Siemens SIMATIC S7-1200 Function Manual page 139

Control mode V1
Physical quantity
Select the unit of measurement and physical quantity for the setpoint and process value in
the "Controller type" group. The setpoint and process value will be displayed in this unit.
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.
PID_3Step does not work with negative proportional gain. Select the check box "Invert
control logic" to reduce the process value with a higher output value.
Examples
● Opening the drain valve will reduce the level of a container's contents.
● Increasing cooling will reduce the temperature.
Start-up behavior after reset
To change straight to the last active mode after restarting the CPU, select the "Enable last
mode after CPU restart" check box.
PID_3Step will remain in "Inactive" mode if the check box is cleared.
Setpoint V1
Procedure
Proceed as follows to define a fixed setpoint:
1. Select "Instance DB".
2. Enter a setpoint, e.g. 80° C.
3. Delete any entry in the instruction.
Proceed as follows to define a variable setpoint:
1. Select "Instruction".
2. Enter the name of the REAL variable in which the setpoint is saved.
Program-controlled assignment of various values to the REAL variable is possible, for
example for the time controlled change of the setpoint.
Process value V1
PID_3Step will scale the value of the analog input to the physical quantity if you use the
analog input value directly.
You will need to write a program for processing if you wish first to process the analog input
value. The process value is, for example, not directly proportional to the value at the analog
input. The processed process value must be in floating point format.
PID control
Function Manual, 03/2017, A5E35300227-AC
Using PID_3Step
5.3 PID_3Step V1
139