Siemens SIMATIC S7-1200 Function Manual page 250

Instructions
9.1 PID_Compact
Substitute output value
In the event of an error, PID_Compact can output a substitute output value that you define at
the SubstituteOutput tag. The substitute output value must be within the output value limits.
Monitoring signal validity
The values of the following parameters are monitored for validity when used:
● Setpoint
● Input
● Input_PER
● Disturbance
● ManualValue
● SubstituteOutput
● Output
● Output_PER
● Output_PWM
Monitoring of the sampling time PID_Compact
Ideally, the sampling time is equivalent to the cycle time of the calling OB. The PID_Compact
instruction measures the time interval between two calls. This is the current sampling time.
On every switchover of operating mode and during the initial startup, the mean value is
formed from the first 10 sampling times. Too great a difference between the current sampling
time and this mean value triggers an error (Error = 0800h).
The error occurs during tuning if:
● New mean value >= 1.1 x old mean value
● New mean value <= 0.9 x old mean value
The error occurs in automatic mode if:
● New mean value >= 1.5 x old mean value
● New mean value <= 0.5 x old mean value
If you deactivate the sampling time monitoring (CycleTime.EnMonitoring = FALSE), you can
also call PID_Compact in OB1. You must then accept a lower control quality due to the
deviating sampling time.
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Function Manual, 03/2017, A5E35300227-AC
PID control