PID

[oliverq]

How do you implement the values of the input in actual run?

 

[pidtuner]

Hi,

I am guessing you mean how to obtain the input/output data required in the first step of the PID Tuner.

If that is the case, then you need to find where the PID code is executed (if is a PLC then find the PID block). Then record:

• The output of the PID block is what the PID Tuner needs for the input column.
• The input of the PID block (which is the process measurement) is what the PID Tuner needs for the output column.
• The time of the experiment.

As you see input and output are inverted between the PID block and what the Tuner expects.

Now to obtain the experiment data : With the PID block turned off, execute an experiment by making a manual step change on the PID output (what would be the Tuner input) and record all the data of that experiment. If you can make more steps, the better. Try steps up and down if possible. Then load that data into the PID Tuner.

Once you load that data, save your project and share your link here, so I can further assist you.

[farr]

Hi, can you explain how to use the time column? should my time unit be in seconds ? minutes ? and what should be the format of time. if you can give an example that would be great. thanks

[pidtuner]

It should always be in seconds, and as time goes, it should always be increasing. To see an example, simply load the test data included in the tool. See the tutorials as well:

https://www.youtube.com/channel/UCkRD7FztiFOdX50BUsOkcSQ

[farr]

Thanks. I have a very slow process. (PV updates every 15-18 min). I have the open loop test data for around 20 hours. How would you recommend entering that data? and interpreting its results

 

[pidtuner]

Then go and convert your time values to seconds. Then fill up the table in the pidtuner, placing the values on each corresponding column. Once the table is complete, verify the charts display the data as expected to confirm that you introduced the data correctly.

Then click the “Save” button and copy the link that the pidtuner shows you. Paste that link here and then I can help you move forward.

[farr]

I think i got the time correctly converted in excel. Here is the project link.

PV is pressure in kpa 0-105 kpa

output is 0-5% range

two operating ranges for pressure. either 89-92 kpa or 99-101 kpa

https://pidtuner.com/#/wFvcVzelhf

[pidtuner]

Hi, Sadly your data is not correct, these are the issues:

• Your data is undersampled. You need to sample faster so the process output response dynamics due to a step change in the process input are clearly visible.
• Your output process curve does not seem to be correlated to your process output curve. I see no correlation between the input of the process and the output. Are you sure you have selected the correct variables in your data.

Take a look at the sample data provided by the tool, you can see a clear correlation between a step change in the input and a slow transitioning on the output to another level. This is what you should be able to see on your data.

Also make sure your PID is turned off during the experiment and that your process input is exactly what would get out of your PID and the process output is exactly what would get in to the PID.

[farr]

Hi,

The data sampling is limited by the pressure analyzer output time delay. It only analyze the sample every 15-20 min. The results are not like a pressure transmitter, they differ from one reading to another.

All of the data is taken when the master loop (pressure) is in manual and flow loop (slave in cascade). You can see as the pressure loop output (0-5%) increases the pressure loop pv (kpa) increases as well.

The goal here is not to be exactly at the pressure setpoint but to be near it. Under these conditions can a 1st order response be trusted from PID tuner?

 

[pidtuner]

Then your test data is still incorrect, what you need is to do is:

• Wait until the process output has settled in one operating point
• Apply one single step in the process input.
• Wait until the process output settles again into another operating point.

The data you shared is not useful, because you make many small changes in the input and do not let the output settle. What we need is one single input step and record how the output settles from one stable value into another one caused by the input change.

[Author]

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