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Storing Data on Projectile

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DerAlbi

Sat Sept 12 2015, 10:12PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Hi guys..
I would like do discuss, if its doable to store some information on a coilguns projectile.
The goal would be to imprint some magnetic remanence and to store maybe 20 bits along 40mm projectile and if doable maybe multiple times for some redundancy.

Problems:
0) Is it doable at all on iron projectiles?
1) impact. When the projectile impacts somewhere the vibration can delete the remanent magnetic fields. (it however needs multiple hits to a hard surface if my tweezers become magnetic and i want to get rid of that annoying magnetism)
2) Stability. Will the data degenerate over time?

How to do it?
The projectile must pass through a coil setup that leave a localized magnetic field on the surface. How would it look like?
I think it would look like a tiiiny toroid cut open so that the magnetic flux path is closed by the projectile. If there is an AC-current applied across such a "writer coil" the projectile have that magnetic field within it. The direction of the current should determine how the magnetic cells are oriented.
This would only use a tiny strip of the projectiles circumference and should enable multiple data-regions.
Of course all this must be done after acceleration because the acceleration stages would destroy any data with their brutal saturation levels.
Since i aim for 100m/s, a 40mm projectile would take 400Âµs. Storing 20 bits means that i need a data throughput of 50kbit/s onto solid iron. Uhhm. Doable?
Ok, that can be relaxed by using parallel datapaths. but that would be a big compromise. :-/

Read back?
Slide the projectile along the writer coil setup and measure the induced voltage. There should be some. Its low and possibly noisy. but it should be present. The more bits stored the better the error correction and the higher the redundancy the worse the projectiles condition can get.

If you have any resources on how this could be practically achieved, brainstorming is on!
I am only at the idea stage here, so no practical experience from my side

klugesmith

Sun Sept 13 2015, 03:48AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

What;s the skin depth in steel for the frequency of your magnetic writing signal? I wonder if there's any benefit to having the binary data modulate the frequency of a carrier with multiple cycles per bit?

Are you familiar with induction hardening? Heat-treatable steel shafts and gear teeth can be hardened on the outside surface only, The induction frequency is high enough for the workpiece skin to become incandescent, then be quenched, before the inside of the part gets hot.

How about adding some solid ferrite, or magnetic recording tape, around the body of your projectile, or as an extension on the tail end?

You could play with magnetic writing at 100 m/s using a pneumatic gun, I bet much easier to make than a a coil gun.

DerAlbi

Sun Sept 13 2015, 03:27PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

There is no additional media to play with. just the bare Iron....

Skin depth is around 10um at 50kHz but that only represents the electrical resistivity of a equal thick iron tube.. i am not sure right now if that skin depth has anything to do with the magnetic penetration. Still.. 10um should be enough to at least interact with the most outside magnetic domains.

Your induction hardening example was a hint that i could so some equal stuff to writing a CD but with eddycurrent heating instead of using a laser? I think the frequency for that would be much higher again and there the skineffect really kicks in.. ok. but thats what you want.. hmmh. not sure.

Dr. Slack

Sun Sept 13 2015, 03:34PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

I think this is one of those applications where we must ask why? Would pits ablated in the side by a laser suit your application better?

DerAlbi

Sun Sept 13 2015, 11:23PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

The goal is to link a projectile to a gun. And even reused projectiles. Laser is allways complicated due to safety reasons. It must be doable without costly equipment and stuff, and without touching the projectile physically.

johnf

Mon Sept 14 2015, 06:38PM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

Deralbi
why not a bar code and optical reader

klugesmith

Mon Sept 14 2015, 08:14PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Why not a bar code and optical reader?

Albi could durably mark steel projectiles by electrolytic etching.

A bar code stencil for one-time use can be made from narrow sticky tape. No longer used for PCB layout, but today available as "pinstriping tape" or "graphic chart tape".

Less durable, but much easier to apply, would be barcodes printed on sticky-backed paper label material.

One search for projectile identification found:

DerAlbi

Wed Sept 16 2015, 10:22AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

So everyone has an idea on how to do it different usings ways i excluded or that contradict design goals

What is the actual problem i will run into with magnetic storage? Can someone answer that?

Dr. Slack

Wed Sept 16 2015, 12:03PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

DerAlbi wrote ...

So everyone has an idea on how to do it different usings ways i excluded or that contradict design goals

What is the actual problem i will run into with magnetic storage? Can someone answer that?

I guess that's the problem with not actually telling people what fundamental problem you're trying to solve

If it's to allow the authorities to match a projectile to a, well, let's call it an electromagnetic accelerator, after a murder for instance, like barrel rifling and marks on a slug can do, then there are all sorts of problems with the accelerator marking it as it goes through. A knowledgable person could disable the write head. The writing depends on the surface properties of the slug, whereas the acceleration fields are much slower and penetrate far deeper. A slug made from different materials in concentric tubes could be designed to be unmarkable.

Ash Small

Wed Sept 16 2015, 01:03PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I think the idea here is to prevent people from using projectiles from a different supplier in order to evade the projectile from being traceable.

What's required is a system that will uniquely mark any ferrous projectile.

This rules out any form of 'special preparation' of projectiles.

The thread is, after all, titled 'Storing Data on a Projectile'.

The thing is though, how would you prevent people from disconnecting that part of the device that does the marking?

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