7.2.7 Auto Tune at Setpoint - Heat/Cool
It is sometimes necessary to tune at the actual setpoint being used. This is allowable in 800 range controllers and the sequence of operation is described below.
|
Period |
Action |
|
A |
Start of Auto Tune. A test is done at the start of autotune to establish the conditions for a tune at setpoint.
The conditions are that the SP must remain within 0.3% of the range of the controller. Range is defined as ‘High Scale Range Value’ – ‘Low Scale Range Value’ for process inputs or the range defined for temperature inputs. |
|
A to B |
The output is frozen at the current value for one minute and the conditions are continuously monitored during this period. If the conditions are met during this period autotune at setpoint is initiated at B. If at any time during this period the PV drifts outside the condition limits a tune at setpoint is abandoned. Tuning is then resumed as a tune from above or below setpoint depending on which way the PV has drifted.
Since the loop is already at setpoint there is no need to calculate a Tune Control Setpoint – the loop is forced to oscillate around the Target Setpoint |
|
C to G |
Initiate oscillation - the process is forced to oscillate by switching the output between the output limits. From this the period of oscillation and the peak to peak response is measured. PID terms are calculated |
|
G to H |
An extra heat stage is provided and all heating and cooling power is turned off at H allowing the plant to respond naturally.
Measurements made during this period allow the relative cool gain ‘r2G’ to be calculated. |
|
I |
Auto Tune is turned off and the process is allowed to control at the target setpoint using the new control terms. |
For a tune at setpoint Auto Tune does not calculate cutback since there was no initial start-up response to the application of heating or cooling. The exception is that the cutback values will never be returned less than
1.6*PB.