Fisher 4195 KB Series Instruction Manual page 30

4195KA, KB, and KC 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
closer to the nozzle. The result of this interaction is
that, when the process pressure changes, proportional
action temporarily reduces the gain of the controller for
improved stability. The process pressure then slowly
returns to set point, as pressure in both bellows equal-
izes via the reset action.
Moving the set point indicator changes the distance
between the nozzle and flapper as does a change in
process pressure, except that when the set point is
changed, the nozzle moves with respect to the flapper.
The proportional band adjustment positions the nozzle
on the flapper. Increasing (widening) the proportional
band moves the nozzle to a position on the flapper
where less input and more feedback motion occurs,
which decreases the gain of the controller. Decreasing
(narrowing) the proportional band moves the nozzle
4–10
RESET BELLOWS
PROPORTIONAL
BELLOWS
FLAPPER
NOZZLE
Figure 4–5. 4195KB Series Controller Schematic
PROPORTIONAL
BAND ADJUSTMENT
RESET
VALVE
SUPPLY
PRESSURE
TO
PROPORTIONAL
BELLOWS
toward a position where more input and less feedback
motion occurs, which increases the gain. The control-
ler action is changed from direct to reverse by turning
the proportional band adjustment to position the
nozzle on the flapper quadrant to a point where the
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 pressure causes a decrease in
output pressure.
A 4195KC 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
pressure systems. A proportional-plus-reset-plus-rate
controller responds to a change in process pressure
as follows:
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