Orientation Tracking Setup

[Nicholas Vandenberg]

Hi all,

I recently started working with some IMU data that I'm trying to use with Moco, the good news being that there's a ton of data available but it's a bit like drinking out of a fire hose. The Xsens system we're using gives a ton of information and I've been able to adapt some conversion code from the native MVNX files to kinematics .mot files. However, (and this seems silly to say) I can only pull joint angle data from the MVN model joints which leaves gaps in various OpenSim model DOFs that don't have data to track (i.e., pelvis_tilt/list/rotation); resulting in solutions that aren't super great.

I have a question regarding the MocoOrientationTrackingGoal. I've used a variant of this goal in the past to assign a states trajectory for reference for the torso orientation. The description of this goal says that it can also take a trajectory in quaternion format, representing the orientation reference data which is provided in an array for all of the MVN model segments in each captured frame.

IMU_data_frames.png (50.3 KiB) Viewed 877 times

I'm wondering if it's as straightforward as building a TimeSeriesTable of the quaternion data and adding a Pelvis_Orientation tracking goal to my problem? Or am I missing something else?

Thanks!
Nick

[Nicholas Bianco]

Hi Nick,

Yes, that should work! It's been a while since I wrote that cost term, so hopefully everything works as expected.

One quick thing: you might run into issues with the Matlab bindings not supporting a TimeSeriesTable of Quaternion or Rotation objects. I believe we fixed that recently, but let me know if it still doesn't work.

Best,
Nick

[Nicholas Vandenberg]

Hey Nick,

I've tried working it up, pretty sure I've pulled the right section to construct the table itself and wrote up the goal:

Pel_OTG_code.png (11.85 KiB) Viewed 832 times

Is this what you were referring to about the table not being supported? I tried it with the TableProcessor after trying just the TimeSeriesTableQuaternion and normal TimeSeriesTable.

Pel_OTG_err.png (14.32 KiB) Viewed 832 times

If it's helpful I also included the .mot file that I made with the orientation data. I'm not sure if the issue is the type of table I'm trying to make or the data I'm trying to put in it.

Best,
Nick

[Nicholas Bianco]

Hey Nick,

You can't load that data directly into a TimeSeriesTableQuaternion, since it's currently just a table of "scalar" columns. But you can load the data into a TimeSeriesTable, and then "pack" it into a table of quaternions.

Code: Select all

pelvisTable = TimeSeriesTable('TFAB_02_SSWlk04_Xsens_PelQuat.mot');

% The "pack" method will take each 4 consecutive columns and pack them into a Quaternion object. It will choose a % column name based on a common prefix across all columns.
pelvisTableQuaternion = pelvisTable.pack();

% If you know the suffixes for each group of 4 columns, you can specify them during the pack operation.
suffixes = StdVectorString();
suffixes.add("_e0");
suffixes.add("_e1");
suffixes.add("_e2");
suffixes.add("_e3");
pelvisTableQuaternion = pelvisTable.pack(suffixes);

-Nick

[Nicholas Vandenberg]

That makes sense, thanks! I've got that worked in now but I'm getting an error about too many output arguments when I use the pack function?

[Nicholas Bianco]

Could you post a screenshot of the error message?

[Nicholas Vandenberg]

Here's the error I'm seeing

Pel_OTG_err.png (3.33 KiB) Viewed 791 times

[Nicholas Bianco]

Ah, sorry -- you need to use the method "packQuaternion".

Code: Select all

pelvisTableQuaternion = pelvisTable.packQuaternion();

[Nicholas Vandenberg]

Gotcha, that makes sense. I'm seeing this now, is it because there's header info or something? Not sure what's still missing.

[Nicholas Bianco]

You need to provide column labels that have a common pattern so that OpenSim can auto-detect how to pack and name the columns. For quaternions, you could have column labels that end with "_e#", where # = 0, 1, 2, 3. For example: "pelvis_e0", "pelvis_e1", "pelvis_e2", "pelvis_e3". Then you can provide these suffixes (as in the example code above), or let OpenSim try to auto-detect (which it should be able to do after renaming).