MagLev C

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FawnOnyx
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Re: MagLev C

Postby FawnOnyx » January 25th, 2014, 6:26 pm

nah, i was not referring to you actually, was more of a general question, i have a surplus of over 200 1-7/8 inch by 7/8 inch by 1/2 inch ceramic magnets each with a 3 pound lift force and i thought i might be able to make a track for my schools other team and i am absolutely stumped as to how i do this. the only way that makes any sense is to anti-align them and as far as i know that would demagnetize them very quickly. i've seen other teams use ceramic magnets so i was wondering how people do it. Do people really anti-align them like that and does the demagnetization actually happen or is it not noticeable?
My teammate and I have build two tracks out of ceramic magnets in the last year. The demagnetization was definitely noticeable over time - when we build the first track about 13 months ago, it comfortably levitated 2000g, and it now only supports about 1200g. We're planning to build a third track next month using stronger ceramic magnets such that we won't have any significant demagnetization problems before states.
Interesting, I've noticed something really similar with our track, except it's demagnetized unevenly so there are magnetic hills and valleys messing up our consistency. Then again we've been using it a lot for a year now and it's time for a new one. Does anyone have any tips for ceramic magnet maintenance though? I suppose not leaving magnets near the track for prolonged periods of time helps.
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MomCoach
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Re: MagLev C

Postby MomCoach » January 30th, 2014, 7:04 pm

I ran Maglev at an invitational a couple weeks back. It was very depressing. Only 6 of the 19 vehicles didn't have construction violations. Nearly all of them were in the propeller shielding.

I did find one problem with the scoring spreadsheet. It doesn't allow a weight less than 250g (or over 2000g), even though that is just a construction violation. You have to change the data validation for the weight cell to allow weights outside the allowed range.

Also a hint on rheostats. If you put a resistor in parallel to the rheostat, you effectively decrease the range of the rheostat. My kids put a 10 ohm resistor (a 10 watt rated one) in parallel to a 25 ohm rheostat. The effective range is 0 to 7.1 ohms. If you just use a 25 ohm rheostat, the usable range is a tiny fraction of the total movement -- very little control possible. The first resistor they tried was only 1/4 watt (made a nice smoke cloud when they turned on the system).

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Re: MagLev C

Postby joeyjoejoe » January 31st, 2014, 4:29 am

What type of violations did you see with the shielding? Was it not present or just insufficient? This was a question we had when building ours. We used fabric netting with 1/8 inch holes to shield ours and if you held the back of the car with one hand and forced a 1/4 in dowel through the front of the fan with the other hand you could lightly touch the blade. It would just take a great deal of force. Still, not one cut finger this year so it must be working!

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Re: MagLev C

Postby MomCoach » January 31st, 2014, 5:57 pm

About half were no shielding at all. I think they just used a vehicle from last year. Several shielded the front, but left the back open or had a gap in the front material that was big enough to let a dowel through. I never 'pushed' the dowel -- if the holes weren't big enough, that was fine by me. Several used a ducted fan with just a cover on the front and back. One of my team's trial models started with that, but the material had too few openings and they didn't get any usable thrust from the motor.

In my view, the rule should be that a 3/8 or 1/2 inch dowel, 4 inches long can't get to the blades -- that's a lot more like a finger than a 1/4 inch dowel, but that's just my opinion.

I'm running the event again at an invitational next week. I hope some of the teams have read this year's rules…...

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Re: MagLev C

Postby chalker » January 31st, 2014, 7:25 pm

.....

I did find one problem with the scoring spreadsheet. It doesn't allow a weight less than 250g (or over 2000g), even though that is just a construction violation. You have to change the data validation for the weight cell to allow weights outside the allowed range.
...
Rule 5.b.ii explicitly states vehicles aren't to be run unless they meet all construction specs. Thus the spreadsheet is correct in not allowing you to enter weights outside that range.

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darkwinters
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Re: MagLev C

Postby darkwinters » January 31st, 2014, 9:31 pm

Just a question (and this may have been clarified earlier): If we're allowed to use variable resistors (i.e. potentiometers/rheostats), what is the point of the rule disallowing integrated circuits?

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Re: MagLev C

Postby joeyjoejoe » February 2nd, 2014, 6:48 pm

Just curious if anybody has been able to get 15s with a 2kg car. With our car at 1kg or even 1.2kg, we can make it pretty reliably. It is still a little iffy at the upper end (>12s) due to the magnetic hills and valleys mentioned on this thread. BTW, this track issue only occurred 2 months after building our track and therefore probably has to do with the aforementioned demagnetization. (note to self: build track in January next time).

At or around 2kg however, our car will not move after setting our rheostat unless its given a slight nudge (obviously a violation).

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Re: MagLev C

Postby Master_Sneeze » February 3rd, 2014, 8:09 pm

This may sound very uninformed, but I recently purchased a potentiometer for our vehicle. It's rated for 25 ohms, but I'm fairly certain I burned it up considering the smell that came from it. So my question is how do I determine the necessary rating for the potentiometer? Our current set up is using two 9 volts in parallel, but we could easily drop down to one if necessary.

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Re: MagLev C

Postby iwonder » February 3rd, 2014, 8:10 pm

Ahh, 25 ohms may be a little much resistance depending on your motor, but the issue you had was the power rating. Look for a potentiometer with a higher power rating next time, yours if probably 1/4W or so (it should say on it), maybe go up to 1/2W or 1W.
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Re: MagLev C

Postby joeyjoejoe » February 6th, 2014, 10:00 am

Just a question (and this may have been clarified earlier): If we're allowed to use variable resistors (i.e. potentiometers/rheostats), what is the point of the rule disallowing integrated circuits?
I'm not sure either but you could build your own if you wanted. Just make sure you don't use ICs. Resistors, capacitors, inductors, diodes and transistors should be just fine.


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