Siemens SIMATIC S7-1200 Function Manual page 349

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 maximum time that the actuator is moved in one
direction is 110% of the motor transition time. 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.
Control logic
An increase of the output value is generally intended to cause an increase in the process
value. This is referred to as a normal control logic. For cooling and discharge control
systems, it may be necessary to invert the control logic. PID_3Step does not work with
negative proportional gain. If InvertControl = TRUE, an increasing control deviation causes a
reduction in the output value. The control logic is also taken into account during pretuning
and fine tuning.
See also
Configuring PID_3Step V1 (Page 138)
PID control
Function Manual, 03/2017, A5E35300227-AC
Instructions
9.2 PID_3Step
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