Model with closed kinematic chain

[Priyanshu Agarwal]

Hi,

I have a problem in which the model has a closed kinematic chain. It comes out that for modeling such a problem two joints need to be associated with a body. However, Joints in OpenSim only supports one CustomJoint within the body definition. Is there a way to simulation such a problem?

Thanks.

[Ayman Habib]

Hi Priyanshu,

To close a loop in OpenSim (=Simbody) you need a constraint. You're free to choose where to put that and what type to use but you can easily split a body into two and add a WeldConstraint between them. Other splits/constraints are also possible and will lead to same dynamics but may affect which questions you can answer so keep that in mind.

Hope this helps,
-Ayman

[Priyanshu Agarwal]

Hi Ayman,

Thank you for your help.

I have one more question in this regards. I have defined a constraint between two bodies such that the angle of two are always constrained. In such a scenario if I perform the Inverse Dynamics Analysis, how the force interaction between the two bodies is considered? To be more precise at what point the force is transferred from one body to another?

Thanks.

[Ayman Habib]

Hi Priyanshu,

Inverse Dynamics does not mess around with the Multibody system dynamics, Constraints and Joints are treated rather symmetrically (though the methodology used internally is to solve the unconstrained system then project the solution to satisfy the constraints but I don't think this would matter in the results) but basically you can assume that the systems of equations are solved simultaneously. Hope this answers your question.

You might also find it useful to read the Simbody user manual/documentation at https://simtk.org/docman/view.php/47/23 ... Manual.pdf and the discussion forum under the simbody project on simtk.org.

Good luck,
-Ayman

[Priyanshu Agarwal]

Thank you Ayman for sharing the document. It is very useful.

Regards,
Priyanshu