Carbolite nanodac - Manual (Page 93)

nanodac™
91
MC27 –EN–1.04
AUTOTUNE (Cont.)
EXAMPLE 1: AUTOTUNE FROM BELOW SP (HEAT/COOL)
The point at which Automatic tuning is performed (Tune Control Point) lies just below the setpoint at
which the process is normally expected to operate (Target Setpoint). This ensures 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 after a 1 minute settling period (point ‘B’ in the figure below).
Examples:
If Target Setpoint = 500°C and Initial PV = 20°C, then the Tune Control Point is 380°C.
If Target Setpoint = 500°C and Initial PV = 400°C, then the Tune Control Point is 475°C.
This is because the overshoot is likely to be less as the process temperature approaches the target
setpoint.
Figure 6.4.5a shows the auto tune sequence.
Figure 6.4.5a Autotune heat/cool process
KEY
A Start of Autotune
A to B Heating and Cooling off for one minute allows steady state conditions to be
established.
B to D First heat/cool cycle to establish first overshoot. Cutback low (CBL) value
calculated from the overshoot magnitude (unless CBL set to ‘Auto’).
B to F Two cycles of oscillation allow peak-to-peak value and oscillation period to be
determined. PID terms are calculated.
F Heating is switched on.
G Heating (and cooling) are switched off allowing the plant to respond naturally.
Measurements over the period F to G are used to calculate the Relative Cool Gain
(R2G). Cutback High is calculated from the equation (CBH = CBL

R2G).
H Autotune is turned off and the process is allowed to control at the target setpoint
using the new control terms.
Note: Autotuning from above SP is identical except that heating and cooling are reversed.
Target SP
True control point
High output
Zero output
Low output
First
overshoot
Peak to
Peak
It is not guaranteed that
PV will not exceed SP
PV
Hysteresis
OP
B – A = 1 minute