Contact computation during CMC

[Daniel Krueger]

Hi,
I am currently working on a model to simulate a person getting up from a chair. Since there is no experimental data available the idea is to prescribe the kinematics of only a subset of the DOF (e.g. stretching the knees = getting up) and use CMC to determine the rest. Therefore I have to model the contact between the feet and the ground using Hunt-Crossley forces. However when I start CMC it seems that there is no computation of contacts. The model falls through the floor and cmc fails. When I replace the Hunt-Crossley contact by a contraint it work!
I wonder if there is a possibility to evaluate contacts during a cmc run?

best regards,
Daniel
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Chair of Engineering Design
University of Erlangen-Nuremberg /Germany

[Ajay Seth]

Under the Actuators and External Loads tab in the CMC Tool, do you have replace model's force set checked? If so all forces in the model are removed and only those in the additional force set file(s) will be applied. If appending reserves and residual actuators, then the contact force should be present.

Please note that CMC was designed to use known (measured) ground reaction forces and this was before OpenSim introduced contact forces. Therefore, it is highly likely that CMC will be unable to match the accelerations in the desired motion and will fail.

Please, let us know. Thank you for giving it a try.

[Daniel Krueger]

Hello,
the option replace/append external forces is set to "append". This doesn't seem to be the problem. However CMC keeps on failing in my case. I will have to figure out a different approach.
Thank you so far..
Daniel

[Dario Cazzola]

Hi Daniel,

I know this is a very old post so this might not be helpful.

I did experience a similar problem using forward dynamics. Ajay is perfectly right. If you append forces and residuals you do get contact forces from Force Reporter.

Being a CMC forward simulation it should be valid also in the case (?)..

Cheers

Dario