharging a capacitor 80 and thus applying a trig-gering source to the trigger pulse and commutating circuit 82, more specifically, to one of two SCRs contained in the commutat-ing circuit. The output of a first SCR in the commutating circuit supplies one of the sustaining signals to the AND gate. As soon as the time set by the timing circuit 102 has ended, the timing circuit will supply a turn-off signal to a second SCR in the com-mutating circuit, thereby removing the sustaining si~nal from the AND gate and simultaneously the signal through the relay circuit from the on-off SCR, thus terminating operations.
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Turning now to Fig. 7, adjustable reference temperature source circuitry and associated circuitry described in the block diagram of Fig. 6 will be more specifically described. ~or purposes of this invention, it should be understood that devices such as AND gate 84, inverter 90, comparator circuit 94, and infrared sensitive detector 92 will not be described in detail as the specific structure of these circuits are well known and the specific structure as such does not form an important aspect of this inventi`on. The voltage dividing network or adjustable reference temperature source 96 consists of resistances 56 and 62.
An additional resistor 60 may be placed in parallel with resistor 62 .
.
Resistor 62 will be variable as by a precision potentiometer 61. This potentiom;eter establishes the desired temperature level. When resistance 60 is utilized, it is understood that the resistance value of 62 is large in comparison to that oE 60, thus inserting a neglibible influence on the 15 value of 60. Resistance 58 represents the resistance which, when connected in parallel to 62, will lower the value of the difference of the .
reference voltage across 62. For a desired 5% difference between final and intermediate temperature levels, the value of reslstance 58 in `
Fig. 7 would have to be approximately 18 times -that of resistance 60.
The connection of resistance 58 in parallel to resistance 62 is accomplished by the switching device 98, which includes a relay contact 64 in con- ~ :
Junction with relay 28 in the commutating circuit. The relay contact 64 being normally open. When the silicon controlled rectifier 32 is triggered into conduction by means of a starting signal to the main inverter circuit, relay 28, as part OI the switching device 98, will be energized and contact 64 w;ll be closed. A resistance 26 is connected ' ~5333~
in parallel across ~he relay winding 28 for purposes of stabil- i ity of operation. The reference voltage appearing between sl.ider 61 and the negative terminal of the supply voltage is compared with the output of the sensing detector 92. .
The output of the detector 92 and the reference `
temperature source 96 are thus fed into a voltage comparator circuit 94 as soon as the temperature corresponding to the reduc-ed reference voltage in circuit 96 is obtained, the differential ~
amplifier type voltage sensing comparator circuit 94 will send l -10 a turnoff signal to the turnoff gates of the inverter and si-multaneously initiate a timing circuit 100 consisting primarily of a unijunction transistor 48 and a time constant determining network 46 and 50. The AND gate 84 is connected at the output .:
of the comparator circuit 94 and the output of the AND gate energizing the relay 66 but only if both output signals from the ..
comparator circuit and an additional input signal corresponding .i .
to the voltage drop across the resistance 34 in~ the circuit of :: :
the silicon controlled rectii~ier 32 are present. Under these conditions, when the first temperature level T2(1) is reached, t,,`
relay 66 will be energized closing the normally open contact 42 :;
from the time delay circuit 100. At the end o:E the selected time i: :
delay, such as (t2 - tl), as determined by the value of resistance 46 and capacitor 50, unijunction 48 will be triggered into con-duction, driving a signal across resistance 40 which will trigger ` ::
SCR 38 thus turning off SCR 32. When 32 is turned off, relay 28 will no longer be energized and contact 64 will open thus `
restoring the reference signal appearing across 60 respectively`~:.
across the slider and negative terminals of potentiometer 61 to ~. .
its original value and corresponding to the final desired tem- .
30 perature T2 (2) "
1~5333~3 From here on, because the AND gate 84 does not receive the addition-al signal from the voltage drop across resistor 34, only the out-put of the differential voltage sensor of comparator circuit 94 will be operative, applying turnoff signal to the inveL-ter ~hell the final temperature is achieved. If an additional dwell time is required after reaching the final -temperature, an additional timer circuit such as 102 can be used which is triggered from the turnoff pulse appearing across resistance 40 when 38 is turned on thereby turning off 32. This timer will then permit in the normal m~nner to operate the inverter for any desired additional time period and for purposes of providing a dual time for the heated components.
The operation of the second timer 102, in conjunction :~
with the commutating unit 82', is now described as follows. At the end of the first timing cycle, timer 100 will provide a signal pulse across resistor 40, thereby triggering SCR~38 in the commutating unit 82, thus turning off SCR-34. The same triggering pulse appearing across resistance 40 of 100 will also supply a ~ .
starting pulse to the gate of SCR-32' in commutating unit 82', thereby initiating the operation of timer 102 in the same manner as timer 100 was initiated by a pulse applied to the gate of SCR-32 in commutating unit 82. Note tha-t the comparator circuit 94 is still supplying a signal to AND gate 84 in the same manner as was the case in the first temperature range. A-t -the same time, a voltage differential appearing across resistance 34' of .
the communicating unit 82', associated with the second timer 102, is supplied to the AND gate 84, therewith satis~yin~ the condition that only when hoth a signal from the comparator out- .
put of 94 and voltage across 34' are present will the AND gate be operative in such a manner that it supplies a positive D.C.
signal through the relay circuit 66 to the on-off SCR-86. In fact, this means that once the first temperature is reached, corresponding to a situation whereby contact 64 is closed, placing resistance 58 effectively in parallel with resistance 60 of the reference signals, thereby establishing control for the first temperature level, operation at this first temperature level will be determined for a time duration corresponding to the setting of resistance 46 in timer 100, and after timer 100 has been turned oEf by virtue of a trigger pulse to the gate of SCR-38 in the commutating unit 82, both the co~utating circuit 82' and, at the same instant, also timing circuit 102 is energized. This corresponds to an opening of the contact 64 .
in 98, which means that during the time period established by timer 102 the system will regulate to a higher temperature determined by the setting of 61 on potentiometer 62 and ~overned ~
by the new higher voltage appearing across resistance 60. ;.
Figure 8 shows an alternate embodiment of the adjust-able reference tempera-ture source 96 which utilizes an operation ;~
transistor 98a replacing the relay 28 and contact 64 ln the above embodiment. As.soon as 98a is triggered into conduction, resistance 58a will be connected to the bottom of 62a. Accord-ingly, the resistance in series with 56a will be the parallel combination of 58a and 62a, thereby effectively reducing -the voltage across 62a and establishing an output sicJnal corresponding to a lower temperature level, such as l'2(1).
..
Background of the Invention Many attempts have been made recently to develop a new technology of securing various mechanical elements to base structures by using adhesive rather than mechanical fasteners.
Two general classes of adhesive have been utilized in this technology, namely pressure sensitive and hea-t activated -~
adhesive. In the case of heat activated adhesive, the use of induction heating techniques were the logical choice as the energy source for activating the adhesive when at least one of the elements to be bonded had ferromagnetic charac-teristics. The main advantage of induction heating techniques over more customary heating methods is the ability to raise the temperature of metallic surfaces to high levels in a relatively short time. Due to the fact that heat is cJen-erated in the body to be heated and does not rely on heat transfer from the heat source to the body, the process is not associated with a general time lag characterizing other methods. Because of the short heating times involved, it becomes of utmost importance to devise techniques which would permit the exact control of the final temperature, either for the p~lrpose of terminating the heating process at the instant when the final temperature was reached or to be able to main-tain a desired final temperature at a constant level for an arbitrarily selected time period.
A special problem in this regard arises in the application where the ferromagnetic material to which the paxts must be bonded is covered with a nonconductive, non-ferromagnetic layer, such as for instance a layer of paint or other coating. All such coatings will have an upper temperature limit which cannot be exceeded without severely 5333~
. . .
damaging the coating. At the same time, it is desirable to maintain the temperature o~ the coating at a temperature su~ficient to activate the adhesi~e for a certain period of time in order to obtain satisfactory bonding. However, only the surface temperature of the coating and not the temperature at the interface between the coating and the sheet of ferro- ;~
magnetic material is accessible for purposes of measurement and control. Previous methods utilized to control the final temperature of a surface incorporated what is generally referred to in control engineering as "proportionate control methods" i.e. reducing the power input to the induction heating generator as a function of the rate of temperature rise or using the two position method, particularly in con-nection with high power vacuum tube generators, using magnetic amplifiers or similar techniques to disengage the high voltage rectifier system supplying power to the oscillator tube. There ;
are also several methods developed in industry where solid state generators were used and control was exercised by the '-use of power input to the workpiece, using pulse width modu-lation techniques or similar methods. In one example of prior art methodsj control is exercised by changing the repetition rate of pulses having e~ual energy content.
The major problem associated with previous methods is ~ `
the difficulty in avoiding overshooting the desired temperature levels or the complexity of circuitry to accomplish such a .~ ;
goal. In either event it is apparently necessary to establish control settings by rather complex experimental methods.
Summary of the Invention In accordance with one aspect of the invention, a U-3~ shaped inductance core is integrated as a unit with a sensing element and this core unit is then operatively associated with an inverter either of the parallel or series type. ---~, 2 ,',,,~ .
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In one broad aspect the invention comprehends an induction heating and temperature responsi~e control appara-tus including an inverter circuit. An induction core unit including a generally U-shaped ferromagnetic core with an exiter winding wrapped thereabout is in operative connection with the inverter circuit and means integrated with the core monitors the surface temperature of a workpiece as it is heated by virtue of the magnetic circuit formed with the workpiece by the core. Means are provided which are responsive to the temperature, and monitoring means automatically con-trols~ the output of the inverter so as to stop the supply of temperature inducing current to the core at a predetermined surface temperature of the workpiece. The means controlling ;~
the inverter oubput includes means for automatically and successively controlling the inverter to maintain a first predetermined temperature for a predetermined time period followed by a subsequent period of operation to increase the temperature to a second temperature level and maintain the second temperature level.
Another aspect of the invention pertains to a method of heating, by induction, a sheet of ferromagnetic base material which is coated with a nonconducting, non-ferromagnetic surface material, such as paint. The steps include heating the base material and simultaneously monitoring the temperature of the surface material, auto-matically stopping the heat input to the base material when temperature of the surface material reaches a first, pre-determined temperature level which is lower than a second, predetermined desired temperature level, allowing the tem-perature of the base material to equalize with the tem-perature of the surface material at the first temperature level, automatically resuming heat input to the base material until the surface temperature reaches the second predetermined ~3~
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temperature level, retaining the second temperature level for a predetermined time interval and therea~ter deactivating the induction heating apparatus.
More particularly the object is to heat by induction without exceeding a critical temperature related to the , deterioration of the coated paint layer. Such heating is accomplished by considering the different rates of heat relatable to both the paint la~er and the ~erromagnetic material, monitoring the surface temperature of the paint layer, turning the power off to eliminate the energy input to ;, the system when the temperature sensed at the paint layer is lower than the desired temperature and also at a temperature which reflects an interface temperature below the maximum tem-perature. The first predetermined temperature is maintained at a constant level for a fixed period of time after which the power is activated to further increase the temperature of both the paint and the ferromagnetic material in such a manner as to provide a temperature differential between the two which is very small, thus serving to eliminate the possibilities of overshooting.
The circuitry for accomplishing this heating method basically includesan infrared sensing detector, an operational amplifier comparator which accepts the output from the infrared .; .
detector and also accepts output from an adjustable reference ~
.. ... .
source which is calibrated in terms of temperature. The adjustable reference source is essentially a voltage dividing circuitry supplied from a constant voltage source which is responsive to a preset timing circuit. The operation of the ;; .
timing circuit in conjunction with the reference source is ~-such as to turn off the power at a predetermined tempera-ture for a given time span after which the voltage input to the com-parator from the reference source changes and is directly related to a slightly higher temperature level which now is the i 7 ~ 3 -4-3~
desired temperature level. Such a circuit thus allows a dwell period which will thereafter permit the surface layer to be heated to its desired temperature in such a manner as to greatly reduce the temperature differential between the paint layer and the ferromagnetic material. `
Other objects and features, as well as a full under-standing of the invention, will appear by reference to the ;
following detailed description, drawings and claims. `
~rief Description of the Drawings Fig. 1 is a partial perspective view of the core integrated with fiber optic sensing element and in position to heat a workpiece which has been coated with a nonconductive material.
Fig. 2 is a greatly enlarged cross-sectional view of a workpiece coated with a paint-type material.
Fig. 3 is a simplified circuit representing the electrical analogue having characteristics similar to the heat-ing characteristics of the workpiece section.
Fig. 4 is a curve representing the behavior of the voltage relative to time elapse of the electrical analogue.
Fig. 5 is a graphical representation of the time/tem-perature relationship occurring during heating by the method of the present invention.
Fig. 6 is a circuit schematic in block diagram form of the two-level heating system of the present invention.
Fig. 7 is a detailed schematic circuit diagram of a preferred embodiment of the two-level temperature control of the present invention.
Fig. 8 is a detailed schematic circuit of an alternate embodiment of the adjustable reference temperature portion of the instant invention.
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Description of the Preferred Embodirnent .... .
Turning now to Fig. 1, there is shown a preferred embodiment of the heating core unit 10 which includes a U-shaped core element 12 with an exciter coil 14 wound about, inducing a current therein. This coil 14 could, for example, be connected directly to the secondary winding of a transformer of a parallel inverter, e. g. an inverter of the MacMurray type. An example of such a core and power supply appropriate for purposes of this invention may be seen in U. S. Patent 3, 816, 690 issued on June 11, 1974. It will be seen that the core unit 10 will have integrated therein a fiber optic element extending through the bridge of the core, positioned by a holding structure 18 so that .
the receiving extremity of the fiber optic element looks at the workpiece to be heated at a posi-tlon Intermediate the legs oE the core in order -to obtain correct and valid tempera-ture measurements simultaneous with the heating of the workpiece. The entire unit thus described may there-.,~
after be potted in a conventional manner.
. . .
.`
The workpiece to be hea-ted 20 comprises a sheet oE ferromagnetic material 24 covered by a surface layer of paint or other nonconducting, nonferromagnetic material 22. It is this particular situation that the present invention is primarily concerned with. The fastening oE some device to the workpiece, and more specifically to the surface of the paint layer 22, through a bonding layer of heat actlvatable adhesive must be done cognizant of certain problems. All coa$ings, such as layer 22, have an upper temperature limit which cannot be exceeded without severely damaging the coating. All heat activatable adhesives have a certain minimum temperature which must be obtained in order ' ~, -6- i ~5333~ `
to effec-t bonding. In the e:xample described herein, the heat for activating the adhesive is obtained through contact wi-th the adhesive .
on the upper surface of the paint layer 22. Therefore, this upper .
surface 22 must be raised to a certain minimum temperature. It is clear that the temperature which the layer 22 is subjected to may necessarily be controlled within a narrow limit.
:;:
To reach the necessary activation temperature in the example -.
described, the sheet material 24 is heated by induction heating techniques using a core such as unit lO. With :~eference to Fig. 2, the thickness of the sheet 24 is generally small enough to justify the assumption that the temperature on the lowermost surface Sl and the interface surface S2 are equal and represented by temperature Tl when applying energ~ to it from an induction heating source. As heat is generated in the metallic sheet 24, it transfers this heat by heat conduction through the paint layer 22 to the heat activatable adhesive , layer. Because the paint layer does have a certain heat transfer coefficientJ the temperature Tl on the interface surface S2 w.lll be higher than the temperature T2 at the upper surface of the paint.
a temperature sensing device such as one utlli.zing the fiber 20 optic element 16, obviously, has only access to the outer surface S3 o:E the paint, IJnder circumstances where it is essential not to exceed a certain temperature in order not to damage the paint, it is evident that controlling to this temperature limit by sensing the -temperature on the outer surface of the paint is insufficient because the interface .
25 temperature Tl may be considerably higher. As an exa~ple of such ~.
, -7- ::
3;~
condition, the activation temperature of an adhesive may be 285E
and the damaging temperature of the paint be 3002F. Controlling the outer paint surface by means of a temperature sensor set not to exceed 30ûF would be useless in as much as the actual interface tempera-ture Tl would probably be far in excess of 300F. Actual measurements of the heat transfer coeEficient of certain thermo~
plastic paints indicate that the temperature gradient between the interface of the paint and metal layer and the outer surface of the -paint might be as~ high as 15-20F.
,' ': ' `; ' ~ The temperature sensing problems described above may be represented by an electr~cal analogue circu~t shown in Fig. 3 which describes the charging of a capacitor C through a reslstance R from ~ .
a constant voltage source El. If -the initial charge on the capacitor C
corresponds to a voltage Eo, then the behavior Oe the voltage E across ;~
the capacitor is given by the standard equation (1~ with Figure 4 representing its characteristic curve.
:E = E~ e /RC ) + E e ~ t /RC
Using this équation and substituting temperatures for voltage T ~ Tl (1 - e t/RC) ~ To e - t/RC
Equation lA will accurately describe the transEer situation, where RC
this time denotes the thermal time constant of the nonconduct~ng, non-ferrorrlagnetic layer 22. HoweYer, equation lA does not apply to the actual heating situation described above inasmuch as the temperature at the interface Tl Itself increases as a function of time when power is absorbed in a continuous manner at a constant rate. We might, however, assume that this increase takes place as a linear function .
~)5333~
of time. Tl, thus, will rise linearly with time from an ini-tial value To. If a thermal equilibrium existed at the start o~ the heat-ing process, -the surface temperature T2 of the coating 22 will rise from the same initial value but in a nonlinear manner, due to the finite value o~ the time constant RC. Obviously, Tl will rise at a more rapid rate than T21 and it is this behaviour of the surface temperature T2 which is the source of the problem to be solved by this invention.
It can be shown -that the temperature differential~ T exist-10 ing at any time t, measured in seconds, is described by the following equation.
~ ~ ( -t/RC ) (2) RC again denotes the thermal time constant of the coating 22 and o~
denotes the rate of temperature rise in degrees per second for a given constant value of absorbed power at the interface. Signific-antly, this equation No. 2 does not contain the term T of the initial temperature T , meaning that the temperature gradient between the interface surface S2 and the outer surface S3 of the coating is, a-t any given instant, independent of the value of the initial temper- t 20 ature existing at the instant when high frequency energy is again applied. It is this relationship which leads to the introduction of the two-level temperature control oE the present invention.
Reference to Figure 5 will facilitate the description of the two-level temperature control method of this invention. ~t time to both the interface temperature Tl and temperature T2 at outer surface S3 of the paint layer are substantially equal and identified as To. Upon application of heat energy by induction means to the sheet material 2~, the two temperatures Tl and T2 will rise as a function of time shown in this Figure 5. A predetermined temperature -~
30 level T2(1) of the paint layer 22 is monitored by means S~lCh as a infrared sensitive detector including a fiber optic elemellt. This predetermined temperature T2(1) will be lower than the final desired .~. . .
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temperature T2(2) which will be the activation temperature of the adhesive and will be determined utilizing the relationship of . "
equation 2 taking into account Tl(maX ) as the temperature not to be exceeded at the interface and the unavoidable temperature differential ~ T. When the temperature T2(1) is reached, T
will be higher than T2 but less than Tl(max )~ a control system .
maintains this temperature T2(1) substantially constant for a predetermined length of time, for example (t2 - tl), to permit equalization between the interface temperature Tl and the tem- .
perature T2. Once equalization of T2 and Tl has been reached, power is applied again at the same rate and continued until the .
surface temperature T2 has reached the desired final level T2(2) and is there monitored automatically for a second predetermined . .
additional length of time, such as (t4 - t3). During the time period (t2 - tl), power may be alternately turned off and on while the interface temperature Tl drops as a result of cooling ;. .
by radiation and conduction to surrounding areas, including to the nonmetallic coating 22. During this time period, when the power is basically off, heat flowing from the higher temperature interface surface S2 compensates for the reduction of the surface temperature T2 due to cooling thereof. In actual practice, the time interval (t2 - tl) will be relatively short in comparison with the final dwell time (t4 - t3), the last-mentioned time interval being determined by the chemical characterist:ics of -the heat activated adhesive. For most such adhesives, the dwell time necessary to obtain a reliable and satisfactory bond is in the order of magnitude of 5 to 15 seconds, a time which is long in comparison to that needed for the equalization of temperatures ~.
on the surface and the interface at the lower temperature level.
At timP t2, the surface and interface temperature are .. :~
equalized and power is applied once more allowing the interface temperature Tl to rise linearly with the same slope as occurring ~
in the interval (tl - to).
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: . . . . . . . .
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~ owever, during the interval (t3 - t2)~ the automatic -temperature monitor control has been changed to permit the surface temperature to ri~e -to its selected final level T2(2). It is important to note that at the end of the time period t4, ~T(2) will be far smaller than the ~T(l) existing before at T2(1~, thus enhancing a smooth control without overshoot.
- Figure 6 shows a simplified block diagram of the equipment use'd to implement the above described two control method. The entire unit may be operated from the 60 Hz alternating current line. A full wave rectifier 79 supplies the inverter 90, through an on-off SCR 86 and a ~.
filter cirouit 88 with D. C. current. The inverter 90 may be of the parallel inverter type driven at its gate by a 10 kHz trigger oscillator 91. A horseshoe type inductor coil 12 is coupled to the inverter and applied directly to the workpiece 20. The surface temperature of the ~' workpiece is .monitored by a fiber optic ele:ment 16. The infrared : ~: 15 radlation collected by the fiber optic element is supplied -tc~ an infrared sensltive detector 92 which may include a photo sensitive device capable ~
of being activated by infrared radiation, a chopper amplifier, operating , .
at a frequency of 400 cycles or thereabouts ancl an A. C. to D. C. converter to whlch a linearizing ampllfier is added to make the outpu-t of the infrared detector directly proportional to the temperature ~ensed by the fiber ~;
optic element 16. Appropriate analogue to digital converters might be added to supply a digi-tal read-out 104 for monitoring the temperature.
The analogue output of the infrared detector 92 is supplied to one input :
of a comparator circuit 94, which is usually one of a standard operational .
amplifier or any one of comme:: cially available, high sensit;vity voltage comparators. The other inpu-t of the comparator is connected to the ' ~:
~L~5333~
output of an adjustable reference -temperature source 96, which is calibrated in units of the temperature monitored by the fiber optic ~-element. As was explained above, it is desirable to be able to adjust the reference voltage to two different levels. The first temperature level, such as T2(l)~ is selected to be somewhat bélow the final desired surface temperature T2(2). To obtain thls two-level operation, the reference source is adjustable to two output voltage ranges in such a rnanner that the adjustable voltage correeponding to a given positon of a potentiometer slider con-tact is always a certain predetermined , :
-10 fraction when operating on the first range, corresponding to the lower .
- temperature level T2(l), then the same slider position corresponding to a higher temperature level corresponding to the desirecl final temperature T2(2). A switching device 98 permits the change in the - reference voltage. This is accomplished by an output pulse derived .
from the output terminal of the comparator 94 when the first temperature level is obtained. When the infrared detector senses the first temper-j , .
ature level, the output pulse of the comparator 94 actuates the switching device 98 and a similar pulse is applied to a timing circuit 100 whlch permits the temperature to remain at the first level, such as T2(1~
for a length of time selected by the adjustment of the timing circult, such as (t2 - tl). Once the switching device is positioned so that the output voltage of the reference temperature source 96 corresponds to a final higher temperature level, the comparator will once again generate a pulse when the final temperature is reached. During desired dwell times, such as (t2 - tl) and (t~ - t3), the temperature of the workpiece is kept constant by an on-off regulation of the inverter power, by means , 33~3 of supplying a proper signal to the on-off SCR 86. This is accomplished by a relay circuit 66 energized by the impulses of ` an AND gate 84. The AND gate 84 operating in such a manner that the relay circuit 66 can be operated only durin~ the duration of the time cycles of either timing circuits 100 or 102 and also upon receipt of a proper pulse from the comparator 94. The relay circuit will be closed, energizing a D.C~ source included in the relay circuit and supplying a D~C. triggering gate signal to the gate of the on-off SCR 86 which operates in the zero switching manner. Each time the temperature of the workpiece 20 is such that it supplies an output signal to the infrared detector which is below the set output voltage of the refere~ce source, the relay circuit 66 will opexate to apply a positive D.C. gate volt-age to SCR 86 by supplying power to the filter 88 and inverter 90.
As soon as the predetermined temperature is obtained, the output signal of comparator 94 will invert and relay circuit 66 will operate in such a manner as to remove the gating signal from the on-off SCR 86, thereby ceasing to supply power to the inverter.
The entire operation is started by depressing a start push button 81, thereby discharging a capacitor 80 and thus applying a trig-gering source to the trigger pulse and commutating circuit 82, more specifically, to one of two SCRs contained in the commutat-ing circuit. The output of a first SCR in the commutating circuit supplies one of the sustaining signals to the AND gate. As soon as the time set by the timing circuit 102 has ended, the timing circuit will supply a turn-off signal to a second SCR in the com-mutating circuit, thereby removing the sustaining si~nal from the AND gate and simultaneously the signal through the relay circuit from the on-off SCR, thus terminating operations.
t , `
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Turning now to Fig. 7, adjustable reference temperature source circuitry and associated circuitry described in the block diagram of Fig. 6 will be more specifically described. ~or purposes of this invention, it should be understood that devices such as AND gate 84, inverter 90, comparator circuit 94, and infrared sensitive detector 92 will not be described in detail as the specific structure of these circuits are well known and the specific structure as such does not form an important aspect of this inventi`on. The voltage dividing network or adjustable reference temperature source 96 consists of resistances 56 and 62.
An additional resistor 60 may be placed in parallel with resistor 62 .
.
Resistor 62 will be variable as by a precision potentiometer 61. This potentiom;eter establishes the desired temperature level. When resistance 60 is utilized, it is understood that the resistance value of 62 is large in comparison to that oE 60, thus inserting a neglibible influence on the 15 value of 60. Resistance 58 represents the resistance which, when connected in parallel to 62, will lower the value of the difference of the .
reference voltage across 62. For a desired 5% difference between final and intermediate temperature levels, the value of reslstance 58 in `
Fig. 7 would have to be approximately 18 times -that of resistance 60.
The connection of resistance 58 in parallel to resistance 62 is accomplished by the switching device 98, which includes a relay contact 64 in con- ~ :
Junction with relay 28 in the commutating circuit. The relay contact 64 being normally open. When the silicon controlled rectifier 32 is triggered into conduction by means of a starting signal to the main inverter circuit, relay 28, as part OI the switching device 98, will be energized and contact 64 w;ll be closed. A resistance 26 is connected ' ~5333~
in parallel across ~he relay winding 28 for purposes of stabil- i ity of operation. The reference voltage appearing between sl.ider 61 and the negative terminal of the supply voltage is compared with the output of the sensing detector 92. .
The output of the detector 92 and the reference `
temperature source 96 are thus fed into a voltage comparator circuit 94 as soon as the temperature corresponding to the reduc-ed reference voltage in circuit 96 is obtained, the differential ~
amplifier type voltage sensing comparator circuit 94 will send l -10 a turnoff signal to the turnoff gates of the inverter and si-multaneously initiate a timing circuit 100 consisting primarily of a unijunction transistor 48 and a time constant determining network 46 and 50. The AND gate 84 is connected at the output .:
of the comparator circuit 94 and the output of the AND gate energizing the relay 66 but only if both output signals from the ..
comparator circuit and an additional input signal corresponding .i .
to the voltage drop across the resistance 34 in~ the circuit of :: :
the silicon controlled rectii~ier 32 are present. Under these conditions, when the first temperature level T2(1) is reached, t,,`
relay 66 will be energized closing the normally open contact 42 :;
from the time delay circuit 100. At the end o:E the selected time i: :
delay, such as (t2 - tl), as determined by the value of resistance 46 and capacitor 50, unijunction 48 will be triggered into con-duction, driving a signal across resistance 40 which will trigger ` ::
SCR 38 thus turning off SCR 32. When 32 is turned off, relay 28 will no longer be energized and contact 64 will open thus `
restoring the reference signal appearing across 60 respectively`~:.
across the slider and negative terminals of potentiometer 61 to ~. .
its original value and corresponding to the final desired tem- .
30 perature T2 (2) "
1~5333~3 From here on, because the AND gate 84 does not receive the addition-al signal from the voltage drop across resistor 34, only the out-put of the differential voltage sensor of comparator circuit 94 will be operative, applying turnoff signal to the inveL-ter ~hell the final temperature is achieved. If an additional dwell time is required after reaching the final -temperature, an additional timer circuit such as 102 can be used which is triggered from the turnoff pulse appearing across resistance 40 when 38 is turned on thereby turning off 32. This timer will then permit in the normal m~nner to operate the inverter for any desired additional time period and for purposes of providing a dual time for the heated components.
The operation of the second timer 102, in conjunction :~
with the commutating unit 82', is now described as follows. At the end of the first timing cycle, timer 100 will provide a signal pulse across resistor 40, thereby triggering SCR~38 in the commutating unit 82, thus turning off SCR-34. The same triggering pulse appearing across resistance 40 of 100 will also supply a ~ .
starting pulse to the gate of SCR-32' in commutating unit 82', thereby initiating the operation of timer 102 in the same manner as timer 100 was initiated by a pulse applied to the gate of SCR-32 in commutating unit 82. Note tha-t the comparator circuit 94 is still supplying a signal to AND gate 84 in the same manner as was the case in the first temperature range. A-t -the same time, a voltage differential appearing across resistance 34' of .
the communicating unit 82', associated with the second timer 102, is supplied to the AND gate 84, therewith satis~yin~ the condition that only when hoth a signal from the comparator out- .
put of 94 and voltage across 34' are present will the AND gate be operative in such a manner that it supplies a positive D.C.
signal through the relay circuit 66 to the on-off SCR-86. In fact, this means that once the first temperature is reached, corresponding to a situation whereby contact 64 is closed, placing resistance 58 effectively in parallel with resistance 60 of the reference signals, thereby establishing control for the first temperature level, operation at this first temperature level will be determined for a time duration corresponding to the setting of resistance 46 in timer 100, and after timer 100 has been turned oEf by virtue of a trigger pulse to the gate of SCR-38 in the commutating unit 82, both the co~utating circuit 82' and, at the same instant, also timing circuit 102 is energized. This corresponds to an opening of the contact 64 .
in 98, which means that during the time period established by timer 102 the system will regulate to a higher temperature determined by the setting of 61 on potentiometer 62 and ~overned ~
by the new higher voltage appearing across resistance 60. ;.
Figure 8 shows an alternate embodiment of the adjust-able reference tempera-ture source 96 which utilizes an operation ;~
transistor 98a replacing the relay 28 and contact 64 ln the above embodiment. As.soon as 98a is triggered into conduction, resistance 58a will be connected to the bottom of 62a. Accord-ingly, the resistance in series with 56a will be the parallel combination of 58a and 62a, thereby effectively reducing -the voltage across 62a and establishing an output sicJnal corresponding to a lower temperature level, such as l'2(1).
..
Claims (11)
1. An induction heating and temperature responsive control apparatus in combination including an inverter circuit, an induction core unit, including a generally U-shaped ferro-magnetic core with an exiter winding wrapped thereabout and in operative connection with the inverter circuit and means integrated with the core to monitor the surface temperature of a workpiece as it is heated by virtue of the magnetic circuit formed with the workpiece by the core, further including means responsive to the temperature, monitoring means for automatical-ly controlling the output of the inverter so as to stop the supply of temperature inducing current to the core at a pre-determined surface temperature of the workpiece, said means controlling the output including means for automatically and successively controlling the inverter to maintain a first pre-determined temperature for a predetermined time period followed by a subsequent period of operation to increase the temperature to a second temperature level and maintain said second temperature level.
2. An induction heating apparatus including a two-level temperature control including a power supply inverter operatively connected to energize an induction core unit, the induction core unit including a temperature sensing element operative therewith including means to convert temperature level sensed to a directly related output voltage, an adjustable, voltage-dividing, reference temperature source, voltage comparator circuit accepting and comparing voltages from the temperature sensing means and the adjustable reference temperature source and including means for producing a first signal when said voltages are equal, means responsive to said first signal including a first time circuit to control the inverter to maintain a first temperature level, switch means responsive to said first signal to increase the value of the reference temperature source after a first predetermined time delay created by said first timing circuit following the initiation of the first signal, means to activate a period of operation of the inverter to increase the temperature to a second, higher temperature level until a second signal is produced by the voltage comparator circuit, at the end of the operating period at the higher temperature level wherein the heating apparatus automatically sensing the temperature level of the workpiece and is capable of controlling the temperature thereof at two levels.
3. An induction heating apparatus in accordance with Claim 2, wherein the adjustable reference temperature source comprises a voltage dividing network including means to vary the value of one of the divided series combined resistors and select the reference temperature level, means placing a further resistor in parallel with said one resistor to decrease the voltage appearing across the said one resistor to temporarily reduce the value of the voltage output from the reference temperature source.
4. An induction heating apparatus in accordance with Claim 3, wherein the means to vary the value of one of the divided series combined resistors is a potentiometer device wherein the desired reference temperature level may be input thereto.
5. An induction heating apparatus in accordance with Claim 2, including a second timing circuit responsive to said second signal operative to turn off the inverter after a second predetermined time delay following the initiation of the second signal.
6. An induction heating apparatus in accordance with Claim 5, wherein said second predetermined time delay is longer than said first predetermined time delay.
7. An induction heating apparatus in accordance with Claim 2, wherein the induction core unit includes a fiber optic element integrated therewith operatively connected to an infrared sensitive detector which includes a linearizing amplifier as the means to convert temperature level sensed by the fiber optic element into an output voltage to be compared with the reference temperature source in the comparator circuit.
8. A method of heating, by induction, a sheet of ferromagnetic base material which is coated with a nonconducting, nonferromagnetic surface material, such as paint, including the step of heating said base material and simultaneously monitoring the temperature of the surface material, automatically stopping the heat input to the base material when temperature of the surface material reaches a first, predetermined temperature level which is lower than a second, predetermined desired temperature level, allowing the temperature of the base material to equalize with the temperature of the surface material at said first temperature level, automatically resuming heat input to the base material until the surface temperature reaches the second predetermined temperature level, retaining the second temperature level for a predetermined time interval and thereafter deactivating the induction heating apparatus.
9. The method of Claim 8, wherein the heating of the base material is accomplished by induction heating with the induction heating apparatus being turned off when the temperat-ure of surface material reaches said first temperature level and is subsequently turned on when the temperatures of the surface material and base material are substantially equal.
10. A device for sensing and controlling temperature of a workpiece which is being heated by a selectively operable energy source, including a temperature sensing element and means to convert the temperature level sensed to a directly related output voltage, an adjustable, voltage-dividing, reference temperature source, voltage comparator circuit accepting and comparing voltages from the temperature sensing means and the adjustable reference temperature source and including means for producing a first signal when said voltages are equal, means responsive to said first signal including a first timing circuit to control the energy source to maintain a first temperature level, switch means responsive to said first signal to increase the value of the reference temperature source after a first predetermined time delay created by said first timing circuit following the initiation of the first signal, means to activate a period of operation of the energy source to increase the temperature of the workpiece to a second, higher temperature level until a second signal is produced by the voltage comparator circuit.
11. The device of Claim 10, including a second timing circuit responsive to said second signal operative to turn off said energy source after a second predetermined time delay following the initiation of the second signal.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US05/566,003 US4032740A (en) | 1975-04-07 | 1975-04-07 | Two-level temperature control for induction heating |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1053338A true CA1053338A (en) | 1979-04-24 |
Family
ID=24261035
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA248,296A Expired CA1053338A (en) | 1975-04-07 | 1976-03-19 | Two-level temperature control for induction heating apparatus |
Country Status (12)
| Country | Link |
|---|---|
| US (1) | US4032740A (en) |
| JP (1) | JPS51123934A (en) |
| AU (1) | AU506299B2 (en) |
| BR (1) | BR7602056A (en) |
| CA (1) | CA1053338A (en) |
| DE (1) | DE2614193A1 (en) |
| ES (1) | ES446744A1 (en) |
| FR (1) | FR2307432A1 (en) |
| GB (1) | GB1549235A (en) |
| IE (1) | IE42974B1 (en) |
| IT (1) | IT1063856B (en) |
| SE (1) | SE414107B (en) |
Families Citing this family (34)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DK142586B (en) * | 1977-07-07 | 1980-11-24 | Topsil As | Apparatus for zone melting of a semiconductor rod. |
| US4521659A (en) * | 1979-08-24 | 1985-06-04 | The United States Of America As Represented By The Administrator Of The National Aeronautics & Space Administration | Induction heating gun |
| US4396221A (en) * | 1980-10-06 | 1983-08-02 | The Regents Of The University Of California | Hinged vehicle window assembly |
| US4546226A (en) * | 1982-04-29 | 1985-10-08 | Entron Technologies, Inc. | Method and apparatus for the conveying and radio frequency processing of dielectric materials |
| GB2122390B (en) * | 1982-05-24 | 1985-07-24 | Michael Leonard Kelly | Electronic thermostat |
| EP0114420B1 (en) * | 1983-01-24 | 1986-08-20 | ATELIERS DE CONSTRUCTIONS ELECTRIQUES DE CHARLEROI (ACEC) Société Anonyme | Method of manufacturing a commutator for a d.c. electric machine |
| DE3518946A1 (en) * | 1985-05-25 | 1986-11-27 | Robert Bosch Gmbh, 7000 Stuttgart | Device for joining parts by means of a heat-activatable adhesive |
| USRE33644E (en) * | 1985-06-28 | 1991-07-23 | Metcal, Inc. | Ferromagnetic element with temperature regulation |
| US4769519A (en) * | 1985-06-28 | 1988-09-06 | Metcal, Inc. | Ferromagnetic element with temperature regulation |
| US5248865A (en) * | 1989-12-18 | 1993-09-28 | Tyler George W | Apparatus for induction heating of bearings or the like |
| US5160396A (en) * | 1991-02-11 | 1992-11-03 | Engineering & Research Associates, Inc. | Low thermal inertia heater |
| US5319170A (en) * | 1992-10-20 | 1994-06-07 | Belmont Instrument Corporation | Induction fluid heater utilizing a shorted turn linking parallel flow paths |
| US5350902A (en) * | 1993-05-12 | 1994-09-27 | The United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration | Induction heating coupler |
| US5414247A (en) * | 1993-12-29 | 1995-05-09 | The Boeing Company | Hot melt induction heater and method |
| US5672290A (en) * | 1995-03-13 | 1997-09-30 | Raychem Corporation | Power source and method for induction heating of articles |
| JP2002503162A (en) | 1997-05-16 | 2002-01-29 | ウポノール イノベーション アクチボラゲット | Fusion bonding equipment |
| US5908575A (en) * | 1997-05-16 | 1999-06-01 | Gas Research Institute | Method of inductively fusion joining plastic pipes |
| SE518499C2 (en) * | 2001-02-02 | 2002-10-15 | Tetra Laval Holdings & Finance | Apparatus for preparing a package or packaging material |
| US6953919B2 (en) | 2003-01-30 | 2005-10-11 | Thermal Solutions, Inc. | RFID-controlled smart range and method of cooking and heating |
| JP2005015906A (en) | 2003-06-30 | 2005-01-20 | Kikuchi Co Ltd | Induction-heating method for thin sheet-made article and apparatus therefor |
| US8803044B2 (en) * | 2003-11-05 | 2014-08-12 | Baxter International Inc. | Dialysis fluid heating systems |
| US7573005B2 (en) * | 2004-04-22 | 2009-08-11 | Thermal Solutions, Inc. | Boil detection method and computer program |
| US7444205B2 (en) * | 2004-10-29 | 2008-10-28 | Neil Desmond | Modular self structuring and computing system |
| US7696458B2 (en) * | 2005-06-03 | 2010-04-13 | Illinois Tool Works Inc. | Induction heating system and method of output power control |
| US7731689B2 (en) | 2007-02-15 | 2010-06-08 | Baxter International Inc. | Dialysis system having inductive heating |
| US11000407B2 (en) | 2007-08-07 | 2021-05-11 | Belmont Instrument, Llc | Hyperthermia, system, method, and components |
| US7819835B2 (en) | 2007-08-07 | 2010-10-26 | Belmont Instrument Corporation | Hyperthermia, system, method and components |
| US8758544B2 (en) | 2010-10-29 | 2014-06-24 | Pilkington Group Limited | Method and apparatus for forming a vehicle window assembly |
| US10137257B2 (en) | 2016-11-30 | 2018-11-27 | Belmont Instrument, Llc | Slack-time heating system for blood and fluid warming |
| US10507292B2 (en) | 2016-11-30 | 2019-12-17 | Belmont Instrument, Llc | Rapid infuser with vacuum release valve |
| US10485936B2 (en) | 2016-11-30 | 2019-11-26 | Belmont Instrument, Llc | Rapid infuser with advantageous flow path for blood and fluid warming |
| US11141104B2 (en) | 2017-06-22 | 2021-10-12 | General Electric Company | Infant warming system having ECG monitor and method for providing resuscitation assistance |
| US10596054B2 (en) | 2017-06-28 | 2020-03-24 | General Electric Company | Infant warming system and method |
| DE102020200111A1 (en) * | 2020-01-08 | 2021-07-08 | Volkswagen Aktiengesellschaft | Device for joining at least one connecting section of a workpiece arrangement |
Family Cites Families (10)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US2267001A (en) * | 1940-12-16 | 1941-12-23 | Ohio Commw Eng Co | Method and apparatus for drying paint |
| GB835825A (en) * | 1956-10-29 | 1960-05-25 | Ajax Magnerthermic Corp | Method and apparatus for the induction heating of billets and the like |
| US2988623A (en) * | 1958-03-17 | 1961-06-13 | Ajax Magnethermic Corp | Method and apparatus for induction heating of billets and for determining average temperature thereof |
| US3183294A (en) * | 1962-04-09 | 1965-05-11 | Ohio Crankshaft Co | Temperature control apparatus |
| US3591752A (en) * | 1969-12-08 | 1971-07-06 | Reynolds Metals Co | Apparatus for measuring the conductor and shield temperature of high voltage cable |
| US3740859A (en) * | 1969-12-29 | 1973-06-26 | Interlake Steel Corp | Drying system |
| US3710062A (en) * | 1971-04-06 | 1973-01-09 | Environment One Corp | Metal base cookware induction heating apparatus having improved power supply and gating control circuit using infra-red temperature sensor and improved induction heating coil arrangement |
| US3781504A (en) * | 1971-12-29 | 1973-12-25 | Gen Electric | Induction cooking appliance including temperature sensing of inductively heated cooking vessel by radiation detection means |
| US3816690A (en) * | 1972-09-18 | 1974-06-11 | Illinois Tool Works | Induction heating apparatus |
| US3845268A (en) * | 1973-02-12 | 1974-10-29 | Boeing Co | Induction heater apparatus and system |
-
1975
- 1975-04-07 US US05/566,003 patent/US4032740A/en not_active Expired - Lifetime
-
1976
- 1976-03-19 CA CA248,296A patent/CA1053338A/en not_active Expired
- 1976-04-02 DE DE19762614193 patent/DE2614193A1/en not_active Withdrawn
- 1976-04-05 GB GB13659/76A patent/GB1549235A/en not_active Expired
- 1976-04-06 AU AU12724/76A patent/AU506299B2/en not_active Expired
- 1976-04-06 ES ES446744A patent/ES446744A1/en not_active Expired
- 1976-04-06 BR BR2056/76A patent/BR7602056A/en unknown
- 1976-04-06 FR FR7609951A patent/FR2307432A1/en active Granted
- 1976-04-06 SE SE7604025A patent/SE414107B/en unknown
- 1976-04-06 IT IT21957/76A patent/IT1063856B/en active
- 1976-04-07 IE IE729/76A patent/IE42974B1/en unknown
- 1976-04-07 JP JP51038307A patent/JPS51123934A/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| BR7602056A (en) | 1976-10-26 |
| JPS51123934A (en) | 1976-10-29 |
| IE42974B1 (en) | 1980-11-19 |
| JPS615270B2 (en) | 1986-02-17 |
| FR2307432B1 (en) | 1979-08-31 |
| SE7604025L (en) | 1976-10-08 |
| AU506299B2 (en) | 1979-12-20 |
| ES446744A1 (en) | 1977-10-01 |
| SE414107B (en) | 1980-07-07 |
| IT1063856B (en) | 1985-02-18 |
| AU1272476A (en) | 1977-10-13 |
| US4032740A (en) | 1977-06-28 |
| FR2307432A1 (en) | 1976-11-05 |
| GB1549235A (en) | 1979-08-01 |
| DE2614193A1 (en) | 1976-10-21 |
| IE42974L (en) | 1976-10-07 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |
Effective date: 19960424 |