P controller: This is used with integrating
control paths (for example batch neutrali‐
sation). If control deviation becomes less,
then the control of the actuator becomes
less (proportional relationship). Once the
setpoint is reached, then the control
output is 0 %. However, the setpoint is
never exactly reached, thereby creating a
constant control deviation. Oscillations
can occur when stabilising large changes.
PI controller: This is used with non-inte‐
grating control paths (for example in-line
neutralisation), where oscillation has to be
avoided and where no remaining control
deviation may occur, i.e. the setpoint
always has to be maintained. A constant
addition of metering chemicals is required.
It is not a malfunction if the controller does
not stop when the setpoint is reached.
PID controller: This controller type has the
properties of a PI controller. Due to the
differentiating control part, it also offers a
certain level of foresight and can react to
forthcoming changes. It is used when
measurement spikes occur in the meas‐
urement curve and these have to be
quickly regulated out.
P, PI and PID controllers are continuous
controllers. The control variable can have
any value within the control range from
-100 % … +100 %.
Manual: If
[Manual]
controller type is
selected, then a control variable within a
range of -100 % … 100 % can be entered.
This function is used to test the cabling
and the actuator.
Off: The control function is deactivated.
The device works only as a measuring
transducer.
Setpoint
The setpoint specifies the target value for
control. The controller attempts to keep
the deviation between the setpoint and
measured value as small as possible.
Ctrl output limitation (control variable limi‐
tation)
Ctrl output limitation specifies the max‐
imum control variable to be output. This
makes sense if an actuator is over-dimen‐
sioned and must not be opened to 100%.
[Ti]
The time
[Ti]
is the integral action time of
the I-controller (integral controller) in sec‐
onds. The time
[Ti]
defines the time inte‐
gration of the control deviation from the
control variable. The smaller the time
[Ti]
,
the greater the effect on the control vari‐
able. An infinitely long time
[Ti]
results in a
pure proportional control.
The time
[Ti]
is the time that an I-control
needs to reach the equal size of control
variable change that a P controller,
although a P controller achieves this
change immediately.
Td
The time Td is the differentiation time of
the D-controller (differential controller) in
seconds. The D-controller reacts to the
rate of change of the measured value.
Glossary
174
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