SMAR fy303, Operation, Maintenance & Instruction Manual

Page: 21 / 114

Section 2
2.1
OPERATION
Functional Description - Output Module
The main parts of the output module are the pilot, servo, Hall Effect sensor and the output control
circuit.
The control circuit receives a digital setpoint signal from the CPU and a feedback signal from the
Hall Effect sensor.
The pneumatic circuit is based on a well-known and widely adopted technology, which is described
on item Nozzle-and-Vane and Spool.
Figure 2.1 - Pneumatic Transducer Schematic
A piezoelectric disk as flapper in the pilot stage. The flapper is deflected when the control circuit
applies a voltage. A small stream of air flowing through the nozzle is obstructed causing an increase
in pressure in the pilot chamber; this is called the pilot pressure.
The pilot pressure is too low, with flowing capacity, and for this reason it must be amplified in the
servo section. The servo section includes a diaphragm in the pilot chamber and a smaller one in the
spool chamber. The pilot pressure applies a force at the pilot chamber's diaphragm which, in the
equilibrium state, will be equal to the force applied by the spool valve at the smaller diaphragm
which is in the spool chamber.
Therefore, upon every position change caused by the positioner, the pilot pressure increases or
decreases as explained in the pilot stage section; such change in pilot pressure causes an upward
or downward valve travel which alters the pressure at output 1 and output 2 until a new equilibrium
is reached, which results in a new valve position.