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4194A, B, and C Series
SET POINT
INDICATOR
PROCESS POINTER
REMOTE SET POINT
CONNECTED HERE
INPUT ELEMENT
CONNECTED HERE
CONNECTING
LINK
BEAM
SUPPLY PRESSURE
OUTPUT PRESSURE
NOZZLE PRESSURE
RESET PRESSURE
46A9764–C
B1908–4 / IL
direction of the flapper motion versus input motion is
reversed as shown in the flapper detail of figure 4-5 or
4-6. With the controller in the reverse-acting mode, an
increase in process differential pressure causes a de-
crease in output pressure.
A 4194C Series controller also has a rate valve as
shown in figures 4-6 and 4-7. This valve is an adjust-
able restriction that momentarily increases the control-
ler gain to accelerate the corrective action for slow
responding systems. A proportional-plus-reset-plus-
rate controller responds to a change in process differ-
ential pressure as follows:
First, the rate action delays the proportional ac-
tion just long enough to allow the controller to respond
4–10
RESET BELLOWS
PROPORTIONAL
BELLOWS
FLAPPER
NOZZLE
Figure 4-5. 4194B Series Controller Schematic
PROPORTIONAL
BAND ADJUSTMENT
RESET
VALVE
SUPPLY
PRESSURE
TO
PROPORTIONAL
BELLOWS
to the change quickly with high gain, but not long
enough for the high gain to cause instability.
Then, the low gain provided by the proportional
action keeps the system stable. Finally, reset action
slowly increases the gain and returns the process dif-
ferential pressure toward the set point.
Anti-Reset Windup (suffix letter F)
Operation
Anti-reset windup is available on all 4194B and C Se-
ries controllers and is designated by the letter F in the
type number. The differential relief valve operates
when the difference between proportional bellows
pressure and reset bellows pressure reaches a prede-
REVERSE ACTION
QUANDRANT
FEEDBACK
LINK
FEEDBACK
MOTION
DIRECT ACTION
QUADRANT
INPUT MOTION
OUTPUT PRESSURE
TO FINAL CONTROL
ELEMENT
RELAY
TO
RESET
TO-
BELLOWS
NOZZLE
SUPPLY
RESET
PRESSURE
VALVE
DIFFERENTIAL
RELIEF
VALVE
FLAPPER
PIVOT
OUTPUT
RELAY
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