Note: Descriptions are shown in the official language in which they were submitted.
~1'73~3~lS
Background of the Inventlon
The process of cooking in a conventional gas or electric
- oven is relatively uncomplicated. Generally, temperature and
time are the only two cooking parameters considered. Normally,
the oven is preheated to a given temperature and the food is
placed in the oven for a specified time period which is some-
times determined by the weight of the food. For example, it
may be preferable to cook a turkey at 350~F for 20 minutes per
pound. Generally speaking, the heat at the surface of the food
gradually travels inward by conduction raising the temperature
of the interior and causing physical changes which are part of
the cooking process. Because this cooking process is relatively
slow and is always limited by the temperature of the oven so
that there can be no thermal runaway, there is a reasonable
tolerance in the selection of the cooking parameters. For
example, a deviation of 10 minutes per hour or 25F in temper-
ature may not have a significant impact on the palatability of
i the cooked food. This tolerance has contributed to a general
confidence of most cooks of their ability to accurately select
~0 temperature and time, even in new situations. Another con-
tributing factor is exposure in that most cooks grew up in
homes where all of the cooking was done in conventional gas or
electric ovens.
The microwave oven has evolved in the last two or three
decades. Although consumer acceptance has greatly increased
as has the percentage of households with microwave ovens, some
consumers are still reluctant to buy or use microwave ovens
because they don't have the general confidence in their ability
`~`
to operate them; they feel intimidated by the sometimes com-
plicated directions for using them. They no longer have the
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t~39~.S
comfortable parameters of temperature and time to select. The
. ~
introduction or indoctrination into a relatively new cooking
process is complicated by the rate at which foods cook. More
specifically, because a microwave oven cooks so fast, an error
of a few minutes in the selected cooking time can be a substan-
tial percentage of the required cooking time and can result in
a substantial difference in the doneness of the food. Further-
more, the temperature of the food body is not limited by the
temperature of the oven; temperature runaway can occur. Accor-
dingly, microwave oven manufacturers have expended considerableeffort in research and development of apparatus and methods for
simplifying the user task of determining the cooking parameters
for microwave ovens. Simplified user operation would presumably
expand the consumer marketplace.
One prior art approach was to provide a temperature probe
which the user inserts in the food body. The oven is then per-
mitted to remain on until the internal temperature rises to a
selected value. This method has the disadvantage of the incon-
venience of inserting the probe especially in frozen food.
Also, there has been difficulty positioning the probe and the
food in the oven and connecting the cable attached to the probe
to a jack in the cavity. However, the most serious drawback is
that the measure of internal temperature in a food body heated
by microwave energy is not an indication of doneness. Actually,
the food body should generally be heated up to approximately
160F and then held there while the cooking process occurs.
Other types o approaches have used various sensors in
the oven cavity to monitor the cooking characteristics of the
food. None of these approaches has met with total consumer
acceptance1
.' ` ,
3~
Summary of the Invention
The invention discloses a microwave oven comprising a
conductive cavity, a source of microwave energy coupled into
the cavity, means for providing a first signal derived from the
weight of an object positioned within the cavity, means for
providing a second signal determined by an operator control
panel selector indicating the initial temperature of the object,
and means responsive to the first and second signals for con-
trolling the source of microwave energy. Preferably, the con-
trolling means comprises a microprocessor. Also, it may bepreferable that the means for providing a first signal comprise
a weight sensitive device such-as a scale positioned below the
. .
cavity and having means for transferring weight in the cavity
to the device. Preferably, the selector selections for initial
temperature are frozen temperature, refrigerator temperature,
room temperature or heated temperature. The first three tem-
peratures are easily determined by the operator. For example,
frozen temperature may be at-0F, refrigerator temperature at
approximately 40F and room temperature at approximately 65F.
; 20 The heated temperature may preferably be 160F. By knowing
the weight or the food,~and the initial temperature of the
-~ food, the microprocessor is able to execute an equation which
determines the time required to produce the required number of
BTUs in the food to raise the temperature to a predetermined
temperature or to process the food through a cooking cycle.
It may be preferable that a directive microwave radiator be
positioned within the cavity and coupled to the source of
~; microwave energy. The directive radiator would ensure that a
~` substantial portion of the microwave energy be incident on the
food before being reflected from the walls.
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~73~S
The invention may also be practiced by a microwave oven
comprising a conductive cavity, a magnetron for supplying
microwave energy to the cavity, and means responsive to inputs
derived from the initial weight and initial temperature of an
object placed within the cavity for controlling the magnetron
wherein a control panel having operator selectable means for
inputting initial temperature information of said object to
said controlling means and means coupled to said object within
the cavity for providing an input signal to the controlling
means, said signal being derived from the weight of said object
within the cavity are provided.
The invention may also be practiced by a microwave oven
comprising a conductive cavity, a magnetron for supplying
microwave energy to the cavity, operator actuable control means
for providing a first signal indicative of the initial tempera-
ture of a food body placed in the cavity, a scale coupled to
the cavity for providing a second signal corresponding to the
weight of the body, and means responsive to the first and
second signals for determining the time duration of microwave
exposure for a predetermined heating or cooking function and
for controlling the magnetron in accordance with the time
duration.
The invention discloses a method for cooking with a
~; microwave oven comprising the steps of weighing a food body
within the microwave cavity, providing a first signal derived
` from the weighing to a microprocessor, inputting a second signal
; determined by the initial temperature of the food body to the
same microprocessor, calculating the time period of microwave
energy exposure to raise the temperature of the food body from
the initial temperature to a predetermined temperature or to a
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cook state and controlling the magnetron in accordance with the
calculated time period.
The invention may also be practiced by a microwave oven
comprising an outer housing, a conductive cavity positioned
- therein along with a magnetron for supplying microwave energy
to the cavity, a weight sensitive device positioned within the
housing in a chamber below the cavity and having vertical
columns protruding through holes in the floor of the cavity,
a plate positioned withln the cavity and supported by the
vertical columns so that weight of a food body resting on the
plate is detected by the weight sensing device, said weight
sensing device comprising means for providing a first signal
corresponding to the weight supported by the vertical columns,
a microprocessor, a control panel coupled to the microprocessor
and having operator selectable means for inputting a second
signal to the microprocessor, the second signal corresponding
to whether said food body has an initial temperature which is a
~! frozen temperature, refrigerator temperature, room temperature
or heated temperature, and the microprocessor controlling the
~0 magnetron in response to the first and second signals.
The invention also discloses a microwave oven comprising
a conductive cavity, a magnetron for supplying microwave energy
to said cavity, a microprocessor, means for providing a first
signal to said microprocessor wherein the first signal corre-
sponds to the weight of an object to be exposed to microwave
energy in the cavity, means providing a second signal to the
microprocessor wherein the second signal corresponds to a
heating function to be performed by the oven which function is
to raise the temperature of the object from approximately
~ 30 refrigerator temperature to approximately room temperature,
-- 5 --
, ~
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3~LS
and the microprocessor in response to the first and second
signals determines a time duration of exposure of said object
to microwave energy for performing said function and for con-
trolling said magnetron in accordance with said time duration.
It may be preferable that the first signal is provided by a
scale coupled to the cavity and the second signal is provided
by an operator actuable control panel. It may also be prefer-
able that the refrigerator temperature be approximately 40F
and the room temperature be approximately 65F. Also, the
function may be defined as raising the temperature of the food
body from approximately room temperature to a predetermined
heated temperature wherein the heating temperature may be
160F. Further, the function may be defined as taking an
object with an initial temperature of 160F and holding it at
approximately that temperature for a time period sufficient to
cook the object.
The invention also discloses a microwave oven comprising
a conductive cavity, a magnetron for supplying microwave energy
to the cavity, a weight sensing device for sensing the weight
~0 of an object positioned in the cavity, a microprocessor, and a
control panel having an operator actuable control for providing
an input to said microprocessor from said weight sensing device,
said signal corresponding to the weight of a dish in said cavity
`- in which dish a food body is to be positioned.
`'`'
r` 30
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17391S
Brief Descrip~ion of ~he Drawings
The following detailed description of preferred and
alternate embodiments of the invention will be more easily
understood with reference to the drawings wherein:
; FIGURE 1 is a front elevation partially cut away of a
microwave oven using the invention to advantage;
FIGURE 2 is a side elevation of the microwave oven of
Figure 1 taken along line 2-2;
FIGURE 3 is a top view of the microwave oven of Figure 1
taken along line 3-3;
FIGURE 4 is an alternate embodiment of the compliant
member, light source and light sensitive device of Figures 1,
v 2 and 3;
FIGURE 5 is a bloc~ diagram of a microwave oven system
embodying the invention;
FIGURE 6 is an expanded view of the control panel of the
microwave oven of Figure l;
FIGURE 7 is a software flow diagram of the programming of
. a microwave oven embodying the invention;
FIGURE 8 is a state diagram of the microwave oven using
the flow diagram of Figure 7;
FIGURE 9 is the interrupt scheme used in conjunction with
the software flow diagram of Figure 7;
FIGURE 10 is an alternate embodiment of the microprocessor
. and associated hardware used in Figure 5;
FIGURE 11 is a side elevation view of the scale embodied
in a bottom fed oven; and
FIGURE 12 is a top view of 'he bottom fed oven of Figure
~;j 11 taken along lines 12-12.
.
'739~5
Description of the Preferred Embodiment
. ~
Referring to Figure 1, there is shown a partially cut
away microwave oven having a heating cavity 10 containing a
food body 12 positioned therein through an access opening pro-
vided by a door (not shown). In the present description, it
is believed unnecessary to show and describe well known and
conventional parts such as, for example, the door seal struc-
ture. It is preferable that microwave energy at 2450 MHz from
a conventional magnetron 14 be coupled through waveguide 15 to
a rotating primary radiator 16 which has a pattern characterized
in that a substantial portion of the energy is absorbed by the
food before being reflected from the walls of the cavity. More
specifically, primary radiator 16 comprises a two-by-two array
of antenna elements 16a where each element is an end driven
half wavelength resonating antenna element supported by a length
of conductor 16b perpendicular to the elements and the upper
wall of the microwave oven cavity. Parallel plate microstrip
transmissi~n lines 16c connect each of the support conductors i
to a center junction 16d axial to rotation. ~t the junction,
a cylindrical probe antenna 9 is attached to the radiator 16
~ structure. Probe antenna 9 which has a capacitive hat 7 is
`~ supported by a plastic bushing 17 positioned within the wave-
guide. The bushing permits rotation of the probe antenna and
`~ radiator around the axis of the probe antenna. Microwave
energy introduced into waveguide 15 by output probe 13 of
magnetron 14 excites probe antenna 9. Energy couples down
probe antenna 9 which functions as a coaxial conductor through
hole 19 in the upper wall of the oven cavity. The upper wall
of cavity 10 is shaped to form a dome 27 having a flattened
conical shape extending outwardly in the wall to provide a
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~ .~73~3~5
:
nearly ci~cular recess partially surrounding the directive
rotating radiator and provides uniform energy distribution in
the product being heated. The dome returns microwave energy
reflected from the food body toward a circular area in the
middle area of the microwave oven cavity. It is preferable
that air from a blower (not shown) used to cool the magnetron
be circulated through the cavity to remove vapors. It may be
preferable that this air be channeled into waveguide 15 and
passed through apertures 21 in the wall of the dome to provide
rotation of radiator 16. Radiator 16 is connected to fins 23
to provide a suitable force surface for the air driven rotation.
The fins may be fabricated of a plastic nonlossy material~
Other paths may also be used to direct the air from the blower
to the fins. Also, in lieu of the air driven method, an elec-
tric motor (not shown) may be used to provide rotation of the
radiator. Grease shield 25 is transparent to microwave energy
and provides splatter isolation from the rest of the cavity.
Control panel 30 which is shown in detail in Figure 6
provides keyboard functions which are inputs to the control
microprocessor 32 and display functions by which the micro-
processor indicates status to the user. Any of a number of
conventional keyboard switches and displays could be used. It
may be preferable that well known capacitor touch pad switches
be used for the keyboard. Also, it is preferable that the
display provide digital read out of parameters such as time
and a simultaneous indication of what keyboard entries have
been selected. Specific functions of the control panel will
be described in detail later herein.
Positioned below the floor 18 of the cavity is a scale 20.
The scale has four vertical support pins 22 which respectively
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~73~
protrude through holes 24 in the floor of cavity 10 in the
proximity of the corners. Supported on the pins is plate 26
which rests approximately one inch above the floor of the cavity
at the corners. Typically, the plate is made of a pyrex glass
material which is transparent to microwaves. The microwaves
pass through the glass, strike the floor of the cavity and are
reflected back up into the food body from the bottom side
This allows the microwave energy to enter the food body from
all sides. Also, the plate may provide some protection for the
magnetron if the oven is accidentally turned on when there is
no load in the cavity. Although the glass plate may be removed
for cleaning, it should always be in the oven during operation.
The weight of the glass plate and any food bodies and dishes
placed thereon is transferred through support pins 22 to scale
;~ 20.
It is desirable that substantially no microwave energy
pass through the four pin holes 24 into chamber 28 below the
cavity which houses the scale. Accordingly, the pin holes 24
which may preferably be circular, are less than one half wave-
length in circumference. More specifically, the holes areslightly larger than the pins which are approximately one
quarter inch in diameter. To minimize inaccuracies in scale
weighings, it is important that there be as little friction as
possible for a pin as it moves up and down through a hole; this
may be accomplished by selecting tolerances that accurately
- position the pins to be concentric with their respective holes
and by using materials that have low coef~icients of friction.
It is preferable that the pins be fabricated of a microwave
transparent material such as a ceramic to provide a microwave
choke through the holes. If a pin were metallic, the structure
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7~S
i would exhibit the properties of a coaxial line with the outer
conductor being the surface of the hole and the center con-
ductor being the pin. Microwave energy would pass even though
; the size of the outer conductor was below cutoff.
Scale 20 comprises four rigid lever arms 36. Each lever
arm has an inverted V-bracket 37 on one end to support the arm
from a knife edged fulcrum 40. At the other end, each arm is
attached to a second arm by a semicircular pivot pin 41 so that
there can be vertical motion at the joint of the arm pair
between the fulcrums at the opposite ends. The pairs of lever
arms 36 so described are positioned parallel to each other so
that each arm of the pair has a corresponding arm in the other
`~ pair. The corresponding arms are rigidly attached by a V shaped
cross bar 43 running perpendicular to the connected lever arms.
In the preferred embodiment, each arm is approximately seven
inches long and the cross bars which are fourteen inches long
are attached approximately one inch from the fulcrums. The
scale was designed with these dimensions so that it would fit
in chamber 28 and the pins would protrude through holes 24 at
appropriate places. The compliant member 44 which resists
downward motion of the lever arms at the pivot pin 41 joint
~, is a rlexible metal strip that is supported in cantilever
fashion from block 46. Rod 48 is attached rigidly and perpen-
dicular to one of the lever arms near the pivot pin joint.
The rod has a disk 50 on the end which rests on compliant
member 44.
As described earlier herein, the weight of plate 26 and
any objects placed thereon is transferred to scale by pins 22
which protrude into the cavity through holes 24 in the bottom
cavity wall. Pins 22 are attached to rectangular brackets 52
3~.73~3~S
which limit the upward movement of the pins through holes 24.
The rectangular brackets 52 are rigidly connected at inside
bottom points of V-shaped cross bars 43 adjacent to the respec-
tive lever arms. Regardless of the distribution of downward
force between the four pins 22, the force is transferred in
approximately the same ratio by the cross bars to the lever
arms on the compliant member side of the scale. Rod 48 couples
the force from the lever arms through disk 50 to the compliant
member 44. ~s the weight and corresponding downward force is
increased, the flexible compliant member bends more; the com-
pliant member is analogous to a spring. The vertical position
`:`
~` of the unsupported end of the compliant member is therefore a
function of the weight exerted on pins 22. The unsupported end
of compliant member 44 is bent downward to form a shade member
57 that shields a particular portion of light beam 54 from being
incident on light sensitive device 56. As the weight on plate
26 is increased so that the unsupported end of compliant member
44 bends further downward, a greater portion of the light beam
is blocked from being incident on light sensitive device 56.
~0 Light sensitive device 56 may preferably be a phototransistor
which provides an analog voltage which is a function of the
light incident upon it. The source 58 of the light beam 54 may
be a light bulb as shown or more preferably a light emitting
diode as shown in the alternate embodiment of Figure 4. It
may be preferable to position a concave lens between the source
of light and the light sensitive device to focus the beam of
light to a relatively small area. Accordingly, the intensity
within that area would be varied rather than varying the area
of light incidence.
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3~ ~73~
Referring to Figure 4, an alternate embodiment of the
compliant member is shown. The light source 58 which may
preferably be a light emitting diode is attached to the un-
supported end of compliant member 44 which is attached in
cantilever fashion to block 46. The light beam 54 is directed
towards light sensitive device 56. A shade member 57a is
positioned between light source and the light sensitive
i device. As a downward force is exerted on the compliant
member by rod 48 through disk 50, an increased portion of the
light beam is blocked by shade member 57a. Accordingly, as
the weight exerted downward on pins 22 is increased, the
analog voltage at the output of light sensitive device 56 is
decreased. A different type of shade member may preferably be
used which would block the upper portion of the light beam
from being incident on the light sensitive device. In this
case, as more weight is placed on the scale causing the beam
to point further downwardt a greater part of the beam would be
incident on the light sensitive device because it is not
blocked by the shade member. This would mean that the output
voltage from the light sensitive device would increase as a
function of increasing weight on the scale.
Scale 20 provides a means for providing microprocessor 32
with an input indicative of the weight of objects in cavity 10.
A substantial advantage of scale 20 so described is that it can
be installed in commercially available microwave ovens without
significant retooling. More specifically, in the particular
microwave oven to which the scale was embodied, chamber 28 had
a height of 3/8 inches in the center and approximately 1 1/2
inches at the corners and edges. Figures 1 and 2 have not been
drawn to scale. The corners and edges of the floor 18 of cavity
1~7~S
10 have always been raised so that a food body supported on
plate 26 would be elevated from the conductive surface of the
; floor where dielectric losses would be very low. The scale
which has a height of approximately one inch has its structure
in a rectangular shape with nothing in the center so that it
fits around the perimeter of chamber 28 where the height is
approximately 1 1/2 inches. Furthermore, ~ecause there is no
structure in the center of the scale, it can be adapted for
; use in a bottom fed microwave oven as will be described later
herein with reference to the alternate embodiment of Figures
11 and 12.
Referring to Figure 5, there is shown a block diagram of
a microwave oven embodying the invention. Scale 20 provides an
input indicative of the weight of the food body to microwave
processor 32. Using the weight of the food body along with
other input parameters, the microprocessor determines the
output profile of the magnetron in time and power and controls
the operation thereof.
Still referring to Figure 5, the analog voltage output
from the light sensitive device of scale 20 is coupled to
multiplexer 60 which operates under control of microprocessor
32. The function of multiplexer 60 is to provide the micro-
processor with a means of selecting which of a plurality of
analog inputs is provided to analog to digital converter 62
for conversion to a digital signal that is acceptable for
input to the microprocessor. An example of another analog
input is from a conventional microwave temperat~re probe.
The reference clock for microprocessor 32 is provided
by clock 64. Conventionally, clock 64 comprises an AC filter
. ~
~.~7~
connected to the 60 H~ AC power line and a zero crossing
detector, the output of which is coupled to the microprocessor.
Referring to Figure 6, there is shown an expanded view of
control panel 30 of Figure 1 which panel comprises keyboard 63
- and display 65. As stated earlier herein, it may be preferable
that the keyboard switches be conventional capacitive touch pad
switches. Typically, a touch panel interface may be connected
between the keyboard and the microprocessor; the interface is
of conv~ntional design and is included in many commercially
available microwave oven models. Similarly, a high voltage
driver interface may be connected between the microprocessor
and displays of control panel 30 to provide lighted indicators.
The keyboard includes touch pads 69 numerically labeled 0--9,
functionally labeled CLOCK, READY TIME, DISH WEIGHT, THAW,
WARM, ~EAT, COOK PROGRAM, STIR TIMER, REDUCED POWER, TIMER,
and push switches 67 labeled START, STOP/RESET and LIGHT. The
display includes digital read outs ~6, function indicator
lights 68 associated with functionally labeled touch pads, and
digital read out 70 associated with the COOK PROGRAM function
~0 pad.
In operation, touch pads labeled 0-9 may generally be
used conventionally to enter data for well ~nown functions
into the microprocessor. For example, when the microwave oven
is not being used, digital read outs 66 display the time of
day. To change the time of day, the user pushes numerical
pads corresponding to the desired time; this time is displayed
in digital read outs 66. Then, when the user pushes CLOCK,
the displayed time is entered into the microprocessor and
becomes the new time of day. Another example is to use the
numerically labeled pads to display the amount of time food
- 15 -
:`
is to be cooked. Upon pushing START, the display time counts
down un~il the oven shuts off. The THAW function pad is used
-- to activate the microprocessor to control the magnetron so
that the food is raised from frozen food at 0F to thawed food
at 40F. The WARM function pad is used to activate the micro-
processor to control the magnetron so that the food is raised
from 40F to 65F. The HEAT function pad is used to activate
the microprocessor to control the magnetron so that the food
is heated from 65F to 160F. The COOK PROGRAM function pad
is used to activate the microprocessor to control the mag-
netron so that the food at 160F is taken through the cooking
process which may or may not raise its temperature to above
160F. In other words, the THAW, WARM, HEAT and COOK inputs
are indicative of the initial temperature of the food. Before
initiating cooking, the COOK PROGRAM which is appropriate for
the particular food being cooked may be selected by touching
an appropriate numerical pad and then touching COOK PROGRAM.
The selected program is displayed in digital read out 70.
When in a cook-by-weight mode which will be described in detail
herein, the REDUCED POWER pad may be touched to activate TEMP
HOLD which decreases the duty cycle of the magnetron. The 1/2,
1/4 and 1/8 indicators are activated by successive touchings
of the REDUCED POWER pad during conventional cook-by-time
operation. The READY TIME function pad is used to program the
~ microwave oven to come on at a future time. The STIR TIMER is
u used to sound an alarm and shut off the oven at a time when
the food is to be stirred or other action taken within the
oven. The TIMER function is used as a count down clock to an
alarm for timing which may or may not be associated with the
microwave oven. The START button initiates execution of a
~. ~73~L5
.:
particular selected programmed subroutine which turns the mag-
netron on. The STOP/RESET button causes the magnetron to be
turned off. Successive pushings of the LIGHT button causes a
- light (not shown) illuminating the cavity to be turned on and
off.
The use of microprocessors to control microwave ovens has
become common in the last decade, In fact/ most if not all of
the industry leaders offer top-of-line microwave ovens that
are microprocessor controlled. In general, the microprocessor
receives inputs from a keyboard and sensors and provides output
signals which control the magnetron and drive the display. In
Figure 5, a new sensor which is a scale coupled to weigh objects
within the cavity has been added. However, the selection of
an appropriate microprocessor and the programming of it to
perform specified functions is well known to those skilled in
the art. The early microprocessor controlled ovens typically
used standard commercially available processor integrated
circuits and the application program was provided in a read
only memory (ROM); these systems generally required many input/
~0 output components to interface the microprocessor to the system.
These interfaces are well known to those skilled in the art.
In recent years, a continuing trend within the microwave oven
industry is to use customized integrated circuits for control-
ling microwave ovens. The large volume of these specialized
~ integrated circuits has enabled the suppliers to spread the
- engineering development costs of transforming user requirements
to circuits over a large number of integrated circuits thus
reducing the cost of the individual unit. Furthermore, there
is a continuing trend to integrate more functions onto a single
silicon integrated circuit eliminating many of the discrete
7~5
electronic components and interface hardware such as digit and
segment drivers, analog to digital converters, multiplexers,
zero crossing detectors, AC filters, and touch panel interfaces.
With the foregoing as a background and turning agaln to Figure
5, microprocessor 32 in the preferred embodiment is a customized
integrated circuit developed by well known techni~ues and fur-
nished by any one of a number of electronic suppliers. The
integrated circuit has interface functions integrated into it.
Even multiplexer 60 and analog to digital converter 62 could
have been included in the microprocessor integrated circuit
such that analog signals are connected directly to the chip.
An alternate embodiment of microprocessor 32 will be given
later herein.
Still reerring to Figure 5, microprocessor 32 receives
inputs from scale 20 and keyboard 63 of control panel 30. In
addition to performing many conventional functions such as,
for example, cooking for a set time, cooking at a set power,
monitoring a temperature probe, and monitoring an interlock,
microprocessor 32 performs a new function which relates to sim-
~0 plified user operation. More specifically, the microprocessoruses the weight of the food as weighed within the cavity along
with the initial temperature of the food to determine how long
the food should be cooked.
Referring to Figures 7, 8 and 9, software flow diagrams
for the programming of microprocessor 32 in accordance with
the invention are shown. Many conventional functions such as
monitoring an interlock are not included in the discussion
herein but the inclusion of them in the flow diagrams and the
pr,ogramming of microprocessors from flow diagrams in general
is well known to those skilled in the art. Referring first
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.
3~3: L5
;
to Figure 7, after POWER UP, the microprocessor is initially
` RESET~which includes a number of conventional software house-
cleaning procedures such as initialization of ou~put channels.
The following equation is used to CALCULAT~ HEATING TIMES.
.
[HUS] [FW + (~W)(SHD)]
Heating Time = ---------------------------~
[OPL] [PLS] [CF]
where HVS is Heat Units Selection, FW is Food
Weight, DW is-Dish Weight, SHD is Specific Heat of Dish, OPL
is Oven Power Level, PLS is Power Level Selectian and CF is
Coupling Factor.
The first term in the heating time equation is Heat
Units Selection which is expressed in BTUs per pound of food.
It has been found that the required heat units per weight unit
of food is in part a function of the temperature range over
which the food is to be heated and chemical and/or physical
changes taking place within the food. By a ~ery simplified
user input from the keyboard, this term of the equation is
determined. More specifically, referring again to Figure 6,
the user indicates the initial temperature state of the food
by touching THAW which as labeled is for frozen foods (0F),
WARM which as labeled is for cold foods (40F) such as out of
the refrigerator and/or HEAT which as labeled is for food at
room temperature (65F). Touching of more than one of these
pads initiates a separate cycle for each function and a
separate calculation of the heating time equation for each
cycle. For the THAW cycle, 100 BTUs per pound is entered into
the equation; for the WARM cycle, 25 BTUs per pound is entered
into the equation; for the HEAT cycle, 100 BTUs per pound is
entered into the equation; and for the COOK cycle, 25-250 BTUs
.~ -- 1 9
~1~7;3`~;;
per pound is entered into the equation depending on the COOK
PROGRA~ that is selected by touch pads and that is displayed
within the COOK PROGRAM touch pad. Although the Heat Units
Selection entry into the equation for COOK determines the heat
ing time for a maximum power level, that time will be increased
by a specific factor if a REDUCED POWER setting is selected.
In other words, the same number of BTUs for the cooking task
are delivered but over a longer period of time for more
delicate cooking or simmering.
The second term in the heating time equation is [Food
Weight + (Dish Weight) ~Specific Heat of Dish)~. The presence
of the Food Weight in the equation is obvious; the multiplica-
tion of its units tpounds) by the units of Heat Units Selection
(BTU per pound) yields BTUs for the numerator of the equation
which when divided by the units (BTUs per minute) of the de- j
nominator, gives the quotient in minutes which are the desired
units. The inclusion of (Weight Dish) (Specific Heat of Dish)
is to compensate for a certain portion of the heat which is
provided to the food being transfered to the dish by conduction.
In other words, more heat must be delivered to the food than
might be thought necessary because some of it is lost by con- I
- duction to the dish. For user simplicity, the specific heat
of the dish in the calculation of the heating time equation is
assumed to be a constant of 0.2 for the WARM and HEAT cycles
where the temperature of the dish is raised by conduction as
the temperature of the food rises. Eor the THAW and COOK
cycles, the specific heat of the dish is set equal to zero to
eliminate the product of it and dish weight from the equation;
with THAW, the BTUs transferred to raise the temperature of the
dish is insignificant compared to the BTUs to thaw the food
-- ~0 --
`"
:~ &~3
`' and with COOK, which starts at 160F, there is no appreciable
rise in temperature. Although a more exacting expression of
- the heat lost by the food (and accordingly the additional heat
required to be delivered to it) would also include the specific
heat of the food and heat rise in gases in the cavity, empirical
analysis has showed that the assumptions were adequate for
proper operation of the oven using the heating time equation.
In operation, when the light indicator on the DISH WEIG~T pad
is on, it is indicative that a dish weight is stored in the
microprocessor. Therefore, to commence a new cooking process
with a new dish, the DISH WEIGHT pad is touched and the light
indicator goes out; this erases the previous dish weight from
the microprocessor memory a~d "zeros the scale". The weight of
the dish may then be set up for entry into the microprocessor
by either entering it through the numerical tooch pads if it
is known or by placing the dish without food in the oven where
it depresses the scale. With a second touching of the DISH
WEIGHT pad, the indicator light thereon goes on indicating
that the new dish weight has been entered into the micro-
processor. It may be preferable that the analog voltage atthe output of light sensitive device 56 be somewhat linear
with the weight that is placed on the scale. With this being
the case, a linear analog to digital converter properly scaled
can be used so that the microprocessor directly samples weight
in pounds. If the analog voltage is not linear with weight
such as being inversely proportioned as the embodiment of
Figure 1, it can be compensated for in the microprocessor by
such conventional techniques as a lookup table. For accuracy
of weighing, it may be preferable that at a weighing time,
the microprocessor take a plurality of weight samples, discard
3~
high and low weights, and average the remainder of the weights.
The weight of the food is calculated by the microprocessor by
.
using the weighing immediately prior to the START button being
` pushed and subtracting the weight of the dish after zero
adjustment.
The first term in the denominator of the heating time
equation is Oven Power Level. In the microprocessor calcula-
tion, it has been assumed that this value is a constant of
725 watts or 41.2 BTUs per minute. In actual operation, there
is generally an error in this assumption. Even ovens of the
same model and manufacturer will typically vary over a range
of 100 watts from unit to unit. It is this inconsistency of
output power that has caused producers of prepared foods to
specify in the microwave cooking directions on the box that
microwave processing times may vary; this is true even though
the characteristics of the food product are well defined and
can easily be empirically determined. Furthermore, output
power may vary as a function of the AC line voltage. The
error in the assumption of 725 watts as the output power can
~0 be minimized by attempts to normalize ovens to that value.
The second term in the denominator of the heating time
equation is Power Level Selection. If the REDUCED POWER pad
has not been used to select TEMP HOLD, a value of 1 is used
for PLS in the heating time equation. If the RED~CED POWER
pad has been used to select TEMP HOLD, 0.3 plus 0.04 per pound
of food is input to the equation. For example, if the food
weighs 1 pound, the magnetron will operate at 34 percent of
full power. Further, if the food weighs 2 pounds, 38 percent
of full power will be outputted. This is implemented by
~0 decreasing the duty cycle of the magnetron~ In the past, it
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~73~
was generally accepted that just as some foods cook better
conventionally at lower rather than higher temperatures, some
foods cook better at reduced microwave energy power levels.
Accordingly, most microwave ovens provide many power level
selections. As part of the development of the cook-by-weight
process, it was found most important to determine the total
number of BTUs required for the particular food and then
deliver them; however, the rate at which microwave energy is
supplied is not so critical. In fact, the TEMP HOLD feature
provides only one reduced power level setting and that is a
function of the weight of the food. Generally, the reduced
power of TEMP HOLD is used to best advantage with food having
a large volume where the microwave energy penetration to the
center of the food is greatly reduced. Additional cooking time
may be desirable to permit heat in the outer portion of the
food to conduct toward the center for more uniform heating and
cooking. It has been found that the most appropriate reduced
power setting is one which holds the food at temperature which
for lightweight foods is approximately 30 per cent of full
power. The additional 4 percent per pound in the PLS formula
compensates for larger food bodies having greater surface areas
~' and therefore greater heat losses that must be compensated for
to maintain temperature. The assllmption that food surface and
size generally relates to~weight has been empirically tested.
The last term of the heat time equation is Coupling
Factor. Not all of the microwave energy output from the mag-
netron is coupled into the food. Some of the energy is lost
in the system such as in the walls, waveguide, and the plate.
The per cent of total energy (assumed to be 725 watts) that is
converted into heat in the food is in part a function of the
~73~5
food surface a~ea and its absorptivity. For example, if one
potato takes four minutes to cook, two potatoes will generally
take less than eight minutes or twice that. This i5 because
as the load is increased, a larger percentage of the total
power is absorbed by the food. It has been found that the
distribution of energy into the food with respect to losses is
approximately expressed by the following formula.
Food Weight
Coupling Factor = -------------------
Food Weigh'c ~ K
In essence, the constant K can be viewed as losses of the oven
;~ expressed in terms of weight. Constant K has been assigned the
value of 0.1. Accordingly, if the food weighs 0.1 pounds, the
coupling factor is one half or the heating time is increased by
a factor of 2 over which it would have otherwise been. If,
however, the food weighed 1.0 pounds, the heating time would
only be increased by a factor of 1.1. In Figure 5, micro-
processor block 32 indicates that the heating time per weight
unit decreases as a function of increasing weight because of
the improved coupling of microwave energy into the greater
food mass.
The discussion of the calculation of the heating time
equation has assumed that certain keyboard entries such as data
relating to initial temperature of the food and sensor entries
such as the weight of the food be available to the micro-
processor. The required information which is provided before
the initial calculation and is periodically updated is provided
by interrupts as shown in Figure 9. At the 60 Hz line crossings
and at the time midpoints therebetween as indicated by the 8.3
MICROSECOND DELAY, the processor is interrupted at which time
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~3~
it DISPLAYS PARAMETERS, LIGHTS DISPLAY, SCANS KE~BOARDS and
SELECTS A/D CHANNEL. At these times, the present keyboard data
and scale data in the microprocessor memory is upda~ed.
Referring again to Figure 7, after the calculation of the
heating time equation for the specific operational parameters,
the program branches at IS STATE ACTIVE? The active state is
defined in ~igure 8 which shows the relationship between oven
states. More specifically, after power up, the microprocessor
automatically goes to a reset state as described earlier with
reference to Figure 7. Next, the microprocessor automatically
goes to the idle state wherein the heating time equation is
continually calculated. The microprocessor stays in the idle
state until the start button of control panel 30 is pushed~
At that time, the microprocessor goes to an active state where
it stays until either the stop button is pushed or the cooking
function is finished. If the stop button is pushed, the micro-
processor goes to à hold state from which it can return to .he
active state by pushing the start button 90 or back to the
reset state as initiated by a second pushing of the stop button.
Again referring to Figure 7, if the state is not active, the
heating time calculation is executed again. If the state is
active, the next question is ANY COOK-sY-WEIGHT FUNCTIONS?
These functions as described with reference to control panel
30, are THAW, WARM, HEAT and COOK. If none of these has been
selected but the processor is active, it is indicative that
the function is cook-by-time. If one or more of the weight
functions has been selected, the software initially tests to
see IS THAW FLAG SET? If it has, the processor controls the
magne~ron which is cycled on and off, the cycle duty time being
a function of the weight of the food. More specifically, the
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~1'7;35~
THAW function as described earlier provides magnetron on-time
to supply 100 BTUs per pound to the food. The power level is
always 100 per cent. If the food weighs less than 3 pounds,
100 BTUs per pound is provided with an equivalent off time
before the next function. If the food weighs 3 or more pounds
but less than 10 pounds/ the same 100 BTUs per pound is supplied
but in increments of 25 BTUs per pound with time intervals
therebetween equal to the time required to supply 50 BTUs per
pound. If the-food weighs 10 or more pounds, it is the same as
the previous sentence except the off time intervals are equal
to the time to supply 75 BTUs per pound. Also, the flash thaw
indicator on control panel 30 is flashed. As described with
reference to Figure 9, this action would be taken at the 60 Hz
line crossings and the midpoints therebetween. Also, the thaw
time would count down. At the end of the thaw cycle or if no
thaw cycle was selected, the microprocessor determines IS WARM
FLAG SET? If it has, the microprocessor turns on the magnetron,
flashes the warm indicator and counts down until the end of the
warm cycle. At the end of the warm cycle or if no warm cycle
was selected, the microprocessor determines IS HEAT FLAG SET?
If it has been, the microprocessor turns on the magnetron,
flashes the heat indicator and counts down until the end of
the heat function. At the end of the heat cycle-or if no heat
; cycle was selected, the microprocessor determines IS COOK FLAG
SET?~ If it has been, the microprocessor turns on the magnetron,
: flashes~--the cook indicator and counts down until the end of the
cook cycle. At the completion of this cycle or if the cook flag
was not set, the software flow returns to the reset subroutine.
Again referring to Figure 5, the microprocessor, using
techniques well known to those of ordinary skill in the art,
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~ ~739~5
controls the power supply 68 for the magnetron 14. When the
cooking time is finished, the microprocessor controls power
supply 71 to shut off magnetron 14. If the cooking time is to
be performed at reduced power, the microprocessor regulates the
~ duty cycle of the magnetron. The microprocessor simultaneously
; provides a visual indication on display 65 of the time for the
magnetron to be on and what functions have been selected.
The invention provides a significant advance in the
~` microwave heating art in that it is a major step towards one
button simplified operation. Many former problems associated
with the user determining cooking parameters have been over-
come. The weight of the food which is provided automatically
by a scale in the oven is entered into a microprocessor which
is programmed to calculate the proper cooking time and then
controls the magnetron in addition to giving an operational
status to the user through displaysO
Referring to Figure 10, an alternate embodiment of the
circuit of Figure 5 is shown. As described earlier herein,
for commercial applications, it may be preferable that the
~0 microprocessor control be provided by a customized integrated
circuit which includes therein many of the interface functions.
The embodiment of Figure 10 shows a general purpose micro-
processor with ancillary hardware and interfaces coupling it
to the microwave oven control panel, sensors, and magnetron
control. An example of microprocessor 100 that could be used
is a MOS Technology, Inc. MCS6502~ As shown in Figure 10, the
microprocessor is connected to data bus 102 which typically
comprises eight lines which may be connected to MCS6502 pins
26-33, respectively. The microprocessor is also connected to
address bus 10~ which typically comprises sixteen lines which
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:
~ ~73~5
; may be connected to MCS6502 pins 9-25~ respectively. Conven-
tional initiating circuitry (INIT) 106 is used only at power up
time by the microprocessor and may be connected to input pins
6 and 40 of microprocessor MCS6502. Further, a conventional
crystal clock (CLOCK GENerator) 108 is required and may be
input to the microprocessor on pins 37 and 39. Line 110 is
used to provide the clock to peripheral interface devices 112
and 114, program memory (ROM~ 116 and data memory (RAM) 118.
Microprocessor 100 provides the same functions as the micro-
processor described with reference to Figure 5. The program
memory 116 which preferably is a read only memory stores the
operational program. The task of writing the program from the
requirements given herein with reference to Figures 7, 8 and 9
are well known to one skilled in the art. Microprocessor 100
provides addresses to address bus 104 to fetch instructions
from program memory 116 and data from data memory 118 which
is a random access memory. Write enable and other control
functions are provided from microprocessor l00 to data memory
118 or peripheral interface devices 112 and 114 on control bus
120.
Peripheral interface devices 112 and 114 allow micro-
processor 100 to read data from keyboard 63, to test the
state of sensors and switches, display the results of internal
operations and control the magnetron. Example peripheral
interface devices 112 and 114 are MCS6522's which may have pins
21-40 connected to control, timing, interrupt, data bus and
address bus. Peripheral interface device 112 provides interface
~` for control panel 30 which includes keyboard 63 and displays
65. Keyboard inputs to the microprocessor are provided by a
conventional matrix scan technique. More specifically, the
;`i - 28 -
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~ 73~
~eyboard comprises a matrix of switches which may be of the
contact or capacitive touch variety. For the control panel of
Figure 6, a 4 x 6 matrix would be sufficient; however, a larger
-` matrix will be described and it is assumed that it may contain
functions not discussed herein. Output signals are sequentially
provided to the columns of the matrix and the rows are sensed
and decoded. In detail, pins 10-17 of MCS6522 are connected to
e~ght lines 124 connected to high current output buffer 126 and
segment output port 128. At the output of high current output
buffer 126 which may, for example, be a 74LS374, eight lines
130-138 as indicated connect through eight amplifiers 139 to
the keyboard. Sequence column scanning pulses are provided on
lines 130-138; the rows of the matrix of switches of the key-
board are sensed by lines 140 which are connected to pins 1-9
of peripheral interface device 112. The sensed data is decoded
whereby microprocessor determines which switches of the switch
matrix of keyboard 63 are closed.
Digital displavs 65 are scanned which means that each
digit is driven for a short period of time, such as two milli-
seconds, in sequence. The entire display is scanned at a ratewhich the eye cannot detect. Lines 130-138 are coupled through
driver circuits, two circuits in Figure 6 being representative
of eight in the embodiment. Each conventional circuit as shown
comprises Vcc which is typically +5 volts, Rl which may be
1.5 K ohms, R2 which may be 1.0 K ohms, and transistor Q.
. These sequenced driver circuits determine which digit of the
display is activated. The data that determines which segments
of a particular digit are on is determined by the output of
segment output port 128 which is coupled to lines 142-150
through resistorS R3 to displays 65~ An example of a segment
- 29 -
1~739:~5
output port is an MC3482. The data and scan pulses time share
lines 124, the enable control to port 128 and buffer 126 being
provided on lines not shown by peripheral interface device
114 on pins 3 and 4, respectively.
Microprocessor 100 controls the output of magnetron
through peripheral interface device 114. More specifically,
outputs from peripheral interface device 114 on lines 160 are
connected to high current output buffer 162 which may be, for
example, a 74LS374. As shown in Figure 10, two of the outputs
of buffer 162 are connected to conventional optical isolators
164 and 166 which may be, for example, MOC3010s. A LOW voltage
(logical 0) at the input of an optical isolator causes the
internal resistance of its output to be a short circuit.
In response to a control signal from optical isolator
164, triac 168 is turned on energizing filament transformer
173. In response to a control signal from optical isolator
166, triac 169 is turned on energizing high voltage power
supply 171 which typically comprises a regulating transformer
in accordance with well known practice. In operation, fila-
ment transformer 173 energizes the filament of magnetron 73
` and high voltage power supply 171 provides approximately 4000 volts to the plate of the magnetronO
Omitted from Figure 10 are many common features such as,
;` for example, interlocks, a blower and fuses. Light emitting
diode 180 which is part of scale 20 directs light towards
photodetector 182. As described with reference to the preferred
embodiment, the analog voltage output of the photodetector is
preferably substantially linear with the weight of the food in
the microwave cavity. The analog voltage output on line 134
is transferred to analog to digital converter 186 which upon
;
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` ~` `
. ; '
~73~1S
command from the microprocessor through peripheral interface
114 on line 188 provides a pulse output which has a time
duration determined by the analog voltage input. Information
derived from this pulse is transferred through peripheral
interface device 114 to microprocessor 100 on data bus 102.
By counting the duration of the pulse, microprocessor 100
determines the weight on the scale.
Referring to Figures 11 and 12, respective side and top
views show scale 20 embodied in a bottom fed microwave oven
such as a conventional microwave electric range. Electric
heating element 200 is positioned towards the bottom of cavity
202 of a microwave electric oven. The microwave energy is pro-
vided by magnetron 204 which has an output probe 206 which is
inserted directly into well 208 of the cavity. The microwave
energy couples from the output probe 206 through a directive
radiator 210 having three antenna ports 212. The microwave
energy propagates through microwave transparent cover 214.
Choke structure 216 prevents microwave energy from leaking out
of the gap between well 208 side walls and the floor of the
cavity. A blower 218 directs air across the fins of magnetron
204 and up ~nto well 208 by duct 220 through apertures 222.
The flow of air may be used to provide a force on veins 224 of
radiator 210 to provide rotation. Scale 20 which is the same
as that described with reference to Figures 1, 2 and 3, is
supported by brackets 230 extending downward from the oven
floor as shown in Figure 11. Because scale 20 substantially
; forms a rectangle without structure in the interior, the bottom
fed microwave source can be positioned in the middle of the
oven floor without 5tructural interference. Pins 22 protrude
~' 30 through holes in the floor of the oven to support plate 26.
31 -
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:`
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The pins 22 in this embodiment are longer than the embodiment
of Figures 1, 2 and 3 so as to rise above electric heating
element 200. Pins 22 may also provide support for oven racks
so as to provide a weight indication of food bodies placed on
them.
This concludes the description of the preferred embodi-
ment. The reading of it however will bring to mind many
modifications to one skilled in the art without departing
from the spirit and scope of the invention. Accordingly, it
is intended that the scope of the invention be limited only
by the claims.
.,
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