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Hydraulic and Pneumatic Knowledge
Fluid Power Equipment
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A rubber diaphragm separates the actuator housing into two air
chambers. The upper chamber receives supply air through an opening in the top of
the housing.
The bottom chamber contains a spring that forces the diaphragm
against mechanical stops in the upper chamber. Finally, a local indicator is
connected to the stem to indicate the position of the valve.
The position of the valve is controlled by
varying supply air pressure in the upper chamber. This
results in a varying force on the top of the diaphragm.
Initially, with no supply air, the spring forces the
diaphragm upward against the mechanical stops and holds the
valve fully open. As supply air pressure is increased from
zero, its force on top of the diaphragm begins to overcome
the opposing force of the spring. This causes the diaphragm
to move downward and the control valve to close. With
increasing supply air pressure, the diaphragm will continue
to move downward and compress the spring until the control
valve is fully closed. Conversely, if supply air pressure is
decreased, the spring will begin to force the diaphragm
upward and open the control valve. Additionally, if supply
pressure is held constant at some value between zero and
maximum, the valve will position at an intermediate position.
Therefore, the valve can be positioned anywhere between fully
open and fully closed in response to changes in supply air
pressure.
A positioner is a device that regulates the
supply air pressure to a pneumatic actuator. It does this by
comparing the actuator’s demanded position with the control
valve’s actual position. The demanded position is
transmitted by a pneumatic or electrical control signal from
a controller to the positioner. The pneumatic actuator in
Figure F1 is shown in Figure F2 with a controller and
positioner added.
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