Mode Of Operation Cont_S - Siemens SIMATIC S7-1200 Function Manual

Instructions
9.4 PID basic functions
9.4.2.2

Mode of operation CONT_S

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 normalizes 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
small constant oscillation due to the manipulated variable quantization (for example, due to a
limited resolution of the manipulated value by the control valve), a dead band is applied to
the error signal (DEADBAND). With DEADB_W = 0, the dead band is switched off.
PI step algorithm
The instruction operates without position feedback. The I-action of the PI algorithm and the
assumed position feedback signal are calculated in one integral action (INT) and compared
with the remaining P-action as a feedback value. The difference is applied to a three-step
element (THREE_ST) and a pulse shaper (PULSEOUT) that generates the pulses for the
control valve. The switching frequency of the controller can be reduced by adapting the
response threshold of the three-step element.
Feedforward control
A disturbance variable can be added at the DISV input.
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Function Manual, 03/2017, A5E35300227-AC
PID control