Siemens SIMATIC S7-1200 Function Manual page 348

Instructions
9.2 PID_3Step
If OutputPerOn = FALSE and FeedbackOn = FALSE, you cannot limit the output value. The
digital outputs are reset with Actuator_H = TRUE or Actuator_L = TRUE, or after a travel time
amounting to 110% of the motor transition time.
The output value is 27648 at 100% and -27648 at -100%. PID_3Step must be able to
completely close the valve. Therefore, zero must be included in the output value limits.
Note
Use with two or more actuators
PID_3 Step is not suitable for use with two or more actuators (for example, in heating/cooling
applications), because different actuators need different PID parameters to achieve a good
control response.
Substitute output value
If an error has occurred, PID_3Step can output a substitute output value and move the
actuator to a safe position that is specified in the SavePosition 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:
● Setpoint
● Input
● Input_PER
● Feedback
● Feedback_PER
● Output
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 = 0800 hex).
PID_3Step is set to "Inactive" mode during tuning under the following conditions:
● New mean value >= 1.1 x old mean value
● New mean value <= 0.9 x old mean value
In automatic mode, PID_3Step is set to "Inactive" mode under the following conditions:
● New mean value >= 1.5 x old mean value
● New mean value <= 0.5 x old mean value
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Function Manual, 03/2017, A5E35300227-AC
PID control