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Sliding contact coilgun

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mogallin

Thu Dec 25 2008, 02:17PM

Registered Member #1883 Joined: Thu Dec 25 2008, 01:58PM
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Posts: 15

Im quite new to this thing with coil & railguns and I'm thinking of building a small one myself...

I know there is quite an advantage in having multiple stages in a coilgun, only this thing with timing and optical sensors seem a bit to advanced for me. So my thought was that the switching might be done somwhat like this:

If im not totally wrong this would, sort of, provide 100 of overlapping stages being switched on and of automaticly by the projectile, though at the cost of a higher friction...

Since nobody else seem to be trying this out I am thinking its just a stupid idea however i cant find anyone having tested it and confirmed that its not working.

Would it work?

rp181

Thu Dec 25 2008, 06:06PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Posts: 1529

I would work, but you would need a SCR/IGBT gate to make contact. If you tried to dump the whole bank through sliding contacts, it would weld/vaporize and waste energy.
Optical triggering is much simpler than you think, all you need is a LM339 IC, 2 resistors, and the photodetectors.
In fact, i was going to post a page on optical triggering on my website (i JUST got a delay circuit working).
No one does this because of the difficulty of putting the contacts, when you can just use optical. I will post on this thread later when i update my site.

mogallin

Thu Dec 25 2008, 07:08PM

Registered Member #1883 Joined: Thu Dec 25 2008, 01:58PM
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Posts: 15

Thanks,
ill have a look around and see if I can find a nice tutorial for this optical triggering :)

rp181

Thu Dec 25 2008, 08:31PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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I wasn't able to find one, ile try and put one up on my site today.

Barry

Thu Dec 25 2008, 09:45PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

rp181 wrote ...

I would work, but you would need a SCR/IGBT gate to make contact. ...If you tried to dump the whole bank through sliding contacts, it would weld/vaporize and waste energy.

rp181, I don't understand, please explain? Why not just lower the input energy or increase contact area until it doesn't vaporize.

Mogallin, thanks for the pointer to Phil Putnam's documents on electromagnetic launchers. As it turns out he offers a whole series of documents in some sort of 2007 EM launch contest:
* Home Page - Lifeboat Foundation EM Launch Competition
* Chapter 0 - Introduction
* Chapter 1 - Milestones in Cannon Launch to Space
* Chapter 2 - Forces in Electromagnetic Launchers
* Chapter 3 - Railguns
* Chapter 4 - Sliding Contact Coilguns
* Chapter 5 - Copper and Superconductor Materials
* Chapter 6 - Rules and Seed Money

Fascinating reading!
Thanks and Merry Electromagnetic Christmas! ~~Barry~~

rp181

Thu Dec 25 2008, 10:43PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Posts: 1529

Lower input energy means less power, and larger contact area would mean larger bore. It could be made to work, but it would be much simpler to use a solid state switch with its gate connected to the sliding contact, unless cost is a major issue.

Barry

Fri Dec 26 2008, 06:10AM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

The document shows the moving coil contact is completing the power path. It must carry full current for both coils.

Look again at Figure 4-2 in the PDF file. Unfortunately it doesn't show all connections but here's the setup.
* The stator coil is only connected to one power terminal at one end.
* The armature coil is connected to the other power terminal, either through flexible wire or an external rail.
* The sliding contact completes the circuit.

This arrangement guarantees the stator to the left of the armature is 'on'. As the armature moves, additional 'on' coil length is added to the stator. It really doesn't make any sense to hypothesize a solid state switch; are you going to add a switch for each turn of the stator coil? Not on my budget!

I'm not suggesting this represents a very practical coilgun design, but the concept is simple and elegant and cheap. It is a huge advantage over railguns to have multi-turn stator and and armature coils, which improves the mutual inductance and coefficient of coupling. Phil Putnam says "the mutual inductance gradient is 300 times larger than that of a typical simple railgun."

Besides, if railguns can carry such large current in sliding contact then this isn't much different. Surely similar techniques with brushes and rails can be applied here. But a solid state switch? I don't see how or where it applies.

Barry
Why don't you ever see the headline "Psychic wins lottery".

mogallin

Fri Dec 26 2008, 11:20AM

Registered Member #1883 Joined: Thu Dec 25 2008, 01:58PM
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Posts: 15

Thanks alot for your great replies!

What i was thinking was to use the idea of sliding contacts in a more traditional rail/coilgun having a rather normai projectile except for it conducting some electricity between one rail and one point of the coil and then from another point of the coil to another rail so that there would be a magnetic field moving just in front of it. The rails should ofc be inside the coild so that the projectile wouldn't have to have a hole in it.
94263223qd5

(just a quick sketch i made; orange is conductor, dark grey is some metal attracted by magnetism & the light grey some other material not attracted by magnetsim. Also it should have been white inside the coil, not light grey)

A totally of topic question:
Would a relay work as switch in a normail one stage coilgun? ( It was used in this guide:

)

rp181

Fri Dec 26 2008, 03:19PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

A realy might work. How much energy are you planning to use?

mogallin

Fri Dec 26 2008, 04:07PM

Registered Member #1883 Joined: Thu Dec 25 2008, 01:58PM
Location:
Posts: 15

Either 2 or 3 capacitors from disposable cameras, guess they are 300v 100uF each (im not sure if one of them has gone bad from bad treatment...)
or Ill buy something. (thinking of a Ellytkond LP 470uF/400V, its from a swedish site if the name is somewhat confusing)

And speaking of buying capacitors, will all work or just some? I am wondering because on the one I've found it says it suits filters in power supplies, or at least that is how it translates into english.

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