Tuning Installation and Operation Handbook
4.2
AUTOMATIC TUNING
Two automatic tuning procedures are provided in the 2416:
•
A one-shot tuner
which automatically sets up the initial values of the parameters listed in Table 4-1 on the
previous page.
•
Adaptive tuning
which continuously monitors the error from setpoint and modifies the PID values if necessary.
4.2.1
One-shot Tuning
The ‘one-shot’ tuner works by switching the output on and off to induce an oscillation in the measured value. From
the amplitude and period of the oscillation, it calculates the tuning parameter values.
If the process cannot tolerate full heating or cooling being applied during tuning, then the level of heating or
cooling can be restricted by setting the heating and cooling power limits in the ‘
oP
’ list. However, the measured
value
must
oscillate to some degree for the tuner to be able to calculate values.
A One-shot Tune can be performed at any time, but normally it is performed only once during the initial
commissioning of the process. However, if the process under control subsequently becomes unstable (because
its characteristics have changed), you can re-tune again for the new conditions.
It is best to start tuning with the process at ambient temperature. This allows the tuner to calculate more
accurately the low cutback and high cutback values which restrict the amount of overshoot, or undershoot.
4.2.2
How to tune
1. Set the setpoint to the value at which you will normally operate the process.
2. In the ‘
Atun
’ list, select ‘
tunE
’ and set it to ‘
on
’.
3. Press the Page and Scroll buttons together to return to the Home display. The display will flash ‘
tunE
’ to
indicate that tuning is in progress.
4. The controller induces an oscillation in the temperature by first turning the heating on, and then off. The first
cycle is not complete until the measured value has reached the required setpoint.
5. After two cycles of oscillation the tuning is completed and the tuner switches itself off.
6. The controller then calculates the tuning parameters listed in Table 4-1 and resumes normal control action.
If you want ‘Proportional only’, ‘PD’, or ‘PI’ control, you should set the ‘
ti
’ or ‘
td
’ parameters to
OFF
before
commencing the tuning cycle. The tuner will leave them off and will not calculate a value for them.
4.2.3
Typical automatic tuning cycle
4.2.4
Calculation of the cutback values
Low cutback
and
High cutback
are values that restrict the amount of overshoot or undershoot that occurs during
large step changes in temperature (for example, under start-up conditions).
If either low cutback, or high cutback, is set to ‘
Auto
’ the values are fixed at three times the proportional band,
and are not changed during automatic tuning.
4.2.5
Adaptive tune
Adaptive tuning is a background algorithm, which continuously monitors the error from setpoint and analyses the
control response during process disturbances. If the algorithm recognises an oscillatory, or under-damped,
response it recalculates the
Pb
,
ti
and
td
values.
Adaptive tune is triggered whenever the error from setpoint exceeds a trigger level. This trigger level is set in the
parameter ‘
drA.t
’, which is found in the Autotune list. The value is in display units. It is automatically set by the
controller, but can also be manually re-adjusted.
Adaptive tune should be used with:
1. Processes whose characteristics change as a result of changes in the load, or setpoint.
2. Processes that cannot tolerate the oscillation induced by a One-shot tune.
Adaptive tune should not be used:
1. Where the process is subjected to regular external disturbances that could mislead the adaptive tuner.
2. On highly interactive multiloop applications. However, moderately interactive loops, such as multi-zone
extruders, should not give a problem.
Time
Setpoint
Temperature
4-2 HA025041 issue 12 Aug 14
Summary of Contents for XST-2-0-12-1V1-E28
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