Functional Operation - ABB AV3 Instruction 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:
The upward force of the signal diaphragm assembly.
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The downward force from the range spring.
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The internal input signal pressure determines the diaphragm
force. The Type AV3/4 positioner internally generates the input
signal pressure; the AV3 uses a current to pneumatic converter,
the AV4 uses a voltage pulse to pneumatic converter. The con-
verter in both cases consists of a servo and EMI/electronics
assembly. The assembly contains electronic circuitry. Applying an
input signal (four to 20 milliamp or voltage pulse) to the converter
produces an input signal pressure to the signal diaphragm.
The range spring force is a function of the shape and position of
the cam. The cam is coupled to the cam shaft, the cam shaft 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 (a pulse or some value between four and
20 milliamp), 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 which ultimately
changes the position of the actuator. 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 exists once again.
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 input control signal (current value or
voltage pulse). For linear motion actuators, the feedback mecha-
nism consists of:
Drive rod which follows the motion of the actuator.
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Adjustable-length, swivel-ended connecting link which transmits
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the motion of the drive rod to an adjustable drive arm on the posi-
tioner.
Camshaft and cam which are rotated through an angle by the
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arm.
A function of the cam is to permit characterization of actuator posi-
tion versus input signal.