PID Loop Operation
Maintenance
and Troubleshooting
8–44
PID Loop Operation
When the loop tuning observations are complete, the loop controller computes To
(bump period) and Xo (amplitude of the PV). Then it uses these values to compute
Kpc (sensitive limit) and Tpc (period limit). From these values, the loop controller
auto tune function computes the PID gains and the sample rate according to the
Ziegler-Nichols equations shown below:
P = 0.45 * Kpc
I = 0.60 * Tpc
D = 0.10 * Tpc
PID tuning: PI tuning:
P = 0.30 *Kpc
I = 1.00 * Tpc
Sample Rate = 0.014 * Tpc Sample Rate = 0.03 * Tpc
D = 0
Kpc = 4M / (Ï€ * Xo) Tpc = 0
M = amplitude of output
Auto tuning error – if the auto tune error bit (bit 13 of Loop Mode and Alarm status
word V+06) is on, please verify the PV and SP values are within 5% of full scale
difference, as required by the auto tune function. The bit will also turn on if the
closed-loop method is in use, and the output goes to the limits of the range.
NOTE: If your PV fluctuates rapidly, you probably need to use the built-in analog filter
(see page 8–45) or create a filter in ladder logic (see example on page 8–46).
In tuning cascaded loops, we will need to de-couple the cascade relationship and
tune the loops individually, using one of the loop tuning procedures previously
covered.
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 loop PV is actually the overall
response of the cascaded loops together.
Tuning
Cascaded Loops
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