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

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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.

Accepted Answer

Yifeng Tang
Yifeng Tang on 30 Apr 2021
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.

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