Magnets, how do they work?

Sorry, couldn't resist the reference.

Been looking at magnets for HardWired II to make a relatively flipper-proof configuration (idea is to make it so 'heavy' that flippers can't lift it), which sorts of magnets are best for the job?

I found these, http://e-magnetsuk.com/magnet_produc..._mounting.aspx (The E1021 ones, 50kg pull) and wasn't sure how suitable they'd be to say, bolt on the underside of the machine. The plan was to have four, one on each corner if the weight allows for it. Alternatively, I did spot this one http://www.first4magnets.com/circula...0kg-pull-p3497 which a bit bigger and presumably heavier with twice the pull of the others I saw. Also quite a lot more expensive.

Would either of these be suitable at all, and if so, which one would be the better choice?

Thanks in advance!

Just to say, Hannibalito 3, not the most powerfull or efficient flipper around can lift around 400 kg on the tip of the flipper.

Hannibalito 4, can't do as much, because the arm would buckle, but in theory, that one can lift 2 tons over 50mm with the ram alone.

If you add that much weight with magnets, your drives will have an issue with that. Or if the axles or chassis bend and ground you

The problem with magnetic hold-down is that the magnets have to be really close to the floor to have much effect. That stated hold-down force of 50Kg will be when the magnet is attached to a steel surface. Magnetic attraction decreases with the cube of the distance so you will get far less attraction when the magnets have a reasonable floor clearance of (say) 5mm.

The other problems are getting high-sided by an uneven floor - it would be really embarrassing to get your bot stuck to the floor. You will also sweep up every magnetic item on the floor and this could also high-side the bot if you pick up something large.

Japanese sumo bots use magnetic hold-down very successfully but their arenas are super flat and kept clean.

Rather than a permanent magnet I'd use an electromagnet or something which can be switched off, otherwise moving the robot might be a problem...but as Mario and Nick said, there are a whole lot of other factors against their use.

Below I linked two magnets which look like they got the power to hold to the ground, but I got no idea of their own weight and power requirements to make them work...add to that a hefty price tag!

http://www.ebay.co.uk/itm/1100-LB-Ho...item27dbf396f0

http://www.ebay.co.uk/itm/2000kg-440...item27d8e7dbde

I recall seeing one of those door electromagnets at a place I worked; the cable to it was quite thin, so the current requirement can not have been much.

There is nothing wrong with using magnets, at least in theory, to make a robot seemingly heavier. But in the arena, a rambot with a shallow wedge knocking into your machine at 15mph will very likely unstick it, as will a flipper. I imagine most FP flippers could lift a robot "weighing" 80kg quite easily anyway, as Mario says.

The main issue I see is the strain on your own robot, if you go for serious downforce. From a mechanical point of view, your robot's drivetrain and chassis has to be strong and stiff enough to prevent buckling under the forces, which is entirely possible (Grant's new axe feather will have around 100kg in downforce, note it doesn't actually have any tyres, just aluminium!), but will take some good design and construction. From an electrical standpoint, the strain is even greater, the drive motors will have to work much harder (effectively you're driving a heavyweight with featherweight kit), which will at very least shorten their lifespan if not burn them out on first testing. Because the motors are working so hard, you speedos will have to be able to handle the current draw, and your battery beefy enough to supply it. Of course all wiring would have to be able to move this current around too.

Point being it's not quite as simple as popping a few magnets under your bot and thus becoming flipper-proof.

Having a little in the way of downforce to give you the edge in grip over the opponent is viable, though. Even just a few kg of pull will be within the range of the average feather drive and will give you a serious advantage over other machines.

edit: oops, I wrote short story, sorry!

I see, I might rethink that plan then. Had no idea flippers were that powerful for one, and it sounds like the downsides of using enough magnets to retrieve satellites from orbit probably outweigh any benefit they may bring, at this level of engineering on my part anyway.

Would it be better to, say, go with much lower levels of downforce over each wheel to help improve grip, or just going for better wheels to get that improvement? All I'm thinking is that my machine, at the moment anyway, is heavily focussed on pushing other robots about, and whilst 4 drills will be able to manage that, I'd not stand a chance up against say, Satanix's beefy drivetrain, or most of the others using the tougher motors... I was debating detachable side skirts like Storm II as well (albeit at the risk of looking suspiciously too similar to it) for when I come up against flippers, but it's the worry of them getting caught between floor panels...

4 drills will out push any robot of any drivetrain if it gets underneath. So, good grip is one thing, but unless you can wedge under another bot to make use of it, it's pointless. Focusing on design is probably best for now.

Enough downforce to make a difference is easily enough added later on; they don't weigh that much when you're only trying to get (gap between steel floor and magnet allowed for) 3-4kg pull.

Tormenta 2 is just 4 drills, 24:1 gearboxes and 100mm wheels, and whilst it's not particularly powerful, it gets underneath and shoves other machines around nicely.

Using magnets for extra grip is very plausible if its done right. Ellis is only partly right about magnetic hold-down adding strain the the drive train; there will certainly be extra force on the gearbox bearings and adding outside axle bearings is always a good idea, magnetic hold-down or not. While the bot is driving in a straight line the only extra stress on the drive train is the extra rolling resistance of the wheels, which will not be much. 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.

The extra strain comes when a 4 wheeled bot tries to turn with skid steering; the wheels won't break traction so easily and the motors will draw more current. This wouldn't apply to a two wheeled bot as its not skid steering. All bot designs would use more current in a pushing match as the wheels have more grip and need more current to break traction.

IMHO, adding around 5 to 10 Kg of magnetic hold down to a two wheeled bot would give it really useful traction while not increasing the current draw and drivetrain stress too much. The NIB magnets needed for that probably wouldn't be too large or expensive either; well worth some experimentation!

OK, I am getting interested in trying this out - another possible use for magnetic hold-down in on two wheeled vertical spinners. Imaging being able to turn sharply without gyro dancing! If I can get that to work on my new spinner bot, it would be a real game changer against most other designs.

How do people think the hold-down force can be tested on the bench? I have a number of salvaged NIB magnets, several electronic and mechanical scales, lots of junk to make jigs from and a short attention span.

Stick the magnets to required surface and try to pull them off with newton meter.

See, I totes know how to magnets, I was uh... just testing you. ;)

"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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Originally Posted by overkill

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.

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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!

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

On the magnet thing I do like the idea of a rolling magnet idea, the magnet being round will always contact the arena floor for maximum attraction. then use normal wheels for drive. there are lots of problems with this idea like how to turn the robot or what happens if the magnet picks up some steel like loose nuts or washers. but there are ways to mitigate this.

The pushing power is down to the friction between the tyres and the arena floor. by increasing the attraction between the floor and the tyres the pushing power will increase, but also the drive transmission load will be greater!

I say give it a go! Invent something new and refine the idea.

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Originally Posted by Flag Captured

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?

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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.

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

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.

And how many arena's have a steel floor?

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 :)

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 ;)

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!

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Originally Posted by Mouldy

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

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https://www.youtube.com/watch?v=KiSFeRcQ-Sg
http://cdn.memegenerator.net/instanc...0/44444359.jpg

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!

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.

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

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