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Microcontroller Switching?

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currentkills91

Wed Jun 15 2011, 09:43AM

Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
Location:
Posts: 265

Hey everyone, I was looking around at things I could do to increase the efficiency of my coilgun rifle, and I came across the whole microcontroller switching idea.

I could use the microcontroller to turn the MOSFETs, or IGBTs on when the IR beam is broken, and then off right when I wanted to, helping to increase overall efficiency a little bit.

I was looking around for some microcontrollers, and came across this one. Would this do the job for me? I honestly have not the slightest amount of knowledge when it comes to these things. Hopefully you guys here can point me in the right direction. (:

Artikbot

Wed Jun 15 2011, 11:25AM

Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137

Why not using a simple comparator?

If you use an LM311 and connect the open emitter pin to the gate of your SCR/IGBT it will work as good as a microcontroller. And probably faster.

Concerning the 'closing when I want' thing, probably a small variable capacitor could help you achieve that 'desired length' without the need of a microcontroller.

Saz43

Wed Jun 15 2011, 03:54PM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

Artikbot wrote ...

Why not using a simple comparator?

If you use an LM311 and connect the open emitter pin to the gate of your SCR/IGBT it will work as good as a microcontroller. And probably faster.

Concerning the 'closing when I want' thing, probably a small variable capacitor could help you achieve that 'desired length' without the need of a microcontroller.

Also, to control the pulse width you can use a simple 555 IC which just requires a single resistor and capacitor to "program". The Microcontroler itself won't increase your efficiecny in any way a 555 couldn't. What the microcontroller is usefull for is if you have so many coil stages and so many logic functions on your coilgun that it gets to the point where it's easier to use one microcontroller as opposed to many many ICs.

If you want to increase efficiency, some additional measures you might consider are:

1. Put Iron end-caps on your coil(s) to improve magnetic coupeling. This works only for lower voltages (~<100V)
2. Use a string of series diodes (as opposed to just one diode) in anti-parallel to improve shutoff current quenching
3. Use a diagonal half-bridge to improve shutoff current quenching
4. Reduce working voltage and use multiple coil stages
5. Take simple measures like winding your coil neatly and tightly, use short fat wires or copper busbars instead of long wires to connect your coil to your capacitor bank, minimize length and diameter of the barrel.

currentkills91

Wed Jun 15 2011, 08:40PM

Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
Location:
Posts: 265

I was thinking of using a microcontroller because i'm thinking of moving up to 4 stages for my gun...and I always seem to have bad luck with 555's -.-

Artikbot

Wed Jun 15 2011, 08:50PM

Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137

No real need to use 555s to trigger...

4 stages? LM2901 and job solved. Very fast comparator, both collector and emitter open. Although it's better (simplifies circuitry) to just have open emitter.

Search for a quad comparator with open emitter only, or invent a bit with an LM2901.

I keep insisting on the open emitter because you can directly drive the SCR/IGBT gates with no need of additional circuitry, besides of the obvious resistor.

Edit: Ooops! Head went bonkers, confused LM2901 with LM339 :S

currentkills91

Wed Jun 15 2011, 09:09PM

Registered Member #3831 Joined: Thu Apr 14 2011, 02:54PM
Location:
Posts: 265

Yeah, i'm using the LM339 right now to trigger the three stages...I suppose I could stick with it, I just though it would be cool to start getting into microcontrollers.

Artikbot

Wed Jun 15 2011, 09:31PM

Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137

Cool, but not the best idea... Unless you switch with massive IGBTs and control pulse lenghts and so on.

But if you simply dump everything on the caps to the coils, there's no real need to use a micro.

You've made me think about something...

A cool micro implementation would be sensing thje projectile, opening the IGBT, sensing it again, measure the speed, then sense it again (second accelerator coil), measure speed again, then sense it again (final accelerator) and calculate average muzzle speed, muzzle energy based upon a given initial mass, and interpolate an acceleration in the barrel.

That would be one epic masterpiece of an idea.

And AFAIK, not complicated to do, if you get a fast enough micro (or even two micros, one sensing stuff and opening coils, the other one doing the calculus and storing the results).

The Arduino platform would be good, the PICAXE might be a tad too slow (talking about sensing projectile, opening gates and having to interpolate functions together)

big5824

Wed Jun 15 2011, 10:56PM

Registered Member #1687 Joined: Tue Sept 09 2008, 08:47PM
Location: UK, Darlington
Posts: 240

Id personally go down the microcontroller route, if you use the built in comparators most have you will be able to get it running only a few microseconds slower than a standalone comparator. Plus, the microcontroller will open up inumerable new possibilities, such as plotting project speed data throughout the acceleration, coil firing delays, etc.

As for the choice of micro, if you consider yourself competent at coding I would jump straight to a dspic or something of similar level, however if you have never programmed before then yes start with an arduino. If you do choose the dspic then feel free to pm me any questions, as I've been learning how to use them over the past few months so most of the info is still fresh in my mind :)

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