Prerequisites; Auto-Hysteresis And Auto-Deviation - Siemens Simatic S7 Series System Manual

PID loops and tuning

11.2 Prerequisites

11.2 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 can cause the output
value to attempt to exceed the minimum or the maximum range limit.
If this happens, the generation of an auto-tune error condition results, and the determination
of less than near optimal suggested values certainly results.
11.3

Auto-hysteresis and auto-deviation

Hysteresis parameter
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.
Note
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.
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System Manual, 09/2015, A5E03822230-AC
S7-200 SMART