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• 9 - PID Loops Simatic 505 SoftShop
Max Time
Max Time is a time in minutes. When a SmarTune session is started, a
timer is set to this value. If that timer expires before the session has
completed, the session is aborted with an error (see Table 9-4).
Noise Band
When electrical signals are converted to values, they vary randomly by
insignificant amounts. An insignificant amount is application dependent.
Noise Band gives a value in engineering units denoting the boundary
between a significant and an insignificant change. If a PV value differs
from a prior value by a Noise Band or greater amount, then a PV change
has occurred. Otherwise the PV is considered unchanged. An incorrect
Noise Band setting could cause some errors and warnings in Table 9-2. A
correct setting may be calculated from hardware specifications, or
determined by experiment and observation, or both.
Step Change
SmarTune works best with a PV change of about 7%. Changing Mn
proportional to the ratio between Step Change and PV span accomplish
this change. Step Change is specified in engineering units of the PV. If a
PV span is 0 to 60 degrees and Step Change is 5 degrees, then Mn would
be changed by about 2667 (5/60 * 32000). Due to round-off error, the
actual value might be slightly different. This example is based on a Mn
span of 0 to 32000. If a 20% offset on output is selected for a loop, a Mn
change of about 2133 (Mn span of 25600) would be accomplished. See
Table 9-2 for possible warnings and errors associated with Step Change.
Wait Time
The SmarTune sample algorithm looks for a PV to change by Step
Change or to quit changing. Wait Time is required to determine when a
PV has quit changing. If a PV value does not change by a Noise Band
amount within a Wait Time period, then it has stopped changing.
PIN
PIN and PIN Variable are provided to force a two-step procedure to be
followed before a loop is tuned. To use this feature, PIN and PIN
Variable must both be set. If PIN is a zero or PIN Variable is a null, then
SmarTune activation is a one-step procedure dependent only on Start
Variable. If both are specified, PIN Variable must equal PIN or a
SmarTune session will not be started or queued.+
Automatic Download
If Automatic Download is true, a loop tuning session is accomplished
with minimum additional support. After tuning values are calculated,
three actions are taken:
• Calculated Kc, Ti, and Td are written to a loop.
• The loop is changed to its prior mode.
• The loop’s SP is assigned its prior value.
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