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jnbrex

Sun Oct 16 2011, 02:19AM

Registered Member #3950 Joined: Wed Jun 15 2011, 12:45AM
Location:
Posts: 51

I am entering this year's Intel ISEF science fair and am doing my project on electromagnetic projectile accelerators (coil g's).
My dependent variable is efficiency. I am limiting myself to a single stage for simplicity's sake. What are your ideas for some independent variables? I currently plan to measure:

â€¢ Wire Gauge
â€¢ Number of layers
â€¢ Coil length vs projectile
â€¢ Pulse time (maybe, difficult)
â€¢ Temperature (Liquid N2, Dry Ice and alcohol)

Any additional ideas will be greatly appreciated!
P.S. Please do not mention the g-word. I do not want my project to be disqualified as a weapon.

Forty

Mon Oct 17 2011, 03:40AM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

external iron is a good one.
also projectile material.

Adam Munich

Mon Oct 17 2011, 04:38AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

How about making a disklauncher? It's a hell of a lot more fun than a coilg.

Legit_bacon

Mon Oct 17 2011, 04:41AM

Registered Member #4034 Joined: Thu Jul 28 2011, 10:41PM
Location: somewhere in the Southern hemisphere
Posts: 138

I recently did a science fair on this, here are some of mine:
- Contact switch vs solid sate switching
- multiple stages
- Projectile, ie large small, material ect.

As for how to messure them, the best way is to measure velocity with a crono, but scone I, and probably you don't have one of those, I shot it a a roll of duct tape on m y floor and messured how far it rolled. This will depend on the power of of your g.. I mean accelerator ;)

Forty

Mon Oct 17 2011, 03:03PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

lol that's a fun way of making a ballistics pendulum. gotta account for the rotational kinetic energy and rolling friction though if you want to determine the energy accurately. for comparative studies all you'd have to look at was the distance it rolled though i guess.
If you want a more exciting way of measuring, you could always do the/our amateur way of "how many can sides does it shoot through"
One fairly accurate way that i can think of would be to take a metal block of known mass and put it on a uniform surface like a sheet of glass. tilt the sheet up until it the mass slides and record the angle. do that a few times so that you're sure you've got the right angle, and from that you can calculate the coefficient of static friction. then you can set the sheet at a known angle and time how long it takes the mass to slide down it from a known initial position. do that a few times and you can find the coefficient of kinetic friction. Now with those two frictions, you can set the sheet level and shoot at the mass with your LEMA (linear electro-magnetic accelerator) and from the distance it slides, accurately determine the energy of the projectile.
that may sound like a lot of work, but you can just do the calculations once and then put the equations into excel to have it calculate the energy for all your other trials. Also, the added steps of calculating the friction coefficients and how to derive the energy from the distance slid would make nice, interesting, scientific 'filler' for your paper/presentation.

Since your reported result will be efficiency, which will be calculated from the energy in and the energy of the projectile out, you'll need an accurate way of knowing the energy in. I would have a voltmeter on the capacitors at all times and record the voltage immediately before the shot, and immediately after. from those voltages and the capacitance (which should also be measured at some point, or calculated via RC time constant) you can find the energy stored in the capacitors before and after the shot, and the difference of those would be your energy input. Of course for comparative studies of the end efficiency, you could just record the initial voltage on the capacitors and assume all the other conditions either don't matter, or are a side effect of the independent variables (ie, changing the coil will probably change the energy left in the capacitors after the projectile leaves the coil, so the energy input would be different, but your results would just report that for a given capacitance and voltage, coil X transferred Y amount of energy into the projectile.)

This sounds like a fun experiment (evident in the fact that i wrote so much already.) Too bad I can't do interesting stuff like this for my classes. I'm stuck doing 'torque on a magnetic dipole in a uniform magnetic field / force on a magnetic dipole in a gradient magnetic field' and have to type the whole damn thing up in Latex.

ZakWolf

Mon Oct 17 2011, 06:37PM

Registered Member #3114 Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608

"CoilG" is a nickname it is really called an "electromagnetic accelerator" call it that instead of a "CoilG"

jnbrex

Tue Oct 18 2011, 12:27AM

Registered Member #3950 Joined: Wed Jun 15 2011, 12:45AM
Location:
Posts: 51

This is a very serious project. I think that external iron and projectile material are both great ideas (thanks, Forty). The mass of the projectile(s) will be measured using an extremely accurate electronic balance before each firing. The velocity will be determined using photogates, and then the kinetic energy will be found using Ke=mv^2/2. The capacitance of the capacitor bank will be measured using an LCR meter before each charging and firing. A voltage meter will be connected to the capacitor bank during charging and firing. Every single aspect of this will be very scientific, accurately and precisely.
I am looking for ways to improve the efficiency of electromagnetic projectile accelerators, and plan to find which factors produce the highest efficiency, then combine them to make a (relatively) highly efficient accelerator. This will probably include LN2 cooling, high-quality projectile material, and maximum magnetic field density and optimally controlled pulse time.
I had an idea that a slotted projectile might have greater efficiency by reducing eddy currents on the surface of the projectile. Does anyone have any experience with that? I am not sure whether the effect will be great enough to be measurable, or a waste of time.
Doing some pre-experimenting (normal steel projectile, normal coil) i have achieved efficiencies of 3.65% which I believe is relatively high on the scale of non-military electromagnetic projectile accelerators.
Thank you for all the help, and any more suggestions (especially out-of-the-box) will be greatly appreciated.
P.S. The title of this project will most likely be "Improving the Efficiency of Electromagnetic Projectile Accelerators"

Turkey9

Tue Oct 18 2011, 02:00AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

I don't really think that LN2 cooling will be a good idea. It will at most decrease the resistance of your coil making it tend more towards the underdamped LCR circuit. With electromagnetic accelerators, the most efficiency is had with a critically damped system. The inefficiencies come from converting the energy in the magnetic field to the kinetic energy of the projectile.

Although science fair is all about the process and a great project can be had from a situation others would call a failure.

I would definitely try the slotted projectile. I saw a paper a while back about a couple students that built an accelerator for their senior design project in college. It broke 20% efficiency if I remember correctly. You should also try powdered iron as a projectile material.

One of the biggest losses comes from the energy wasted in the emf quenching diode. If you could figure out a way to return that energy to the capacitors, your efficiency would sky rocket.

Forty

Tue Oct 18 2011, 06:53AM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

The best projectile i've come up with is a solenoid plunger. it's laminated, made of a high permeability metal, and already perfectly shaped with a pointed tip.
see examples here

although none of them look coated like mine. you could laminate them easily. using these instead of my nickel cobalt plated steel rounds instantly added about 25% more projectile energy for the coil i was using.

I haven't experimented with slotted/powdered projectiles because i like to have the rounds be sturdy and easy to reproduce. I suppose a mold for epoxy+powdered iron rounds wouldn't be very difficult, but i'd be worried that they'd shatter on impact.
The 'V-switch' circuit will return the back emf energy to the capacitors, but it won't do much in the way of imparting more energy to the projectile. It depends whether you want to improve the efficiency of the whole system, or just the increase the energy transferred to the projectile.

Turkey9

Tue Oct 18 2011, 11:37PM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

I believe the V-Switch is just a system that is able to turn off the main SCR at the right time, not return the energy to the capacitors. It's main advantage is that the coil doesn't need to be tuned perfectly in order to prevent suck back.

To get the best efficiency, a square wave is desired. This is really hard to do when dealing with thousands of amps but can be accomplished using an H-bridge and a very large bank of capacitors. The idea is to only use a small fraction of the capacitor energy so that the discharge is fairly uniform.

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