C1 Controllers and Transmitters
Instruction Manual
September 2009
22
ARROW DOWN
−
RELIEVES
ON DECREASING OUTPUT
ARROW UP
−
RELIEVES
ON INCREASING OUTPUT
(OUTPUT AT SUPPLY
DURING SHUTDOWN)
(OUTPUT AT ZERO
DURING SHUTDOWN)
CONSTANT SUPPLY
PRESSURE
TO FINAL
CONTROL
ELEMENT
EXHAUST
RESET BELLOWS
RESTRICTION
RESET VALVE
GE23697
−
A
GE34724
−
A
E1063
SENSED
PRESSURE
PROPORTIONAL
BELLOWS
SENSED PRESSURE OUTPUT PRESSURE NOZZLE PRESSURE RESET PRESSURE
DIFFERENTIAL
RELIEF VALVE
CANTILEVER
SPRING
PRESSURE
SETTING DIAL
PROPORTIONAL
BAND ADJUSTMENT
KNOB
PRESSURE
SETTING KNOB
Figure 14. Schematic of Reverse
−
Acting Proportional
−
Plus
−
Reset Controller with Anti
−
Reset Windup
proportional band adjustment, the shorter the
effective length of the cantilever spring. The spring
rate of the cantilever spring increases as its length
shortens, allowing less motion to be transferred from
the bellows to the beam and flapper for a given
change in output pressure.
Proportional
−
Plus
−
Reset Controllers
Action of a proportional
−
plus
−
reset controller is
similar to that of a proportional
−
only controller
except that feedback from the controller output
pressure is piped to a reset bellows as well as to the
proportional bellows as shown at the right in
figure 13.
With an increasing controller output pressure,
pressure in the reset bellows increases. Increases in
reset bellows pressure moves the beam and flapper
closer to the nozzle, starting another increase of
pressure throughout the system. Pressure buildup
continues until the controlled pressure is brought
back to the set point. The reset valve is adjustable to
vary the amount of delay in the reset action. Closing
the reset valve increases the delay in reset action.
Controllers with Anti
−
Reset Windup
During a prolonged difference between set point and
the controlled variable, such as encountered with
intermittent control applications (e.g., batch
temperature control or wide open monitors on
pressure control), reset ramps the controller output
to either zero or full supply pressure; this condition is
reset windup. When the controlled variable crosses
the set point, there will be a delay before the
