simtk.org

Hey Ayman,

Do you know where the ID tool exerts its forces? I'm not sure if it's at
(1) the joint frames connecting the bodies that move relative to each other
(2) at the CMs of the individual bodies that are connected by a joint
(3) at the origins of the body frames
(or)
(4) somewhere else..?

If it's 1,2, or 3 above, then which body does it refer to? The parent or the child?

Thanks!
adila

Hi Adila,

ID takes a model and optionally a set of Forces (e.g. GRF) and a motion as input and produces as output the forces/moments acting along the model's "Coordinates" to reproduce the motion.
So to answer your question:
1. For inputs the forces are whatever you specify (Bushing, GRF, Point Forces etc.)
2. For outputs these are the forces/moments acting along the coordinates (close to option 1 in your description). A joint describes the motion between two frames and the is parametrized by the joint's coordinates.

Hope this explains,
-Ayman

Thanks Ayman, your explanation was perfect.

On a similar note, does Forward Dynamics exert its forces along the model's generalized coordinates as well?
and, is the ForceReporter reporting these forces (along the generalized coordinates), or some other frame of reference.

Thanks!
adila