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google-deepmind / mujoco

Multi-Joint dynamics with Contact. A general purpose physics simulator.
https://mujoco.org
Apache License 2.0
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Soft Robot Simulation: (a) Example XML files, and (b) difference between Composite and Deformable soft objects #1630

Closed kurtenkera closed 5 months ago

kurtenkera commented 5 months ago

Hi,

I'm a PhD researcher and I'm trying to use MuJoCo for soft-robot simulation.

I have two questions:

(1) I'm looking for some example soft-robot XML files to learn how to model my own soft-robot in MuJoCo - are there any prominent examples? I can see in the MuJoCo menageries that there are several rigid robot XML files which seem to use obj2mjcf to decompose an OBJ file of a rigid robot into various sub-meshes, which can then be used to create an XML file. Can something analogous be done for soft robots, but using the flexcomp element instead of the rigid geom element? This is hard for me to imagine, since by definition a soft-robot isn't a series of rigid links connected by various joints, so I don't see the need to generate several flexcomp sub-meshes for a soft-robot (unless I'm mistaken)?

(2) I'm struggling to understand the differences between composite objects and deformable objects. From reading through these two links, it seems like deformable objects can more reliably model soft-bodies, but I'd like to understand the actual differences?

I appreciate any help/feedback/comments dramatically! Thanks in advance.

quagla commented 5 months ago

Hi, the answer to (2) is clear: please use deformable objects, we will deprecate 2D/3D soft bodies from composite since they use only spheres at mesh vertices for collisions.

For (1), you have to think of flexcomp as a collection of body's (one per mesh vertex), so it's not an alternative to geom. A deformable objects introduces 3*nvert degrees of freedom, while a geom introduces none. We have some examples here https://github.com/google-deepmind/mujoco/tree/main/model/plugin/elasticity I think the one you may want to look at is trampoline_flex since it shows how to pin a soft object to a body.

Let me know if you have more questions.

kurtenkera commented 5 months ago

Hi @quagla, for flexcomp we reference 3D mesh files. So is the 'nvert' you're referring to the number of total nodes in the 3D mesh? Or only the total number of vertices included for collisions/in the convex hull of the 3D mesh?

quagla commented 5 months ago

The total number. It's really the total number of independent deformation modes of the soft object.