make unitree go2 mjx model compatible with hfields

[7oponaut]

I had two issues when attempting to use unitree_go2/go2_mjx.xml in scenes with hfields such as google_barkour_vb/scene_hfield_mjx.xml:

1. the margin attribute in the go2 class default section results in the following error:

NotImplementedError: (mjtGeom.mjGEOM_HFIELD, mjtGeom.mjGEOM_SPHERE) margin/gap not implemented.

2. the impratio="100" option setting makes the scene unstable with mjx acceleration (specifically, I encountered nan qpos values)

This PR removes these two attributes, resulting in no margin and an impratio="1" default.

I don't have a full understanding of how these changes affect the simulation. Please determine if they should be merged.

@saran-t requested this PR after encountering my post about the instability on X.

p.s. My version of the file has additional changes which might be helpful

• The go2-floor contact is way too slippery imo, it looks unrealistic to me. I 5x'ed friction on the feet of the go2 model, since they have higher priority than the floor.
• The tracking camera is too high for me. I set its z coordinate to 0.5 to center the go2 better.

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[kevinzakka]

Thanks for these @7oponaut. We should probably check if the same changes should be made to the other Unitree MJX variants. Will wait on @erikfrey to review!

[erikfrey]

Both of these changes look good. Although you can use impratio for pyramidal friction, it's more stable for elliptic cone friction.

The reason you are seeing more slippage and had to raise the friction is probably due to removing impratio, but that's OK. I'll leave it to you and @kevinzakka to discuss whether to update friction, as he knows better than me where those values came from.

[kevinzakka]

@erikfrey I'm okay with increasing friction, but since elliptic is now supported by MJX, should we not switch to impratio + elliptic?

[erikfrey]

As a rule of thumb, for locomotion you can rely on pyramidal friction - it's cheaper to simulate but still transfers to real just fine.

[kevinzakka]

Gotcha, then all good for increasing friction!

[kevinzakka]

@erikfrey Can we get this merged in?

[kevinzakka]

@7oponaut Can you sign the CLA?

[erikfrey]

@kevinzakka yes, once CLA is signed.

[7oponaut]

Sorry for taking your time, I was under the impression that I am contributing to an open source repository.

I am not comfortable with signing the CLA.

I'm taking issue with 3) in the CLA: I'm not a lawyer or a Google employee and I don't want to deal with litigatory issues in case Google later commercializes any of this repository and gets sued for whatever reason.