Sunday, April 10, 2011
Saturday, April 9, 2011
How it's done volume 2: Using force calculations to design coils
Building accelerator coils is a pain. For starters, they take forever to build. Most importantly though if you build the firing tube and it's coils too small you won't get the power you need to accelerate but if you build them too large the coils won't fire quickly enough.
Why is that? Most serious coil guns use SCRs. You'll have a capacitor, you want to discharge it into the coil. If you just use a switch, the amperage involved is probably more than the 20A most switches are rated for so the switch will be destroyed (in many cases these machines work at thousands or tens of thousands of amps). In fact, this current is generally too strong to use even normal power mosfets. But never fear, there is a product called an SCR. It's essentially a diode that doesn't conduct in either direction till you turn it on. And it happens to be able to survive extremely amperages compared to other devices. The way it does this is that it only turns on.
The amperage limits of other electrical components often comes from heating. The wattage dissipated by a transistor is the voltage across it times the amperage through it. If either is small, there is no heat dissipation. If both are significant, that's when the heat is on. And predictably, if a transistor is partially conducting, it will be taking on a lot of heat. Now SCRs only turn on. Thus, they go very quickly from zero amperage and high voltage to high amperage and zero voltage. This keeps them from building up much heat and thus you can get megawatt switches for double digit prices.
So now you've got a coil that can fire. However, if it fires too long, the projectile will be leaving out the other side and get pulled back in. Thus, you'll have to design the coils to fire for just long enough to pull the projectile in but to have been expended after that. That is why the coils must be neither too large or too small.
How many times should I wind the wire around the firing tube to make the coils? 10? 10,000? How to know? Should I be using 10 gauge wire? 30 gauge wire? Here's where some seriously shoddy math comes in. Essentially, doing some calculations about the expected inductance, discharge time, etc. I don't want to go into the math because it's complex and most people don't give a damn. So I'll just link you to the spreadsheet that you can plug numbers in to calculate. That said, the spreadsheet won't save you. It's not ballistics where you'll find out within 1% where the projectile will land. This will get you within 2x or 4x in either direction. Plug in numbers and then look at the timings to make sure they're not telling you the rise and fall times are too far off. Then do the same thing with the next coil and so on for ever coil you'll be making. For me, that boiled down to about the settings in the spreadsheet.
When you decide on a set of parameters, build a coil or two and fire it. Get a triggering scope to check if the timing you had is what you expected.
Remember those other firing tubes at the top? Well they're not being used for a reason. Countless hours making parts that can't get up to and surpass the very difficult to beat 60m/s marker.
How it's done volume 1: Getting and optimizing for force
Check out wikipedia for the basics... You read it? Good. Now that you understand that, let's talk about engineering this gun. Our objective is to get it to fire a fast moving projectile. Chemical guns get a good velocity. It should work like that. But with magnetics. And not being so damn loud.
When considering a dynamic system (aka: shit moving), it's often helpful to consider the static case (nothing is moving). In our static case, we have a projectile near an electromagnet. During this moment, the electromagnet is exerting a force on the projectile. If the force is allowed to move the projectile some small distance, the projectile will pick up Energy = Force * Distance from it. (You may ask where the energy comes from in this case. If we observed the coil during this time, the current through it would have decreased. As the now magnetized projectile moves closer it causes current in the opposite direction of the existing current.)
So how do we build this machine to get the maximum force? After all, it looks like getting the most force is what we need to get the most energy in our projectile. If this where a chemical gun, that force could be computed as the pressure on the back of the bullet. The pressure * the area of the back of the bullet * the length of the barrel will give you the expected energy of the bullet. In that case, we could just have to add more powder to increase the pressure. We could also increase the area of the back of the bullet. This is common in many very high speed guns.
Shittily, knowing the force we're imparting is ridiculously complex with a magnetic weapon. For starters, the magnetic field strength is based on the current in the coils which is both dynamic and changes with the movement of the projectile. Secondly, the force is difficult (impossible?) to compute just from the construction of the coils and especially hard as the projectile is moving through them.
So if don't know how to find the force, we should at least be able to aim for getting more of it. I recommend downloading this Finite Element Method Magnetics (FEMM) magnetic simulator to get a sense of how the magnetic fields will work. once you get the hang of it, it'll also tell you what sort of magnetic field strength to expect given your setup. In particular, you should notice that it's hard to increase the magnetic field strength beyond 2T (or rather it doesn't increase as fast after that). This is called magnetic saturation. From what I understand, that's essentially when all the iron dipoles are already pointing in the same direction (and thus there are no more to give a boost to the field being applied to them).
If you don't download that and use it, I'll give you a hint: There's a stronger field when there's iron around the coil as well. So the first thing we now know is that to make the strongest accelerator, we need to have magnetic shielding around our coils.
And that is the point of what we've learned with this post: If you want to get more power, put magnetic shielding around your coils.
Thursday, April 7, 2011
Firing an electromagnetic rifle at a piece of plexiglas
Not as impressive as the plate firing but this was actually my first demo of the accelerator outside the basic test setup I had.
How that plate is doing
The plate is actually mostly bent. At the time of firing it was actually just taped on both sides to the target and then had a larger steel plate about 4" behind it. Based on the shape of the impact, I'd guess that it actually bent till the first place touched the back on and then the projectile pinched them. I wonder if we could have gotten penetration if it had been better mounted.
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