22
…2 OPERATOR LEVEL
Note 1. The time taken to complete auto-tune depends upon the system response time.
Note 2. Time Proportioning – the cycle time must be set prior to running an auto-tune. The
cycle time is not changed by the auto-tune.
2.8.2 '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.
t
SPT
PV
Auto-tune
complete
Controlling to
Set Point
B – Process response during auto-tune
+2%
– 2%
t
PV
A – Stable process before auto-tune
SPT
< ±10 span
12 hours max.
Fig 2.7b Typical ‘At Set Point’ Auto-tune Cycles
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