21
5.7.1
Auto-tuning Page
Information on Initial Conditions.
• 'Start-up' Tuning – the controller is placed in the Manual control mode with the control output value set to give a
stable
process variable at least 10% of the engineering range below the control set point.
• 'At Set Point' Tuning – may be initialized in the automatic mode but the process variable must be close to the
required
set point and stable. The control output must also be stable. However, for best results the Manual
control mode can be
used to stabilize the output and the process value. The output must be adjusted slowly
to allow process response to the
change, to bring the process variable to the required control set point. The
closer the process is to the set point, the more
effective the auto-tuning cycle.
Page Header – Auto-tune.
The
switch can be programmed to jump to this frame (
Set Up Function Keys
Page
,
ADVANCED CONFIGURATION LEVEL
). If the switch is used, the display
reverts to the first frame of the
Operating Page
on leaving this
Page
.
Auto-tune Type
The two tuning facilities (
'Start-up'
and
'At Set point'
) are used to calculate
automatically the Proportional, Integral and Derivative terms required.
'
Start-up
' tuning is used from initial start-up or when there is a large change in
set point value.
'At Set point'
tuning is used when the process is close to the required set point.
Select the auto-tune type required.
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 auto-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 auto-
tuning to operate correctly. In practice, use as large a step size as can be
tolerated.
'At Set Point' Tuning
– the controller output changes by plus and minus the
output step size from its initial starting value when auto-tuning is executed. If
the output step size is too large to allow this its value is reduced,
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 crossing either of the auto-tune limits (see following frames).
Set the step size required.
Continued on next page.
tUNE
At–tYP
AUtO
StrtUP
StrtUP
At SPt
StEP
50
50
0
At
Set point
or
100
0
Start-up
5 CONTROL OPERATION…
Summary of Contents for COMMANDER 1900 Series
Page 42: ...40 NOTES ...
