Carbolite nanodac - Manual (Page 97)

nanodac™
95
MC27 –EN–1.04
AUTOTUNE (Cont.)
FAILURE MODES
The conditions for performing an autotune are monitored by the Tune menu parameter ‘State’. If
autotune is not successful error conditions are read by this parameter as follows:
Timeout Set if any one stage is not completed within an hour. Possible causes are the loop
being open circuit, or not responding to the controller demands. Some heavily
lagged systems may produce a timeout if the cooling rate is very slow.
TI Limit This is set if Autotune calculates a value for the integral term which is greater than
the maximum allowable (99999 seconds). This indicates that the loop is not
responding or that the tune is taking too long.
R2G Limit Error occurs if the calculate value of R2G is outside the range 0.1 to 10.0. R2G limit
can occur if the gain difference between heating and cooling is too large, or if the
controller is configured for heat/cool, but the heating and/or cooling device is
turned off or not working correctly.
10.1.1 Relative Cool Gain in Well Lagged Processes
In the majority of processes Relative Cool Gain R2G is calculated by the autotune algorithm as described
in the previous sections.
There are occasions, however, where an alternative algorithm may be preferred. These are processes
which are heavily lagged, where the heat loss to ambient is very small so that natural cooling is extremely
slow, and certain high order plants, those that need derivative, Td. This algorithm is known as R2GPD and
has been added to controllers from firmware version V4.10.
The type of algorithm is selected using the parameter 'Tune R2G' found in the Auto-Tune list. The choices
are:-
Standard This is the default as described in example 2 in section B2.4.5. and is suitable for
use on most processes. The benefit of this algorithm is that it is relatively quick.
However, in the type of process described in the previous paragraph, it can
produce values which are not ideal. These values are generally identified by R2G
equal to or very close to 0.1.
R2GPD If the process is known to be heavily lagged or produces values such as those above
then R2GPD should be selected. This algorithm extends the autotune period by
putting the controller into proportional plus derivative mode (PD) and uses the
output power demand value during this period to determine the Relative Cool
Gain.
Off The automatic calculation of Relative Cool Gain can be turned off and the value
entered manually as described in section B2.4.6.