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.
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):
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
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