Samson 4763, Mounting And Operating Instructions

1

Design and principle of

operation

The electropneumatic positioner is used for

the correlation between the valve stem posi-

tion (controlled variable x) and the input sig-

nal (reference variable w) received from the

controller. In this case, the input signal ac-

cepted from the control device is compared

to the travel (valve stem position) of the con-

trol valve, and a pneumatic signal pressure

(output variable y) is delivered.
The positioner consists of an i/p converter

unit (21) and the pneumatic section includ-

ing the lever (1), shaft (1.1) and range

spring (6), plus the control system compos-

ing nozzle, flapper and booster. The input

signal (e.g. 4 to 20 mA) is directly fed to the

i/p converter unit and converted to a pro-

portional air pressure signal p

e

. Any change

of the input current signal causes a propor-

tional change of the air pressure p

e

sent to

the pneumatic control system.
The air pressure p

e

, in turn, produces a

force which acts on the surface of the mea-

suring diaphragm (8) and is compared to

the force of the range spring (6). The motion

of the diaphragm (8) is transferred to the

flapper (10.2) via the feeler pin (9.1), and

the nozzle (10.1) releases pressure. Any

change of either the air pressure p

e

or the

valve stem position causes the pressure to

change in the booster (12) connected down-

stream of the nozzle. The signal pressure p

st

which is released causes the plug stem to as-

sume a position based on the input signal.
The adjustable volume restriction Q (14) and

Xp (gain) restriction (13) are used to opti-

mize the control loop. The range spring (6),

which can be exchanged, is assigned to

both the rated valve travel and the nominal

voltage of the input signal.

6

EB 8359-2 EN

Design and principle of operation

4

21

14

13 10.2 7

1
15

8

5 6.1

6

3

Fig. 2 · Positioner with cover removed