Note: Descriptions are shown in the official language in which they were submitted.
~2~4~3~ DRUG
AUTOMATIC DEFROST SENSING ARRANGEMENT FOR MICROWAVE OVEN
BACKGROUND OF THE INVENTION
. . _ _ . .
The present invention relates broadly to a
system and method for defrosting objects in a microwave
oven. More specifically, the present invention relates
to a system and method for distinguishing between the
frozen state and -the thawed state of an object being
heated in the cooking cavity of a microwave oven to
detect the transition from the frozen state to the
thawed state, and using such information to control
oven operation in a defrost operating mode.
It is well known in the art that the dielectric
constant for water is substantially greater than that for
ice to the extent that the response to the microwave
excitation system to ice in the cooking cavity generally
approaches that of a no-load condition. Since generally
foods contain a large percent by weight of water, the
dielectric constant for a food load in its thawed state
is typically substantially higher than the dielectric
constant for the same load in its frozen state. In many
domestic microwave cooking oven designs in present use,
the ratio of input power to reflected power back to the
magnetron is sensitive to variations in the dielectric
constant of the food load being heated in the oven.
In such ovens, it is known to monitor the microwave
input reflection coefficient in the oven -to detect the
change in reflection coefficient indicative of the
beginning of the transition of the food load from its
frozen state to its thawed state. One example of
such a control system and method can be found in US.
Patent 4,210,795, to Lent. In the Lent system the
magnetron power output level is switched from a high
-- 1 --
I 9 DRUG
to a low level upon detection of a reflection
coefficient less than a predetermined reference
value indicated that the food load in the oven has
begun to thaw.
Such an approach works satisfactorily in
those microwave ovens which are particularly
sensitive to changes in the dielectric constant of the
food object being heated. However, the cooking
performance of a microwave oven would be greatly
enhanced if operating parameters of the excitation
system for the oven would be relatively insensitive
to variations in dielectric characteristics of food
loads heated therein. An example of one such oven
is described in United States Patent Number
4,458,126, issued July 3, 1984 to Dills et at.
In the Dills et at oven during normal operation,
changes in such magnetron operating parameters as
the voltage standing wave ratio and the phase of
the standing wave in the wave guide for foods in the
frozen and thawed states, respectively, are relatively
indistinguishable when sensed by a sensor in the wave-
guide. ions, an arrangement such as that described
in the Lent patent would require a very high
precision measurement system capable of resolving
very small changes in the measured parameters.
From the foregoing, it is apparent that
the more a microwave oven system is optimized to
provide a relatively stable magnetron operating point
for food loads over a wide range of dielectric
constant values, the more difficult it becomes to
distinguish the frozen state from the thawed state
for foods being defrosted in the oven based upon the
So DRUG
difference in the operating parameters measurable in the
wave guide. It would be desirable, therefore, to
provide a defrost detection system for such an oven,
which system effectively distinguishes between thawed
and frozen states of food objects heated therein as a
function of the change in the dielectric constant
as the food object converts from its frozen state to
its thawed state.
It is therefore a primary object of the
present invention to provide a method and a system for
distinguishing between the frozen state and the thawed
state of a food load being heated in the cavity for a
microwave oven which in normal operation demonstrates
relatively little variation in voltage standing wave
ratio and phase for loads over a wide range of
dielectric constant values, including ice and water.
SUMMARY OF TOE INVENTION
The present invention provides a system and
method for defrost detection particularly applicable
to a microwave oven having an excitation system which
normally exhibits relatively little change in voltage
standing wave ratio and phase for loads of widely
varying dielectric constant values. In accordance with
the invention, means are provided for the periodic
introduction of a discontinuity in the wave guide coupling
microwave energy from the source to the cooking cavity.
The discontinuity is effective to cause a substantial
change in the magnitude and phase of the electromagnetic
field in the wave guide for food objects in the frozen
state, while causing relatively little change in these
parameters for the same food objects in the thawed state.
Hence, the presence of the discontinuity in the wave-
-- 3
~Z4S37 9 DRUG
guide provides a readily detectable difference in field
strength at the sensor location in the wave guide
between an object in its frozen state and the same
object in its thawed state.
Sensing means responsive to the strength
of the electromagnetic field at a predetermined
location in the wave guide generates an output signal
representative of field strength at that location.
Defrost detection means sample the output signal from
the sensing means while said discontinuity is present
in the wave guide to detect a predetermined relation-
ship between the signal and a reference which when
detected indicates the food load has converted from
its frozen state to its thawed state. The periodic
introduction of the discontinuity temporarily trays-
forms the excitation system to a modified system which
is particularly sensitive to the difference in the
dielectric constant of loads in the frozen state
relative to loads in the thawed state to the extent
that difference in phase and the voltage standing wave
ratio between the frozen and thawed states of the food
load are readily resolvable without resort to costly
high precision sensing circuitry.
BRIEF DESCRIPTION OF THE DRAWINGS
While the novel features of the invention
are set forth with particularity in the appended
claims, the invention both as to organization and
content will be better understood and appreciated
from the following detailed description, taken in
conjunction with the drawings in which:
FIG. 1 is a front perspective view of a
microwave oven;
S37 9 DRUG
FIG. 2 is a front schematic sectional view
of the microwave oven of FIG. 1 taken along line
2-2;
FIG. 3 is a schematic side view partially
in section of the microwave oven of FIG. 1 with portions
removed to illustrate details of the illustrative
embodiment of the present invention;-
FIG. 4 is an enlarged view of the detector shown in FIG. 2 mounted in a section of the wave guide
for making electrical field strength in the wave guide
of the oven;
FIG. 5 is a Wreck diagram for the excitation
system of the oven of FIG. 1 showing the operating
region for the magnetron under different load conditions;
FIG. 6 is a partial enlarged view of the oven
of FIG. 3 taken along lines 6-6 showing details of the
means for introducing a discontinuity into the oven
wave guide in accordance with the present invention;
FIG. 7 is a partial enlarged view taken along
lines of FIG. 6 showing additional details of the
means for introducing a discontinuity into the wave guide
of the oven;
FIG. 8 is a simplified schematic circuit
diagram of that portion of the microprocessor based
control system of the oven of FIG. 1 illustratively
embodying the present invention; and
FIGS. PA and 9B are flow diagrams illustratively
embodying alternative defrost mode control algorithms
implemented in the microprocessor in the circuit of FIG.
8 in accordance with one aspect of the present invention.
DETAILED DESCRIPTION
Referring now to FIGS. 1-3, there is shown a
microwave oven designated generally 10. The outer
4~37 gD-RG-14976
cabinet comprises six cabinet walls including upper and
lower walls 12 and 14, a rear wall 16, two side walls
18 and 20, and a front wall partly formed by hingedly
supported door 22 and partly by control panel 24. The
space inside the outer cabinet is divided generally
into a cooking cavity 26 and a controls compartment
28. The cooking cavity 26 includes upper wall 30, a
bottom wall 32, side walls 34 and 36, the rear cavity
wall being cabinet wall 16 and the front cavity wall
o'er a /
' 10 being defined by the interface 38 of door 22. my
dimensions of cavity 26 are 16 inches wide by 13.67
inches high by 13.38 inches deep. The support plate
pie r owe I
40 of microwave _ dielectric material such
as that commercially available under the trade mark
PYROCERAM OR NEWSROOM is disposed in cavity 26
substantially parallel to bottom cabinet wall 14.
Controls compartment 28 has mounted therein
a magnetron 42 which is adapted to produce microwave
energy having a center frequency of approximately
2450 MHz at output probe 44 thereof when coupled to
a suitable source of power (not shown) such as the
120 volt AC power supply typically available at
domestic wall receptacles. In connection with the
magnetron 42, a blower designated generally 46
provides cooling airflow for channeling airflow over
the magnetron cooling fins 48. The front facing
opening of the controls compartment 28 is enclosed by
control panel 24. It will be understood that numerous
other opponents are required in a complete microwave
oven, but for clarity of illustration and description
only those elements believed essential for a proper
understanding of the present invention are shown and
~Z4~7 9 DRUG
described. Such other elements may all be conventional
and as such are well known to those skilled in the art.
The excitation system for oven 10 is a dual
feed system having a rotating antenna 50 supported from
top cavity wall 30 and a slotted radiating chamber 52
extending centrally along the bottom wall 32 of cavity
26.
The source of microwave energy for the
excitation system of oven 10 is magnetron 42. Microwave
energy from magnetron output probe 44 of magnetron 42
is coupled to the antenna 50 and the slotted wave guide
52, respectively, by wave guide means comprising a
central section 52 which houses magnetron output probe
44, a first section 56 extending generally centrally
along the upper cavity wall 30 to couple energy from
probe 44 to antenna 50 and a second section 58 running
in a vertical direction generally centrally along
cavity side wall 34 to couple energy from probe 44 to
chamber 52. A rounded step 60 formed at the junction
of first and second sections 56 and 58, respectively,
divides the power from magnetron 42 between these
sections, matches the impedance of the system to the
magnetron, and facilitates excitation of the antenna
50 and the slotted chamber 52 in phase.
Wave guide section 56 is of generally
rectangular cross section and generally formed by
member 62 of generally U-shaped cross section and top
cavity wall 30. End wall 64 of section 56 provides
a short circuit termination for section 56. Second
wave guide section 58 is also of generally rectangular
cross sectiGnbeing generally formed by member 66 of
U-shaped cross section and side wall 34. The end wall
Z~53~
DRUG
68 of section 58 remote from magnetron 42 forms a
standard 45 degree transition bend to guide energy
propagated in section 58 to opening 70 which opens
into radiating chamber 52. The 45 degree bend
provides a low loss transition with no phase change
nor power dissipation. Both sections are dimensioned
to support a Tell propagating mode. Specifically, the
width (the dimension running front to rear of the
cavity) is more than one-half but less than one guide
wave length and the height is less than one-half
guide wave length. The height of sections 56 and 58
is nominally .75 inches and the width is nominally 3.6
inches.
Central wave guide section 54 is a generally
rectangular enclosure which is formed on top and sides
by extension of member 62 beyond cavity 26 and on the
bottom by support flange 76. Section 54 serves as a
launching area for microwave energy radiated from
magnetron probe 44 enclosed therein. Conductive end
20 wall 78, spaced approximately 3/4 inch from probe 44,
provides a short circuit wave guide termination. The
spacing is in accordance with magnetron manufacturer
recommendations for proper operating characteristics.
Section 54 is of the same width as sections 56 and 58
but of significantly greater height (on the order of
two inches) with an open end facing the rounded step
60 formed at the intersection of side wall 34 and
top wall 30.
Energy radiation from probe 44 within central
section 44 propagates to the vicinity of step 60 where
sections 56 and 58 join section 54. At this juncture,
the energy splits with a first portion propagating in
~4S37
DRUG
the first section 56 and a second portion propagating
in the second section 58, the fraction of the total
energy apportioned to each being a function of the
impedance presented to the magnetron at the entrance
to each section. It is believed that the curve step
at 60 (radius of curvature nominally .64 inches)
forms a junction which renders the sending impedance
for both sections 56 and 58 more sensitive to antenna
and food load impedance variations than would be the
0 case with a more conventional bifurcator or power
of the type projecting sharply into the
junction region for power splitting.
Antenna 50 comprises a center fed microwave
strip line 80 extending substantially parallel to top
cavity wall 30 vertically spaced from top wall 30
by a nominal distance of 1/4 inch (approximately .05
free space wavelengths). Strip line member 80 is
terminated at each end by vertical radiating members
82 and 84 which extend in a direction away from top
wall 30 at an angle to strip line 80 to provide
predominantly TM mode excitation in the cavity. As
the antenna rotates, it passes through positions of
optimum coupling of certain modes in the cavity.
In oven 10, members 82 and 84 extend at an angle of
90 degrees relative to strip line member 80.
Strip line member 80 and radiating members
82 and 84 are formed from a metallic strip preferably
of approximately 1/2 inch (.1 free space wavelength)
in width and approximately .025 incus (.006 free
space wavelengths) in thickness. The length of
each of radiating members 82 and 84, respectively,
is nominally one inch (slightly less than 1/4
Irk
f ED RG-14976
free space wavelength). Dimensions Lo and Lo are
preferably selected equal so that the radiating members
82 and 84 are fed in time phase with each other. The
length for Lo and Lo in oven 10 is a nominal length of
four inches (approximately 7/8 free space wavelengths)
to provide the desired impedance match for radiating
members 82 and 84. Energy from wave guide section 56 is
coupled to strip line member 80 by conductive metallic
antenna probe 86. Antenna probe 86 includes a
cylindrical portion 88 terminating at one end in an
impedance matching capacitive cap 90 which extends into
the interior of wave guide section 56 for coupling
therewith. Probe 86 is located at an integral multiple
of 1/6 guide wavelengths from end wall 64 of guide
section 56 for tight coupling in accordance with known
design practice to contribute to the desired high
sending impedance at the entrance to section 56.
Wave guide section 56 extends a distance of 4/6 guide
wavelengths beyond probe 86 to provide structural
support to top cavity wall 30. The extent of penes
traction by probe 86 into guide section 56 is adjusted
to provide the desired coupling. The maximum extent
being limited by requirement for sufficient clearance
between cap section 90 and upper wall 68 of guide section
64 to prevent arcing. In the illustrative embodiment,
this gap is nominally set at .12 inches. Capacitive
cap 90 provides the desired equivalent electrical
length for probe 86 for good impedance matching and
coupling of energy from wave guide 56. Probe 86 is
rotatable supported in top cavity wall 28 by a
dielectric bushing 92. A microwave energy transparent
antenna cover 94 of truncated conical configuration is
-- 10 -
~45;~7 9 Draggle
provided to enclose the antenna 50 to protect it from
mechanical interference with items placed in cavity 24
and to keep it clean.
Antenna 50 is rotated by electric motor 96
which is drivingly coupled to antenna 50 by a pulley and
belt arrangement including pulley 98 supported from
antenna probe 86 and pulley 100 supported from drive
shaft 102 of motor 96. Pulleys 98 and 100 are drivingly
coupled by drive belt 104.
Microwave energy is coupled to the lower region
of cavity 26 by rectangular radiating chamber 52 which
extends centrally along the bottom wall of cavity 24.
Chamber 52 is formed by a channel member of generally
U-shaped cross-section having a top wall 106 and integral
side walls 108. The U-shaped member is suitably secured
to a flat central section 110 of the bottom wall 32 of
cavity 26, such as by welding. Energy from wave guide
section 58 enters chamber 52 through open end 70.
Chamber 52 is terminated at its opposite end by end
wall 112 which provides a short circuit termination for
chamber 52. The height and width dimensions of chamber
52 are chosen in the conventional manner herein before
described with reference to wave guide sections 56
and 58 to support a Tell mode varying with the width
being as thin as those sections and the height being
nominally .79 inches. Chamber 52 extends across a
substantial portion of cavity 26 so as to provide the
desired energy distribution pattern. However, the
exact length thereof is chosen to provide the proper
impedance back to the entry port of wave guide section
58.
537 9 DRUG
Top wall 106 of chamber 52 has formed therein
an array of radiating slots 114 arranged to establish a
particular substantially stationary radiation pattern
in the cavity 26. Specifically, the slots are arranged
to provide a radiating pattern which provides cooking
regions of relatively high energy density which fill
in areas of the antenna radiating pattern of relatively
low energy density. Each of radiating slots 114 is
constructed as a non-resonant slot, that is, -the
longitudinal axis of the slot is oriented crosswise
to the direction of propagation in chamber 52. The
dimensions of the slots are chosen to be evenly
distributing the energy along the radiating chamber and
to provide the desired impedance matching. Specifically,
slot length is chosen at less than one-half the guide
wavelength so as to provide non-resonant slots. This
assures that energy is relatively evenly distributed
along the length of chamber 52 rather than radiating
from those slots nearest the entrance of the chamber.
As herein before described, support plate 40
is disposed in cavity 26 for supporting food items to be
heated in the cavity. The spacing of plate 40 above
chamber 52 is selected for desired impedance matching
This spacing significantly affects energy intensity
at the bottom of food loads supported on plate 40. A
nominal spacing of approximately .18 inches was
selected for the oven of FIG. 10 to provide satisfactory
performance for a wide range of typical food load sizes.
Support plate 40 also serves as a refracting member for
energy radiated from radiating chamber 52 as well as
energy reflected from bottom cavity wall 32. The
refracting function of plate 40 tends to laterally
~24S37 9 DRUG
spread the energy radiating pattern radiated from slots
114 to more widely distribute this energy in cavity 26.
Bottom wall 32 of the oven cavity has
surfaces 116 and 118 which are bent or sloped upwardly
from flat central section 110 to front and rear walls,
respectively, of the cavity. These surfaces operate
primarily to reflect microwave energy from the
antenna 50 upwardly and centrally toward the food to
be heated which is usually located in the central
portion of the oven. To this end, the reflective
surfaces are bent upwardly at an angle to the horizontal
of between three and fourteen degrees. The exact angle
is chosen based on various parameters such as dielectric
constants of typical foods to be cooked in the oven and
its location in the oven cavity. In oven 10, this angle
is about 8 degrees to the horizontal.
It has been empirically determined that for
most food loads satisfactory cooking performance for
the dual feed system of the oven of FIG. 1 is
achieved when more power is radiated from the top than
from the bottom. Thus, in designing the excitation
system for oven 10, those parameters bearing on the
impedance presented at the entrance to each wave guide
section such as guide wave lengths, antenna parameters,
and slot configurations, have been selected in accordance
with standard design practices to provide impedance
matching which results in greater apportionment of
the energy from the magnetron being coupled to
antenna 50. In oven 10, these parameters are selected
to provide a high impedance at both points with the
relative impedance being balanced to provide a nominal
power spread of 50-75 percent of the total power
~Z24~37
DRUG
I
-` going to section for most loads.
As herein before briefly described, the energy
delivered to the central wave guide section 54 from
magnetron 42 is split between wave guide sections 56
and 58 as a function of the impedance presented by
the junction of each with central section 54. The
impedance presented to magnetron 42 by antenna 80 at
the entry port of guide section 56 varies with time
as the antenna rotates. The initial impedance
presented by slotted chamber 52 at the entry port
for the guide section 58 at the beginning of -the
cooking cycle is a function of the food load parameters
the size, dielectric constant, etc. In addition, as
the food cooks, certain parameters such as the
dielectric constant change, altering the impedances
at both entry ports, but particularly at the entry
port to the second section as seen by magnetron 42.
Hence, the fractional apportionment of energy to the
guide sections 56 and 58 varies as the impedances
presented at their respective entry ports change and
thus adapt initially to the food load and also changes
as the food load characteristics change during the
cooking process.
Additional details as to the structure and
manner of operation of oven 10 may be found in
US. Patent No. 4,458,126, issued July 3, 1984
to Dills et at, entitled, "Microwave Oven with Dual
Feed Excitation System", mentioned briefly in the
Background discussion.
As discussed briefly in the Background
section, it is an object of the present invention
to provide a system and method for implementing a
- 14 -
~L2~4S37
9~-RG-14976
defrost mode in such a microwave oven. In a
satisfactory defrost mode, a frozen food load is
converted from a frozen state in which the food
object is essentially a solid or brittle unworkable
mass to a thawed state in which the food object is
sufficiently thawed to be malleable or pliable
enough for manipulations typically associated with
food preparation such as forming ground meat patties
or meat balls, or having centers that could be pierced
or broken apart with a fork but not sufficiently heated
to begin actual cooking of the food object. Hence,
for purposes of the description to follow, the terms
"frozen state" and "thawed state" will be understood
to connote the following characteristics of the food
object to which-the terms are applied. An object is
considered to be in its frozen state when it is
essentially of solid or hard consistency, typically
at a temperature of less than 15F. An object is
considered in its thawed state when sufficiently
malleable or pliable to enable manipulation for food
preparation and typically at a temperature in excess
of 25F. For example, a 2-lb. mass of hamburger in
its frozen state will be a single solid unworkable mass.
In its thawed state, the user will be able to readily
break the mass into smaller pieces. Ice particles
may be present in the mass, but the meat will be
sufficiently thawed to be hand workable. For
different types of foods, the temperature of the food
object in each state may be different; hence, a workable
consistency rather than an actual temperature of the
food object is the characteristic of primary concern
when defrosting objects in a microwave oven.
- 15 -
~2.24~37 DRUG
In accordance with the present invention,
the transition of a food item from its frozen state to
its thawed state is detected by monitoring the field
strength in the wave guide. To this end, a crystal
field detector 120 is mounted to wave guide section
56. As best seen in FIG. 4 detector 120 comprises a
crystal detector 122 supported in a generally cylindrical
base support member 124 suitably secured to wave guide
wall 62. Crystal 122 may be a standard crystal
detector such as that commercially available and
identifiable by the designator IN 32. A probe 126
extends from crystal 122 into the interior of
wave guide section 56 through aperture 127 formed in
wave guide wall 62. Probe 126 and crystal 122 are
enclosed within base member 124 by a dielectric sleeve
128. Crystal 122 is electrically coupled to the
control circuitry via coaxial cable 129, comprising an
inner conductor 130; a dielectric core 131 surrounding
I
it- inner conductor 130, a conductive shield layer I and
a protective dielectric outer layer 133 of a polyvinyl
chloride material or similar material. A conductive
spring member 134 is secured at one end to inner
conductor 130. The opposite end engages contact
surface 135 of crystal 122. When fully assembled,
spring member 134 is compressed between conductor 130
and contact surface 135 to ensure good electrical
contact with crystal 122. A generally cylindrical
coupling member 136 for connecting coaxial cable 129
to detector 120 is connected to cable 129 at its
annular neck portion 137 of reduced diameter. The
inner conductor 130 and dielectric core 131 of
coaxial cable 129 are received with neck portion 137
- 16 -
537
DRUG
with the shielding layer 132 and outer protective layer
133 extending over the exterior of neck portion 137.
A ring clamp 138 secures these layers to the neck. The
opposite end of coupling member 136 is threaded onto
base member 124. The electromagnetic field in wave-
guide 56 produces a voltage in the probe of the field
detector proportional to and representative of
field strength in the guild e at the probe.
The design of oven 10 is such that during
normal operations the field strength sensed by
detector 120 will show relatively little difference
between a given food load in its frozen state and
in its thawed state. Hence, in its normal operation
it would be difficult to reliably identify the
transition from the frozen state to the thawed state.
The Wreck diagram of FIG. illustrates the
operating characteristics of the excitation system
of oven 10 in terms of the performance characteristics
of magnetron 42. The performance characteristics of a
magnetron such as output power and operating frequency
depend on the load presented to the magnetron comprising
the wave guide, the cavity and objects placed in the
cavity. The Wreck diagram of FIG. 5 is a polar plot,
the coordinates of which express load impedance in terms
of phase and magnitude of the reflection coefficient at
the magnetron probe.
The Wreck diagram includes an arrangement of
concentric circles which indicate the reflection factor
or voltage standing wave ratio (VSWR); the center
indicating a VSWR of 1, and the progressively larger
circles indicating higher VSWR's as shown. A series
of straight lines radiate from the center calibrated
~Z~537 DRUG
in fractional wavelengths as indicated on the perimeter
of the chart, indicating the phase position of the first
minimum of the electric field relative to the magnetron
probe.
During normal operation of oven 10, the
operating point for substantially all typical food loads,
thawed or frozen, heated in cavity 26 lies in the cross-
hatched region designated A, corresponding to standing
wave ratios in the range of 1~1 to 1.6 with relative phase
of .30 to .40 guide wavelengths. In normal operation, a
given food load both in its frozen state and in its
thawed state would have an operating point in this region.
Hence, it would be very difficult to reliably distinguish
between frozen and thawed states by monitoring field
characteristics in the wave guide when operating in this mode.
In accordance with the present invention,
this difficulty is overcome by periodically introducing
a discontinuity into the wave guide and sampling the field
strength detected by detector 120 with the discontinuity
present in the wave guide. It has been empirically
determined that in the oven of FIG. a discontinuity
may be located in wave guide section 58 which introduces
a significant difference in both the standing wave ratio
and the phase of the standing wave in wave guide section
56 for a food object in its frozen state relative to that
same food object in its thawed state. This discontinuity
in effect significantly shifts the operating point of
the excitation system of the Wreck diagram (FIG. 5) for
a food object in its frozen state relative to that for
the same food object in its thawed state. In oven 10,
the operating point for thawed food objects is
relatively unaffected by the introduction of the
- 18 -
4~;37
DRUG
discontinuity. However, for frozen food objects both
the magnitude of the VOW and the phase of the standing
wave are substantially altered, introducing a readily
detectable difference in field strength at the sensor
between frozen and thawed state. More particularly,
when the discontinuity is introduced into ~aveguide,
the operating point for a frozen food load falls in the
crosshatched region designated B in FIG. 5, while the
operating point for the same load in its thawed state
continues to lie in Region A. Region B is characterized
by standing wave ratios on the order of 1.75 to 5 and
phase angles on the order of .10 to .15 guide wavelengths.
The introduction of the discontinuity in effect forces a
phase shift for the field in the wave guide on the order of
one quarter guide wavelength for a food load in its
frozen state relative to its thawed state. Hence, by
locating a sensor at a position along the wave guide where
the field strength is a maximum for a frozen food load
with the discontinuity present, field strength at that
location for the same food load in its thawed state will
be a relative minimum, rendering the transition from
frozen state to thawed state readily detectable without
the need to resort to high resolution sensing devices.
In the illustrative embodiment, as best
seen in FIGS. 3, 6 and 7, the means for introducing a
discontinuity into the wave guide comprises discontinuity
plate 144 suitably secured in wave guide section 58 for
movement between a neutral position in which the plane
of plate 144 is essentially parallel to the longitudinal
axis of the guide, i.e., parallel to the direction of
propagation of energy in the guide; and an active
position in which the plane of plate 144 is generally
-- 19 --
53~
DRUG
perpendicular to the longitudinal axis of the guide,
transverse to the direction of energy propagation.
In the neutral position, plate 144 has no substantial
effect on energy propagation in the guide. In the
active position plate 144 provides a short circuit
termination for guide section 58. The position of
plate 144 is controlled by short stroke low power
solenoid 146 suitably mechanically linked to plate 144.
Plate 144 comprises a generally rectangular
planar member formed of 20 gauge sheet metal, having a
length slightly less than the width of guide section 58
to permit ample clearance and prevent arcing. Similarly,
the width of the member 148 is slightly less than the
height of guide section 58. The corners of the member
are tapered to minimize potential arcing. Mounting pins
150 and 152 extend from opposite ends of plate member 148,
through small apertures formed in opposing side walls of
section 58, the longitudinal axes of said pins defining
the axis of rotation of plate 144. Dielectric bushings
20 154 and 156 rotatable support pins 150 and 152,
respectively, in the side wall apertures. Control arm
158 extends from pin 152 as part of the mechanical linkage
to solenoid 146. Linking member 160 couples the free end
of control arm 158 to solenoid plunger 162. FIG. 6
illustrates the neutral position for discontinuity 144
with solenoid plunger 162 extended in full lines.
The active position for discontinuity 144 with plunger
162 withdrawn is illustrated in phantom.
When in its active position, discontinuity 144
effectively halts energization of slotted chamber 52,
thereby altering the characteristics of the excitation
system of oven 10 to the extent that, as herein before
- 20 -
453~ DRUG
described, the operating point on the Wreck diagram
shifts to Region B when frozen objects are present in
the cooking cavity.
The positioning of discontinuity 144 along
the length of wave guide section 158 and the positioning
of detector 120 along the length of wave guide section 56
interactively effect the performance of the defrost
detection arrangement provided by the present invention.
The position of discontinuity 144 is empirically selected
at a location which forces a one-quarter guide wavelength
shift in the phase of the standing wave in the guide
section 56 when an object in its frozen state is being
heated in cavity 26. Detector 120 is empirically positioned
at a maximum field strongly point for the frozen food load
with discontinuity 144 in its active position. Hence, as
the food approaches its thawed state, the operating point
shifts from Region B to Region A, as the quarter wavelength
phase shift decreases. As the phase shifts toward that
for food in a thawed state, the voltage maximum point shifts
away from detector 120. Hence, the transition from the
frozen state to the thawed state may be detected by
detecting a decrease in the field strength sensed by detector
120 below a suitably selected reference level. In the
oven of FIG. 2, detector 120 is positioned approximately
midway between antenna probe 86 and the entry port of
guide section 56. Discontinuity plate 144 is located
approximately midway between the entry port of guide
section 58 and open end 70 of radiating chamber 52. The
exact position for each is empirically determined by
adjusting each to detect a maximum voltage or field
strength at the detector location when the oven is
operating in Region B of the Wreck diagram
- 21 -
~Z2~3~ 9 D-RG-14g76
Referring now to the simplified schematic
circuit diagram of FIG. 8, field detector 120 is
incorporated in a microprocessor based control
arrangement which illustratively embodies the
apparatus and performs the method of the present
invention. The circuit of FIG. 8 includes a field
sensing circuit 164 for sensing the field strength of
the electromagnetic field supported in the wave guide
at detector 120 and generating a voltage signal having
a magnitude representative of the sensed field strength;
a microprocessor 166 for sampling and processing the
voltage signal from sensing circuit 164 and controlling
magnetron energization to implement a defrost mode
algorithm; a magnetron power circuit 168; a solenoid
circuit 170 responsive to microprocessor 166 to
selectively position discontinuity member 144; and a
user alert circuit 172 responsive to microprocessor
166 for providing an audible signal to the user signifying
the end of the defrost mode.
Microprocessor 166 is a standard TAMS 2670
Series OK microprocessor of the type readily commercially
available from Texas Instruments. The ROM of microprocessor
166 has been customized to perform the desired control
functions for microwave oven 10.
Field sensing circuit 164 includes detector
120 which as herein before described comprises probe
126 extending into waveguidesection 58. Probe 26 is
electrically connected to field sensitive crystal 122.
The output signal from detector 120 may be on the order
of millivolts. Hence, this signal is coupled to
amplifying circuit 174 by suitably shielded coaxial
cable 129. Amplifying circuit 174 includes a
~;Z;Z 4537
DRUG
conventional operational amplifier 178 and a voltage
divider network comprising resistors 176 and 180 which
determine the gain of the amplifier network. The
non-inverting input of amplifier 178 is coupled to
inner conductor 130 from detector circuit 164. Resistor
180 is coupled between the output of conventional
amplifier 178 and its inverting input. Resistor
176 is connected between the inverting input and ground.
The values of resistors 176 and 180 are selected to
provide the desired gain in accordance with standard
circuit design practice. The amplifier voltage signal
VOW which appears on line 182 is proportional to the
sensed field strength at detector probe 126. VOW is
coupled to an appropriate input port of microprocessor
166 via line 182.
Microprocessor 166 is programmed to periodically
generate a control signal effective to switch discontinuity
member 144 into its active position for sampling periods
of predetermined duration and to monitor the output signal
provided by field sensing circuit 164 during these periods.
A suitable sample control signal is coupled via line 184
from an output of microprocessor 166 to solenoid coil 186
I
of solenoid 146 via conventional driver circuit I. When
I
coil is energized solenoid plunger 162 is retracted and
discontinuity plate 144 is in its active position. In the
illustrative embodiment, the frequency of the sampling
interval is on the order of three samples per minute with
a duration of the sampling period on the order of one
second.
Detector monitoring means responsive to the
voltage signal provided by field sensing circuit 164 on
line 182 is provided in the circuit of FIG. 8 by
_ 23 -
I DRUG
microprocessor 166 which is appropriately programmed
to detect a magnitude of Vow indicative of a field
strength less than a predetermined reference level.
VOW less than the reference level is indicative of the
occurrence of the transition of the state of the food
object in the cavity from its frozen state to its
thawed state. The microprocessor is further programmed
to control operation in the defrost mode in response to
the detection of the voltage signal indicative of the
transition being detected in a manner to be described
hereinafter.
User alert circuit 172 comprises a
conventional oscillator circuit 190 operative to drive
speaker 192 at an audible frequency in response to a
suitable control signal on line 194 from microprocessor
166. An audible tone is generated upon termination of
the defrost mode to indicate to the user that the
defrost cycle has ended.
Magnetron power circuit 168 is connected
between lines Lo and Lo to provide power to magnetron
42. Lines Lo and Lo are adapted for coupling to a
power supply such as that provided by a standard 120
volt 60 Ho domestic power receptacle. Power circuit
168 includes power transformer 196 having a high
voltage secondary 198 connected to energize magnetron
42 through a half-wave voltage doubler circuit
comprising series capacitor 200 and rectifying diode
202 connected across the magnetron anode and cathode
terminals 204 and 206, respectively, and oppositely
poled with respect thereto. Secondary winding 208
of transformer 196 is connected as a filament winding
to heat the cathode of magnetron 42. Primary winding
- 24 -
~2~537 DRUG
210 of transformer 196 is connected across lines Lo
and Lo. Power to primary winding 210 is controlled
by trial 212 connected in line Lo. Gate terminal
214 of trial 212 is coupled to output line 216 to
microprocessor 166 via conventional opto-coupler circuit
218. Suitable control signals are provided to gate
terminal 214 by microprocessor 166 to operate
transformer 196 in a duty cycle control mode.
operation of the circuit of FIG. 8 will now
be described with reference to the flow diagrams of
FIGS. PA and 9B. FIGS. PA and 9B illustrate
alternative algorithms for controlling oven operation
in the defrost mode in response to the field strength
signal from field sensing circuit 164. These diagrams
illustrate algorithms which can be implemented in the
Read Only Memory (ROM) of microprocessor 166. From
these diagrams, one of ordinary skill in the programming
art can readily prepare a set of instructions for
permanent storage in the ROM of microprocessor 166.
It is of course to be understood that other portions
of the microprocessor ROM may be utilized to implement
additional oven control algorithms. Since the details
of such additional algorithms add nothing to the
understanding of the present invention, such details
have been omitted for brevity and simplicity.
It will be recalled that the objective of
defrost mode operation is to convert a food load
from its frozen state to its thawed state while
avoiding cooking the food. The algorithm of
FIG. PA simply turns off the magnetron, ending
the defrost cycle, upon detection of an output
voltage less than the predetermined reference
~2~37 9 DRUG
signifying that the food load has assumed its thawed
state.
Since frozen food items tend to thaw from
the outside inwardly, this algorithm will work
better for relatively thin food items which
typically will be thawed to the center, when
sufficiently thawed to cause the field strength
voltage signal to drop below the reference level.
However, for relatively thick or bulky items such as
roasts, surface thawing may progress to the point
of causing the sought voltage decrease to occur
even though -the center region of the food item may
not be thawed as completely as preferable. The
algorithm illustrated in FIG. 9B is applicable to
a broader class of food loads. In this algorithm,
the time required for the food load to become
sufficiently thawed to cause the sought decrease in
voltage to occur is measured. Upon detection of
this condition, hereinafter referred to as the
transition point, the operating power of the magnetron
is reduced to avoid cooking the thawed regions but
extends the cycle for an additional time period
at a lower power setting to allow further thawing.
The duration of this additional time period is
determined as a function of the initial time period
required to reach the thawed state. In this way,
certain characteristics of the food load affecting
the thawing time for the particular load are
automatically taken into account.
Referring now to FIG. PA, the first
algorithm, will be described in greater detail.
- 26 -
37 9 D-RG-l4g76
This program will be entered periodically, according
to the desired sampling rate. As herein before
described, in the illustrative embodiments the
sampling rate is three samples per minute. Thus,
this routine will be executed every twenty seconds
when the oven is operated in the defrost mode. Upon
entering the routine an output signal is provided
on microprocessor output line 184, energizing
solenoid 146, and causing discontinuity plate 144
to be placed in its active position (Block 220).
After a suitable delay on the order of 500 Millie
seconds (Block 221) to allow conditions to
stabilize, the voltage signal VOW from field strength
circuit 164 is read in (Block 222). An internal
analog to digital conversion is performed by
microprocessor 166, and Inquiry 224 determines
whether the signal VOW is less than reference voltage
OR. It will be recalled that reference voltage
OR corresponds -to a voltage level indicative of a
typical food load in its thawed state. If the
answer to Inquiry 224 is No, the microprocessor simply
exits the defrost routine (Block 226). If the
answer is Yes, signifying a load in its thawed state,
the magnetron is turned off, ending the defrost cycle
(Block 228), and a signal is provided on output line
194 (FIG. 8), enabling oscillator 190, thereby
providing an audible signal (Block 230) to the user
signifying the end of the defrost cycle.
Referring now to FIG. 9B, this routine
is also periodically entered in accordance with
the desired sampling rate, which in this embodiment
is once every twenty seconds. Upon the entering the
~9L~37 DRUG
routine, Inquiry 232 first checks the state of a
defrost flag. If the transition point characterized
by VOW OR has not yet been reached, the answer to
Inquiry 232 will be No, and the program continues
by actuating the discontinuity (Block 234) after a
suitable delay (Block 235) inputting VOW (Block 236)
and comparing V with OR (Inquiry 238), as herein-
before described with reference to FIG. PA. If VOW
is not less than OR, indicating the transition point
has not yet been reached, timer T is incremented
(Block 240) to measure the time required to reach the
transition point. The program then exits the routine
(Block 242). This sequence is repeated every twenty
seconds until the answer to Inquiry 238 is Yes,
signifying the transition point has been detected.
At this point, T represents the time elapsed from
the beginning of the defrost cycle to detection of
the transition point. This value is multiplied by
a constant K (Block 244) to compute the duration
for the ensuing reduced power portion of the defrost
cycle. The constant K is a predetermined factor
empirically found to provide satisfactory results
for the size and type of food loads to be defrosted.
A factor in the .2-.4 range is considered suitable
for most food loads likely to be defrosted in a
domestic microwave oven. As a further refinement
of this algorithm, factors could be empirically
determined for each of several categories of items
to be defrosted, with the constant employed during
any particular defrost cycle being selected by user
input of food item category information when
selecting the defrost cycle.
- 28 -
537 DRUG
Next, the defrost flag is set (Block 246).
The setting of the defrost flag enables the sampling
portion of the defrost subroutine to be bypassed for
the remaining portion of the defrost cycle by Inquiry
232. Timer T is reset (Block 248) to measure the
duration of the remaining portion of the cycle. The
power level for the magnetron is reduced -to a lower
level for the balance of the cycle to avoid surface
cooking of the food item (Block 250). In -the
illustrative embodiment, the power level is set at
approximately 30 percent of full power for the initial
portion of the defrost cycle. The power level is
reduced following detection of the transition point
to a level in the 15-20 percent of full power range
for the remainder of the defrost cycle. Inquiry 252
controls the duration of the second or final portion
of the defrost cycle by comparing time T with TO
the duration computed for the second portion of the
cycle in Block 244. If the elapsed time T is less
than TO (No to Inquiry 252), the timer is incremented
(Block 254), and the program exits the subroutine
(Block 256). When T is greater than TO (Yes to
Inquiry 252), the magnetron is turned off, ending
the defrost cycle (Block 258), and a signal is output
to oscillator circuit 190 (FIG. 8) to generate an
audible tone alerting the user that the defrost cycle
has been completed (Block 260).
While specific embodiments of the method
and apparatus of the present invention have been
illustrated and described herein, it is realized
that modifications and changes will occur to those
skilled in the art to which the invention pertains.
- 29 -
53~ 9 D RUG l 4 9 7 6
For example, the sensor location and the sampling
circuitry are designed to respond to a decrease in
field strength as the thawing of the food causes the
phase of the standing wave in the wave guide to shift.
Alternatively, by selecting a different sensor
location, the system could be designed to detect a
relative minimum field strength signal for the
frozen state and a relative maximum field strength
signal corresponding to the thawed state. It is
therefore to be understood that the appended claims
are intended to cover all such modifications and
changes as fall within the true spirit and scope of
the invention.
- 30 -
