Force DOF measure in tutorial 4a

[Anne Koelewijn]

Hi,

I had a quick question about the second DOFmeasure in tutorial 4a, where Geyer and Herr's controller is used for walking. The first one makes sense to me because there the ankle angle is limited between -60 and 60 degrees. However, I do not understand the physical meaning of the second measure which is applied to the knee angle:

Code: Select all

DofMeasure {
	weight = 0.01
	threshold = 5
	dof = knee_angle
	force { min = 0 max = 0 abs_penalty = 1 }
}

What does the force constraint mean?

Thanks for your help!
Anne

[Thomas Geijtenbeek]

Hi Anne,

This is a penalty for excessive knee hyperextension. Instead of looking at the knee angle though, we look at the magnitude of the knee limit force. In the OpenSim model, this is modeled through a CoordinateLimitForce, which proportionally increases after the knee angle crosses a certain threshold. The DofMeasure in SCONE adds a penalty based on the average magnitude of this moment, with a threshold of 5Nm. If the average is below the threshold, the penalty will be zero, to allow for some minimal knee limit force.

Hope this helps!