Becker HPP-SB, Instruction Manual, Page: 4 / 26

The positioner is a force-balanced instru-
ment that provides a control valve position pro-
portional to a pneumatic input signal. The con-
trol valve will
open on loss of input signal . The
energy to operate the control valve is obtained
from the differential between the force created
by the pneumatic loading of one side of the
cylinder and the opposing spring
(encapsulated in the actuator spring car-
tridge). In steady-state, the force imposed on
the balance beam by the input signal dia-
phragm and the range spring are equal.
Therefore, the top and bottom balance valves
of the positioner will be at their closed posi-
tions. The positioner does not feed gas to the
cylinder or exhaust the loading pressures from
the cylinder. When the spring force and cylin-
der force are equal, the control valve remains
stationary.
HPP-SB Positioner
“How-It-Works”
An increase in the input signal pressure re-
sults in the opening of the lower balanced
valve due to an imbalance in the beam forces.
This increases the pressure in the cylinder,
further closing the valve and compressing the
actuator spring. The actuator feedback rod
stretches the range spring, increasing its ten-
sion. 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 when the actuator
spring and cylinder forces again balance. The
sensitivity adjustment of the positioner
changes the positioner’s deadband (the
change in input signal necessary to create a
certain amount of change in output signal)
.
Principles of Operation
Close on Increasing Signal – Actuator Fails Open
HPP-SB 4 September 1999