PID Loop Operation
Maintenance
8–41
PID Loop Operation
The controls for the auto tuning function use three bits in the PID Mode 2 word V+01,
as shown below. DirectSOFT32 will manipulate these bits automatically when you
use the auto tune feature within DirectSOFT. Or, you may have ladder logic access
these bits directly for allowing control from another source such as a dedicated
operator interface. The individual control bits let you to start the auto tune procedure,
select PID or PI tuning, and select closed-loop or open-loop tuning. If you select PI
tuning, the auto tune procedure leaves the derivative gain at 0. The Loop Mode and
Alarm Status word V+06 reports the auto tune status as shown. Bit 12 will be on (1)
when during the auto tuning cycle, automatically returning to off (0) when done.
PID Mode 2 Setting V+01
013456789101112131415 2Bit
Auto Tune Function
Auto Tuning
Controls
0=closed loop,
1=open loop
0=PID tuning,
1=open PI tuning
Start Auto Tune
(0 to 1 transition)
013456789101112131415 2Bit
Loop Mode and Alarm Status V+06
Auto Tune
Active
Auto Tune
Error
Auto Tuning
Status
Open-Loop Auto Tuning – During an open-loop auto tuning cycle, the loop
controller operates as shown in the diagram below. Before starting this procedure,
place the loop in Manual mode and ensure the PV and control output values are in
the middle of their ranges (away from the end points).
Process Variable
Loop
Calculation
Manufacturing
Process
Setpoint Value
Control
Output
S
Error Term
+
–
PLC System
Open Loop
Auto Tuning
Step Function
Process Variable
Response
NOTE: In theory, the SP value does not matter in this case, because the loop is not
closed. However, the firmware requires that the SP value be more than 205 counts
away from the PV value before starting the auto tune cycle (205 counts or more
below the SP for forward-acting loops, or 205 counts or more above the SP for
reverse-acting loops).
When auto tuning, the loop controller induces a step change on the output and
simply observes the response of the PV. 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.
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