joints and coordinates definition

[Giordano Valente]

Hi,
I'm using *.vtp geometries converted from stl formats to build an Osim model using v1.8. These geometries have different locations and orientations from the *.vtp's in the geometry folder. Defining the joints and the coordinates, the bodies don't rotate around joints centers. Defining just a body and the ground (and acting on the locations values in CustomJoint) the body keeps rotating around the Osim axes. Any suggestions?
Thank you.

Giordano Valente

[Peter Loan]

Hi Giordano,

You should be able to adjust the <location> values
in the CustomJoint to get the desired kinematics.
The <location_in_parent> values specify the location
of the joint center in the parent body's reference
frame, and the <location> values specify the joint
center in the child's reference frame. For
example, if you're defining a hip joint and your
femur bone (and body) has its origin in the middle
of the bone, your <location> values should define
the translation from the middle of the bone to the
center of the femoral head. When you change the
hip angles, the femur should then rotate about
the femoral head.

Regards,
Peter

[Katja Oberhofer]

I have a question related to this thread. The .osim file doesn't seem to contain any information about the location/origin of a body's reference frame. Where does this information come from? Is it somehow stored in the .vrt file? I am looking at the gait2354_simbody.osim file. Tibia is loaded as child of femur. Femur is loaded as child of pelvis. The femur coordinate system seems to have its origin at the knee joint. However, the location of the knee joint has not previously been specified with respect to any coordinate system.

[Katja Oberhofer]

Found the answer myself. The CustomJoint 'knee_r', being local coordinate system of Body 'femur_r', is translated in x and y with respect to parent 'pelvis' so that the translational DOF in the knee can be captured. Body 'tibia_r' is then loaded as child of 'femur_r' at [0 0 0] location_in_parent body coordinate system 'knee_r'. In case anyone else had the same question....

Cheers, Katja

[Ayman Habib]

Hi Katja,

Since this question comes up a lot, I went ahead and added an image to our Wiki, explaining the relation.

http://wiki.simtk.org/opensim/FrontPage ... Frames.jpg

Hope this helps,
-Ayman

[Katja Oberhofer]

Thanks Ayman, pictures are always useful!

I think the question mainly comes up because of the knee joint, which doesn't have a fixed position with respect to pelvis.

Cheers,

Katja