ContactMesh not working with input stl file from solidworks.

[Ratna Sambhav]

Hello,
I need a little bit of help in understanding why the ContactMesh used by me is not working properly. I made a sphere in solidworks, saved it in stl format, Part4.stl. Then, I made the body of a ball and body of a flat surface, using API command of OpenSim. I activated contact on the surface using ContactHalfSpace command. For the body of the ball, I activated contact by using ContactMesh('Part4.stl') command. I kept the ball above the surface and simulated. Problem is ball is passing through the surface. I tried using ContactSphere and it worked. But my main concern is to import SolidWorks model and activate contact on this, which I don't see working here.
(It is just an example, I am trying to run before I implement it in my project where I have to import an upper limb exoskeleton design made in solidworks and calculate contact force between the musculoskeletal model and the device, which will be used as user's intention in control loop implemention)

Here is my code:
surface = Body('surface',1,Vec3(0),Inertia(1));
surf_geo = Brick(Vec3(5,0.1,5));
surf_geo.setColor(Vec3(0));
surface.attachGeometry(surf_geo);
model.addBody(surface);
joint = WeldJoint('joint',model.getGround(),Vec3(0,0.9,0),Vec3(0,0,0),surface,Vec3(0),Vec3(0));
model.addJoint(joint);

ball = Body('ball',1,Vec3(0),Inertia(1));
sphere = Mesh('Part3.stl')
sphere.setColor(Vec3(1));
ball.attachGeometry(sphere);
model.addBody(ball);
joint = FreeJoint('joint',model.getGround(), Vec3(0,5,0), Vec3(0, 0, 0), ball,Vec3(0), Vec3(0, 0, 0));
model.addJoint(joint);

floor = ContactHalfSpace(Vec3(0), Vec3(0, 0, -pi/2), surface,'floor');
contact = ContactMesh('Part3.stl',Vec3(0,0,0),Vec3(0,0,0),ball,'contact');
% contact = ContactSphere(1, Vec3(0, 0, 0), ball, 'contact');

stiffness = 1.e8; dissipation = 0.5; friction = [0.1, 0.1, 0.1];
contactForce = HuntCrossleyForce();
contactForce.setStiffness(stiffness);
contactForce.setDissipation(dissipation);
contactForce.setStaticFriction(friction(1));
contactForce.setDynamicFriction(friction(2));
contactForce.setViscousFriction(friction(3));
contactForce.addGeometry('floor');
contactForce.addGeometry('contact');

% Add the contact-related components to the model.
model.addContactGeometry(floor);
model.addContactGeometry(contact);
model.addForce(contactForce);

[Ayman Habib]

Hi Ratna,

The difference between a ContactMesh and a ContactSphere is that the ContactSphere has sidedness and triangulation built in while a mesh can have a variety of issues as it supports a more general shapes, examples:
- Normals pointing in the wrong direction or not aligned
- Mesh not closed for whatever reason, or have degenerate triangles.
You can use some third party tools (e.g. MeshLab) to figure out the issues with the Mesh/STL file. Also make sure the sidedness of the halfspace is correct (I assume it is the same as the one you used with ContactSphere).

Hope this helps,
-Ayman

[Ratna Sambhav]

Thankyou for your kind attention Ayman Habib.
From now on, I'll be using MeshLab for model import, for avoiding any issue related to mesh. My current problem was solved by using ElasticFoundationForce instead of HuntCrossleyForce.
Thanks