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
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.)
All 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|
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.