Technical data

Table Of Contents
SFBs for Integrated Control
System Software for S7-300/400 System and Standard Functions - Volume 2/2
27-20
A5E00739858-01
The duration of the positive or negative pulses is calculated from the input variable
(in %) multiplied by the period time.
IPeriod time =
100
INV
PER_TM
The following figure shows a symmetrical curve of a three-step controller (ratio
factor = 1).
Duration o
f
th
pos. pulse
-100 %
100 %
PER_TM
PER_TM - P_B_TM
P_B_TM
Continuous Off
Cont inuous On
Duration of the
neg. pulse
Three-Step Control Asymmetrical
Using the ratio factor RATIOFAC, the ratio of the duration of positive to negative
pulses can be changed. In a thermal process, for example, this would allow
different system time constants for heating and cooling.
The ratio factor also influences the minimum pulse or minimum break time. A ratio
factor < 1 means that the threshold value for negative pulses is multiplied by the
ratio factor.
Ratio Factor < 1
The pulse duration at the negative pulse output calculated from the input variable
multiplied by the period time is reduced by the ratio factor.
Dur. of positive pulse =
100
INV
PER_TM
Dur. of negative pulse =
100
INV
PER_TM
RATIOFAC