refer from: [computation -> force generation]
（https://roboti.us/forum/index.php?threads/custom-mesh-jittering-in-mujoco-environment-in-openai-gym.3860/）
The collision mechanism uses the convex hull of the mesh, and generates a single contact with the box. When the convex hull has large flat areas, this single contact has to run around (over time steps) to prevent penetration over the entire surface.
There are two solutions to this problem:
1. Replace the ground box with a plane (or is it already a plane?) and use MuJoCo 2.0. The latest version of the collision detector generates multiple contacts between a mesh and a plane, which results in more stable simulation. But this only works for plane-mesh, not for box-mesh.
2. The better solution is to break the mesh into several meshes, and include them as multiple geoms in the same body. Then MuJoCo will construct the convex hull of each sub-mesh, resulting in multiple contact points (even without the special plane mechanism mentioned above) and furthermore it will be a better approximation to the actual object geometry.
拆分网格为多个子网格，根据模型的自由度整除3增加子网格的数量
Break it into even more meshes. At rest, each contact point immobilizes 3 DOFs. A rigid body has 6 DOFs, and your object has an extra internal DOF, so that makes 7. So you need 3 active contacts, i.e. the object should be broken into at least 3 sub-meshes.