3572 And 3576 Valve Positioners - Fisher 3570 Instruction Manual

3570 Positioners
September 2015

3572 and 3576 Valve Positioners

Refer to the schematic diagram in figure 7, which shows the 3572 positioner mounted on a 472 pneumatic piston
actuator. For the 3576 positioner, the principle of operation is identical to the 3572 positioner but the actuator can be
a direct or reverse acting pneumatic diaphragm actuator.
Figure 7. Schematic Diagram of Fisher 3572 Positioner with a 472 Pneumatic Piston Actuator
RANGE SPRING
BIAS SPRING
CYLINDER
CR4006‐A
A1084‐1/IL
The pneumatic output signal from a control device is piped to the positioner bellows. For explanation purposes,
assume this signal has increased. The bellows expands and moves the beam, which pivots around a fixed point and
covers the relay nozzle. The nozzle pressure in the relay increases due to the restriction created by the beam covering
the nozzle. Through relay action, the pressure above the piston overcomes the force exerted by the actuator spring,
and the piston moves downward. This changes the valve plug position.
In the 3572 positioner, piston movement is fed back to the beam by means of a range spring, which is connected to
the beam and the piston rod extension. As the piston rod extension moves downward, the range spring is extended
until the torque of the beam balances the torque exerted by the instrument bellows.
18
INPUT
SIGNAL
R
SUPPLY
PISTON
YOKE
INPUT SIGNAL PRESSURE
SUPPLY PRESSURE
TOP CYLINDER PRESSURE
NOZZLE PRESSURE
BELLOWS
REVERSED
POSITION
RELAY A
R = RESTRICTION
Instruction Manual
D200137X012