How can I get symbolic transfer function from block diagram?

Hi,

I have a block diagram of a model, and it is unstable. Due to the nature of the system that I am modelling, I cannot use any controllers to stabilize the system, but rather I need to find which variable (design parameter) is affecting the instability. Currently the model has only one pole at (0,0.000408) that causes the instability. My approach was to see the transfer function in symbolic format so I can see which parameter is contributing to the RHP pole so I can modify them. When I linearize the system, it only gives TF with numbers already calculated.

Any suggestion or method to get TF in symbolic format would be great!

Oh and the system is quite complicated and large, and I couldnt use Mason's rule with any confidence.

[Azzi Abdelmalek]

can you post your model?

Answer 1

[Ryan G]

Unfortunately none of the control design software supports symbolic representations. If you were able to get this done, which I'm not fully sure you could, you would be stuck doing everything by hand.

However, I believe you can use the Compensator Design Tool for this task if you model the system in Simulink. The workflow I would use would be as follows:

1) Select the appropriate I/O 2) Open the tool (as seen in the link above) 3) Select all of the blocks (gains, transfer functions, integrators etc) in the tunable blocks window. This is how you'll end up determining what you want to tune. 4) Click the tune blocks button. Once you get through the prompts, where you'll definately want to see the root locus, you can manually tune any one of those parameters you select in step (3) to see how this effects the design.

5) (optional) If the model instability comes from a transfer function you may have to break it up into components. However, I believe you can utilize steps 3 and 4 to adjust the poles and zero's directly.