Burkert 8693 - Schematic representation of the process control

22
Type 8692, 8693
english
Description of System
4 TYPE 8693 POSITIONER WITH PROCESS CONTROLLER
If the positioner is operated with process controller Type 8693, the aforementioned position control becomes the subordinate
auxiliary control circuit; this results in a cascade control. The process controller in the main control circuit of the positioner has
a PID function. The process set-point value (SP) is specified as set-point value and compared with the actual value (PV) of the
process variable to be controlled. The position measuring system records the current position (POS) of the pneumatic actuator.
The position controller compares this actual position value with the set-point value (CMD), which is determined by the process
controller. In case of a control deviation (Xd1), a pulse-width modulated voltage signal is sent to the control system as a manipu-
lated variable. If there is a positive control difference in single-acting actuators, the air inlet valve is controlled via output B1. If
the control difference is negative, the bleed valve is controlled via output E1. In this way the position of the actuator is changed
until control difference is 0. Z2 represents a disturbance variable.
Valve opening
Continuous
valve
Control system
solenoid valves
Position measuring
Position con-
troller
CMD
Xd1
B1
E1
P
K
Z1
POS
Position control circuit
+
-
Process variable
Process
Position
control
circuit
Transmitter
Process controller
Process set-
point value
SP Xd2
CMD
Valve
opening
Z2
PV
+
-
Fig. 6: Signal flow plan of process controller