How Cont_C Works - Siemens SIMATIC S7-1200 Function Manual

Instructions
9.4 PID basic functions
9.4.1.2

How CONT_C works

Setpoint branch
The setpoint is entered in floating-point format at the SP_INT input.
Process value branch
The process value can be input in I/O or floating-point format. The function CRP_IN converts
the I/O value PV_PER to a floating-point format -100 to +100 % in accordance with the
following rule:
Output of CRP_IN = PV_PER * 100 / 27648
The PV_NORM function scales the output of CRP_IN according to the following rule:
Output of PV_NORM = (output of CRP_IN) *PV_FAC + PV_OFF
PV_FAC has a default of 1 and PV_OFF a default of 0.
Forming the error signal
The difference between the setpoint and process value is the error signal. To suppress a
minor sustained oscillation due to manipulated variable quantization (e.g. with a pulse width
modulation with PULSEGEN), the error signal is applied to a dead band (DEADBAND). With
DEADB_W = 0, the dead band is switched off.
PID Algorithm
The PID algorithm operates as a position algorithm. The proportional, integral (INT), and
differential (DIF) actions are connected in parallel and can be activated or deactivated
individually. This allows P, PI, PD, and PID controllers to be configured. Pure I controllers
are also possible.
Manual value processing
It is possible to switch over between manual and automatic mode. In manual mode, the
manipulated variable is corrected to a manually selected value.
The integral action (INT) is set internally to LMN - LMN_P - DISV and the derivative action
(DIF) is set to 0 and synchronized internally. Changeover to automatic mode is therefore
bumpless.
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Function Manual, 03/2017, A5E35300227-AC
PID control