Parameterize a TransformAxis

[Nicolas Eydieux]

Dear OpenSim community,

Is it possible to parameterize a TransformAxis ? Actually, I would like to define an axis which depends on a coordinate with a sort of CoordinateCouplerConstraint.

Thanks per advance,
Nicolas.

[Michael Sherman]

Hi, Nicolas. To parameterize an axis you need to provide a coordinate that controls it. That means introduce a new body and joint, then use couplers to express the dependency of the new joint's dofs on others.

It is fine for the introduced body to be massless and inertialess if it is connected to massful bodies -- in that case it is just introducing an additional frame. The general rule-of-thumb for when you can get away with a massless or inertialess body is to imagine applying a torque (or force) to each of that body's joint dofs. What would be the resulting acceleration? If it would be infinite then the body is not allowed. If it would be finite due to the presence of other bodies connected outboard to the massless one, then that is well-conditioned and OK.

So for example a massless body that does not serve as the parent for any other body is not allowed.

Regards,
Sherm

[Nicolas Eydieux]

Hi Sherm,

Thank you first for your quick answer. I post this topic for the problem of massless bodies that I would like to replace by parameterized axis. Thanks to your answer, I know understand well why certain body are not allowed. Finally, I can keep massless bodies with massful bodies connected outboard what solves my problem.

Thank you again,
Nicolas.