Error While Adding In HuntCrossleyForce

[FawazMallick]

So I am trying to design an exoskeleton that attaches at the hip, this exoskeleton has two parts: hip brace and 1 thigh brace for the right leg( modeled separately). Using an actuated lower body opensim model ( with actuators, constraints and force sets defined) I used opensim creator to import my hip and thigh brace stl files and add it onto the model. Now the thigh brace interacts with the thigh, I used a sphere body to simulate the circumference of the thigh, and then using a contact mesh for the thigh brace and contact sphere for the sphere body representing the thighs circumference. I tried adding a HuntCrossley Force between these two bodies so that they may interact with one another and I may see how it affects the gait cycle by running inverse kinematics in opensim. However I get the following error when adding the force:

[error] exception: thrown while drawing the model editor
exception: error detected while trying to apply a property edit
exception: SimTK Exception thrown at ContactGeometry_TriangleMesh.cpp:466:
Bad call to Simbody API method ContactGeometry::TriangleMesh::Impl::TriangleMesh::Impl(): Each edge must be shared by exactly two faces.
(Required condition 'forwardEdges.size() == backwardEdges.size()' was not met.)

[error] Exceptions typically happen when an invalid edit is made to the model
[error] attempting to rollback the model edit to a clean state
[error] model rollback succeeded

Please help, this urgent and I would really appreciate suggestions

[adamkewley]

Hi @FawazMallick ,

That error message is propagated from simbody, which OpenSim internally uses for physics calculations. Simbody's official documentation for triangle meshes is available here:

• https://simbody.github.io/3.7.0/classSimTK_1_1ContactGeometry_1_1TriangleMesh.html#details

The error message you're seeing is from here:

• https://github.com/simbody/simbody/blob/c8e14d2708c1be3fdc7c4a7e67ad66d615bd4906/SimTKmath/Geometry/src/ContactGeometry_TriangleMesh.cpp#L464

The error you're seeing is because (it seems) simbody specifically requires that meshes can't have shared edges. Whether your mesh does or doesn't is a side-effect of the modelling procedure. STL files shouldn't have this issue (iirc, they don't contain indices). At a quick glance, the linked algorithm looks like it may also encounter problems when given a mesh with an odd number of faces/triangles. However, I can't speak for code that's external to the Creator project.

Anecdotally, users have mentioned issues using TriangleMesh contact geometries in the past. However, most users rarely provide a follow-up or explanation when they get it working, so I can't say what the solution is.