Thanks @peteroznewman, . Sorry, I have two follow up questionsn1) How would your suggestion be different from just inserting a displacement BC and imposing the zero displacement condition on the normal axis (using the global coordinates, without creating a new coordinate system)n2) What if I needed the surface to move in the surface normal direction, with the only restriction that all nodes on that surface remain in plane if they move?n

@tbaba One way to do that is by creating a new coordinate system for that plane. If you make the Z axis of that new coordinate system normal to the plane, then you would use a Displacement boundary condition of Z = 0 and set the coordinate system in the Details window to the new coordinate system instead of Global. Leave the other two directions Free. This BC would allow the nodes on that surface to move in X and Y and rotate about Z in the new coordinate system, which is what a Planar constraint does.https://forum.ansys.com/discussion/comment/98671#Comment_98671