Origin for ground reaction torques in Gait2354

[William Thompson]

I am trying to understand the ground reaction torques in the provided Gait 2354 example. About what axes are the ground reaction torques contained in the file "subject01_walk1_grf.mot" applied? The documentation at

http://simtk-confluence.stanford.edu:80 ... t%29+Files

states the following: Columns 14-16 represent the x, y, and z components of the right foot's ground reaction moment vector (typically in the model coordinate system).

What exactly does that mean? Where is the origin of the model's coordinate system about whose axes these moments are applied? It is definitely not the ground origin or else the torques would have to be much higher than those in the referenced file.

Bill

[Michael Mauersberger]

Hi Bill,

I wondered about this question as well. But if you look at "subject01_walk1_grf.xml", you will see that both the point of application and the direction of force or torque is expressed in ground coordinates, hence in global coordinates.

I further guess that these torque moments exist because there is only one ground reaction force applied but in reality there is a pressure distribution over the foot sole inducing the described torque moments. But maybe you knew it already...


Regards
Michael

[jimmy d]

(typically in the model coordinate system).

I can see why this confusing. Just think of it as the global coordinate system. Model coordinate system just means the global coordinate system of your model.

[William Thompson]

Here is what is still confusing. The torque about z axis, ground_torque_z, does not appear to be the vertical force, ground_force_vy, times the distance from the origin on the x axis, ground_force_px, while the feet are making contact with the ground. If everything is in global coordinates, I would expect to see larger torques when a 700N force is applied about half a meter from the z axis. Perhaps I am misinterpreting something.

[jimmy d]

The point of application, and direction, should be in the global coordinate frame. All the forces acting between the foot and the ground can be computed as a single ground reaction force vector and a free moment. This is what people typically compute and apply to the model.