Bumpers and Magnets

[Tristan Lall]

I think this could work.

But, there are right and wrong ways of doing this. If it were me, I'd design it so that sliding forces (in all directions in the plane of the backing) are resisted by some sort of edges—for example, if the magnets are recessed into the frame, and the bumper backing has steel plates that stick out slightly and fit snugly into the recesses. I'd also spread those points of contact so that pushing on the top edge can't lever the bumper away at the bottom edge. Finally, I'd pick very strong magnets. Not too strong that they can hurt you (much), but strong enough that you can't pull it apart by hand. Then I'd devise some sort of lever action or sliding mechanism to disengage the magnets from within the frame. (For example: pull a tab to slide the magnets upward and out of the recesses, then the bumper falls off. Another example: design cams that push really hard on the bumper backing, using some sort of suitable mechanical advantage. It might look strange if you have to pull out an extra-large Allen key to take your bumpers off, but it will be worth it.)

As for your proposed size of magnet, you should do the calculations (and have those calculations handy at inspection). With enough of those (my gut feeling without doing the math is 16 to 24 per side), this could work.

The standard in the rules is "a rigid fastening system [that forms] a tight, robust connection to the main structure/frame (e.g. not attached with hook-and-loop or tie-wraps)...[which is] designed to withstand vigorous game play." I don't think that categorically excludes magnets.

Originally Posted by GaryVoshol

But I'm not an inspector.

You (as a referee) could always invoke T07 to enforce a robot rule, especially if the bumper is clearly not up to the job during gameplay.

[DonRotolo]

I am an inspector.

If you stand on the bumper and it doesn't come off, it's 'robust'
Any magnet I can imagine, through 1/8" aluminum, will not be nearly strong enough.

If I were you, save the magnet money and use something else.

[Tristan Lall]

Hopefully these diagrams clarify what I was thinking. One set of several steel plates is mounted to the bumper plywood with countersunk screws (not shown). The other steel plates hold the magnets, and fit down into vertical slots against one face of the frame rail.

As described above, you don't want the magnets to be obstructed by 0.125 in of aluminum. You want the magnet, directly between two steel plates, making contact with both. The first plate is the backing of the magnet (on the robot), and the second plate is attached to the bumper.

As for magnets, choose something like this, then pot it in metal-filled epoxy (e.g. Devcon or JB Weld) in the recesses of the magnet backing plate. It has over 19 lb_f per magnet in this configuration. To pull it straight off, you'd need 475 lb_f (likely more than the pullout strength in the wood). To lever it off by pushing at the top edge (e.g. in a robot-to-robot collision), you'd need something like 237 lb_f, not accounting for the resistance offered by the edge of the bumper-mounted steel plates in their slots, or the fact that the opposing robot would be pushing on a rounded pool noodle. If you stand on it, the downward component of the force goes into the slots in the frame, while the magnets pick up the horizontal component.

As for the magnet backing plates, they pull straight up, with a wiggling motion, to remove the bumpers. (Grease their faces to make this easier.)

There's a very rough STEP CAD file attached with details. (No tolerances!)