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430J Coil Gun

430J Coil Gun

Coil gun muzzle

Specs


Bank Energy 434.5 J
Bank Voltage 450 V
Bank Capacitance 4.7 mF
Charger 15 W Boost Converter
Coil Inductance 80 µH
Coil Resistance 180 mOhms
Estimated Peak Current 1.4 kA
Projectile Weight 17 grams
Average Projectile Speed 27 m/s
Average Efficiency 1.45 %
Peak Efficiency (at 200V bank voltage, 11g projectile) 2.65%

Intro

A coil gun is among the easiest to construct and most reliable electromagnetic projectile accelerators. Consisting of a coil/solenoid and high current source of electricity. How can a hollow coil and electricity accelerate things? A ferrous projectile is placed partway into the coil. When the trigger is triggered the high current source is connected to the coil, causing large currents to flow through the coil. In practice the high current source is a capacitor bank which can quickly discharge it's stored energy, resulting in a short term high current source. The magnetic field strength in a solenoid is proportional to the current flowing through the coil, and the larger the field strength the larger the acceleration will be on ferrous materials. When the coil is energized the field strength becomes so strong the projectile is sucked into the center of the coil. The trick here is to stop the current just before the projectile reaches the center of the coil, otherwise it would stick in the coil. The speed which the projectile gained while being attracted to the coil is what now propels it forward and out of the coil. The physics behind the basic functioning aren't too complicated. Switching currents approaching 1-2kA isn't an easy task, and all mechanical solutions have large losses. For a practical trigger the SCR is used, which can handle the required pulse currents quiet and efficiently. Components can be seen below.

Coil gun lash up  Coil gun schematic

It doesn't take many components to put together a single stage coil gun, but due to the high energy involved a single error could result in a violent failure. The peak current, discharge time, reverse current/ voltage can be calculated, and are characteristic for each coil gun. A coil gun makes up an RLC circuit, which makes determining the characteristics easy. Barry has made an excellent simulator here -> RLC Simulator By simply plugging in the component values one can predict how the circuit will operate. This is what my coil gun looks like:

RCL simulation of coil gun

The SCR should have a pulse current rating at least 10% over the estimated maximum current. The 50RIA120 can pass 1.5kA for 8ms, which is perfect for my coil gun. The reverse diodes are there to protect the capacitor from being reverse charged by the energy in the coil when the SCR turns off.  They give a path for the negative current pulse, and should be rated for the estimated current. The 68 ohm resistor and 12V battery simply give some initial current to turn the SCR on. Once on it latches until the current falls below a certain level. The coil is wound on a non-conductive and non-magnetic form, which allows the projectile to pass freely through the coil. The coil former should be as thin as possible, but keep in mind that the coil shrinks with great force when firing, so the barrel must be strong. For charging the capacitor bank I use a boost converter, which is able to charge the bank to 430V in 30 seconds from a 12V source.

Coil and Projectiles

Coil   Projectiles

This is what the coil looks like. External iron/steel is used to confine the magnetic field and help concentrate it on the projectile, while holding the coil together. The wire needs to be pretty thick to sustain the high currents without shattering. 15 AWG should hold up to 1.5kA for short durations. My coil uses 17AWG and it gets warm after just a few shots. The coil is 5.5 cm long, and I used 10 meters of wire. No idea how many turns or layers though. The inductance with no projectile was measured to 80µH, which allows for a short high current pulse. The projectiles I used weighed 9, 11 and 17 grams. The projectiles should fill as much of the barrel as possible in order to experience the most force. I used thick projectiles in hopes that they would not saturate as fast, and thus absorb as much energy as possible.

Actual coil gun

Full view of Coil gun  Loading Mechanism  Pwned Soup Can

The completed coil gun in all its ugliness, and no, I couldn’t have made it look better or I would have. A locomotive look was not what I was aiming for. The switch hanging out the side is the firing switch, which needs replacement. I didn’t have the proper type at hand. At least this coil gun is portable. The loading mechanism will hold the projectile in place, it's not to complicated yet. A magnet is glued to the bottom of the runway to hold the projectile. Once in place the projectile stays there, even when the gun is shaken upside down. The magnetic is too weak to restrict the projectile while firing, and actually helps by holding it in place until the magnetic field has built up enough. The coil is held together and fastened with zip-ties. Ghetto. I tried sending the blunt 17 gram slug through, but that only resulted in a huge bulge in the bottom of the can. However the 17 gram projectile was able to fly sideways straight through a shoe-box without the box so much as flinching, which was one of the most awesome shots I've ever seen.

Measuring Projectile speed and calculating efficiency


There are three common ways of measuring speed. One is to make a speed-trap, but that's the most elaborate. If you have a microphone, a PC with audio editing software and two sheets of paper there's an easier way. Basically set the two sheets up at a known distance, put the microphone at the first sheet and shoot through the paper while recording. When you look at the audio signal later there will be two spikes when each sheet was penetrated. The time between these spikes will be the time the projectile used to cover the known distance. V = (D / (T - D / S))
Where D is the distance between the papers, T is time and S is the speed of sound, 340m/s. This is most accurate with higher energy projectiles, because some kinetic energy is lost to penetrate each sheet. Not much, but with low energy coil guns enough to seriously slow the projectile down. My coil gun made some muzzle noise when firing, which allowed me to ignore the first sheet. For the second sheet I used a tin can which made a much more audible sound on impact. Of course muzzle noise isn't the best indicator of the start time, since the projectile uses some time to accelerate.
Sound recording of shot
The final and simplest method is fairly accurate and only requires a stop-watch. You point the coil-gun straight up and shoot, while measuring the time between firing and when the projectile hits the ground. With some basic high school physics one finds that the initial speed is given by: V = 9.81 * (T / 2) Optionally one can record the shot with a microphone and hope it picks up the sound of the projectile landing. I used all three methods when testing my coil gun and there were no big gaps between the measured speeds. Once the speed and mass is known it's a simple matter to calculate the kinetic energy the projectile had. E = 0.5 * M * V^2 For quick calculations I've made a spreadsheet calculator available for download.

Youtube Video



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This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.

Disclaimer: I do not take responsibility for any injury, death, hurt ego, or other forms of personal damage which may result from recreating these experiments. Projects are merely presented as a source of inspiration, and should only be conducted by responsible individuals, or under the supervision of responsible individuals. It is your own life, so proceed at your own risk! All projects are for noncommercial use only.