TransformAxis with respect to time?

[Ryan Byrne]

Hi all,

Does anyone know of a method or function I can used to transform an axis (TransformAxis) with respect to time? I am attempting to translate the center of rotation of a joint, but would like to do it with respect to time if possible (as opposed to a coordinate). I realize I can create a translational coordinate and prescribe it that way, but I am trying to keep my coordinates strictly rotational as of now.

Thanks,
Ryan

[Christopher Dembia]

I'm pretty sure the only way to prescribe the value of a degree of freedom is to use a coordinate's prescribed motion. What is the larger problem you hope to solve? Why do you want only rotational DOFs?

[Ryan Byrne]

Hi Christopher,

So I have been working on developing a spine model with 6 DOF lumbar joints. The kinematic inputs are from experimentally measured data (translation and rotation of joint with respect to time). The kinematics are originally prescribed with respect to time using "prescribed_function". When running SO with locked translational coordinates, the muscle activations look good. When any translational coordinate is unlocked at a joint, none of the muscles activate at all and the CoordinateActuators and Bushing elements at the joint carry all of the load. I'm not exactly sure of the reasoning - I assume it has to do with the SO algorithm.

The muscles do activate when only rotational coordinates are prescribed and the origin of the joint is translated using TransformAxis. This would require me to transform our 6 DOF kinematic data to 3 DOF + center of rotation location. This I can do, however the issue with this is that I cannot find a way to move the center of rotation with respect to time.

Thoughts?

[Ryan Byrne]

Just to add - I have tried translating the center of rotation using the TransformAxis with respect to the prescribed rotational coordinates. The issue here is that there are cases where there are multiple instances of the same rotational value at different times - each with a different translational value.