Marathon Computer AACC 2000 - Manual (Page 54)

Marathon Monitors Inc.
AACC 2000 Carbon Nov. 1, 1997
54
AUTOMATIC TUNING
Two automatic tuning methods are provided in the AACC 2000:
•
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.
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 process variable. This allows the
tuner to calculate more accurately the low cutback and high cutback values which restrict
the amount of overshoot, or undershoot.
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 process variable 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.