Is there a bug in Python ChLinkLockPointLine?

[Jose]

Hello. 

I am new to Chrono and have been playing with MbD and constraints using Python.

It seems to me there is a bug in ChLinkLockPointLine. When I initialize it, it seems to constrain the point to the X axis instead of the Z, which is, as far as I understand, the standard behavior among MbD solvers. It is as if the rotation matrix I give to it was internally rotated 90 deg. Something similar happens with the output forces. They are rotated 90 degrees. Is this a bug, or am I getting something wrong? I have managed to make the other joints work correctly and compared them with other solvers, so not sure if I am getting the in-line constraint wrong. I temporarily fixed it by passing a rotated ChFramed, and by rotating the output forces as well. 

Kind regards.

[Radu Serban]

Jose,

 

There is really no “standard” among MBD solvers, as each is free to adopt whatever convention they want.

 

Having said that, this particular joint does not fit with the convention we strived for in Chrono: all joints that have 1 degree of freedom should associate that with the Z axis (e.g., a revolute joint has its rotation axis along Z, a prismatic joint allows relative motion along the Z axis, etc.), while all joints that have 2 DOFs associate those with the X and Y axes (e.g., a universal joint has its “cross” aligned with the X and Y axes, a PointPlane joint allows motion in the X-Y plane, etc.).

As it is currently implemented, ChLockPointLine defines the constraint line along the X axis. We will change this in a future commit for consistency.

 

--Radu

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