Siemens SIMATIC Manual page 30

Continuous Temperature Controller FB 58 "TCONT_CP"
Rule of Thumb for the CYCLE and CYCLE_P Sampling Times
The controller sampling time should not exceed 10 % of the calculated integral time
constant of the controller (TI): CYCLE ≤ TI/10
For an adequately accurate manipulated variable resolution, make sure that the
following relationship applies: CYCLE_P ≤ PER_TM/50.
Rule of Thumb for the Pulse Repetition Period PER_TM
The pulse repetition period should not exceed 20 % of the calculated reset time of
the controller (TI):
PER_TM ≤ TI/5
Example of the Effects of the Parameters CYCLE_P, CYCLE and PER_TM:
PER_TM = 10 s, CYCLE = 1 s, CYCLE_P = 100 ms.
Every second, a new value is calculated for the manipulated variable, every
100 ms, the value is compared with the pulse length or break length output up to
now.
•
When a pulse is output, there are two possibilities:
The calculated value of the manipulated variable is higher than the pulse
-
length/PER_TM up to now. The pulse is then extended.
The calculated value of the manipulated variable is less than or equal to
-
the pulse length/PER_TM up to now. In this case, a pulse is no longer
output.
•
If no pulse is output, there are then also two possibilities:
The value (100 % - calculated value of the manipulated variable) is higher
-
than the break length/ PER_TM up to now. The break is then extended.
The value (100 % - calculated value of the manipulated variable) is less
-
than or equal to the break length/PER_TM up to now. A pulse is then
output.
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PID Temperature Control
A5E00125039-02