Jacobians using Simscape (numerical or symbolic)

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ENRICO
ENRICO 2024-8-18
评论： ENRICO 2024-8-24
I'm working with a Simscape model and I'm trying to get a numerical or possibly analytical form Jacobian for my system (given specific frames).
Actually I'm using multibody, but problem could be of general interest. Better if using geometrical approach. I know that some toolbox already perform same operation and give it using geometrical approch.
I tought using a numerical differentiation would be an answer, but is by far too complex, heavy and clumsy to carrry out for my purpose, thus I was looking for an alternative method to be used in one single run.
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ENRICO
ENRICO 2024-8-22
Well, that's the matter in the end.... I know how to build up a jacobian if I've the symbolic model, but using simscape directly fropm simulink is not given free access to final model (basing on my knowledges), unless it's obvously calcualted beneath, and used for code generation. So I ths should be nice to have a symbolic representation, at least where possible of a simscape model, since is a different representation of same system, and in any case solution would be of numerical more than closed form nature. Given references are not giving me a solution to my needs
Sahas
Sahas 2024-8-23
Hi @ENRICO,
Could you clearly outline the requirements and specify the exact workflow you need? This will help me assist you more effectively :)

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回答（1 个）

Yifeng Tang
Yifeng Tang 2024-8-20
The Jacobian of the system of equations is formulated by the solver, but I don't think it's available explicitly in any form. In Model Settings -> Solver, if you select an implicit solver (daessc, ode23t, etc.), you'll see an option for Jacobian method, where you can choose between analytical and numerical (perturbation) ways of storing the Jacobian. The Jacobian is needed for the nonlinear iteration in implicit solvers, but no it's not visible.
The solver profiler tool can tell you how many times the solver decided to recompute the Jacobian during the simulation, but still, the Jacobian itself isn't available in any form outside the simulation. At least I don't know of a way.
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Yifeng Tang
Yifeng Tang 2024-8-24
Oh, I wasn't talking about another way to get the jacobian. I'm wondering what is the purpose of obtaining the Jacobian. Are you trying to improve the numerical performance of the model? Choose the best solver? I'm trying to see if there are other suggestions I can provide since the actual Jacobian isn't available.
ENRICO
ENRICO 2024-8-24
Ah! Ok. There are many reasons in truth. Jacobians are much more used and useful in mechanics than in many other fields, so it's wothy have it, especially when dealing with complex systems. Essentially for a rgid body system most of quantities of interest could be derived from. Specifically in my case I want ot use it as part of building a regressor matrix numerically.

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