User manual

PID Loop Operation
(DL450 Only)
Maintenance
8--45
PID Loop Operation (DL450 only)
DL405 User Manual, 4th Edition, Rev. A
Open--Loop Auto Tuning
During an open--loop auto tuning cycle, the loop controller operates as shown in the
diagram below. Before starting this procedure, place the loop in Manual Mode and
ensure the PV and control output values are i n the middle of their ranges (away from
the end points).
NOTE: In theory, the SP value does not matter in this case, because the loop is not
closed. However, the requirement of the firmware is that the SP value must be more
than 5% of the PV range from the actual PV before starting the auto tune cycle (for
the DL450, 12 bit PV should be 205 counts or more below the SP for forward--acting
loops, or 205 counts or more above the SP for reverse--acting loops).
When auto tuning, the loop controller induces a step change on the output and
simply observes the response of the PV. From the PV response, the auto tune
function calculates the gains and the s ample time. It automatically places the results
in the corresponding registers in the loop table.
The following timing diagram shows the events which occur in the open--loop auto
tuning cycle. The auto tune function takes control of the control output and induces a
10%--of--span step change. If the PV change which the loop controller observes is
less than 2%, then the step change on the output is increased to 20%--of--span.
S When Auto Tune starts, s tep c hange output m = 10%
S During Auto Tune, the controller output reached the full scale positive
limit. Auto Tune stopped and the Auto Tune Error bit in the Alarm word
bit turned on.
S When PV change is under 2%, output is changed at 20%. Open Loop
Auto Tune Cycle Wave: Step Response Method.