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2.13.2 'At Set Point' Auto-tune
If the process variable is within 10% of the set point, 'at set point' tuning is carried out.
•
'At set point' tuning – manipulates the control output to produce a controlled oscillation of the
process.
A step change of ±10% of the starting output value is applied initially. This is adjusted to give an
•
amplitude of oscillation 3 times the noise level.
•
Once the amplitude and period of oscillation are consistent (minimum 2 cycles, maximum 4
cycles) PID parameters are calculated.
•
If no errors exist the controller enters auto mode and begins to control the process using the new
PID parameters.
•
If an error occurs during the auto-tune, the controller reverts to manual mode with the control
output set to the default output value. An error message is displayed in the operator level –
see Table 2.1.
PV
+2%
– 2%
A – Stable process before auto-tune
Note. The time taken to complete auto-tune depends upon the system response time.
Notes For Special Cases.
Cascade Control – the slave loop must be tuned before the master loop. The slave must be
placed into local set point mode (cascade disabled) and the slave set point adjusted to the
required value prior to tuning.
Feedforward Control – during an auto-tune with a controller with feedforward the feedforward
signal is not applied. The feedforward gain and bias values are not changed by the auto-tune and
must be adjusted separately.
Time Proportioning – the cycle time must be set prior to running an auto-tune. The cycle time is
not changed by the auto-tune.
SPT
< ±10 span
Fig 2.12b Typical 'At Set Point' Auto-tune Cycles
2 OPERATOR LEVEL...
PV
t
B – Process response during auto-tune
12 hours max.
SPT
Controlling to
Set Point
Auto-tune
complete
t
31
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