Principles of Operation
Close on Increasing Signal
The positioner is a force-balanced instrument that provides a control valve posi-
tion proportional to the pneumatic input signal. The control valve will
open
on loss
of input signal. The energy to operate the control valve is obtained from the differen-
tial between the supply and discharge pressures. In steady state, the forces im-
posed on the balance beam by the input signal diaphragm and the range spring are
equal; therefore, the distance between the top and bottom seats and nozzles in the
positioner is equal. This causes the cylinder top and bottom pressures to be equal,
and the control valve is stationary. An increase in the input signal pressure results
in the lower seat being pushed into the lower nozzle due to an imbalance in the
beam forces. At the same time, the top seat moves away from the top nozzle.
This increases the pressure in the cylinder bottom, while the pressure in the cylin-
der top decreases. The control valve begins to close. The actuator rod stretches
the range spring, increasing its tension. This force, which opposes the force on the
balanced beam caused by the increasing input signal, continues to increase until
the balance beam forces are in equilibrium. At this point the valve is in the correct
position for the applied input signal. The positioner’s sensitivity adjustment effects
the responsiveness of the positioner nozzles. The greater the seat and nozzle sen-
sitivity, the greater the accuracy of the positioner.
HPP-4
5
September 1999
Power Gas and Cylinder Bottom
Cylinder Top
Exhaust
Instrument Signal
HPP-4 Positioner
“How-It-Works”
