User manual

PID Loop Operation
(DL450 Only)
Maintenance
8--65
PID Loop Operation (DL450 only)
DL405 User Manual, 4th Edition, Rev. A
When tuning cascaded loops, you will need to de--couple the cascade relationship
and tune the minorloop, using one of the loop tuningprocedures previously covered.
Once this has been done, have the minor loop in cascade mode and auto tune the
major loop (see Step 4).
1. If you are not using auto tuning, then find the loop sample rate for the minor
loop, using the method discussed earlier in this chapter. Then set the sample
rate of the major loop slower than the minor loop by a factor of 10. Use this as a
starting point.
2. Tune the minor loop first. Leave the major loop in Manual Mode, and you will
need to generate SP changes for the minor loop manually as described in the
loop tuning procedure.
3. Verify the minor loop gives a critically--damped response to a 10% SP change
while in Auto Mode. Then we are finished tuning the minor loop.
4. In this step, you will need to get the minor loop in Cascade Mode, and then the
Major loop in Auto Mode. We will be tuning the major loop with the minor loop
treated as a series component its overall process. Therefore, do not go back and
tune the minor loop again while tuning the major loop.
5. Tune the major loop, following the standard loop tuning procedure in this
section. The response of the major l oop PV is actually the overall response of the
cascaded loops together.
Tuning Cascaded
Loops