Principle Of Operation; 3570, 3570C, 3570P, 3570Pc, And 3571 Valve Positioners - Fisher 3570 Instruction Manual

Instruction Manual
D200137X012

Principle of Operation

3570, 3570C, 3570P, 3570PC, and 3571 Valve Positioners

Refer to the schematic diagram in figure 6. 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 simultaneously uncovers the nozzle of relay B and covers the nozzle of
relay A. The nozzle pressure in relay A increases due to the restriction created by the beam covering the nozzle.
Through relay action, the pressure to the top of the piston increases. At the same time, relay B reacts to the change in
beam position to decrease the pressure to the underside of the piston. These unbalanced pressures move the actuator
piston down.
In the 3570 and 3570C positioners, the piston movement is fed back to the beam by means of a range spring, which is
connected to the beam and to the piston rod extension. In the 3570P, 3570PC, and 3571 positioners, the feedback is
provided to the range spring by a cable or wire that is connected to the actuator‐valve stem connector. The downward
movement of the piston rod extension extends the range spring until the torque on the beam balances the torque
exerted by the instrument bellows.
Figure 6. Schematic Diagram of Fisher 3570 Positioner with a 470 Pneumatic Piston Actuator
RANGE SPRING
BIAS SPRING
CYLINDER
A1067‐1
As the input signal decreases, the reverse action takes place. The bellows contracts, and as the beam pivots, it covers
the nozzle of relay B and uncovers the nozzle of relay A. Through relay action, the pressure below the piston increases
and the pressure above the piston decreases to move the piston upward.
RELAY "B"
R
INPUT
SIGNAL
SUPPLY
SUPPLY
YOKE
INPUT SIGNAL PRESSURE
SUPPLY PRESSURE
TOP CYLINDER PRESSURE
BOTTOM CYLINDER PRESSURE
NOZZLE PRESSURE
BELLOWS REVERSED POSITION
RELAY "A"
R
R = RESTRICTION
3570 Positioners
September 2015
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