Siemens SIMATIC S7-1200 Function Manual page 314

Instructions
9.2 PID_3Step
Monitoring the PID_3Step sampling time
Ideally, the sampling time is equivalent to the cycle time of the calling OB. The PID_3Step
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 (ErrorBits = 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_3Step in OB1. You must then accept a lower control quality due to the
deviating sampling time.
Sampling time of the PID algorithm
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 the PID algorithm represents the time between two calculations of the output value. It
is calculated during tuning and rounded to a multiple of the cycle time. All other functions of
PID_3Step are executed at every call.
Measuring the motor transition time
The motor transition time is the time in seconds the motor requires to move the actuator from
the closed to the opened state. The actuator is moved in one direction for a maximum time of
Config.VirtualActuatorLimit × Retain.TransitTime/100. PID_3Step requires the motor
transition time to be as accurate as possible for good controller results. The data in the
actuator documentation contains average values for this type of actuator. The value for the
specific actuator used may differ. You can measure the motor transition time during
commissioning. The output value limits are not taken into consideration during the motor
transition time measurement. The actuator can travel to the high or the low endstop.
The motor transition time is taken into consideration in the calculation of the analog output
value as well as in the calculation of the digital output values. It is mainly required for correct
operation during auto-tuning and the anti-windup behavior. You should therefore configure
the motor transition time with the value that the motor requires to move the actuator from the
closed to the opened state.
If no relevant motor transition time is in effect in your process (e.g. with solenoid valves), so
that the output value has a direct and full effect on the process, use PID_Compact instead.
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Function Manual, 03/2017, A5E35300227-AC
PID control