SMAR FY301, Instruction, Operations & Maintenance Manual

Page: 25 / 82

Section 2
2.1
OPERATION
Transducer Functional Description
The main parts of the output module are the pilot, servo, Hall effect position sensor and the output
control circuit.
The control circuit receives a digital setpoint signal from the CPU and a feedback signal from the
position sensor.
The pneumatic circuit is based on a well-known and widely adopted technology, which is described on
item baffle and nozzle and Spool Valve.
PILOT
RESTRICTION
PILOT
DIAPHRAGM
SPOOL
DIAPHRAGM
AIR SUPPLY
NOZZLE
PIEZO VANE
SPOOL
PILOT CHAMBER
VENT 2
OUTPUT 2
OUTPUT 1
VENT 1
SPRING
Figure 2.1 - Schematic Pneumatic Transducer
A piezoelectric disk is the 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 a small 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.
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 balance is reached (new
valve position).