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Coilgun with multiplex drive circuit

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LightningPhil:
Dear All,

Years and years ago I was into meddling with coilguns. There was a discussion regarding multiplexing switches to reduce the number needed to drive a lot of coils. I don't suppose anyone else remembers such?  If so, answers on a post card  :)

The goal is to launch 12 mm bearings, with many stages, for fun. It's catapult amo and is unlikely to go as fast as a catapult would launch it.

There are no plans to take this off the bench and will certainly keep the muzzle energy below the UK legal limit for a rifle. So safe and legal...

Main motivation is completing a project I dreamed of now I have the skill set to do it (and access to COMSOL).  While using it as a good excuse to learn to use an FPGA to monitor projectile position, control the coil drive circuit and analyse the performance of each shot.

Since they have lots of pins... Keen on having above 50 coils, each 12 mm long. If each is driven by its own bridge, then that's 200 switches. But... the dusty memory suggested a multiplexed idea may allow a vast decrease in switches and associated drive circuitry.

While the max speed it could launch and still be legal is just shy of 68 m/s, keeping muzzle energy down to just a couple of J would also keep currents down and capacitor energy requirements also. This is for FPGA fun and ticking a teenage project off.. Something like 25 m/s would be great.

So if anyone has ideas of how to multiplex coils or info of the mysterious patent (which I can't find on Espacenet) that would be awesome.

Cheers,
Phil

jbb:
Do you need to drive the coils with -ve voltage? If not, first candidate for switch reduction would be to go for an arrangement like a two transistor forward converter, which can only drive +ve voltage but is still able to recover the inductor energy back into the DC bus.

I thoroughly recommend isolated gate drivers for something like this, because the big switching transients will be all over the place, making ‘ground’ a moving target.

Something else I’ve always wondered about is a tuned LC approach, where each coil is fired by a single SCR / thyristor. Once fired there would be a resonant current rise, peak and fall back to zero at which point the thyristor turns itself off. Slight catch: the capacitor voltage would go from positive at start, to zero at peak current and then negative so you’d need to use film or ceramic caps.

jbb:
Aaaand I've nerd-sniped myself.

Attached are some entirely non-optimised simulations to illustrate the point. Both deliver fairly similar current to the coil, but they use quite different methods.

Edit: for the resonant type, the next coil stage can just flip the polarity of the coil & SCR...

Edit: switches are totally wrong, I just used what I had lying around.

LightningPhil:
Thanks Jbb,

The two transistor forward converter is likely the way to go.  And yes, only need to drive forward voltage and then turn off, ideally as fast as possible - which this is helpful with.

The multiplex circuit if I remember correctly operated like an array of two transistor forward converters, with coils aranged between them such that if there were N pairs of switches (forming a single effective converter with each pair), they could drive N^2 coils.  Each coil has to have a diode in series to avoid being driven in reverse (I think).  It's been about 20 years.  Suprised I remember anything of it.  Shall also try and draw it.

So with 8 pairs of switches, should be able to drive 64 coils.

Time to fire up LTSpice too  :)

LightningPhil:
Attached is a circuit with 4 pairs of switches and 16 coils. Drawing more is faff... But should be fairly obvious how this can be expanded. Think I've remembered enough of the diagram from the patent now.. Still wish I could find it!

Anyhow, If it turns out it wasn't patented, then it can't be now  ;D

So, with N (the number of switch pairs) being 3 or higher, more coils can be driven than there are switches. Rather keen on trying N=8 and going with 64 coils now.

We have to imagine gate drivers and a big fat capacitor involved in Pbus in the circuit, keeping voltage reasonably stiff during the chain of small discharges.

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