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User Manual 3200 Series
64 Part No HA028651 Issue 11.0 July-12
11.3.5 Autotune from Below SP – Heat/Cool
The point at which Automatic tuning is performed (Tune Control Point) is designed to operate just below the setpoint at
which the process is normally expected to operate (Target Setpoint). This is to ensure that the process is not significantly
overheated or overcooled. The Tune Control Point is calculated as follows:-
Tune Control Point = Initial PV + 0.75(Target Setpoint – Initial PV).
The Initial PV is the PV measured at ‘B’ (after a 1 minute settling period)
Examples: If Target Setpoint = 500
O
C and Initial PV = 20
O
C, then the Tune Control Point will be 380
O
C.
If Target Setpoint = 500
O
C and Initial PV = 400
O
C, then the Tune Control Point will be 475
O
C.
This is because the overshoot is likely to be less as the process temperature is already getting close to the target setpoint.
The sequence of operation for a tune from below setpoint for a heat/cool control loop is described below:-
Period Action
A Start of Autotune
A to B Both heating and cooling power remains off for a period of 1 minute to allow the algorithm to establish steady state
conditions.
B to D First heat/cool cycle to establish first overshoot.
‘CBLO’ is calculated on the basis of the size of this overshoot (assuming it is not set to Auto in the initial conditions).
B to F Two cycles of oscillation are produced from which the peak to peak response and the true period of oscillation are
measured. PID terms are calculated
F to G An extra heat stage is provided and all heating and cooling power is turned off at G allowing the plant to respond
naturally.
Measurements made during this period allow the relative cool gain ‘R2G’ to be calculated.
‘CBHI’ is calculated from CBLO*R2G.
H Autotune is turned off at and the process is allowed to control at the target setpoint using the new control terms.
Autotune can also occur when the initial PV is above SP. The sequence is the same as tuning from below setpoint except
that the sequence begins with full cooling applied at ‘B’ after the first one minute settling time.
E F C D
A – B = 1 min.
G H
First
overshoot
Peak
to
Peak
Low Output
Zero Output
High Output
Target Setpoint
Tune Control Point
Hysteresis
H - End of
Autotune
A - Start of
Autotune
B