retrieve force platform channels from c3d

[Alice Mantoan]

Hi all,

I'm trying to read c3d files using btk for the first time.

I would like to ask if there's a way to get the analog channels (in terms of the number of the channel) where force data are stored.

I did it with c3dserver, so I was wondering if there's the same chance also with btk.

I need it to separate force plate data from all the other analog data, regardless force plate 's type.

Thanks a lot.

Regards,

Alice

[fabien leboeuf]

hi alice

if you only want the analog channels, there is the elementary function : btkgetanalogs returning a matlab struture indexing force, emg...

that's all.

regards

Fabien

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Fabien Leboeuf
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[Arnaud Barré]

To complete the answer of Fabien, you can also use the function btkGetForcePlatforms [1] which returns a structure with all the informations (channels, corners, origin, calibration matrix) related to the force platforms in your acquisition.

If you want to compute the forces, moments and PWA (similar to COP without any assumption about horizontal null force), you can use the functions btkGetGroundReactionWrenches [3] which do it for you.

Arnaud

[1] http://b-tk.googlecode.com/svn/doc/Matlab/0.3/classbtk_get_force_platforms.html
[2] http://b-tk.googlecode.com/svn/doc/Matlab/0.3/classbtk_get_force_platform_wrenches.html
[3] http://b-tk.googlecode.com/svn/doc/Matlab/0.3/classbtk_get_ground_reaction_wrenches.html

[Alice Mantoan]

Thanks a lot for your quick replies.

I'm sorry, but perhaps my question was not clear.

I've already tried the solutions that you suggest, but with these functions I lost the information about the association between the analog channels and the force plate output.

In the c3d file is indeed stored in FORCE_PLATFORM\CHANNEL the number (1 to 6 or 1 to 8) of analog channels used for each force platform output (the order or the connection of force platforms in the analog channels is not always the same among different laboratories).

The functions you suggest returns something like this: analogs.Fx1, analogs.Fy1,...but if i have to manage data from different laboratories, different force plates, with different analog connections, the order or the name of force plate analog channel may be different, so I need the information of which analog channels are dedicated to force platform data.

Btk allows to do a lot of thing without knowing this, but unfortunately i'm managing data in C3D files at a lower level than btk.

Hope my question is more understandable now.

Thanks a lot

Best regards,

Alice

[Arnaud Barré]

If you want to access directly to the content of the parameter FORCE_PLATFORM:CHANNEL, you can use the function btkGetMetaData [1]. Using the following Matlab command:

% Matlab code
% h is a handle to an acquisition
md = btkGetMetaData(h, ‘FORCE_PLATFORM’,’CHANNEL’)
md.info.values

you will have access to the analog channel indices used by the force platforms.

Regards,

Arnaud

[1] http://b-tk.googlecode.com/svn/doc/Matlab/0.3/classbtk_get_meta_data.html

[Alice Mantoan]

That's what I needed!!It works!

Thanks really a lot!!!

Kind regards,

Alice