Siemens SIMATIC S7-1200 Function Manual page 178

Using PID_Temp
6.2 Configuring PID_Temp
PID algorithm sampling time
The controlled system needs a certain amount of time to respond to changes in the output
value. It is therefore not advisable to calculate the output value in every cycle. The sampling
time of "PID algorithm" represents the time between two calculations of the PID output value.
It is calculated during tuning and rounded to a multiple of the PID_Temp sampling time (cycle
time of the cyclic interrupt OB). All other functions of PID_Temp are executed at every call.
If you use OutputHeat_PWM or OutputCool_PWM, the sampling time of the PID algorithm is
used as the period duration of the pulse width modulation. The accuracy of the output signal
is determined by the ratio of the PID algorithm sampling time to the cycle time of the OB. The
cycle time should be no more than a tenth of the PID algorithm sampling time.
The sampling time of the PID algorithm that is used as the period duration of the pulse width
modulation at OutputCool_PWM depends on the method for heating/cooling selected in
"Basic settings Output":
● If the cooling factor is used, the "sampling time of the PID algorithm for heating" also
applies to OutputCool_PWM.
● If PID parameter switching is used, the "sampling time PID algorithm for cooling" applies
as the period duration for OutputCool_PWM.
If the sampling time of the PID algorithm and thus the period duration of the pulse width
modulation is very high when OutputHeat_PWM or OutputCool_PWM is used, you can
specify a deviating shorter period duration at the parameters
Config.Output.Heat.PwmPeriode or Config.Output.Cool.PwmPeriode in order to improve
smoothness of the process value.
Dead zone width
If the process value is affected by noise, the noise can also have an effect on the output
value. The output value may fluctuate considerably when controller gain is high and the
derivative action is activated. If the process value lies within the dead zone around the
setpoint, the control deviation is suppressed so that the PID algorithm does not react and
unnecessary fluctuations of the output value are reduced.
The dead zone width for heating or cooling is not set automatically during tuning. You have
to correctly configure the dead zone width manually. The dead zone is deactivated by setting
the dead zone width = 0.0.
When the dead zone is switched on, the result can be a permanent control deviation
(deviation between setpoint and actual value). This can have a negative effect on fine tuning.
If cooling is activated in the basic settings and PID parameter switching is selected as the
method for heating/cooling in the output settings, the dead zone lies between "Setpoint -
dead zone width (heating)" and "Setpoint + dead zone width (cooling)".
If cooling is deactivated in the basic settings or the cooling factor is used, the dead zone lies
symmetrically between "Setpoint - dead zone width (heating)" and "Setpoint + dead zone
width (heating)".
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Function Manual, 03/2017, A5E35300227-AC
PID control