PID Loop Operation
Maintenance
and Troubleshooting
8–40
PID Loop Operation
S Over-damped response – the gains are too small, so gradually increase
them, concentrating on the proportional gain first.
S Under-damped response – the gains are too large. Reduce the integral
gain first, and then the proportional gain if necessary.
S Critically-damped response – this is the the optimal gain setting. You
can verify that this is the best response by increasing the proportional
gain slightly. the loop then should make one or two small oscillations.
PV
10% of
SP range
SP
over-damped response
critically-damped response
under-damped response
Now you may want to add a little derivative gain to further improve the
critically-damped response above. Note the proportional and integral gains will be
very close to their final values at this point. Adding some derivative action will allow
you to increase the proportional gain slightly without causing loop oscillations. The
derivative action tends to tame the proportional response slightly, so adjust these
gains together.
Autotuning is initiated within DirectSOFT32. You can use autotuning to establish
initial PID parameter values (autotuning is not run continuously during operation).
Whenever a substantial change in loop dynamics occurs (mass of process, size of
actuator, etc.), you will need to repeat the tuning procedure to derive the new gains
that are 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 DL05 is not intended to perform as a replacement for your process knowledge.
The loop controller offers both closed-loop and open-loop methods. If you intend to
use the auto tune feature, we recommend you use the open-loop method first. This
will permit you to use the closed-loop method of auto tuning when the loop is
operational (Auto Mode) and cannot be shut down (Manual Mode). The following
sections describe how to use the auto tuning feature, and what occurs in open and
closed-loop auto tuning.
Auto Tuning
Procedure
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