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Best method for a multistage?

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LAN_403

Killa-X

Wed Dec 03 2008, 03:18AM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

Well, I guess it's coming to the time where I'm considering adding a 2nd coil. Whats the best way to trigger the 2nd coil on the exact time for best force? By optical light sensors or physical touch using the bullet to complete the circuit?

Also.. If i want 2 coils.. Is it best to have a fairly small one to boost the object, followed by a large one to accelerate? or should they be fairly the same size?

Would be my first multistage when I actually start on it. It's been awhile since I've updated my single stage test coil since I got this new laptop. Work will resume how ever soon. :)

Turkey9

Wed Dec 03 2008, 03:36AM

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

just add the next coil and tune it for the most acceleration. Use optical sensors to trigger the coil because a mechanical method, although simpler, would slowly wear down and also is less precise.

Myke

Wed Dec 03 2008, 04:30AM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

You want the second coil to have a faster rise time and decay time so that the projectile is accelerated while entering the coil. You want the current to decay quickly so that there isn't any suck back. To get faster rise and decay times you can use a higher voltage and/or less inductance for the rise time and less capacitance for the decay time.

I would just say experiment to see what works best. Then you can fine tune it with calculations (or you could do it the other way around to get an idea of the values you want).

Barry

Wed Dec 03 2008, 02:50PM

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

I'm a big supporter of optical detectors, rather than mechanical. The problem is that we want to get microsecond timing out of it, but mechanical systems are generally more on millisecond levels. For example, that's one reason we use SCR's instead of relays.

Of course repeatability is the main issue; as long as it's reasonably fast and any built-in delay is absolutely constant from shot to shot, then you can always adjust the distance to the coil to compensate for the delay.

Cheers, Barry
What do elves learn in 1st grade? The elf-abet

Killa-X

Wed Dec 03 2008, 08:33PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
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Yeah, The first I heard of optical sensors, I was thinking it could be too slow, like a possible delay, also "outside world" lighting might effect.

So like, whats the real design here. A Light sensor (photocell?) and a LED? So when the light is on fully, it has less resistance on the 9V output so it triggers a SCR..? Because I know in mechanical, you dont want the "bullet" to be a spark-gap ignite. You want it to trigger a SCR..

Other than that, I guess with most I seen online, people tend to either have almost similar coil sizes, or 1 big and 1 small..The small usually being the primary.

rp181

Wed Dec 03 2008, 09:40PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Posts: 1529

A photo transistor with IR LED works great. A LM339 comparator is good to trigger.

KLH

Thu Dec 04 2008, 07:04PM

Registered Member #1819 Joined: Thu Nov 20 2008, 04:05PM
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Posts: 137

Killa-X wrote ...

So like, whats the real design here. A Light sensor (photocell?) and a LED? So when the light is on fully, it has less resistance on the 9V output so it triggers a SCR..?

Like rp181 said, you should use a phototransistor (or a photodiode). A photocell is simply too slow, especially when you build higher power, higher speed coilguns. A phototransistor should be fast enough for most applications, but the fastest coilguns will need the even-faster photodiode.

Phototransistors can also be used almost exactly like photocells. Photodiodes need a current-to-voltage amplifier with high gain (since photodiodes act like current sources).

To trigger the SCR, record the voltage from the phototransistor/diode when the projectile is between the sensor set and use that voltage to trigger a comparator.

big5824

Thu Dec 04 2008, 09:21PM

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

what about an LDR? would that switch fast enough?

rp181

Fri Dec 05 2008, 01:02AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

LDR's are slower than transistors. Heres a schematic i made for triggering, that is attached to a delay circuit which you can adjust for best performance (via changing capacitors and resistors). I havnt tested it yet though.

It's on the bottom of the page.

youngcoilgunner

Fri Dec 05 2008, 04:51AM

Registered Member #1633 Joined: Tue Aug 12 2008, 04:21AM
Location:
Posts: 45

dude, thats one heck of a schematic.

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