Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVEMENTS IN DIELECTRIC CONDITIONING
SPECIFICATION
The present invention relates generally to improvements in
the batch treatment of materials to redistribute and usually
reduce their moisture by subjecting them to radio frequency
energy. More particularly, the present invention relates to
methods and apparatus for the automatic control of the coupling
of radio frequency power sources to materials and the
termination of treatment cycles when a certain pre-determined
average moisture level has been reachedO
In the batch treatment of materials such as plywood veneer,
skins and leather, paper products, sheets oE natural and
synthetic materials, a valuable consideration is obtaining
uniform results in the moisture content of the materials after
the completion of the treatment cycle. With conventional
treatment, however, such uniformity is achieved either by
careful monitoring of the moisture content during the treatment
cycle, with appropriate instrumentation, or by subjecting all
batches to a uniform treatment cycle if the size and moisture
characteristics of each batch as originally loaded in the
dielectric treatment apparatus, is closely controlled. When the
moisture content is monitored, the information is usable for
manually adjusting the coupling or power level of the radio
frequerlcy energy source and termination of the treatment cycle.
The fact that considerable skill, judgment and experience are
necessary for controlling the dielectric treatment process in
order to obtain acceptably uniform product, results in
unacceptably high operating labor costs. On the other hand,
since, in most cases, the materials being treated are frequently
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irregular in terms of moisture content, size and density,
assembling uniform batches for dielectric treatment is
impractically expensive.
In the present application, there is disclosed a controller
for a dielectric treating apparatus having a radio frequency
energy source and means for coupling the source to a moist
dielectric drying load in discrete steps in both directions, to
increase and decrease the coupling to maintain a pre-determined
power level to the load. The controller încludes means for
individually and separately counting the discrete adjustments in
each direction and means for interrupting the operating cycle of
the apparatus in response to reaching a pre-determined
combination of increasing and decreasing adjustments.
It is accordingly a general object of the present invention
to improve the accuracy of control of dielectric treatment
processes and apparatus.
Another object is to provide automatic control of
dielectric treating processes and apparatus, which automatic
control is effective even when successive batches being operated
upon are o~ di~ferent sizes , densities and moisture contents
within relatively broad limits.
- Still another more specific object is that of obtaining
uniform moisture content o~ the treated materials at the end o~
the operating cycle.
In the achievement o~ the foregoing objects, a feature of
the invention relates to means incorporated in the apparatus for
adjusting coupling in discrete steps to maintain radio
frequency input to the load at or near full power within narrow
limitsl as water contained in the load is changed in either
temperature or quantity. With the passage of time after a load
is first subjected to radio frequency energy and while the
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application of radio frequency power continues, the water
content under conventional uncontrolled conditions is first
heated to its boiling point by the energy and then evaporates.
As the temperature of the water in the load rises, there is
greater absorption of energy by the load. However, after the
boiling point is reached, water begins to evapoate and as the
quantity of water in the load is decreased, absorption of energy
is decreased. These conditions of changing energy absorptionby
the load with conventional uncontrolled processes and apparatus
are uneconomical since the apparatus only operates at or near
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full power during a relati~ely short portion of a treatment
cycle. However, by reducing coupling in discrete steps during
heating of the water toward its boiling point and thereafter
similarly increasing coupling as water evaporates, in accordance
with the present feature, it is possible to operate apparatus
within a narrow range at or close to full power and tnus render
the apparatus far more efficient in its operation.
According to a related feature of the invention, the number
of steps in reducing the coupling is counted and thereafter the
steps increasing the coupling are subtracted from the reducing
steps to establish the termination of the treatment c~cle so as
to obtain a pre-determined moisture content of the processed
load.
The foregoing objects and features will be more fully
understood and appreciated from the following detailed
description of an illustrative embodiment of the present
invention, taken in connection with the accompanying drawings in
which:
Fig. 1 is a view in perspective and partly in cross-section
o~ an apparatus which is useful for carrying out the present
invention;
Fig. 2 is a schematic diagram of an electric circuit
associated wîth the apparatus of Fi~. l;
Fig. 3 is a graphic representation of the effect of the
temperatue of a load upon power absorption, without making any
adjustment either in coupling or in output of the r. f. power
supply;
Fig. 4 is a graphic representation of the effect of the
loss of moisture upon power absorption, again without making any
adjustments in controls either of coupling or power;
Fig. 5 is a graphic representation of the combined effect
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on power absorption by a typical moisture containing load, of
increases in the temperature of both solids and water and of the
loss of water through evaporation;
Fig. 6 is a graphic representation of power absorption by a
moisture containing load, plotted against time without any
adjustments in controls either of coupling or r. f. power
output;
Fig. 7 is a graphic representation similar to Fig. 6 but
showing a plurality of discrete adjustments, first to near
maximum power input, then a plurality of adjustments in coupling
including several initial downward steps and, near the end of
the treatment cycle, several upward steps;
Fig. 8 is a view in front elevation and partly in
cross-section o an adjustable capacitor and its actuating
means, forming a part of a coupling adjusting means according to
the present invention; and
Fig. 9 is a graphic represent:ation of the effect of
differences in resonant frequency of load and r. f. power
source.
Turning now to the drawings, particularly Fig. 1, there is
shown an apparatus disclosed and described in detail in United
States Letters Patent NOn 4,296,555, issued to me on October 27,
1981 and including a housing adapted to re-conditioning batches
of plywood veneer with r. f. energy to redistribute and usually
reduce the average moisture content. Usually, at the start o~
the operaing cycle of the apparatus, the average moisture of
individual plywood veneer sheets varies greatly from sheet to
sheet and in different areas of the same sheet and the
compressed heights of successive batches are frequently variable
to a substantial degree. In addition, because of warping and
distortion in imperfectly conditioned sheets, the density of
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successive batches is unpredictable.
The apparatus of Fig. 1, indicated generally at 20,
includes an enclosure into which batches of veneer are
introduced on a conveyor belt 24 to a position between a fixed
electrode 26 and a movable electrode 28. In order to control
the redistribution of moisture in individual sheets of veneer
and in the batch as a whole, the movable plate is raised and
lowered by a motor 30 as more fully explained in my above
identified Patent, Associated with the enclosure 22 and the
electrodes 26 and 2~ is an electric circuit including an r. f.
power unit indicated generally at 32 and coupled to the
electrodes.
There is shown in Fig. 2. a schematic representation of the
circuit including the r. f. power unit 32 and associated
controls according to the present invention. The r. f. power
unit is ill the form of an oscillator circuit including a triode
36 having a cathode 38, a grid 40 and a plate or anode 42.
Voltage i5 supplied to the plate 42 by a d. c. power supply 44
and the plate is connected to a tank circuit comprising an
inductance 45 and a variable capacitor 46 through a coupling
condenser 48. R, f. energy is coupled to the load from the
junction of the tank circuit through an inductance 50 to the
movable electrode 28, the fixed electrode 26 being grounded.
Paralleling the electrodes and a load 52 i5 another inductance
54.
The apparatus above described in connection with Fig, 1 and
the circ:uits already described in connection with Fig. 2 are
essentially like or the equivalent of those disclosed in my
above identified Patent. Control devices now to be described
are useEul, however, with a wide variety of different apparatus
for treating various moisture containing materials with r, f.
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energy.
Before proceeding with a description o~ the circuitry
employed according to the present invention for controlling the
application of r. ~. power to moisture containing loads and the
termination of operating cycles of dielectric conditioning
apparatus, phenomena upon which the present invention is based
will first be described in connection with Figs. 3 through 6~
There is shown in Fig. 3, a graph representing the power
absorption by a load of moist material in a typical apparatus,
10 the plate voltage of the r. f. power oscillator having been
adjusted for coupling approximately fifty percent o~ ma~imum
available power to the load at the start of the operating cycle.
~s the water contained in the material is heated by the r. ~.
energy, power absorption by the load increases until one hundred
percent of available power is reached at the boiling point of
the water, if the initial adjustment of plate voltage has been
accurately proportional to the water content of t:he material and
no ~urther adjustments are made to the apparatus. Fig. 4 is a
graphic representation of the e~fect of the loss of moisture
20 through evaporation a~ter the boiling point of the water is
reached and r. f. energy starting at one hundred percent of
available power continues to be applied with no further
adjustments in the controls of the r. f. power oscillator.
Under these conditions, the power transfer to the load declines
with the loss of moisture until, when the load is completely
dry, the power absorption is down to less than twenty-five
percent. There is shown in Fig. 5 the combined effect of loss
of moisture and heating of the material being treated by the
application o~ r~ f. energy. Since the loss of moisture, which
30 reduces energy absorption, is being partially counteracted by
heating of the material, which causes increa~ed r. E. energy
~979~)3
absorption, the load absorbs about fifty percent of the
available power when the material is dry but heated. Fig.
illustrates the changes in the power absorption by a moist load
in a dielectric conditioning or treating apparatus, as a result
of changes in the temperature and quantity of water during a
typical operating cycle, without changing any controls in the r.
f. power: supply. Starting with a plate voltage adjustment of
the oscillator to provide approximately fifty percent absorption
of maximum available power by the load, with the heating of the
water content, the absorption rises to one hundred percent when
the water reaches the boiling point and begins to evaporate.
One hundred percent absorption is reached i~ the initial
adjustment has been accurate for the average moisture content of
the load. The water continues to evaporate during a somewhat
balanced period indicated in Fig. 6 between index lines 60 and
62 representin~ respectively the reaching of the boiling point
o~ water, the point of maximum energy absorption and a slight
increase in load temperature as approximately one percent of the
moisture is l~st through evaporationO After passing the point
represented by the index line 62, there is little further
increase in load temperature but a rapid loss of moisture as
energy ahsorption declines to a value below seventy-five percent
of available power.
The treatment of moist materials without adjustment of the
apparatus, as depicted in Fig. 6, is inef~icient and much more
time consuming than is the case with automatic controls in
accordance with the present invention. In addition, as will be
seen, the present devices and methods permit the accurate
control of the final moisture content of a load within broad
limits of initial height or thickness, density and moisture
content. The control, as shown in Fig. 7, is accomplished by
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03
initially increasing coupling of r. f. energy to the load in a
first step 64, which raises the absorption of r. f. energy by
the load to within ten percent of maximum available power.
Thereafter, as the water in the material is heated and
accordingly absorbs more r. f. energy, a full power condition is
reached, which causes a downward adjustment step repeated
several times during a normal dielectric treatment cycle. The
degree of dryness in the processed load is determined by
difference in the number of discrete adjustments downward in the
initial part of the operating cycle of the apparatus and upward
near the end of the operting cycle. Thus, if there are many
more downward than upward adjustments~ the load will have a
greater moisture content at the end of the treatment cyc~le.
For controlling the operation of the apparatus by discrete
adjustments, there is provided, as seen in Fig. 2, a circuit
including an add and subtract counting switch assembly 68,
ha~ing add and subtract circuits 70 and 72 respectively. The
assembly 68, which is of a commercially availabe type, may be so
adjusted that a pair of no~mally closed conta~ts 68-1 operated
by and forming a part of the assembly will open after a certain
number of pulses coun~ed in the add mode, have been reduced to
~ero by an eaual number of pulses counted in the subtract mode.
The initial adjusting step 64 and all succeeding adjusting steps
are controlled in their magnitude and timing by a meter relay 74
connected in the circuit to measure plate current of the
oscillator triode 36. The relay 74 includes normally-open
contacts 74-1 and 74-2 which close repectively to increase and
decrease coupling of the load to the r~ f. power supply. Thus,
if the power absorbed by the load is less than the pre-set
minimum, a condition which obtains when the apparatus is first
energized with a new load, the contacts 74-1 close and energize
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a field winding 76 of a motor 78r shown in Fig. 8, which is
thereby actuated to adjust the variable capacitor ~6 to increase
the coupling between the load and the r. f. power supply, as
will hereinafter be explained. After the initial adjustmnent of
the capacitor 46, each time that power absorption reaches a
pre-set upper limit of the meter relay, the contacts 74-2
cloase, causing the energization of a second field winding 80 of
the motor 78, thereby actuating the motor to adjust the
capacitor ~6 to reduce the coupling between thé load and the r.
E. power supply. For the sake of simplicityr the windings 76
and 80 associated with the operation of the motor 78, have been
describd in general functional terms and illustrated as directly
connected to the capacitor 46. It will be readily understood,
however, that ~he windings 76 and 80 are part of the motor 78,
which is of the reversible type and that the energization of one
winding causes the motor to rotate in one direction, while
energi~ation of the other winding causes rotation in the
opposite direction, rotation in either direction thus making the
necessary adjustments in the capacitor 46. It will also be
understood that the capacitor 46 is typically a commercially
available hermetically sealed adjustable type having a plurality
of concentric plates 82 and its adjustment either to increase or
decrease capacity is hy rotation of a shaft by the motor 78
through driving and driven gears 86 and 88 respectively. The
manner in which adjustments of the capacitor 46 change the
coupling of the r. f. power supply to the load 52 will be
explained below~
Returning now to Fig. 2, there is shown a counter 86 having
two pairs of normally open contacts 86-1 and 86-2 located
respectively in series with the add circuit 70 and the subtract
circuit 7~. The counter 86 is of a commercially available type
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in which the normally open contacts close after the mechanism
has received a pre-set number of electrical impulses. Thus, iE
the mechanism 86 is set for two impulses before closing the
contacts 86-1 and ~6-2, the adding and subtracting circuits 70
and 72 may only be energized a~ter the motor 46 has been
actuated two times to reduce the coupling, as indicated by an
index marker 88 in Fig. 7. Thereafter, each actuation of the
motor ~6 to decrease the coupling of the load to the r. f. power
supply, will be counted by the adder circuit 70 through the
closed contacts 74-2 and 86-1. ~fter a last decoupling
adjustment indicatd at 90 in Fig. 7, the water in the load
reaches the boiling point without requiring a further decoupling
adjustment because the power absorbed by the load never quite
reaches the upper limit set by the meter relay 74. Following the
reaching of the boiling point of water in the load, there is an
extended period during which power absorption by the load
declines slowly as the loss of moisture through evaporation,
which tends to reduce power absorption is balanced in part by a
rise in the load temperature whi~h tends to increase power
absorption. It will be noted at this point that the period
represented in Fig. 7 be~ween reaching the boiling point of
water and a first upward step 92 in the coupling, is generally
comparable to the period indicated between index markers 60 and
62 in Fig. 6, representing uncorrected coupling between r. ~.
power supply and load. However, the time required to reach the
boiling point of water represented by the interval between the
start of the cycle and the final dowmward adjusting step 90 in
Fig. 7 is much shorter than the corresponding period between the
start of the cycle and the index 60 representing the boiling
point o~ water in the self-regulating cycle graphically
illustrated by Fig~ 6. The same is true of the later parts of
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the two cycles in which the interval between the first upward
adjustment 90 and the end of the controlled cycle, depicted in
Fig. 7, is actually considerably shorter in duration than the
corresponding period between the index marker 62 and the end of
the uncontrolled cycle depicted in Fig. 6, for loads of
comparable size and moisture content. The reason for the
reduced duration of initial and final periods of the controlled
cycle over the corresponding periods of the uncontrolled cycle
is that more power is being coupled into the load by the step
adjustments in the cycle depicted by Fig. 7 during these
periods, than the load would receive naturally in accordance
with the uncontrolled cycle of FigO 6. It will accordingly be
realized that time represented along the x axis of Figs. 6 and 7
is only generally comparable during the central portion of the
two cycles and that the two figures have been shown as of
similar overall length to facilitate juxtaposition for purposes
of comparing the two cycles in their parts and not to indicate
overall equa]ity of duration of the two cycles. Stated another
way, the two graphs of Figs. 6 and 7 are not intended to portray
either equal or linear time bases.
The mode of the step adjustments in coupling, which have
been discussed above, are the result of adjustments in the
operating frequency of the r. ~. power supply to coincide more
closely with the resonant freqllency of the load. In Fig. 9 t the
frequency response or absorption capability of the load is
depicted by a curve 96 which represents the voltage which may be
coupled to the load at a part of the frequency spectrum. On the
other hand, a curve 98 ~epresents the voltage available from the
r~ f. power supply over another part of the frequency spectrum~
As the load is affected by the r. f. energy, being heated and
losing moisture, its resonant frequency changes as the
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dielectric constant, which determines its effective capacity, is
correspondingly changedO Assuming the relative conditions of
load resonant frequency and r. f. power frequency depicted in
Fig. 9, heating of the load takes place in that area bounded
between the upper shoulder 100 of the r. f. power oscillator
curve and the lower shoulder 102 of the load curve. If power
transfer from source to load is to be increased, the frequency
of the power source is increased by reducing the capacity of the
capacitor 46 in steps, as already described, so that the output
of the source more closely coincides with the resonant frequency
of the load. On the other hand, decreases in the operating
frequency of the r. f. power source are similarly caused by step
increases in the value of the capacitor 46 also as already
described.
From the foregoing, it is seen that the present control
devices and methods are effective for producing improved
e~ficiency and accuracy in the use of dielectric heating
apparatus applied to moisture containing materials. It will
also be appreciated that the reference to the apparatus of my
patent has been for the purpose of illustrating the present
control devices and methods applied to a complete treatment
apparatus but that the present control devices and methods are
much more broadly applicable to a wide variety o~ treatment
processes which require efficiency in the use of the apparatus
and accurate and automatic control to produce uniform moisture
content in treated products, starting with initial loads which
vary broadly in such chararacteristics as size, density and
moisture content. It is accordingly not intended that the
foregoing description and accompanying drawings be taken as
limitations, but rather that the scope of the invention be
interpreted in terms oE the appended claims.
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