Magnets, how do they work?

[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.

[blazerbotics]

John made some magnetic wheels years ago for Beta, but like the robot, they never saw the arena!

[mark_m]

If you want to see what magnets can do for you, have a look at the US bot Shag: http://www.youtube.com/watch?v=Erf3J8WR_6o

Its a 12lb bot with the drive train and traction of a 30pounder.

[maddox10]

And how many arena's have a steel floor?

[Mouldy]

Robochallenge arena... they do all the uk champs so its there you want to use the full potential...

few tests from 540 when i first built it with magnets;

Driving verticle

http://www.youtube.com/watch?v=bpei2vKm8kA

Owning as a pusher... can see how it drove like it was on rails, and when other robots hit it, its like they drove into a brick wall... infact if i put the robot down on the floor... i physically couldnt push it sideways it had that much grip...also shows how the drive motors used to react to so much traction;

http://www.youtube.com/watch?v=IvPACmkn41c

The next video shows how planted it made the robot when hitting into NST... doesnt budge at all... unfortunetly this is a good/bad thing... as all the force goes into making him fly off / straight into the body... which caused the armour to jam into the wheels;

http://www.youtube.com/watch?v=b1sVgYnukNo

The last video i found kinda wierd... it was due to the fact the magnet was so close to the drive motor i think (around 10mm away from a speed 900) or maybe it was so close to the esc...

Either way i had to ground out the magnets effect in the end as shown in the video by attaching a piece of hardox to the magnet, which in turn was held onto the removeable lid of the machine.... if i didnt do this id totally loose power to one side of the drive.

http://www.youtube.com/watch?v=c2WH4VxD39I

so in summary... make sure you balance out the size of your magnet to the power of your drive... if i built 540 again, id probably use a similar size magnet (50x50x20..BIG) but id go with a square wheel base with 4 off RS775 banebots on 16:1 with 75mm wheels or similar... would make for a pretty awesome pusher...

...but pushers are lame and still get owned by 720

[Flag Captured]

How much downforce does 540 end up with in those videos? To barely move after a hit from NST is pretty incredible! It definitely looks like there's a lot of performance gain on that side of things, looking into some solutions for higher powered drive, since I think drills might be usable but end up causing a few problems...

Would there be much of a performance drop by having two smaller magnets instead of one larger one? HardWired has a removable weapon bay with a linear actuator in at the minute, and will eventually have other things like a small drum on there or what have you, so I can't really have a huge magnet on the underside of that since it'll probably wreck havoc with the weapon drives - the idea would be that I'd have a smaller magnet on each side in between the two wheels either side of the machine to keep it away from everything it'd affect as much as I could. Those wheels would most likely also have some kind of thick silver steel axle as well as bearings to help ease the load, but I'm still not sure yet.

Realistically, it's not probably going to be as solid a pusher as 540 or Stag, but it'd more be as a support to the lifting capabilities - pushing stuff around is the main aim, but if I can have significantly improved grip and stability, then that'd improve performance tenfold.

Oh, and Mouldy - that sounds like a bit of a challenge there

[Mouldy]

found this calculator very useful, you can work it all out yourself;

http://www.kjmagnetics.com/calculato...calcType=block

my magnet was N45grade 50X50X20mm- 5mm ground clearance, then mounted on 2mm ti base, giving a distance of 7mm from the steel plate (arena floor) its being attracted to.

I wouldnt suggest you bother getting magnets if you have a standard drill drive, your wasting your time and asking for trouble.
The location of magnet relative to the wheels is very important... for example if you had a wedge shape robot with wheels at the back, magnet by the flipper tip... it would take the traction away from your wheels and you'd probably just sit wheel spinning.

Ideally you want a magnet next to each wheel... or if this isnt possibly, bang on in the centre of the wheel base, making sure your base is rigid enough not to flex, but obviously none magnetic so the attraction isnt effected. Have fun!

[Revsmin]

...but pushers are lame and still get owned by 720

https://www.youtube.com/watch?v=KiSFeRcQ-Sg

[Flag Captured]

That calculator is exactly what I needed, thanks!

Definitely not either, I'd like to move away from drill drives - as nice as it is to have a cheap, ready made drive, they're pretty problematic even in the capacity I've used them, and I've never even fought before...
I'm thinking things through with a baseplate too, not sure which'd be the most suitable material for the job within my budget - thinking of having a rather thick 6082 T6 (I think?) aluminium baseplate if weight allows, so hopefully that'll be suitable to mount a magnet to the underside of, or something along those lines - I've seen a few magnets designed with holes pre-drilled, so that might be a way to go. Not worked out specifics yet with the drive, but I'm going to try and have a magnet near each wheel individually if I can since that'll probably give the best results - might have a play around with some designs and see what works best on paper, based on that - thanks!

[overkill]

Thanks for the link Dave! K&J have exactly what I need and their shipping is less than half of the shop I usually use.

[Mouldy]

Microgravity... After you became immobile I decided to slam drum first into the walls to try break it prior to the champs... Glad I did as it highlighted a few issues I managed to iron out before the uk's... Our fight also showed how pants the engagement used to be lol

And as for the calculator I found it pretty accurate within a few kg's of force

[Revsmin]

Yeah, your engagement in that fight was terrible. That fight helped me find major issues with my robot too, mostly the ground clearance, but also the scoop that might as well have been cardboard :P