Adding an external force for ID with a Moving Path Point

[Carolin Renz]

Hi everybody,

I want to simulate a passive knee flexion in a knee simulator. The femur und tibia are fixed in steel cylinders, which are then connected to a hip and ankle joint assembley. The hip joint assembley can move down vertically. In order to keep the ground reaction force constant, there are servomotors and steel cables fixed to the tendons, immitating the muscles. These forces shall simply be simulated as external force vectors applied to the respecting point.

For the simulaion I'm using the knee defined in the Gait 2392 Model.

I want to solve an Inverse Dynamics problem and my problem is now, that the Point, which the force is applied to is a Moving Path Point depending on the knee angle. If I understood right, in ID I can only define the Path Point's position according to time.
How can I apply a force to a point depending on the knee angle (in ID).

Can anyone help me with this problem?
Thanks a lot

Carolin

[jimmy d]

Hi Carolin,

there are some expression based forces that can be defined. The list of current force types can be found on doxygen; https://simtk.org/api_docs/opensim/api_ ... Force.html

Not sure if the specific force type you are describing is available, though the ExpressionBasedCoordinateForce() may be most useful.

Hope that helps,
-james