Prerequisites; Auto-Hysteresis And Auto-Deviation - Siemens SIMATIC S7-200 System Manual

Prerequisites

The loop that you want to auto-tune must be in automatic mode. The loop output must be
controlled by the execution of the PID instruction. Auto-tune will fail if the loop is in manual mode.
Before initiating an auto-tune operation your process must be brought to a stable state which
means that the PV has reached setpoint (or for a P type loop, a constant difference between PV
and setpoint) and the output is not changing erratically.
Ideally, the loop output value needs to be near the center of the control range when auto-tuning is
started. The auto-tune procedure sets up an oscillation in the process by making small step
changes in the loop output. If the loop output is close to either extreme of its control range, the
step changes introduced in the auto-tune procedure may cause the output value to attempt to
exceed the minimum or the maximum range limit.
If this were to happen, it may result in the generation of an auto-tune error condition, and it will
certainly result in the determination of less than near optimal suggested values.

Auto-Hysteresis and Auto-Deviation

The hysteresis parameter specifies the excursion (plus or minus) from setpoint that the PV
(process variable) is allowed to make without causing the relay controller to change the output.
This value is used to minimize the effect of noise in the PV signal to more accurately determine
the natural oscillation frequency of the process.
If you select to automatically determine the hysteresis value, the PID Auto-Tuner will enter a
hysteresis determination sequence. This sequence involves sampling the process variable for a
period of time and then performing a standard deviation calculation on the sample results.
In order to have a statistically meaningful sample, a set of at least 100 samples must be acquired.
For a loop with a sample time of 200 msec, acquiring 100 samples takes 20 seconds. For loops
with a longer sample time it will take longer. Even though 100 samples can be acquired in less
than 20 seconds for loops with sample times less than 200 msec, the hysteresis determination
sequence always acquires samples for at least 20 seconds.
Once all the samples have been acquired, the standard deviation for the sample set is calculated.
The hysteresis value is defined to be two times the standard deviation. The calculated hysteresis
value is written into the actual hysteresis field (AHYS) of the loop table.
Tip
While the auto-hysteresis sequence is in progress, the normal PID calculation is not performed.
Therefore, it is imperative that the process be in a stable state prior to initiating an auto-tune
sequence. This will yield a better result for the hysteresis value and it will ensure that the
process does not go out of control during the auto-hysteresis determination sequence.
The deviation parameter specifies the desired peak-to-peak swing of the PV around the setpoint.
If you select to automatically determine this value, the desired deviation of the PV is computed by
multiplying the hysteresis value by 4.5. The output will be driven proportionally to induce this
magnitude of oscillation in the process during auto-tuning.
PID Auto-Tune and the PID Tuning Control Panel Chapter 15
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