CA2046775C - High frequency heating apparatus and electromagnetic wave detector for use in high frequency heating apparatus - Google Patents

Abstract

Abstract A high frequency heating apparatus and an electro-magnetic wave detector for use in the apparatus are so arranged as to estimate the condition of a food article placed in a heating chamber by detection of microwaves within the chamber. By controlling the position of an antenna, and employing a method of processing signals of the microwaves detected, it becomes possible to achieve stable detection with high reliability.

Description

2~46775 HIGH FREQUENCY HEATING APPARATUS AND ELECTROMAGNETIC
~AVE DETECTOR FOR USE I~N HIGH FREOUENCY HEATING APPARATUS
The present invention relates generally to a high frequency heating arrangement, and, more particularly, to high frequency heating apparatus or microwave oven or the like, that is capable of automized cooking, for example, the thawing of food articles etc. by controlling the functioning of the apparatus by an estimation of the state of the food article based on detection of the state of the electromagnetic waves within a heating chamber. The invention also relates to an electromagnetic wave detector for use in such a high ~requency heating apparatus. - ~ -Recently, there has been a tendency towards the automization of cooking, for example, by automatic thawing of a food article by utilization of a high frequency heating apparatus.
The conventional practice has been for the operator to input the weight of the food article by keys (referred to as "time-auto"), or he finds the weight of the food article using a weight sensor that automatically detects the weight, whereby to heat the article only for a preliminary time set for each weight of such article. There has also been proposed another arrangement in which an antenna is disposed within the heating chamber to find the proper heating time by utilizing the characteristic that the microwave power detected by the antenna not being absorbed by the food article varies inversely with the weight of the article, for exampla, in Japanese Patent ~aid-Open Publication Tokkaisho No. 52-2133.
To enable the prior art to be described with the aid of a diagram, the figures of the drawings will first be listed.
Fig. 1 is a schematic diagram showing the general construction of a conventional high frequency heating apparatus:

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Fig. 2 is a diagram showing the general construction of a high frequency heating apparatus according to one preferred embodiment of the present invention;
Fig. 3 is an exploded perspective view showing portions of high frequency heating apparatus according to an embodiment of the present invention;
Fig. 4 is a top plan view showing on an enlarged scale one embodiment of antenna means and a cletecting circuit, which may be employed in the arrangement of Fig. 3;
Fig. 5 is a characteristic diagram showing the relation between the temperature of a food article and the degree of absorption of electromagnetic waves;
Fig. 6 is a characteristic diagram showing the relation between ideal food article temperatures and detecting circuit outputs;
Fig. 7 is a characteristic diagram showing the relation between the heating time according to food articles -and detecting circuit outputs;
Fig. 8 is a characteristic diagram showing the -relation between weights of the food article and detecting circuit outputs;
Fig. 9 is a schematic side sectional view of a high `~
frequency heating apparatus for explaining the relation between the antenna fixing position and food article position;
Fig. 10 is a schematic side sectional view of a high frequency heating apparatus according to another embodiment of the present invention;
Fig. 11 is a view similar to Fig. 4, which shows another embodiment thereo~:
Fig. 12 is an equivalent circuit diagram showing the antenna, detecting circuit and a smoothing circuit;
Fig. 13 is a frequency characteristic diagram of impedance for a micro-strip line;
Fig. 14 is a ~ilter characteristic diagram for a smoothing cixcuit;
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Fig.s 15(a) and 15(b) are characteristic diagrams of output waveforms according to the presence or absence of the smoothing circuit;
Fig. 16 is a fre~uency characteristic diagram showing the degree of amplification of a general amplifier;
Fig. 17(a) is a top plan view of an electromagnetic wave detector which may be employed in high frequency heating apparatus of the present invention;
Fig. 17(b3 is a cross section taken along the line XVII(b)-XVII(b) in Fig. 17(a);
Fig. 18 is an equivalent circuit diagram for the detecting circuit and the smoothing circuit;
Fig. l9 is a characteristic diagram showing the relation between food article weights and detecting circuit outpu~s;
Fig. 20 .is a voltage-current characteristic diagram for a diode;
Figs. 21(a), 21(b) and 21(c) are respective diagrams for micro-strip lines;
Figs. 22(a), 22(b) and 22~c) are time-charts showiny the functioning of the detecting circuit; ~ -Fig. 23 is a ~orward direction voltage-current characteristic diagram for a Schottky barrier diode;
Fig. 24 is a temperature characteristic diagram of the reverse recovery time of the Schottky barrier diode: -Fig. 25 is an input/output characteristic diagram ~or the detecting circuit: and Fig. 26 is a characteristic diagram sho~ing the variation rate of output by temperature with respect to the input of the detec~ing circuit.
In the known arrangement of Fig. 1, a ~rozen food article 2 is placed in a heating chamber 1, and electro- -magnetic waves, i.e. microwaves, represented by an arrow 4 are applied thereto from a microwave radiating portion 3. In this case, some part 5 of the microwaves ~ that are not absorbed by ~ -the food article 2 is selected by an antenna 6 in the heating chamber 1. After being detected by a detecting circui~ 7 this . ,~":.......................................................... ' ' : :

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data is fed to a control section 8. Since the amount of microwaves detected by the antenna 6 varies inversely with the weight of the food article 2, the weight of the food article 2 can be detected in this way, thus making it possible to set optimum heating time.
However, this conventional detecting means has had various problems. In the first place, when a weight sensor is used, there has been the disadvantage that the time weight tends to be influenced by the weight of a dish or container employed. :
In the case where the antenna is employed, the detection level tends to vary with the antenna construction, the detection circuit construction, and the interconnection between them, or it may be influenced by an external electromagnetic field, resulting in unstable factors in determining the subsequent heating sequence based on the weight estimation. Consequently, there has been the problem that an optimum finished state could not be reliably achieved. ` -Accordingly, an object of the present invention is to provide high frequency heating apparatus in which control -of the apparatus by the detection of the state of the electro-magnetic waves ~y an antenna and a detecting circuit is sufficiently stable to provide an optimum ~inished cooking ~ -condition.
Another object of the present invention is to provide an electromagnetic wave detector that is usable for achieving such high frequency heating apparatus.
In accomplishing these and other objects, according to one aspect of the present invention, there is provided high frequency heating apparatus comprising a heating chamber for accommodating a food article to be heated therein, a microwave radiating means for radiating microwave energy to heat the ;
food article, an antenna means located in a top portion of said heating chamber for detacting part of the microwave energy within said heating chamber, a detecting circuit for detecting electric power as detected by said antenna means, ~
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and a control section for controlling functions of various appliances by an output from said detecting circuit.
The invention also consists of high frequency heating apparatus comprising a heating chamber for accommodating a food article to be heated therein, a microwave radiating means for radiating microwave energy to heat the food article, an antenna means located in the vicinity of an opening formed in a wall of said heating chamber for detecting part of the microwave energy within said heating chamber, a detecting circuit for detecting electric power as detected by said antenna means, and a control section for controlling functions of various appliances by an output from said detecting circuit, said apparatus being so arranged that leakage power in the vicinity of said opening, said antenna means and said detecting circuit is less than 1/10 o~ a rated power of the parts constituting said detecting circuit where said antenna means and said detecting circuit are actually mounted.
The invention also consists of high frequency heating apparatus comprising a heating chamber for accommodating a food article to be heated therein, a microwave radiating means for radiating microwave energy to heat the food article, a power source for supplying electric power to said microwave radiating means, an antenna means for detecting ~ .
part of the microwave energy within said heating chamber, a detecting circuit for detecting electric power as detected by said antenna means, a smoothing circuit ~or smoothing an output of said detecting circuit, an amplifying section for ampli~ying an output of said smoothing circuit, and a control :
section for controlling functions of various appliances by an - output from said amplifying section.
The invention also consists o~ high frequency apparatus comprising a heating chamber for accommodating a food article to be heated therein, a microwave radiating means for radiating microwave energy to heat the food article, an antenna means for detecting part of the microwave energy within said heating chamber, a detecting circuit having micro- :

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strip lines and chip parts including a detecting diode, for detecting electric power as d~tected by said antenna means, and a control section for controlling functions of various appliances by an output from said detecting circuit.
The invention also consists of high frequency heating apparatus comprising a heating chamber for accom-modating a food article to be heated therein, a microwave radiating means for radiating microwave energy to heat the food article, an antenna means for detecting part of the microwave energy within said heating chamber, a detecting circuit for detecting electric power as detected by said antenna means by employment of a Schottky barrier diode, and a control section for controlling functions of various appliances by an output from said detecting circuit.
The invention also ~onsists of an electromagnetic wave detector for use in a high frequency heating apparatus, which comprises a double-sided substrate prepared by applying copper foils onto opposite faces of a substrate material, and an antenna means for detecting electromagnetic waves, and a detecting circuit having micro-strip lines and chip parts including a detecting diode provided on said substrate. -:
There is shown in Fig. 2, a high frequency heating apparatus Hl according to one preferred embodiment of the present invention, which generally includes a heating chamker 1 defined by a top wall la, side walls lb and a bottom wall lc for accommodating a food article 2 to be heated therein, a microwave radiating means 3, i.e. a magnetron or the like, mounted on one of the side walls lb for directing microwave energy to the food article 2 for heating, an antenna 6 located on the top wall la of the heatinq chamber 1 for detecting part - of the microwave energy within said heating chamber 1, a detecting circuit 7 coupled with the antenna 6 for detecting the electxic power detected by the antenna 6, and a control section 8 provided to control the functions of the apparatus -based on the output from the detecting circuit 7.
With the above arrangement, upon turning on the power the microwaves indicated by the arrow 4 are radiated .1 .~,. . .
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from the means 3 towards the article 2 in the chamber 1. Some portion 5 of the microwaves 4 that is not absorbed by the article 2 passes through an opening 10 in the top wall la of the chamber 1 via a cover 9 of a resin material over the opening 10, and is detected by the antenna 6. This antenna is made of copper foil located on a printed circuit board 11 and is located on the top wall lb above the opening 10, as will be described in more detail later, and is conn~cted to the detecting circuit 7 on the reverse face of the printed circuit board 11 for detection. Thereafter the data is sent to the control section 8 by leads 12 as the output of the detection circuit 7.
According to the detPcted amount, the control section 8 chec~s the state of the food article 2, and finds the optimum thawing time, also controlling the functioning of the microwave radiating means 3 and a fan 13 for cooling the latter.
Referring also to the exploded perspective view of Fig. 3, the construction around the detecting circuit 7 will now be described in more detail. ---Fig. 3 shows one example of mounting the detecting circuit 7 and the antenna 6 on the top wall la of the heating -chamber 1, the printed circuit board 11 having the detecting circuit 7 and the antenna 6 respectively located on the upper and lower faces thereof. For grounding the board 11 is soldered to four soldering projections 15 on a metallic plate 14, while a cover plate 16 with a rectangular box-like portion and folded edges with screw openings is applied thereover for shielding the microwaves. The plate 14 is fixed to a metallic support member 17 that in turn is fixed by spot-welding to the top wall la of the chamber 1. Screws 18 extend through screw openings of the cover plate 16 to be threaded into corre-sponding holes of the support member 17. The cover 9 of the resin material for covering the opening 10 in the top wall la is fixed to the underface of such top wall by engagement of projections formed on the opening cover 9 and corresponding holes in the top wall la.
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With this construction, the printed circuit board 11 and consequently the detecting circuit 7 are positively grounded by soldering to the metallic plate 14, the metallic plate 14 and the metallic support member 17 being fully short-circuited, while the metallic support member 17 is perfectly short~circuited to the wall la of the chamber 1 by welding.
Therefore, not only a high positional accuracy is achieved for mounting of parts, with positive grounding therebetween, but any stress to the detecting circuit can be suppressed, since lo the stress caused by the screw tightening is absorbed by the metallic plate 14c Fig. 4 shows a top plan view of one example of the printed circuit board 11 as viewed from the side of the -:
detecting circuit 7. In Fig. 4, dotted lines represent patterns on the reverse face of the board 11, while the one-dotted chain line circles denote patterns on the reverse face without any resist (i.e. the grounding portions to be soldered to the metallic plate 14 as stated earlier with reference to Fig~ 3). Microwaves detected by the antenna 6 are led into the detecting circuit 7 via a through-hole 19 extending -through the board ll so as to be detected by said detecting --circuit 7 which consists of a chip, such as Schottky barrier ~ -diode 20 etc. and micro-strip lines, and thus signals in the form of a D.C. current are transmitted through the lead lines 12.
Referring further to the characteristic diagrams of Figs. 5 to 7 showing the principles for detection of thawing in the apparatus, the product of the specific dielectric constant Er and the dielectric loss tan ~ varies as in Fig. 5 when a food article is uniformly heated and simultaneously raised in temperature. In Fig. 5, the abscissa shows the temperature T of the food article, and the ordinate denotes Er tan ~, which is an index indicating how the food article is capable of absorbing microwaves. In the diagram it is indicated that the microwaves are not readily absorbed at freezing temperatures but ars easily absorbQd in the vicinity of O'C. In other words, the microwaves detected by the -20~fi77~
antenna not being absorbed by the food article are increased during freezing, but decreased in the vicinity of ooc. From this fact, the diagram o~ Fig. 6 is obtained, in which the temperature T of the food article is plotted on the abscissa and the output V of the detecting circuit on the ordinate. As is seen from Fig. 6, in the case where the food article shows a uniform temperature rise, thawing detection is possible at an inflection point of the detection output. In an actual case, however, the heating is not uniform, portions where lo microwaves are concentrated and other portions where -microwaves are not concentrated being combined. Therefore, in the resultant waveform, a number of curves as in Fig. 6 overlap each other, with thawing not being completed at the inflection point at all times.
Accordingly, what are actually effective are the initial value of the detecting circuit output and initial variation rate. The initial value is generally inversely proportional to the weight of the food article, and, for example, in the case of a small amount of food, the absorption of microwaves is slight, with a large initial detecting circuit output; whereas in the case of a large food article, the absorption of microwaves is large, with a small initial detecting circuit output. In the case of a food article at a low temperature ~-20C), the initial variation rate of the detecting circuit output is large, whereas for a food article at a medium temperature (-10C), the initial variation rate of the detecting circuit output tends to be small.
In the diagram of Pig. 7 showing a typical example7 time t is plotted on the abscissa, while the detecting circuit output V is shown on the ordinate. Curve (a) represents a -food article o~ small amount at a low temperature, and curve (b) denotes a food article of large amount at a medium temperature.
Based on the principles described so far, correla-tion between the weight m and the initial output Vs is obtained, the initial output variation rate being set as a parameter as shown in Fig. 8, whereby to effect a weight ~ ;

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~ 2~6775 judgement and an initial temperature judgement of the food article. In Fig. 8, curve (c) relates to a low temperature food article with a large variation rate, wh:ile curve (d) denotes a medium temperature food article with a small variation rate. Needless to say, it is arranged that, by effecting cooking with an optimum heating time set per unit weight and initial temperature in the control section 8, extremely stable thawing detection can be realized as compared with a weight sensor or the like which may involve erroneous detection.
Referring further to Fig. 9, the r~lation among the fixing position of the antenna, the placing o~ the food article, and the output o~ the detecting circuit will now be described.
Fig. 9 shows an example of high frequency heating apparatus in which antennas 21 and 22 are respectively disposed in a side wall lb and the top wall la. For the side wall antenna 21, if the food article 2 is placed to the side, as at A and/or B in the chamber 1, an extremely large difference may result in the detected amount. More speci~ically, in the chamber 1, since microwaves are mixed in a complex fashion, and are absorbed into the article 2 to a certain extent, the microwave distribution in the vicinity of -the article 2 is still more distorted. Accordingly, although the microwaves in the chamber 1 may be totally detected when the food article 2 is at position B, the detected amount becomes unreliable if the article 2 is located at position A, since the microwaves in the vicinity of the side antenna 21 are disturbed. This is attributable to the fact that the difference between the distance el from the side wall antenna 21 to the article 2 at position A and the distance Q2 from said antenna 21 to the article 2 at the position B, is large -t~l ~ e2).
On the other hand, with the antenna 22 disposed in the top wall la of the chamber 1, since its distances ~3 and e4 to the articles 2 at the positions A and B are close to or approximately equal to each other (e3 - e41, the microwaves in '..'~ ' r 20~677~

the chamber 1 can be totally detected at any time.
Furthermore, since the food article generally has a long horizontal dimension, while being short in the vertical direction, the top wall is the best location for the antenna.
The apparatus Hl of the embodiment described with reference to Figs. 2 to 4 has the ~ollowing effects:
Since the antenna 6 and the detecting circuit 7 are covered by the metallic plate 14, the metallic cover plate 16, the metallic support member 17 and the wall surface of the chamber 1, etc., there is no leakage of microwaves towards the outside or introduction of noise from the outside, so that stable detecting performance can be achieved.
Owing to the antenna 6 being located in the vicinity of the opening 10, so as to be protected by the opening cover 9, the antenna 6 is not hit directly by scattered matter ~rom the food article 2, thus eliminating a cause of errors, such as a dielectric constant variation, etc. around the antenna 6.
Similarly, since the antenna 6 and the detecting -;
circuit 7 are not directly mounted on the wall face of the chamber 1, a temperature rise of the walls of the chamber 1 due to cooXing is not readily transmitted thereto. With favourable ventilation therearound, thermal destruction of the parts constituting the detecting circuit 7 or influence over ~ -their temperature characteristic does not readily take place, thus further assisting stable detection.
Since the antenna 6 is constituted by patterns on the same substrate as that for the detecting circuit 7, extremely ~avourable dimensional accuracy can be achieved for stable matching with respect to the detecting circuit 7, with consequent reduction of scattering of the characteristics.
In the above construction, even in actual cooking or under the worst operating condition, leakage power therearound is ~owld to be less than lOmN/cm2 when such leakage power is measured by a leakage meter, which leakage is below 1/10 with respect to the part of 1/lOW rating, and thus, there is no excessive input to the detecting circuit, without generating 2~L677~

of leakage power which might cause harm to the operator or cause erroneous functioning.
In the printed circuit board for the embodiment of Fig. 4, although a Schottky barrier diode of 250mW rating, and other parts are employed, the chip parts main:Ly employed are normally of a rated power of lOOmW to 500mW, with the rating values tending to be lowered as the temperature rises. In this high frequency heating apparatus, even i~ the wave leakage was less than 1/10 of the rated value in practica, further investigation must be made through study o~ the actual temperature conditions and power consumption of each chip part.
Referring to Fig. 10, there is shown a hiyh frequency heating apparatus H2 according to a second embodiment of the present invention. In addition to the arrangement for the apparatus H1 described earlier with reference to Fig. 2, the apparatus H2 is so arranged as to smooth the output of the detecting circuit 7 by a smoothing circuit 23, the output of the smoothing circuit 23 being fed to an amplifying circuit 24 to be processed a~ter having been converted into a voltage of a level easy to control. Based on the signal after amplification, the control section 8 gives instruction to an inverter power source 25 for controlling the microwave radiating section 3.
Fig. 11 shows an example o~ a printed circuit board llB employed in the apparatus H2 of Fig. 10. The construction o~ the board llB will now be explained in addition to the arrangement of the printed circuit board 11 described earlier with reference to Fig. 4.
In Fig. 11, the microwaves transmitted through the antenna 6 consisting of the patterns on the reverse face of the substrate 11 (shown by dotted lines~ are fed into the -detecting cixcuit 7 (i.e. the upper side portion ~rom a line D-D' in Fig. 11) so as to be detected by the detecting circuit 7 which is constituted by chip parts, such as the Schottky barrier diode 20, etc., and the micro-strip lines, and, after being smoothed by the smoothing circuit 23 ~surrounded by two-~ ~ - .

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2~775 dotted lines), is fed to the amplifying section 24 ~Fig. lo) through lead wires 12.
Reference is also made to Fig. 12 showing an equivalent circuit for the printed circuit board llB of Fig. 11.
In Fig. 12, the antenna 6 is connected to a parallel connection of a capacitor CL and a resistor RL through a resistor RD, the Schottky barrier diode D (20) and the micro-strip line LL ~30), while a junction between the antenna 6 and the resistor RD is connected to a parallel connection o~ a capacitor CB and a resistor RB through a micro-strip line LB -(29). A junction between the diode 20 and the micro-strip line LL (30) is connected to a micro-strip line CS (28), thereby constituting said detecting circuit 7 (shown -surrounded by the one-dotted chain line). A junction between the micro-strip line 30 and the parallel connection of the capacitor CL and resistor RL is connected to an output 12 through a resistor RH (26), with a capacitor 27 across the output to form the smoothing circuit 23.
As was also stated with reference to Fig. 11, the microwaves transmitted from the antenna 5 to the detecting circuit 7 are detected by the Schottky barrier diode 20 and smoothed by the smoothing circuit 230 The micro-strip line 28 -~
is so designed as to short-circuit to ground an output of a frequency in the vicinity of the centre frequency of the electromagnetic waves in the signal, and is thus considered to be a capacitor in terms of high frequency waves. Meanwhile, the micro-strip lines 29 and 30 are each so designed as not to transmit tAe output of the frequency in the vicinity of the centre frequency of the electromagnetic waves to the ~; -subsequent stage, and are each considered to be an inductance in terms o~ the high frequency waves. Xowever, since signals --other than the centre frequency of the electromagnetic waves cannot be removed by the microwave-strip lines 28, 29 and 30 and are therefore transmitted to the subsequent process, they are smoothed by the smoothing circuit 23. Due to the fact that the inverter power source 25 is employed in the present 2~16775 embodiment, high frequency oscillations having switching frequencies (at 20 to 30 KHz in general) as an envelope are employed, and, in order to remove abnormal switching frequencies, the cut-off frequency fc is set at about 13 KHz.
Such conditions are shown in Fig. 13 and thereafter.
Fig. 13 shows the frequency characteristics of ths impedance for the micro-strip lines 28, 29 and 30, with the impedance Z being plotted on the ordinate, and the frequency f on the abscissa. In Fig. 13, curve (a) relates to the micro-strip line 28, the impedance approaching O in the vicinity ofthe centre frequency fo of the electromagnetic wave. Curve (b) relates to the micro-strip lines 29 and 30, the impedance being greatly increased in the vicinity of fo. This diagram shows that the impedance varies when the frequency deviates from a narrow band region of the micro-strip line, and thus, the smoothing circuit 23 is required at a subsequent stage.
Fig. 14 shows the cut-off state of the smoothing circuit.
In Fig. 14, when the frequency f for the abscissa is low, a ratio V2/V1 of the input V1 to the output V2 of the smoothing circuit 23 for the ordinate is OdB (i.e. it is completely passed), but upon a rise of the frequency, the ratio V~/V1 falls greatly (i.e. to cut off). According to one preferred embodiment of the present invention, cut-off is -3dB
at 13 KXz as described earlier, and the switching frequency fl t-30KHz) of the inverter power source 25, and signals of frequencies thereabove are completely cut off.
Figs. 15(a) and 15(b) are graphical diagram~ showing the output of the detecting circuit 7 as it is smoothed by the smoothing circuit 23, and not smoothed thereby, with the ordinate representing the output V of the detecting circuit 7, and the abscissa denoting time t.
Fig. 15(a) relates to the case where smoothing is effected, only the envelope for the commercial power source ~ -frequency (60Hz) remaininy.
Meanwhile, in Fig. 15(b) without smoothing, an oscillation of 30 KHz is noticed, with the 60 Hz set as the .
~,~, '.', ', ,' 2~775 envelope. By the provision of the smoothing circuit 23 the frequency is lowered to oscillation at the low frequency of 60 Hz.
The frequency of the electromagnetic waves or microwaves as detected by the antenna 6 is equal to the oscillatiny freq~lency of the microwave radiating portion 3 having the power source frequency as the envelope, and the output as detected by the detecting circuit 7 assumes a rectified waveform containing a high frequency component, with the amplitude only in the positive dir~ction. If this output is fed to the amplifying section 24 at the subsequent stage, as it is, there is the disadvantage, as shown in Fig. 16, that the degree of amplification is lowered as the frequency is raised due to the frequency characteristic (the abscissa represents frequency f, and the ordinate the open voltage gain Av) of the amplifying section 24 (i.e. a general purpose operational amplifier), and the signal becomes unreliable.
Moreover, if the output of the detecting circuit 7 con~aining the high frequency component is fed to the amplifying section 24 at the subsequent stage through lead wires and patterns in a roundabout passage, noise tends to be picked up, with consequent deterioration of the detecting accuracy. The favourable e~fect of the smoothing circuit 23 can thus be clearly understood.
Subsequently, the electromagnetic wave or microwave detector including the printed circuit board 11, metallic plate 14, and metallic cover 16 will be described in more detail with reference to Figs. 17(a) and 17(b).
As seen from the cross section of Fig. 17(b), in an electromagnetic wave detector having a top plan view as in Fig. 17(a~, the metallic cover 16 is temporarily fi~ed to the --metallic plate 14 by inserting inturned edges 31 of the cover 16 into corresponding openings in the metallic plate 14 for subsequent folding of said edges 31, with the printed circuit board 11 (or llB~ being located therebetween (Fig. 3). Such an electromagnetic wave detector can be moved to any place as ~ -a unit containing the lead wires 12, and has a sufficient ~
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:: . -20~1~775 resistance against noise. In this construction, the printedcircuit board can be prepared by a glass thermosetting material, 1uoroplastic material or the like having a small high frequency loss, and fo~ned with copper oil patterns on opposite faces. Reference is further made to the equivalent circuit of the detecting circuit 7 in Fig. 18.
In Fig. 1~, the antenna 6 is connected to the parallel connection of the capacitor CL (36) and resistor RL
(33) through a resistor RD (32)/ the Schottky barrier diode D
(20) and the micro-strip line LL (30), while the junction between the antenna 6 and the resistor 32 is connected to the parallel connection of the capacitor CB (35) and the resistor RB (34) through the micro-strip line LB (29), the junction between the diode 20 and the micro-strip line 30 being connected to the micro-strip line CS (28). The junction between the micro-strip line 30 and the parallel connection of the capacitor CL (36) and the resistor 33 is connected to one output lead 12 through the resistor RH (26), with the junction between the resistor 26 and the output lead 12 being connected to the other output lead 12 through the capacitor 27.
The function of the detecting circuit 7 will now be described with reference to Fig. 18. ~ -When microwaves are directed from the antenna 6 in the detecting circuit 7, since it is so designed that, with ~ ~
respect to the centre frequency of the microwaves, the micro- ~ -strip lines 29 and 30 become "open" (infin}te impedance~ and the micro-strip line 28 is short-circuited to ground, the high frequency waves are grounded by the micro-strip line 28 through the resistor 32 and the Schottky barrier diode 20. In this case, the output in the positive direction as rectified by the diode 20 flows through the load resistance 33 as a D.C.
current. For forming a D.C. closed loop, the same current also flows through the resistor 34, thus forming the loop as in 34-32-20-33. Thus, the half wave rectified waveform obtained by the current flowing through the load resistance is smoothed by the resistor 26 and capacitor 27 and transmitted - -to the output 12. By way of example, it is to be noted that -~: - .

2~4~775 the parts other than the micro~strip lines in the detecting circuit 7 are all chip parts.
The reason for providing the resistor 32 will now be explained with reference to Figs. 19 and 20. When an electromagnetic wave detector is employed in a high frequency heating apparatus, the amount of power to be detected by the antenna 6 and the detecting circuit output vary largely with the conditions of the food article to be placed in the heating chamber. By way of example, as shown in Fig. 19 in which the abscissa represents weight, and the ordinate denotes the detecting circuit output, a food article having a low initial temperature is represented by curve (a), while one having a high initial temperature by curve (b). The variation extends from Vs min to Vs max.
Fig. 20 shows the voltage-current characteristic of the Schottky barrier diode 20. On the assumption that the resistor 32 (connected in series with the Schottky barrier diode 20) is absent for functioning in a range between O and V
in Fig. 20, the sensitivity (variation rate or inclination) in the vicinity of V1 dif~ers extremely from that in the vicinity of 0. Thus, a judgement without linearity is made, such that a light load as shown in Fig. 19 is easily detected, but the sensitivity is low with respect to a heavy load. On the other hand, when it is so arranged to lose some of the electric power at the resistor 32 by the insertion thereof, the Schottky barrier diode 20 functions in the range between 0 and VZ, and a large linearity can be imparted as compared with the case where the resistor 32 is not present.
Subsequently, the micro-strip lines referred to earlier with reference to Fig. 18 will be explained.
Fig. 21(a) shows a case where a load impedance ZL(38) ~ -~
is connected to a micro-strip line 37 have a length e with a ~ -characteristic impedance Zo. In this case, the impedance Zi is generally represented by -,, "^ ~' ` '~' .

2 ~4~75 +'Z tan~Q l (1) ~ = [Ag] J

where Ag represents a wavelength on the substrate.
When the relation of equation (1) is applied to Fig. 18, the micro-strip line 28 is of a so called open-stub design, not connected with a load impedance as represented by Fig. 21(b). In this case, if equation (1) is simplified by putting Z~ ; ~, Zi = -j Zo cot B e.
Since the pattern length e is set to be equal to Aq , the relation will be Zi = jzocOt ~T = O :
In other words, the condition of short-circuiting is established in terms of the high fre~uency waves.
The micro-strip lines 29 and 30, etc. in Fig. 18 may be considered as in Fig. 21(c), and when capacitors having a relatively large capacity are selected for the capacitors 35 and 36 (or the capacitor 39 of Fig. 21), the load impedance Z~ = l approaches 0, and the resistor, etc. connected in ~
j~C . ' , ' parallel thereto can be neglected. In other words, equation (1) can be simplified as follows by setting ZL=O, : . .

Zi - jZOtanBe Since the pattern length e is selected to be Aa, the relation will be /~ , Zi = jZOtan ~ =
2 ~
i.e. to be "open" in terms of high fre~uency waves. - --::
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19 20~77~ ~
The function referred to earlier with reference to Fig. 18 is thus realized, so that the high frequency waves are not transmitted to the load resistance RL.
The characteristics of the detecting circuit 7 will now be described with reference to the time-charts o~
Figs. 22(a), 22(b) and 22(c) showing the funct:ioning of the detecting circuit 7.
When the input Vin from the antenna 6 is of a sine wave voltage as shown in Fig. 22(a~, the voltage VD applied to the Schottky barrier diode 20 will be as shown in Fig. Z2(b), and voltage component determined by a forward voltage VF as at A-A' in the forward direction remains. Such voltage component becomes large as the forward voltage VF increases, and varies according to the temperature characteristic of said voltage VF. Meanwhile, the current iD flowing through the Schottky barrier diode 20 will become as shown in Fig. 22(c), and in the positive direction, the current at A''-A''' increasing and decreasing according to the temperature rise of the forward voltage VF. on the other hand, in the negative direction, as shown at B-B', the current component determined by the reverse restoration time trr (Fig. 24) remains during the high frequency wave period. Such current component increases as ~-the time trr increases, and varies to correspond to the temperature characteristics of the time trr. In other words, ~
it is indicated that the rectifying function is lost as the ~ -forward voltage VF and the reverse restoration time trr become ;~
larger. Accordingly, it is seen that employment of the Schottky barrier diode having a smaller forward voltage VF and reverse recovery time trr than a fast recovery diode is more effective. - -Reference is made to Fig. 23 showing VF-IF
characteristics (i.e. forward direction voltage - current characteristics) of the Schottky barrier diode, in which curve --~
(a) represents the characteristics at normal temperature, while curve ~b) denotes the characteristics at high temper-ature. From the diagram of Fig. 23, it is seen that the ~ -voltage when the same current is following is reduced by the - -.. ,, - -:

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temperature rise, or the current when the same voltage is being applied is increased by the temperature rise, with the variation rate in the low voltage range being particularly large~
Fig. 24 shows the temperature characteristics of the reverse restoration time trr for the Schottky barrier diode, from which it is observed that the reverse restoration time trr of the ordinate is increased as the temperature T of the abscissa is raised.
From the characteristics of Figs. 22 to 24 as described so far, the input/output characteristics of the detecting circuit 7 will become as shown in Fig. 25, in which the abscissa represents incident power Pin as detected by the antenna 6, and the ordinate denotes the average output Vout of the detecting circuit at that time, the characteristic at normal temperature being represented by (c), and the characteristic at high temperature by (d). In other words, with a temperature rise during the low input period, the -output increases, since the positive current increases due to the characteristic of the forward voltage ~F, while during the high input period, the output is reduced, since the reverse direction current by trr is increased, although the variation due to the forward voltage VF is reduced. The above fact is represented in Fig. 26, when presented in graphical form as the variation rate of the outputs during normal temperature and high temperature. In Fig. 26, the abscissa represents incident power Pin, and the ordinate denotes the variation rate as obtained by dividing the diff2rence between the output at high temperature and the output at normal temperature by -~
the output at normal temperature. Curve te) represents a diode having small temperature characteristics for VF and trr (e.g. the Schottky barrier diode), while curve (f) denotes a diode with large temperature characteristics for VF and trr (e.g. a fast recovery diode).
With respect to high frequency heating apparatus having a rapid temperature rise due to repeated cooking, etc., ~ --it is clear that th- Schottky barrier diode is preferable to . -- . . .
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20~6~75 maintain the detection accuracy. It is to be noted that from the viewpoint of designing, the range for using the detecting circuit is set to be in the vicinity of a point g in the diagram of Fig. 26.
By the arrangements described so far, favourabls ~ffects as follows can be obtained.
(1) Since the antenna is mounted in the top wall portion of the heating chamber, microwaves within the heating chamber can be most effectively detected on the whole without depending on the position(s) where the food article~s) is placed, and thus, stable cooked conditions can be achieved.
Furthermore, particularly in a less expensive apparatus for general family use, the microwave radiating portion and the suction/exhaust port, etc. are provided in the side wall of -the heating chamber in many cases, and by the above construction of the present invention, the antenna is not readily affected by heat and noise of the microwave radiating : . .
portion and by hot air from the exhaust port, etc., whereby to realize detection with high reliability.
(2) Owing to the fact that the antenna is located in the --vicinity of an opening formed in the top wall of the heating chamber, instead of being disposed within the heating chamber, - -a temperature rise due to concentration of microwaves into the -antenna itself or excessive input to the detecting circuit, etc., can be suppressed for high dependability, while -obstruction or danger to a user from the antenna protruding - -into the heating chamber is prevented.

(3) Since the power leakage in the vicinity of the opening, antenna and detecting circuit is reduced to less than 1/10 of the rated power of the parts constituting the detecting circuit, any over-input to the detecting circuit can be prevented, and, therefore, the constituent parts are not readily damaged and adverse influence on users is eliminated ~ --for safety. Moreover, there is not possibility that the leakage power cause noise in or mal~unction of external appliances. ~ `

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(4) By the arrangement to smooth the output of the detecting circuit, it is possible to send a signal to the amplifier at the next stage after suppressing the high frequency wave component remaining in the rectified waveform of the microwaves detected by the antenna, and, therefore, any influence over the *requency characteristic O:e the ampli~ier can be prevented, and stable signal detection and signal processing are possible irrespective of variations of the oscillating frequency of the microwave radiating portion.

(5) Due to the employment of an inverter power source, the detecting circuit output has the oscillation of the switching frequency in the envelope of the power source -frequency, and the signal tends to easily pick up noise or readily generate noise. Accordingly, besides the effect referred to in item (4), since the switching frequency can be suppressed by the provision of the smoothing circuit, the noise factor can be excluded ~or extremely stable detection.
~6) Since the detecting circuit is constituted by the micro-strip lines and chip parts, it is easy to prevent high frequency waves from being transmitted to the circuitry after the diode, and, thus, any adverse effect on the matched state by the attachment of the chip parts in the circuitry after the diode can be suppressed as far as possible. Thus, unnecessary high frequency loss in the chip parts can be avoided, while the characteristic of the detecting circuit becomes very stable, resulting in accurate detection, since no high frequency waves are carried by the detecting output.
(7) As the chip resistance is connected in series with the detecting diode, the linearity of the input/output characteristics of the detecting circuit is increased, thus making it possible to detect with a stable accuracy irrespective of the state of the food article.
(8) Since the open-stub micro-strip line is short-circuited to ground with respect to the centre frequency of the electromagnetic waves and is located on the output side of the detecting diode, the high frequency waves are consumed at the open-stub portion so as not to be transmitted to the - , . .

20~6775 subsec~ent stage. As a result, the effect in item (6) above can be obtained.
(9) Since the Schottky barrier diode is employed as the detecting elemenk, variation of the detecting output due to the temperature ~haracteristics of the forwarcl voltage VF and reverse recovery time trr is small for effecting extremely stable detection. Moreover, the rectifying function is superior, since trr is small for good sensitivity of input and output, and a large output can be achieved even if the lo detecting amount at the antenna is reduced. More specifi-cally, for obtaining an output of the samP level, since the leakage waves towards the surrounding portion can be reduced as well as the detecting amount of the antenna, noise generation for external appliances is decreased to eliminate erroneous functioning, while the apparatus is highly safe for the user.
(lo) As the antenna and the detecting circuit including the micro-strip lines and the chip parts are constructed on -the same substrate, stable matching is achieved between the -antenna and the detecting circuit for detection of the electromagnetic waves with high accuracy.
(11) Since the chip resistance is connected in series with the detecting diode, the linearity of the input/output characteristics for the detecting circuit is increased, thereby making it possible to effect detection with stable accuracy without depending on the input level.
(12) As also stated in above items (8) and (6), in another aspect of the present invention, an open-stub micro-strip line short-circuited to ground with respect to the centre frequency of the electromagnetic waves to be detected - is provided on the output side of the detecting diode, the high frequency waves being consumed at the open-stub portivn so as not to be transmitted to the subsequent stage.
Accordingly, any adverse effect on the matched state by the attachment of the chip parts in the circuitry after the diode can be suppressed as ~ar as possible. There~ore, unnecessary ~-high frequency loss at the chip parts can be avoided, while . . ,:

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the characteristic of the detecting circuit becomes very stable for accurate detection, since no high frequency waves are carried by the detecting output.
(13) As was also stated in the above item (9), in a further aspect of the present invention, as a Schottky barrier diode is employed as the detecting element, variation of the detecting output due to the temperature characteristics of the forward voltage VF and the reverse recovery time trr is small for achieving stable detection. Moreover, the rectifying function is superior, since trr is small for good sensitivity of input and output, and a large output can be achieved even if the detecting amount at the antenna is small.
Although the present invention has been fully described by way of example with reference to the accompanying -~ -drawings, it is to be noted here that various changes and modifications will be apparent to those skilled in the art.
Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as included therein.
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Claims (20)

1. A high frequency heating apparatus comprising:
a heating chamber having an inner top wall, an inner bottom wall, and inner side walls, said inner bottom wall for resting a food article thereon;
microwave generating means for generating microwave energy within said chamber for heating the food article;
an aperture in said inner top wall for passing microwave energy therethrough;
a receiving antenna means, located in close vicinity to said aperture and external said heating chamber, for receiving said microwave energy passing through said aperture;
a detecting circuit, coupled to said receiving antenna means, for detecting an amount of said microwave energy received by said receiving antenna means; and, control means, coupled to said detecting circuit, for controlling operating functions of the high frequency heating apparatus in accordance with said amount of said microwave energy detected by said detecting circuit.

2. A high frequency heating apparatus as recited in claim 1, wherein said control means is further for determining a thawing state of a food article based on said amount of said microwave energy detected by said detecting circuit.

3. A high frequency heating apparatus as recited in claim 1, wherein said microwave generating means includes a microwave radiator located at one of said inner side walls of said chamber.

4. A high frequency heating apparatus as recited in claim 2, wherein said microwave generating means includes a microwave radiator located at one of said inner side walls of said chamber.

5. A high frequency heating apparatus as recited in claim 1, further comprising a substrate member having opposite first and second surfaces, said first surface confronting said aperture and having said receiving antenna means disposed thereon, said second surface having said detecting circuit disposed thereon.

6. A high frequency heating apparatus as recited in claim 4, further comprising a substrate member having opposite first and second surfaces, said first surface confronting said aperture and having said receiving antenna means disposed thereon, said second surface having said detecting circuit disposed thereon.

7. A high frequency heating apparatus as recited in claim 5, further comprising a smoothing circuit, operatively interposed between said detecting circuit and said control means and disposed on said second surface of said substrate member, for smoothing an output of said detecting circuit.

8. A high frequency heating apparatus as recited in claim 6, further comprising a smoothing circuit, operatively interposed between said detecting circuit and said control means and disposed on said second surface of said substrate member, for smoothing an output of said detecting circuit.

9. A high frequency heating apparatus as recited in claim 1, wherein said detecting circuit comprises plural micro-strip lines and plural chip component parts including a detecting diode which is operatively coupled to said receiving antenna means.

10. A high frequency heating apparatus as recited in claim 5, wherein said detecting circuit comprises plural micro-strip lines and plural chip component parts including a detecting diode which is operatively coupled to said receiving antenna means.

11. A high frequency heating apparatus as recited in claim 1, wherein said detecting circuit comprises a chip component Schottky barrier diode.

12. A high frequency heating apparatus as recited in claim 9, wherein said detecting diode is a chip component Schottky barrier diode.

13. A high frequency heating apparatus as recited in claim 10, wherein said detecting diode is a chip component Schottky barrier diode.

14. A high frequency heating apparatus as recited in claim 9, further comprising a chip component resistance connected in series with said receiving antenna means and said detecting diode.

15. A high frequency heating apparatus as recited in claim 13, further comprising a chip component resistance connected in series with said receiving antenna means and said detecting diode.

16. A high frequency heating apparatus as recited in claim 9, wherein one of said plural micro-strip lines is an open-stub micro-strip line which short-circuits to ground with respect to a center frequency of said microwave energy and which is connected to an output side of said detecting diode.

17. A high frequency heating apparatus as recited in claim 13, wherein one of said plural micro-strip lines is an open-stub micro-strip line which short-circuits to ground with respect to a center frequency of said microwave energy and which is connected to an output side of said detecting diode.

18. A high frequency heating apparatus as recited in claim 1, wherein a microwave energy in a vicinity of said receiving antenna means and said detecting circuit is less than 1/10 of a rated power of component parts constituting said detecting circuit.

19. A high frequency heating apparatus as recited in claim 5, wherein a microwave energy in a vicinity of said receiving antenna means and said detecting circuit is less than 1/10 of a rated power of component parts constituting said detecting circuit.

20. A high frequency heating apparatus as recited in claim 10, wherein a microwave energy in a vicinity of said receiving antenna means and said detecting circuit is less than 1/10 of a rated power of component parts constituting said detecting circuit.