Technical data

Table Of Contents
SFBs for Integrated Control
System Software for S7-300/400 System and Standard Functions - Volume 2/2
27-2
A5E00739858-01
Description
Apart from the functions in the setpoint and process value branches, the SFB/FB
implements a complete PID controller with continuous manipulated variable output
and the option of influencing the manipulated value manually.
In the following, you will find a detailed description of the sub-functions:
Setpoint Branch
The setpoint is entered in floating-point format at the SP_INT input.
Process Variable Branch
The process variable can be input in the peripheral (I/O) or floating-point format.
The CRP_IN function converts the PV_PER peripheral value to a floating-point
format of -100 to +100 % according to the following formula:
Output of CPR_IN = PV_PER
100
27648
The PV_NORM function normalizes the output of CRP_IN according to the
following formula:
Output of PV_NORM = (output of CPR_IN) * PV_FAC + PV_OFF
PV_FAC has a default of 1 and PV_OFF a default of 0.
Error Signal
The difference between the setpoint and process variable is the error signal. To
suppress a small constant oscillation due to the manipulated variable quantization
(for example, in pulse duration modulation with PULSEGEN), a dead band is
applied to the error signal (DEADBAND). If DEADB_W = 0, the dead band is
switched off.
PID Algorithm
The PID algorithm operates as a position algorithm. The proportional, integral
(INT), and derivative (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 and D controllers are also possible.
Manual Value
It is possible to switch over between a manual and an automatic mode. In the
manual mode, the manipulated variable is corrected to a manually selected value.
The integrator (INT) is set internally to LMN - LMN_P - DISV and the derivative unit
(DIF) to 0 and matched internally. This means that a switchover to the automatic
mode does not cause any sudden change in the manipulated value.