Note: Descriptions are shown in the official language in which they were submitted.
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POLYMERIC PROCESSING SYSTEM
FOR PRODUCING OPHTHALMIC LENSES
S
BACKGROUND OF THE INVENTION
Methods and systems for making an ophthalmic lens by curing a thin layer of
resin on
an optical preform, wafer, or single focus lens we well known, as discussed in
U.S. Patent No
5,219,497. The methods for selecting the location of the additional optic and
the curing lens
materials are discussed in U.S. Patent 5,178,800. Additionally, the use of a
combination of heat
and light to polymerize a monomer resin layer onto a single vision lens is
known, as discussed
in U.S. Patent Nos. 5,470,892 and 5,147,585. All of these patents are
incorporated herein by this
1 S reference.
In concert with a controlled thermal profile, the lens making systems control
the duration
and time of light provided from the curing lamps to produce an ophthalmic
lens. Typically, the
light is modulated with mechanical shutters to~ control the amount of
radiation exposure.
Mechanical shutters, however, do not necessaril;r permit as fme a control over
the modulation
of the light as desired. Additionally, the mechanical parts comprising the
shutters are susceptible
to breaking and thus may present a reliability problem with respect to the
lens making system.
Additionally, some lens making systems use liquid crystal spatial light
modulators (LCSLMs),
which tend to heat up during operation of the system due to their inherently
limited transmission
in the clear state.
Additionally, known lens making systems are relatively inefficient in managing
thermal
energy. As a result, lens casting systems typically require excessive cooling
with multiple fans
and/or circulating pumps.
Another problem faced by practicing optometrists and lens manufacturers using
known
lens making systems is the maintenance of substantial inventories of single
focus lenses and
other optical preforms. Additionally, there is always a concern regarding
potential mistakes
made in selecting from multiple types of lens materials, resins, and
associated cure cycles for the
systems.
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Consequently, a need exists for an improved ophthalmic lens making system.
WO 98/28126 PCT/CTS97/23667
SUMMARY OF THE INVENTION
The present invention, therefore, providers an improved ophthalmic lens making
system
designed to minimize the disadvantages associated with the prior art. The
system includes a
~que combination of components for achieving fast, efficient ophthalmic lens
production using
a combination of heat and ultraviolet and/or visible: light to cure a layer of
resin on a single focus
lens. In particular, the present invention provides a lens making system
capable of in-office
processing that produces a high quality ophthahmic lens with very short cure
times, typically
between 15 and 40 minutes, depending on the optical resin composition. This
system is
The system is controlled by a microprocessor, permitting the development of a
lens
information stored in the system. The presence of a modem in the system
greatly facilitates
automatic reordering and subsequent stocking of the lens making material.
The system provides a high degree of consistent thermal and ultraviolet andlor
visible
maximizing the light source efficiency of the system by separating the lamps
from the curing
chamber and by providing an air flow design that keeps the lamps cools.
Specifically, the
-2-
mfg ~,~em having a personal computer- based architecture. The use of a
personal computer-
particularly well suited for making bifocal and multifocal ophthalmic lenses,
but can also be used
to cure a uniform layer of resin on a single focus lens.
based system facilitates the implementation of tv~ro important features of the
present invention.
An optical scanner or bar code-scanning wand o:r pen is provided for
automatically reading in
information on the resin tube and on the single vision lens envelope,
permitting the system to
keep track of all the prescriptions processed on the system. Scanning in this
information also
allows the microprocessor to check compatibility between the materials and the
resin and to
ensure that the appropriate cure cycles are activated.
Additionally, the system is provided with a modem to allow remote accessing of
the
radiation that produces very repeatable curing results. These results are
improved by
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cooling fan of the power supply for the system is utilized to provide
substantially all the
necessary cooling for the system. This eliminates the need to include
additional cooling fans or
circulating pumps, and has the added benefit of providing a much quieter
system.
The system also provides a mechanism for temporally modulating the light from
the
curing lamps by controlling the power supplied to a set of electronic ballasts
used to control the
curing lamps. The use of electronic ballasts to modulate the light sources
overcomes the
problems associated with the modulating means used in the prior art, such as
mechanical shutters
and LCSLMs.
The system also provides a unique and highly efFlcient method of managing
thermal
energy with very low power requirements. As part of the thermal management
system, a divider
plenum is provided in the curing chamber of the; apparatus and separates hot
air entering from
the heating element fcom return air flowing out of the curing chamber. The
divider plenum may
be positioned within the curing chamber to adjust the temperature balance
within the chamber.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features and advantages of the present invention will be
appreciated as
the same become better understood with reference to the following Detailed
Description when
considered in connection with the accompanying; drawings, wherein:
FIG. I is a schematic perspective view of a lens making system according to
the present
invention;
FIG. 2 is an electronic block diagram of the electronic system for the lens
making system
of FIG. 1;
FIG. 3 is a schematic fiont view of the thermal and optical systems of the
lens making
system of FIG. 1;
FIG. 4 is a side cross-sectional view of a curing chamber of the lens making
system of
FIG. 1;
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FIG. 5 is a front cross-sectional view of the curing chamber of the lens
making system
of FIG. 1;
FIG. 6 is a back cross-sectional view of the lens making system of FIG. 1;
FIG. 7 is a plot of selected thermal charau~teristics of the lens making
system of FIG. 1,
including oven temperature, mold temperatures, and lamp temperatures, along
with a lamp duty
cycle for a representative cure cycle; and
FIG. 8 is a schematic illustration of the amtomatic inventory and processing
parameter
features of the lens making system of FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to the figures, a presently preferred embodiment of the lens
making system
is illustrated in FIG. 1. The system 10 has a computer-based architecture and
includes a curing
chamber for producing ophthalmic lenses, especially bifocal and multifocal
lenses, using a
combination of heat and actinic radiation, specifically ultraviolet and/or
visible light, to cure a
layer of resin on a single focus lens. The curing; chamber includes an
insulated oven 12 and
ultraviolet and/or visible light sources 14 (FIG. 3) positioned above and
below the oven 12. The
light sources 14 provide ultraviolet and/or visible light to the interior of
the oven, where a mold
tray 16 is located. As will be described in more detail below, electronic
ballasts 18 are provided
to control the light applied by the lighting sources. to the curing chamber.
Molds containing the resin that is to be cured onto a single vision lens are
located on the
mold tray 16. A dispenser assembly 20 allows for resin to be automatically
dispensed into the
molds of the mold tray. In a presently preferred embodiment, the dispenser
assembly 20 includes
a stepper motor 22 (FIG. 2) and a plunger that discharges resin from a
disposable syringe within
the system. The stepper motor 22 is preferably capable of carefully dispensing
resin to the
nearest 0.01 milliliter. Additional dispensing assemblies 24 may be added to
increase the
dispensing capabilities of the system.
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These components are contained within an outer shell 11 of the system 10,
which is
preferably constructed fiom cold rolled steel, but can also be made from other
suitable materials,
such as aluminum or plastic.
A back-lit liquid crystal display 26 indicates the system status, and allows
the system to
prompt a user for information. Other system inforimation, such as lamp hours,
cure status, and
oven conditions are displayed on light emitting diodes 28 (LED's) along the
front of the status
indicating panel 30 of the system. A keypad 2S~ on the front of the system
permits a system
operator to select various options and enter data.
An optical sensor or bar code-scanning wand 31 facilitates the automatic entry
of
pertinent information into the system, such as resin and lens material data,
that is relevant to the
processing parameters of the lenses. Additionally, a modem 33 (FIG. 2) within
the system
facilitates automatic reordering and subsequent stocking of materials used in
the ophthalmic lens
making process, such as single vision lenses, wafers, and resins. The system
10 also includes
a floppy disk drive 32 for installing and upgrading; system software.
Turning now to FIG. 2, a preferred embodiment of the electronic control system
is
illustrated. The heart of the electronic system of the present invention is a
motherboard 34,
preferably equipped with Intel 386, 486, or 586 rnicroprocessors. The use of
the motherboard
32 permits the lens making system to have a personal computer-based (PC-based)
architecture,
which has several distinct advantages over other noncomputer-based lens making
systems
currently available. For example, a PC-based architecture provides the ability
to develop and
3 0 m~~ software for the system in a high level language, such as C or C++.
Another advantage
of a PC-based architecture is the availability of additional and supplemental
hardware designed
for the personal computer. By utilizing modular "plug and play" technology
available for the
personal computer, new and important features may be added to the lens making
system with
minimal investment in development time and associated expense, such as the bar
code-scanning
wand 31 and the modem 33, which communicates with the motherboard 34.
An interface board 36 provides a communication link between the motherboard 34
and
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the rest of the system through a standard AT buss. The primary purpose of the
interface board
36 is to provide the necessary logic link between the AT bus and the
components of the system.
S
The interface board 36 provides the digital logic to a set of switching
electronics 38 where all
logic levels used for power switching are optically isolated from the rest of
the digital
electronics. For example, optically isolated drives 40, 42 are used to turn
power transistors, such
as metal oxide semiconductor field-effect transistors (MOSFETs), on and off to
connect or
remove 12 VDC power to the electronic ballasts 18.
A heating element 44 for the oven 12 is powered by 115 VAC or 240 VAC, and is
also
controlled via optically isolated switching of a TRIAC with built in zero
crossing detection. The
zero crossing detection prevents unwanted noise during AC switching. The zero
crossing
detection also facilitates pulse width modulated control of the heating
element 44 to provide
consistent and efficient heating of the system.
A DC motor 46 is used to rotate an oven f ~n 48 that circulates the heat
within the curing
chamber. The logic levels used to control this motor are also optically
isolated and controlled
via high power MOSFETs. Additionally, a thermal sensor 50 located within the
curing chamber
provides temperature information directly to the interface board 36.
A mufti I/O board 52 is used to provide connmunication between the motherboard
34 and
a hard drive 54 of the system's computer. The hard drive 54 is used to store
the system software
and the system operator's material usage history. The mufti I/O board 52 also
provides
communication to the floppy disk drive 32, such as a 3.5 inch disk drive,
which is used to install
and upgrade the system software. Additionally, the mufti I/O board 52 provides
a
communication port to the bar code-scanning wand 3 l, which is used to enter
information about
the lens and resin directly into the system via bar codes on the resin tubes
and lens envelopes.
An optical sensor 56 is used to limit the motion of the stepper motor 22 of
the dispensing
assembly 20. The dispensing assembly 20 is activated by pushing a fill switch
58, which is
connected directly to the interface board 36.
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Additionally, the keypad 29 allows the system operator to input commands
directly to
the interface board 36, which passes the commands to the motherboard 34. The
backlit LCD
display 26, which provides system status information, is also connected to the
interface board,
which passes information to the LCD display from the motherboard.
The power supply 55 for the system is a standard PC power supply that has
enough
capacity to operate the PC components, such as the motherboard, the hard and
floppy drives, as
well as the electronic ballasts. This allows for a :much simpler system design
that requires only
a single power supply. Moreover, as discussed below, the cooling system has a
simple, yet
elegant design in which the cooling fan 57 of the power supply provides
substantially all of the
necessary cooling for the lamps and various electronic components in the
system, including the
electronic ballasts.
FIG. 3 illustrates a presently preferred embodiment of the optical and thermal
systems
of the present invention. A highly controlled combination of heat and
ultraviolet and/or visible
light is used to polymerize a liquid resin on a single focal lens placed in
the curing chamber.
Preferably, the light sources 14 are separated and thermally insulated from
the heat source of the
system. As can be seen from FIGS 3-6, the ligl;~t sources 14 are located above
and below the
curing oven 12. Placement of the light sources .outside the heated oven
maximizes the lamps
efficacy by allowing the lamps to remain at a :much lower temperature than the
oven. For
example, although the oven may be as hot as 220 degrees Fahrenheit, the lamps
remain
somewhere between 80 and 120 degrees Fahrenheit. Maintaining the lamps at
lower
temperatures is important for long lamp life and I;ood lamp e~cacy.
The light sources 14 are preferably either ultraviolet and/or visible
fluorescent tube
lamps, such as Phillips PLS-9W/08, PLS-9W/10, or PLS-9W/27, with a maximum
illumination
between 300 and 400 nanometers. Although the lamps will operate at different
line
3 S frequencies(50 or 60 Hz), operation at the higher frequency provides
better lamp efficacy.
Electronic ballasts 18 are provided to modulate the ultraviolet and/or visible
light from
the light sources 14. The use of electronic ballasts provides a clean method
for modulating the
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lights. By switching the DC power supplied to the: electronic ballasts, the
lamps are easily turned
on and offto meter the amount of ultraviolet and/or visible light that is
applied to the resin. This
is preferred over the use of liquid crystal spatial light modulators and
mechanical shutters for the
reasons already stated. As described above, MOSFETs are preferably provided to
connect or
remove 12 VDC power to the electronic ballasts 18. The electronic ballasts
also provide
consistent AC power to the lamps, regardless o~f variations in line voltage or
frequency. In a
presently preferred embodiment, the electronic ballasts are Bodine 12TPL7-9E
or equivalent, and
produce 20 to 30 kHz of AC power from a 12 VDC source. As mentioned above, the
electronic
ballasts are preferably powered by the 12 V rail of the PC power supply.
The oven 12 is constructed from a durable material, such as stainless steel or
aluminum,
and is thermally insulated with a suitable material, such as fiberglass or
high temperature foam
rubber. Since the light sources 14 are placed outside of the curing oven,
optical ports 60 are
constructed in the oven 12 to allow the transmission of light into the curing
chamber. In the
embodiment illustrated in FIGS. 3-6, two optical ports 60 are provided in each
of an upper and
lower surface 62, 64, respectively, of the curing oven 12 to permit light to
reach the molds in the
curing oven. The optical ports 60 are preferably constructed from high
temperature glass with
an optical transparency of at least 90% at 360 to ti00 manometers. To secure
the optical ports in
position, metal rings constructed from stainless steel or anodized cold rolled
steel and high
temperature foam rubber gaskets are used.
The optical ports 60 are preferably surrounded by collimating reflectors 66 to
ensure that
the light entering the oven 12 is uniform. Uniform light intensity is critical
for uniform curing
of the resins used to form the additional optic on the single focus lenses.
Referring now to FIGS. 4-6, additional details of the curing chamber are
illustrated. FIG.
4 is a side cross-sectional view of the curing oven 12, which is covered with
a layer of fiberglass
or foam insulation 68. Optical ports 60 allow the ultraviolet or visible light
to enter the curing
oven from above and below. The mold tray 16~ is positioned within the curing
oven 12, and
includes a pair of molds 70 held in place by a metal tray 72. The molds 70 can
be either glass
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or plastic, and are preferably ground from crown glass or molded from
transparent plastics, such
as polycarbonate, polypropylene or a polyethylene polypropylene copolymer.
During operation of the curing oven 12, liquid resin and a pair of plastic,
single vision
lenses are placed in the molds 70. The oven is heated by blowing air with the
squirrel cage fan
48 across the resistive heating element 44. The fan 48 is driven by the DC
motor 46, and is
designed to pull air in towards the motor along its axis of rotation, and to
blow air outward in all
directions. A specially designed dividing plenwm 74 located between the
heating element and
the molds facilitates even circulation of the heated air throughout the oven.
Turning now to FIG. 5, a front cross-sectional view of the curing oven 12 is
illustrated.
In this view, the divider plenum 74 is shown to be shorter than the width of
the oven 12, by
about 45%. This allows the hot air being blown across the heating element 44
to enter the oven
12 evenly across the top 75 of the oven and the return air from the bottom of
the oven to be
drawn out evenly from the lower left 77 and right 79 hand sides of the oven.
It should be noted
that the position of the divider plenum 72 can be moved to the left or the
right relative to the
oven to adjust the temperature balance within the oven.
Additionally, the thermal sensor 50 is located in an upper 81 right hand comer
of the
oven 12 . The location of the thermal sensor 50 is important to achieving an
accurate and
representative measurement of the oven temperature. In general, the placement
shown in FIG.
5 is consistent with counterclockwise rotation of the oven fan 48, since this
allows for a
measurement point with the most laminar air flow.
Additional features of the cooling system are illustrated in FIG. 6, which is
a back cross-
sectional view of the entire system. A plurality o:F louvers 80 are placed on
the side walls of the
system to allow cool air into the system. The louvers 80 are partially covered
with a set of duct
work 82 that directs air flow over the lamps while preventing unwanted light
leakage through
the louvers. Cool air is drawn through the ducts and forced across the lower
half of the system
by the cooling fan 57 on the system power supply 55. By controlling the air
movement in this
manner, the coolest air is pulled across the lamps 14, removing the heat
generated by the lamps
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and keeping the lamps operating at a very high efficacy. The remaining air
flows across the
system electronics, which are arranged on a chassis 84 mounted to the inside
lower left hand side
85 of the system 10. The chassis 84 provides easy access to the serial port of
the mufti I/O board
52 used to connect the bar code-scanning wand 3~ 1 to the system.
Additionally, the modem 33
is also readily accessible through the back of the chassis 84.
The cooling system works in conjunction with the optical and thermal systems
to provide
proper curing of the resins. It should be noted that: proper curing of the
resins is also affected by
the thermal control of the oven. In a presently preferred embodiment, oven
temperature is
carefully controlled by a Propartional, Integral, Derivative (PID) control
algorithm. The PID
control algorithm is described in Appendix A, attached hereto, which is
incorporated herein by
this reference. This algorithm is used to determine the correct duty cycle for
pulse width
modulation control of the electrical power applied to the heating element. In
this manner, the
temperature set point can be reached with minimal overshoot. Pulse width
modulated control
of the heating power also allows the maximum duty cycle to be adjusted to the
different AC
voltage levels used throughout the world. Preferably, a heating element that
provides 350 watts
of power at 120 VAC at a maximum duty cycle ~of 100% is employed in the
system. Because
power is proportional to the square of the voltage, the same heater provides
350 watts of power
at 240 VAC, with a maximum duty cycle of 25%. In this manner, the same heating
element can
be used for 120 VAC or 240 VAC and still provides all the thermal energy
required to cure the
resin. As a result, the lens making system provided herein can be used
throughout the world.
~ general, the thermal control of the curing oven needs to be within certain
limits to
achieve consistent and repeatable curing results. In a presently preferred
embodiment, the
system has the following thermal control characteristics: ( 1 ) the actual air
temperature in the
oven is controlled to within +/- 5 degrees Fahrenheit of the set point
temperature throughout the
3 S cure cycle; (2) the variation in left to right mold temperatures is never
more than 7.5 degrees
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Fahrenheit; (3) the temperature ramp rate is preferably controlled to be no
more than I 5 degrees
Fahrenheit/minute; and (4) the temperature in the oven should not exceed 220
degrees
Fahrenheit.
Similarly, the lamp intensity needs to be ~iwithin certain limits to achieve
consistent and
repeatable curing results. In a presently preferred embodiment, the lamp
intensity is between
2000 and 3000 millijoules per square centimeter at a nominal wavelength of 390
nanometers.
Through repeated experiments with a variety of materials and lens styles, it
has been determined
that this level of thermal and optical control, and these limits, are required
to maintain consistent
results in the final product. Properties such as scratch resistance, adhesion,
and optical and
cosmetic quality may be otherwise adversely affected.
FIG. 7 is an illustrative example of a representative cure profile for a pair
of ophthalmic
lenses produced by the present invention. The upper dashed line 87 represents
the temperature
set point that was programmed into the software of the system. This
temperature profile is
determined by a variety of factors, including the resin chemistry, the type of
lens material and
the type of lens design. The sold upper line 89 is the actual oven
temperature, measured from
the center of the oven with a J-type thermocouple. Additionally, the middle
dashed line 91
represents the left mold 70a temperature and the middle solid line 93
represents the right mold
70b temperature.
As the lines indicate, the mold temperatures are very consistent from left to
right, as a
result of the carefully balanced air flow. The mold temperatures significantly
lag the air
temperature due to the large thermal masses of the glass or plastic molds.
This is an important
factor to consider in the development of thermal profiles for making lenses.
FIG. 7 also illustrates the lamp profile of i:he system during the curing
process. During
the first five minutes of the curing process, no lights are turned on, and the
mold, lenses and resin
3 5 are left in the dark and allowed to warm up slightly. The second five
minutes of the cycle allows
for a very gradual ramp in oven temperature, with simultaneous flashing or
blinking of the
actinic radiation sources from the top and bottom. of the oven. After the
first ten minutes, the
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lamps are fumed on continuously while the oven continues to ramp in
temperature until the final
dwell temperature is reached.
S
While the oven is heating, it is important to monitor other temperatures
within the curing
chamber, as they can indirectly affect the curing process. The lower solid
line 95 in FIG. 7
represents the air temperature near the center of vthe upper lamps in the
system. It is important
to note that this solid line indicates that the temperature near the upper
lamps 14a is slowly but
steadily falling for the first ten minutes of the cure cycle, despite the fact
that the oven is actually
heating from approximately 115 degrees Fahrenheit to 130 degrees Fahrenheit
during that same
period. This suggests that there is very little heat transfer between the oven
and the lamps. After
the first ten minutes when the lamps are continuously on, it is clear that the
air temperature near
the lamps is slowly rising up to approximately 1.20 degrees Fahrenheit at the
end of the cure
cycle. This suggests that the major source of heat near the lamps is the lamps
themselves.
The lower dashed line 97, near the bottom of the graph in FIG. 7, represents
the air
temperature near the center of the lower lamps 1.4b in the system. While
following a similar
trend as the upper lamps, the lower lamp temperature never exceeds 105 degrees
Fahrenheit due
to slightly better air circulation at this location.
The longer dashed line 99 near the bottom of FIG. 7 represents the air
temperature near
the digital electronics inside the system. Here the temperature stays
relatively low, less than 110
degrees Fahrenheit throughout the entire cure cycle. As indicated in FIG. 7,
very little thermal
energy actually leaks from the curing oven 12 into the rest of the system.
Turning to FIG. 8, the use of the scanning wand 31 and modem 33 components of
the
system 10 will now be described in more detail. In conjunction with the PC-
based architecture
of the system, these components allow the present invention not only to track
and reorder
inventory automatically, but also to generate processing parameters for the
ophthalmic lenses to
be produced. For example, a resin tube 100 may include bar coded information
102 about the
resin chemistry, the expiration date, the batch numt>er, as well as a unique
identification number.
Similarly, bar coded information 104 on a single vision lens envelope 106 may
include, for
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example, information on the material type, lens distance, and astigmatic
power, as well as a
unique identification number. By scanning this iinfonnation into the system
with the bar code-
s
scanning wand 31, the system operator can provide the system with the
necessary information
for complete record keeping on the materials used by the system, thereby
allowing the tracking
of ali prescriptions processed on the system. Additionally, this information
permits the system
to check compatibility between materials and ream automatically and ensure
that appropriate
cure cycles are activated.
The encoded data is sent directly into the system via the serial port on the
back of the
system. A modem line 33 allows the system to be accessed remotely; thus all of
the lens casting
records can be downloaded to a central office location 110. Once the central
office 110 reviews
the casting log from a system location, the exact materials required for
restoring the system
operator's inventory can be shipped by ground o~r air transportation,
depending on the system
operator's requirements. This provides an important advantage to both the
system operator and
the material supplier. The system operator can nnaintain a smaller inventory
of materials if he
can rely on quick restocking from the manufacturer. Additionally, the material
supplier can
schedule production runs based on a running average of all system usage
throughout its regional
market. Moreover, remote access of the casting log will permit the material
supplier to keep on-
site inventory of materials current, without having to do an on-site physical
audit at the system
operator's location.
While various embodiments of this invention have been shown and described, it
would
be apparent to those skilled in the art that many more modifications are
possible without
departing from the inventive concept herein. It is, therefore, to be
understood that, within the
scope of the appended claims, this invention may be practiced otherwise than
as specif cally
described.
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i
APPENDIX A
INNOTECH LENS SYSTEM CHAMBER PID CONTROL ALGORITHM
Preface
In order to precisely control (regulate) the Oven temperature to desired
temperatures
(setpoints), an adjustable mathematical equation is implemented. This equation
is called a
Proportional, Integral, Derivative (PID) equation. The Derivative component is
not used. The
PID equation has a Proportional component (Pterm which is directly
proportional to the error
value between the Setpoint (desired) temperature: and the Actual (measured)
temperature, and
an Integral component (lterm which represents a history of the error value).
There are 3 modes
of Heat Regulation, the Warm-up mode, the Default mode, and the Curing mode.
The Integral
term is computed in 2 ways depending on the Heat Regulation mode.
The PID equation is:
PIDout = Pterm + Iterm
where, depending on the Heat Regulation mode,
Pterm = PconstWARM * Error
lterm * IconstWARM * (Average of last 6 Errors)
or,
Pterm = PconstDEFAULT* Error
Iterm = IconstDEFAULT* (AveraF;e of last 6 Errors)
or,
Pterm = PconstCURE* Error
Iterm = ItenmPREVIOUS + [Error * (2/IconstCURE))
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The Oven Heat is turned ON once and OFF once during every SSOms time period.
This
SSOms time period is divided into 50 1 lms increments. At the start of a given
SSOms time period,
the Oven heat is turned ON. Then based on the PID out value, the Oven Heat is
fumed OFF at
1 ofthese 50 possible l lms increments and it remains OFF until the start of
the next SSOms time
period. The time that the Oven Heat is ON and the time that the Oven Heat is
OFF during a
SSOms time period is called the Duty Cycle. For e:~cample if the Oven Heat is
turned ON for one
half (25* 1 lms = 275ms) of the SSOms time period, the Duty Cycie would be SO%
for one fifth
(IO* 11 ms - 1 l Oms) it would be 20%. This ON and OFF Duty Cycle is repeated
every SSOms
unless the Oven Heat is commanded to remain Ol~F.
The Pconst and Iconst values are modifiable in the Utilities Menu. They can be
adjusted
from 0 to 99. For Pconst, the higher the value the; greater the corrective
Proportional response
to the error. For Iconst during the WARM-UP and DEFAULT modes, the higher the
value the
greater the corrective integral response based on the immediate history of the
error. For Iconst,
ding the CURING mode, the lower the value the quicker the corrective responses
will "zero-
in" on the value necessary to produce zero error.
Compute the Proportional and Integral tenors of the PID equation.
Pterm - PconstCURE * TEMPerr
Iterm = ItermPREVIOUS + [(TEMPerr *(1/IconstCURE)]
where ItermPREV I OUS is the calculated from previous average
measurement
Compute PID Output Value
Three time each second, the A/D Heat Sensor value is read and saved in a 3
element
circular buffer and the average of these 3 latest readings is computed.
At the start of the next SSOms time period, the new measured temperature is
computed
based on the current average A/D Heat Sensor reading value. This measured
temperature value
is computed as follows:
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Assume that:
A/D reading - 0 is equivalent to 70 °F
A/D reading - 255 is equivalent to 230 °F
So the total temperature span = 230 °F - 70 °F = 160
°F
TEMPmeas = 70 °F + x
where x = a value from 0 °:F to 160 °F
This can be seen as the ratio:
X °F - (,~ where y = the A/D reading value (0-255)
160 °F 255
So TEMPmeas - 70 °',F + x °F
70 °1F + (y * (160°F/255)]
For WARM-UP OR DEFAULT mode:
Compute the Error and Average Error between the current Setpoint
temperature and TEMPmeas.
TEMPerr = TEMPset - TEMPmease
ERRORbuff = ERRORbuff+ TEMPerr
(ERRORbuff always contains the sum of the last 6 TEMPerr's).
ERRORavg = ERRORbuff/6
Compute the Proportional and Integral terms of the PID equation. The constants
will be either DEFAULT or WARM depending on the Heat Regulation mode.
Pterm - PconstDEFAULT * TEMPerr
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Item - IconstDEFAULT *)_sRRORavg
PIDout -Pterm + Iterm
For CURING mode:
TEMPerr = TEMPset - TEMPmea;s
Compute Duty Cvcle
The Duty Cycle is base on the PID output. 'When starking a SSOms period,
OFFcnts = the
number of 11 ms increments (out of a possible 50) until the heat will be fumed
OFF.
Observe the following ratio:
OFFcnts - PIDout
50 500
and therefore,
Duty Cycle = OFFcnts/50 x 100%
The value 500 is the maximum PlDout value allowed. This means that a PIDout
value
greater than or equal to 500 is equivalent to 50 11 ms increments or a 100%
Duty Cycle.
The Duty Cycle is also attenuated (reduced) depending on the AC voltage level
supplied
to the Oven Heater. If the AC voltage - 120 VAC', then no attenuation of power
(Duty Cycle),
is necessary. But an AC voltage of 240 VAC (twice the voltage = 4 times the
Wattage, a 4/I
ratio) requires a 4-to-1 attenuation of power (Duty Cycle) supplied to the
Oven Heater.
So,
OFFcnts(attenuated) _ [x * (100 - ATTEI\fconst)]/100
and,
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Duty Cycle = OFFcnts(attenuated)/50
In summary,
At the start of every SSOms period, the number of 11 ms periods (out of 50)
until the
Oven Heat is to be fumed off is calculated based on a PID output value
(possibly attenuated).
The Oven Heat is then turned ON (if at least 1 1 lrns period of ON time is
calculated). When the
calculated number of l lms periods passes, the Oven Heat is turned OFF and it
remains OFF until
this cycle is repeated at the beginning of the next SSOms period.
20
30
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