Carbolite nanodac - Manual (Page 95)

nanodac™
93
MC27 –EN–1.04
AUTOTUNE (Cont.)
EXAMPLE 3: AUTOTUNE AT SP (HEAT /COOL)
It is sometimes necessary to tune at the actual setpoint being used as shown below.
For a tune at setpoint, autotune does not calculate cutback since there was no initial start up response to
the application of heating or cooling. Cutback values of less than 1.6

PB will not be returned.
Figure 6.4.5c Autotune at setpoint
A Start of Autotune. A test is done at the start of autotune to establish conditions
for a tune at setpoint. Conditions are that SP must remain within 0.3% of the
range of the controller if ‘PB Units’ (
Setup menu ) is set to ‘Percent’, or +1
engineering unit (1 in 1000) if the ‘PB Units’ is set to ‘Eng’. Range is defined as
‘Range High’ – ‘Range Low’ for process inputs or the thermocouple or RTD range
defined in section A3 for temperature inputs.
A to B The output is frozen at he current value for one minute, and the conditions are
continuously monitored during this period. If the conditions specified above are
met, then an autotune at setpoint is initiated at ‘B’. If PV drifts outside the
condition limits at any time during this period, tuning at SP is abandoned, and
tuning resumes as a ‘tune from above’ or ‘tune from below’, depending on the
direction of drift. Since the loop is already at setpoint, a Tune Control setpoint is
not calculated; the loop is forced to oscillate about the Target SP.
C to G The process is forced to oscillate by switching the output between the output
limits. The oscillation period and the peak-to-peak response are determined, and
the PID terms calculated.
G to H An extra heating stage is initiated, then all heating and cooling are switched off at
H, allowing the plant to respond naturally. The relative cool gain (R2G) is
calculated.
I Autotune is switched off and the process is allowed to control at the target
setpoint using the newly calculated terms.
Target SP
High output
Low output
Peak to
Peak
PV
Hysteresis
OP
B – A = 1 minute
Zero output