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Description  |
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RELATED APPLICATIONS
This application is related to patent application Ser. No. 08/131,231,
entitled "Imaging color sensor", by Campo et al., filed Oct. 4, 1993, now
abandoned, assigned to the assignee of the present invention and herein
incorporated by reference.
FIELD OF THE INVENTION
This invention relates to the color of compounded polymer(s) and, more
particularly, to a system for closed-loop feedback control of the color of
the compounded polymer(s).
BACKGROUND OF THE INVENTION
Contemporary plastics are typically comprised of one or more base polymers
or resins, one or more colorants or colorant additives, and other
additives. Such additives may include, for example, fiberglass for
structural reinforcement, flame retardants, plasticizers, or mold release
agents. The plastics are manufactured by mixing these constituents,
usually by machine, to form a substantially homogeneous polymer mixture.
In this context, a substantially homogeneous compounded polymer mixture is
distinguished from a polymer mixture having a substantially uniform color.
In addition to the heat produced by mixing these constituents, termed
"shear heat", other external heat may also be supplied. The resulting
material, frequently produced in the form of strands, webs, bars, sheets
or films, to name only a few possible shapes, may, after at least partial
solidification of the mixture, then be pelletized to produce a final
polymer product.
Experience has shown that the color of the resulting polymer product may
depend upon several factors. These include, among others, the
concentration and type of colorants, the base resins employed and their
concentration by weight, the temperature history during mixing, and the
ultimate degree of constituent inter-mixing achieved during processing.
Thus, variations in color between polymer products may arise for a large
variety of reasons. For example, color may vary among products due to
polymer product formulation or recipe differences. Likewise, color
variations may exist between lots for a given product formulation or
recipe due to, for example, machine-to-machine differences. Furthermore,
color differences may exist within lots due to changing raw material
characteristics, changing operating conditions, and inaccuracies and other
anomalies in processing, such as differences in the constituent feed
rates. Thus, a need exists for a reliable and effective means or method of
controlling the color of compounded polymer(s) while the compounded
polymer(s) are in-process and, thus, bringing a production lot of the
compounded polymer(s) to the desired color and substantially maintaining
that color throughout the production run.
SUMMARY OF THE INVENTION
One object of the invention is to continually monitor the color of the
compounded polymer or polymers during manufacture and automatically adjust
colorant additive addition rates, or other constituent addition rates, to
efficiently and quickly obtain the desired polymer color while the
compounded polymers are still in-process. In particular, such measurements
should occur before any pelletization and/or packaging.
Another object of the invention is to provide a system for controlling the
color of the compounded polymer(s) and thereby reduce both compounder down
time and scrap material production.
Yet another object of the invention is to maintain the color of the
compounded polymer(s) within a desired specification throughout the
production run while also accommodating varying or changing raw material
properties and other variations in processing conditions.
Briefly, in accordance with one embodiment of the invention, a system for
controlling the color of compounded polymer(s) comprises: a compounder for
inter-mixing the constituents of the compounded polymer(s) to produce a
substantially homogeneous mixture; a sensor for measuring the color of the
substantially homogeneous mixture at predetermined intervals; a
controller, responsive to the sensor, for determining the appropriate
colorant additive addition rate(s); and a colorant additive feeder,
responsive to the controller, for providing the colorant additive(s) to
the mixture at rates substantially prescribed by the controller.
BRIEF DESCRIPTION OF THE DRAWINGS
The subject matter regarded as the invention is particularly pointed out
and distinctly claimed in the concluding portion of the specification. The
invention, however, both as to organization and method of operation,
together with further objects and advantages thereof, may best be
understood by reference to the following detailed description when read
with the accompanying drawings in which:
FIG. 1 is a schematic diagram illustrating one embodiment of a system for
controlling the color of compounded polymer(s) in accordance with the
invention.
FIG. 2 is a block diagram illustrating a relationship between a polymer
compounding process and signals representing the color of the compounded
polymer(s).
FIG. 3 is a block diagram illustrating one embodiment of a controller for a
system for controlling the in-process color of compounded polymer(s) in
accordance with the invention.
FIG. 4 is a block diagram illustrating another implementation of the
embodiment illustrated in FIG. 3.
FIGS. 5a-5e are, respectively, plots illustrating the change in CIE
Laboratory color parameters due to a change in the amount of the
particular colorant for a nominal formulation of GE NORYL color number
50133.
FIGS. 6a to 6c are, respectively, plots illustrating an actual and
simulated open loop pulse response for one embodiment of a system for
controlling the color of compounded polymer(s) in accordance with the
invention.
FIGS. 7a and 7b are, respectively, plots illustrating a simulation of a
closed loop response to an initial color error for the embodiment of a
system for controlling the color of compounded polymer(s) in accordance
with the invention having the open loop pulse responses illustrated in
FIGS. 6a to 6c.
DETAILED DESCRIPTION OF THE INVENTION
Plastics, such as thermoplastic polymers or thermoset polymers, may be used
in a number of different commercial products. Industries employing
polymers include the printing industry, the paint industry, the fabric
industry, and the plastic industry. In a number of these products and
industries, the color of the polymer product may be important. In such
industries, a manual procedure is typically used to adjust the amount of
colorant(s) or colorant additive concentration(s) to achieve the desired
polymer product color for a production run in which a polymer or several
polymers are compounded. In the context of the invention, the term
colorant or colorant additive refers to any additive to a mixture of
polymer product constituents that affects the polymer product color by
itself or in combination with the other constituents. This procedure
usually involves preparing a blend of base resin(s) or polymer(s),
colorant(s), such as, for example, solid pigments, liquid pigments or
dyes, and other additive(s), according to a nominal recipe, sampling this
blend, compounding the blend in a laboratory machine to generate a
pelletized polymer product, injection molding the pellets to obtain a
plaque of substantially uniform color, measuring the plaque color in a
laboratory spectrocolorimeter, comparing the plaque color to the product
"standard plaque," computing an addition of colorant(s) to correct the
color, and adding this correction in colorant(s) to the blend.
This sequence is typically repeated until the laboratory scale machine
produces a molded plaque of a nominally acceptable color. A sample of the
suitably adjusted blend of resin(s), colorant(s) and other additives is
then compounded on a production scale machine. Again pellet samples are
collected, injection molded to produce plaques, measured with the
spectrocolorimeter, and compared to the desired product standard. Any
differences, which may arise from processing differences between the
laboratory scale compounder and the production scale compounder, are again
manually compensated by an addition of colorant(s) to the blend. Although
the concentration of other constituents other than colorants may also be
modified, typically this is not effective or economical for modifying the
color of the polymer product. These adjustments continue in an iterative
fashion until the desired product color is achieved on the production
machine. Once the desired polymer product color is achieved, the entire
blend is compounded without further adjustment. Because of the substantial
time and effort involved in each of these colorant adjustments, it may be
advantageous to reduce the number of adjustments required to achieve the
desired polymer product color. For example, typically two hours is
required to complete an iteration of the adjustment procedure on a
production compounder.
Attempts have been made to provide accurate predictions of the effects of
the addition of colorant(s) on polymer product color. These predictions
may be provided by commercially available software tools based on various
implementations of the Kubelka-Munk color theory, such as explained in
Judd and Wyszecki, Color in Business Science and Industry, John Wiley &
Sons, New York, 1975; Billmeyer, J. and Saltzman, M., Principles of Color
Technology, John Wiley & Sons, New York, 1981; and Wyszecki and Stiles,
Color Science: Concepts and Methods, Quantitative Data, and Formulae, 2d
ed, John Wiley & Sons, New York, 1982. These software packages, such as,
for example, MTS available from MTS Colorimetric, Cergy-Ponttoise, France,
typically provide initial colorant loading or concentration recipes to
match customer color requirements and are also used to facilitate the
calculation of colorant addition adjustments to eliminate differences in
color between a plaque molded from production samples and the "standard
plaque." These programs typically require measurements of reflectance
spectra from both the sample and standard plaques, and are typically used
with dedicated spectrocolorimeters. Despite efforts to customize them for
specific products or manufacturing sites, these programs generally are not
able to provide an adequate prediction of the effects of colorant loading
or concentration on polymer product color and several iterations of the
color adjustment process are, therefore, often required even when these
software tools are employed in the process to obtain the desired polymer
product color. Thus, state of the art color adjustment procedures have
several drawbacks. The procedures are time-consuming, require manual
intervention, require extremely accurate predictions of polymer product
color, often result in using excessive colorant or excessive colorant
additions, and also provide no compensation for shifts in polymer product
color that may occur during a production run.
FIG. 1 is a schematic diagram of an embodiment 100 of a system for
controlling the color of compounded polymer(s) in accordance with the
invention. System 100 includes: a compounder, 105, for compounding the
polymer(s), colorant additive(s), and even other additive(s) to produce a
compounded polymer mixture; a sensor, 110, for continually monitoring the
color of the substantially homogeneous compounded polymer mixture,
in-process, at predetermined intervals; a colorant additive feeder, 130,
for providing one or more colorant additives to the compounded polymer
mixture at substantially predetermined colorant addition rates, and a
controller, 120, responsive to the sensor, for controlling the colorant
addition rate of the colorant additive feeder for each colorant additive
or for combinations of colorant additives. System 100 may further include
a sensor 107, such as a thermocouple or an infrared radiation sensor, for
measuring the temperature of the in-process compounded polymer mixture at
substantially the same time that sensor 110 measures the color of the
mixture. Likewise, a temperature sensor may measure the temperature of
liquid bath 170, if desired. Controller 120 may be coupled to and
responsive to such a temperature sensor and use the measured temperature
to compensate the measured color or alternatively, the target color for
the effect of temperature upon the color of compounded polymer mixture.
Compounder 105 may include, as illustrated, a base resin reservoir and
feeder 160, and a production extruder 140. Nonetheless, other examples of
compounding machines include kneading machines, mixers, including banbury
type internal mixers, mixing rolls and single or twin screw extruders.
Likewise, as suggested earlier, FIG. 1 illustrates liquid bath 170 for
in-process cooling of the mixture after it exits the extruder, such as
through an aperture in a die incorporated at one end of the extruder, as
illustrated; however, such a bath, although convenient to solidify the
in-process mixture rapidly, may be excluded from alternative embodiments.
For example, air cooling may alternatively be employed. As illustrated,
the colorant additive feeder or colorant feed system 130 is in physical
association with the production extruder, in this embodiment by a channel,
passage, or other material handling connection between the colorant
additive feeder and the production extruder. In general, the base resin
reservoir and feeder and colorant additive addition feeder may comprise
any one of a number of materials handling apparatus, such as described in
Unit Operations of Chemical Engineering, written by W. L. McCabe and J. C.
Smith, and available from McGraw-Hill (1976). Extruder 140 mixes the
polymer product constituents received from base resin supply 160, and from
other sources, such as colorant additive addition feeder 130. Likewise,
other additives may be fed, such as fiber for reinforcement, flame
retardant, etc. These may be fed, for example, from the same feeder that
feeds the colorants or from a separate feeder, depending on the particular
embodiment. Thus, various feeders may feed constituents to extruder 140,
and the extruder mixes the constituents to provide the compounded polymer
mixture.
As illustrated, colorant feeder 130 responds to colorant feed rate
adjustments provided by controller 120. Colorant feed system 130 thus
feeds additions of colorant(s) or colorant additive(s) into the production
extruder, which mixes the constituents and thereby results in the
adjustment of the in-process color of the compounded polymer mixture. In
response to the controller, feeder 130 may either increase or reduce the
rate of addition of colorant additives, thus, in many instances,
conserving the use of such colorants and avoiding waste. As illustrated,
the mixture may leave the production extruder through a die. The die may
incorporate apertures of various shapes and sizes to produce various
forms, such as cylindrical strands, webs, sheets, bars, pipes, or
channels, to name a few possible shapes. In this particular embodiment, as
suggested earlier, strands of material exit the die and are provided to
liquid bath 170 in order to cool and partially harden the mixture.
Typically, water will be employed. At this point in the process, and as
illustrated, sensor 110 may obtain color information about the mixture
from optical signals reflected from the product. These optical signals may
be transformed by the sensor to electrical signals and provided to the
controller in order to determine one or more adjustments to the colorant
addition feed rate.
It will now be appreciated that in general the color of an object, such as
a polymer mixture or polymer product, may be specified by no less than
three independent color parameters or color signal values. See, for
example, the previously referenced Judd and Wyszecki text. Each of these
three parameters or signal values may, therefore, be adjusted individually
to affect color and in the context of the invention these three parameters
are referred to as the three dimensions of color space.
For the embodiment of a system for controlling the color of compounded
polymer(s) illustrated in FIG. 1, system 100 may be initialized with a
target product color and a nominal colorant feed rate, typically resulting
in a particular amount of colorant per pound of final product. This
procedure is performed once and thus provides the nominal starting point
or initialization of the system. Once provided an initial nominal
combination of polymer(s), and colorant(s) or colorant additive(s), the
system may comprise hardware and software components to implement
continual colorant additive adjustments to subsequently realize the
desired polymer product color during production or compounded polymer
processing. Likewise, the hardware and software components may be
implemented so as to determine the initialization or initial formulation
as well.
Although many different procedures may be employed to provide an initial
starting point for the system, such as by a completely automated
procedure, the "closer" the system initialization places the system
operating point to the desired in-process polymer mixture color, the more
quickly the desired in-process color will be realized. Nonetheless, one
advantage of a system for controlling the color of compounded polymer(s)
in accordance with the invention is the fact that the initial formulation
may not initially achieve the desired in-process color and the system may
automatically adjust the formulation to obtain the desired target color
in-process. In order to achieve this initialization more effectively, one
possible initialization methodology is provided hereinafter.
A polymer product formulation may be provided in terms of desired color,
base resin composition or concentration, colorant additive composition or
concentration, and other additive concentration. Thus, a nominal product
specification or formulation requires selecting the appropriate
additive(s), colorant additive(s) and polymer(s) in nominal concentrations
which theoretical computations indicate will achieve the desired target
polymer product color based on the Kubelka-Munk theory, while also
providing the flexibility to adjust the polymer color, as the need arises
during processing, by varying the relative amount(s) of colorant
additive(s). It will be appreciated that the relative amount of polymer(s)
may theoretically also be adjusted to affect color, although this may not
be practical or economical.
Likewise, an appropriate target color for the polymer product, as measured
by sensor 110, must be specified. The problem is made more complex because
of differences between various methods of measuring color and the effects
of injection molding on polymer product color. Therefore, the target color
for the compounded polymer product as measured by sensor 110 may not be
the same color as the "standard" plaque for the polymer product.
Obtaining a nominal product formulation of selected polymer(s),
colorant(s), and other additive(s) is conventional and may be performed by
any one of a number of commercial available products, such as the
aforementioned MTS. Other examples of commercially available software
include OPTIMATCH.TM. Plastics Color Formulation System available from
MacBeth, a division of Kollmorgen, Corp., in Newburgh, N.Y. The result of
such a procedure is a list of n colorant additive(s) and nominal
concentration(s), x.sub.i0, for each, where i is a non-zero integer up to
n. Although the concentration of the polymers or base resins and other
additives may also be specified per pound of final product, these
concentrations are usually not adjusted beyond the initial formulation or
at least not in-process, unlike colorant concentrations.
Ideally, this recipe would result in a polymer product with a reflectance
spectra or spectral reflectance curve substantially identical to that of
the polymer product standard, that is, a non-metameric match, although
this is unlikely to occur in actual practice. Instead the predicted
spectral reflectance may correspond to three color space parameters or
signal values, X.sub.o, Y.sub.o, and Z.sub.o. It will be appreciated that
for a given illuminant, the color of an object may be decomposed into
three such signal values, such as described in the aforementioned Judd and
Wyszecki text and as previously described regarding the three dimensions
of color space. For convenience, in the context of the invention, the
tristimulus signal values indicating the color of an object are employed,
although the invention is not limited in scope to this particular color
signal formulation. For example, transformations may be employed to
produce other color signal formulations. Likewise, RGB color signals
(i.e., red, green and blue color signals such as those produced by an RGB
video camera and defined by National Television System Committee (NTSC)
standards as set forth, for example, in Television Engineering Handbook,
K. Blair Benson, Editor, McGraw-Hill, 1986) may alternatively be employed,
such as described in aforementioned U.S. Pat. No. 5,559,173, issued Sep.
24, 1996.
One aspect of the nominal colorant formulation is obtaining a formulation
which allows flexible modification of the nominal recipe to produce
"arbitrary" colors "near" the target polymer product color. This feature
is not provided or even recognized by any of the known commercial software
product formulation tools, Thus, one aspect of a system for controlling
the color of compounded polymer(s) in accordance with the invention
includes a methodology for providing a quantitative measure of the
"controllability" of the color of the compounded polymer mixture about a
particular nominal recipe based on properties of the formulation obtained
from a linearized version of the Kubelka-Munk equations, as described
hereinafter. This measure provides an indication of the ease with which
the nominal recipe may be modified to produce arbitrary colors in a
neighborhood around the target polymer product color. Such
controllability, and its associated quantative measure, is useful in a
system for controlling the color of in-process compounded polymer(s) in
that disturbances, changes, or anomalies in operating conditions may shift
the color of the polymer mixture in arbitrary directions away from the
target color during production or compounded polymer processing. In order
to correct these in-process color shifts, the feedback controller in a
system for controlling the color of compounded polymer(s) in accordance
with the invention has the capability to shift the in-process mixture
color in the reverse direction, back "towards" the target color, by
adjusting one or more colorant addition feed rates.
The Kubelka-Munk theory provides a nonlinear relationship between colorant
additive concentrations, c.sub.i, and "color space" (i.e., any well-known
3-dimensional color scale, such as any of those defined by the
international standards organization known as CIE (Commission
International de l'Eclairage) which includes tristimulus values X, Y, Z
and 1976 CIE L*a*b* values L, a, b) of the form
X=f.sub.1 (c.sub.1, c.sub.2, . . . , c.sub.n) (1)
Y=f.sub.2 (c.sub.1, c.sub.2, . . . , c.sub.n) (2)
Z=f.sub.3 (c.sub.1, c.sub.2, . . . , c.sub.n) (3)
where X, Y, Z are the previously described color space parameters or signal
values and f.sub.i are mathematical relationships that depend, at least in
part, on the chosen light source or illuminant and the "standard
observer", as explained in greater detail in Judd and Wyszecki.
In the case that the color space of interest is defined by CIE tristimulus
values, equations (1), (2), (3) could take the form:
##EQU1##
where k is a normalizing constant given by
##EQU2##
R(.lambda.) is the sample reflectance at wavelength .lambda., in
percentage,
S(.lambda.) is the illuminant's relative spectral power at wavelength
.lambda., and
x(.lambda.), y(.lambda., z(.lambda.) are the color matching functions for
the selected observer evaluated at wavelength .lambda..
As is well known in the art, the reflectance R(.lambda.) for a sample
containing one or more colorants can be determined for a given sample from
the ratio of the absorption and scattering coefficients of the mixture via
##EQU3##
where the ratio
##EQU4##
is given by
##EQU5##
where C.sub.i is the weight fraction of colorant i in the sample,
k.sub.i is the absorption coefficient for colorant i in the sample, and
s.sub.i is the scattering coefficient for colorant i in the sample.
It is important to note that many other "color spaces" can be derived from
the CIE tristimulus values X, Y, Z defined here. These are well known in
the art and include, among others: CIE L*, a*, b*; CIE L*, u*, v*; FMC;
FMC-II, and CMC. It will be appreciated that any of these well-defined
transformations of X, Y, Z could be applied and the following develppment
would proceed unmodified in principle.
Presuming that CIE tristimulus, X, Y, Z is the color space of interest, the
expansion of these relationships in a conventional Taylor series about a
nominal colorant recipe c.sub.10, . . . , c.sub.n0, provides the following
simplified mathematical representation.
##EQU6##
where X.sub.0, Y.sub.0, and Z.sub.0, are, the nominal color space values.
In a particular embodiment, these nominal color space values could be
tristimulus signal values. O(c-c.sub.0).sup.2 denotes mathematical terms
of "order" two and above, and the matrix, G, of steady state gains is
provided by:
##EQU7##
For small changes in the nominal recipe, the higher-order terms in equation
(4) should not contribute significantly to changes in the in-process
mixture color and the gain matrix, G, characterizes the effects of
colorant additive concentration or loading changes on the in-process
mixture color. Thus, arbitrary changes in the in-process color around, or
in the vicinity of, the target polymer product color may be achieved if G
has full row rank. The matrix G provides a quantitative measure of the
"difficulty" in achieving the colorant loading or concentrations
corresponding to modifications in the desired in process color. This
quantitative measure is related to the property of matrix G referred to as
the "condition number" and is explained in more detail in Matrix
Computations, by C. H. Golub and C. F. Van Loan, available from Johns
Hopkins University Press (1983) and Linear Algebra and its Applications,
by G. Strang, available from Academic Press (1980). The condition number,
as is well known in the art, is the ratio of the largest singular value of
a matrix to the smallest non zero singular value of the matrix. In those
situations where G has an infinite condition number, G has rank
deficiency. When the condition number of G is large with respect to unity,
solutions to the linear system of equations (4), with higher order terms
neglected, exist but are difficult to obtain numerically and the resulting
solution is likewise sensitive to changes in the entries in G. See, for
example, the aforementioned Golub and Van Loan text. Since inaccuracies in
G are inevitable as a result of limitations in the Kubelka-Munk theory and
for other reasons, this situation implies a formulation which, in the
context of the invention, is "practically unadjustable." That is, in
relation to the amount of colorant necessary, the in-process mixture color
is difficult to adjust or modify. Thus, the formulation is either actually
unadjustable or practically unadjustable. When the condition of G is
moderate relative to one, however, reliable solutions to the linear system
of equations are relatively easy to obtained numerically. For example, a
condition number above 1000 is probably excessive, whereas a condition
number below 100 is probably not. Nonetheless, it will now be appreciated
the acceptability of the condition number may depend, at least in part, on
the amount of uncertainty associated with the entries of G.
The condition number, therefore, provides a quantitative measure of the
in-process color formulation flexibility. Whether a nominal recipe is
practically unadjustable in the context of the invention in one approach
may be resolved by incorporating the condition number measure during
colorant additive formulation in conjunction with other colorant
formulation techniques, such as those provided by commercially available
polymer product color formulation software.
As previously indicated, in determining the initial product formulation, a
second aspect is the identification of an appropriate target for the
product color, as measured in-process by sensor 110. Likewise, depending
on the particular application of a system for controlling the color of
compounded polymer(s) in accordance with the invention, the target color
may be changed or modified during a single production run. Two alternative
approaches for providing a target product color are either preparing a
"production standard" or developing a transformation that relates the
sensor measurements to the ultimate plaque or desired polymer product
color.
A production standard is a physical sample of the polymer product generated
in a previous lot which, when molded, results in a plaque identical to the
polymer product color standard. This production standard is then measured
by sensor 110 and its color, as measured, used as the target color for
production. Depending upon the type of sensor employed, it may be useful
to measure both the physical production standard and the in-process
polymer product or compounded polymer mixture substantially
simultaneously. Thus, in such an embodiment, sensor 110 need only provide
a differential color measurement and need not produce absolute color
measurements of significantly high accuracy and precision. By yet another
technique, it may be sufficient to measure the production standard when
the lot is begun, store the generated measurement signals and use these
stored signals as the control target throughout the production run.
A second approach to the determination of target color for sensor 110
involves developing a transformation which relates the measured color of
the in-process polymer product, such as strands, or webs, to the color of
the plaque molded under known conditions from that polymer product. Many
aspects of the operating environment may account for this color
difference, such as, for example, the temperature of the polymer mixture
at the time of color measurement. Thus, it may be desirable to include a
temperature sensor to measure the temperature of the polymer mixture, or a
variable correlated with this temperature, at the time of measurement.
Likewise, it will be appreciated that either the target color or the
measured color of the compounded polymer mixture may be adjusted or
compensated to account for any color variations due to temperature. Once a
transformation is obtained, the inverse of this transformation may then be
applied to the polymer product standard plaque color to produce a target
color for the signal measurements obtained by the sensor. This approach
would be most effective where a consistent relationship exists between
polymer product color measured in-process and the plaque ultimately
obtained from that product. It will now be appreciated that once a plaque
or polymer product color standard is available, the initialization of a
system for controlling the color of compounded polymer(s) in accordance
with the invention may be completely automated.
As illustrated in FIG. 1, system 100 includes sensor 110 which measures,
in-process, compounded polymer color. Depending on the type of compounding
machine used, the product may be in the form of, for example, multiple
thin strands, a flat web, or sheets. The color sensor may take a number of
different forms. Any sensor capable of repeatedly measuring the polymer
mixture color may be employed, including tristimulus (3-filter)
colorimeters, spectrophotometric colorimeters, and electronic video or
still frame camera-based systems. For example, a generic RGB video camera
may suffice. One color sensor embodiment is described in more detail in
aforesaid U.S. Pat. No. 5,559,173, issued Sep. 24, 1996 entitled "Imaging
Color Sensor." In any case, the sensor should have the capability of
producing three-dimensional signal measurements in real-time that provide
color information.
Another component of a system for controlling the color of compounded
polymer(s) includes colorant additive feeder 130. Potential feeding
methods include: feeding powders or pellets directly to the compounder,
such as at the feed throat; feeding single pigment dispersion pellets to
the compounder, such as at the feed throat; feeding liquid colorants to
the compounder, such as at the feed throat; or injecting liquid colorants
into the compounder at other locations than at the feed throat. Examples
of devices adequate for handling such liquid or solid materials include
belt feeders, vibratory feeders, loss of weight feeders, pneumatic
conveyers, peristaltic pumps, gear pumps, positive displacement pumps and
centrifugal pumps, to name only a few. Such a feeder should have the
capability to feed separate streams of colorants to the compounding
machine. Nonetheless, particular colorants may constitute a pre-mixed
blend of constituents. Furthermore, it may prove economical to pre-mix a
substantial portion of the mixture and only vary, through closed loop
feedback control, a selected number of colorants in relatively small
amounts to obtain the desired target color. It is desirable that the feed
rate for each colorant additive be independently adjustable from the feed
rate for any other colorant additive so that the colorant additive feeder
may be responsive to the controller to provide one or more colorant
additives to the compounded polymer(s) mixture at independently
adjustable, substantially predetermined, colorant addition rates.
Yet another component of a system for controlling the color of compounded
polymer(s) in accordance with the invention is controller 120, which
determines appropriate colorant addition rates in response to changes in
the sensor signals that indicate changes in the measured in-process
polymer mixture color. This feedback control may be implemented using
conventional process control hardware, such as programmable logic
controllers, (PLCs) or distributed control systems (DSCs). For effective
operation, one embodiment of a system for controlling the color of
compounded polymer(s) may employ a set of discrete time difference
equations, such as may be used to realize linear, time-invariant feedback
control. It will be appreciated, however, that any one of a number of
different feedback control techniques may be employed, such as, for
example, fuzzy logic feedback control, such as described in Fuzzy Logic
and Control: Software and Hardware Applications, edited by M. Jamshidi, N.
Vadiec, and T. J. Ross, and available from Prentice Hall (1993), or neural
network feedback control, such as described in Neural Networks for
Control, edited by Miller, Sutton, and Werbos, and available from MIT
Press (1990). A neural network closed loop feedback control computes
changes in pigment concentrations based on sensor values using a highly
interconnected set of simple computing units (known as "neurons" in the
prior art). Similarly, a fuzzy logic closed loop feedback control
constitutes another way of developing a relationship between color signal
values and required pigment concentration adjustments using what are known
as fuzzy sets to define the sensor values and pigment levels, and simple
rules for defining the mapping of sensor values within a given range to
pigment adjustments.
Fuzzy logic itself is a theoretical framework in which imprecise or
qualitative knowledge can be collected and manipulated; it is particularly
suited to capturing expert knowledge expressed in linguistic terms and
applying it in a systematic way. Its primary aim is to provide a formal,
computationally-oriented system of concepts and techniques for dealing
with modes of reasoning which are approximate rather than exact. It is a
generalization of traditional (or "crisp") logic which employs rules
relating an antecedent and consequent in the form
if (antecedent) then (consequent)
in which the anteedent and consequent can take on only the values TRUE and
FALSE such as, for example,
if .DELTA.E<1 then color is acceptable.
Fuzzy logic provides a framework for dealing with antecedents and
consequents which correspond to imprecisely defined conditions such as,
for example,
if color is too light then add more dark pigment.
Nonetheless, the particular embodiment of this invention is based on a
dynamic model relating changes in colorant or colorant additive feed rates
to changes in compounded polymer color measured in-process. This
relationship may be based upon a mathematical model providing a
description of the signals that may be produced during a production run to
represent polymer product color, such as provided hereinafter. In this
particular embodiment, the controller is implemented so as to provide a
single adjustable parameter, selected externally, to specify the desired
closed-loop speed of response of the system, although the scope of the
invention is not limited in this respect.
A technique to accomplish this is now provided. To provide a useful
technique for closed loop feedback control, assumptions are first made
regarding: 1) the effects of colorant feed rate changes on colorant
concentration in the polymer mixture; 2) the effects of colorant
concentration on the polymer mixture color; and 3) the time delay
associated with transportation of the polymer mixture from the compounder
to the sensor. It will now be appreciated that these assumptions may not
reflect the precise operation of the system; however, these assumptions
have proven suitable for effective operation of a system for controlling
the color of compounded polymer(s) in accordance with the invention. FIG.
2 is a block diagram illustrating this approach in which signals provide
color information at different points in the compounding process.
It is assumed, for the sake of convenience, that the colorants or colorant
additives do not react or interact in the compounder. As a result, the
colorant mixing dynamics, block 101 in FIG. 2, may be described by the
following frequency domain matrix equation:
##EQU8##
where: s is | | |
