
Simatic 505 SoftShop
9 - PID Loops
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173
Calculate Derivative
If Calculate Derivative is false, only Kc and Ti are calculated, and Td is
set to zero. If Calculate Derivative is true, Kc, Ti, and Td are calculated.
Safe Output
Safe Output is a Mn value that will not cause any harm to a process. The
default is to use the loop Mn value just prior to a tuning session start.
High Stop
If a PV goes above High Stop, Mn is set to Safe Output and an error is
declared (see Table 9-4).
Low Stop
If a PV goes below Low Stop, Mn is set to Safe Output and an error is
declared (see Table 9-4).
Largest Gain, Largest Reset, Largest Rate
If a calculated value is larger than a configured value, then it is reduced
to a configured value and a warning is declared (see Table 9-4).
Smallest Gain, Smallest Reset, Smallest Rate
If a calculated value is smaller than a configured value, then it is
increased to a configured value and a warning is declared (see Table 9-4).
Activation Time Slice, Calculation Time Slice
These two values set how much impact SmarTune will have on PLC scan
time. If zero in all configurations, a default will be used (2 milliseconds).
Otherwise, in each category, the largest value specified will be used.
Activation Time Slice controls how responsive SmarTune is to tuning
session requests. Increase this value if SmarTune is taking an excessive
amount of time to start a tuning session. Remember that as this value is
increased, PLC scan time will increase.
Calculation Time Slice determines how much real time it will take to
calculate tuning parameters. It is possible a calculation might take 20
seconds or more of PLC time. If a PLC has a scan time of 10
milliseconds and Calculation Time Slice is 2 milliseconds, then a 20-
second calculation would take about 120 seconds in real time: (10ms +
2ms) / 2ms * 20s = 120s. The above formula is an algebraic
simplification of: Xs / (2ms / 12ms) = 20s where X is real time in
seconds. This value should be increased if a SmarTune session takes an
excessive amount of time with a status of calculating (see Table 9-4 and
Status Variable). Remember that as this value is increased, PLC scan time
will increase while a SmarTune session is calculating.
PID Documentation
505 SoftShop provides a simple way to document PID Loops while you
are creating or editing loops. The maximum number of characters you can
enter for each header is 16K.