moving forward to the edge

Hello,

my problem is, i have an gameobject, i placed it on a table. i want my object to move forward until it finds itself on the edge of the table. When such thing happens, i want my gameobject to move backward, to simply go back until the he finds himself on the edge again.

I have added a rigidbody and gravity, but i have problem, that OnCollisionExit triggers to late, when my GameObject is already to far. i wanted to check for partialy being over edge by checking the contactCount but it also doesnt work (sometime i get 20 contacts on collision, sometimes i got 7, and i cannot find rule, what amount of contact count means that my gameobject is partialy over edge). Im moving my GameObect with transform.position.forward, and gameobject is a simple cube.

Any hints? im begginer in Unity