System and process for controlling dielectric ovens

A control system for controlling the heating of a product in a dielectric oven comprises at least one dielectric heating circuit including an electromagnetic energy source, such as a triode vacuum tube, having an anode and a resonant circuit including at least one inductor and at least a pair of capacitors. Each capacitor includes two capacitor plates and one of these capacitor plates is moveable, such that each pair of capacitors forms a variable capacitor in which the product to be heated is a dielectric. The system also includes at least one ammeter for measuring actual anode current at the anode. A motor is used to increase or decrease a distance between the plates of at least one of said capacitors, thereby adjusting the electromagnetic energy delivered to the product. A processor receives ammeter measurements, whereby the distance between the pair of capacitor plates is adjusted to increase or decrease the actual anode current. The processor also receives, stores, and retrieves a requested anode current and compares the requested anode current to the actual anode current to determine whether to increase or decrease the distance between the pair of capacitor plates. Alternatively, the electromagnetic energy source may have a duty cycle adjusted by a keying circuit or an anode voltage adjusted by a voltage control device, or both. The processor may include a timer, whereby an average actual anode current is measured. The processor may receive, store, and retrieve a requested average anode current and compares this current to the actual average anode current to adjust the electromagnetic field electrically by either increasing or decreasing the duty cycle or increasing or decreasing the anode voltage of the electromagnetic energy source, or both, and thereby increase or decrease the average actual anode current.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to systems and processes for controlling the heating 
of a product, such as cooking foodstuffs, in a dielectric oven. Further, 
it relates to systems and processes for controlling dielectric ovens 
having multiple support levels for heating commercial quantities of the 
product. Particularly, it relates to systems and processes for controlling 
the current flow within capacitor plates which produce an electromagnetic 
field providing electromagnetic energy to the product. 
2. Description of the Related Art 
Commercial ovens are commonly convection ovens utilizing a slow convection 
heating process to heat products. Dielectric ovens, however, heat a 
product due to the electric, i.e., dielectric, losses caused when the 
product is placed in a varying electromagnetic field. If the product is 
homogeneous and the electromagnetic field is uniform, heat may develop 
uniformly and simultaneously throughout the mass of the product. 
Dielectric ovens are known, and examples of such ovens are disclosed in 
U.S. Pat. No. 4,812,609 to Butot; U.S. Pat. No. 4,978,826 to DeRuiter et 
al.; and U.S. Pat. No. 4,980,530 to Butot, which are incorporated herein 
by reference. Such ovens may operate in a frequency range of 2 to 40 Mhz. 
Referring to FIG. 1a, a dielectric oven 200 may be fitted with guide racks 
202 for stacking a plurality of trays 204 carrying a product 206 to be 
heated. These racks 202 also may function as electrodes for producing an 
electromagnetic field. A variable air capacitance 212 is created between 
tray 204 containing product 206 and electrodes 202 to control the 
electromagnetic energy applied to product 206. 
Dielectric ovens may utilize an oscillating circuit or circuits having 
specially designed electromagnetic energy sources, such as power tubes. 
Such energy sources may be coupled and supply current to guide rack 
electrodes 202 via contacts 205 which project through an oven housing 209 
into heating cavity 208. The oscillating circuit(s) generally provide a 
substantially fixed distribution of voltage and power within a heating 
cavity. Thus, longer heating times may be required for heating greater 
quantities of products. Further, frequencies at which the ovens are 
operated are dependent on the characteristics of the product being heated. 
Referring to FIG. 1b, although dielectric ovens 200' may handle a plurality 
of vertically stacked trays 204', which permit products 206' to be heated 
at multiple levels 210' within a single heating cavity 208', only a single 
pair of electrodes 202' may be provided to apply the electromagnetic 
energy for heating. Thus, when a number of different heating levels 210' 
are used, the amount of energy applied to product 206' in each tray 204' 
may be reduced, and heating may take longer. As discussed above, 
electromagnetic energy sources may be coupled and supply current to 
electrodes 202' via contacts 205' which project through an oven housing 
209' into a heating cavity 208'. A variable air capacitance 212' may be 
created between tray 204' (and product 206') and electrodes 202' to 
control the energy applied to product 206'. 
A dielectric oven may include a heating cavity for receiving a tray 
containing the product, an electromagnetic energy source; oscillating 
circuit for producing an electric signal, and an electrode configuration 
for producing an electromagnetic field in the cavity to apply energy from 
the oscillating circuit to the product. Such ovens are broadly operable 
for increasing the energy applied from the oscillating circuit to the 
product, without increasing the operating voltage of the electromagnetic 
energy source or the frequency of its operation. These ovens may include a 
plurality of oscillating circuits having substantially similar resonant 
frequencies. 
The oscillating circuits may receive power from a power robe in order to 
establish respective oscillating signals. More particularly, at least 
first and second oscillating circuits may be provided, and the electrode 
configuration may include at least first and second electrodes, each of 
which is a component of one of the at least two oscillating circuits. The 
product may be bracketed between electrodes of a capacitor in the 
oscillating circuit. The oscillating circuit is arranged to provide a 
voltage across the capacitor which is twice the voltage across the power 
source, thus permitting doubling of distance between the electrodes of the 
capacitor without reducing the electromagnetic field strength and 
increasing of quantities of the product which may be heated between the 
capacitor electrodes. 
Each of the oscillating circuits may also include an inductance and a 
capacitance. The capacitance includes a pair of capacitors respectively 
formed between two capacitor plates, i,e., the electrodes of the 
oscillating circuit, and another pair of plates, for example, wall 
portions of the heating cavity. The two electrodes of each oscillating 
circuit may be oriented to produce an open electromagnetic field between 
them. In this configuration, electrode pairs form a pair of 
interconnecting load capacitors between the electrodes of the oscillating 
circuits. The dielectric of the load capacitors includes the product 
placed between the electrodes of the capacitors, i.e., within the 
capacitance. 
This configuration produces an open electromagnetic field between the 
electrodes of each of the pair of interconnecting (load) capacitors. The 
open electromagnetic field has a power intensity distribution determined 
by the dielectric characteristics of the product, while permitting the 
electromagnetic energy source to operate at a substantially constant power 
level. Further, the use of the load capacitors as connectors between the 
oscillators isolates the frequency of oscillation of the oscillating 
circuits from the effects of the dielectric characteristics of the 
product. Thus, both the power intensity and the frequency of the power 
transferring signals are maintained more nearly constant, with reduced 
variations caused by the dielectric characteristics of the product being 
heated. 
A variable air capacitance or air "gap" may be included between the 
electrodes of the interconnecting capacitors and the product for 
controlling the energy applied to the product between the load capacitors. 
See FIGS. 1a and 1b. Nevertheless, such a variable air "gap" may interfere 
with the rapid insertion and removal of trays containing products for 
heating. Further, because of the speed with which products may be heated 
in a dielectric oven, manual control of the dielectric oven may be 
difficult and inefficient. Because heating may include the thawing and 
cooking of frozen foodstuffs, accurate control of the oven prevents uneven 
or inadequate heating. 
SUMMARY OF THE INVENTION 
Thus, a need has arisen for a more efficient system for controlling the 
heating of a product in a dielectric oven. It is an object of this 
invention that a variable capacitor is adjusted to control the 
electromagnetic field produced by the heating circuit and, thereby, to 
control the heating of the product. It is a feature of this invention that 
the capacitance may be mechanically adjusted by varying the distance 
between capacitor plates, e.g., a pair of capacitor plates whereby the 
strength of the electromagnetic field between the plates is adjusted. It 
is also a feature of this invention that the average current supplied to 
the plates during the heating cycle may be adjusted electrically, either 
by varying the grid voltage supplied to the generator or by varying the 
anode voltage supplied to the generator. The electrical and mechanical 
methods of adjustment may be combined to control the electromagnetic field 
produced by the dielectric circuit. It is a particular advantage of 
embodiments employing electrical adjustment of the power delivered to the 
food product that fewer moving parts are used than in a mechanically 
adjusted system or a system combining electrical and mechanical 
adjustment. Further, an electrical adjustment system is generally more 
reliable and easier to manufacturer and maintain. 
It is yet another object of this invention that sensors may directly 
measure the temperature of the product within the electromagnetic field. 
It is an advantage of this invention that the sensor(s) may measure the 
temperature of the product while it is being heated to permit constant 
monitoring of the performance of the dielectric oven. It is a feature of 
this invention that the progress of the product's heating, e.g., product 
temperature, may be measured by means of an infrared sensor or by passing 
a fluid-filled tube through the product and measuring the temperature 
change in the fluid. A suitable fluid has a low dielectric loss constant, 
but relatively high thermal conductivity, such as oil or air, so that the 
fluid is heated by the product, rather than the electromagnetic field 
produced in the oven. 
It is still another object of this invention that the sensors may 
indirectly measure the heating of the product within the electromagnetic 
field. Again, it is an advantage of this invention that the sensor(s) may 
measure the temperature of the product while it is being heated to permit 
constant monitoring of the performance of the dielectric oven. It is a 
feature of this invention that the progress of the product's heating may 
be measured by determining the temperature or humidity difference between 
air drawn into the oven and air exhausted from the oven. 
An embodiment of the invention is a control system for controlling the 
heating of a product in a dielectric oven. The control system comprises at 
least one dielectric heating circuit including an electromagnetic energy 
source, such as a triode vacuum tube, having an anode. The system also 
comprises a resonant circuit including at least one inductor and at least 
a pair of capacitors, wherein each capacitor includes two capacitor plates 
and at least one of the capacitor plates is moveable. Each pair of 
capacitors forms a variable capacitor in which the product to be heated is 
a dielectric. At least one ammeter measures the actual current at the 
anode. A motor increases or decreases a distance between the plates of at 
least one of the capacitors, thereby adjusting the electromagnetic energy 
applied to the product. A processor, such as a microprocessor, receives 
ammeter measurements, whereby the distance between the pair of capacitor 
plates is determined, and receives, stores, and retrieves a requested 
anode current. The processor also compares the requested anode current to 
the actual anode current to determine whether to increase or decrease the 
distance between the plates of at least one of the capacitors, thereby 
increasing or decreasing the actual anode current, and instructs the motor 
to adjust the distance between the plates. 
In another embodiment, a control system controls the heating of a product 
in a dielectric oven. The system comprises at least one dielectric heating 
circuit including an electromagnetic energy source having an anode and a 
selectable duty cycle and a resonant circuit including at least one 
inductor and at least a pair of capacitors, wherein each capacitor 
includes two capacitor plates. Each pair of capacitors forms a variable 
capacitor in which the product to be heated is a dielectric. At least one 
ammeter measures an actual current at the anode. A keying device, such as 
a grid block keyer, adjusts the duty cycle. A processor includes a timer 
and receives ammeter measurements, whereby the actual average anode 
current is determined. It also receives, stores, and retrieves a requested 
average anode current; compares the requested average anode current to the 
actual average anode current to determine whether to increase or decrease 
the duty cycle, thereby increasing or decreasing the actual average anode 
current; and instructs the keying device to adjust the duty cycle. 
In a third embodiment, a control system controls the heating of a product 
in a dielectric oven. The system comprises at least one dielectric heating 
circuit including an electromagnetic energy source having an anode and a 
resonant circuit including at least one inductor and at least a pair of 
capacitors, wherein each capacitor includes two capacitor plates. Each 
pair of capacitors forms a variable capacitor in which the products to be 
heated are a dielectric. At least one ammeter measures an actual current 
at the anode. A voltage control device, such as a motorized variac or a 
triac, controls a first voltage provided to a power supply, such as a 
transformer, which provides a second or anode voltage at the anode of the 
electromagnetic energy source. A processor includes a timer and receives 
ammeter measurements whereby the actual average anode current is 
determined. It also receives, stores, and retrieves a requested average 
anode current, compares the requested average anode current to the actual 
average anode current to determine whether to increase or decrease the 
second or anode voltage, thereby increasing or decreasing the actual 
average anode current, and instructs the voltage control device to vary 
the first voltage to the power supply. 
In still another embodiment of the invention, the control system may 
comprise a combination of the components of the embodiments described 
above. 
A further embodiment of the invention is a process for controlling the 
heating of a product in a dielectric oven comprising a processor, an 
electromagnetic energy source, such as a triode vacuum tube, having an 
anode, and a resonant circuit including at least one inductor and at least 
a pair of capacitors. Each of the capacitors has a pair of capacitor 
plates, and the product is located between at least said pair of 
capacitors. The process comprises the steps of requesting an anode current 
and measuring an actual anode current. Further, it comprises the steps of 
comparing the requested anode current to the actual anode current to 
determine whether to increase or decrease a distance between at least one 
pair of capacitor plates, thereby increasing or decreasing the actual 
anode current, and adjusting the distance between the at least one pair of 
capacitor plates. 
In another embodiment of the process of this invention, a process for 
controlling the heating of a product in a dielectric oven comprises a 
processor, an electromagnetic energy source having an anode and a 
selectable duty cycle, and a resonant circuit including at least one 
inductor and at least a pair of capacitors. Each of said capacitors has a 
pair of capacitor plates, and the product is located between at least said 
pair of capacitors. The process comprises the steps of measuring an anode 
current, selecting a duty cycle for the electromagnetic energy source, and 
determining an actual average anode current and a requested average anode 
current. Further, the process comprises the steps of comparing the 
requested average anode current to the actual average anode current to 
determine whether to increase or decrease the duty cycle, thereby 
increasing or decreasing the actual average anode current, and adjusting 
the duty cycle. 
In yet another embodiment of the process of this invention, a process for 
controlling the heating of a product in a dielectric oven comprises a 
processor, an electromagnetic energy source having an anode and a resonant 
circuit having at least one inductor and at least a pair of capacitors. 
Each of said capacitors has a pair of capacitor plates, and the product is 
located between at least said pair of capacitors. The process comprises 
the steps of measuring an anode current, selecting an anode voltage for 
the electromagnetic energy source, and determining an actual average anode 
current and a requested average anode current. Further, the process 
comprises the steps of comparing the requested average anode current to 
the actual average anode current to determine whether to increase or 
decrease the anode voltage, thereby increasing or decreasing the actual 
average anode current, and adjusting the anode voltage. 
The process may also include a combination of the adjustment of the 
distance between capacitor plates and the adjustment of the duty cycle or 
anode voltage, or both, to increase or decrease the electromagnetic field 
strength. Although various combinations of these steps are possible, in at 
least one embodiment, larger variations in energy applied to the product 
are accomplished by changing the position of the capacitor plates while 
smaller variations of energy applied to the product are accomplished by 
adjusting the duty cycle or anode voltage. 
Other objects, advantages, and features will be apparent when the detailed 
description of the invention and the drawings are considered.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 2, a schematic diagram of a preferred embodiment of the 
control system 1 is depicted. Control system 1 is comprised of a 
dielectric oven 10 which may receive a plurality of product trays 12. Each 
of product trays 12 is placed in an oven cavity 11 and between a pair of 
capacitors 13. The electrodes (not shown) thus produce an electromagnetic 
field providing energy from a electromagnetic, high frequency generator 22 
to the food product (not shown) in product tray 12. 
Referring to FIG. 3, each of capacitors 13 is formed by a pair of capacitor 
plates 13a and 13b. At least one of capacitor plates 13a or 13b is 
moveable. For example, plate 13b may be attached to a positioning bar 60. 
Bar 60 may move plate 13b forward to reduce the separation 62 of plates 
13a and 13b, i.e., increase the capacitance and increase the energy 
applied to the product, or its movement may be reversed to increase 
separation 62 of plates 13a an 13b, i.e., decrease the capacitance and 
reduce the energy applied to the product. In the embodiment depicted in 
FIG. 3, capacitor plates 13a and 13b are substantially outside of an oven 
cavity 64 and are separated from cavity 64 by an oven liner 66 fabricated 
from a material with a low dielectric loss constant, such as a polyester 
or polycarbonate resin. The electromagnetic field (not shown) is produced 
between a pair of electrodes 68 mounted on product tray 12 (only one 
shown), wherein each electrode 68 is electrically connected to a plate 13a 
by, for example, a contact 70. Contact 70 passes through liner 66 and into 
cavity 64. Product tray 12 is also fabricated from a material with a low 
dielectric loss constant, such as a polyester or polycarbonate resin, and 
in another embodiment, product tray 12 may be supported by an electrode 
extending from plate 13a. 
Referring again to FIG. 2, a voltage control device 19 supplies a first 
voltage to a high voltage power supply 20 which provides or second or 
anode voltage, e.g., 5500 volts DC, to electromagnetic, high frequency 
generator 22 comprised of a triode vacuum tube 23 and associated resonant 
circuitry. Generator 22 operates at a frequency determined by the resonant 
circuit including inductor assembly 24 and capacitors 13. At least one 
ammeter measures current at an anode 23' in generator 22 which is an 
indirect measure of the power delivered between capacitor plates 13a and 
13b. 
Referring to FIG. 4, a dielectric oven includes oven cavity 64. Oven cavity 
64 may have air intake ports 72 and at least one air exhaust port 74. As 
shown in FIGS. 2 and 4, at least one temperature sensor 16 may measure the 
temperature difference between air entering intake ports 72 and exiting 
exhaust port 74 or the temperature change over time (e.g., .DELTA.T) at, 
for example, exhaust port 74, or both. Intake ports 72 and exhaust ports 
74 may be equipped with intake and exhaust fans (not shown), respectively, 
to improve ventilation within oven 10. Further, at least one humidity 
sensor 18 may measure the difference between the humidity of air entering 
intake ports 72 and exiting exhaust port 74 or the humidity change over 
time (e.g., .DELTA.H) at, for example, exhaust port 74, or both. By 
measuring temperature or humidity, or both, in this way, it is possible to 
monitor oven performance. For example, the humidity difference between air 
entering intake port 72 and air exiting exhaust port 74 or the humidity 
change over time in air exiting exhaust port 74 is an indication of the 
progress of the heating of the product, such as a foodstuff. The 
difference in air temperature or humidity, or both, may be attributed to 
dielectric losses in the product or water vapor, e.g., steam, produced 
during heating of the product. Further, if the product is heated in a 
heating liquid, e.g., water, product heating may be monitored by 
determining the temperature of the heating liquid or the vapor, e.g., 
steam, present in the oven or in the air exhausted from the oven as a 
result of the heating of the heating liquid. 
During the early phases of heating, the humidity in the exhaust air will 
increase. Eventually, however, humidity released from the product will 
stabilize and decline. The measurements obtained by temperature sensor 16 
or humidity sensor 18, or both, are transmitted to a microprocessor 
control board 40. Further, as depicted in FIG. 4, interlocks 76 may be 
located around an access 78 to oven 10, so that when access cover 80 seals 
access 78, interlocks 76 are closed and a closure signal is sent to 
control board 40. Because of dangers in the dielectric heating of products 
and the possible leakage of electromagnetic energy from the dielectric 
oven, it is preferable that the oven be equipped with interlocks 76 to 
prevent its operation, e.g., prevent the flow of current from generator 22 
to one of capacitor plates 13a or 13b, when oven access 80 is open. 
As discussed above, generator 22 has a selectable duty cycle. A duty cycle 
is the ratio of working time to total time for an intermittently operating 
device. It may be expressed as a percentage. For example, if the total 
time for one cycle is one second and generator 22 supplies current for 
0.33 seconds during that cycle, generator 22 has a 33 percent duty cycle. 
This duty cycle is adjusted by a grid block keyer 28, and keyer 28 is 
controlled by control board 40 to adjust the duty cycle of generator 22. 
Keyer 28 may key the dielectric heating circuit by applying negative grid 
bias 29 at several times the cutoff value to the grid of a power tube 23' 
during key up conditions. For example, when the key is down, the blocking 
bias is removed, and normal current flows through the keyed circuit. 
As previously discussed, voltage control device 19 is connected in series 
with the available line voltage and the primary windings of power supply 
20. Signals generated by microprocessor 40 instruct voltage control device 
19 to adjust the amplitude of voltage delivered to power supply 20, 
whereby the anode voltage delivered to anode 23' of power tube 23 is 
varied. 
In addition to temperature and humidity data, microprocessor control board 
40 also receives anode current measurements 25 transmitted from ammeter 14 
and generator 22. Microprocessor control board 40 also senses the grid 
current 26 in generator 22 and senses the state 27 of and controls the 
intake fan(s) and the exhaust fan(s) (not shown) by controlling the flow 
of current from generator 22. 
The separation 62 of capacitor plates 13a and 13b (see FIG. 3) is adjusted 
by means of motors, such as upper motor 30a and lower motor 30b. Such 
motors receive adjustment instructions from microprocessor control board 
40 and supply feedback on their operation to microprocessor control board 
40. Motors 30a and 30b may be powered by current supplied from an 
alternative power source (not shown). 
Product identification, product heating parameters, and operational safety 
limits may be input to microprocessor control board 40 from a control 
panel and displays 50. Control panel and displays 50 may include various 
means for inputting information including a keyboard, a touch pad, a touch 
screen, a bar code reader, or the like. Information input to 
microprocessor control board 40 may be received by, stored in, and 
retrieved from storage components, such as RAMs or EPROMs. Control panel 
and displays 50 may also be used to review heating performance sensor 
measurements received by microprocessor control board 40 and to monitor 
the operation of keyer 28 and motors 30a and 30b. Control panel and 
displays 50 may further be used to monitor the status of interlocks 76. 
The flow charts of FIGS. 5-9 depict embodiments of a main control process 
logic loop (FIGS. 5a and 5b), a state routines flow chart (FIG. 6), a flow 
chart depicting electromagnetic energy output control using moveable 
capacitor plates and a related capacitor plate position control flow chart 
(FIGS. 7a and 7b), a flow chart depicting electromagnetic energy output 
control using duty-cycled grid blocking and a related flow chart for 
control of grid duty cycle (FIGS. 8a and 8b), and a flow chart depicting 
electromagnetic energy output control using variable anode voltage and a 
related flow chart for control of anode voltage (FIGS. 9a and 9b). The 
first of these figures shows a main loop, FIG. 5a, and an alternate main 
loop using cooking performance sensors, FIG. 5b. Referring to FIG. 5a, 
main loop A begins with a power-up and initialize operation 2. In this 
operation, various tasks may be performed. In general, however, power is 
supplied to microprocessor control board 40 and various internal 
diagnostic tests are performed. Power supply 20 and generator 22 are 
activated, and keyer 28 transmits the initial duty cycle to generator 22. 
Moreover, upper and lower motors 30a and 30b position the moveable 
capacitor plates to the "home" or power-up position. Finally, all sensors 
and measuring devices are zeroed, and diagnostic tests and internal checks 
are performed on control panel and displays 50. 
After the control system has been powered-up and initialized, the state 
routines operation 3 is performed. This operation is described in greater 
detail in FIG. 6. State routines operation 3 involves the selection of the 
"Cook" or "Idle" state of operation for the control system. Generally, in 
the Cook state, the system heats products. In the Idle state, however, the 
system remains powered-up and initialized and ready to receive products or 
to commence or resume heating products already placed within the 
dielectric oven. 
User input/output (I/O) operation 4 allows an operator to key inputs into 
the control system and to display system outputs using control panel and 
displays 50. In this operation, heating parameters, such as heating time, 
thaw time, or anode current for new products may be requested, or 
preprogrammed heating parameters may be requested by identifying the 
product type or the quantity of products to be heated, or both. In this 
operation, the number of heating levels to be utilized may also be input. 
For example, heating parameters may be input to instruct the oven to heat 
a foodstuff at a lower electromagnetic field strength for a prescribed 
period in order to gently defrost the foodstuff and then to gradually 
increase the electromagnetic field strength until a higher cooking 
intensity is reached. 
In update inputs operation 5, the sensors and interlocks, i.e., door 
switches, are checked. This operation involves a status check of each 
sensor and interlock to insure that it is operational. Further, sensors, 
such as ammeter 14, may be read. Sensors also may be updated with new 
safety limits (I.limit) or tolerances (I.tol). Safety limits include 
limits on the amount of anode current which may be supplied to plates 13a 
or 13b. This prevents current overloads and overheating of the dielectric 
heating circuits. Further, such safety limits may protect the generator 
from short circuits. Tolerances may be input to the control system to 
prevent duty cycle, anode voltage, or plate distance adjustments as a 
result of insignificant variations or fluctuations in the capacitance. 
Associated with update inputs operation 5 is condition error check 
operation 6. In this operation, errors identified during update inputs 
operation 5 may be corrected. Some error conditions may be due to 
improperly input updates. Other errors may be due to faulty sensors or 
interlocks. Diagnostic checks may be run on selected sensors and 
interlocks from condition error check operation 6. Some sensors or 
interlocks may be replaced while others experiencing programming problems 
may be corrected from control panel and displays 50. 
Once operations 4, 5, and 6 have been completed and all sensors and 
interlocks have been checked and all heating parameters have been 
requested, input, or updated; electromagnetic energy output control 
operation 8 is performed. This operation is described in greater detail in 
FIGS. 7a and 7b, 8a and 8b, and 9a and 9b. The steps involved in operation 
8 are dependent on the method of controlling the capacitance, e.g., using 
moveable capacitor plates (FIGS. 7a and 7b), using duty-cycled grid 
blocking (FIGS. 8a and 8b), or using variable anode voltage (FIGS. 9a and 
9b). Once operation 8 has been completed, however, the other output update 
operation 9 which involves a status check of other control system outputs, 
such as lapsed heat time, may be completed. 
Referring to FIG. 5b, an alternative embodiment of the main loop B using 
cooking performance sensors is depicted. This main loop is essentially 
identical to main loop A depicted in FIG. 5a. Nevertheless, main loop B of 
FIG. 5b is intended for use with a dielectric oven equipped with cooking 
performance sensors. In the cooking performance input update operation 7, 
cooking performance sensors, such as temperature sensor(s) 16 and humidity 
sensor(s) 18, are provided with predetermined safety limits, such as high 
temperature or high humidity limits, and efficiency limits, such as an 
optimum temperature or optimum humidity curve profile. These limits allow 
the system to identify preferred, improper, or unsafe cooking performance 
conditions and to monitor the cooking performance of the dielectric oven 
when containing various products. With the exception of this operation, 
main loop B of FIG. 5b is identical to main loop A of FIG. 5a. Thus, in 
either main loop, when the update of other input operation 9 is complete, 
the system returns to perform state routines operation 3. 
Referring to FIG. 6, state routines performance operation 3 is depicted in 
greater detail. At step 100, the system determines whether it is in the 
Cook state. If the system is in the Cook state, the system determines 
whether the Cook state variables have been initialized. In step 102, the 
system determines whether the substate value equals zero (Substate=0). If 
the substate variable equals zero, no Cook state variables have been 
initialized, and the Cook state variables are then initialized, as 
indicated in step 104. Once the Cook state variables have been 
initialized, the substate value is set at 1 (Substate=1), as indicated in 
step 106. If, however, it is determined at step 102 that the substate 
value is not equal to zero (Substate=0), the state routines operation 
skips steps 104 and 106 and proceeds to step 108. In step 108, various 
cooking parameters, such as the cook timer, the cook stages for heating 
products in multiple stages, and the cook cycle for repetitive heating, 
are monitored. 
In step 110, the requested anode current (I.req) which is essentially a 
measure of requested electromagnetic energy output is set into the control 
system. This may be accomplished by microprocessor control board 40 
retrieving heating parameters previously stored in its storage components, 
or new data or parameters may be input to a control system. Alternatively, 
a measure of electromagnetic energy output may be determined by 
microprocessor control board 40 using a heating parameters algorithm based 
on the type and quantity of product placed in the dielectric oven. During 
state routines operations, the control system determines whether the cook 
cycle has been completed or cancelled, as indicated in step 112. If the 
cook cycle has not been completed or cancelled, the control system returns 
to the main loop. However, if the cook cycle has been completed or 
cancelled, the control system first switches to the Idle state and 
requests initialization of Idle state variables by setting the substate 
value to zero (Substate=0) and then returning to the main loop. 
If, however, it was determined at decision step 100 that the system is not 
in the Cook state, the control system proceeds to step 120 and determines 
whether the control system is in the Idle state. If as a result of steps 
100 and 120, it is determined that the system is neither in the Cook state 
nor Idle state, the system is in an invalid or unidentified state. If the 
heating system does not recognize the state, the system defaults to the 
Idle state, as indicated in step 140. In addition, initialization of Idle 
state variables is requested by setting the substate value to zero 
(Substate=0). 
Once it is determined that the system is in the Idle state, the control 
system determines in step 122 whether the substate value is set at zero. 
If the substate value is not already set at zero, the requested plate 
current is set to zero (I.req=0) in step 128. This insures that no 
electromagnetic energy output occurs in the Idle state. If, however, the 
substate value already equals zero (Substate=0), the Idle state variables 
are initialized in step 124. After the Idle state variables have been 
initialized in step 124, the substate value is set to one (Substate=1) in 
step 126. 
With the Idle state variables initialized and the requested plate current 
set at zero (I.req=0), the system determines whether the cook cycle has 
started in step 130. If the cook cycle has not started, the state routines 
operation 3 is complete, and the control system returns to the main loop. 
If, however, the cook cycle has started, the state is reset to the Cook 
state, and the substate value is zeroed (Substate=0) in step 132 before 
the system returns to the main loop. 
As discussed above, the electromagnetic energy output control operation 
utilized by the control system is determined by anode current. The 
electromagnetic field strength may be controlled by varying separation 62, 
i.e., the distance, between plates 13a and 13b (FIGS. 7a and 7b), by 
adjusting the duty cycle of generator 22 (FIGS. 8a and 8b), or by 
adjusting the anode voltage (FIGS. 9a and 9b). 
EM Energy Output Using Moveable Plates 
Referring to FIGS. 7a and 7b, the electromagnetic energy output control 
operation and the plate position control operation are depicted. In step 
200 of FIG. 7a, it is first determined whether all safety inputs are 
satisfactory. In this step, the system determines whether all monitored 
values are within acceptable levels and whether all interlocks 76 are 
closed. Specifically, the system determines whether the temperature and 
humidity sensed by temperature sensor 16 and humidity sensor 18, 
respectively, are within acceptable ranges for cooking performance. If any 
of the safety outputs is unsatisfactory, the system confirms that 
electromagnetic energy output is off, as indicated in step 202. The 
control system then instructs motors 30a and 30b to place capacitor plates 
13a or 13b in the "home" position of step 204. The "home" position for the 
plates may be the position at which the capacitance generated by the pair 
of capacitors is the minimum capacitance achievable using the moving 
plates. The system then returns to the main loop and continues main loop 
operations. 
If all safety inputs are satisfactory, the system proceeds to step 206 in 
which it determines whether the anode current measured (I.anode) by 
ammeter 14 is less than the safety limit for anode current (I.limit). If 
the measured or actual anode current equals or exceeds the safety limit 
for anode current (I.anode.gtoreq.I.limit), the system again confirms that 
electromagnetic energy output is off in step 202 and causes the capacitor 
plates to be placed in the "home" position of step 204. However, if the 
actual anode current is less than the safety limit for anode current 
(I.anode&lt;I.limit), as indicated in step 208, the system next determines 
whether the requested anode current is greater than zero (I.req&gt;0). If 
requested anode current is not greater than zero (I.req.ltoreq.0), once 
again the system confirms that electromagnetic energy output is off, 
requests that the capacitor plates be placed in the "home" position, and 
returns to the main loop. 
If requested anode current is greater than zero (I.req&gt;0), the control 
system determines whether electromagnetic energy output has already been 
turned on, as indicated in step 210. If electromagnetic energy output is 
not on, the system determines in step 212 whether the plates are in the 
"home" position. If electromagnetic energy output is not on and the plates 
are not in the "home" position, again, the control system confirms that 
electromagnetic energy output is off and places the plates in the "home" 
position. Nevertheless, if the plates are in the "home" position of step 
212, the system turns electromagnetic energy output on, as indicated in 
step 214, and starts the delay timer for the "power on" (PwrOn) delay as 
indicated in step 216. 
The "power on" delay insures that the control algorithm does not react 
until transient effects of turning the generator on or moving the 
capacitor plates have subsided. It also prevents damage to the system from 
occurring when the generator is turned on and immediately turned off. The 
delay requires that the generator be turned on for some minimum period of 
time, e.g., about 0.5 seconds, to insure that the generator is not damaged 
by rapid on/off shifting. Preferably, all of the electrical or 
electromechanical components of the control system are equipped with such 
delays. While these delays are not necessary for the operation of the 
system, they improve its longevity. 
If the system determines in step 210 that electromagnetic energy output is 
already on, the system proceeds to the plate position control operation 
depicted in FIG. 7b. In step 300 of FIG. 7b, the system determines whether 
the plates are being moved forward by motors 30a or 30b. If the plates are 
being moved forward, the system determines in step 302 whether the plates 
are at their forward limit. The forward limit is the plate separation at 
which the greatest capacitance is generated between a pair of capacitors 
in a dielectric heating circuit. If the plates are at their forward limit, 
the system stops the plates forward movement, as indicated in step 304, 
and starts the delay timer for the "minimum stop" (MinStop) delay of step 
306. As mentioned above, whenever an electrical or electromechanical 
component is stopped or started, a delay timer may be started to insure 
that the component is not damaged by rapidly activating and deactivating 
it. After the "minimum stop" delay has been initiated, the system returns 
to the electromagnetic energy output control operation of FIG. 7a. 
If, however, the system determines in step 302 that the plates are not at 
their forward limit, the system then determines whether any delay timer 
has expired, as indicated in step 308. If the delay timer has not expired, 
the system returns to the electromagnetic energy output control operation 
of FIG. 7a and confirms, as indicated in step 218, that electromagnetic 
energy output is still on. Nevertheless, if the delay timer has expired, 
the system proceeds to step 310 and determines whether the measured anode 
current is greater than or equal to the requested anode current 
(I.anode.gtoreq.I.req). If the actual anode current equals or exceeds the 
requested anode current, the control system proceeds to step 304 and stops 
the forward movement of the plates. However, if actual anode current does 
not equal or exceed requested anode current (I.anode&lt;I.req), the system 
returns to the electromagnetic energy output control operation and 
confirms in step 218 that electromagnetic energy output remains on. 
If the plates are not moving forward, the control system determines whether 
the plates are moving in reverse, as indicated in step 312. If the plates 
are moving in reverse, the system determines whether the plates have 
reached their reverse limit. See step 314. If the plates are at the 
reverse limit, the control system stops the plates, as indicated in step 
304, and starts the delay timer for the "minimum stop" delay. The reverse 
limit is the opposite of the forward limit. It is the plate separation at 
which the least capacitance is generated between a pair of capacitors in a 
dielectric heating circuit. 
Nevertheless, if the plates are not at the reverse limit, the control 
system proceeds to step 316 and determines whether the delay timer has 
expired. Again if the delay timer has not expired, the system returns to 
the electromagnetic energy output control operation and confirms that 
electromagnetic energy output is still on. If, however, the delay timer 
has expired, the system determines whether actual plate current is less 
than or equal to requested anode current (I.anode.ltoreq.I.req). If actual 
anode current is less than or equal to requested plate current, the system 
stops the reverse movement of the plates and starts the delay timer for 
minimum stop delay. If actual anode current is greater than requested 
anode current (I.anode&gt;I.req), the system returns to the electromagnetic 
energy output control operation and confirms that the electromagnetic 
energy output remains on. 
If the control system determines that the plates are neither moving forward 
nor reversed, the plates are stopped, as indicated in step 320. While the 
control system may determine that the plates are neither stopped nor 
moving, this is an invalid system mode, as indicated in step 322, and the 
system instructs the plates to stop and starts the "minimum stop" delay 
timer. When such an invalid mode is detected, the system returns to the 
electromagnetic energy output control operation and confirms that 
electromagnetic energy output remains on. 
Nevertheless, once the plates are stopped, the system proceeds to step 324 
and determines whether the delay timer has expired. As discussed above, if 
the delay timer has not expired, the system returns to the electromagnetic 
energy output control operation and confirms that electromagnetic energy 
output is still on. If, however, the delay timer has expired, the system 
determines whether the actual anode current is less than the requested 
anode current less some acceptable tolerance (I.anode&lt;I.req-I.tol). This 
acceptable tolerance of step 326 is an amount that reflects deviations 
that are small enough, such that plate position adjustments are not 
desirable. If the actual anode current is less than the requested current 
minus the acceptable tolerance, the system determines whether the plates 
are at the forward limit, as indicated in step 328. If the plates are at 
the forward limit, the system returns to the electromagnetic energy output 
control operation and confirms that electromagnetic energy output is still 
on. 
If the plates are not at the forward limit, however, the system instructs 
the motor(s) to move the plates forward, as indicated in step 330, and 
starts the delay timer for the "minimum forward" (MinFwd) delay. If actual 
anode current is not less than requested anode current minus the 
acceptable tolerance (I.anode.gtoreq.I.req-I.tol), the system determines 
whether actual anode current is greater than requested anode current plus 
the acceptable tolerance (I.anode&gt;I.req+I.tol). If actual anode current is 
neither less than requested anode current minus the acceptable tolerance 
or greater than actual anode current plus the acceptable tolerance, as 
indicated in steps 326 and 324, the system returns to the electromagnetic 
energy output control operation and confirms that electromagnetic energy 
output is still on. If actual anode current is greater than requested 
anode current plus an acceptable tolerance (I.anode&gt;I.req+I.tol), the 
system determines whether the plates are at the reverse limit, as 
indicated in step 336. If the plates are at their reverse limit, the 
system returns to the electromagnetic energy output control operation and 
confirms that electromagnetic energy output is still on. However, if the 
plates are not at their reverse limit, the system instructs the motor(s) 
to move the plates in the reverse direction, as indicated in step 338. The 
system also starts the delay timer for the "minimum reverse" (MinRev) 
delay. 
EM Energy Output Using Duty-Cycled Grid Blocking 
The system may use duty-cycled grid blocking to adjust average power of a 
generator, as disclosed in FIGS. 8a and 8b. In this embodiment, the system 
again checks to insure that all safety inputs are satisfactory in step 400 
of FIG. 8a. As described above, safety inputs include all heating 
performance parameters and oven interlocks 76. If all safety inputs are 
not satisfactory, the system confirms that electromagnetic energy output 
has been turned off, as indicated in step 402. Further, the system 
requests the "off" grid duty cycle, as indicated in step 404. The "off" 
grid duty cycle is equivalent to a zero percent duty cycle. The duty cycle 
measures the output on the grid blocking circuit. When a zero percent duty 
cycle is initiated, the grid is blocked, and energy is not applied to the 
product. Conversely, at a 100 percent duty cycle, the grid is completely 
unblocked, and full power is applied to the product. 
If all safety inputs are satisfactory, the system next determines whether 
the actual anode current is less than the safety limit for anode current 
(I.anode&lt;I.limit), as indicated in step 406. Once again, if the actual 
current is greater than or equal to the safety limit for anode current 
(I.anode.gtoreq.I.limit), the system confirms that electromagnetic energy 
output is off and initiates the "off" grid duty cycle. If actual anode 
current is less than the safety limit for anode current, however, the 
system determines whether a requested average anode current is greater 
than zero (avg. I.req&gt;0). If the requested average anode current is less 
than or equal to zero (avg.I.req.ltoreq.0), the system again confirms that 
electromagnetic energy output is off and initiates the "off" grid duty 
cycle, as indicated in steps 402 and 404. If the average requested anode 
current is greater than zero (avg.I.req&gt;0), however, the system determines 
whether electromagnetic energy output is already on, step 410. If 
electromagnetic energy output is not on, the system proceeds to step 412 
and initiates the "starting" grid duty cycle. The system then turns 
electromagnetic energy output on and starts the delay timer for the "power 
on" (PwrOn) delay. 
Returning to step 410, if electromagnetic energy output is already on, the 
system proceeds to the grid cycle control operation depicted in FIG. 8b. 
Initially, the control system determines whether the delay timer has 
expired, as indicated in step 500. If the delay timer has not expired, the 
system returns to the electromagnetic energy output control operation and 
confirms that electromagnetic energy output remains on. If, however, the 
delay timer has expired, the system proceeds to step 502 and determines 
whether an actual average anode current is less than the requested average 
anode current minus an acceptable tolerance (avg.I.anode&lt;avg.I.req-I.tol). 
If the actual average anode current is less than the requested average 
anode current minus an acceptable tolerance, the system determines whether 
the duty cycle equals the "maximum duty cycle limit" (MaxLimit) as 
indicated in step 504. The maximum limit may be the 100 percent duty cycle 
or the full power transmission duty cycle. Alternatively, an additional 
safety factor may be built into the grid duty cycle determination. For 
example, the maximum limit may be set at about 80 percent duty cycle to 
protect the anode from current overload. 
If the system determines in step 504 that the duty cycle equals the maximum 
limit, the system returns to the electromagnetic energy output control 
operation and confirms that electromagnetic energy output is on. However, 
if duty cycle is less than the maximum limit, the duty cycle is increased, 
so as to approach the maximum limit. Nevertheless, if after being 
increased, the duty cycle exceeds the maximum limit, it is decreased to 
equal the maximum limit (MaxLimit), as indicated in step 512. On the other 
hand, if the duty cycle is not greater than the maximum limit, the start 
delay timer for the "minimum duty cycle increase" (Minlnc) delay is 
initiated, as shown in step 510, and the system returns to the 
electromagnetic energy output control operation. 
If the actual average anode current is greater than or equal to the 
requested average anode current minus an acceptable tolerance 
(avg.I.anode.gtoreq.avg.I.req-I.tol), the system proceeds to step 514 and 
determines whether the actual average anode current is greater than the 
requested average anode current plus an acceptable tolerance 
(avg.I.anode&gt;avg.I.req+I.tol). If the actual average anode current is 
neither less than the requested average anode current minus an acceptable 
tolerance nor greater than a requested average anode current plus an 
acceptable tolerance, the system returns to the electromagnetic energy 
output control operation and confirms that the electromagnetic energy 
output is still on. However, if the actual average anode current is 
greater than the requested average anode current plus an acceptable 
tolerance (avg.I.anode&gt;avg.I.req 30 I.tol), the system determines whether 
the duty cycle equals the minimum limit. As with the maximum limit, the 
minimum limit may be a zero percent duty cycle which keeps the grid 
blocked, so that energy is not applied to the product. Alternatively, 
however, it may be something greater than a zero percent duty cycle, such 
as an about 20 percent duty cycle, so that some energy is generated to 
allow the heating process to continue. This may prevent cooling of the 
product when the generator is operating at the minimum limit. 
If the duty cycle equals the minimum limit, the system returns to the 
electromagnetic energy output control operation and confirms that the 
electromagnetic energy output is on. However, if the duty cycle is greater 
than the minimum limit, the system decreases the duty cycle, as indicated 
in step 518, to reduce the duty cycle toward the minimum limit. If after 
this decrease in the duty cycle, the duty cycle is less than the minimum 
limit, as indicated in step 520, the system proceeds to step 524 and sets 
the duty cycle equal to the minimum limit (MinLimit). If, however, it is 
determined in step 520 that the duty cycle is greater than or equal the 
minimum limit, the system proceeds to step 522 and starts the delay timer 
for the "minimum duty cycle decrease" (MinDec). Whether the duty cycle is 
greater than or equal to the minimum limit after steps 520 and 524, the 
start delay is initiated, and the system returns to the electromagnetic 
energy output control operation and confirms that electromagnetic energy 
output is still on before returning to the main loop. 
EM Energy Output Using Variable Anode Voltage 
The system may use anode voltage control to adjust the average power, as 
disclosed in FIGS. 9a and 9b. In this embodiment, the system again checks 
to insure that all safety inputs are satisfactory in step 600 of FIG. 9a. 
As described above, safety inputs include all heating performance 
parameters and oven interlocks 76. If all safety inputs are not 
satisfactory, the system confirms that electromagnetic energy output has 
been turned-off, as indicated in step 602. Further, the system requests 
the "off" anode voltage, as indicated in step 604. The "off" anode voltage 
is equivalent to a zero anode voltage. When a zero anode voltage is 
initiated, energy is not applied to the product. 
If all safety inputs are satisfactory, the system next determines whether 
the actual anode current is less than the safety limits of anode current 
(I.anode&lt;I.limit), as indicated in step 606. Once again, if the actual 
anode current is greater than or equal to the safety limit for anode 
current (I.anode.gtoreq.I.limit), the system confirms that the 
electromagnetic energy output is off and initiates the "off" anode 
voltage. If actual anode current is less than the safety limit for anode 
current, however, the system determines whether a requested average anode 
current is greater than zero (avg.I.req&gt;0). If the requested average anode 
current is less than or equal to zero (avg.I.req.ltoreq.0), the system 
again confirms that electromagnetic energy output is off and initiates the 
"off" plate voltage, as indicated in steps 602 and 604. If the average 
requested anode current is greater than zero (avg.I.req&gt;0), however, the 
system determines whether electromagnetic energy output is already on, 
step 610. If electromagnetic energy output is not on, the system proceeds 
to step 612 and initiates the "starting" anode voltage. The system then 
raises anode voltage until electromagnetic energy is detected and starts 
the delay timer for the "power on" (PwrOn) delay. 
Returning to step 610, if electromagnetic energy is already on, the system 
proceeds to the anode voltage control operation depicted in FIG. 9b. 
Initially, the control system determines whether the delay timer has 
expired, as indicated in step 700. If the delay timer has not expired, the 
system returns to the electromagnetic energy output control operation and 
confirms that electromagnetic energy output remains on. If, however, the 
delay timer has expired, the system proceeds to step 702 and determines 
whether the actual average anode current is less than the requested 
average anode current minus an acceptable tolerance 
(avg.I.anode&lt;avg.I.req-I.tol). If the actual average anode current is less 
than the requested average anode current minus an acceptable tolerance, 
the system determines whether the anode voltage equals the "maximum anode 
voltage" limit (MaxLimit) as indicated in step 704. The maximum limit may 
be the maximum voltage obtainable from the power supply or, alternatively 
a limit with a built in safety factor to protect the anode from 
over-voltage conditions. 
If the system determines in step 704 that the anode voltage equals the 
maximum limit, the systems returns to the electromagnetic energy output 
control operation and confirms that electromagnetic energy output is on. 
However, if the anode voltage is less than the maximum limit, the anode 
voltage is increased, as indicated in step 706, so as to approach the 
maximum limit. Nevertheless, if after being increased, the anode voltage 
exceeds the maximum limit, as indicated in step 708, the anode voltage is 
decreased to equal the maximum limit, as indicated in step 712. On the 
other hand, if the anode voltage is less than or equal to the maximum 
limit, the system proceeds to step 710 and starts the delay timer for the 
"minimum anode voltage increase" (Minlnc), and the system returns to the 
electromagnetic energy output control operation. 
If the actual average anode current is greater than or equal to the 
requested average anode current minus an acceptable tolerance 
(avg.I.anode.gtoreq.avg.I.req-I.tol), the system proceeds to step 714 and 
determines whether the actual average anode current is greater than the 
requested average anode current plus an acceptable tolerance 
(avg.I.anode&gt;avg.I.req+I.tol). If the actual average anode current is 
neither less than the requested average anode current minus an acceptable 
tolerance nor greater than a requested average anode current plus an 
acceptable tolerance, the system returns to the electromagnetic energy 
output control operation and confirms that the electromagnetic energy is 
still on. However, if the actual average anode current is greater than the 
requested average anode current plus an acceptable tolerance 
(avg.I.anode&gt;avg.I.req+I.tol), the system determines whether the anode 
voltage equals the minimum limit (MinLimit). As with the maximum limit, 
the minimum limit may be zero anode voltage, so that energy is not applied 
to the product. Alternatively, however, it may be something greater than 
zero, such as 50 percent of the maximum anode voltage, so that some energy 
is generated and the heating process allowed to continue. This may prevent 
cooling of the product when the generator is operating at the minimum 
limit. 
If the anode voltage equals the minimum limit in step 716, the system 
returns to the electromagnetic energy output control operation and 
confirms that the electromagnetic energy output is on. However, if the 
anode voltage is greater than the minimum limit, the system decreases the 
anode voltage, as indicated in step 718, to reduce the anode voltage 
toward the minimum limit. If after this decrease in the anode voltage, the 
anode voltage is less than the minimum limit, as indicated in step 720, 
the system proceeds to step 724 and sets the anode voltage equal to the 
minimum limit. If, however, it is determined in step 720 that the anode 
voltage is greater than or equal the minimum limit, the system proceeds to 
step 722 and starts the delay timer for the "minimum anode voltage 
decrease" (MinDec). Whether the anode voltage is greater than or equal to 
the minimum limit after steps 720 and 724, the start delay is initiated, 
and the system returns to the electromagnetic energy output control 
operation and confirms that electromagnetic energy is still on before 
returning to the main loop. 
Although a detailed description of the present invention has been provided 
above it is to be understood that the scope of the invention is not to be 
limited thereby, but is to be determined by the claims which follow.