12
Product description
Type 3281
5.3.2
Process controller
The additionally implemented PID controller can, except for the position control, also be used to conduct a
process control for purposes of cascade control.
Positioner
Position control loop
CMD
CTRL
POS
Z1
POS
Valve opening
+
Xd1
-
Position sensor
Continuous valve
Electromotive
actuator
Set-point
position
Process
controller
Position
control
loo
p
SP
CMD
Z2
PV
Measured
quantity
+
Xd2
-
Transmitter
Process
Set-point
position
Valve
opening
Fig. 3:
Signal Flow Diagram Process Controller
The process controller is integrated into a control loop. The set-point position of the valve is derived from the
process set-point value and the process actual value, with the control parameters (PID controller). The process
set-point value can be specified by an external signal.
For process control, the position control mentioned above becomes an auxiliary control loop, which creates
a cascade control. The process controller in the main control loop has a PID function. The process set-point
value (SP) is specified as the set-point value and compared with the actual value (PV) of the measured variable
being controlled. The position sensor records the current position (POS) of the electromotive linear actuator.
This actual position is compared by the positioner with the set-point value (CMD) specified by the process
controller. If there is a control difference (Xd1), the actuating variable (CTRL) is used to change the actual
position (POS) and thus the valve opening. Z2 is a disturbance.
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