User manual

PID Loop Operation
(DL450 Only)
Maintenance
8--47
PID Loop Operation (DL450 only)
DL405 User Manual, 4th Edition, Rev. A
The following timing diagram shows the events which occur in the closed--loop auto
tuning cycle. The auto tune function examines the direction of the offset of the PV
from the SP. The auto tune function then takes control of the control output and
induces a full--span step change in the opposite direction. Each time the sign of the
error (SP y PV) changes, the output changes full--span in the opposite direction.
This proceeds through three full cycles.
*Mmax = Output Value upper limit setting. Mmin = Output Value lower limit setting.
* This example is direct--acting.
When set to reverse --acting, the output will be inverted. When the loop tuning
observations are complete, the loop controller c omputes T
o
(bump period) and X
o
(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 Zeigler--Nichols
equations shown below:
Kpc = 4M / (π £ X0) Tpc = 0
M = Amplitude of output
PID Tuning PI Tuning
P = 0.45 £ Kpc P = 0.30 £ Kpc
I = 0.60 £ Tpc I = 1.00 £ Tpc
D = 0.10 £ Tpc D=0
Sample Rate = 0.014 £ Tpc Sample Rate = 0.03 £ Tpc
Auto Tuning Error
In open--loop tuning, if the auto tune error bit (bit 13 of loop Mode/Alarm s tatus word
V+06) is on, please v erify the PV and SP values are at least 5% of full scale
difference, as required by the auto tune function.
NOTE: If your PV fluctuates rapidly, you probably need to create a filter in ladder
logic (see example on page 8--54).