Functional Operation - ABB AV1 Operating Instructions Manual

DESCRIPTION AND OPERATION

FUNCTIONAL OPERATION

FUNCTIONAL OPERATION
2 - 2
Type AV positioners operate by balancing opposing forces. Figure
2-1 shows a diagram of the Type AV positioner. A balance beam,
hinged at one end and connected to the pilot valve at the other, is
acted upon by two forces:
• Upward force of the signal diaphragm assembly.
• Downward force from the range spring.
The input signal pressure determines the diaphragm force. The
Type AV1 positioner uses an external input pressure (either 20.7
to 103.4 kilopascals (3.0 to 15.0 pounds per square inch gage) or
20.7 to 186.2 kilopascals (3.0 to 27.0 pounds per square inch
gage)). The Type AV2 positioner uses a current to pneumatic con-
verter to generate the input signal pressure.
The range spring force is a function of the shape and position of
the cam. The cam is coupled to the cam shaft that is connected
through linkage (or coupling) to the actuator. Therefore, range
spring tension is a function of the actuator position.
A change in input signal changes the force exerted by the signal
diaphragm, moving the balance beam, in turn moving the pilot
valve. The pilot valve supplies and/or exhausts air to the actuator
that ultimately changes its position. The change in actuator posi-
tion is fed back to the positioning cam. The positioning cam
moves, changing the tension of the range spring until a balanced
condition once again exists.
The positioner is normally located in a control loop (Fig. 2-2)
between the controller and the actuator.
Actuator position is fed back to the positioner for comparison with
the position commanded by the input control signal (direct signal
pressure or a current value). For linear motion actuators, the feed-
back mechanism consists of:
A drive rod that follows the motion of the actuator.
•
• An adjustable-length, swivel-ended connecting link that trans-
mits the motion of the drive rod to an adjustable drive arm on
the positioner.
A camshaft and cam that are rotated through an angle by the
•
drive arm. A function of the cam is to permit characterization of
actuator position versus input signal.