Description of the Firmware
4–7
IP244
C79000–B8576–C860–02
The manipulated variable (S
k
) is obtained as follows
S
k
= K
P
(p
k
+ i
k
+ d
k
),
converted to a value as a percentage of the sampling time.
The individual branches can be disabled by setting the appropriate parameters T
N
, T
D
to 0. If you
do not require the P-branch, the gain K
R
must be entered as 0; K
P
is then internally set to 1.
To avoid the wind-up effect, the integration is interrupted when the manipulated variable reaches
100 % or when the manipulated variable of a 2-step controller is less than –12% (this only applies
to a purely heating controller). For a purely heating controller, the I-action can never be negative.
For a purely cooling controller, the same applies in reverse: +12% and the I-action is always
negative. The derivative branch contains a first order filter to damp the control action.
The calculated manipulated variable is converted to a heating or cooling time in multiples of 50 or
60 ms depending on the heating-cooling ratio. In Pt 100 operation, this is converted to a multiple
of 80 ms (if the special function is selected, the time base is then a multiple of 55 ms).
1.1.2 Actual Value Processing
The analog inputs (controller inputs) are read in cyclically and checked for line breaks. Depending
on the selected type of sensor, the signal is linearized according to a stored characteristic curve
and a temperature value is calculated (exception: linear sensors). In the case of thermocouples,
a reference junction compensation is carried out on the module. Thermocouples supply a voltage
proportional to the temperature difference (T
1
–T
2
) across the thermocouple. To determine the
absolute temperature, the temperature of the reference junction must be compensated.
The following equation applies for the thermal e.m.f.: U
Th
=
k · (T
1
– T
2
) [V]
(k = constant dependent upon type of thermocouple).
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