Damper Adjustment; Principle Of Operation; Maintenance - Fisher 4070 Instruction Manual

DAMPER
ADJUSTMENTS
A7230 / IL
Figure 11. Damper Adjustment
10. Replace the positioner front cover.
11. If you used the zero adjustment opening, remem-
ber to install the zero adjustment cover to ensure the
unit is sealed.

Damper Adjustment

The built in dampers provide a simple means of ad-
justing the actuator travel speed. The damper adjust-
ments are located on the connecting block as shown
in figure 11. For maximum actuator travel speed adjust
dampers to the minimum damping position.
Double acting actuators—adjust only the OUTLET
damper. Set the SUPPLY damper to the minimum
damping position.
Single acting actuators—adjust both dampers for
desired operation.

Principle of Operation

Refer to figure 12.
The pneumatic positioner operates on a force balance
principal. Force is originated by the signal pressure
transmitted through a diaphragm on to the balance
arm. The opposing force is achieved through the feed-
back spring and is proportional to the position of the
lower arm. The lower arm position is determined by
the position of the cam which is secured to the spindle
and connected to the actuator shaft thus providing the
feedback from the actuator/valve. When these two
forces are equal, the balance arm and the spool in the
Type 4060 & 4070
7
8
3
A7231 / IL
Figure 12. Principle of Operation
pilot valve is in a neutral position—the complete unit is
in a balanced position. Air is supplied to the pilot valve
through port S, and controls the air flow through ports
C1 and C2
Assume an equilibrium position. An increasing control
pressure deflects the diaphragm 1 down, compressing
the feedback spring 3. The balance arm 2 moves the
spool 7 in the pilot valve 8 furnishing supply air to the
actuator, while at the same time air is exhausted from
actuator and is vented to atmosphere through the pilot
valve and the OUT port.
With increasing supply air, the linear motion of the ac-
tuator stem rotates the positioner spindle 6. The
spindle and cam 5 rotation forces the lower arm 4 up-
wards compressing the feedback spring 3. This motion
continues until the two forces are equal and the unit is
in an equilibrium position.

Maintenance

WARNING
Avoid personal injury from sudden re-
lease of process pressure. Before
mounting the positioner on a valve in
service:
Disconnect any operating lines pro-
viding air pressure or a control signal to
the actuator. Be sure the actuator can-
not suddenly open or close the valve.
1
2
5
6
4
9