Best way to approach this? (Logix5000)

Hi All,

I'm just looking to hear some different idea's on how I could approach this problem with Logix5000.

I have a vessel with a level transmitter and level controller(PIDE Block) on it. The vessel has an inlet ontop, and an outlet on the bottom, the level control valve is on the outlet line.

With my level controller i'm looking to keep level at x setpoint while dumping out of the bottom. This is all working fine, HOWEVER, this is the part i'm a little confused with. I have a flow transmitter right before the level control valve, what i'm essentially trying to do is while the level controller is dumping, i want to keep the flow rate out of the vessel within a range, i.e 10 to 100 units of flow.

What would be the best way to approach this issue of keeping the flow rate within a range of x to z while the level controller is modulating the level control valve in your opinion?

Many Thanks in advance to anyone who replies with some ideas

Put the PID into manual and force the CV to a value that gives you the maximum flow rate. Do the same for the minimum flow rate. Use those values as upper and lower limits on the PID's CV.

Steve Bailey said:

Put the PID into manual and force the CV to a value that gives you the maximum flow rate. Do the same for the minimum flow rate. Use those values as upper and lower limits on the PID's CV.

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Steve,

I really appreciate the reply and this is the exact reason I posted. I could think of a way to do this but more than anything the reason that drove me to post was to hear other ideas and more importantly better approaches than one I would come up with.

I just had one of those "that clicked" moments after i read your post. Very cool, so simple. IS there any specific name for this method with respect to control?

I'm thinking that absolute CV limits will only work for one set of conditions. If anything changes then the limits change.

Without seeing the system my initial thought is that some form of a low select control will be better. Do you have access to the AB knowledgebase? There is a downloadable example there of a scheme known as a low select loop. The example shows how to sync the seperate PIDE blocks by wiring the selected CV back to the other block's CVPrevious.

TConnolly said:

I'm thinking that absolute CV limits will only work for one set of conditions. If anything changes then the limits change.

Without seeing the system my initial thought is that some form of a low select control will be better. Do you have access to the AB knowledgebase? There is a downloadable example there of a scheme known as a low select loop. The example shows how to sync the seperate PIDE blocks by wiring the selected CV back to the other block's CVPrevious.

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TConnoly,

Those were my thoughts as well but I really do like the idea in general, I would not of thought of that tonight that's for sure.

Correct me if I'm wrong here, which i assume I am, but if i did do a low select, I would have my LIC and a FIC and the FIC would require a fixed setpoint. See the problem is, I can't have a fixed setpoint even if it was in the middle of the flow rate range. What has to happen is that while the level controller is controlling level in the tank, the outlet flow must be any rate within that range.

However i'm pretty sure i've misinterpreted what you were saying so please correct me if i'm wrong. I'm curious though, what is the result of wiring the selected CV to the other PIDE blocks CVPrevious?

Many thanks for the reply

Hi Joel - What about setting it up as a Cascade Contoller, i.e. have a Level PID whose CV Output is the Setpoint input to your Flow PID?

The way I would implement this would be to configure my CVmax/CVmin on my LIC to the ranges you want to limit your flow control to (minus some small deadband). Then the SP on your FIC is tied to the LIC CV, and will modulate your control valve to keep flow within the allowable range in an attempt to keep the level on setpoint.

One thing to keep in mind with this approach is that Loop Tuning becomes a little more difficult. The rule of thumb to follow is that you want your master loop (in this case your LIC) to be tuned about 4 time slower than the slave loop (Flow). If you go this route and want further tuning tips I might be able to give you a little more detail.

Good Luck!

matger said:

Hi Joel - What about setting it up as a Cascade Contoller, i.e. have a Level PID whose CV Output is the Setpoint input to your Flow PID?

The way I would implement this would be to configure my CVmax/CVmin on my LIC to the ranges you want to limit your flow control to (minus some small deadband). Then the SP on your FIC is tied to the LIC CV, and will modulate your control valve to keep flow within the allowable range in an attempt to keep the level on setpoint.

One thing to keep in mind with this approach is that Loop Tuning becomes a little more difficult. The rule of thumb to follow is that you want your master loop (in this case your LIC) to be tuned about 4 time slower than the slave loop (Flow). If you go this route and want further tuning tips I might be able to give you a little more detail.

Good Luck!

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Mat - thank you for taking the time to reply. I believe what you mentioned would actually work in this situation, very cool and good to know for future reference as well. I'm going to setup the cascade tomorrow morning or afternoon and give this a go. If I running into any tuning troubles when the process is live I will give you a message but I think from the tuning advice that you've given me I will be fine.

Glad I posted on here, learned a few things from just a few minutes of time. Thanks to everyone who posted.

Cheers

CVPrevious is the result of the previous CV computation of the PID. When you have a low select you can be in a situation where you are bouncing back and forth between the PID controlling tank leval and the PID controlling maximum flow rate. You want to be able to switch seamlessly between the two loops. Wiring each loop's output back to the other loop's CVPrevious accomplishes that. See the technote in the AB KB about it. In you case you'll need a low select and a high select.

If you want to control flow out of the vessel then you should have the flow transmitter on the discharge side.

The simplest way to bracket the flow is to clamp max and min valve positions. For a given level (head) you can calculate those from the Cv of the valve.