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Thread: Rigid body rotations using rigid joints

  1. #1
    Join Date
    Jul 2017
    Posts
    11

    Default Negative Jacobians at random points

    He there

    I am attempting to get a knee extension model working, but there are some basics I can't get working before I even get there.

    I am first prescribing a displacement and pushing the patella into the femur to create contact, in the second step, just to test my contact I then apply a force pushing the patella into the femur, and I then get negative jacobians at arbitrary points on my femur cartilage that are not in contact with the patella cartilage.

    Does anyone have any ideas why I might be getting this problem only in the second step?

    Ultimately I would like to actually prescribe a motion simulating knee bending while simultaneously applying a quadriceps force, but I can't seem to get past applying a simple compression force. 1255.zip

    Thank you,
    Regards,
    Julian
    Last edited by julianatk; 09-15-2017 at 05:16 AM.

  2. #2
    Join Date
    Dec 2007
    Posts
    705

    Default

    Hi Julian,

    The reason that you are getting this problem is because, in the second step of the analysis, you are prescribing nodal forces on one of the contact surfaces (the patella). Instead, you should prescribe a force on the rigid body attached to the patella articular layer (Physics->Add rigid constraint->Prescribed rigid force). If you modify the model that way, it will no longer produce this problem.

    Best,

    Gerard

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