Cascade Loop Tuning

Hello Guest,

Did this just start happening? Was there a change to the system? If so, what was it?

The SPLLAG has me a bit concerned as to your problem. It also seems very unlikely that both PID operations would have the same tuning parameters. Find out what SPLLAG means - the manual will have that info. If you can't find it, call Foxboro and let us know what it means. At that point, we can start to sort things out.

I know you are anxious to find the solution to your problem. Registering with CR4 will gain you at least one thing: you will automatically be sent an email the minute someone replies to your post. You don't get this unless you register. Welcome to CR4 and will wait to see your answers to my questions.

Mike

Mikerho,

Thanks for your reply. I will login to CR4 and get access to get email notifications for answers.

SPLLAG is the SetPoint Lead-Lag. Lead works in the PD for damping and the Lag works on the PI for the steady state error.

In the case of using SPLLAG then the gain formula:

Gain = (PB/100) * SPLLAG

If not

Gain = (PB/100)

This is just my opinion based on tuning various loops. Keep in mind all loops are different. Normally I wouldn't use a derivative on the pressure control and .5 is quit a large derivative. What could be happening is when your master temperature controller is interacting with your setpoint on the pressure controller the derivative is then causing the pressure to start swinging.

On your temp control you have a small gain of 250 which is a good starting point. Is your system repeats per minute or minutes per repeat? If it's repeats per minute then the .1 integral is a good place to start, if minutes per repeat then the .1 is way too fast and I would go to a 5 or or even 10 to start with and then tinker with with the numbers from there to see what works best.

I always like to start with a slow loop and speed it up from there. Play with the tuning parameters and get a feel of how the system reacts to the changes. Sometimes it takes days to get a loop to control properly.

As you wrote, it is strange that both PID have the same gains. Depending on the process characteristics, reducing the gains of the master loop should help. There is little risk of doing more harm if it is already hitting the limits. Be careful anyway and make sure that you are staying within safe operating limits. Usually, slow PID are more stable. Once the system is stable, you can verify the slave loop tuning with small step responses then move to the master loop. Use your favorite tuning method.

As a previous responder said, try to figure out the story behind this peculiar tuning. There might be a good reason for this or it might be a mistake. Anyhow, it is important to know what happened to fix this for good.

You say everthing is OK when you put the pressure control in manual.This tells me the load is pretty constant.

If so why do you need derivative action at all?

I would increase the gain of pressure controller and delete derivative action and see.

What is the single critical process parameter needing to be controlled?

There is no point in controlling the pressure as well, if the temperature is the desired control parameter. This is evidenced by the stability of the temperature loop while the pressure loop is in manual.