meshcat | Remotely-controllable 3D viewer , built on top of three.js | Frontend Framework library

The NaNs that you're seeing in the parameter group correspond to the world body, and although they look suspicious I don't think they are the root of your problem. The world body doesn't have a valid set of inertial parameters (thus they're set to NaN), and is handled as a special case in the code. As it is written in the current implementation, the body parameter API is ubiquitous for every body. Each body has a set of parameters in the context, regardless of whether they are valid. This might be a point of contention for special bodies ('world'), so I've opened up an issue to discuss.

The problem you're having now is because your base_main link (and thus your entire robot) is a free floating body. You're not allowed to have a free floating tree of zero mass links attached with a joint to another free floating tree of links with non-zero mass because any torque applied at the joint connecting them (Joint_base_yaw in your case) would cause an infinite acceleration on the inboard bodies. To solve this:

• (Option 1): Add a fixed joint between base_main and world in your URDF file.

Here is one solution. I don't love it, but it accomplishes the stated goal. Note the changes:

1. saving and publishing only one animation
2. naming the model in the MultibodyPlant with a distinct name for each sim
3. setting delete_prefix_on_load=False so it doesn't clear the old geometry

Here is my quick reproduction, in case it helps. I've confirmed that setting a very small target accuracy results in the same numbers, set the initial angular velocity down so it looks more reasonable, confirmed that the gravity is properly being zeroed, etc.

Copying my suggestion from the issue:

If you set the LCM_DEFAULT_URL environment variable to a different value for each (visualizer process, simulator process) pair, the messages will be transmitted on different socket addresses and not interfere with each other.

(For the simulator, you can also set the URL in the DrakeLcm constructor, though the environment variable is often easiest when launching several processes.)

The default value for LCM_DEFAULT_URL is shown here:

drake/lcm/drake_lcm.cc

Line 22 in 20d799e constexpr const char* const kLcmDefaultUrl = "udpm://239.255.76.67:7667?ttl=0";

Using a different port number, for example, would provide for independent process pairs.

env LCM_DEFAULT_URL=udpm://239.255.76.67:7666?ttl=0 bazel-bin/tools/drake_visualizer &

or

export LCM_DEFAULT_URL=udpm://239.255.76.67:7666?ttl=0

bazel-bin/tools/drake_visualizer &

bazel-bin/stuff/my_simulator &

Here's a simple example:

https://github.com/RobotLocomotion/drake/blob/165da3960ce2aede5558b27a6102d66bafa70b03/manipulation/util/geometry_inspector.py#L169

The header of that file also has some useful instructions.