Samson 3767, Mounting And Operating Instructions

Page: 38 / 68

38 EB 8355-2 EN
Operation
5 Operation
5.1 Tuning the positioner
mounted onto the control
valve
Starting point and reference variable
When adjusting the positioner directly at the
control valve, the travel (opening angle) must
be adapted to the reference variable.
With a reference variable, for example, 4 to
20 mA, the valve must move through its en-
tire travel range from 0 to 100 % (Fig. 13,
left).
For positioners for rotary actuator, an open -
ing angle, for example, 0 to 70° must be as-
signed to the reference variable.
The starting point is based on the CLOSED
position of the valve.
Depending on the actuator version ("actua
-
tor stem extends" or "actuator stem retracts")
and the operating direction of the positioner
(>> or <>), this starting point can be either
the lower or upper range value (4 or 20 mA)
of the reference variable.
The reference variable range and thus the
upper range value determine the travel of
the valve.
In split-range operation (Fig. 13, right), the
control valves work with smaller reference
variable ranges. The controller output signal
is used to control two control valves, dividing
it such that the valves move through their en
-
tire travel range at half the input signal
range each (e.g. first valve set to 4 to
12 mA, second valve set to 12 to 20 mA). To
avoid overlapping, allow for a dead band of
± 0.5 mA as shown in Fig. 13.
The starting point (zero) is adjusted at the
zero adjuster (6.2); the span, i.e. the upper
range value, is adjusted at the span adjuster
(6.1).
During the adjustment, connect a suitable
ammeter to the signal input and apply air to
the supply air input.
NOTICE
When the positioner is controlled by
a computer whose signal is limited,
e.g. between 4 to 20 mA, set the po -
sitioner to the range from 4.5 to
20 mA.
This is the only way to ensure that the
actuator is completely vented and the
valve completely closed when the
controller issues a 4 mA signal.
For operating direction <>, set the
range to 4 to 19.5 mA.