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8.3
P
ROPORTIONAL
(P)
CONTROL
In case of proportional control the value of the Proportional band (
) is not zero but the Integral time (
)
and the Derivative time (
) are set to zero.
The Proportional band can be set within the range of 1 to 100%.
The Proportional Control will be accomplished by changing the time of the relay’s energised or
de-energised state within the cycle time. The cycle time of the relay (
) can be set within the range of
1 to 120 s.
Proportional range: the control range of the cycle time. Out of the proportional range the relay is always
energised or de-energised.
The value of the Proportional range: q =
(%) *
M, where
M = measurement range.
The position of the Proportional range compared to the Set Value (SV) depends on the percentage value
set in the
control parameter.
If
value = 0 %, the whole range is under the SV.
If
value = 50.0 %, the proportional range is symmetrical to the SV.
If
value = 100 %, the proportional band is over the SV.
Process Value
( )
PV
Set Value
(S )
V
Reduce
value
Increase
value
8.4
PID
CONTROL
PID control is the most commonly used control mode because the best controlling accuracy can be
achieved with it. Similar to the Proportional (P) Control described in the previous chapter, the PID control
will be accomplished by changing the relay’s energised and de-energised states within its cycle time (
).
Since the determination of the optimal P-I-D parameters are difficult and time consuming manually the
usage of the Autotuning is recommended.
Devices with analogue output (4-20 mA) PID control should be only used. Prior to start the control process
it should be determined which control characteristics (cooling or heating) is required.