Commissioning Liquisys M CLM223/253
42 Endress+Hauser
7.4.3 Current input
For the "Current input" function group, you require a relay card with a current input which
is not available in the basic device version. With this function group, you can monitor
process parameters and use them for feedforward control. For this purpose, you must
connect the current output of an external measured variable (e.g. flowmeter) to the 4 to
20mA input of the transmitter. The following assignment applies:
Flow in main stream Current signal in mA Current input signal in %
Flowmeter start of measuring range 4 0
Flowmeter end of measuring range 20 100
Flow monitoring in the main stream
This arrangement is particularly practical if the sample flow through a flow assembly in an
open outlet is independent of the flow in the main stream.
This permits signaling of an alarm condition in the main stream (flow too low or has
completely stopped) and triggers dosing switch-off even if the medium flow is maintained
due to the method of installation.
A0008923
25 Alarm signaling and dosing switch-off by the main stream
A Flow in main stream F Flow restoration
B Relay contacts of PID controller Z2 Delay for controller switch-off, see field Z2
C Alarm relay Z3 Delay for controller switch-on, see field Z3
D Flow below switch-off limit Z 4 or flow failure 0 Off
E Flow alarm 1 On
Feedforward control to PID controller
You can optimize control on control systems with very short response times by measuring
the medium flow rate in addition to the oxygen content. Then apply this flow rate value (4
to 20 mA) as feedforward control to the PID controller.
Feedforward control is a multiplying function as illustrated in the figure below (example
with factory setting):
To Next Page
Loading...
