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Registered Member #3950
Joined: Wed Jun 15 2011, 12:45AM
Location:
Posts: 51
Why do you think you are getting better efficiency with increased projectile velocity? Are you actually measuring the velocity after each stage with photogates?
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
Wow, nice job! It's cool to see a really well built coilgun at that low voltage.
To figure out energy input, I'd try to measure it. If your entire circuit could stand to have slightly more resistance, say, .01 ohms, you could add in a current sense resistor. Scope the voltage and current (according to the sense resistor) and multiply then together to get power over time. Then integrate to get total energy. You should even be able to see the pulses from each coil and thus be able to determine individual stage energy. But don't forget to account for the power lost in the sense resistor. This would be a much better method than running simulation and making assumptions. The measurements could even be done with a simple sound card oscilloscope if care is taken to get the right voltage levels; the pulses of current into your coils shouldn't have too high of a frequency.
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
jnbrex wrote ...
Why do you think you are getting better efficiency with increased projectile velocity? Are you actually measuring the velocity after each stage with photogates?
As the projectile speeds up, it induces a stronger voltage (E=B*length*velocity) in the coils as it passes through. This induced voltage in the coil is the power stolen by the projectile, so the faster it goes the better it takes energy from the coil. Also, the same induced voltage impedes current flow through the coil, which reduces I^2*R losses further improving efficiency.
I add stages one by one, taking three measurements with the speed trap each time and using the average. It's pretty consistent so it gives a measure of the energy added by each stage.
Turkey9 wrote ...
Wow, nice job! It's cool to see a really well built coilgun at that low voltage.
To figure out energy input, I'd try to measure it. If your entire circuit could stand to have slightly more resistance, say, .01 ohms, you could add in a current sense resistor. Scope the voltage and current (according to the sense resistor) and multiply then together to get power over time. Then integrate to get total energy. You should even be able to see the pulses from each coil and thus be able to determine individual stage energy. But don't forget to account for the power lost in the sense resistor. This would be a much better method than running simulation and making assumptions. The measurements could even be done with a simple sound card oscilloscope if care is taken to get the right voltage levels; the pulses of current into your coils shouldn't have too high of a frequency.
Thanks! That would be the best method. But I don't have an oscilloscope, and this will be my last project (at least for a while) so buying one isn't a good move for me right now. The simulation should be fairly close and if anything my efficiency guesses are conservative so it's kind of an "at least it's doing this good" ballpark sort of thing.
Registered Member #3950
Joined: Wed Jun 15 2011, 12:45AM
Location:
Posts: 51
I've been doing a research project recently, and adding a current sense resistor was very beneficial. This resistor only has a resistance of 0.005 ohms and can easily withstand 2000 amp + peak currents. Putting in a current sense resistor on every stage should let you calculate the energy that goes into the coil and the length of the pulse. The manufacturer part number for the resistor is PWR4412-2SCR005F. I got some from newark.com. If you do decide to buy some get extra because they are fragile!
Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
this may be what you are doing, but i cant tell. are you using the solenoid to knock the slug into place? because what if you just positioned the solenoid so that it's center is the distance required from the main coil for shooting. that way if you put enough current through the solenoid, you could hold the slug in place until the very last millisecond. this way you could potentially hold the gun vertical without any changes in slug positioning.
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
IamSmooth wrote ...
I probably missed this, but how are you determining the proper timing for each stage?
Simplpe IR LED and IR transistor. The projectile and the coil are the same length so the timing is close to perfect for each shot.
ben123324 wrote ...
this may be what you are doing, but i cant tell. are you using the solenoid to knock the slug into place? because what if you just positioned the solenoid so that it's center is the distance required from the main coil for shooting. that way if you put enough current through the solenoid, you could hold the slug in place until the very last millisecond. this way you could potentially hold the gun vertical without any changes in slug positioning.
I'm not sure what you're describing... But the solenoid pushes the ammo out of the magazine, and into the path of the first stage IR trigger. It will work no matter how you hold the gun.
Registered Member #2901
Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837
Oops, didn't realise this thread was olllllld ... one remark about the original never build design, that mosfet would have been fried because of inrush current if it was switched on at the wrong time.
Registered Member #4237
Joined: Tue Nov 29 2011, 02:49PM
Location:
Posts: 117
Well, since this thread got Necro'd, I might as well make a comment: I love your coilgun! Really sleek design. I love the power rails on the side of the barrel, I might incorporate those into my design. I might have missed it, but how many joules is your cap bank on? I also have a few other questions: Can we see a picture of the projectile? Have you done simulations to see on which interval of the individual coils you get the most "bang for your bucks" in therms of force per current?
I'm getting really inspired by this, and I can't wait to get building on my 9-stage this summer. I've planned, done FEMM simulations, written simulators, and it's finally time to get building... EDIT: Also, how do you secure the coils to the barrel, and the barrel to the main plate? I'm having some trouble coming up with a secure way to do that.
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
Pinkamena wrote ...
Well, since this thread got Necro'd, I might as well make a comment: I love your coilgun! Really sleek design. I love the power rails on the side of the barrel, I might incorporate those into my design. I might have missed it, but how many joules is your cap bank on?
Thanks I'm glad you like it!! There's no capacitors.
wrote ...
I also have a few other questions: Can we see a picture of the projectile? Have you done simulations to see on which interval of the individual coils you get the most "bang for your bucks" in therms of force per current?
I haven't done any FEMM simulations, just RL circuit simulations and the basic kinematics equations for projectile speed.
wrote ...
I'm getting really inspired by this, and I can't wait to get building on my 9-stage this summer. I've planned, done FEMM simulations, written simulators, and it's finally time to get building... EDIT: Also, how do you secure the coils to the barrel, and the barrel to the main plate? I'm having some trouble coming up with a secure way to do that.
Sweet! If I've helped inspired you then that's awesome, I hope your project goes well and you learn a lot. The coils are just wound around the barrel tight enough to stay put, and kinda held in-place by the inter-coil spacers that hold the sensors. They do slip a bit if you twist them. I've got some nails hammered into the board that the barrel fits neatly inbetween. It's tight enough that the friction holds it all in. Right now I'm in the difficult phase of building the ammo-magazine and fitting the whole thing into a gun.
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New Coilgun Project
