simtk.org

Hi,

is it possible to enforce constraints between parts if one side of the constraint is massless?
An example would be one massless pendulum in 'ExampleCustomConstraint.cpp', which results in 'nan's on my machine.
I suppose that this fails because the constraint forces need some non-zero inertia elements to act on.
Is there a way to implement such a constraint in Simbody?

Thank you very much!

The rule to follow is this:

Imagine the system with just the bodies and joints (constraints removed). That tree-structured system defines the mass matrix and must be non-singular.

So you can weld massless bodies together if they are part of the tree system (i.e. using a weld joint rather than a weld constraint). There still must be no way in the tree system to apply a force to any joint dof that would cause infinite acceleration.

Once you have a non-singular body/joint tree, you can put constraints anywhere you want.

Regards,
Sherm

Hi Sherm,

thank you for the prompt reply. I will modify my model to avoid the singularity.

Regards,
Simon