Instruction Manual
D200156X012
4194HA, HB, HC Controllers
July 2018
33
Figure 4‐3. Schematic of Operation for Fisher 4194HC Controller
SET POINT
INDICATOR
PROCESS INDICATOR
REMOTE SET POINT
CONNECTED HERE
INPUT ELEMENT
CONNECTED HERE
CONNECTING
LINK
BEAM
FLAPPER
NOZZLE
PROPORTIONAL
BELLOWS
RESET BELLOWS
PROPORTIONAL
BAND ADJUSTMENT
SUPPLY
PRESSURE
RELAY
OUTPUT PRESSURE
TO FINAL CONTROL
ELEMENT
REVERSE ACTION
QUADRANT
FLAPPER
PIVOT
FEEDBACK
LINK
FEEDBACK
MOTION
DIRECT ACTION
QUADRANT
INPUT MOTION
FLAPPER DETAIL
SUPPLY PRESSURE
OUTPUT PRESSURE
NOZZLE PRESSURE
RESET PRESSURE
RELAY
TO NOZZLE
TO
RESET
BELLOWS
TO
PROPORTIONAL
BELLOWS
DIFFERENTIAL
RELIEF VALVE
SUPPLY
PRESSURE
OUTPUT
PROPORTIONAL‐PLUS‐RESET‐PLUS‐RATE CONTROL
PROPORTIONAL‐PLUS‐RESET‐PLUS‐RATE CONTROL
WITH ANTI‐RESET WINDUP
46A9764‐B
C0593‐3
PROPORTIONAL PRESSURE
RATE VALVE
RESET VALVE
RATE VALVE
RESET VALVE
The controller action is changed from DIRECT to REVERSE by turning the proportional band adjustment knob to
position the nozzle on the flapper quadrant to a point where the direction of the flapper motion versus input motion is
reversed (see flapper detail in figure 4‐2). With the controller in the reverse‐acting mode, an increase in process
differential pressure causes a decrease in output pressure.
A 4194HC controller also has a rate valve (figure 4‐4), which is an adjustable restriction that momentarily increases the
controller gain to accelerate the corrective action for slow pressure systems. A proportional‐plus‐reset‐plus‐rate
controller responds to a change in process differential pressure as follows:
D
First, the rate action delays the proportional action just long enough to allow the controller to respond to the
change quickly with high gain but not long enough for the high gain to cause instability.