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The adjustment of the PID control parameters can be done in automatic mode and auto-adaptive mode.
Auto-tuning
is initiated at the
operator request.
Auto-adaptive tuning
is initiated by the controller whenever the control performance is lower than desired.
6.1
AUTO-TUNING
At the start of the auto-tuning process, the controller has the behavior of an ON/OFF controller. It applies minimum and maximum
performance to the process. Throughout the tuning process, the controller performance is refined until it is completed, already under
optimized PID control. It is started after you select one of the options
FAST
,
FULL
,
SELF
,
rSLF
or
TGHT
, set in the
ATUN
parameter (
Ctrl
>>
Atun
).
The process can be seen below:
Example of auto-tuning
6.2
AUTO-ADAPTATIVE TUNING
The auto-adaptive tuning is started by the controller whenever its performance is worse than what was found after the previous tuning. To
activate performance supervision and auto-adaptive tuning, you must set the
ATUN
parameter (
Ctrl
>>
Atun
) in
SELF
,
rSLF
or
TGHT
mode.
During auto-adaptive tuning, the behavior of the controller depends on its performance. If the mismatch is small, you will hardly notice the
tuning. If the mismatch is large, the auto-adaptative tuning is like the auto-tuning mode, applying the minimum and maximum performance
to the process in ON/OFF control.
The process can be seen below:
You can select the tuning type in the
ATUN
parameter:
Example of auto-adaptative tuning
•
oFF
: The controller does not perform auto-tuning or auto-adaptive tuning. The PID parameters will not be automatically determined or
optimized by the controller.
•
FAST
: The controller will perform the auto-tuning process only once, returning to
oFF
mode after it is finished. In this mode, the tuning
will be completed in less time, but will not be as precise as in
FULL
mode.
•
FULL
: This mode performs the same process as
FAST
mode, although the tuning is more precise and slower, resulting in better PID
control performance.
•
SELF
: The process performance is monitored, and auto-adaptive tuning will start whenever the performance becomes unsatisfactory.
After an adjustment cycle, the controller will start collecting data about the process. This helps it to create a reference, which is used
to assess the need for further adjustments.
This phase is proportional to the response time of the process.
•
RSLF
: The controller performs the auto-tuning process and returns to
SELF
mode. Normally, this mode is used to force immediate auto-
tuning of a controller that is operating in
SELF
mode.
•
TGHT
: Similar to
SELF
mode. In addition to auto-adaptive tuning, performs auto-tuning whenever the controller is set to
RUN
=
YES
or
when the controller is powered up.
Whenever you change the
ATUN
parameter to a value other than
oFF
, auto-tuning will be immediately started by the controller (if the
controller is not set to
RUN
=
YES
, tuning will start when it transitions to this condition). For the auto-adaptive tuning to work properly, it is
essential to perform auto-tuning.
The auto-tuning and auto-adaptive tuning methods are suitable for most industrial processes. However, there may be processes or even
situations where the methods will not be able to determine the controller parameters properly. This results in undesired oscillations or
leads the process to extreme conditions.
The oscillations imposed by the tuning methods may be intolerable for certain processes. You must consider these effects before using the
controller and take preventive measures to ensure the integrity of the process and users.
6
PID PARAMETERS