User manual

PID Loop Operation
(DL450 Only)
Maintenance
and Troubleshooting
8--40
PID Loop Operation (DL450 only)
DL405 User Manual, 4th Edition, Rev. A
PID Loop Tuning
Once you have set up a PID loop, it must be tuned in order for it to work. The goal of
loop tuning is to adjust the loop gains so the loop has optimal performance in
dynamic conditions. The quality of a loop’s performance may generally be judged by
how well the PV follows the SP after a SP step change. It is important to keep in mind
that understanding the process is fundamental to getting a well designed control
loop. Sensors must be in appropriate locations and valves must be sized correctly
with appropriate trim. PID control does not have typical values. There isn’t one
control process that is identical to another.
Manual Tuning vs. Auto Tuning
You may enter the PID gain v alues to tune your loops (manual tuning), or you can
rely on the PID processing “engine“ in the CPU to automatically calculate the gain
values (auto tuning). Most experienced process engineers will have a favorite
method; the DL450 will accommodate either preference. The use of auto tuning can
eliminate much of the trial--and--error of the manual tuning approach, especially if
you do not have a lot of loop tuning experience. However, performing the auto tuning
procedure will get the gains close tooptimal values, but additional manual tuning can
get the gain values to their optimal values.
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 tune 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.
Whether you use manual or auto tuning, it is very important to verify basic
characteristics of a newly--installed process before attempting to tune it. With the
loop in Manual Mode, verify the following items for each new loop.
S Setpoint y verify that the SP source can generate a setpoint. Put the
PLC in Run Mode and leave the loop in Manual Mode, then monitor the
loop table location V+02 to see the SP value(s). (If you are using the
ramp/soak generator, test it now).
S Process Variable y verify that the PV value is an accurate
measurement, and the PV data arriving in the loop table location V+03
is correct. If the PV signal is very noisy, c onsider filtering the input either
through hardware (RC low--pass filter), or using the filter in this chapter.
S Control Output y if it is safe to do so, manually change the output a
small amount (perhaps 10%) and observe its affect on the process
variable. Verify the process is direct--acting or reverse acting, and check
the setting for the control output (inverted or non--inverted). Make sure
the control output upper and lower limits are not equal to each other.
S Sample Rate y while operating open--loop, this is a good time to find
the ideal sample rate (see Configure the PID Loop beginning on page
8--25). However, if you are going to use auto tuning, the auto tuning
procedure will automatically calculate the sample rate in addition to the
PID gains.
Open--Loop Test