Magnets, how do they work?

[daveimi]

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

[Ellis]

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

[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


IMG_0105.jpg
IMG_0106.jpg
IMG_0107.jpg
IMG_0113.jpg
IMG_0117.jpg

[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

[craig_colliass]

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.

[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:

_2082059.JPG

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:

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



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;



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;



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.



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