Siemens SIMATIC S7 System Manual page 588

PID loops and tuning
11.4 Auto-tune sequence
Notice how the auto-tuner switches the output to cause the process (as evidenced by the PV
value) to undergo small oscillations. The frequency and the amplitude of the PV oscillations
are indicative of the process gain and natural frequency.
Based upon the information collected about the frequency and gain of the process during the
auto-tune process, the ultimate gain and ultimate frequency values are calculated. From
these values, the suggested values for gain (loop gain), reset (integral time), and rate
(derivative time) are calculated.
Note
Your loop type determines which tuning values are calculated by the auto-tuner. For
example, for a PI loop, the auto-tuner will calculate gain and integral time values, but the
suggested derivative time will be 0.0 (no derivative action).
Once the auto-tune sequence has completed, the output of the loop is returned to its initial
value. The next time the loop is executed, the normal PID calculation will be performed.
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System Manual, V2.3, 07/2017, A5E03822230-AF
S7-200 SMART