Super Systems 3 Series - Manual (Page 53)

Series 3 Operations Manual
53
11.3.3 Automatic Tuning
Auto Tune automatically sets the following parameters:
Proportional Band ‘PB’
Integral Time ‘Ti’ If ‘Ti’ and/or ‘Td’ is set to OFF, because you wish to use PI, PD or P only control, these terms will
remain off after an autotune.
Derivative Time ‘Td’
Cutback High ‘CBHI’ If CBH and/or CBL is set to ‘Auto’ these terms will remain at Auto after an autotune, i.e. 3*PB.
For autotune to set the cutback values, CBHI and CBLO must be set to a value (other than Auto)
before autotune is started.
Autotune will never return cutback values which are less than 1.6*PB.
Cutback Low ‘CBLO’
Relative Cool Gain ‘R2G’ R2G is only calculated if the controller is configured as heat/cool.
Following an autotune, ‘R2G’ is always limited to between 0.1 and 10. If the calculated value is
outside this limit a ‘Tune Fail’ alarm is given.
Loop Break Time ‘LBT’ Following an autotune, ‘LBT’ is set to 2*Ti (assuming the integral time is not set to OFF). If ‘Ti’
is set to OFF then ‘LBT’ is set to 12*Td.
Auto tune uses the ‘one-shot’ tuner which works by switching the output on and off to induce an oscillation in the process value.
From the amplitude and period of the oscillation, it calculates the tuning parameter values. The autotune sequence for different
conditions is described in section 11.
11.3.4 To Start Autotune
In operator levels 2 or 3, set the ‘AUTO-TUNE ENABLE’ parameter 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 if ‘PB.UNT’ is set to
‘percent’ or +1 engineering unit (1 in 1000) if the ‘PB.UNT’ is set to ‘Eng’. Range is defined as ‘Range High Limit’ to
‘Range Low Limit’ for process inputs or the range defined for temperature inputs.
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If the PV is just outside the range stated above the autotune will attempt a tune from above or below SP.
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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.
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If an Autotune cannot be performed an error message, Etun, will be flashed in the display