Siemens Simatic S7-1500 - Page 467

Instructions
9.4 PID basic functions
PID control
Function Manual, 03/2017, A5E35300227-AC
467
Asymmetrical three-step control
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.
Ratio factor < 1
The pulse duration at the negative pulse output, calculated by multiplying the input variable
by the period duration, is multiplied by the ratio factor.
Positive pulse duration = INV /100 * PER_TM
Negative pulse duration = INV / 100 * PER_TM * RATIOFAC
The following figure shows the asymmetrical characteristic curve of the three-step controller
(ratio factor = 0.5):
Ratio factor > 1
The pulse duration at the positive pulse output, calculated by multiplying the input variable by
the period duration, is divided by the ratio factor.
Positive pulse duration = INV / 100 * PER_TM / RATIOFAC
Negative pulse duration = INV / 100 * PER_TM