User manual

PID Loop Operation
(DL450 Only)
Maintenance
and Troubleshooting
8--44
PID Loop Operation (DL450 only)
DL405 User Manual, 4th Edition, Rev. A
The auto tuning feature for the DL450 loop controller will only run once each time it is
enabled in the PID table. Therefore, auto tuning does not run continuously during
operation (this would be adaptive control). Whenever there is a substantial c hange
in loop dynamics, such as mass of process, size of actuator, etc., the tuning process
will need to be repeated in order to derive new gains required for optimal control.
WARNING: Only authorized personnel fully familiar with all aspects of the
process should make changes that affect the loop tuning constants. Using the
loop auto tuning procedures will affect the process, including inducing large
changes in the control output value. Make sure you thoroughly consider the
impact of any changes to minimize the risk of injury to personnel or damage to
equipment. The auto tune in the DL450 is not intended to be used as a
replacement for your process knowledge.
Once the physical loop components are connected to the PLC, auto tuning can be
initiated within DirectSOFT (see the DirectSOFT Programming Software Manual),
and it can be used to establish initial PID parameter values. Auto tuning is the best
“guess“ the CPU can do after some trial tests.
The loop controller offers both closed--loop and open--loop methods. The following
sections describe how to use the auto tuning feature, and what occurs in open and
closed--loop auto tuning.
The controls for the auto tuning function use three bits in the PID Mode 2 word V+01,
as shown below. DirectSOFT will manipulate these bits automatically when you use
the auto tune feature within DirectSOFT. Or, you may have your ladder logic access
these bits directly for allowing control from another source such as a dedicated
operator interface. The individual control bits allow 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) during the auto tune cycle, automatically returning to off (0) when
done.
Auto Tuning
Procedure