Carbolite nanodac - Manual (Page 91)

nanodac™
89
MC27 –EN–1.04
10. Appendix B TUNING CONSIDERATIONS
If a process includes adjacent interactive zones, each zone should be tuned independently with the
adjacent zones at operating temperature.
It is recommended that a tuning process be initiated when the PV and setpoint are far apart. This allows
start up conditions to be measured and cutback values to be calculated more accurately. Cutback is not
set for ‘Tune at setpoint’.
In a programmer/controller tuning should only be attempted during dwell periods and not during ramp
stages. If a programmer/controller is tuned automatically the controller should be placed in ‘Hold’ during
each dwell period whilst autotune is active.
Note: Tuning, carried out in dwell periods which are at different extremes of temperature may
give different results owing to non linearity of heating (or cooling). This may provide a convenient
way to establish values for Gain Scheduling.
If an auto tune is initiated there are two further parameters (High Output’ and ‘Low Output’) which need
to be set. These are found in the ‘
Tune ’ menu.
High Output Sets a high output limit to be imposed during autotune. Must be

Output High,
set in the Output menu.
Low Output Sets a low output limit to be imposed during autotune. Must be

Output Low, set
in the Output menu.
The above values must be set correctly, otherwise sufficient power to achieve SP might not be available
during tuning, and the tune will eventually fail.
10.1 Autotune
Autotune automatically sets the following PID menu parameters:
PB Proportional band.
Ti Integral time. If previously set to ‘Off’ Ti will remain off after an autotune.
Td Derivative time. If previously set to ‘Off’ Td will remain off after an autotune.
CBH, CBL Cutback high and low values. If either is set to ‘Auto’, it will remain so after auto
tuning. In order that Autotune set the cutback values for the user, a value other
than ‘Auto’ must be selected before Autotune is initiated. Autotune never returns
cutback values less than 1.6

PB
R2G Calculated only if the unit is configured as Heat/Cool. Following an Autotune, R2G
lies between 0.1 and 10. If the calculated value lies outside this range, a ‘Tune Fail’
alarm is set.
LBT Loop break time. Following an autotune, LBT is set to 2

Ti (if Ti was not
previously set ‘Off’), or to 12

Td (if Ti was previously set to ‘Off’).