Inverse Dynamics in Closed-Chain Systems:

[Musa Audu]

I am wondering if anyone can help us out with solving inverse dynamics (ID) problem in systems with closed kinematic chains. A good way to solve the problem is to prescribe a number of generalized coordinates that equal the number of free degrees-of-freedom of the system. The advantage of doing this is that one could get joint moments that are symmetric with respect to a bilaterally symmetric system if the chosen prescribed coordinates are symmetric. We have not seen any examples on how to do this in SimBody and OpenSim. If we do ID in OpenSim GUI by presenting a motion file in which we prescribe a reduced number of generalized coordinates, OpenSim warns that it is filling in the rest with their default values. While this is OK for open-chain systems, it is not necessarily the bets for closed-chain systems. Any ideas on how to resolve this problem in both SimBody and OpenSim will be greatly appreciated.

[Michael Sherman]

My suggestion would be to focus on doing this just in Simbody at first, to handle one problem at a time. If you can get that to work, then a separate question is how to get OpenSim to set up a system the same way.

[Musa Audu]

Thanks Sherm. Could you provide us any hints on how to proceed in SimBody - anything similar to the sdfulltrq() function in SDFAST that has been defined in SimBody? Thanks.
Musa

[Michael Sherman]

Hi, Musa. Simbody's SimbodyMatterSubsystem::calcResidualForce() is the Simbody equivalent of sdfulltrq().
Here is the documentation.

Regards,
Sherm

[Musa Audu]

Excellent! Thank you Sherm.