Inverse dynamics problem

[Ashley Heers]

Hello, everyone! I am new to OpenSim. I have a model of a bird forelimb, with kinematic data as well as external load data (aerodynamic forces) for one wingstroke cycle. The kinematic and force data are already smoothed, and I think my joints are good (they work fine in Maya, anyways). When I use the inverse dynamics tool, it runs quickly and fine, but when I plot the results, the moments jump all over the place (from positive values to negative values). Filtering makes no difference, and I can't for the life of me figure out what's going on. This is happening at every joint, and in every direction. Any suggestions? I've attached a screenshot of some of the results.

Thank you so much for any help!!

P.S. My bird only weighs 500 g, and the forces are small (total force ~1.5 N). Does OpenSim have any problems with forces this small? The bird is also flapping very quickly...

[jimmy d]

Hi Ashley,

I have seen these issues before when the capture frequency was low. In that particular case, the person had forces and coordinate values at the same 100hz.

Perhaps try to increase the (apparent) capture frequency of the force data and see if that improves it.

Let us know how that goes,
-james

[Ashley Heers]

Thanks! That particular issue turned out to be a filtering problem after all, so problem fixed. Now, however, I'm running into a different error, even with the (apparent) sampling frequency (external load) increased. Inverse dynamics is working fine, but static optimization is not. I've tried adjusting every parameter, I've added all the appropriate reserve / residual actuators, and for some reason the static optimization tool doesn't like some of my joints - it works for the wrist and elbow but not for anything more proximal. I keep getting "acceleration constraint violations" or an error that says, "On entry to DLASD4 parameter number -1 had an illegal value". What does this mean? I can't find any reference to it in the User Guide or forum.

Thanks!

Ashley

[Jack Zhao]

Dear Ashley:
The problem seems like a bug about DLASD4 in SO that OpenSim 3.3 has fixed. You can search for relevant keywords about your error message and SO to verify this.


Thanks & Regards,
Jack