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Definition of custom intertia properties in SimMechanics
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Dear support,
I want to define custom inertia properties in my SimMechanics-model. I am not quite sure if the terms "Moments of Inertia" and "Products of Inertia" are used in a correct way in the SimMechanics documentation. At least it is not explained clearly what the user is supposed to fill in the dialogue box.
From my CAD-modell I derived an inertia tensor in the form:
[J_11 J_12 J_13
J_12 J_22 J_23
J_13 J_23 J_33]
I use exactly the same coordinate system in my SimMechanics-model to make the import of data easier. The coordinate systems of imported stl-file is taken as the reference frame in SimMechanics. It is neither a principal coordinate system nor the origin is located at the centre of mass.
"The key parameters for a SimMechanics Body block are its mass and inertia tensor relative to the centre of gravity of the body."
I doubt that. It makes more sense to me when the inertia properties are relative to the reference frame.
My question is: What am I expected to fill in for "Moments of Inertia" and "Products of Inertia"?
![](https://www.mathworks.com/matlabcentral/images/broken_image.png)
The documentation which can be found in the link below says the following:
Quote:
Moments of Inertia
Enter the principal moments of inertia as a 3-vector in the frame R of the solid. In the drop-down list, choose units. The default is kg*m^2 (kilograms-meters2).
Products of Inertia
Enter the products of inertia as a 3-vector in the frame R of the solid. In the drop-down list, choose units. The default is kg*m^2 (kilograms-meters2).
Principal moments are not equal to J_11, J_22, J_33 in the general case. Does this mean that I have to derive the principal moments of inertia from my inertia matrix (which are the eigenvalues)? If this is the case: In which order do I have to list the principal moments in the dialogue box (Can be found under the point "Custom Inertia" in the doc.).
I guess that the products of inertia are equal to J_12, J_13, J_23. Again: In which order do I have to put the values into the vector required in the dialogue box?
In that respect the documantation page I referred to says:
Quote:
"The products of inertia are (I23, I13, I12)."
Is this the right order?
I am a bit confused that there does not seem to be a simple way to get my inertia tensor directly into SimMechanics. It is a problem of conventions.
I hope you can help and clarify what I need to fill in the dialogue box.
I apologize for my limited English writing skills as I am not a native speaker.
Kind regards.
Jakub
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Respuestas (2)
Pouya Jamali
el 27 de Nov. de 2012
Diagonal elements of the inertia tensor are moments of inertia and the others are products of inertia. You should consider that this matrix is symmetrical so it just has 6 independent elements. If you have a CAD model it's better to use smlink to import the geometry automatically instead of fill in the inertia of the model parts manually.
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