Design And Principle Of Operation; Communication - Samson 3785 Mounting And Operating Instructions

Design and principle of operation

1. Design and principle of operation
The digital PROFIBUS-PA positioner is de-
signed for attachment to pneumatic control
valves and is used to ensure a correspond-
ence between the valve stem position (control-
led variable) and the control signal (reference
variable). It compares the digital output signal
to the travel of the control valve, and delivers
a corresponding pressure signal as the output
variable. For this purpose, auxiliary supply air
is required, with a pressure of 1.4 to 6 bar.
Auxiliary energy is supplied via bus-powered
PROFIBUS-PA segment based on IEC 61158-
2 standard transmission technology.
The positioner essentially consists of an induc-
tive, non-contact displacement sensor system
and an electrically controlled valve block
comprising the two on-off valves and an elec-
tronic unit. This unit contains two microcontrol-
lers for processing the control algorithm and
managing the PROFIBUS communication.
Whenever a deviation between the actual
valve travel (actual value) and the reference
variable (set point) occurs, the microcontrol-
ler produces binary pulse-pause modulated
signals to control the two on-off valves,
each of which is assigned a downstream
amplifier. One of these valves controls
the exhaust air, and the other one controls
the supply air.
The supply air valve (3) switches the supply
air (7, supply air pressure 1.4 to 6 bar) to
the actuator (filling). The exhaust air valve
(4) controls the air exhausted from the actua-
tor to the atmosphere (venting). These on-off
valves can either have the switching states -
permanently open, permanently closed - or
generate single pulses of changing widths.
With the two valves being controlled, the
plug stem moves to a position correspond-
8
EB 8382-1 EN
ing to the reference variable. If there is no
system deviation, both the supply air and
the exhaust air valve are closed.
As a standard feature, the positioner is
equipped with a binary input for floating
contacts which additionally signals the
switching state of another field device via
PROFIBUS.
Activating the write protection switch lo-
cated in the hinged cover prevents the posi-
tioner settings from being overwritten by
PROFIBUS communication.
Positioner with forced venting function:
The positioner is controlled via a 6 to 24 V
signal, causing the signal pressure to be ap-
plied to the actuator. If this voltage signal
decreases, the signal pressure is shut off
and the actuator is vented. The springs con-
tained in the actuator move the valve to its
fail-safe position. The forced venting is in-
stalled in all positioners and can be acti-
vated/deactivated as required over a
switch. See chapter 4.3 for more details.
1.1 Option
As a supplement to the standard positioner
version, the positioner can also be retro-
fitted with limit switches. Two proximity swit-
ches can be used to indicate the valves' end
positions in fails-safe circuits.

1.2 Communication

The positioner is completely controlled via
digital signal transmission according to the
PROFIBUS-PA Profile Class B based on EN
50170 and DIN 19245 Part 4.
Data is transmitted as bit synchronous cur-
rent modulation at a transmission rate of