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diaphragm is forced down, which opens the feed valve and
allows main air into the branch chamber. When the pilot pressure
decreases, the pilot chamber diaphragm rises, closing the feed
valve. If the pilot chamber diaphragm rises enough, it lifts the
bleed valve off the feed valve disc, allowing air to escape from
the branch chamber through the vent, thus decreasing the
branchline pressure. Main air is used only when branchline
pressure must be increased and to supply the very small amount
exhausted through the nozzle.
SIGNAL AMPLIFIER
In addition to the capacity amplifier, pneumatic systems also
use a signal amplifier. Generally, modern amplifiers use
diaphragms for control logic instead of levers, bellows, and
linkages.
A signal amplifier increases the level of the input signal and
provides increased flow. This amplifier is used primarily in
sensor-controller systems where a small signal change from a
sensor must be amplified to provide a proportional branchline
pressure. The signal amplifier must be very sensitive and
accurate, because the input signal from the sensor may change
as little as 0.74 kPa per kelvin.
Another use for a signal amplifier is to multiply a signal by
two to four times so a signal from one controller can operate
several actuators in sequence.
FEED AND BLEED SYSTEM
The "feed and bleed" (sometimes called "non bleed") system
of controlling branchline pressure is more complicated than
the nozzle-flapper assembly but theoretically uses less air. The
nozzle-flapper system exhausts some air through the nozzle
continually, whereas the feed and bleed system exhausts air
only when the branchline pressure is being reduced. Since
modern nozzle-flapper devices consume little air, feed and
bleed systems are no longer popular.
The feed and bleed system consists of a feed valve that
supplies main air to the branch line and a bleed valve that
exhausts air from the branch line (Fig. 5). Each valve consists
of a ball nested on top of a tube. Some pneumatic controllers
use pressure balance diaphragm devices in lieu of springs and
valves. A spring in the tube continually tries to force the ball
up. The lever holds the ball down to form a tight seal at the end
of the tube. The feed and bleed valves cannot be open at the
same time.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
PNEUMATIC CONTROL FUNDAMENTALS
SETPOINT
SENSING
ADJUSTMENT
FORCE
EXH
M
Fig. 5. Feed and Bleed System.
A force applied by the sensing element at the sensor input
point is opposed by the setpoint adjustment spring and lever.
When the sensing element pushes down on the lever, the lever
pivots on the bleed ball and allows the feed ball to rise, which
allows main air into the chamber. If the sensing element reduces
its force, the other end of the lever rises and pivots on the feed
ball, and the bleed ball rises to exhaust air from the system.
The sensor can be any sensing element having enough force to
operate the system.
SENSING ELEMENTS
BIMETAL
A bimetal sensing element is often used in a temperature
controller to move the flapper. A bimetal consists of two strips
of different metals welded together as shown in Figure 6A. As
the bimetal is heated, the metal with the higher coefficient of
expansion expands more than the other metal, and the bimetal
warps toward the lower-coefficient metal (Fig. 6B). As the
temperature falls, the bimetal warps in the other direction
(Fig. 6C).
A. CALIBRATION TEMPERATURE
B. INCREASED TEMPERATURE
C. DECREASED TEMPERATURE
METALS:
Fig. 6. Bimetal Sensing Element.
63
BRANCHLINE
PRESSURE
PRESSURE
CHAMBER
BLEED VALVE
FEED VALVE
C2382-1
HIGH COEFFICIENT OF EXPANSION
LOW COEFFICIENT OF EXPANSION
C1087
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