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Description  |
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BACKGROUND OF THE INVENTION
This invention relates generally to an apparatus for automatically
preparing batches of a mixture containing several ingredients, and more
particularly to an apparatus for automatically catalyzing and supplying
resinous coatings for paperboard.
Coating, dipping and related operations are generally most efficient and
cost-effective when done as continuous, on-line processes. Thus, efforts
have been made to develop machines for automatically mixing ingredients
and providing continuous supplies of coating to the line. In the case of
resinous coatings for paperboard, this has been particularly difficult.
Resinous coatings are extremely corrosive and will rapidly destroy any
delicate mechanisms such as sensors which are in regular contact with the
coating or its various components. Moreover, once the resinous coating is
mixed its viscosity increases rapidly and it will harden into a glassy,
unmanageable mass if left standing for any appreciable period. Even where
this is not allowed to happen, deposits of hardened resin will tend to
accumulate on the sides of the supply apparatus, creating shadow-readings
on level probes and other sensors which operate by direct contact, and
also rapidly clogging any bottlenecks, such as pipe bends or metering
valves, in the apparatus. Thus it will be seen that a metered mix-control
system such as the one described in U.S. Pat. No. 3,620,915 issued Nov.
16, 1971 to Keyes, IV et al, which requires contacting sensors and
metering valves for flow control, would be unsuitable for use with
resinous coatings.
The relative proportions of the ingredients in a resinous coating must be
carefully controlled. For example, the catalyst rarely represents more
than a few percent of the total solids content of the coating, but the
addition of too much catalyst may cause the coating to solidify in the
pipes of the supply system while the addition of too little may result in
an unacceptably tacky coated web. While a resinous coating need contain
only three ingredients, i.e. a concentrated resin, a catalyst, and water,
small amounts of other ingredients such as defoamers, wetting agents,
viscosity and pH modifiers, curing agents and scavengers are often added
and it is desireable to be able to precisely control the amounts added,
both to achieve optimum results and to control the costs of what are
generally expensive ingredients.
SUMMARY OF THE INVENTION
Thus, it is an object of this invention to provide an automatic mixing
apparatus which can detect and accurately control the addition of both
very large and very small amounts of ingredients.
It is yet another object to provide an apparatus which is not susceptible
to corrosion or clogging.
It is yet another object to provide an apparatus which will automatically
sense and respond to downstream resin demand so as to insure a continuous
coating supply at the applicator.
It is still another object to provide an apparatus capable of automatically
correcting for deviations from the target amounts for critical ingredients
so as to maintain correct overall ingredient ratios.
Briefly, the present invention is an apparatus and method for preparing a
mixture, such as a resinous coating, in batches and providing a continuous
supply of the mixture at a feed station. Two or more ingredients are
supplied from separate sources through separate selectively operable
supply valve means to a mixing tank which may also contain or be connected
to agitator means. Dumping means are provided for selectively emptying the
mixing tank contents into a run tank. Each tank is connected to
gravimetric means for providing a monitoring signal indicative of the
weight of the tank contents. In the preferred embodiment, the monitoring
signals are provided by suspending each tank from a load cell. Logic
control means responsive to the monitoring signals are provided for
selectively activating the supply valve means, the agitator means, and the
dumping means. The control means include comparator means for comparing
the actual weight of each ingredient added with a precalculated target
weight and means for calculating the target weights of subsequently added
or non-critical ingredients based on the actual added weights of
previously added critical ingredients in order to maintain correct
ingredient ratios.
OUTLINE OF DRAWING
For a better understanding of the invention, as well as other objects and
further features thereof, reference is made to the following detailed
description to be read in conjunction with the annexed drawing, whose
single FIGURE is a block diagram showing a preferred embodiment of an
automatic mixing apparatus in accordance with the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Turning now to the drawing, an apparatus indicated generally by the
reference numeral 20 has been depicted in schematic form. The apparatus
includes a mixing tank, 24, and a run tank, 25, connected by a mixing tank
valve, 34. The mixing tank chosen should be at least large enough to hold
one batch of solution and the run tank large enough to hold one batch plus
a reserve amount sufficient to satisfy demand while a fresh batch is being
prepared. The batch size should be chosen on the basis of expected usage
rate, large enough that short falls will not occur but not so large that
the coating material will remain unused in the run tank for extended
periods.
Ingredients are supplied to the mixing tank from two or more independently
controllable sources. For the purposes of illustration, it will be assumed
that the mixture being prepared, 40, is a catalyzed resin, and that it has
three ingredients, 41, 42, and 43 which are, respectively, an undiluted
resin, water, and a catalyst. Thus, undiluted resin, 41, may be supplied
to mixing tank 24 from source 21 through resin supply valve 31. Catalyst
43 may be supplied from source 23 through catalyst supply valve 33 and
water 42 may be supplied from source 22 through water supply valve 32. It
should be understood, however, that while the apparatus of this invention
is particularly well adapted for use with resinous coatings and other
mixtures which are corrosive, quick-setting or both, it may be used for
other mixtures as well.
It is an advantage of this invention that the apparatus may be used, with
the addition of a suitable number of supply sources and valves, for
mixtures having any number of ingredients. The ingredients may be supplied
in almost any measurable form, as, for example, liquids, powders,
granules, gasses, or extrudable gels. Moreover, the ingredients need not
be added in identical forms. Thus, for example, a first ingredient might
be supplied as a powder, a second as a liquid, and the third as a gel. And
while means 31, 32, and 33 have been described as supply valves it will be
obvious that other means of starting and cutting off the flow of an
ingredient to the mixing tank may be appropriate and are also within the
scope of the invention.
Means, 51, are provided for gravimetrically monitoring the contents of
mixing tank 24. Similarly, means, 52, are provided for gravimetrically
monitoring the contents of run tank 25. In the preferred embodiment
illustrated in FIG. 1, these means are provided by suspending each tank
from a load cell. Alternatively, each tank might rest upon a scale.
Gravimetric monitoring means have two advantages over the more
conventional level monitors. First, gravimetric monitors provide more
accurate and reliable readings, particularly where small amounts are
involved. This is particularly important where one ingredient, such as a
catalyst, must be added in a quantity which is very small in proportion to
the amounts of other ingredients. It also prevents waste. Second,
gravimetric monitors, unlike level probes, are non-contacting sensors.
Thus shadow readings caused by coating of the sensors will not occur nor
will the gravimeter monitors be easily damaged by corrosive mixtures.
In the preferred embodiment, selectively operable agitator means, 26, for
agitating the contents of mixing tank 40 are also provided.
Logic control means, 10, preferably in the form of a digital computer, are
provided for selectively operating the supply valves, 31, 32, and 33 the
agitator means, 26, and the mixing tank valve, 34. The computer receives a
mixing tank monitoring signal, 101, from the mixing tank load cell, 51,
and a run tank monitoring signal, 52, from the run tank load cell, 52. To
provide essentially continuous monitoring, it is preferred that the load
cells 51 and 52 be sensed and the current values of monitoring signals 101
and 102 be read into the computer several times a second. Less frequent
sensing might be required if the tanks were filled and emptied rather
slowly.
The computer, 10, also operates on a number of predetermined parameters. It
is preferred that these parameters be manually input, so that they can be
changed as the need arises, but the preset parameters might also be
"burned in". In the preferred embodiment, the predetermined parameters are
a prep request trigger level, 110, a run tank refill level, 111, a mixing
tank abort level, 112, a batch size, 113, a received resin solids content,
114, a target resin solids content, 115, a received catalyst solids
content, 116, a catalyst to resin solids ratio, 117, a minimum agitation
time, 118, and a minimum settling time, 119.
The computer, 10, includes calculator means for calculating target
ingredient weights based on the predetermined parameters and the actual
weights of previously added ingredients as indicated by the mixing tank
monitoring signal, 101. It also includes comparator means for comparing
actual with target values, storage means and, preferably, a timer.
In the preferred embodiment, the control sequence is as described below.
Prior to the initiation of the automatic resin preparation sequence, the
computer calculates initial target weights for each ingredient based on
the input values of predetermined parameters 113, 114, 115, 116, and 117.
Letting V.sub.k be the value of predetermined parameter k, X be the resin
target weight, Y the water target weight and Z the catalyst target weight,
the computer calculates initial values for X, Y, and Z in accordance with
the following algorithms:
##EQU1##
The target set sequence may be done automatically whenever a resin prep
sequence is initialed, but it is preferred that the target set be done
once when the apparatus is started up and thereafter be triggered only by
a manually input change in the value of one of the predetermined
parameters.
In the first run tank test mode, the initial state in the resin prep
sequence, the logic control means, 10, compares the values of the run tank
monitoring signal 102 and the predetermined prep request trigger level
parameter 110. When the weight of the contents of the run tank falls below
the predetermined trigger level, the computer, 10, closes mixing tank
valve 34 by means of output signal 134. The computer 10, then compares the
values of the mixing tank monitoring signal and the predetermined mixing
tank abort level parameter 112. If the weight of the mixing tank contents
exceeds the abort level, the apparatus shuts down and, preferably displays
an appropriate warning signal. Otherwise the resin supply valve, 31, is
opened by output signal 131. The computer then sets an initial target
weight, T.sub.o equal to X.sub.i, the previously calculated resin target
weight. The computer then compares the values of the mixing tank
monitoring signal 101 with T.sub.o : When the weight of the mixing tank
contents is at least as great as T.sub.o the resin supply valve is closed
by output signal 131.
When valve 31 has been fully closed, the actual weight of the resin in the
tank will be somewhat greater than the target weight X.sub.i. This
phenomenon is known as overshoot and is caused primarily by the finite
closure time of valve 31, although signal lag time, the period between
consecutive tests of mixing tank weight, and valve clogging may all be
contributing factors. An important feature of this invention is that it
provides automatic overshoot compensation. The key ingredient or
ingredients are added first and the target weights for the other
ingredients are then recalculated based on the actual added weights of the
key ingredients so that critical ingredient ratios can be maintained
within close tolerances. This automatic adjustment is believed to be a
unique feature of the apparatus, not previously available in batch-type
mixing apparatus.
When valve 31 is closed, the mixing tank is allowed to settle for
predetermined minimum time (based on a comparison between a timer and
predetermined settling time parameter 119), before the current value of
the mixing tank monitoring signal 101 is read into the computer 10. This
value will be X.sub.A, the mixing tank. The computer then calculates a new
target weight, T.sub.1, based on the actual amount of resin added in
accordance with the following algorithm:
##EQU2##
Water supply valve, 32, is then opened by output signal, 132. The computer
then compares the value of the mixing tank monitoring signal, 101, with
T.sub.1 and when the weight of the mixing tank contents is at least as
great as T.sub.1, the water supply valve is closed by output signal 132.
The computer 10, then activates the agitator means 26 by output signal 136
and the resin/water combination is agitated for a predetermined minimum
time (based on a comparison between a timer and predetermined mixing time
parameter 118). When this time is up, the agitator is turned off by output
signal 136 and the mixing tank is again allowed to settle for the
predetermined minimum settling time before the current value of the mixing
tank monitoring signal is read into the computer. This value will be
X.sub.A +Y.sub.A, the sum of the actual weights of resin and water added
to the mixing tank. The computer then calculates a new target weight,
T.sub.2, based on this value in accordance with the following algorithm:
##EQU3##
Catalyst supply valve, 33, is then opened by output signal, 133. The
computer then compares the value of the mixing tank monitoring signal,
101, with T.sub.2 and when the weight of the mixing tank contents is at
least as great as T.sub.2, the catalyst supply valve is closed by output
signal 133.
The computer 10, then activates the agitator means 26 by output-signal 136
and the resin/water/catalyst combination is agitated for a predetermined
minimum time (based on a comparison between a timer and predetermined
mixing time parameter 118). When this time is up, the apparatus enters,
the second run tank test mode. In this mode, the computer, 10 compares the
values of the run tank monitoring signal, 102, and the run tank refill
level parameter, 111. So long as the weight of the run tank exceeds this
preset refill level, the mixing tank valve 34, will remain closed and
agitator means 26 will be activated by signal 136 to prevent settling of
the mixture in the run tank. When the weight of the run tank drops below
this preset level, the agitator means will be turned off by signal 136 and
mixing tank valve 34 will be opened by signal 134. Since the refill signal
is never tested unless the resin is fully ready, there is no danger that a
half-mixed batch will be dumped into the run tank.
While a preferred embodiment of the invention has been shown and described,
it will be appreciated that various changes and modifications may be made
therein without departing from the spirit of the invention.
For example, while certain predetermined parameters have been described in
connection with the preferred embodiment, it will be obvious that others
are also within the scope of the invention and that more or fewer than the
number described may be used. Thus, if the mixing time were relatively
short in comparison to the average time for depleting the run tank, a
single request parameter might be used in place of prep request trigger
level 110 and run tank refill level 111. On the other hand separate
agitation times might be established for each step in the mixing process
instead of a single minimum agitation time. And ingedient/batch weight
target ratios may be used instead of target solids contents, or ingredient
target weights may be input directly.
And while in the embodiment described the resin was the key ingredient to
which both catalyst and water additions were targeted, it will be
understood that an ingredient addition can be targeted to any
earlier-added ingredient or combination of earlier added ingredients.
Thus, for example, the catalyst might have been keyed to both the water
and the resin actual weights in accordance with the following algorithm:
##EQU4##
And a fourth ingredient, W, a water softener might be added on the basis
of actual water usage only, in accordance with the following algorithm:
##EQU5##
These variations and others like them will be obvious to those skilled in
the art.
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