17
…6.2
Self-tuning Page
Self tune Type
Select the Self-tune type required:
AtSPt
(At Set Point) use when the process is
close to the required set point.
StrtUP
(Start-up) use for self-tune from initial start-
up, or for self-tune when there is a large
change in set point value.
Output Step Size
The output step size is a percentage of the control output.
'Start-up' Tuning – the larger the step size used the quicker
the self-tuning process is performed, but the greater the
overshoot (above the calculated trip point). If too small a step
size is used, the response may be too slow for the self-tuning
to operate correctly. In practice, use as large a step size as
can be tolerated up to a maximum of 100%.
'At Set Point' Tuning – the controller output changes by plus
and minus the output step size from its initial starting value
when self-tuning is executed. If the output step size is too
large to allow this its value is reduced. Maximum is 50%.
Example – If the controller output value = 30% and the
selected step size = 50%. The step size is reduced to 30%.
The step size should be large enough so that the amplitude
of the process variable excursions are at least four times
larger than the hysteresis parameter to allow the best
possible response data. The output step size must be
small enough to avoid the process variable crossing either
of the self-tune limits (see following frames).
Hysteresis Value
The hysteresis value is entered in engineering units and is
used to determine when to change the controller output
value.
'At Set Point' Tuning only – when the process variable
crosses the hysteresis band (plus and minus the
hysteresis value), the controller output changes by plus
and minus the
StEP
value from its initial starting value.
The hysteresis value should be set as small as possible
but larger than the noise in the process variable signal, to
allow the best possible response data.
Set the hysteresis value required (in engineering units).
Continued on next page
6
TUNING LEVEL…
StrtUP
HYSt
10
StEP
100.0
St-tYP
At SPt
StrtuP
