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After building a small 6mm coil gun I decided it was time to upscale... To a .50cal...
Youtube video of it in action.
This would mean a dramaticaly larger capacitor bank and equally large solid state switches. Whilst under construction I was approached by a Professor in the Engineering Department asking whether I could adapt it for use as a demonstration piece on electromagnetic acceleration. It only involved a few safety measures so I agreed and they gave me funding for components. The big buy was going to be a high current and voltage solid state switch. Luckily a Mitsubishi CM400HU-24F IGBT was supplied by a member of staff in the department who works on the power systems in trains.
Youtube video of it in action.
This would mean a dramaticaly larger capacitor bank and equally large solid state switches. Whilst under construction I was approached by a Professor in the Engineering Department asking whether I could adapt it for use as a demonstration piece on electromagnetic acceleration. It only involved a few safety measures so I agreed and they gave me funding for components. The big buy was going to be a high current and voltage solid state switch. Luckily a Mitsubishi CM400HU-24F IGBT was supplied by a member of staff in the department who works on the power systems in trains.
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Signing UpStep 1The Theory
A coil gun works on the principle of electromagnetic attraction. The coil gun itself is akin to an air cored solenoid. A ferromagnetic armature is placed at the breach of the coil and is electromagnetically attracted towards the centre of the coil when a current is passed though the coil. When the armature reaches the centre of the coil it will start actively decelerating if the current is still passing through the coil as it is magnetically attracted to the midpoint of the coil. This means that the current pulse length needs to be of a finite length in order for the armature to be accelerated up to the centre of the coil and not ‘sucked back’ as it tries to leave the coil via the other end. This is more specifically a ‘reluctance coil gun’, a diagram is shown below.
This is the biggest disadvantage to coil guns, the current pulse length needs to be controlled in order to cut the power when the armature reaches the middle of the coil.
The second largest challenge to overcome with coil guns is delivering the electricity as quickly as possible. It stands to reason that the more energy you can put onto the coil gun, the more energy will be transferred into kinetic energy in the armature.
The main losses in this system are the eddy currents in the flyway tube and the projectile/armature, these can be minimised by slotting the flyway tube or using a non-conductive material. The projectile must be ferromagnetic thus this means that limiting eddy currents cannot be achieved by using a non-conductive material. The best solution to this is to use a powdered Iron in epoxy resin matrix. Although this improves initial efficiency it also removes the ability for the gun to be operated as an ‘inductance coil gun’.
This is the biggest disadvantage to coil guns, the current pulse length needs to be controlled in order to cut the power when the armature reaches the middle of the coil.
The second largest challenge to overcome with coil guns is delivering the electricity as quickly as possible. It stands to reason that the more energy you can put onto the coil gun, the more energy will be transferred into kinetic energy in the armature.
The main losses in this system are the eddy currents in the flyway tube and the projectile/armature, these can be minimised by slotting the flyway tube or using a non-conductive material. The projectile must be ferromagnetic thus this means that limiting eddy currents cannot be achieved by using a non-conductive material. The best solution to this is to use a powdered Iron in epoxy resin matrix. Although this improves initial efficiency it also removes the ability for the gun to be operated as an ‘inductance coil gun’.
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You can make a coilgun more powerful by putting more energy in. That isn't quite as easy as it sounds though. You could increase the length of your projectile, the coil, and increase the size of your Cap bank in accordance with this. You can also place a secondary coil after the first to further accelerate the projectile when you reach the limit of what can be done with one coil. You can then keep stacking coils but each coil adds much less power than the one preceding it so it's diminishing returns.
The US military are researching them for use in long range weapons for ship but the big problem is that when they try and fire it, the recoil would sink the ship! They were also considered in WW2 for shooting down aircraft as the projectiles are cheaper and easier to produce. All it needs is electricity to fire them and it's rate of fire is almost unlimited, storage of projectiles is less dangerous because there are no explosives involved.
http://search.digikey.com/us/en/products/FGA180N33ATDTU/FGA180N33ATDTU-ND/1916864
I would have to buy 15 IGBTs or so to put in parallel to handle the amps, and that would cost around 60 bucks, but I was wanting to build another coilgun/railgun, and though as fun as manually triggering it is, it looses a lot of bang. I built a railgun before, now Im thinking coilguns are more efficient. Eh, only difference is rails or coil, if I change my mind Its only gonna cost me like 5 bucks.
So about 15 in parallel wold work?
The best method I have found is to stretch out two pieces of paper one meter part and put a microphone in the middle of them (out of the projectile flight path of course). Record the sound on your computer as you fire the coil gun and then use audio editing software to measure the time between the two bangs as the projectile hits each sheet.
Alternatively you can put it 1m off the ground and shoot it horizontally into sand. You can see then measure to the imprint where it landed and use SUVAT equations to calculate the initial velocity.
Finally you could video it shooting in front of a background of equally spaced black and white strips and measure how many it goes over per unit time or how much time it take for it to travel over say ten of 1cm width.
Does this help?
I have done the camera approach and a standard camera films at around 25frames per second so over a one meter length it could measure up to 25m/s. I found that at 21m/s it was inaccurate as the projectile was blurred from the lengthy exposure time of my camera. You don't need to slow it down. You need to see it frame by frame with the frame time stamp to 100ths of a second. I used a free trial of Adobe's software available from their website. These others are also available:
http://www.desktop-video-guide.com/top-5-free-video-editing-software-review.html
As long as you can guarantee that the projectile will pass through your two light gates and you can get that signal into the computer then that will be your most accurate and easy option. I would posticulate that the PT signal would be easier to analyse than the microphone method as it will have less noise and a clearer step change as the beam is broken.
Most sound cards run at 48kHz so over 1cm you could measure up to 480m/s though accuracy would be much better at velocities closer to half that.
You should also make your measurements as large as possible as if you can only place your PT's accurate to +/-1mm then you have 4mm of uncertainty in the PT's position which is 40% of the measure so your velocity is effectively an estimate. If it were 100mm then it would be 4%, 1000mm 0.4%uncertainty, you get the idea. Also it means that the time would be longer between the gates so you could measure that more accurately in the software.
At 450v you could use full wave rectified mains maybe from a 12v-240v inverter passed in series through a neon tube on the AC side of the bridge. This would only charge to around 340v though. a little short of your 450v capability. You could use two in series to get 680v then use a charge regulator to limit it to 450v. Or simply watch the voltage across the caps and cut it before it goes over 450v.
Your coil and your projectiles are undoubtedly what are costing you performance. Firstly your 5/16 bolt has a thread which will mess up your flux linkage from the coil to the projectile and effectively introduces an air gap. It will also have more friction and air resistance. You want a smooth non-electrically conductive, ferro-magnetic projectile with flat ends. you can buy ferrite cores off ebay in short lengths.
Your coil will also be stunting its performance. I don't have my simulator on this computer so I can't tell you what coil you want right now but the dimensions of yours seems wrong to me. Firstly the L/D ratio is too high. Your coil should be the same length as your projectile and your projectile should be roughly 5-9 times as long as it is diameter. 4"=100mm is too long. 5/16"=7.9mm so 40-72mm (2"-3") is about right.
At that much power you also want thicker wire, 14AWG or thicker. I can't be sure without checking the maths but three layers seems to few to me. My experience suggests that somewhere around double that would be better.
Are you going for raw power, as in projectile kinetic energy, Piercing power, as in pressure per unit area, or projectile velocity? Each is a different coil and projectile. High velocity wants a light weight projectile and a longer coil with fewer layers. Raw power means you want a large diameter heavy projectile with a short fat coil. Your talking over a 0.50cal. For piercing power there is a compromise to be drawn between diameter which you want thinner, and weight which you want higher but you can't increase the length too much...
Finally it sounds like your coil is badly formed. your plastic tube may be too thick, your projectile may not be a snug enough fit in the barrel or too tight and stick. Your coil should have end plates too that the coil is formed between so it is nice and square in cross section. You should coil it nice and tight and glue each layer in place, one at a time. Try to minimise the gap between the coil and the projectile. The easiest way to do this is to use your projectile as the coil form. just wrap a piece of paper or plastic around it first, one thin layer.
To be honest I know nothing about Static Transfer Switches. This could be inefficient and waste energy compared to solid state alternatives. I appreciate that as these voltage and currents they can be rather expensive though. I'm using a pneumatically actuated spark gap switch with two ali blocks and a copper peg with a morse taper submerged in mineral oil. No current and voltage limits in my range and loses are minimal.
If you want any further help just ask :)
http://205.243.100.155/frames/shrinkergallery.html
Bert is a very cool maker of strange things , (like the magnetic gun), when he makes his items he will erase your cred cards magnetic strips and the end result is a hoot.
thanks fer th efeed back
Getting to the point: They are variations on a 55mm long, 13mm diameter solid soft iron bar. They weigh from a little over 40g.
Right, you have a couple of options.
First you could use full or half rectified mains. The simplest way to do this is to use a diode from the live followed by a mains rated switch and then a light bulb fitting with a 20W light bulb in it. In the UK you'll get just under 340v DC out. In the US if the mains is 120v then you'll get 170V which isn't a lot of use to you. If you use this method you need to use a voltmeter to monitor the charge level and make sure you don't overcharge them. Also put a couple of mega ohm resistor across the capacitor bank to make sure it can't be left charged and will naturally discharge itself.
You could use 12v DC to 240v AC inverter to source your mains. they're available on the net for about £20 and will run off any 12v source though a decent size lead acid or gel cell would be recommended.
If either of the above options are used then a diode bridge could be used to double the charge rate with full wave instead of half wave rectification..
The other way of doing it is to use a step up buck boost converter but they are a lot more complex to make and explain here.
You could also just link up several camera flash chargers in parallel or add a second 1.5v battery. This is a good way to blow up circuits though so keep a few spares. Camera flash chargers aren't designed for that kind of punishment and don't usually last that long.
Hope this helps, If you want more help then let me know and I'll draw you some circuit diagrams when I get home.
470uf capacitor wired in parallel. and i couldn't find a solid state switch that will
suit it. please help
(If you have tested the range; how long is the effective range?)
It shoots at 21.25m/s so by some simple SUVAT over 20m it will drop about five meters. So you'd have to aim up a little, around 14 degrees to hit your target at that range but when it hits it would still go through a beer can.