CA1105094A - Digitally programmed microwave cooker - Google Patents

Abstract

DIGITALLY PROGRAMMED MICROWAVE COOKER

A microwave cooker digitally programmed in any of a plu-rality of different cycles whose digital programs are selected from a front panel display. Programs include a cook cycle, a defrost cycle, and/or a defrost and cook cycle, in which a body of refrigerated food to be heated is subjected to microwave energy for a predetermined time in the oven, and then allowed to set for a predetermined time to allow heat produced in the food body by said microwave energy to at least partially dis-perse throughout the body thereby equalizing the temperature of different food body regions and then applying additional microwave energy for a predetermined time to cook said food body.

Description

_ckgrolmd of the Invention Digital programming of various functions such as industrial processes or resistance heating of conventional elec~ric stoves is known. However, such devices do not have the same fail-safe requirements as microwave cookers. For example, prevention of excess electromagnetic radiation is a problem with conventional electric stoves and, hence, con-ventional digitally controlled sequences o:E control circuits may be used.
Microwave cookers, however, required that a frozen or partially frozen food body be cooked with a time sequence.
A timer is manually reset a number of times to apply the microwave energy and to then allow the :Eood body to set for a period to transfer heat by conduction to those portions of the food body which absorb less microwave energy due, for example, to having ice crystals formed therein which have a lower loss at the microwave frequency of 2.~5 KM~I predominantly used for microwave cooking. Attempts to avoid the necessity for manually resetting the timer at the end of each application of defrosting microwave energy have been tried, for example, by providing additional mechanical timers which are interconnected by additional relay contacts to produce various sequences of opera-tion, but such deviccs have provecl e~pensive and h~ve required additional switch contacts which can be a source oE maintenance.

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. Summary of ~he Invention In accordance with this invention, fail-safe mechanical switch systems.are incorporated in the digitally controlled power circuit~y. Such switch systems are normally actuated prior to the flow o curren* therethrough and, accordingly, such switches do not switch against çurrent with attendan~
arcing or sparking.
More specifically, se~uencing of the power on and off is . achieved in accordance with this inven~ion by digitally con-trolled semiconductor switches such as ~hyristors ~o controlan alternating curren~ signal supplied to a power supply such as the primary of a high voltage transformer which energizes a source of microwave energy such as a magnetron ~hose output is isupplied to a cooking cavity. Mechanical switches actuated by : a closure mernber for an access opening to said cooking cavity are connec~ed.directly în series with said semiconductor switch and a power source so that power cannot be supplied to the power supply in the event that the door is open. In addition, :~ means are provided for sensing ths position o~ the mechanical : 20 ~ interlock switches ~o disable the semiconductor switch in the event that th~ door is open and one.o the mec}~anically actu-ated switches sticks closed. More speci~ically, a latch lnter-lock switch and a irst interlock switch have additional con~acts thereon connecting an open door sensing component through a :
: seco~d interlock switch and a thermal sensing structure to the . ground side o an inpUt power line so that power cannot be supplied ~o: the transformer.
This invention urther provides time sequencing of the .microwave powe~ by~ a computer.which is manually programmed by . ~ .
touching pads on a front panel.to selec~.a program in said : ~ - 2 ~

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computer. The selected cycle, such as a defrost cycle, a defrost and cooking cycle, or a cooking cycle, is ed from the output from said computer to a power relay system for applying energy to a power supply such as a transformer providing high voltage to a microwave generator such as a magnetron.
This invention further provides that the power relay systcm is preferably a semiconductor structure, such as a thyristor, so that no mechanical contacts are opened or closed to switch power to the magnetron on and of since door interlock and safety protection switches have all been actuated prior to actuation of the semiconductor power relay by the computer.
This invention further provides that the computer may consist of a large number of computer functions, the majority of which are formed on a semiconductor chip or chips as an integrated or hybrid circuit requiri.ng low input control power and, hence, easily accentuated from capacitive pads on a ront panel board having a plurality of different pads thereon which are labeled in accordance with the time and desired defrost mode to be selected. Preferably, the panel indicates the mode or cycle which has been selected and/or the time for each mode of the food processing sequence.
According to the invention there is provided a microwave heating system comprising: means comprising a magnetron for supplying microwave energy to a multimode cavity having a movable closure member; a power supply ;.
for said magnetron; means for supplying power to said power supply from an alternating current source compris.ing a switch mechanically actuated by closure of said movable closure member; means comprising a semiconductor switch connected in series with said mechanically actuated switch for con-trolling the supp~y of power to said magnetron from said power supply in sequences of different average microwave power levels comprising means or : pulsing the supply of power to said magnetron at different pulse repetition :30 rate duty cycles; said controlling means further conprising a clock having a ~'' :
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frequency substantially higher than the frequency of said power source; and means for manually selecting at least one of said different pulse repetition rate duty cycles.

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Brief Description of the Dr~ s Other and further objects and advantages of the invention will become apparent as the description thereo~ progresses, re~erence being had to the accompanying drawings wherein:
FIG. 1 illustrates a fron~ elevation view.o~ a microwave oven incorporating the invention;
FIG. 2 illustrates a schematic diagram o~ the control . system in this învention; and FIGS. 3A through 3D illustrate diag~ams o~ digi~ally pro-:L0 grammed timing sequences for the system o~ FIG. 2 , ' , ,~
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Description of ~he Preferred Embodiment Referring now to FIGS. 1 and 2, there is shown a microwave oven cavity lO having a food bod~ 12 positioned therein through a door 14 supplied with microwave energy from a magnetron 16 via a waveguide 18. While the r.~agnetron 16 may generate micro-wave en0rgy of any desired frequency, a frequency of 2.45 KMH
is preferred The csoking cavity 10 has in~ernal dimensions which are many times the free space wavelength of said frequency.
. so that many diferent resonan~ modes ma~ be produced therein, and such modes may.be cyclically distributed by a mode stirrer 20 driven by any desired means in accordance with well-known practice.
Magne~ron 16 is supplied with power at a voltage of, for example, 4000 voIts rom a high voltage power supply 22 com-prising a transformer 24 having a primary winding 26, a high voltage secondary winding 28 and a ilament winding 30. Fila-ment winding 3~ is connected to the filament 32 of the magnetron 1~ whose anode 34.is grounded,and any spurious oscillations emanating rom the magnetron 16 via the leads ~o the cathode 32 are suppressed~by capacitors 38.. The high voltage winding 28 . has:one end connected to ground through a current sensing re-slstor 40 and ~o ilament.winding 30 through an energy storage condenser 42 to one side of magnetron ~ilament 32. A hal wave rectiier 44 is connected from ground to the same side o the filament 32 as ~h~ colldenser 42, with the rectifier 44 being poled such that when the ungrounded end of winding 28 is posi-tive, current flows through rectiier 44 to charge condenser 42, and when ~he voltage across winding 28 reverses, filament 32 is driven negative to cause conduction of magnetron 16, partially ~0 dischar~ing condenser 42~ Operation of such a high voltage .

circuit for magnetron power supplies is shown in greater detail in U.S. Patent No. 3,396,342 in which the transformer is pref-- erably selected to have saturation characteristics at the maximum desired voltage to at least partially ompensate ~or changes in the input voltage level to the primary winding and/or . different secondary load conditions occurring, for exampleJ
during wa~m-up or.~roughout the life o the magnetron.
Power is supplied to input winding 26 from a conventional 110 volt 60 cycle source ~o a plug 50. One side of plug 50 is connected to one side o a thermally ac~uated element 52 which is heated by the resistor 40 so that in the event excess current is drawn by the magnetron 16 or other elements of the power supply 22 for more than a prede~ermined time, ~hermal energ~
generated by resistor ~0 will cause element S2 to open thereby.
. de-energizing the.circuit. The o~her side of element 5Z is connected to one side of a similar thermally actuated element 54, and the other side of element 54 is connected through a resistor 56 which is adjacent to element 54 and which will : transfer heat thereto i~ current flows through resistor 56 when the oven is enérgized. The other end o resistor 56 is con-.
nected througll an interlock switch 58 actuated by the door 14 and opened when t.he door 14 is closed. The opposite terminal o switch 58 from that connected to resistor 56 is connected to , a second interlock.switch 60 actuated by door 14. When the ; door 14 is Glosed, switch 60 contacts ~erminal 64 o, control : circuit 62.
The other side of the plug 50 from tha~ connec~ed to pro~
tection element 5Z is connected to one side of a thermally actuated..protection element 66 mounted on, or adjacent to, the ; 30 anode~34 o~ magnetron.16 and adapted to open i~ the temperature : '. ' :: ~ 6 -''. ': . . .. ~ . . ..

of magnetron 16 exceeds a predetermined temperature. The other side of element 66 from that connected to the plug S0 is con-nected to a power input terminal 68 of control 62 through a protection element 70 located adjacent to and sensing the heat ing cavity 10. Control circuit 62 connects ~erminal 68 to terminal 64 when it is . desired ~o energize the transformer 24 to supply mi rowave energy to the oven. The terminal 64 is connected to one end of transformer primary winding 26 through interlocX switch 60 a~d a latch switch 74 mechanically actuated by a latch solenoid 76 which prevents the door 14 from being opened when solenoid 76 is energized. Solenoid 76 is connected between the grounded bus connected to thermal element 54 and terminal 64.
A blower motor 80 lS connected between the grounded bus and the junction between latch switch 74 and winding 26 so that when motor 80 is energized by the circuit 62, the blower sup-plies cooling air to the magnetron 16 and also to the cavity 10 to turn a fan~shaped mode stirrer 20 rotatably supported on a shaft.
The end o~ winding 2~ opposite to that connected to switch 74 is connected to ground through a semiconductor switch 84 such as a triac having a control ~lectrode 86 to which a con-~rol signal is s~ppli~d from a terminal 82 o control circuit 62 when it is desired to energize transformer 24.
To lnitiate operation o~ the circuit, a normally open start button 90 is pushed to momentarily connect terminal 6S
through a normally closed contact of a stop button 92 to a terminal 94 of circuit 62. Terminal 9~ is connected through a second normally closed section 96 of the stop button, mechan-ically gang~d to section 92, to a terminal 98 of circuit 62 .

which actuates a relay in circuit 62 ha~ing a set o normally open contacts 102 actuated by a solenoid 104. Contacts 102 when closed supply power from terminal 68 to terminal 64 which energizes latch solenoid 76 and supplies power ~hrough interW
lock switch 60 and latch switch 74 to the ungrounded end of winding 26.
The position of the left interlock switch 60,and the latch interlock switch 74 is detected by additional contacts on those switches which a~e contacted by those switch0s when the door 14 is opened and the interlock switch 58 is closed9 as shown in FIG. Z, thereby supplying a ground potential to an open door ; detect circuit 106 in control circuit 62. The OlltpUt of circuit 106 prevents operation of a start detector circuit 108 supplied from terminal 94 when door 14 is open.
Upon pushing of ~he start button 90 wi~h door 14 closed~
power is supplied to latch solenoid 76, closing latch 74 and opening the contact of latch 74 connected to the circuit 106.
The door 14 is ~hen locked shut, and, since interlock swltch 60 ; ~ has been actuated to remove the ground from door detect circuit106, a star~ signal is supplied to a digital computer 100 in circuit 62 which actuates relay solenoid 104 through a triac 103, closing relay contacts 102 to bypass the start button 90 thereby holding latch solenoid 76 energized until the program sequence of computer l00 ends, at which time solenoid 104 is de-energîzed, opening contacts 102, de-energizing latch solenoid 76 and permit~ing the door to be opened. Power to solenoid 104 is also interruptable by pushing ganged stop but~ons 92 and 96.
~; ~ Prom the~foregoing, it may be seen that even upon failure of ~he digital computer circuit 100, the semiconductor switch 30~ ~84 or the semiconduc~or swi~ch ~; the o~en once energized :
~;: - ... :, .. ,. :, .. .. . -cannot be opened to produce radiation leakage un~il power ls removed from the latch solenoid 76 which, under these circum-. . .
stances, is dlrectly in parallel wi~h the input to the trans-fo-rmer 26. Accordingly, with such a circuit, multiple program-ming may be achieved usin~ semiconductor circuitry while still ; retaining the fail-safe conditions of the mechanical interlock structures required by safety standards.
Digital computer 100 can be any general purpose digi~al computer having sufficient memory to retain the desired program 10 and may be located either on the oven or, if desired, may be a central computer for a commissary supplying the cooking control .programs. A special purpose computer could also be used with the particular digital componen~s preferably sslected from standard digital components in which large numbers o~ circui~s having dîfferent functions are formed on a semiconductor chip ,j~ fio~77ect, and/or a plurality of semiconductor chips or ~ ed on a substrate and packaged as a unit. It is also contemplated, however, that any of the conventional digital techniques em-: ploying core logic, bipolar semiconductor logic or MOS logic may be used.
Digital computer 100 is shown herein by way of example as having.a digital computing section 110 which performs memory functions, programming f-mctions, s~quencin~ control ~mc~ions and, for example, contains a.master oscilla~or clock which may.
; . have a frequency o~ 100 KHæ. A second section of khe computer 100 is the display input and output section 112 which supplies data from the computer section 110 to a binary to a digital code display panel driver 120. The output of driver 120 pro-vides light~actuating signals to a plurality of regions 144 of display 122 lndicating coo~ing.functions and digital code to , ' g ~: .
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four locations on a four-position number display 142 whicll are connected to the appropriate number signals at unit 142.
Input program data signals, which are produced by touching panel pads 1~-~0 of display panel 122, are sensed by a panel pad sensor and binary to decimal encoder 124 which supplies a level sensing signal to all the pads 140. The sensed pad causes data identifying the pad ln digital form to be sent to section 112 of the computer 100.
An output drive section 114 of computer 100 ener-gizes the semiconductor switches 106 and 86 as well as a bell alarm 126 to indicate the end of the cycle. Alarm 126 may be - de-energized by means of a switch 128, if desired.
A section 116 of computer 100 is a coincidence de-: tector to which a square wave input is supplied from a pha.se shifter clipper 118 supplied with a 60 cycle sihe wave voltage from term:inal 68. The output signal from phase shifter clipper 118 which is a sine wave clipped to form said square wave is preferably delayed by 90 or ~/2 radiansJ and is supplied to co-incidence detector 116 where it is compared with clock pulses ~ 20 from section 110 to energize switch 84 through section 110 and : output section 114 only when a positive going excursion of the .
wave :Erom phase shifter 118 appears and to de-energize switch 84 only when a negative excursion o:E the output of phase shifter clipper 118 occurs so that the phase shift of the alternat-ing current suppli.ed the trans:Eormer 26 during the start-up alld shutdown of the transformer is accurately controlled. The ~.
precise phase shift is preferably chosen to minimize input currènt to the transformer and depends upon the transformer design and the value of the condenser 42. In the design se-lected for optimum operation, condenser 42 discharges at least ~.

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partially during the por~ion of the 60 cycle wave when ~he rectifier 44 is nonconduc~ing ~nd magnetron 16 is conducting.
Under these conditions, values are chosen such that a phase c/P~
shift of 90 of the phase shifter~118 produces substantially ` reduced star~ing current surges to the power supply 22 thereby reducing peak currents which might otherwisa damage in~erlock switches or blow fuses and reduces power line interference to ` ~adios or television sets.
The de~energization of the transformer 24 only during a negative going excursion of the output of phase shifter ~
insures the residual flux in the transformer 26 will be in the reverse direc~ion from that produced by starting current and3 hence, the possibility that the ~ransformer will be driven into saturation and drawing large peak currents during s~art-up due to residual transformer core 1ux during repeated start-up program sequences is avoided.
A light 130, positioned in oven cavity 10, is energized by switch 132 ~anged to swi~ch 58 ~o turn on ligh~ 130 when door 14 is open. If desired, an auxiliary switch 134 may be manually 23 closed to turn light 130 on during cooking when door 14 is closed to obser~e the oven interior through a transparen~ glass over aper~ured metal in ~he front of door 14.
Display 122 as shown in FIG. 2 is a flat glass panel having touch pads indicated at 1~0 for numbers and coolcing functions, a digital read-out section 142 and individual lights 144 for cook~ 146 for hold, 148 for defrost, 150 for slo cook, and 152 for timer. ;The panel 122 is thus cleanàble easily and does not attrac~ dirt and grease from cooking activi~ies.
The pads 140 are ac~uated by touching with a finger which reduces the~level of a carrier signal having a frequency of from .

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30 to 300 KHz supplied by encoder 124 through pads 140 to le~el detectors in encoder 124 connected ~hrough separate lines to each of the pads 140. Each particular pad level de~ected eauses . a read-only memory in encoder 12~ to send a digi~al code ~o ~he digital computer 100, coded wit~ the iden~ity of the pad sensed, whic.h stores the code.o~ the number or ~unction.
Referring now to FIGS~ 3A and 33, ~here is shown a time sequence illustrating an example of defrosting and cooking a food body using sequential operations programmed into the com-puter 100. Time in minutcs is shown on the diagram for defros~ing and cooking a food body such as a roast of beef. PIG. 3A
illustrates *he wave~orm output ~rom ~he computer 100 control-llng ~he switch 84. .P~..3B shows the waveform o~ ~he averags mlcrowave power.progr~m selec~ed ~or a defrosting and cooking sequence.
. . . To implement the de~rost and cook sequence~ body 12 is put in the oyen 10 and the door 14 is closed, supplying power from . one side o~ the line through the interlock switch sys~em at the an 80 and a side of the transformer primary winding 2S. The de~rost pad 140 is then touched supplying a level detector signal to encoder 124 to store.the defros~ function digita.l code in the memory.o~ compul.er 100. A number, such as 0800, is sequentially supplied by touchillg the pads 0, 8, 0 and 0 ~hereby s~oring the . time o~ 8 minutes and 0 seconds for the de~rost unction in the : .. computer~ The second ~unction to be carried out is, then pro grammed into ~he compu~er. For example, t~ cook ~unc~ion is selected by touching pads l40 corresponding ~o ~he desired cook ...time, such as 19 5, 3 and 0? to.select the time of l5 minutes - .~ and 30 seconds.
30~ start but~on 90 is ~hen pressed a~d power is supplied " '' ' , , , ' ' ' ' ' i .~ 12 -.

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to the transformer ~ energizing magnetron 16 to deliver micro-wave energy to the oven 10. As shown in FIGo 3A, a pulse 158 is supplied from computer driver 114 having a leading edge 160 synchronized with a pulse of the computer clock and at the peak positive potential o~ the 60 cycle alternating current supplied to plug S0 to start magnetron 16. Ater producing microwave - energy ~'or 20 seconds, ~agnetron 16 is turned off by the trail-ing edge 162 of pulse 158 at the peak negati~e excursion of the alterna~ing curren~ voltage to the current plug 50. A~ter a wait of 40 seconds, pulse 158 reoccurs to re-energize trans-, ormer 24 and again supply microwave energy to the oven 10.
This ~unction cycle continues or the time selected for defrost so that the microwave energy is supplied to the oven as,bursts of energy having a duty cycle of one-third. At the end o~ the 8-minute time period selected for de~rost, the computer 100 automatically' resets to hold the magnetron o~f for the same time programmed for defrost.
, The curve 3B shows the average microwave power in the oven ~ ,~ for the control program of FIG. 3A. Since during the defrost - 20 function duty cycle the power is on one-third of the time, the averaga power is shown at 170 as one-third o~ the full power of 700,wat~s. The periods of ~ait between pulses of microwave power permit portions of the food which have absorl)ed more ~nergy ~rom the microwave field in the oven than adjacent ~egions thereof to transfer the enexgy by conduction to such adjacent regions to melt, ~or example, ice crystals in the body. During the subsequent hold period 172 with no microwave '~ energy supplied to the oven, thermal gradienk throughout the ~food body levels still more to insure, for example, that small portions of the roask 12 are defrosted~

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During the cook time period 164 as shown in FI~. 3A, the microwave energy is tu~led on at time 166 and off at time 168, resulting in full average mîcrowave power being supplied to the oven, as shown in FIG. 3B, in region 174 beginning at poin~
176 and ending at point 178. Fo.llowing expiration of the cook-ing cycle, compu~er 100 supplies a signal to ~he alarm 126 and de-energizes the relay solenoid 104 by de-energizing the input to triaf ~4~, which removes power from solenoid ~6 and permits the door 14 to be open. In addition, the fan 80 is de-energized stopping ~he mode stirrer 200 The sequentially processed food body.12 may be then removed from the oven 10 by opening the . door 14.
~ Alternatively, if a defrost and slow cook program is de-; sired, the defrost pad 140 is.touched and ~ime pads touched.
. For example, as shown in FIG. 3C, to produce 12 minutes o~`
defrost, the pads 1, 2, 0 and 0 are touched, the slo cook pad is touched and.the slow cook time of, ~or example, 11 minutes ana 30 seconds is selected by touching the pads 140 or 1, 1, : 3 and 0.
. 20 .S~art button 90 is pressed and, as shown in ~IG. 3C, 20-second on pulses 180 separated by 40-second .off periods are . supplied ~o the computer by switch 8~. During this 12-minute period; a.digital code sent through driver 120 lights area 148 .
labelled defrost. Pollowing expiration o~ the defrost period, an automatic repeat of the time is supplied by the computer with no pulses supplied the swi~ch ~ so that body 1~ in the oven is allowed to set for the same number of seconds as de-frost with the area 146 labelled hold being illuminated.
Pulses 182 are then supplied to switch 84 for *he cook unction .- .. . ~ . , .
for 11.-1!2 minutes. This produces ~he avexage microwave power~

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in the oven shown by curve 3D by portions 182, 184 and 186 for defros~, hold and slo cook, respectivelyO
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This compietes the description of the embodiments of the invention disclosed herein. However, many modifications thereof will be apparent to persons ski~led in the art without depart-ing from the spirit and scope of the in~ention. For example, any desired form o display media, such as those used in mini-computers, can be used, and any desired microwave genera~or, power supply or digi~al computer circuitry can be used. Accord-ingly7 it is intended that this invention be not limited to theparticular details illustrated herein~ except as de~ined by the appended claims.

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Claims (4)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A microwave heating system comprising: means comprising a magnetron for supplying microwave. energy to a multimode cavity having a movable closure member; a power supply for said magnetron; means for supplying power to said power supply from an alternating current source comprising a switch mechanically actuated by closure of said movable closure member; means comprising a semiconductor switch con-nected in series with said mechanically actuated switch for controlling the supply of power to said magnetron from said power supply in sequences of different average microwave power levels comprising means for pulsing the supply of power to said magnetron at different pulse repetition rate duty cycles; said controlling means, further comprising a clock having a frequency substantially higher than the frequency of said power source;
and means for manually selecting at least one of said different pulse repetition rate duty cycles.

2. The system in accordance with claim 1 wherein: said means for controlling said power comprises means for generating trains of control pulses at a pulse repetition frequency higher than said power source.

3. The system in accordance with claim 2 wherein:
control of said microwave power levels comprises control of the duty cycle of said control pulses.

4. The system in accordance with claim 1 wherein said control system comprises a digital computer.