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6 Month Gauss Cannon Project

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blackgrunge

Mon Dec 29 2008, 08:37AM

Registered Member #1889 Joined: Mon Dec 29 2008, 07:36AM
Location:
Posts: 55

So I've been working on this thing for a local science competition and I happened to stumble across this site on my way through the web. Very nice forum for this kind of stuff its nice to see so many people interested in this kind of thing. But I digress. I've been working on this one for about 6 month but I've been playing with the idea for about 3 years. It seems that only a few of the members on this forum have really reached a serious level in their designs. Though I did come across Yohan's project in the archives. I've posted this to see if I could impart some knowledge and answer any questions people have.

My setup is bordering on lethal so I hope mr. moderate man won't have problem with me talking about it. Currently I am competing in a competition so I won't be able to release all the ideas I've developed but here's some less specific stuff.

I have two power sources, one for testing out triggering circuitry and another for test firing. The first is rated at 450v 3x1500uf and the second which is being shipped as we speak is an upgrade from the old test firing equipment. The new power source is 400v @ 8x3900uf a piece which is just short of 2.5kJ; the equivalent of about a jar and a half of peanut butter (just to put things in perspective). I'm charging with a voltage multiplier coupled with a capacitor filter for low voltage peaks that I made myself. I'm currently working on building a new one that has a voltage divider built in that will use a variac resistor so that I can adjust the voltage instead of charging to 330v like I am now.

I'm using a series of high power SCRs unlike the playskool ones that you can get 500 for $30

. They are rated for 1400v rms and can handle (so far) pulses of 700 Amps. I'm not sure about their maximum surge capability but I guess with the new capacitor bank I'm getting I'll find out. I've mounted the SCRs with SCR mounts into a square Nylon rod so they aren't just lying on the table. The nylon during rapid fire holds up just fine for about 9 shots with the 1st capacitor bank but I wouldn't push it much beyond that.

My coils are optically triggered with a series of SCRs and shut-off SCRs that actually turn the SCR off by throwing an oppositely polarized pulse across the anode and cathode. Its working out pretty well but the more current that is pushed through the SCR the harder it becomes to shut off this way so I'm gonna have to start adding capacitors to the shut off circuit to ensure i'm getting a larger enough pulse. I've stripped down the gun so now it only houses one coil because I needed to do some more fine tuning. The barrel is made of carbon fiber which provides good strength and a relatively small wall thickness while also cutting down on eddy currents. The IR sensors I use I've tuned so that they are so sensitive that they pick up on body heat in a completely dark room. They are paired with a true IR emitter not a red light that people like to call an IR emitter, the beam is completely invisible and has a very focused beam. As a triggering circuit I'm using a hex inverting schmitt trigger logic gate that has a very high noise immunity. I'm running its outputs to transistors that boost the output current and then into an opto-isolator that isolates my SCR gate.

Here's some photos I took recently they are all clickable thumbnails since they are really high resolution and I am too lazy to resize:

Baby Capacitor Bank

SCRs

Got Logic?

*The brains of the operation*

Trigger Circuit (Logic Gate is out of socket because of soldering)

Low Voltage Coils (Just for Fun)

The Real Coil

What was left down range

Voltage Multiplier and Supplies

*IC sockets*

Backyard Skunkworks

Mon Dec 29 2008, 10:04AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

Cool setup man!

Of course given your statement of "bordering on lethal", I've gotta ask what kind of kinetic energy your getting. At 2.5kJ and 1% conversion, your already talking about a good 25 joules. If can get your gun to 5% efficient, you may surpass Donnie James' 7-Day CG cannon at 127J.

And I should point out the peanut butter jar and a half calculation is off, you forgot to divide by 1000 (because "calories" in foor are really kcal, me running a half marathon burns 2,000,000 actual calories, but only 2,000 "food calories"). I always pick at stuff like that...

Once again, cool setup; get some vids up once you get 'em.

uzzors2k

Mon Dec 29 2008, 11:07AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

You actually turn off SCRs? Not bad, don't think I've heard of anyone trying that before. Usually IGBT banks are used when the coil needs to be turned off again. I'm interested in hearing what your efficiency is at too. The down range photo shows some rods stuck in a cardboard box, I was hoping that was wood before I enlarged the photo.

You say you use optical switching, but I only see one coil. Do you plan on upgrading to a multi-stage?

rp181

Mon Dec 29 2008, 01:47PM

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

Whats your trigger circuit made up of? Im also working on one with remarkably similar specs. Thats a great idea to reverse voltage to shut the SCR. But Im thinking that would be a waste of energy and it would be worth it to design a good coil.

blackgrunge

Mon Dec 29 2008, 04:37PM

Registered Member #1889 Joined: Mon Dec 29 2008, 07:36AM
Location:
Posts: 55

@Backyard Skunkworks

Thanks, sorry about the peanut butter calcs I was a little sloppy

. The final design is expected to reach a 10% efficiency and this isn't the final power source either. I am doubling it again so it will end up being 5kJ and it will be a universal capacitor bank, meaning it will power all the coil (got some tricks that'll help me to that

). As far as reaching 10% let's just say I still have a few aces up my sleeve that no one has quite thought of yet...

@Uzzors

Like I mentioned in my post I have a multistage setup but I dialed everything back so that I could refine the coil design and fine tune my setup. I have all the triggers and circuitry to reassemble the multistage but the power supply isn't yet big enough to support more than one stage so it would be a really big waste. The reason its not big enough because as I mentioned in my rely to backward, I am using a centralized power supply that each of the coils draw from. That concept is not seen commonly in coil gun design because the capacitor bank must be relatively large. The upgraded bank will supply me with enough energy to maintain a high amp output over several coils (trust me on this one, I have the calcs to support).

Keep in mind that was only the results from the single stage with the 'baby' capacitor bank charged to 330v. The stripped down single stage setup I have now is capable of much more but for testing purposes there's really no need to push it to the max, not when I'm tuning my trigger circuits, its simply reckless on my part. I have some IGBTs but they are expensive and I've burned through several because they aren't designed to handle 1000amps surges that terminate instantaneously. I have tried to use them but I ended up destroying so many that it wasn't worth the money I was spending. If you or anyone else ever starts playing around with some of the higher level capacitors I recommend that you use GTO SCRs. They are a little less expensive than IGBTs but are much easier to deal with and are specifically built to handle high currents. The reason I don't use them is that the gate has a delay that's in the nanoseconds and I can't afford even those kind of delays in my design. But yah I'll put together the multistage when I'm ready, I want to rewind some new precision coils like the one in the photo. It's all about fine tuning and optimization at this point. I'm really just trying to prove a concept rather than make a big gun because a 30% efficiency speaks much louder than a big hole in the wall as you probably know.

@rp181

If you read the last paragraph of my post I go into the components of trigger system if you have questions beyond that I can PM you or email you some specifics because there are a few personal touches that I have patents pending on.

Don't get me wrong I'm not, not going to tune the coils but the SCR shut off operates on its own set of capacitors. It uses a pulse from a photo flash capacitor that is charged separate from the main capacitor bank. By shutting down the SCR no energy is wasted. The point of this if you read my replies to the other guys above, I'm using a centralized capacitor bank. SCR shut off is immediate and very effective. It saves the unused energy and immensely boosts efficiency by turning off the coil as soon as the projectile reaches the center of the coil. Here's a little website that helped me along my way and I think anyone interested in using this idea I came up with is more than welcomed to try: http://www.uoguelph.ca/~antoon/tutorial/triacs/triacs.html . Its a great method to save some money. Most of the time you have to use GTO SCRs or IGBTs to turn the coils off but with this you are able to save a little money. Now before anyone starts yelling at me about the stupid RCL circuit properties don't fret I address the problem of the coil storing energy. If anyone would like to know how either PM or just reply and I'll see what I can do about going into more depth about it. Its just a lot to talk about.

----------------

I'll post some requested vids in my next reply and I'll be sure to takes some photos of the new capacitor bank!! Hopefully it'll be here soon but at latest its should be here by friday.

guitardude012

Mon Dec 29 2008, 08:36PM

Registered Member #968 Joined: Fri Aug 24 2007, 04:54PM
Location: Norman, Oklahoma
Posts: 104

Uzzors wrote ...

You actually turn off SCRs? Not bad, don't think I've heard of anyone trying that before. Usually IGBT banks are used when the coil needs to be turned off again. I'm interested in hearing what your efficiency is at too. The down range photo shows some rods stuck in a cardboard box, I was hoping that was wood before I enlarged the photo.

You say you use optical switching, but I only see one coil. Do you plan on upgrading to a multi-stage?

I think he has re-invented the V-switch. Looks like that last link is down.

Cool CG =)

blackgrunge

Mon Dec 29 2008, 09:03PM

Registered Member #1889 Joined: Mon Dec 29 2008, 07:36AM
Location:
Posts: 55

Just copy and paste the link into the address bar. I'm not sure why the forum won't link it.

@guitardude012

Could you fill me in on what a V-Switch is?

El_Roberto

Tue Dec 30 2008, 12:13AM

Registered Member #1774 Joined: Wed Oct 22 2008, 02:51AM
Location:
Posts: 135

Nice to see more people getting into coilguns, I'm also building one similar to yours (Less power though). It will be a 6 stage 2Kj gun with the first two stages being 288 joules each and the last four being 360 joules each. Ive only had the ability to build the first stage so far (The electronics store isn't open for another two weeks) and it can shoot through a soda can, Ive tried shooting beer bottles but it just flattens the end of my steel bullets. One way (That Im probably going to use) of turning off the SCR's is to use an IGBT in series with all of the SCR's so that when the IGBT turns of the SCR's will turn off, I think this is a better way because it only uses one more component. Can someone please give us a detailed diagram or something of the V-switch? Ive seen it once but I still dont understand how it works and how the negatively charged capacitor doesn't detonate! Also blackgrunge what diameter bullet are you using? It looks a bit too small, nice setup though, can't wait to see more.

rp181

Tue Dec 30 2008, 03:52AM

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

Doh, acidental double post, delete this one =)

rp181

Tue Dec 30 2008, 03:55AM

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

A V switch uses 2 SCR's for one stage. One of the SCR's switches, and the projectile goes. Then the other switches opposite polarity, to quench the magnetic field.
Is a programmable delay incorporated into your design for each stage? Ive had great results with a optical triggering system using LM339 and 3 555's.
Also,
You should probably run your ideas by people, it might have already been done.
Have you seen stuff on magnetic flux linkage? A colloid of iron particles in epoxy surrounding a coil got 10%.

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