I am preparing force plate data to calculate the joint reaction forces. The digital data shows the forces and COP along X, Y, and Z direction. However, I need ground torques (Mx, My, Mz) to prepare the ground reaction file. Can you please tell me how I can calculate this?
I got a free moment from the force plate. Is it the same as Mz? Thanks
Force plate data for ground Reaction force
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Abdullah Al Masud
- Posts: 19
- Joined: Wed May 30, 2018 9:33 am
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Benjamin Michaud
- Posts: 31
- Joined: Mon May 03, 2010 6:35 am
Re: Force plate data for ground Reaction force
Dear Abdullah,
I am the programmer of ezc3d so I had to code all this things already to make the force platform module, so hopefully I can provide you with some insights
The relation between Force, Moment and CoP is as follow:
Moment = F ^ CoP + Tz
Where, ^ is the cross product, and Tz is the [0; 0; Free moment] vector (please notice that depending on your sign convention, it may by "-" Tz).
To be more specific about the free moment, it is the moment generated at the CoP not related to the force. Since the foot cannot pull on the ground, there is no moment that can occur about the axes on the horizontal plane (that is X and Y). However, the foot is free to rotate about the vertical axis (Z) creating a moment, hence Free Moment.
Going back and forth between Force/Moment and Force/CoP/Tz is pretty straight forward. Please notice though that one assumption must be made when finding CoP from Force/Moment. Indeed, when you try to solve, you find yourself with more variable than equations. To get the right amount of variable, you must make an assumption on one value of CoP. Usually, it is done on the CoPz. This is easy to understand; since your force platform should be leveled, then the z value is always the same (that is the z value of the surface of the force platform). Assuming that, it is trivial to go back and forth between "Force/Moment" and "Force/CoP/Tz"
Hope this helps!
I am the programmer of ezc3d so I had to code all this things already to make the force platform module, so hopefully I can provide you with some insights
The relation between Force, Moment and CoP is as follow:
Moment = F ^ CoP + Tz
Where, ^ is the cross product, and Tz is the [0; 0; Free moment] vector (please notice that depending on your sign convention, it may by "-" Tz).
To be more specific about the free moment, it is the moment generated at the CoP not related to the force. Since the foot cannot pull on the ground, there is no moment that can occur about the axes on the horizontal plane (that is X and Y). However, the foot is free to rotate about the vertical axis (Z) creating a moment, hence Free Moment.
Going back and forth between Force/Moment and Force/CoP/Tz is pretty straight forward. Please notice though that one assumption must be made when finding CoP from Force/Moment. Indeed, when you try to solve, you find yourself with more variable than equations. To get the right amount of variable, you must make an assumption on one value of CoP. Usually, it is done on the CoPz. This is easy to understand; since your force platform should be leveled, then the z value is always the same (that is the z value of the surface of the force platform). Assuming that, it is trivial to go back and forth between "Force/Moment" and "Force/CoP/Tz"
Hope this helps!