[Question]: Gazebo simulator wx series arm inertia problem

[wellyowo]

Question

Hello,

When I run Gazebo with the WX200 simulator, I feel confused about setting up the inertia for three links: ee_arm_link, fingers_link, and ee_gripper_link. All these links have no visual parts and are set with a mass of 0.001 and inertia of 0.001 as well. However, when I look at the inertia visualization in Gazebo, it appears like the picture

I have two questions regarding setting the inertia to 0.001:

1. Why are these three links set with the parameter at 0.001?
2. Will this setup affect the dynamics of the arm? In the official Gazebo tutorial, the inertia visualization box is supposed to fit the visual part of the model. However, it doesn’t in my case. I've tried changing the inertia value to make the visual box smaller, but the gripper model starts shaking if the inertia of these three links is set smaller than 10^-6, 10^-4, and 10^-3, respectively.

Please let me know if this setup won't affect anything, or if there are any considerations I should be aware of.
Thank you.

Robot Model

wx200

Operating System

Ubuntu 18.04

ROS Version

ROS 1 Melodic

Additional Info

No response

[swiz23]

Since those 3 links you mentioned have no visual parts, they should really have zero mass/inertial properties. The reason those links exist are purely for convenience (either to signify the end of the arm/gripper kinematic chain or to provide a common reference frame for the two paddles on each gripper finger). However, as you discovered, Gazebo doesn't play nice with links in the URDF that have no mass/inertial properties. So the 0.001 you see there is just a dummy value. And no, I don't think it will affect the dynamics of the arm. The mass of each of those 'virtual' links are each 1 gram which I would imagine is negligible compared to the inertial properties of the other robot links.