Question: How to detect broken mechanism

[ManuelZ]

Hi, sometimes during an optimization procedure with a Genetic algorithm, the chosen set of parameters makes my mechanism break (probably because with certain parameters, some joints manage to get to some angle that's not intended for them to be in).

Normal mechanism:

Broken mechanism:

What could I do to either, detect this and stop the simulation, or to prevent some joints from going out of safe ranges?

[jgerstmayr]

First, your solver may not converge or have large drift (in case of TrapezoidalInde2 solver). You can use a preStepUserFunction to perform checks (iterating over all relevant joints) at the beginning of every step to detect if some safe ranges are left. The preStepUserFunction, even if written in Python, is usually fast because it is only called once per step.

[ManuelZ]

Ok thanks. I'm doing this inside my SetPreStepUserFunction

u = mbs.GetObjectOutput(joint_idx, exu.OutputVariableType.Rotation)

However, two things:

1. I'm using a ObjectJointRevolute2D, and in the documentation, the point 7.2.33.1 (Description) is empty (other joints do have this section filled, so that I can know what OutputVariableTypes are available for this joint).
2. Trying exu.OutputVariableType.Position I get "ObjectJointRevolute2D has no OutputVariableType 'Position'". Trying exu.OutputVariableType.Rotation gets me CObjectJointRevolute2D::GetOutputVariableConnector not implemented.

Does this mean that isn't currently possible to get output variables from a ObjectJointRevolute2D in this way? Or should I add a sensor first?

[jgerstmayr]

The CObjectJointRevolute2D has currently no output variables. But the values can be easily computed from the associated bodies. The rotation is not available in JointRevolute2D, as the 2D revolute joint can also be attached to position-based markers, which have no rotation. The documentation for this joint is indeed missing, but you can see the implemented methods in the C++ code.