7.2.3 Automatic Tuning
Auto Tune automatically sets the following parameters:-
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Proportional Band ‘PB’ |
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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. |
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Derivative Time ‘Td’ |
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Cutback High ‘CBHi’ |
If CBHi and/or CBLo 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. |
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Cutback Low ‘CBLo’ |
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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. |
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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 sections 7.2.5 to 7.2.7.