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4hv.org :: Forums :: Electromagnetic Projectile Accelerators
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Storing Data on Projectile

Author Post
DerAlbi
Sat Sep 12 2015, 10:12PM Print View
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Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 519
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 400µs. 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
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klugesmith
Sun Sep 13 2015, 03:48AM
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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.
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DerAlbi
Sun Sep 13 2015, 03:27PM
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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.
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Dr. Slack
Sun Sep 13 2015, 03:34PM
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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?
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DerAlbi
Sun Sep 13 2015, 11:23PM
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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.
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johnf
Mon Sep 14 2015, 06:38PM
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Deralbi
why not a bar code and optical reader
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klugesmith
Mon Sep 14 2015, 08:14PM
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Location: Los Altos, California
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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: .
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DerAlbi
Wed Sep 16 2015, 10:22AM
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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?
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Dr. Slack
Wed Sep 16 2015, 12:03PM
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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.
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Ash Small
Wed Sep 16 2015, 01:03PM
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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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DerAlbi
Wed Sep 16 2015, 06:27PM
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Location: Dresden, Germany
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By having a read head exactly after the write head. That disables the gun if that part of the gun does not work properly. That gives you one shot after tempering with the gun. You will need that to test the gun after disassembly.

Projectile marks wont happen since the barrel is the way weaker material.

I told people exactly what the goal here is:
The goal would be to imprint some magnetic remanence and to store maybe 20 bits along 40mm projectile


The projectile comes out of the gun pretty much saturated and "whiped".
One could try to punch holes in that remanent magnetisation by havin a ultra short coil that has a reverse magnetic orientiation than the drive coils.
But i can imagine that this is not focussed enough.
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Ash Small
Wed Sep 16 2015, 09:11PM
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DerAlbi wrote ...

Projectile marks wont happen since the barrel is the way weaker material.


Ceramic barrel?
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Dr. Slack
Wed Sep 16 2015, 09:43PM
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Location: UK St. Albans
Posts: 1588
DerAlbi wrote ...

I told people exactly what the goal here is:
The goal would be to imprint some magnetic remanence and to store maybe 20 bits along 40mm projectile



With respect, that's your proposed solution, not what the fundamental problem you're trying to solve actually is
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DerAlbi
Wed Sep 16 2015, 10:09PM
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Barrel will be some low friction plastic. In a coilgun is not good for efficiency to mechanically alter the projectile. it also does not work when the projectile is shot multiple times (coilgun projectiles are reusable!). There is not much left except magnetic storage.
But i see, no discussion of how one can imprint some magnetic remanence representing data will happen here.
And no, thats not my proposed solution. Its just the way to go in a coilgun and its the thing that needs a solution since i have no idea how to do it properly and if its even possible or what factors determine the "doability" or how i can come to some estimation of how high the data density can be and when this sentence will end...
I just presented my initial thoughts on that, sry if that was confusing.. bad initial description.
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2Spoons
Wed Sep 16 2015, 10:27PM
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Have you considered embedding an RFID memory chip? Like this one:
I have no idea how well it would survive launch in a coil gun, but it could be worth a try.
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DerAlbi
Wed Sep 16 2015, 10:55PM
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Location: Dresden, Germany
Posts: 519
Yeah what a solution. i think RFID with inductive coupling is great near a high permeability material that short out every higher frequency making it the perfect shield for the RFID-Tag. Also fondling around before every shot, preparing the projectile, is really attractive to everyone who wants to shoot a coilgun. Not to say the additional cost of such stuff.
Of course, lets ignore the obvious way to go like using the natural available media for storing information... like.. uuhhmm.. the projectile itself?
I think, i just should use edding and write a serial number on each projectile. thats good.
Or built in a iron rasp inside the barrel. Wasting some joules of the hardly earned kinetic energy is allways a good solution.
Or what about attachting a string with hotglue. That links the projectile to the gun for sure!
Or i just change the coil drive and shoot backwards, that would imprint a dna-sample of the shooter on every projectile. Usefull too. Way to store information!
Why not throw the coilgun, instead of shooting the projectile? That would circumvent the problem too!

Sry, i currently feel like the next post will propose that i should buy a dog that chews on the projectiles.
Am i really so bad at explaining that i am seeking a discussion on how to imprint magnetic information to a solid iron projectile and what i could expect and how to achieve it?
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Ash Small
Wed Sep 16 2015, 11:22PM
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Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4118
My understanding is that data is stored in ferrite, not iron.

Ferrite, in my understanding, comprises numerous seperate 'grains', whereas iron is, in my understanding, some form of crystal lattice compriseing much larger grains, hence the eddy currents, etc.

I'm not sure it's possible to store information in iron in the same way that it's possible to do on magnetic tape, for example.

In my understanding, you need tiny grains in order to magnetise different sections with different polarities.

Even if you could produce such a projectile, others could produce projectiles consisting of a single large crystal of iron, or certainly get close.

That's just my understanding, though
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DerAlbi
Thu Sep 17 2015, 07:56AM
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Location: Dresden, Germany
Posts: 519
Thaaaaaank you for resonse on topic

The "grains" you describe are the magnetic domains within those the magnetism is aligned. See:
As you can see in the pictures the orientation can change within the solid iron.
I cant think that in a hard drive you have purposely single grains. Its surely a single surface isolated by crystal boundarys like within the iron.
I think the material that is specially made for magnetic storage has just a much more optimized hysteresis loop. (so that no accidential magnet field easily destroys the data and so that the remanent field strength is higher for better read out)

I never noticed directly that my projectiles were magnetized after a shot. However i noticed that changing the orientation degraded performance. So a shot does have some "impact" on the projectile.

In the link to wikipedia the paragraph about "Why domains form" stated that it is a "low energy state" when the grain magnetisation does not create a strong external field. That actually would have some impact on how one should write to the projectile: instead of having single oriented large domains, one should try to imprint a frequency modulation. That should than be more stable... right?
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Ash Small
Thu Sep 17 2015, 12:11PM
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And then there is the difference between 'soft' iron and 'hard' iron.

In my understanding, soft iron, ie annealed, is ideal for, say, transformers, because it has no 'magnetic memory', and is not 'permanently magnetised.

Hard iron, ie work hardended, can be permanently magnetised, ie it has a magnetic memory.

I imagine you'd need 'hard iron' in order to permanently magnetise it, but what's to stop people making soft iron projectiles with no magnetic memory?

That again, is just my understanding of some of the issues here.

EDIT: Imprinting a 'low frequency' signal does sound like the way to go, if you can overcome the other issues
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DerAlbi
Thu Sep 17 2015, 02:06PM
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"Low frequency" Lets see: 100m/s and 2mm bit length: so you got 20us time for one bit. Using a frequency fitting at least 4 full waves inside one bit, makes the wavelength 0.5mm so 5us for one period. Thats 200kHz. Aww

Does anyone know what the skin effect does here? i mean: it determines the equivalent thickness for resistivity. but does that mean that magnetic field lines wont penetrate? Will eddy currents inhibit the magnetisation at all?

Hmmh. Do i even need to reprogramm the iron? I mean it comes completeley saturated from the last acceleration coil.... soooo.. all i need to do is to get rid of the magnetisation and thats it allready.

Hard/Soft magnetic: afaik my projectiles are made from steel. that makes them hard magnetic. as i said: the remeber the last magnetisation which showed up in performance problem in my coilgun.
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Ash Small
Thu Sep 17 2015, 04:37PM
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Steel can be soft or hard. This, I think, mainly depends on grain structure, and the easiest way to alter the structure is generally heat treatment.

Transformer steel, for exampl;e, needs to be soft, whereas permanent magnets are hard.
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2Spoons
Fri Sep 18 2015, 05:19AM
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Wow, so fixated on one solution, and so hostile to alternatives. You do realise we are trying to help you find a workable solution?
I think your magnetic imprinting method is likely to fail because:
You will have to overcome a large remanent field in the projectile, resulting from its travel through a bunch of solenoids, unless you can provide a demagnetising coil at the muzzle.
Skin effect will prevent field penetration into the projectile at speed, so if you are very, very lucky you may get a magnetic imprint 5 to 10um deep (based on your 200kHz above).
I think reading that back is going to be exceedingly difficult.
Still, if you want to try then I wish you good luck and success.

And before you dismiss RFID out of hand consider putting the chip on a small plastic standoff on the rear of the projectile to get it clear of the iron, and the read/write coil is mounted in the breech so that data can be written as the projectile is loaded.
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hen918
Fri Sep 18 2015, 07:13PM
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Posts: 417
2Spoons wrote ...

Wow, so fixated on one solution, and so hostile to alternatives. You do realise we are trying to help you find a workable solution?
I think your magnetic imprinting method is likely to fail because:
You will have to overcome a large remanent field in the projectile, resulting from its travel through a bunch of solenoids, unless you can provide a demagnetising coil at the muzzle.
Skin effect will prevent field penetration into the projectile at speed, so if you are very, very lucky you may get a magnetic imprint 5 to 10um deep (based on your 200kHz above).
I think reading that back is going to be exceedingly difficult.
Still, if you want to try then I wish you good luck and success.

And before you dismiss RFID out of hand consider putting the chip on a small plastic standoff on the rear of the projectile to get it clear of the iron, and the read/write coil is mounted in the breech so that data can be written as the projectile is loaded.


I think, for the purpose of identifing a coil-gun projectile that is used in a crime, the NFC/RFID idea is flawed: It can be erased too easily, a distance from the projectile.
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Signification
Sat Sep 19 2015, 10:18AM
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Yeah, simplicity of countermeasures virtually kills this idea.
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DerAlbi
Sat Sep 19 2015, 03:35PM
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Sry 2Spoon, the possibilities are obviously infinite to approach any given problem. The more offtopic it gets the harder it is to get someone who could contibute here to read the thread.

For now: How to design a magnetic writer?

i mean.. somehow the flux needs to go through the projectile thats up to 0.5mm away. (the air gape size due to the barrel)
I think i need something like a horseshoe shaped core. Made from... Ferrite? But i think to store the best possible remanent field, i should dirve the iron to saturation. which will not happen with a core material that saturates earlier than iron. Pure Iron will not work as a core at those frequencies. But if that does not work.. will the projectile work?
The closed loop through the overall 1mm gap must be more attractive to the flux than closing directly inside the horeshoe.
The size alone to satisfy such condition puts a limit on the bitsize.
Also: how to to choose the orientation? Make the field lines parallel to the projectile direction? That would lead to really big physical bits. Or the horseshoe is oriented in such a way that the field lines are perpendicular to the projectiles direction. That could allow for smaller bits but it does not benefit from the previous magnetic imprint of the last acceleration coil.
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Bored Chemist
Sat Sep 19 2015, 08:46PM
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If I got a magnet and shot it from a compressed air cannon into a solid target, I would expect the shock to demagnetise it.

So...?
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Hazmatt_(The Underdog)
Sat Sep 19 2015, 10:44PM
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1. Duplicate the magnetic head from a tape deck, but beef it up for high current
2. You need pulsed DC to write not AC
3. You will need to pulse several amps of DC at very high speed to write to the slug (H-Bridge)
4. You will need to "Erase" the slug before writing to it, so that requires an erase head.
5. You will need an optical gate for the slug to pass through to start the write cycle, its moving pretty fast so spacing is critical
6. Impacts will distort the slug, reading back may be worthless, but that depends on how wide the magnetized section of the slug is, doesn't it?
7. Results?
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