How do I handle "Model unable to assemble" issues?

[adamkewley]

AK: this question was ported from an email. It was edited, redacted, etc. to fit the discussions format.

I am using OpenSim Creator to create an upper extremity exoskeleton model, but when I add the exoskeleton components, I have the following problem (screenshot below). I would like the exoskeleton to do some lifting movements with the elbow, so outside the position of the elbow Skeleton model, I hope that he can be fixed on the upper half of the exoskeleton model, and hope that he can lift up with the elbow, so I added a Freejoint and a WeldConstraint, but after adding it, it always prompts "Model unable to assemble" (see screenshot).

[adamkewley]

The reason that error tends to happen is because the model is requesting an "impossible" situation. For example, using a WeldConstraint to weld A to B, while also using another WeldConstraint to weld A to C, where B and C are spatially separate and cannot be moved next to eachover. It's impossible for the physics engine to give you what you're asking for, and that problem manifests as this error message.

However, in your case, you have a FreeJoint that you would (reasonably) expect to be "floppy" enough to move around when other constraints are applied. E.g. if your topology is something like (numbers here indicate exo parts):

• ArmAboveElbow <---- WeldJoint <---- 1
• 1 <---- WeldJoint <---- 2
• 2 <---- WeldJoint <---- 3
• 3 <---- WeldJoint <---- 4
• 4 <---- WeldJoint <---- 5
• 5 <---- FreeJoint <---- 6
• 6 <---- WeldJoint <---- 7
• 7 <---- WeldJoint <---- 8
• ArmBelowElbow <----> WeldConstraint <----> 8

You might expect the FreeJoint part to be "solved" by the physics engine (i.e. it would satisfy your WeldConstraint by rotating/translating the FreeJoint)?

If that's what you want, then it might be that you have to tell the physics engine that the coordinates in your FreeJoint are "free to satisfy constraints". To do this, click each Coordinate of your FreeJoint in the osim editor and, in the Properties Editor tab, ensure that the is_free_to_satisfy_constraints checkbox is ticked (screenshot below). From what I understand, by checking that box you are effectively telling the physics engine to ignore whatever value you initially set for the coordinate, and that the physics engine is free to modify it if it needs to do so in order to satisfy other constraints (in your case, your double-weld)