Constraints between massless bodies

[Simon Radler]

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!

[Michael Sherman]

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

[Simon Radler]

Hi Sherm,

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

Regards,
Simon