error designing model in Model Predictive Control

Good morning,

I´m trying to define MPC Structure by linearization. I would be grateful if you could tell me how to resolve this error message " plant model cannot be a pure direct feed-through system without additional dynamics" . I have tried using delay blocks as suggested in some posts, but I cannot get it to work. Thank you, best regards.

[Sam Chak]

The error implies that your Plant model is a pure direct feed-through system.

Please clarify if this is true.

[Pablo Horrillo Quintero]

Thanks for your answer.

Yes, that is. I don´t know how to create a small delay in the feed-through system. I have tried with delay blocks or a transfer function in series with the manipulated variables.

[Sam Chak]

Hi @Pablo Horrillo Quintero

I'm unsure if I understand correctly. The MPC algorithm does not work on a very pure direct feed-through system. Adding a delay in the direct feed-through system is not really pure dynamics.

Moreover, if your Plant is a pure direct feed-through system, then you don't need MPC or any other dynamics-based controllers.

In fact, I have never seen a pure direct feed-through system. Perhaps, you can give an example of a direct feed-through Plant.

[Pablo Horrillo Quintero]

Hello Sam,

I'm starting with MPC. Looking in Model Predictive Control Guide Toolbox - Guetting Started - I found this:

My plant is a PV system with batteries. I`m trying to use MPC to obtain the adequate battery power (manipulated variables) from State of Charge (SOC) levels and power references (measured outputs).

Thanks so much for your answer.

[Sam Chak]

Though I'm not familiar with batteries but I remember that the event of charging and discharging of batteries are dynamical processes.

Do you have the transfer function to describe the SOC of the battery?

If your battery model is in State-space, then you have a non-zero "Direct" feed-through matrrix D.

[Pablo Horrillo Quintero]

Sorry, I don´t have the transfer function.

My battery model is a Lithium-Ion battery from electrical/specialized power system/sources lybrary. I have two questions.

• I'm using a PWM modulation technique, ¿is there a problem between PWM modulation and MPC?
• What should be the correct state for the battery?

The use of MPC is only to obtain the power of each battery (there are 3 PV modules and 3 batteries) from the SOC and the power references. Then, the system uses the battery power and the modulation to obtain the gates of the IGBTs for the inverter. I don´t know if this configuration is possible.

Thank you.

Answer 1

[Sayan]

Hi @Pablo Horrillo Quintero,

I understand from the error that your "Plant" model used in the MPC structure in MPC Designer is a pure direct feed-through. The possible reasons for the error and their possible fixes are mentioned below.

• Ensure the "Plant" model used in your model does not contain any direct feed-through between input and output.
• You can try introducing additional intermediary states to the "Plant" model to eliminate direct feed-through.
• Use the "Debug" tool in Simulink to find any error present in the "Plant" model.
• Test the plant model by running it independently to find any errors or warnings in the "Diagnostic Viewer" and resolve them before reconfiguring the MPC designer with the updated model.

You can go through the following MATLAB documentation about the "Algebraic Loop" concept to remove direct feed-through in your model.

https://www.mathworks.com/help/simulink/ug/algebraic-loops.html

Hope this helps in resolving the error.