Why does the "Spatial Contact Force" block not properly constrain two "File Solid" bodies in Simscape Multibody (R2020a and later)?

I have two CAD models that I have imported as STEP files into Simscape Multibody using "File Solid" blocks. I am trying to connect these parts in Simscape Multibody using the "Spatial Contact Force" block. One of these parts is a small pin which is supposed to go inside of a hole on the other part. Whenever I try simulating this system, I notice in Mechanics Explorer that these two parts break away from each other, with the larger part flying out of view (the smaller part stays fixed because it is attached to the world reference frame).

How can I avoid this issue?

Answer 1

[MathWorks Support Team]

The issue you are seeing is due to how contact forces are modeled for "File Solid" blocks. Please see this excerpt from the documentation for the "Spatial Contact Force":

"The geometries exported from the File Solid block or Revolved Solid block are convex hull representations that do not accurately capture the shape of true geometries."

For more information, you can refer to the documentation here:

https://www.mathworks.com/help/sm/ref/spatialcontactforce.html

 

In this case, the convex hull representation of at least one of these bodies does not exactly match its true geometry, and since there is such a tight tolerance between the two bodies, this results in unexpected/excessive contact. To work around this issue, you have two options:

1) Remodel the bodies imported from CAD as other Simscape Multibody bodies that do not use convex hull representation (i.e., not the "File Solid" block or "Revolved Solid" blocks).

2) Model the physical relationship between these two parts using a "Joint" instead of the "Spatial Contact Force" block.