Coil Gun Results and Explanations

Several months ago when describing how the build the coil gun, I mentioned that I would explain some of the “extras” that we implemented in our coil gun project. I will explain the mounting brackets and the opto-interrupters here, as well as why a coil gun works.

How it works
When a current flows through a coil of wire a directed magnet field is induced inside that coil proportional to the current. After the capacitor is fully charged we discharge the capacitor into the coil. This provides a large current in the coil thereby inducing a strong magnetic field. The projectiles that we use are made of steel which contains a lot of iron, a ferromagnetic material. The magnetic field in the coil aligns the magnetic moments in the iron which makes the iron attracted to the center of the coil. The brief discharge of the capacitor finishes before the projectile reaches the middle of the coil so the projectile continues instead of stopping in the middle of the coil.

Coil gun testing and speed rating
Barrel, opto-interrupters, and coil gun mounting bracket I used the mounting brackets (in this case, a CPU heat sink bracket with notches carved into it) to mount the coil gun so that we could consistently measure the muzzle velocity of the projectiles. I then mounted the opto-interrupters in front of the muzzle, so that we could use an oscilloscope to measure the time between when each of the opto-interrupters was blocked by the projectile – thus giving us the projectile speed. The opto-interrupters used here were actually from an old laser printer that I had disassembled a long time ago, which was convenient because it had a small circuit with screw holes. The circuit basically just pulls the voltage up on one of the three wires, and when the opto-interrupter is blocked, it turns on the transistor, which pulls down the output voltage. You could implement this in a variety of different ways (including just using resistors and an opto-interrupter), but this was already built and so we used it. The other two wires you see are for a voltage source (we used 5V.)

oscilloscope timing measurmentAll speed measurements were taken using opto-interrupters that were spaced a half inch apart directly at the end of the coil gun barrel. The voltage output from the opto-interrupters were connected to an oscilloscope, so measuring the time between the voltage dips of each opto-interrupter gives the time it takes a projectile to travel between the two opto-interrupters, which is used to calculate the velocity.

Notice the sharp downward spike at the left of the oscilloscope screen. This spike shows when the coil gun was fired. The spike occurs because electomagnetic noise was induced either on the oscilloscope’s connector wires or on the opto-interrupter circuit (or both). However, I’m pretty sure the oscilloscope’s connector wires are shielded, and notice how the spike goes down to exactly 0 V. Both of these insights make me think that the noise is induced on the opto-interrupter circuit/wiring, not on the oscilloscope.

The projectiles used were steel cylinders of varying lengths and masses. One BB was also tested as noted in the table below.

Mass (g) Time (ms) Velocity
in/sec ft/sec m/sec mph
0.589 1.42 357.2 29.77 9.073 20.30
0.707 1.19 421.5 35.13 10.71 23.95
1.061 1.15 434.0 36.17 11.02 24.66
1.650 1.45 345.7 28.81 8.78 19.64
0.34 (BB) 1.72 290.8 24.23 7.39 16.52

Table 1 – Our Coil Gun Performance

As you can see, the 3rd smallest metal rod achieved the highest velocity. Each of these measurements were taken multiple times, yielding various results that were very consistent, if not exact duplicates. The fastest velocity was over 24 mph (11 m/s). That’s pretty fast, but the mass of the projectile is so small that it doesn’t hurt if it hits you.

6 thoughts on “Coil Gun Results and Explanations”

  1. hello,
    is the force depending on the rotation around the core or just the volts and amps,
    because I made the same thing you have and it can’t kick the ball away. it for robo soccer. do i need to buy a bigger solenoid or just use more amps en volts because i already have 2 2700UF condensators.

    Niels
    Dutch robosoccer junior

    1. The force depends directly on the number of wire windings and the current. However, the voltage plays a major role because the amount of current you get depends on the voltage you apply to the solenoid.

      In general you will need several hundred volts on the capacitors. This high voltage will make it so that when you put the voltage on the solenoid, the current through the solenoid will be able to ramp up very quickly (it will ramp up very slowly with a small voltage across the solenoid). The amount of force on the ferrite projectile corresponds to [number of windings] multiplied by [current through solenoid]. It is more complicated than that, though: if you have too many windings, then the resistance through the wire will be very high and will limit the current too much. If you have too few windings, then the magnetic flux will be very small and there will not be enough force on the projectile.

      Basically, it is a tradeoff. A good starting point is to use several hundred volts (make sure you capacitors are rated for high voltage) and a few hundred wire windings in the solenoid. Then from there you can experiment with using fewer or more wire windings.

  2. Hello, Mike.

    Here in Brazil, you re like a Tech hero, we builded a lot of coilguns to ours Aircrafts, planes and RC copters.

    I live near a Tech Store, there are a lot of components there, so I want to know what can i do to improve the power if the coilgun, but i want the correct numbers, exacly abou Farad and Volts, And the size of the coil, and the battery i can use. (but if i need a power that the batteries doesnt have, no problem, i can use a transformer Plugged on the AC 110v.

    Sry my bad english, thanks and bye.

    1. Hi Erick,

      There has been quite a bit of discussion on what sizes of coils and capacitors to use in the comment section of my other post that describes how to build the coil gun here: http://mike-thomson.com/blog/?p=13 (Read through the post if you haven’t already, and then scroll down to its comments and take a look through those.)

      It basically comes down to a trade-off between the amount of charge the capacitors hold, the voltage level they can handle, and the amount of time it takes the capacitors to discharge. In general, you want the highest number of Farads as possible, but while still using capacitors that are rated for the voltage level that you will use (for example, rated for several hundred volts). The larger the Farad rating, the longer it will take them to discharge, and the further away from the coil the iron projectile should initially be placed. (You want the capacitor the finish discharging at the instant that the projectile has been propelled to the center of the coil.)

      I hope that helps. Let me know if you take pictures of your RC planes & copters with coil guns – that would be very cool to see!

      1. Thanks for answering me, Mike!

        I hope my father give me back my SmartPhone so i can take the pictures, or my WingMan can do it and send me the Photos.

        We re trying 2 things here(we = they, because the only thing i do is to get the ingredients (Capacitors, coil etc., they do the hard work and also intalled on my plane.)

        1_ We need more than 1 “bullet” to shoot, and an lighter Weapon (The coilgun and its Power supply are too heavy to ours little eletric planes!, so we lose speed, mobility, “fuel”, stability, and, course, aim capability.)

        2_ We need improve the CoilGun Power to do an Kind of AA (anti-aircraft guns) with a great precision(here the power supply can be heavy, as the weapon, it will stay on the ground.)

        And thats mine:

        3_ I want to do the Coilgun Myself! i did nt got very well your tutorial, i just translated it and the “Hard work guys” builded two Coil guns (one for they copter, and other for my plane).

        They are still in BETA, so there are fails, like Aim convergency, i cant shoot a moving target, just big groung targets.

        we also tried to use the plane´s power supply and our Li-Po Pack blowed up!

        Thanks and Bye.

  3. Velocity is difficult because the device that creates your flux is slow to do so, it has inductance. That’s why the electronics are there, to get the high voltage. I am working a different problem – instead of velocity I am after momentum.

    Really fun stuff!
    All the best
    Kirk

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