7.2.5 Auto Tune 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 = 500OC and Initial PV = 20OC, then the Tune Control Point will be 380OC.

 

If Target Setpoint = 500OC and Initial PV = 400OC, then the Tune Control Point will be 475OC.

 

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.