Flowserve APEX A9000 Accord - Manual (Page 9)

9
Figure 1
6. Principle of operation
The APEX A9000 positioner is a digital positioner with various options. The positioner consists of three main modules:
1. The microprocessor-based electronic control module includes direct local user interface switches
2. The piezo valve-based electro- pneumatic converter module
3. The infinite resolution valve position sensor.
The basic positioner operation is best understood by referring to figure 1. The complete control circuit is powered by the
two-wire, 4-20 mA command signal. The analog 4-20 mA command is passed to the microprocessor, where it is compared
to the measured valve stem position. The control algorithm in the processor performs control calculations and
produces an output command to the piezo valve, which drives the pneumatic amplifier. The position of the pilot valve in
the pneumatic amplifier is measured and relayed to the inner loop control circuit. This two-stage control provides for
more responsive and tighter control than is possible with a single stage control algorithm . The pneumatic amplifier
controls the airflow to the actuator. The change of pressure and volume of the air in the actuator causes the valve to
stroke. As the valve approaches the desired position, the difference between the commanded position and the
measured position becomes smaller and the output to the piezo is decreased. This, in turn, causes the pilot valve to
close and the resulting flow to decrease, which slows the actuator movement as it approaches, the new commanded
position. When the valve actuator is at the desired position. the pneumatic amplifier output is held at zero, which holds
the valve in a constant position.
7. Mounting and installation
7.1 General
Before starting installation, inspect the digital positioner for any transit damages.
.