Note: Descriptions are shown in the official language in which they were submitted.
~0850'~7
~ACKGROUND OF INVENTION
This invention relates generally to process control systems, and
more particularly to an override process control system in which a selector
station operates in conjunction with two or more electronic controllers, only
one of which governs a final control element, the other controllers being on
a standby basis.
An electronic controller is a component in a process control loop
that is subject to disturbances, the controller acting in conjunction with
other devices to maintain a process variable at a desired value. The factor
controlled may be flow rate, pressure, viscosity, liquid level, or any other
process variable. In operation, the electronic controller receives, in terms
of corresponding input signals, both the process variable and a set point,
and it compares these electrical values to produce an output signal that re-
flects the deviation of the process variable from the set point. This output
signal, when applied to a final control element, will directly or indirectly
govern the process variable.
Thus one input signal to a controller may be derived from a flow-
meter whose reading is converted into a corresponding electrical value, and
the output signal may be impressed on a flow-regulating valve which is caused
to assume an intermediate position between open and closed at which the flow
; rate conforms to the set point. The set point generator may be an internal
component of the controller or a remotely-controlled device. A typical elec-
tronic controller is that manufactured by Fischer ~ Porter Co. of Warminister,
Pa., and described in their Instruction Bulletin ~1974) for the Series
53 EL 4000 Electronic Controller. The disclosure of this bulletin is in-
corporated herein by reference.
Variations in controller action are obtained by adjustment of para-
meters associated with the control modes and are available in several combina-
~osso~7
tions. These modes of control action which are combined to adjust the con-
troller output signal are known as proportional, reset and derivative.
Proportional action produces an output signal proportional to the de-
viation of the controlled process variable from the set point. The amount of
dcviat:ion in terms of percentage required to move the final control element
through the full range is known as the proportional band. Automatic reset
action, also known as integral action, produces a corrective signal proportion-
al to the length of time the controlled variable has been away from the set
point, while derivative action, also known as rate action, produces a correc-
tive signal proportional to the rate at which the controlled variable is
changing. Manual reset action is an operator-actuated potentiometer control-
led to produce a corrective signal directly proportional to the magnitude of
the adjustment.
"Slideback" action permits the transfer from manual to automatic con-
trol without a sudden change in output signal. This means that the operator
can switch the controller from manual to automatic without an instantaneous
jump even when there is a difference between set point and process.
There are some practical situations in which it becomes essential
to provide an override control system to regulate a process with only a single
final control element from two or more process variants that are interdepend-
ent and which must not exceed certain maximum and/or minimum safe limits.
In a situation of this type, control of the process requires that
the system always control from the variable that has the greater tendency at
any instant to depart, from the control point in the undesired direction. In
this system, two or more process variables are related in such a way that
either can be controlled by the same manipulated variable. An override con-
trol system for this purpose requires an override selector station associated
with several electronic controllers respectively responsive to the interde-
pendent process variables.
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10850Z7
One known type of selector station for an override process control
systeM is that manufactured by Fischer ~ Porter Co. and described in their
Instruction Bulletin (1972) for Series 53 EL 3090-B Override Selector Station.
Another known type is the SPEC 200 Automatic Selector Control Systems manu-
factured by Foxboro Corportion and described in their Technical Information
TI 200-225, published September 1973.
In an override control system of the type heretofore known, the final
control element is manipulated by the selector station to prevent two or more
process variables from exceeding specified limits. Each process variable is
fed to a respective electronic controller included in the override system, the
set point of the controller representing a limiting safe operating point (maxi-
mum or minimum) for that variable. All controller outputs are fed to the
selector station which transmits the highest or lowest controller output as
determined by a high-low switch on the selector, to the final control element.
When the proc~ss variable of one or more of the controllers ap-
proaches an unsafe condition, the controller output will start to change to
an extent determined by how unsafe the variable has become, as determined by
the deviation from the set point and by the controller mode settings. At any
given time, the process variable which is most unsafe is that producing the
greatest output change tendency. And since this variable will produce in its
associated controller the highest (or lowest) output, it will be chosen by
the selector station to control the final operator.
To illustrate the operation of a typical system employing an over-
ride selector station of the known type, we shall assume an arrangement in
which fluid is drawn into a compressor and discharged thereby into a process
through a control valve (final control element). In order to avoid cavitation
in a turbine pump, one must prevent loss of suction pressure in the event the
discharge pressure is also low. To this en~, a sensor on the suction side of
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1085027
the compressor transmits a process variable input signal to a "suction" elec-
tronic controller, while a sensor on the discharge side of the compressor
transmits a process variable input signal to a "discharge pressure" electronic
controller.
The output of the "suction" controller as well as that of the "dis-
charge pressure" controller is applied to the override selector station whose
output signal acts to govern the single final control element valve in the
compressor discharge line. Normally, the output of the "discharge pressure"
controller is selected by the override station to adjust the output pressure
of the compressor. But if the suction pressure drops below the set point of
the "suction" controller, the override station will then select this control-
ler to take over control of the valve, in which case the "discharge pressure"
controller is inactive.
In all override control systems of the type heretofore known, the
function of the override station is to select the output signal from one of
several process controllers, and to generate an equivalent output signal which
is applied to the single final control element. A voltage signal proportional
to the selected output signal is transmitted to each of the several electronic
controllers which make up the system. In the then selected controller, this
voltage signal is used as "feedback," whereas in the non-selected controllers,
the voltage signal serves as a reset update to prevent these controllers from
going into "Reset Wind Up."
Thus existing types of override selectors require an output line
from each controller to the selector station and-a second return connection
for the feedback and reset update to each controller internal-feedback ter-
minal. If, therefore, it were decided to convert a process control system
having a group of electronic controllers to operate in conjunction with an
override selector station of the type heretofore known, major wiring changes
would be required, with a resulting prolonged down time.
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Also whil0 existing forms of electronic controllers are capable of
switching over from automatic to manual operation, when such controllers are
associated with an override selector station of the type previously known,
one cannot effect such transfer by switch-over means already included in the
controller. The reason this cannot be done is that in the conventional over-
ride system, the final control element is operated not by one of the control-
lers but by an output signal generated in the override selector station.
It is necessary, therefore, either to incorporate a manual-
automatic transfer means in the override station or to provide a separate
manual/automatic transfer station in the output line, further adding to the
cost of the override installation.
SUMMARY OF INVENTION
In view of the foregoing, it is the main object of the invention
to provide an improved and simplified override system for regulating a pro-
cess with a single final control element from at least two process variables
that are interdependent and which must not exceed acceptable minimum and
maximum limits.
More particularly, it is an object of the invention to provide an
override system of the above type which includes a like number of electronic
controllers each responsive to a respective process variable, the output sig-
nal of the controller which is responsive to the primary process variable
directly influenced by this final control element being applied to this ele-
ment to govern its operation, the other controllers operating on a standby
basis.
A significant feature of the invention is that the several elec-
tronic controllers are monitored by a selector station which, when an unsafe
signal is received from one of the standby controllers, forces the output of
the primary controller to respond to the unsafe signal until such time as the
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~0850Z7
signal is again safe.
Monitoring is effected by an individual line between each controller
and 1:he selector station, over which line a signal is received by the station
regarding conditions prevailing in each controller and over which same line a
signal is returned to effect the desired reset control.
Among the advantages of the invention are the following:
An existing process control system made up of several independent electronic
controllers may be readily converted to an override system by providing a
selector station in accordance with the invention, in which the existing re-
lationship between the primary controller and the final control element is
retained and the other controllers are put on a standby basis, one wire being
connected from a point within each of these controllers to the selector sta-
tion. This same wire is used to transmit signals in both directions between
the selector station and each controller.
No special training is required for operating the primary controller
ln an override system in accordance with the invention, for its operation is
essentially similar to that of any standard electronic controller. Thus
automatic manual transfer is effected at the primary controller. Transfer
between automatic and manual at the primary controller is balanceless bump-
less with slideback, as with a sophisticated standard electronic controller.
OUTLINE OF THE DRAWINGS
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 accompanying drawings, where-
in:
Figure l is a block diagram of an override process control system
according to the invention;
Figure 2 is a schematic circuit diagram of the system.
10~50Z7
DESCRIpTION OF THE INVENTION
The Basic System:
Re~erring no~ to Figure 1, there is shown an override process con-
trol system in accordance with the invention for controlling a process with
a single final control element from three process variables that are inter-
dependent and which must not exceed certain maximum and minimum safe or ac-
ceptable limits. While a system involving three interdependent process vari-
ables is disclosed, in practice the number may be two or more than three.
Since three process variables are involved, the system is provided
with three identical electronic controllers, A, B and C, preferably any of
the several Fischer ~ Porter type - Series 53 4000. Each controller compares
a process variable value with a set point value in a deviation amplifier and
derives therefrom a deviation signal that depends on the difference between
these values. The deviation signal is applied to one input of a differential
amplifier and compared therein with a reset voltage produced in a feedback
path including an auto generator extending between the output of this ampli-
fier and the other input of this amplifier to produce a reset drive voltage.
The output of the auto generator is connected to an output generator
which converts the reset drive signal to a current output signal for operat-
2Q ing a final control element. Each controller is provided with a terminal T
which is connected to the input of the auto generator.
The process which is being controlled involves the controlled flow
of a liquid into a process tank 10, the rate of flow being regulated by a
-~ valve 11 which acts as the final control element in a process control loop
that includes electronic controller A. In order to determine the flow rate,
a magn~tic flowmeter 12 is interposed in the flow line in advance of valve 11
to provide a process variable signal which is applied to controller A, where
it is compared with a set point signal representing the desired process value.
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The controller may be operated manually by pressing button M or automatically
by pressing button A. The set point may be manually adjusted by turning the
wheel 13 of a potentiometer.
As fluid is fed into process tank 10, which we shall assume contains
a liquid bath in which a chemical action takes place, the level of fluid in
the tank varies as well as the viscosity of the bath. It is important not
only that the flow rate of fluid fed into the tank be maintained within cer-
tain acceptable limits, but also that the liquid level in the tank not go too
high or too low, and that the viscosity of the bath also remain within safe
limits.
Hence to sense the liquid level in process tank 10, a level trans-
mitter 14 is provided which supplies a process variable to controller B. To
sense the viscosity of the bath, a viscosity transmitter 15 is provided to
supply a process variable to controller C. The three process variables are
interdependent, for the liquid level and the viscosity depends on the flow
rate.
However, the flow rate process variable is the primary variable,
for its is directly influenced by the setting of the single final control
element 11 of the system, whereas the other variables are secondary, for they
are indirectly affected by this setting.
In the arrangement in accordance with the invention, only controller
A is operative to control final control element 11, and the output current
signals of controllers B and C are not used to carry out a control function.
Controller A is therefore referred to as the working or primary controller,
and controllers B and C as the standby controllers. But all three controllers
are operative to determine whether the process variables to which they are
responsive lie within safe limits, and to generate at its terminal T a reset
drive voltage which depends on the difference between the deviation signal and
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10850Z7
the proportional plus integral feedback voltage.
If, therefore, in normal operation, all three process variables are
within safe limits, working controller A, which responds only to the primary
process variable, will exclusively govern the final control element. But
while the control of the final control element by controller A is always such
as to maintain flow rate within acceptable limits, a situation may arise in
which, despite this fact, an aberration is encountered in which the liquid
level or the viscosity process variable falls below an acceptable minimum or
rises above an acceptable maximum.
It is essential, therefore, when this occurs for the standby con-
troller which responds to the aberration to take over the control of the final
control element. To accomplish this purpose, an override control selector CS
is provided which is coupled by individual lines La, Lb and Lc, to the ter-
minals T of controllers A, B and C, so that only a single connection exists
between the selector and each controller.
Control selector station CS senses the voltage and current on each
of lines La to Lc (or a greater number of lines), and when the reset drive
; voltage from a standby controller increases or decreases to an unsafe level
as compared to the primary controller signal on line La, then the current to
~o the standby controller will move toward zero and change direction. This re-
verse current is sensed by the selector and the standby controller is accepted
as a new input, all other lines including line La to primary controller A
then becoming outputs which follow this new input.
Selector station CS functions as a voltage and current monitor with
respect to each line leading to a controller, with a safe and unsafe direction
of current flow. When the current in a given line is forced in the unsafe
direction by an external factor, circuits in the control selector couple the
voltage of the unsafe signal to the primary controller and to all other lines.
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~08S0Z7
Therefore, the primary controller is forced to respond to the unsafe
signal, this signal being used as a source for the next comparison to produce
an output signal for governing the final control element. The input to the
control selector is the reset drive voltage from the selected controller into
a high impedance circuit. The reset update-voltage is returned to the non-
selected controllers in a very low impedance voltage signal, selection being
automatic, using the currents which are required by the selector station
~plus and minus) to maintain this voltage level.
Simplified Schematic
Referring now to Figure 2, there is shown in simplified schematic
form, an override control system according to the invention which includes
only the primary electornic controller A and one secondary electronic control-
ler B, the two controllers operating in conjunction with control selector CS.
In this instance, the final control element 16 is a valve interposed in a
gravity feed pipe 17 leading from a set of four tanks 1 to 4 whose fluid out-
puts are controlled by manually-operated valves. The blended fluids from
these tanks are fed through valve 16 into a final storage tank 5.
The level of liquid in storage tank 5 is determined by a differen-
tial-pressure level transmitter 18 whose electrical output signal represents
this process variable. The flow rate through gravity pipe 17 is sensed by a
flow transmitter 19 whose electrical output signal represents flow rate and
therefore reflects this process variable.
Since flow rate is directly influenced by the setting of the final
control element 16, this process variable is referred to as the primary pro-
cess variable PVp, whereas the liquid level in final tank 5 is indirectly in-
fluenced by the setting of the final control element and is therefore referred
to as the secondary process variable PVs. Inasmuch as the flow rate into the
final storage tank determines the level of liquid therein, the primary and
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secondary process variables PVp and PVs are interdependent. While only a
single secondary process variable is included in this override system, it is
to be understood that, in practice, two or more secondary process variables
may be involved.
Electronic controllers A and B are preferably of the F ~ P Series
53 40()0 type. Controller A is responsive to the primary process variable PVp
to produce an output signal at output terminal Tl which is applied to actuator
20 for valve 16, thereby creating a conventional process control loop. Pri-
mary process variable PVp is applied to input terminal T2 which is connected
to the input of a deviation amplifier 21 in controller A which serves to com-
pare this variable with a set point SP determined by the adjustment of a local
set point potentiometer 22 connected to grounded input terminal T3, the slider
of the potentiometer being connected to the second input of deviation ampli-
fier 21. This deviation amplifier is a high impedance, highly stable differen-
tial amplifier whose output reflects the difference between the process
variable and the set point.
The output of deviation amplifier 21 is fed to the positive input
of a main amplifier 23 which is essentially a differential dc amplifier.
The output signal from main amplifier 23 is fed through a resistor
Rl ~22K) to an auto generator 24 which, in practice, may consist of a single
stage amplifier. Auto generator 24 provides a signal which goes to an output
generator 25 and serves as a feedback signal to main amplifier 23 through a
reset-proportional band circuit.
The reset-proportional band circuit is defined by a proportional
band adjustment potentiometer 26 connected in series with a reset adjustment
potentiometer 27 between the output of auto generator 24 and the negative input
of main amplifier 23, the slider of potentiometer 26 being connected through
reset capacitor 28 to the negative input. The auto reset feedback circuit is
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108SQZ7
an RC lead network in the feedback path in which the time constant is altered
by changing the value of a variable resistance. The reset circuit serves to
change the value of the controller output signal at a rate proportional to
the deviation divided by the proportional band until the error is reduced to
zero.
In practice, the reset drive voltage at the input of auto genera-
tor 24 which appears at terminal T4 lies in a range of 11.7 to 17.7 volts dc,
the output of this generator being fed to output generator 25 which converts
the voltage from auto generator 24 to a current output at terminal Tl in the
range of 4 to 20 mAdc. This current output is applied to valve actuator 20.
The ~-econda~yprocess variable PVs from level transmitter 18 is
applied to input terminal T2 of electronic level controller B which is identi-
cal to the primary flow controller A, this process variable being compared
with a set point SP as determined by the setting of local set,point potentio-
meter 22 in this controller. HoweverJ the current output signal at terminal
,' Tl of the level controller is not employed to operate a control element but
is fed to grounded output terminal Tx.
A single line La connectes terminal T4 (reset drive voltage) of
primary controller A (flow controller) and a terminal T5 ofthe override con-
trol selector CS, and a single line Lb connects terminal T4 of standby con-
troller B (level controller) and a terminal T6 of the control selector. For
a second standby controller (not shown)J a single line Lc from this controller
goes to terminal T7 of control selector CS. Terminal T8 of the control selec-
tor is the common terminal.
Terminal T5 of control selector CS is connected to the negative
input of a diferential amplifier 29 acting as a primary comparator whose out-
put is connected to the same input via a feedback resistor R2 (4.7K). The
output of comparator 29 is connected through a resistor R3 (lOK) to the
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108SUZ7
negative input of a differential amplifier 30 whose positive input is connect-
ed to terminal T5 to provide a bias voltage thereto. The output of feedback
amplifier 30 is applied to the positive input of comparator 29 to provide a
reference voltage therefor. This reerence RV voltage, which appears on a
bus, is determined by the input to comparator 29 ~the reset drive voltage) and
the output thereof.
Terminal T6 of the control selector which communicates with stand-
by controller B is connected to the negative input of a differential amplifier
31 acting as a secondary comparator whose output is connected to the same in-
put via a feedback resistor R4 ~4.7K) which is the same value as resistor R2.
The output of feedback amplifier 30 is applied to the positive input of second-
ary comparator 31 to provide a reference voltage therefor. The output of com-
parator 31 is connected to one terminal of both diodes Dl and D2. The other
terminals of diodes Dl and D2 are connected through a selector switch 32 to
the output of comparator 29 through resistor R3 and the negative input of feed-
back amplifier 30 to privide for "high" or "low" select operation, depending
on the switch position. During non-override operation, diode Dl or D2 selected
by switch 32 is non-conducting due to reverse bias which appears at the
output of secondary comparator 31.
That is to say, diode Dl forward conducts when there is a lower
voltage at the output of comparator 31, or diode D2 forward conducts when
there is a higher voltage. Switch 32 selects which condition can exist. The
voltage across the selected diode depends on the difference between the output
of comparator 31 and the output of comparator 29 through resistor R3. The
output voltage of comparator 31 is the result of current flow through resistor
R4 as required to maintain a voltage at the negative input of comparator 31
equal to reference voltage RV.
Terminal T7 is connected to one input of another secondary com-
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io850'Z7
parator 33 whose associated circuit is identical to that of comparator 31,
this stage of the control selector being reserved for another standby control-
ler (not shown). ~ence for purposes of understanding the control selector,
we shall consider only the operation of comparator 31.
In operation, control selector CS communicates between two or more
electronic controllers in an override system through an automatic impedance
control circuit in which resistor R3 (lOK) and diodes Dl or D2, acting as non-
linear elements in the input of feedback amplifier 30, determine which com-
parator will set the reference voltage level on the bus line RV.
The reference voltage is initially set by the reset drive voltage
taken from the primary controller A and applied to primary comparator 29.
Each controller is connected as an output to a respective voltage follower.
The selected controller has the same reset drive voltage as the reference
voltage; hence minimal current is communicated to the controller, so that
the control selector appears as a high impedance thereto. The non-selected
controllers are forced to follow via the output from the low impedance voltage
follower. The control selector can select the highest, the lowest or operate
between two or more signals set by the combination of selected high and low.
Control Selector/Flow Controller
Operation in Non-Override Mode
Referring to Figure 2 as representative of any typical control
system, control selector CS senses each controller reset drive voltage. This
voltage is approximately 11.7 to 17.7 Vdc and corresponds to the controller
output current of 4 to 20 mAdc. In practice, the input voltage to the con-
trol selector is limited only by the supply voltage, the reset voltage stated
here being a function of the associated controller.
The high-low selector switches 32 are shown as connected to the
"low" diodes D2. We shall first assume that the system is operating in the
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non-override mode. In this mode, the reset of the secondary or level con-
troller B is forced to track the primary or flow controller A whose reset
drive voltage is at 50% and therefore 14.7 Vdc. This 14.7 Vdc reset drive
voltage is applied to terminal T5 connected to the negative input of compara-
tor 29 where it is compared with reference voltage RV.
If the reference voltage is lower than the reset drive voltage
at terminal T5, the output of comparator 29 will decrease. This decrease will
be sensed by feedback amplifier 30 whose output will cause the reference volt-
age to increase until it equals the reset drive voltage at terminal T5.
Hence the reference voltage on bus RV becomes 14.7 Vdc. This is the same as
the reset drive voltage on primary controller A; consequently there is no load-
ing on comparator 29 and the input impedance of comparator 29 as seen by pri-
mary controller A is high during the non-override mode of operation.
Control Selector/Level Controller
Operation in Non-Override Mode
During the non-override mode, the standby or level controller B
has its reset updated so that it is equal to the reference voltage RV of the
control selector. The deviation of level controller B, not having its own
feedback, will drive its main amplifier 23 "off scale." We shall assume by
way of example that the voltage at the output of main amplifier 23 which is on
one side of resistor Rl ~22K) is + 23 Vdc and that the voltage (reset drive
voltage) at the other side of this resistor is the reset update voltage from
comparator 31 of the control selector which is 14.7 Vdc. Hence a current of
+ 0.377 mAdc is caused to flow through the 22K ohm resistor Rl.
To maintain the reset drive voltage of level controller B at 50%,
the + 0.377 mAdc current from main amplifier 23 is nulled out by a - 0.377
current from the output of comparator 31 in control selector CS through its
4.7 K ohm resistor R4. The output voltage of secondary comparator 31 measures
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108SOZ7
- 1.772 Vdc with respect to the output voltage of primary comparator 29, which
is equal to the reference voltage. Hence selector diode D2 connected to the
output of comparator 31 is reverse biased and is non-conductive.
Automatic Transfer to Override
Control at Level Controller
We shall now assume that the deviation signal at the input to
main amplifier 23 in level controller B is reduced and may even begin to re-
verse itself due to an increase in the liquid level in final tank 5 initially
below the "limit" set point of level controller B. When this happens, main
amplifier 23 will snap out of saturation and take control of its own feedback,
and the current through the 22 K ohm resistor Rl then changes from a positive
current to a small steady negative current at the same 50% output level. Now
the output of level controller B will begin to slowly decrease at its uwn re-
set rate. Because the input impedance of control selector CS has changed,
it looks high to level controller B.
Transfer to Override Mode
At Control Selector
-
Before transfer is completed, reference voltage RV will continue
to follow the output of primary comparator 29 which is set by flow controller
A. The output from secondary comparator 31 will quickly change from - 0.377
mAdc to a small, steady positive current and follow the new reset voltage set
by level controller B. The voltage at the output of secondary comparator 31
has now changed to 1 0.6 vDc with respect to reference voltage RV and the
output of comparator 29, causing diode D2 in the output of this comparator to
be forward biased and turned "on".
When diode D2 is "on," then applied to the negative input of feed-
back amplifier 30 is the output of secondary comparator 31 through diode D2
as uell as the output of primary comparator 29 through the 10 K ohm resistor
R3. Because the impedance of diode D2 is much less than 10 K ohm, effective
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~0850Z7
control of eedback amplifier 30 is relinquished by primary comparator 29 to
secondary comparator 31.
The small steady current which results from the + 0.6 Vdc (for-
ward diode drop) at the output of secondary comparator 31 will flow back to
level controller B, but will not affect its operation. However, reference
voltage RV in control selector CS will now follow the reset drive voltage
derived from level controller B through secondary comparator 31 and feedback
amplifier 30.
Reference voltage RV is no longer under the control of primary
comparator 29; for the reference voltage, which is now decreasing, follows
level controller B. The output current of primary comparator 29 will de-
crease rapidly to a value required to hold the reset update in flow controller
A at the reference voltage level. Current from the output of primary com-
parator 29 is split two ways, a small portion thereof serving to forward bias
diode D2 in the output of secondary comparator 31 to render it conductive,
and the remainder acting to update the reset in flow controller A through
resistor R2 ~
Automatic Transfer to Override
At Flow Controller
Before transfer to the override mode takes place, control selector
2Q CS follows the reset voltage in flow controller A, and the control selector
appears as a high impedance to the output of main amplifier 23 in the flow
controller, even through the 22 K ohm resistance of resistor Rl.
At the time of transfer to override, current begins to communicate
between flow controller A and control selector CS. Main amplifier 23 of flow
controller A tries to maintain control through resistor Rl until this ampli-
fier runs out of voltage, at which point it is then quickly overpowered by
current from the control selector. This happens without a bump at the input
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to auto generator 24 of flow controller A. The control selector now appears
as a low impedance voltage source.
The reset update voltage from control selector CS performs a dual
function, for it not only updates the reset in flow controller A but it forces
the flow controller output to follow the selected level controller so that now
the final control element proceeds to correct for the aberration which led to
override operation. Output current from the flow controller continues to feed
final control element 16, which is now set by level controller B through con-
trol selector CS, until the desired correction is effected, at which point
bumpless transfer back to the non-override mode takes place automatically.
While there has been shown and described a preferred embodiment of
an override process control system in accordance with the invention, it will
be appreciated that many changes and modifications may be made therein with-
out, however, departing from the essential spirit thereof.
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