Technical data

SFBs for Integrated Control
System Software for S7-300/400 System and Standard Functions - Volume 2/2
A5E00739858-01
27-9
27.2 Step Control with SFB 42/FB 42 "CONT_S"
Introduction
SFB/FB "CONT_S" (step controller) is used on SIMATIC S7 programmable logic
controllers to control technical processes with digital manipulated value output
signals for integrating actuators. During parameter assignment, you can activate or
deactivate sub-functions of the PI step controller to adapt the controller to the
process. You can easily do this by using the parameter assignment tool (Menu
path: Start > Simatic > Step7 > Assign PID Control parameters). The online
electronic manual is found under Start > Simatic > Step7 > Assign PID Control
English.
Application
You can use the controller as a PI fixed setpoint controller or in secondary control
loops in cascade, blending or ratio controllers, however not as the primary
controller. The functions of the controller are based on the PI control algorithm of
the sampling controller supplemented by the functions for generating the binary
output signal from the analog actuating signal.
The following applies starting with FB V1.5 or V1.1.0 of CPU 314 IFM:
With TI = T#0ms, the integral component of the controller can be disabled, thus
allowing the block to be used as a proportional controller.
Since the controller works without any position feedback signal, the internally
calculated manipulated variable will not exactly match the signal control element
position. An adjustment is made if the manipulated variable (ER * GAIN) is
negative. The controller then sets the output QLMNDN (manipulated value signal
low) until LMNR_LS (lower limit of the position feedback signal) is set.
The controller can also be used as a secondary actuator in a controller cascade.
The setpoint input SP_INT is used to assign the control element position. In this
case the actual value input and the parameter TI (integration time) must be set to
zero. An application example is temperature regulation by means of heat output
control using pulse-pause control and cooling capacity control by means of a valve.
In this case, to close the valve completely, the manipulated variable (ER * GAIN)
should have a negative setting.
Note
The calculation of the values in the control blocks is only correct if the block is
called at regular intervals. For this reason, you should call the control blocks in a
cyclic interrupt OB (OB30 to OB38). Enter the sampling time in the CYCLE
parameter.