Stability constraint

[Simon Jeng]

Hi,

In order to keep the stability of the model, I want to limit the CoP in the foot support region. Is there any way to realize it in Moco?

Simon

[Ton van den Bogert]

In models of postural control, this is usually done by placing upper and lower bounds on the ankle moment. That is equivalent to bounding the CoP.

Ton van den Bogert

[Simon Jeng]

In models of postural control, this is usually done by placing upper and lower bounds on the ankle moment. That is equivalent to bounding the CoP.

Ton van den Bogert

Hi Ton van den Bogert,

Thanks for your reply. I think ankle moment in a planar model could be expressed as the formula in the attached figure. Fay, Fax, Ma are reaction forces and moment at the ankle. Fcx and Fcy are GRF at CoP. In this expression, Fcy and Fcx are unknown. How could I bound CoP location by bounding ankle moment?

Best wishes,
Simon

[Ton van den Bogert]

Yes, the diagram and equation are exactly right.

During standing, you can assume that y*Fx can be neglected and Fy is equal to body weight. Then you can convert the bounds on x (which depend on foot size) to bounds on Ma.

Ton

[Simon Jeng]

Thanks Ton,

So now in Moco, we can only bound CoP during standing or some static postures. It is not possible for dynamic movement, right?

[Ton van den Bogert]

I am not familiar enough with Moco to answer that question.

My answer was just to inform you about the approximation that is usually made in postural control models. In such models, the foot is "glued" to the ground. To prevent situations where the heel or toe would leave the ground, a bound on the ankle moment is used.

It may be possible to have a more accurate constraint in Moco.

Ton

[Simon Jeng]

That's enough for my research. Thanks again

Simon

[Nicholas Bianco]

Hi Simon,

You can bound ankle moments if you're using a model with torque actuators, but there's not currently a quick way to bound moments (or the CoP) in general yet.

Best,
-Nick

[Ross Miller]

Hi Simon,

I may be misunderstanding your question, but if you want the CoP to only be on the foot, this can be achieved by only defining contact elements on the foot.

Ross

[Simon Jeng]

Hi Simon,

You can bound ankle moments if you're using a model with torque actuators, but there's not currently a quick way to bound moments (or the CoP) in general yet.

Best,
-Nick

Hi Nick,

Thanks, I have done this. I think it's a good alternative for some movements that the foot is fixed on the ground (e.g. squatting, sit-to-stand) now.

Simon