[Simscape Multibody Contact Forces] Fluctuations in Normal & Friction Forces

I am working on a Simscape 3D model of a 2-wheels robot moving inside a rolling pipe:

Pipe is rotating along its own central axis (z);

Robot is also moving around (not along) pipe central axis (z);

Therefore, both robot and pipe are rotating along axis z;

The model is built using multibody and contact block

The contact forces between robot wheels and pipe are modeled using contact block (Sphere to Tube Contact Force (3D));

1_ The problem is that the robot is supposed to be moving only in the plain perpendicular to z axis so the velocity along z axis is supposed to be zero

However, there is some velocity along axis z(vAxis); The values are low but with high-frequency fluctuations. Since friction force is a function of relative velocities, it is also suffering from same fluctuations

2_ Penetration velocity (vrad) is also considered and it has low values but with high-frequency fluctuations. Normal force is defined as a function of pentation velocity and so it is also suffering from same fluctuations.

Has anyone had similar experince?

Any explanation or suggestions in this regard would be highly appreciated.

Answer 1

[Yifeng Tang]

The contact force, when in contact, is modeled as a stiff spring and damper to prevent the penetration and calculate the normal and friction force. So the parameters for the spring and damper can play a role in the vibration you see. From your description, sounds like the spring is stiff enough, but the small magnitude motion isn't damped enough.

You may also consider add a planar constraint to your robot to get rid of the along-axis motion.