Why the dof vel of some joints is not zero when my robot keeps static?

Autonomous Machines Robotics - Isaac Isaac Gym
1

I use torque control for all dof of my robot. When my robot is visually stationary, the angular velocity of some joints is not 0, but jumps between large positive and negative values. Why?
I have try to set “frcition=0” and “damping=0” in the urdf files.
Please help
image


self.dof_vel:  tensor([[ 1.4603e-01, -9.3518e-02, -2.7656e-01,  4.4244e-01,  2.8872e+00,
         -3.3078e+00,  4.5807e-01,  3.0486e-03]], device='cuda:0')
self.dof_vel:  tensor([[-0.1438,  0.0927,  0.2746, -0.4317, -2.8311,  3.3060,  0.0168,  0.3377]],
       device='cuda:0')
self.dof_vel:  tensor([[ 1.4603e-01, -9.3517e-02, -2.7656e-01,  4.4244e-01,  2.8872e+00,
         -3.3078e+00,  4.5807e-01,  3.0486e-03]], device='cuda:0')
self.dof_vel:  tensor([[-0.1438,  0.0927,  0.2746, -0.4317, -2.8312,  3.3060,  0.0168,  0.3377]],
       device='cuda:0')
self.dof_vel:  tensor([[ 1.4603e-01, -9.3517e-02, -2.7656e-01,  4.4245e-01,  2.8873e+00,
         -3.3078e+00,  4.5807e-01,  3.0186e-03]], device='cuda:0')
self.dof_vel:  tensor([[-0.1438,  0.0927,  0.2747, -0.4317, -2.8311,  3.3060,  0.0168,  0.3377]],
       device='cuda:0')
2

Is there any body to help？