Technical data

Table Of Contents
SFBs for Integrated Control
System Software for S7-300/400 System and Standard Functions - Volume 2/2
A5E00739858-01
27-19
Modes
Depending on the parameters assigned to the pulse generator, PID controllers with
a three-step output or with a bipolar or unipolar two-step output can be configured.
The following table illustrates the setting of the switch combinations for the possible
modes.
Switch
Mode MAN_ON STEP3_ON ST2BI_ON
Three-step control FALSE TRUE Any
Two-step control with
bipolar control range
(-100 % to +100 %)
FALSE FALSE TRUE
Two-step control with
unipolar control
range (0 % ... 100 %)
FALSE FALSE FALSE
Manual mode TRUE Any Any
Three-Step Control
In the "three-step control" mode, the actuating signal can adopt three states. The
values of the binary output signals QPOS_P and QNEG_P are assigned to the
statuses of the actuator.
The table shows the example of a temperature control:
Actuator
Output Signals Heat Off Cool
QPOS_P TRUE FALSE FALSE
QNEG_P FALSE FALSE TRUE
Based on the input variable, a characteristic curve is used to calculate a pulse
duration. The form of the characteristic curve is defined by the minimum pulse or
minimum break time and the ratio factor.
The normal value for the ratio factor is 1.
The "doglegs" in the curves are caused by the minimum pulse or minimum break
times.
Minimum Pulse or Minimum Break Time
A correctly assigned minimum pulse or minimum break time P_B_TM can prevent
short on/off times that reduce the working life of switching elements and actuators.
Note
Small absolute values at the input variable LMN that could otherwise generate a
pulse duration shorter than P_B_TM are suppressed. Large input values that
would generate a pulse duration longer than (PER_TM - P_B_TM) are set to 100
% or -100 %.