some bugs about making Spherical joint

[fairytalef00]

Hello,

I'm trying to create a humanoid robot in urdf and implement it in CoppeliaSim.
I realized that I can't dynamically activate the spherical joints, so I built it with 3 revolute joints, as shown in the attachment.

For the mujoco physics engine, I made it without auxiliary masses,
For the bullet physics engine, I added a secondary mass between the revolute joints.

I have a couple questions here.
1) First, for mujoco, I thought I did a good job, but like the video (human_mujoco), it freezes during simulation.
I don't see anything wrong with the dynamics(!), so I don't know what is causing the error.

2) If I use the bullet engine, I understand that I need to add a virtual link (virtual mass) between revolutes, but I have questions such as what should be the mass or inertia of that link and should it be responsive?

- Regarding the dynamic simulation rules(?), I saw that when two objects are connected, their masses should not differ by more than 10 times. However, my concern is whether adding a virtual mass that is within 10 times the mass will cause problems with dynamics and collisions.


There are my files.
https://1drv.ms/f/s!AuvU7BTQNjpWh_Vem8H ... w?e=NaRCxI

Thank you!

[coppelia]

Hello,

in your MuJoCo model, not all joints are controlled in positions. Additionally, make sure to set all joint's dynamic engine property armature to 20: this is the secret sauce most of the time.

When working with Bullet or ODE, you indeed should make sure that masses and inertia are not too different for connected shapes. If you have auxiliary masses in-between, distribute the shape's mass/inertia to the aux. shapes.

Cheers

[fairytalef00]

It was resolved immediately.

Thank you for taking the time to resolve this!!!