PID Loop Operation
Maintenance
and Troubleshooting
8–42
PID Loop Operation
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.
Tangent
Rr = Slope
Process Wave
SP
PV
(%)
Base Line
Lr
(sec.)
LrRr
(%)
Time (sec)
Output Value
(%)
Step Change Dm=10%
PID Cycle
Auto Tune Start Auto Tune End
PID Cycle
Auto Tune Cycle
* When Auto Tune starts, step change output
Dm=10%
* 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.
* When PV change is under 2%, output is changed at 20%.
Open Loop Auto Tune Cycle Wave: Step Response Method
When the loop tuning observations are complete, 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 Ziegler-Nichols equations, shown below:
P = 1.2 * Dm/LrRr
I = 2.0 * Lr
D = 0.5 * Lr
PID tuning: PI tuning:
P = 0.9 *
Dm/LrRr
I = 3.33 * Lr
Sample Rate = 0.056 * Lr Sample Rate = 0.12 * Lr
D = 0
Dm = Output step change (10% = 0.1, 20% = 0.2)
We highly recommend using DirectSOFT32 for the auto tuning interface. The
duration of each auto tuning cycle will depend on the mass of our 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 section on auto tuning).
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