7.2.4 To Start Auto Tune

In operator level 2, set the AUTO-TUNE ENABLE parameter, ‘A.tUn’ to ‘on’.

 

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.

 

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), it may be necessary to tune again for the new conditions.

 

The Auto Tune algorithm reacts in different ways depending on the initial conditions of the plant. The explanations given in this section are for the following conditions:-

 

  1. Initial PV is below the setpoint and, therefore, approaches the setpoint from below for a heat/cool control loop

  2. Initial PV is below the setpoint and, therefore, approaches the setpoint from below for a heat only control loop

  3. Initial PV is at the same value as the setpoint. That is, within 0.3% of the range of the controller. Range is defined as ‘High Scale Range Value’ to ‘Low Scale Range Value’ for process inputs or the range defined in section 5.2.2 for temperature inputs.

 

☻ If the PV is just outside the range stated above the autotune will attempt a tune from above or below SP.

 

☻ If the controller is autotuning and sensor break occurs, the autotune will abort. Autotune must be re-started when the sensor break condition is no longer present.

 

☻ If an Autotune cannot be performed an error message, EtUn, will be flashed in the display (this may take around 2 hours). At the same time the A.tun parameter will show FAIL. It will be necessary to turn Autotune OFF and start again. Autotune will not work if the loop does not respond to changes or, of course, if it is left open.