Design And Principle Of Operation - Samson 4765 Mounting And Operating Instructions

Design and principle of operation

4 Design and principle of oper-
ation
The pneumatic positioner is used to assign
the valve position (controlled variable) to the
input signal (reference variable). The posi-
tioner compares the control signal issued by
a control system to the travel of the control
valve and issues a signal pressure (output
variable) for the pneumatic actuator.
The positioner consists of the lever with shaft
and range spring, the measuring diaphragm
and the pneumatic control system with noz-
zle, flapper plate and booster. In addition, a
pressure gauge can optionally be attached
for the control signal (input) and the signal
pressure (output signal).
The positioner operates according to the
force-balance principle. In this way, the mo-
tion of the actuator stem or the plug stem
(controlled variable x) is transmitted to the
lever (1) and the range spring (6) over the
plate (20). This action twists the range spring
and varies its spring force.
The reference variable, i.e. the control signal
(p
) of the upstream controller, produces a
e
force which acts on the surface of the mea-
suring diaphragm (8) and is compared to
the force of the range spring (6). At the same
time, the motion of the measuring dia-
phragm is transferred to the flapper plate
(10.2) over the feeler pin (9.1), and the noz-
zle (10.1) releases pressure. The supply air is
fed to the booster (12) and flows through the
Xp restriction (13) and the nozzle (10.1) and
hits the flapper plate (10.2).
8
Any change in the control signal p
valve position causes the pressure to change
upstream or downstream of the booster. The
air controlled by the booster (signal pressure
p ) flows through the volume restriction (14)
st
to the pneumatic actuator, causing the plug
stem to move to a position corresponding to
the reference variable.
The adjustable restrictions (13 and 14) are
used to optimize the positioner control loop.
The range spring (6), which can be ex-
changed, is assigned to both the rated valve
travel and the span of the reference variable.
Legend for Fig. 1 and Fig. 2
1 Lever for valve travel
1.1 Shaft
2 Pin
2.1 Nut
3 Sleeve
4 Zero point adjustment
5 Fastening screw
6 Range spring
6.1 Bracket
7 Fastening screw
8 Measuring diaphragm
9 Diaphragm lever
9.1 Feeler pin
10 Nozzle block
10.1 Nozzle
10.2 Flapper plate
11 Cover plate
12 Booster
13 Xp restriction
14 Volume restriction Q
15 Hole for fastening screw
20 Plate
or the
e
EB 8359-1 EN