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huzbum

Sat Apr 12 2008, 04:10PM

Registered Member #1439 Joined: Sat Apr 12 2008, 03:25PM
Location:
Posts: 5

I'm new to this site, but I've been kicking around the idea of a coil nail gun for a while. I intend to build a bench (non portable) coil gun to start with, then revise the design into something portable. Before getting too deep into design (or buying components) I wanted to run some idea's past people who have already built coilguns.

I want to have 3 stages with no timer circuits! My idea is to have 3 1kv capacitor banks the first being about 250uf, the second 500uf, and the third 750uf. My theory is that if triggered at the same time the capacitance will make each stage's magnetic field will collapse in order, and all I would have to do is get the spacing correct.

Any thoughts on this theory?

Also, what would it take (power wise) to reach projectile speeds of 500fps? I intend to use nails with the heads cut off... By the way, I have no concern for efficiency, only projectile speed.

Bjørn

Sat Apr 12 2008, 05:20PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Your theory works but in reality nothing is that simple. getting the spacing correct is difficult since the spacing is not constant. It will depend on temperature and other factors.

You will spend 2 kW of power to launch one projectile every second or about 2 kJ for each shot depending on a lot of factor that is undecided yet. You can do a lot better of you spend more time and make it more advanced.

Have a look here:

Barry

Sun Apr 13 2008, 01:15PM

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

Been there, done that, with a three-stage model.

It turns out that a multistage coilgun with fixed timing doesn't work because the timing is exquisitely sensitive to starting position and other factors. The results were so variable that it might even fire backwards. (Imagine my surprise.) This was my Mark 1 coilgun first attempt and it had other problems, e.g. lots of turns with thin wire resulted in high inductance. But even careful control of timing with digital logic could not overcome the fundamental flaw. A multistage coilgun really needs a position detector at each coil.

Cheers, Barry
The meek shall inherit the earth ... if that's okay with you.

TheMerovingian

Sun Apr 13 2008, 06:28PM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

Alternatively hybrid design using optical sensor and timing (both analog or digital) can work well. I achieved 10% efficiency on my halfbridge coilgun rifle (three stage) usin analog timing (555 with trimmers and poliester caps). This way you regulate only the pulse width and not che phasing with the projectile. Now I am working on a 5 stage multistage dump-n-quench coilgun with digital control (PIC16f876A ) with the possibility of correcting the pulse times during operation, by means on a 4-digit numeric display and 3 button keyboard (MODE, UP,DOWN) that controls ammo in magazine, cap voltage (ADC-on board), cap setpoint, shot counter, password and pulse regulating for each stage

pbfree

Mon Apr 14 2008, 01:06AM

Registered Member #1386 Joined: Tue Mar 11 2008, 08:16AM
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Posts: 13

Start position should be and is the low hanging fruit. That position should be fixed and voltages regulated regardless of triggering method. These two are the easy parts. Struggle here and might as well not build it.

Quantum Singularity

Mon Apr 14 2008, 03:26PM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

Yeah, i'd say if you dont want to mess with timing/optical triggering, then just focus on making the best single stage you can. When you feel up to the timing/triggering challenge, then go for the multi-stage.

Barry

Mon Apr 14 2008, 04:08PM

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

The "best single-stage design" offers some big advantages. It frees you from the complexity of detectors and such, giving you a lot more time to concentrate on all the basic design issues. There are a lot of valuable things to be learned and I find it an advantage to work in a coilgun's simplest-possible-world. Particularly for your first coilgun, it's best to get one stage working well before compounding the complexity with multiple stages.

Maybe it's just me. But I feel that the single-stage coilguns are still so wretchedly inefficient that I need to get this working well before building multiple inefficient stages.

Cheers, Barry
Any e-mail returned to me will be re-sent with a 10KV attachment.

huzbum

Mon Apr 14 2008, 08:37PM

Registered Member #1439 Joined: Sat Apr 12 2008, 03:25PM
Location:
Posts: 5

I intend to build it as a growing experiment. Start with a single stage and optimize that before adding a 2nd stage, then optimize, then add the third stage, and of course optimize...

Barry to me it sounds like your gun lacked consistency. I imagine many variables can be isolated, if at the very least in a bench /experimental scenario.

I guess I need to ask, how consistent/accurate are everyone else's coilguns? I suppose if you can't get a single stage to operate consistently, then you can't make assumptions about when to fire the 2nd stage...

Quantum Singularity

Tue Apr 15 2008, 01:42PM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

I'm sure some of the most sophisticated and professionally built ones are far more consistent but the average ones can be very inconsistent. There are many variables, and many build on top of each other. Even if you only had just one variable that was inconsistent, on a multistage gun each stage would multiply this inconsistency exponentially. Now imagine you have 10 inconsistent variables... Even with a good single stage gun I often measured projectile speeds that varied by at least 5-10%. On a multi stage gun that doesn't use any kind of position sensing of the projectile, if the position of the projectile going into the second stage was even slightly off then it would impact the second stage severely, and the third stage might be completely off.

It really depends on how advanced the design is... but if your going to make it that good then you might as well have all the optical triggers and microprocessor controlled timing, etc. For example, how are you going to charge the caps and how accurate will that be? Will you have a comparator to keep the caps charged to a specific target within 1 volt accuracy? If your just charging them 'by hand' then that's a big variable right there. And if your charger doesn't maintain a specific voltage the caps will slowly bleed even over the course of a few seconds time it takes you to pull the trigger. And electrolytics aren't the most consistent caps either, especially pulse discharging them like we do, I imagine every shot will have slightly different ESR's and ESL's, progressively getting worse over time.

You can see the consistency of my last induction launcher that I did three different days testing:

All tests were done with same coil and same cap but various voltages. Just compare the same voltage shots and resulting velocity for different runs.

I think you are on the right track though, start with a single stage and then later add on as you learn to optimize. I just think in the mean time you should be studying up on optical triggering, timing, etc, not discounting them. It will probably work much better and you'll learn more. Of course many folks here have built there first coilguns before reading about them online like myself, and we have learned from failed or poor designs. You can always build whatever you want even if its less than ideal, were just trying to give you the best advice thats all.

huzbum

Tue Apr 15 2008, 04:13PM

Registered Member #1439 Joined: Sat Apr 12 2008, 03:25PM
Location:
Posts: 5

hmm... point taken... Don't expect consistency... Thanks.

I'm stubborn, so I'll probably try to make it work anyways, but I'll be planning for failure in that department.

At what point do you trigger your 2nd coil? some point before it reaches the coil? or as it's in the coil?

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