16
12.5. AUTOMATIC PID PARAMETER SELECTION
The first step to use the PID parameter selection function is to choose the type of tuning (parameter 37:
tunE
,
chapter 10). The tuning is started automatically when the control starts (after the power supply is switched on, and
by pressing the
[F]
function button, when parameter 50:
Func
=
StSP
, chapter 9.1). Moreover, tuning can be
stopped (
oFF
), and then started (
on
) at any time using the
t-St
function available in the quick menu (chapter
11). During the tuning (when the display shows, alternately with the measured value, the message
tvnE
) the set
value must not be changed (20:
Set1
or 30:
SEt3
when 50:
Func
=
SEt3
).
The value of parameter 37:
tunE
determines the selection of the PID parameter selection method:
a)
37:
tunE
=
Auto
- automatic selection - the controller continuously checks if there are appropriate conditions
for starting the tuning and tests the object in order to select the proper method. The algorithm continuously
forces operation in the PID mode. The necessary condition for initiating the PID parameter selection procedure
is that the current measured value must be located outside of the insensitivity zone defined as the sum of the
values of parameter 38:
Pb
and 21:
H1
in relation to the set value 20:
Set1
, as shown in figure 12.5.
Fig. 12.5. Location of the insensitivity
zone for heating (19:
Fun1
=
inv
)
and cooling (19:
Fun1
=
dir
)
In order to avoid unnecessary activation of tuning, which may slow down the process, it is recommended to set
the highest possible value of
H1
, not less than 10÷30% of the range of variability of the process (e.g. the
measured temperature). Testing of the object with temporary switchoff of the output and the
tunE
message
also takes place in the insensitivity band if sudden changes in the measured value or set value are detected.
The choice of the parameter selection method depends on the nature of the initial conditions. In the case of a
stabilized controlled value, the run-up (quick) method will be selected; in other cases, the oscillation method
(slower) will be selected.
Automatic selection enables optimum selection of the PID parameters for the current conditions at the object,
without the user's involvement. It is recommended for variable value regulation (disturbance of the conditions
determined during the operation due to the change of, e.g. the set value or the weight of the furnace batch).
b)
37:
tunE
=
StEP
– selection of parameters in the run-up phase (response to step function). During
determination of the object's characteristics, the algorithm does not cause any additional delay in reaching the
set value. This method is intended specifically for objects of stabilized initial value of the controlled value (e.g.
temperature in a cold furnace). In order to avoid disturbing stabilized initial conditions, before the automatic
tuning is switched on, the power supply of the operating element (e.g. a heater) should be switched off using an
external connector or the control start/stop function should be used (the
[F]
button). The power supply must be
switched on immediately after the tuning is started, in the output switch-on delay phase. If the power supply is
switched on later, an erroneous analysis of the object and improper selection of PID parameters will result.
c)
37:
tunE
=
oSct
– selection of parameters using the oscillation method. The algorithm consists in measuring
the amplitude and the period of oscillation on a slightly lower level (in the case of heating; higher level - in the
case of cooling) than the set value, thus eliminating the risk of exceeding the target value at the object testing
stage. During determination of the object's characteristics, the algorithm causes additional delays in reaching
the set value. This method is intended specifically for objects of unstable initial value of the controlled value (e.g.
temperature in a hot furnace).
