User manual

PID Loop Operation
(DL450 Only)
Maintenance
and Troubleshooting
8--46
PID Loop Operation (DL450 only)
DL405 User Manual, 4th Edition, Rev. A
When the loop tuning observations are c omplete, the loop controller computes Rr
(maximum slope in %/sec.) and Lr (dead time in sec). The auto tune function
computes the gains according to the Zeigler--Nichols equations, shown below:
PID Tuning SP Range
P = 1.2* nm/LrRr P = 0.9* nm/LrRr
I = 2.0* Lr I = 3.33* Lr
D = 0.5* Lr D=0
Sample Rate = 0.056* Lr Sample Rate = 0.12* Lr
nm = Output step change (10% = 0.1, 20% = 0.2)
We highly recommend using DirectSOFT for the auto tuning interface. The duration
of each auto tuning cycle will depend on the mass of the process. A slowly--changing
PV will result in a longer auto tune cycle time. When the auto tuning is complete, the
proportional, integral, and derivative gain values are automatically updated in loop
table locations V+10, V+11, and V+12 respectively. The sample time in V+07 is also
updated automatically. You can test the validity of the values the auto tuning
procedure yields by measuring the closed--loop response of the PV to a step change
in the output. The instructions on how to do this are in the section on the manual
tuning procedure (located prior to this auto tuning section).
Closed--Loop Auto Tuning
During a closed--loop auto tuning cycle the loop controller operates as shown in the
diagram below.
When auto tuning, the loop controller imposes a square wave on the output. Each
transition of the output occurs when the PV value crosses over/under the SP value.
Therefore, the frequency of the limit cycle is roughly proportional to the mass of the
process. From the PV response, the auto tune function calculates the gains and the
sample time. It automatically places the results in the corresponding registers in the
loop table.