Joint in Opensim Model

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Hilary Cheng
Posts: 12
Joined: Thu Oct 04, 2018 9:10 pm

Joint in Opensim Model

Post by Hilary Cheng » Wed May 29, 2019 12:08 am

Hi,

I would like to know how the Custom Joint relating to Spacial transform in terms of formulation. Is there any paper talking about that?

In Opensim GUI, I found that a coordinate(elv_angle) is trying to Rotate the axis of another coordinate(shoulder_elv).
When I read the attached .osim file, I cannot identify the pattern for the above relationship. Which part of the .osim file stated the relationship between joints/coordinates?

Thanks!!

Regards,
Hilary
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NorthwesternShoulderModel.osim
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Dimitar Stanev
Posts: 1096
Joined: Fri Jan 31, 2014 5:14 am

Re: Joint in Opensim Model

Post by Dimitar Stanev » Thu May 30, 2019 4:08 am

A custom joint is a wrapper around Simbody's function based mobilizer:

https://simbody.github.io/simbody-3.6-d ... ml#details
https://simbody.github.io/simbody-3.6-d ... ml#details

Doxygen:

"When you create it, you specify the number of generalized coordinates, and six Function objects which calculate the spatial rotations and translations based on those coordinates."

publication:

https://simtk.org/docman/view.php/47/15 ... 2-p291.pdf

Then you can also create relationships between the general coordinates. This is achieved through constraints (e.g., CoordinateCouplerConstraint).

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Ajay Seth
Posts: 136
Joined: Thu Mar 15, 2007 10:39 am

Re: Joint in Opensim Model

Post by Ajay Seth » Thu May 30, 2019 1:55 pm

The SpatialTransform in OpenSim describes how the Child frame moves with respect to the Parent frame of a CustomJoint. It specifies the permissible movement (by user provided functions) on 6 axes (3 rotational and 3 translational TransformAxes) parameterized by generalized coordinates (the joint's coordinates). See: https://simtk-confluence.stanford.edu:8 ... els-Joints

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