Marathon Computer AACC 2000 manual One-shot Tuning, How to tune

Marathon Monitors Inc.

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.

AACC 2000 Carbon 54 Nov. 1, 1997