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BACKGROUND OF THE INVENTION
The present invention relates to process control systems and more
particularly to means for controlling the concentration of a constituent
in a product made by mixing components having differing amounts of the
constituent.
Photographic emulsions can be prepared by mixing solutions of silver
nitrate and alkali halide. The properties of such emulsions are, in part,
a function of the defined shape, average size and size distribution of the
silver halide crystals in the emulsion. To produce emulsions having
desirable silver halide crystal characteristics, the silver ion
concentration, acidity, flow patterns, temperature, and addition rate of
the reactants must be closely controlled during the mixing operation.
One known system for controlling the addition rate of the reactants is a
gravity feed system in which control valves are used to limit the rates of
delivery of the silver nitrate and alkali halide solutions. In this
system, a recorder is used to monitor acidity and silver ion
concentration. Where a deviation from set point is sensed, the addition
rates of the solutions are adjusted.
One disadvantage of such a system is that continuous variations in the
primary addition rates of the reactants does nothing to enhance the
overall stability of the process.
In another prior art system, the primary addition rates for the alkali
halide and silver nitrate solutions are controlled by gear pumps in the
primary flow paths for the solutions. The flow rate for each solution is
monitored and controlled by a flow meter. Silver ion concentration is
controlled in this system through the use of separate control flow paths
from the reactant supply vessels to the mixing vessel.
Each control flow path includes a silver ion concentration analyzer, a gear
pump and a flow meter. Where the silver ion concentration is below set
point, the gear pump in the silver nitrate control flow path is energized.
Conversely, where the silver ion concentration is above set point, the
gear pump in the alkali halide control loop is energized. The control
loops operate independently of the primary loop. Since the reactants
addition rate is controlled by a gear pump, which is the only available
means to adjust the flow rate, the limits to the range of motor speeds has
a limiting effect on the turn-down ratio of the maximum to minimum
addition rate through the control flow paths.
SUMMARY OF THE INVENTION
The present invention permits photographic emulsions or other products to
be manufactured by a process during which significant disturbances in a
constituent may occur and be compensated for.
Apparatus for practicing the present invention can control the
concentration of a constituent of a product prepared by mixing at least
one constituent-deficient component and at least one constituent-rich
component in a vessel. The apparatus includes a primary flow path with
first and second metering devices for establishing the primary addition
rates for the constituent-deficient component and the constituent-rich
component. The primary flow path also includes means for driving the first
and second metering devices continuously during the mixing operation. The
apparatus further includes a secondary flow path including a third
metering device for delivering secondary amounts of the
constituent-deficient component upon command and a fourth metering device
for delivering secondary amounts of the constituent-rich component upon
command. Concentration control elements of this apparatus include means
for measuring the concentration of the constituent in the vessel and means
responsive to any deviation within a predetermined range to energize one
of the metering devices in the secondary flow path to add a secondary
amount of whichever component is needed to offset the deviation. This
means responds to deviations outside the predetermined range to alter the
delivery rate of one of the metering devices in the primary flow loop as a
direct function of the magnitude of the deviation.
DESCRIPTION OF THE DRAWINGS
While the specification concludes with claims particularly pointing out and
distinctly claiming that which is regarded as the present invention,
further details and advantages of a particular embodiment of the invention
may be more readily ascertained from the following detailed description
when read in conjunction with the accompanying drawings wherein:
FIG. 1 is a schematic diagram of a photographic emulsion manufacturing
process which incorporates the present invention;
FIG. 2 is a block diagram of the concentration control circuits with two
controllers employed in the control of that process;
FIG. 3 is a block diagram of the concentration control circuits with three
controllers employed in the control of that process; and
FIG. 4 is a compilation of the control signals generated over a wide range
of ion concentration deviations.
DESCRIPTION OF SPECIFIC EMBODIMENTS
Referring now to FIG. 1, the function of the present invention is to
control the rates at which an alkali halide solution in a supply tank 10
and a silver nitrate solution in a supply tank 12 are delivered to a
mixing vessel 38. The solution within the vessel 38 is continuously
agitated by the mixer 99. An outlet conduit from supply tank 10 includes a
tank valve 14 and a filter 16 through which the alkali halide solution is
directed. Most of the filtered alkali halide solution enters a primary
flow path through a conduit 18 although a small amount is diverted to a
secondary flow path through a conduit 20.
The outlet connections from the supply tank 12 for the silver nitrate
solution are similar to the outlet connections for the alkali halide
solution. That is, an outlet conduit from the supply tank 12 includes a
tank valve 22, a filter 24, a primary flow conduit 26 and a secondary flow
conduit 28.
The primary addition rate for each of the solutions is established in the
primary flow path by means of reciprocating metering pumps 30 and 32 in
conduits 18 and 26, respectively. Each of the reciprocating metering pumps
30 and 32 is a conventional pump, the delivery rate of which can be
adjusted by varying either the stroke rate or the stroke length of the
reciprocating element. For example, pumps 30 and 32 could be Model SED MR
2-88-142-SM pumps available from the Milton Roy Co. of Philadelphia,
Pennsylvania.
Both of the reciprocating metering pumps 30 and 32 are driven at the same
stroke rate by a variable speed drive motor 34 operated by speed control
circuit 36. Alkali halide solution metered by pump 30 is delivered to the
mixing vessel 38 through a conduit 40 which includes a back pressure valve
42. Metered amounts of silver nitrate solution are delivered to the mixing
vessel 38 through a conduit 44 having a back pressure valve 46. These
valves may be 1/2" 746-033-50 valves, also available from the Milton Roy
Co.
In the secondary flow path a selectively energized metering pump 48 forces
secondary amounts of alkali halide solution from conduit 20 through a back
pressure valve 49 to a conduit 50 terminating in the mixing vessel 38. A
similar metering pump 52 regulates the secondary flow of silver nitrate
solution from conduit 28 through a back pressure valve 53 to a conduit 54
which also terminates in the mixing vessel 38. Metering pumps 48 and 52
may each be a 201V-40-43-SM Chem Pump available from the Crane Co. of
Warrington, Pa.
The concentration of silver ions in the mixing vessel is measured by means
of a probe assembly 57 immersed in the vessel. The signals developed by
this probe assembly are applied to concentration control circuits, shown
as block 58, but described in more detail with reference to FIGS. 2 and 3.
Where measurements indicate the silver ion concentration deviates from a
setpoint or predetermined value by a relatively small amount, circuits 58
cause either metering pump 48 or metering pump 52 to be energized to
deliver whichever solution is needed to offset the deviation.
Where the ion concentration disturbance is relatively large; that is,
incapable of being corrected when the energized pump is running at maximum
capacity, concentration control circuits 58 generate signals which alter
the stroke length of reciprocating metering pump 30, thereby altering the
primary addition rate for the alkali halide solution. The stroke length
may either be increased or decreased to provide more or less alkali halide
solution as needed to offset deviations in the silver ion concentration.
According to a preferred embodiment of the invention, only the stroke
length of pump 30 is altered by the concentration control circuits 58. The
stroke rate of pump 30 remains the same as that of pump 32 which is
preferably not tied to the concentration control circuits.
Referring now to FIG. 2, a silver billet probe 56 and a silver/silver
chloride reference probe 60 in probe assembly 57 provide input signals to
a silver ion concentration meter 62 which generates a signal representing
the absolute value of the silver ion concentration in vessel 38. The
output of meter 62 is applied to two proportional controllers 70 and 72
through a current converter 66.
Probe 60 may be a Ag/AgCl reference probe available from Beckman
Instruments Inc. of Irvine, California. Silver ion concentration meter 62
is basically a Model 900 pH analyzer also available from Beckman
Instruments. Proportional controllers 70 and 72 are Model No. 62H
Proportional controllers available from Foxboro Co. in Rochester, New
York.
The function of the proportional controller 70 is to offset the signal
representing the measured ion concentration by the magnitude of the
setpoint signal to derive a concentration error signal when there is a
surplus of silver ions. The magnitude of the output signal indicates the
size of the deviation. The output signal of the proportional controller 70
remains at a minimum value when there is a deficiency of silver ions.
The function of the proportional controller 72 is the same as the function
of the proportional controller 70 where there is a deficiency of silver
ions. That is, the output signal from the proportional controller 72 is
proportional to the deviation between the measured silver ion
concentration and the setpoint signal when there is a deficiency of silver
ions. The output signal of the proportional controller 72 remains at a
minimum value when there is a surplus of silver ions.
The silver ion concentration set points for the proportional controllers 70
and 72 are preferably remotely controlled through separate channels one
and two from digital data setpoint circuit 68. The output of controller 70
is applied both to a high signal selector circuit 74 and to a low signal
selector circuit 76. The function of the low signal selector circuit 76 is
to compare the output of proportional controller 70 with a reference
signal furnished by a reference current source 78. Signal selector circuit
76 passes the lower of the two compared signals to a current converter 80.
The output of current converter 80 controls the alkali halide pump 48 in
the secondary flow loop.
The output of the low signal selector circuit 76 is directly related to the
magnitude of the concentration error signal within a limited range, the
upper boundary of which is established by the value of the reference
current provided by source 78. Where the concentration error signal
exceeds the reference signal, the output of low signal selector circuit 76
is a constant having the magnitude of the reference. This constant signal
represents the maximum corrective capacity of the alkali halide pump 48 in
the secondary flow loop.
Auto/manual station 82 is a conventional process control element which
accepts instrument-generated input signals or manual input signals under
the control of an operator. For purposes of this description, it is
assumed that auto/manual station 82 is always operating in the automatic
mode. The signal appearing at the output of high signal selector circuit
74 is passed through station 82 to one input of a summing amplifier 84,
the output of which is applied to a current converter 86. The output of
summing amplifier 84 provides an input to a stroke length control 88 for
the alkali halide pump 30 in the primary flow loop. The primary stroke
length of the alkali halide pump is remotely controlled by a digital data
circuit through channel 3. The signal generated by the high signal
selector circuit 74 alters the stroke length of the alkali halide pump 30
slightly to increase the primary addition rate of the ion alkali halide
solution thereby reducing the silver ion concentration in the vessel. The
stroke length controller 88 is preferably a TAMR Actuator with AMI-MA
control available from the Milton Roy Co.
The high signal selector circuit 74 with its internal reference current
source is part of a system for correcting large errors in the ion
concentration. For ease of description, large errors; i.e., errors which
cannot be corrected primarily through use of the secondary flow loop only,
are referred to as abnormal errors. Errors which can be corrected through
use of the secondary flow loop are described as normal errors. During a
typical emulsion manufacturing process, both normal and abnormal errors
may be expected to occur. Most errors will, or course, be in the normal
range. The operation of the concentration control circuit in correcting
abnormal errors is described in more detail later.
When the output of the PAg meter 62 indicates a deficiency of silver ions,
the output of proportional controller 72 is applied to a high signal
selector circuit 92 and to a low signal selector circuit 94. The low
signal selector circuit 94 compares the output of proportional controller
72 with a reference signal provided by source 96, passing the lower of the
two signals to a current converter 98. The output of current converter 98
controls the energization of the silver nitrate pump 52 in the secondary
flow loop. The low signal selector circuit 94 provides a control signal to
the silver nitrate pump 52 which is linearly related to the magnitude of
concentration within the normal error range, the upper limit of which is
set by the reference signal generated by source 96.
The output of high signal selector circuit 92 is applied to an auto/manual
station 100 which, when operating in its automatic mode passes the signal
to a negative input to the summing amplifier 84. The resulting output from
summing amplifier 84, after conversion in the current converter 86, is
applied to the stroke length control 88 to decrease the stroke length of
the alkali halide pump 30 in the primary flow loop.
The control elements described above, including the low signal selector
circuits 76 and 94 can correct only a limited or normal range of ion
concentration errors. Where a concentration is abnormal; i.e., outside
this range, the high signal selector circuits 74 and 92 are brought into
operation.
Whenever the signal from the PAg meter 62 indicates a surplus of silver
ions, an output from proportional controller 70 is compared to an output
from an internal reference current source in the high signal selector
circuit 74. This circuit passes the higher signal. The minimum output
signal from the high signal selector signal 74 is established by the
internal reference current source with the output of high signal selector
circuit 74 being directly related to any higher output from proportional
controller 70. The selected signal is applied to auto/manual station 82
and to an alarm circuit 102 which notifies the process operator that an
abnormal concentration error has been detected. The output from the
auto/manual station 82, when applied through summing amplifier 84,
converter 86, and the stroke length control 88, increases the stroke
length of the alkali halide pump 30 in the primary flow loop.
Since the output of the high signal selector circuit 74 is directly related
to the magnitude of the abnormal concentration error, the stroke length of
the alkali halide pump 30 is changed linearly over a nearly unlimited
range to alter the primary addition rate of the alkali halide to whatever
extent is necessary to correct the abnormal concentration error.
Where the output of the PAg meter 62 indicates an abnormal deficiency of
silver ions, high signal selector circuit 92, after comparing the output
from proportional controller 72 to a reference current provided by an
internal source, causes an alarm 106 to be energized. The output of the
high signal selector circuit 92, when applied through auto/manual station
100, summing amplifier 84, and current converter 86, also causes the
stroke length of the alkali halide pump 30 to be decreased in direct
relation to the magnitude of the abnormal concentration error. The
decrease in the primary addition rate of the alkali halide will offset the
deficient silver ion concentration.
FIG. 3 shows an alternate approach to controlling the silver ion
concentration. A silver billet probe 55 and a silver/silver chloride
reference probe 61 provide input signals to a silver ion concentration
meter 63 which generates a signal representing the absolute value of the
silver ion concentration in vessel 38. The output of meter 63 is applied
to a three mode controller 65 through a current converter 67 which
converts the meter output current to a magnitude and form usable by the
three mode controller. In one embodiment of the invention, a silver ion
concentration remote programmable set point circuit 69 provides an input
signal to the three mode controller 65 representing the desired value of
the silver ion concentration. Controller 65 is preferably a Model No. 62H
proportional, reset and derivative controller available from Foxboro
Company.
The function of the three mode controller 65 is to offset the signal
representing the measured ion concentration by the magnitude of the
setpoint signal to derive a concentration error signal having a polarity
indicating whether there is a surplus or a deficiency of silver ions and a
magnitude indicating the size of the deviation.
The concentration error signal is applied to a pair of proportional
controllers 71 and 73 which provide signals linearly related to the
magnitude of the error. Where the silver ion concentration is above the
setpoint, the control elements associated with proportional controller 71
are energized, and when the silver ion concentration is below the
setpoint, the control elements associated with proportional controller 73
are energized.
The output signals from the proportional controllers 71 and 73 are applied
to a set of components identical to those appearing within dotted outline
200 in FIG. 2.
Idealized representations of the signals provided by selector circuits 74,
76, 92, 94 appear in FIG. 4. Where the probes and meter indicate an
abnormal deficiency of silver ions, the output of high signal selector
circuit 92 jumps to the internal reference current value and then
increases in relation to the absolute magnitude of the deviation from set
point. The greater the deficiency, the greater the magnitude of the
control signal. The output of selector circuit 92 reaches a minimum level
at the breakpoint between the normal error range and the abnormal error
range and remains at that level for any lesser errors. The output of the
low signal selector circuit 94 is a fixed maximum for any deficiencies in
the abnormal range but is linearly related to errors in the normal range.
Similarly, the output of low signal selector circuit 76 is linearly related
to normal surpluses of silver ions but is a constant signal for abnormal
surpluses. Abnormal surpluses result in the generation of a
directly-related, linear signal from a high signal selector circuit 74.
The breakpoints between the normal and abnormal error ranges are determined
by the values of the reference signals provided by reference current
sources shown in FIG. 2. The signals illustrated in FIG. 4 are based on an
assumption that sources associated with proportional controller 70
generate the same reference signal and that sources associated with
proportional controller 72 also generate identical references signals.
While there has been described what are considered to be preferred
embodiments of the present invention, variations and modifications therein
will occur to those skilled in the art. Therefore, it is intended that the
appended claims shall be construed to include all such variations and
modifications as fall within the true spirit and scope of the invention.
* * * * *
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