Get Free moment through wrapper

[jimmyDunne]

Can't find a method to get the calculated free moments through matlab/python interface.

Would be an important feature for those using btk to export data to OpenSim.

Thanks for all your awesome work!

[Alzathar]

The free moment is available by computing the ground reaction wrench associated with each force platforms. Each wrench contains forces and moments at the point of application. The free moment corresponds to the vertical coordinate of the moment.

Matlab
Within Matlab, you have to use the function btkGetGroundReactionWrenches. For example if you have two force platforms and the vertical axis of the global frame is the Z-axis, you can use the following example

h = btkReadAcquisition('path/to/my/file');
grw = btkGetGroundReactionWrenches(h);
Tz1 = grw(1).M(:,3); % Free moment for the first platform
Tz2 = grw(2).M(:,3); % Free moment for the second platform

Python
It is also possible to compute these wrenches with the Python wrapping, but the way to do is different. You can use the example given in this tutorial. The output of the pipeline (grwc) is a btk.btkWrenchCollection object.

[jimmyDunne]

Thanks for the answer!

Are the wrenches in Nmm rather than Nm? I didn't necessarily think the wrenches were the free moment since the values are in the hundreds when I am expecting walking free moment values ~10 Nm

[Alzathar]

The unit for the moments depends of your acquisition settings. To know the stored unit for the moment, you can use the Matlab function btkGetPointsUnit.

h = btkReadAcquisition('path/to/my/file');
btkGetPointsUnit(h,'moment')

[jimmyDunne]

Thanks! The only two active OpenSim c3d conversion tools are MOtoNMS and c3d2OpenSim.

Both of these projects rely on btk as the c3d reader, but I feel we both under utilize your software. For instance, both projects calculate free moment rather than using the wrenches class.

Im not sure how to really deal with that. May be a conversation for a different medium :)

[Alzathar]

We can continue this conversation on the mailing-list BTK Users if you want. Other BTK/OpenSim users might be interested in this discussion and could contribute too.