Note: Descriptions are shown in the official language in which they were submitted.
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
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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
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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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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
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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
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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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13
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
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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
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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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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
17
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
18
+'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. - -
-
::
' ~
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 - -
.. ,, - -:
.,,.,~, , ~., .
'' , ' '.
7 7 5
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
. -- . . .
. "
20~6~75
21
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. ~ `
.,, ,:
P.
-` ~0~677~
22
(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
23
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
. . ,:
`` 204~7~
24
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.
.
. .
.''"~ ' . .:
. : .
,, .
- ' '
