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my multistaged coilgun idea (pretty sure I don't know what I'm doing)

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Yanom

Fri May 04 2012, 12:07AM

Registered Member #4659 Joined: Sun Apr 29 2012, 06:14PM
Location:
Posts: 158

So I've never studied the physics behind these coilguns before, and I've never built one (want to) but I've had this idea bouncing around in my head for awhile...
So anyways, people tell me timing the coils is the hardest part about making a coilgun. So I thought about this:

2qvun2h

so *i think* a magnet passing through a coil will produce some measureable current. So I thought of the idea of placing "sensor coils" around the barrel of the coilgun to detect the slug as it gets fired. As the slug passees through sensor loop 1, it turns off coil 1 and turns on coil 2. Then when the slug passes through sensor loop 2, it turns off coil 2 and turns coil 3 on.

All the sensor loops are hooked up to a circuit that controls relays on the coil power sources to make it all work

Am I crazy or would this work?

Ben Solon

Fri May 04 2012, 01:03AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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Posts: 600

I haven't done much with coilguns, but I can tell you that it will work only if the slug is magnetized. And you donâ€™t want to be shooting magnets. As I said, I canâ€™t say for sure, but the slug will only hold a very small amount of magnetization if any. You can try with an amplifier to see if you get any readings, but keep in mind that you also need to isolate the main coils discharge signal from the small one, and any ringing in the power coil may also throw it off. You would do better to use optics, because like any drsstc builder can tell you, emf can be a bitch! Light is a completely different type of energy from em, so they donâ€™t interfere with each other. Using an ir led or laser trip circuit would be much more efficient.

Yanom

Fri May 04 2012, 02:17AM

Registered Member #4659 Joined: Sun Apr 29 2012, 06:14PM
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Posts: 158

wrote ...
it will work only if the slug is magnetized. And you donâ€™t want to be shooting magnets.

Ah.

wrote ...
You would do better to use optics

how does that work? I've never seen that setup.

Tetris

Fri May 04 2012, 03:42PM

Registered Member #4016 Joined: Thu Jul 21 2011, 01:52AM
Location: Gainesville, FL
Posts: 660

Maybe you can use a small laser or LED and a photocell circuit to detect the differentiations in light. Wire it so that if there is a lack of light (aka the slug has passed in between the cell and the source, blocking light from it), it sends a voltage to a driver for the coils. That would trigger it, maybe. The circuit is a very simple transistor circuit, though I don't have the circuit on me right now (I'm at school). A simple google search for "night light" circuit, but except for emmitting light, the output tells if the slug is over the photocell. The bright LED or laser (I suggest a laser, even the pet ones from walmart might work) would be a seperate circuit, just powered so that it would constantly be on until turned off. You just need to find a way to make sure that the light triggers the photocell, and the output triggers the other coil to fire.

Turkey9

Sat May 05 2012, 02:23AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

A similar setup is used in V-switch coilguns. A sense coil (actually a small iron core inductor sitting perpendicular to the barrel at the end of the coil) detects when the projectile is close and kills the current in the coil. I don't think it would work very well to trigger the coil, however, because there is a great chance that the projectile won't be magnetized by the time it reaches the next coil. Here is the link

Optical sensors have been used countless times and are basically what HVC was talking about. Use an LED and a phototransistor that matches it. When the projectile breaks the beam, current will stop flowing through the phototransistor and if you know anything about transistors you can easily turn this into a signal to trigger the coil.

Ben Solon

Sat May 05 2012, 02:31AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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Posts: 600

As turkey said, the signal is there or it is not. This can be read multiple ways using either transistors or comparators. It should be easy enough to find, the principle is the same as laser tripwires. The led or laser is always on, and when the slug passes in between it and the sensor, you can read that signal. Sandra include photodiodes, phototransistors, or photoresistors. I lc you still can't find any info, let me know. I can draw up a quick schematic.

Barry

Wed Jun 13 2012, 08:52PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

Yanom wrote ...

So *i think* a magnet passing through a coil will produce some measurable current

It will work. Also, the projectile doesn't have to be a magnet... If the sensor coil carries some DC current then it will magnetize any iron that moves through. This will produce an inductive kick that can be detected and used for triggering coils.

I think these sensor coils would be used more often, except it's generally slightly easier to build an optical trigger system instead.

Cheers, Barry

Josh Campbell

Wed Jun 13 2012, 11:12PM

Registered Member #5258 Joined: Sun Jun 10 2012, 10:15PM
Location: Missouri - USA
Posts: 119

You will have much better luck using optics. The first multi-stage coil I attempted used hall effect sensors and with all the high current pulses and coils around it was very difficult to get them to reject all that other noise. I went with IR beam break detection which is much easier to deal with circuit wise and is not affected by nearby magnetic fields.

Another nice side benefit of using optics is that you get a pulse width that can be used to me sure the speed of your projectile at each sensor. When the projectile passes into the beam you have a state change, then some time later when the projectile is no longer blocking that same beam the state changes back. Allowing you to measure the speed of a projectile which you know the length of.

This was very helpful in that it allowed you to see which stages were performing and which were not so you could adjust them individually rather than blindly tweaking stages and looking for an end result increase in velocity out of the barrel. It also easily illustrates the massive amounts of energy needed for each additional increase in speed.

Where:
KE = kinetic energy
m = mass of object
v = speed of object

This equation reveals that the kinetic energy of an object is directly proportional to the square of its speed. That means that for a twofold increase in speed, the kinetic energy will increase by a factor of four. For a threefold increase in speed, the kinetic energy will increase by a factor of nine. And for a fourfold increase in speed, the kinetic energy will increase by a factor of sixteen. The kinetic energy is dependent upon the square of the speed. As it is often said, an equation is not merely a recipe for algebraic problem solving, but also a guide to thinking about the relationship between quantities (or capacitor bank size lol).

Using identical coils and capacitor banks per stage seems like a good idea at first but when you calculate the amount of energy required to increase your projectile velocity it becomes clear that to get any decent increase over one or two coils will require exponentially larger stages. That being said I don't want to discourage you from making a multi stage coil gun. You will see some increase in velocity in each stage especially if you can tweak each one individually and see the results independently. Good luck and happy shooting!

Edit: Another cheap and dirty way of triggering stages is to insert thin wires into small holes and use the contact closure of the metal projectile to trigger your SCR's. Not my favorite because the wires break or don't work, and you loose the velocity sensing ability of the optical setup.

Yanom

Thu Jun 14 2012, 01:07AM

Registered Member #4659 Joined: Sun Apr 29 2012, 06:14PM
Location:
Posts: 158

2bytes wrote ...

Edit: Another cheap and dirty way of triggering stages is to insert thin wires into small holes and use the contact closure of the metal projectile to trigger your SCR's. Not my favorite because the wires break or don't work, and you loose the velocity sensing ability of the optical setup.

cool. What if.... this "slug switch" was the only thing that lay between the capacitor and the coil? So the entire force of the capacitor was shorted through the slug? This would eliminate the need for any sort of switching circuit.... Would that work?

Josh Campbell

Thu Jun 14 2012, 04:08AM

Registered Member #5258 Joined: Sun Jun 10 2012, 10:15PM
Location: Missouri - USA
Posts: 119

One problem with that is the wires that are inserted into the barrel need to be small enough to not interfere with the slug and using them as the current carrying switch would mean they would burn up. Maybe if you had a conductive barrel that was split in half (similar to the two rails of a rail gun). Then you had multiple divided sections running the length of the barrel so that the walls of the barrel were the switch legs and the slug would complete the circuit touching a larger surface area. Still thats getting more complicated to actually make and I don't' think it would be very efficient anyway.

SCR's are really perfect for this type of switching. You could do it quick and dirty with no other circuitry just use the slug switch to apply a low current gate-to-cathode voltage. The SCR then switches to the on state and remains on until the charge drops below the breakdown voltage regardless of slug location. This was the switch I used before converting to and optical beam break type. It was pretty reliable but the wires often had to be replaced after each firing. I really only used it as a proof of concept for the multistage. The SCR's and caps all stayed the same I just added the optical triggering in place of the poor mans "slug switch".

Edit: I said SCR's are perfect for this type of switching, meaning the wires (or poor mans slug switch, lol). Not for the optical beam break however. I wanted to be able to control the energy release (pulse period) with the ATMEL uP that was measuring velocity. So obviously SCR's with their greedy, stay on till it's drained attitude would not do, MOSFETS and IGBT's took over from there.

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