PID SET UP WITH FRENIC Eco series inverter.
2
In the picture you can see that the signals from the
PID command
and the
PID
Feedback
are subtracted resulting in an error signal. When the
PID command
and
the
PID feedback
signal are the same no correction is needed and the drive will run
at the same frequency. When the
PID command
is larger than the
PID feedback
the
error signal will be positive and the drive frequency will increase until the
PID
Feedback
signal is the same. This is a continuous process. As soon as a difference
in the feedback is detected the drive frequency will be adjusted accordingly.
•
PID command
=
PID feedback
=> No frequency change.
•
PID command
>
PID feedback
=> Frequency is
in
creased.
•
PID command
<
PID feedback
=> Frequency is
de
creased.
The example above is a simple representation of the process. In real world
processes there usually is a time delay between changing the drive frequency and
the response of the PID feedback signal.
3.2 P-control.
The P part of a PID controller is the Proportional part. This means that the action of
the P part only depends on the momentary difference (error) between the set value
and the feedback. The drawback using only a P controller is that the more the
feedback signal approaches the set value, the smaller the action (correction) signal
from P control becomes, slowing the settling of the process. In theory the set value
will never be reached.
3.3 I-control.
With the I (integrating) part of the controller, the effect of the P controller never
reaching the end value can be solved. The I part integrates the difference between
the set and feedback value. Even when this difference is small, after some time, the
correction to the error signal will become larger, helping the process to reach the Set
value.
3.4 D-control.
The D-part (differentiate) part of the controller helps to overcome another drawback
of the proportional controller. This is that the correction signal can never become
larger than the difference between the set and feedback signal (error). The D part of
the controller can amplify the difference (error) a bit more so the correction signal to
the process will become higher (for a short time) and the process will reach the set-
point faster. It is recommended not to use this part unless a good response is not
achieved with the P and I control.
