Magnets, how do they work?

[craig_colliass]

"Been looking at magnets for HardWired II to make a relatively flipper-proof configuration"

A flipper-proof configuration exists it's called Seraph


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[Flag Captured]

Magnetic hold-down is not like adding extra weight to the bot as the bot's inertia stays the same and the effort needed to accelerate it will also be the same.

So does that mean that it won't have any benefits to the machine other than downforce, I.E. if I loaded HardWired (at 13kg) up with 200kg worth of magnets, and ran into another machine, it'd only be like 13kg of robot running into it, not 213kg?

I think that'd be a big improvement too with the vertical spinners and the drums, an improvement in stability would certainly make them a lot deadlier. I'd love to try and get a full-body-spinner using something like that so that a single hit doesn't launch it when you get a really big hit, but I'm sure that there's more complex problems than just magnets would solve!

"Been looking at magnets for HardWired II to make a relatively flipper-proof configuration"

A flipper-proof configuration exists it's called Seraph

Ahhh, I see what you've done there - Unfortunately though, the wheels are about 3 feet too big to work on my design

[overkill]

So does that mean that it won't have any benefits to the machine other than downforce, I.E. if I loaded HardWired (at 13kg) up with 200kg worth of magnets, and ran into another machine, it'd only be like 13kg of robot running into it, not 213kg?

That's correct, your bot would hit with the same kinetic energy as it would with no magnets. The advantage is that your bot would have 213Kg down-force on its wheels. Assuming no wedges are involved, your bot would have around 16 times more traction and would own the other bot. On the downside, your bot would need a battery, ESC and motors capable of delivering 16x more energy to use all that extra traction. I used the EDTsim drive train calculator and with just 20Kg of hold-down the current needed to break traction more than doubled compared to just 13.6Kg

I made these magnetic wheels a few years ago and while I never measured their hold down force, it allowed my 4 wheel bot to drive on the the steel ceiling of my workshop:

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The finished wheels had moulded urethane treads and in use they were a complete disaster! The bot could hardly turn, the treads got torn up and they picked up anything magnetic off the floor, which ripped the treads up even faster. they were also rather heavy for the amount of hold-down they provided.

That brings up another problem with magnetic wheels and round magnets in general: Only the magnet(s) in contact with the floor are doing anything useful. In the case of my wheels, maybe three magnets at the bottom of the wheel were providing any hold-down and the other 9 were dead weight. I could have gotten the same hold-down at 25% of the weight by using fixed magnets on the chassis.