Local Reference Systems - Ground Reference System : rotation and translation

[Giulia Ghielmi]

Hello everyone!
Would anyone be able to help me extract in MATLAB all the information (translation and rotation) concerning local reference system with respect to ground reference system? I need this information to build my OpenSim's model in Abaqus.
To find the translation of local reference system for each joints, I'm using the coordinates returned by Joint reaction Analysis.
Regarding axis rotation I'm encountering some difficulties.
In this example I have the joint between L5 and S1 and directly in OpenSim I can see the translation and rotation about the reference system of sacrum with respect to reference system of lumbar5. But I would need the position of sacrum reference system with respect to the ground reference system. Is there a function in Matlab with which I can extract this information?

The second question is: The rotation of the axe4s of local reference system with respect to another in what order is performed?

Thamks in advance

[Ayman Habib]

Hello,

Typically frames are placed on other frames to form a chain, and the GUI shows the relative translation/rotation between a frame and its "parent" frame. However there are API methods that you can call to compute the transformation between any pair of frames, and then to decompose into translations/rotations, in particular the methods described here:
https://simtk.org/api_docs/opensim/api_ ... Frame.html
to

Code: Select all

getTransformInGround, getPositionInGround, getRotationInGround

and the methods to convert between different frames:

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findTransformBetween, findStationLocationInAnotherFrame

and many others.

The underlying Transform class is documented here
https://simtk.org/api_docs/simbody/3.5/ ... ml#details

Please let us know if you run into problems using the methods above in MATLAB or if there's other functionality that you need.

Best regards,
-Ayman

[Giulia Ghielmi]

Thank you! I'll check all the functions and see if they work