How to extract global moments of inertia and calculate impulses

Andreas_H

Hi all,

I'm looking at trying to extract impulses from bodies in a MBD model. For linear impulses this is relatively trivial but for angular impulses the need to compute the inertia tensor makes this more difficult.

Currently bodies are defined with inertias given in a global coordinate system not the cm. I assume the solver will be in the background calculating how these change as the body moves but as far as I can tell there is no variable to pull these values from during the simulation.

As a result I'm looking at calculating the moment of inertia values using the expression builder + parallel axis and then doing the impulse calculation to get angular impulse about the origin.

Is there a easy way to pull out these values without doing the manual calculation? If not, is there a way to do matrix algebra in the expression builder to avoid individually calculating each value?

The goal is to have angular and linear impulse for any body from t=0 to t=T.

GTT Adam

Hi @Andreas_H,

The inertia values for bodies will not be able to be extracted via a variable during the simulation, so you will need to calculate those on the fly in a similar manner to how you have described.

I would not attempt the calculation using the Expression Builder but rather through a User Subroutine or from a post-processing script. The Expression Builder will not allow matrix algebra, so you would need to write individual expressions for each body which could be cumbersome if your system contains a significant number of bodies.

The User Subroutine would allow you to write Python-based or OML-based scripts to handle the calculations on the fly. Similar scripts could also be written if the results of the impulse calculations are not required during simulation.

Hope this helps!

Adam Reid