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10.3 On-off controller with P behaviour
Note
Setting for an on-off controller: Xp > O.
It is possible to use an on-off controller with P
behaviour as an intermittent pulse regulator or as a
pulse frequency regulator.
10.3.1 Intermittent pulse regulator
Setting for an intermittent pulse regulator:
Note
Xp > O, pulse frequency = 0.
Example:
•
setpoint = 6 mS/cm
•
measuring range 0 to 10
•
Xp = 30
•
control direction = downwards.
Xp determines the size of the proportional band as a
percentage of the measuring range. In the following
example, this means that above 6 mS the relay
switches on, first with a short activation time and
then with an increasing activation time to 9 mS (the
total switching on + off time remains constant).
For greater measured values, the relay remains
constantly switched on.
4 5 6 7 8
0 1
2 3
9
6
7
SP
MV
MV
T
ON
T
ON
T
Intermittent pulse regulator
Fig. 13
SP
setpoint
MV
measured value
The intermittent pulse regulation offers the option to
recreate a constant controlling element using very
simple control elements, e.g. a magnetic valve.
Here, the relationship between switching on and off
is changed depending on the calculated control
variable. The total time (ON+OFF) is constant in
intermittent pulse regulation. The total time can be
set to between 1 and 99 seconds depending on the
control system. A setting of 10 seconds leads to a
good result in most cases.
[mS/cm]
[mS
/cm
]
10
[mS/cm]
[
mS/cm
]
8
9
If the calculated control variable is very low, in some
cases it is possible that the control element attached
here does not show a reaction within this short time.
In this case, it is possible to set a minimum drive
pulse. This can be within the range 0.1 to
9.9 seconds.
The intermittent pulse period must always
Note
be at least twice as long as the minimum
switch-on time!
10.3.2 Pulse-frequency regulator
Setting for a pulse-frequency regulator:
Note
Xp > O, pulse frequency > 0.
Example:
•
setpoint = 6 mS/cm
•
measuring range 0 to 10
•
Xp = 20 or 40
•
pulse frequency = 120
•
control direction = downwards.
Xp determines the size of the proportional band as a
percentage of the measuring range. In the following
example, this means that impulses are output from
6 mS and constantly increase to 8 mS (Xp = 20) or
10 mS (Xp = 40). For higher measured values, the
prespecified maximum number of impulses (here
120/min) is output.
Pulse frequency
120
[1/min]
[1 min]
/
90
Xp = 20
60
30
0
6
7
8
Setpoint
Pulse-frequency regulator
Fig. 14
The pulse frequency regulator in combination with an
appropriate control variable is almost the same as a
continuous controller. The disadvantages of the
impulse-driven control variable only becomes
apparent in the lower control variable range and with
a low maximum pulse frequency. The specification of
the maximum pulse frequency refers to 100 %
dosing flow and can be set within a range from 0 to
180 impulses per minute.
100%
Measured
Xp = 40
value
[mS/cm]
10
9
15
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