MagLev C

erikb
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Re: MagLev C

Postby erikb » September 10th, 2013, 9:55 am

I in general like the changes they made, such as guards for propellers, and no need to go for the most power or mass. I do think that 5 to 15 seconds is a really slow target time range to move a max 2kg maglev just a meter though.
A very easy way to do this: potentiometer. Done and done.
I'm not sure, but wouldn't this cause heat dissipation problems when you resistively current limit the motor? I think most pots are rated for 1/4-1/2 watts, so even for 15 seconds wouldn't it overheat and blow?
There is more then one way to fry a chicken.

Yes, you are going to have a hard time finding a pot that works for this situation. But rheostats are common and cheap.
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Re: MagLev C

Postby chalker7 » September 11th, 2013, 10:40 am

I agree that the event isn't quite as exciting as it was last year, but it was kind of necessary. The rich teams had a huge advantage and the teams with smaller budgets were kind of stuck.

This way, you don't just go and buy the biggest 9V motor you can find and run it with 3000mAh LiPo
I have said this before and I will say this again, money does not buy a winning device. Instead, I encourage you to think of the issue this way: resources contribute to and enable performance. The distinction between money and resources is critical, money is a resource but it isn't the only variable. Teams with more resources like time, access to professionals/equipment, expertise, experience, etc will also produce a quality device even if they don't necessarily have large budgets.

Similarly, don't think of performance simply as the fastest/longest flying/lightest/whatever is relevant one year. Consistency is also a performance metric. A team that professionally mills their chassis out of billet aluminum and has a custom circuit trace etched for their motor/battery is going to have a much more predictable and consistent time than a team that duct tapes cardboard together and twists their wires. In the former case, resources help enable the creation of such a device.

Finally, I don't want to be difficult, but a 3000mAh LiPo means effectively nothing for overall performance, that's just how much charge the battery can carry. What really matters (or mattered) in this event is how quickly it can discharge and how evenly it can discharge. Those ratings are the "C" ratings on a battery. A tiny 100mAh battery rated at 70C would outperform a huge 3000mAh battery rated at 15C in a short race. However, you could get many more runs (30 more, to be precise) out of the big battery. The people who work on these rules do know what they're talking about...sometimes... :)
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erikb
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Re: MagLev C

Postby erikb » September 11th, 2013, 12:50 pm

The rich teams had a huge advantage and the teams with smaller budgets were kind of stuck.
Stuck how? Winning comes down to who looks at the problem and comes up with the best solution. Then works hard at implementing that solution.

When you start out thinking that you can't win because of some reason you won't.

Last year Poudre placed 1,2,3,6 and 9 at nationals, in the build events. And less $100 was spent building the device for each event. The kids had to scavenge and recycle. Only the robotic arm received more and that was because the transmitters and receivers vanished over the summer.

And their was no fancy milling. The mag lev was balsa wood glued to magnets and motors. The track was stolen magnetic strips from a chalk board (we did offer to give them back after nationals) and left over wood from someone's wood project.

It's the solution and the time you put in that wins. Not if you can purchase it.
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Re: MagLev C

Postby GeoChamp96 » September 11th, 2013, 8:57 pm

I agree that the event isn't quite as exciting as it was last year, but it was kind of necessary. The rich teams had a huge advantage and the teams with smaller budgets were kind of stuck.

This way, you don't just go and buy the biggest 9V motor you can find and run it with 3000mAh LiPo
I have said this before and I will say this again, money does not buy a winning device. Instead, I encourage you to think of the issue this way: resources contribute to and enable performance. The distinction between money and resources is critical, money is a resource but it isn't the only variable. Teams with more resources like time, access to professionals/equipment, expertise, experience, etc will also produce a quality device even if they don't necessarily have large budgets.

Similarly, don't think of performance simply as the fastest/longest flying/lightest/whatever is relevant one year. Consistency is also a performance metric. A team that professionally mills their chassis out of billet aluminum and has a custom circuit trace etched for their motor/battery is going to have a much more predictable and consistent time than a team that duct tapes cardboard together and twists their wires. In the former case, resources help enable the creation of such a device.

Finally, I don't want to be difficult, but a 3000mAh LiPo means effectively nothing for overall performance, that's just how much charge the battery can carry. What really matters (or mattered) in this event is how quickly it can discharge and how evenly it can discharge. Those ratings are the "C" ratings on a battery. A tiny 100mAh battery rated at 70C would outperform a huge 3000mAh battery rated at 15C in a short race. However, you could get many more runs (30 more, to be precise) out of the big battery. The people who work on these rules do know what they're talking about...sometimes... :)
Perhaps I phrased that poorly. I was thinking about many of the teams I saw with rare earth tracks and >3kg cars, and how some teams don't necessarily have the same amount of resources (ours, for example. There's no way we would have spent the close to $1,000 some of those tracks can cost). Yes, I know there were many other ways to come up with a good score, but having it be a set time seems to me to be more accessible to all the teams. But hey, I don't know much. It's only my second year in the event.

The point I was trying to make with the batteries is that the 3,000mAh ones are generally more expensive than ones with less capacity of the same brand. Perhaps I phrased that pretty poorly as well. I apologize.

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Re: MagLev C

Postby gorf250 » September 11th, 2013, 10:38 pm

Most pots don't have the power rating for this event, but some are rated for much high power. Just remember, it's not the power from the motor, it's the power that's lost in the resistor, they're typically a large difference.
Based on some super quick (and probably mildly incorrect) calculations using the fact that my maglev drew about 20A last year, and a vehicle this year will need to draw about 4 amps (twice the mass, but ten times slower, I realize that these relationships probably aren't linear but close enough) we can roughly calculate the power consumed by the potentiometer. Using the formula P=I^2 R, a potentiometer with a resistance of around 2.5 ohms would consume about 40W. Plus you would need a pot with an extremely low resistance to avoid drawing all the power in the circuit. Sorry if my calculations are blatantly wrong.
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Re: MagLev C

Postby FawnOnyx » September 11th, 2013, 11:51 pm

Based on some super quick (and probably mildly incorrect) calculations using the fact that my maglev drew about 20A last year, and a vehicle this year will need to draw about 4 amps (twice the mass, but ten times slower, I realize that these relationships probably aren't linear but close enough) we can roughly calculate the power consumed by the potentiometer. Using the formula P=I^2 R, a potentiometer with a resistance of around 2.5 ohms would consume about 40W. Plus you would need a pot with an extremely low resistance to avoid drawing all the power in the circuit. Sorry if my calculations are blatantly wrong.
I agree, that's a pretty sketchy estimate, but I guess it'd be around there. Just looking at the rheostats on mouser that fit those specs though, the cheapest ones are already around 40 bucks, pretty high.
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Re: MagLev C

Postby iwonder » September 12th, 2013, 4:28 am

Based on some super quick (and probably mildly incorrect) calculations using the fact that my maglev drew about 20A last year, and a vehicle this year will need to draw about 4 amps (twice the mass, but ten times slower, I realize that these relationships probably aren't linear but close enough) we can roughly calculate the power consumed by the potentiometer. Using the formula P=I^2 R, a potentiometer with a resistance of around 2.5 ohms would consume about 40W. Plus you would need a pot with an extremely low resistance to avoid drawing all the power in the circuit. Sorry if my calculations are blatantly wrong.
I agree, that's a pretty sketchy estimate, but I guess it'd be around there. Just looking at the rheostats on mouser that fit those specs though, the cheapest ones are already around 40 bucks, pretty high.
You mean like this one?
http://search.digikey.com/scripts/DkSea ... T50-5.0-ND

And your vehicle probably had two motors last year(?) which is also illegal this year, which means a lower battery voltage and it changes the numbers up.
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Re: MagLev C

Postby chalker7 » September 12th, 2013, 7:35 am

Perhaps I phrased that poorly. I was thinking about many of the teams I saw with rare earth tracks and >3kg cars, and how some teams don't necessarily have the same amount of resources (ours, for example. There's no way we would have spent the close to $1,000 some of those tracks can cost). Yes, I know there were many other ways to come up with a good score, but having it be a set time seems to me to be more accessible to all the teams. But hey, I don't know much. It's only my second year in the event.

The point I was trying to make with the batteries is that the 3,000mAh ones are generally more expensive than ones with less capacity of the same brand. Perhaps I phrased that pretty poorly as well. I apologize.
No need to apologize, I was just trying to make a point that resources in general are what produce results. Money is only one of those, and yes, we do think about it a lot. If a team spends 1000 dollars on their track but then don't have enough time to test it, they won't do well. I have seen similar things happen time after time.
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Re: MagLev C

Postby joeyjoejoe » September 15th, 2013, 6:37 pm

This may be a little early in the game, but we have built and are testing an early (and nowhere near final) 2kg prototype that is controlled via rheostat and have noticed a peculiarity that is making long runs (around the 15s mark) very difficult to control.
It seems that, even with friction nearly out of the picture (our car has bearings at the corners to reduce friction with the track), the speed at which the prop needs to rotate to provide the longer run times requires a push to even get it moving at all. Its really weird. We have a "15s" mark on the hand-written dial of the rheostat. When we set it to that mark and turn the car on, the prop moves but the car doesn't. Then, if you merely touch the rear of the car (surely mondo illegal), it starts moving at the prescribed speed. If you then set the prop to the lowest speed that will move the car with no help, it completes the run in around 9 seconds. We thought we'd have this one knocked out quickly so it would be "on to the Mission possible and Scrambler events". Oops!
Just curious if anybody else has seen this.


Also, regarding rule 3h (I know Chalker.... rules clarification), what is your opinion of the following blurb:
"Propellers....... must be shielded from direct contact such that the event supervisor is not able to make contact with the propeller with a standard 1/4" dowel."

Does this merely imply a ducted-fan design? If so, then you could stick a dowell inside the duct to contact the prop easily. Do you think this means that some sort of screen needs to be employed at the ends of the duct to prevent any contact with the prop whatsoever?
Any input is appreciated.

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Re: MagLev C

Postby gorf250 » September 15th, 2013, 7:07 pm

This may be a little early in the game, but we have built and are testing an early (and nowhere near final) 2kg prototype that is controlled via rheostat and have noticed a peculiarity that is making long runs (around the 15s mark) very difficult to control.
It seems that, even with friction nearly out of the picture (our car has bearings at the corners to reduce friction with the track), the speed at which the prop needs to rotate to provide the longer run times requires a push to even get it moving at all. Its really weird. We have a "15s" mark on the hand-written dial of the rheostat. When we set it to that mark and turn the car on, the prop moves but the car doesn't. Then, if you merely touch the rear of the car (surely mondo illegal), it starts moving at the prescribed speed. If you then set the prop to the lowest speed that will move the car with no help, it completes the run in around 9 seconds. We thought we'd have this one knocked out quickly so it would be "on to the Mission possible and Scrambler events". Oops!
Just curious if anybody else has seen this.
I was worried about this when I first saw the rules. In order to take 15s, the car requires a very low force. However, if this force is less that the force of static friction between the car and the side rails, it will be impossible for the car to move, without a push-start from some sort of larger force. I don't know how this could be remedied, without sonehow decreasing the friction between the car and siderails.
As to your secod question, nothing official but I interpreted it as a complete shield encasing the whole fan, so probably some sorr of mesh or screen.
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