Thermcraft XST-6-0-36-3V - Manual (Page 76)

Tuning Installation and Operation Handbook
4-2 2408 and 2404 Controller
AUTOMATIC TUNING
Two automatic tuning methods are provided in the 2408 and 2404:
• 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 temperature. 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 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.