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Multi Staging a coilgun

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Reid

Tue Apr 07 2009, 11:04PM

Registered Member #2064 Joined: Sat Apr 04 2009, 09:26PM
Location: NJ, USA
Posts: 16

Hi guys, I'm working on a small scale coilgun as a private research project with my physics teacher.
I have built and tested a simple, single stage design that fires a 15mm long, 3mm dai. nail shaft at about 20-25 mps.
I am interested in building a dual stage coilgun, but I have no idea how to time the second stage's firing.

What would you suggest?
microcontroller?
photo gate?
other sensor?

thanks, Reid

rp181

Tue Apr 07 2009, 11:22PM

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

You could do it all those ways, but the photogate is the most used.

A microcontroller is generally not used, due to the variability of each shot, which cold make one shot good, and another bad. The absolute simplest way is to position 2 wire brushes, not touching, and facing each other. When the conductive projectile passes through, current flows, and triggers your switch, usually a SCR. A photogate is the most used option. On one side, a LED, usually infrared, is across from a photodiode. A simple comparator circuit (Using a LM311 or LM339) then triggers the switch. Photogates allow flexibility in the timing of switching. The simple way to adjust the "delay" is to slide the photogate up and down between the coils. You could also add a solid state delay circuit, doable with some 555's.

Good luck.

big5824

Wed Apr 08 2009, 12:42AM

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

I strongly advise against the metal contact idea, you will get a lot of bounce which will upset your triggers, and any residue on the projectile could stop it triggering. The photogate idea is much more accurate and reliable.

Bjørn

Wed Apr 08 2009, 01:34AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Coil guns are simple to get working but they are very complex because everything depends on several other variables.

If you want perfect control you have no other choice than using a microcontroller. Even a $3 one today can do timing with a resolution of 15 ns so there will be no variability to speak of.

To have full control over the exit speed you need to take into account the charge of the second capacitor bank (including temperature variations), the velocity of the projectile and the position of the projectile as it enters the second coil. Then calculate the proper timing for the second coil to reach the exact target velocity.

Most people are not interested in accurate control but how many soda cans they can dent, in that case it is enough with moveble photo gates and some trial and error.

hotcrazyfruit

Wed Apr 08 2009, 01:50AM

banned on 5/26/2009
Registered Member #1877 Joined: Mon Dec 22 2008, 02:03AM
Location:
Posts: 147

iv asked this a few times but, would a small capacitor between the gate of an SCR delay it effectively and reliably( it delays same amount each time). i want to build a multi stage but don't have the parts (comparators and 555's). nor do i have a photo transistor. ill get some soon :/

Reid

Wed Apr 08 2009, 02:16AM

Registered Member #2064 Joined: Sat Apr 04 2009, 09:26PM
Location: NJ, USA
Posts: 16

okay, thanks, guys.
what kind of circuit would I need to run a relay-microcontroller operation?

Also, coils question. The coil I am using at present had NO design behind it. I just took a roll of radioshack 22 gauge wire and wound all of it into a 1.5" long, 6+ layer coil. From this, I have been firing (corrected) .5" projectiles.

Given my capacitor bank, (6*80uF @ 330v max), where would you start designing a better coil?

rp181 wrote ...

You could do it all those ways, but the photogate is the most used.

A microcontroller is generally not used, due to the variability of each shot, which cold make one shot good, and another bad. The absolute simplest way is to position 2 wire brushes, not touching, and facing each other. When the conductive projectile passes through, current flows, and triggers your switch, usually a SCR. A photogate is the most used option. On one side, a LED, usually infrared, is across from a photodiode. A simple comparator circuit (Using a LM311 or LM339) then triggers the switch. Photogates allow flexibility in the timing of switching. The simple way to adjust the "delay" is to slide the photogate up and down between the coils. You could also add a solid state delay circuit, doable with some 555's.

Good luck.

just a wild idea, I have been using electrical/light switches to fire my coil, do you think that the circuit in a night light could act as an acceptable photo gate?

[Edit: Doublepost (edit the original instead)]

hotcrazyfruit

Wed Apr 08 2009, 02:37AM

banned on 5/26/2009
Registered Member #1877 Joined: Mon Dec 22 2008, 02:03AM
Location:
Posts: 147

Reid wrote ...

just a wild idea, I have been using electrical/light switches to fire my coil, do you think that the circuit in a night light could act as an acceptable photo gate?

the circuit in a night light is far too slow. but it is the same idea. when the photo transistor detects a deficit in light, it closes the circuit. im not sure on the exact circuitry, but im pretty sure how that works.
EDIT : also i think your coil probably has a bit to much inductance, 1.5 inchs of 22 gauge is alot of turns, and 6 + layers is ALOT of layers even for larger coils and wire. i would recommend bringing it down to about 3/4 " of turns and 2-4 layers. this will ensure a quick pulse and avoid a quick snap to centre.

Bjørn

Wed Apr 08 2009, 02:55AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

There is no real point in going multi stage until you have control over your first stage. A mechanical switch, relay or otherwise does not give accurate enough control so you need to change to transistor switching.

There are hundreds of posts on the forum and in the archives that will tell you everything you need to know. And don't forget the wiki:

rp181

Wed Apr 08 2009, 03:39AM

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

To get a idea on what is happening in your coilgun, you can use barry's RLC simulator:

To find your inductance and resistance of the coil (if a LCR meter is not available), use his inductor sim:

To use a photogate, this circuit works well, though its designed for model train detection:

That particular circuit has 4 optical gates (you could use it for a 5 stage gun)
As said, look into SCR's (Silicon controlled rectifiers). Beware of surge current ratings, as well as the pulse width.

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