Technical data

Table Of Contents
SFBs for Integrated Control
System Software for S7-300/400 System and Standard Functions - Volume 2/2
27-10
A5E00739858-01
Description
Apart from the functions in the process value branch, the SFB implements a
complete PI controller with a digital manipulated value output and the option of
influencing the manipulated value manually. The step controller operates without a
position feedback signal.
In the following you will find the description of the partial 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_PE
R
100
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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, due to a limited resolution of the manipulated value by the actuator
valve), a dead band is applied to the error signal (DEADBAND). If DEADB_W = 0,
the dead band is switched off.
PI Step Algorithm
The SFB/FB operates without a position feedback signal. The I action of the PI
algorithm and the assumed position feedback signal are calculated in one
integrator (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
generator (PULSEOUT) that creates the pulses for the actuator. The switching
frequency of the controller can be reduced by adapting the threshold on of the
three-step element.
Feed Forward Control
A disturbance variable can be fed forward at the DISV input.
Initialization
SFB/FB42 "CONT_S" has an initialization routine that is run through when the input
parameter COM_RST = TRUE is set.
All other outputs are set to their default values.