Additive and multiplicative feedforward
control
Alongside information relating to the
actual measured variable, e.g. the
chlorine concentration, the interference
variable is a further source of information
for the controller that makes it easier for
the controller to provide stable control
during flow processes. During flow pro‐
cesses, both the above parameters
change frequently within wide ranges. If
one parameter variable is not known, then
it is impossible to achieve stable control of
the other parameter variable. If processing
of an interference variable is active, then
the processing of the interference variable
is signalled on the controller's continuous
display under
[NAME OF INTERFERENCE VARIABLE]
and
[UNIT]
with the letter
[ Q]
. Depending
on the configuration, an interference vari‐
able can be effective for one or both
measuring channels
The signal source of the interference vari‐
able can be supplied to the controller via
an analog signal or a frequency (incorpo‐
rated in the basic design of the controller).
Channel 2 should be equipped with equip‐
ment package 2 (one main measured vari‐
able, e.g. chlorine) or equipment package
4 (2 main measured variables, e.g. pH
and chlorine) to process an analog signal.
A frequency signal is connected to digital
input 2 and an analog signal to mA input
2. The interference variable can act on
both channels with accessory package 4,
e.g.:
n mA input at channel 1: chlorine meas‐
urement
n mV input at channel 2: pH measure‐
ment
n Channel 2 analog input: flow signal
Applicational example of additive interfer‐
ence variable
If the addition of a chemical is largely only
dependent on the flow (proportional
dependency), then the addition of an addi‐
tive interference variable proportional to
the interference variable (flow), adds a
proportion of the control variable to the
control variable of the setpoint controller
(setpoint control, that is the comparison,
setpoint: actual value). It is also possible
to completely switch off control of the set‐
point and only provide flow-proportional
metering. The measurement of the main
measured value can be used together
with the limit values as a monitoring func‐
tion.
Applicational example:
You are to chlorinate drinking water. The
required setpoint is 0.3 mg/l (ppm)
chlorine. The volumetric flow of the
drinking water is measured with a flow
meter. The measuring signal of the flow
meter is routed to the controller via a 4 ...
20 mA signal. A chlorine sensor CLE3
continuously measures the chlorine. The
volumetric flow alters within a wide flow
range from 0 ... 250 m³/h. The chlorine
concentration of 0.3 mg/l is achieved
using the proportionality between the
water flow and the added volume of
chlorine (the correct design of the
metering pump according to the chlorine
concentration is a prerequisite). If the
chlorine requirement were now to
increase, caused by a higher flow or
greater depletion (higher temperature,
more germs), then an additional positive
fraction of the setpoint control would be
added to the flow-proportional control vari‐
able. If by contrast, too much chlorine is
Setting the
[Control]
112
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