Pid Loop Tuning; Open-Loop Test - Automation Direct DL05 User Manual

Chapter 8: PID Loop Operation

PID Loop Tuning

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Once you have set up a PID loop, it must be tuned in order for it to work. The goal of loop
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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
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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
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values. There isn't one control process that is identical to another.
Manual Tuning vs. Auto Tuning
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You may enter the PID gain values 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 DL05 will
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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
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experience. However, performing the auto tuning procedure will get the gains close to
optimal values, but additional manual tuning can get the gain values to their optimal values.
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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
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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 be used as a replacement for your process
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knowledge.

Open-Loop Test

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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
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the following items for each new loop.
• Setpoint – verify that the SP source can generate a setpoint. Put the PLC in Run Mode and leave
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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).
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• Process Variable – 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, consider filtering the input
either through hardware (RC low-pass filter), or using the filter in this chapter.
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• Control Output – 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
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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.
• Sample Rate – while operating open-loop, this is a good time to find the ideal sample rate
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(procedure given earlier in this chapter). 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.
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DL05 Micro PLC User Manual, 6th Edition, Rev. A