System identification for open loop unstable plant?

[adamkse]

Hello,

I have been trying to use system identification on labview. My problem is that my plant is open loop unstable and none of the tutorials or examples I have found seem to cover this so I cannot get it to work. Does anyone have any suggestions as to how I should go about it? My plant has an RC servo so it needs a PWM signal to stimulate it.

Thanks

Adam

[Ray.R]

Can you provide more details?  I don't quite follow what you are trying to do...

[adamkse]

I want to control my plant with lead/lag control but I do not know the plant model which makes this very difficult. I cannot theoretically calculate it because the information I need to obtain a numerical transfer function or state space model is unavailable.

I have it controlled with PID at the moment but this is not ideal.

I am trying to use the system identification toolkit to obtain a transfer function but I do not know what to do because the examples do not work for open loop unstable plants.

Adam

[laiwei]

There is a tutorial about closed-loop system id, please see http://zone.ni.com/devzone/cda/tut/p/id/4321 to get some ideas.

[JÞB]

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adamkse wrote:

I want to control my plant with lead/lag control but I do not know the plant model which makes this very difficult. I cannot theoretically calculate it because the information I need to obtain a numerical transfer function or state space model is unavailable.

I have it controlled with PID at the moment but this is not ideal.

I am trying to use the system identification toolkit to obtain a transfer function but I do not know what to do because the examples do not work for open loop unstable plants.

Adam

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Ok Open loop unstable functions cannot be predictably controlled (hence the terms; open loop and unstable,)  Not being able to model the actions is a bit of a problem too.

Somehow, someway you need to get some kind of model or some kind of feedback to either provide stability in the plant or close the loop.

Adding hardware (sensors) is the most likely solution.

tell me more about the plant, I do this kind of thing for a living, so I could offer some ideas. 

[Ray.R]

Thanks Jeff,

You took the words right out of my mouth...  Except for your last sentence..  😉

Please tell us more about the plant so that we can have a better understanding of what you are dealing with.

R

[JÞB]

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Ray.R wrote:

Thanks Jeff,

You took the words right out of my mouth...  Except for your last sentence..  😉

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The "Hardly Working" line-  Yeah- It is a lie

[adamkse]

Hey Guys,

Thanks for the fast replies. I have the plant stabilized with a PID controller at the moment but want to move on to better controllers. I tried the closed loop identification technique but my results do not simulate a stable response. 

My plant is the ball and beam control problem which you can find many links for on the internet that can explain it better than I can. 

I have a sensor and a negative unity feedback loop at the moment.

[Ray.R]

Curious:  Is this some sort of homework?

[adamkse]

Yes, its a project. Although the system identification is not specified part of it. I just want to use it so that I can obtain a model. I have derived the symbolic transfer function but the parameters I need to know are unavailable to me.