Instruction Manual
D103292X012
C1 Controllers and Transmitters
March 2017
28
This controller output pressure change feeds back to the proportional bellows, countering the pressure change in the
nozzle and equalizes the relay diaphragm pressure differential. The relay valve maintains a new loading pressure
according to the change in sensed pressure.
If the proportional band adjustment is at its maximum setting (10), the cantilever spring in the proportional band
assembly has a low spring rate, allowing more feedback motion to be transferred from the proportional bellows for a
change in output pressure. As the effective length of the cantilever is reduced, its spring rate increases, causing less
feedback motion from proportional bellows. Setting the cantilever spring to its maximum length results in a
proportional band of 100%. The lower the 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 controller output responds to the change in controlled variable.
Anti-reset windup minimizes this delay and permits returning the controlled variable to set point more quickly with
minimal overshoot.
As shown in
figure 14 a proportional-plus-reset controller with anti-reset windup includes a differential relief valve.
The valve consists of two pressure chambers separated by a spring-loaded diaphragm.
For the controller shown in figure 14, proportional pressure registers rapidly on the spring side of the relief valve
diaphragm as well as in the proportional bellows, and reset pressure registers on the opposite side of the relief valve
diaphragm. As long as controlled pressure changes are slow enough for normal proportional and reset action, the
relief valve spring will keep the relief valve diaphragm from opening. However, a large or rapid decrease in controller
pressure will cause the relay to exhaust loading pressure from the control device rapidly, and also from the
proportional system and spring side of the relief diaphragm. If this decrease on the spring side of the diaphragm is
greater than the relief valve spring setting, the diaphragm will move off the relief valve orifice and permit the
proportional pressure on the opposite side of the relief valve diaphragm to bleed rapidly into the reset bellows. The
anti-reset windup action also can be reversed to relieve with an increasing proportional pressure.
Differential Gap Controllers
With a differential gap controller, feedback pressure does not counteract the change in flapper position as it does in a
proportional-only controller. Instead, feedback pressure is piped to the bellows located on the side of the beam and
flapper opposite the nozzle (the proportional bellows in figure 13). Then, as controller output pressure increases,
feedback pressure moves the flapper closer to the nozzle to again increase controller output pressure. This process
continues rapidly until the controller output pressure is at the upper range limit. The action of a differential gap
controller is so rapid that output pressure changes from zero to maximum as soon as the switching point is reached.
