High Voltage Forum

Pulse power => Induction Launchers, Coil Guns and Rails guns => Topic started by: MRMILSTAR on November 19, 2019, 05:51:39 AM

Title: Disc launcher attachment
Post by: MRMILSTAR on November 19, 2019, 05:51:39 AM
It appears that I will be making the initial posting in this category. Hopefully, more will follow.

I finished the disc launcher attachment for my pulsed power generator today. It is designed to launch a 3.5" disk drive platter. It consists of 7.75 turns of 9 AWG double-build magnet wire wound in a spiral. It took several attempts before I figured out a way to wind the spiral accurately. The inductance is about 3.5 microhenrys. The resonant frequency with the 100 uF capacitor is about 8.5 kHz.

The coil is recessed into a 4 mm deep hole which I machined into a 0.5" thick sheet of G10. The hole is 3.5" in diameter. The coil was then immersed in 2-part epoxy and covered with a 0.03" thick sheet of G10. The G10 sheet is so thin that you can see the coil through it. A few air bubbles got trapped but that doesn't affect its strength or operation. The coupling to the disc is excellent due to the low 0.03" spacing between the coil and the disk. The final picture shows the disc launcher attachment  installed on my pulsed power generator.

The coil is recessed into the G10 and immersed in epoxy to keep it from flying apart during high-energy operation. I have watched Youtube videos of other disc launchers that mounted the coils free-standing and held in place with tie wraps but they always deform or fly apart at high power. My coil should be able to take at least 1500 Joules of energy but that is probably over-kill for a 3.5" disc. Because of the cold weather I haven't had a chance to test it yet but it should work great.

The final picture shows it installed on my pulse power generator.
Title: Re: Disc launcher attachment
Post by: klugesmith on November 20, 2019, 09:42:20 PM
Nice work there, Mil.

Are you going to work up from low charge voltages, where disk barely moves or rises only a few inches?
Then I hope you can experiment with pancake coils that have more turns, to get slower pulses, and see how the efficiency changes.  You can use much thinner wire for the low-energy experiments.

The system ought to be completely linear, so pulses with capacitor at 10 volts or 1000 volts differ only in amplitude.
There's an important exception.
The time for disk to move away from coil, changing the RLC parameters, gets smaller as you increase the charge energy. 
Eventually the geometry is changing significantly during the main pulse (60 us?). 
Also, the temperature and electrical resistance of the flying shorted turn are higher at the end of the pulse than at the beginning.

Specific example: consider a bifilar winding (spiral of two identical wires in the same plane), initially connected in parallel.  Now if you change the external connection to series, you should get exactly 4x the resistance and 4x the inductance.  Peak ampere-turn product and magnetic field strength should be the same for both connections.  With series connection, the pulses lasts twice as long, but induced currents in platter are only half as large.  Mechanical impulse (force X time) should be about the same.  Let the experiment be done.

The inductance is about 3.5 microhenrys. The resonant frequency with the 100 uF capacitor is about 8 kHz.
Sounds right, and sqrt(L/C) is about 187 milliohms.   LCR discharge waveform will be oscillatory if the circuit R (total of wire + capacitor ESR) is much smaller than that.  Can you show us a scope picture of discharge from 10 volts?   With and without a disk, or two, on the launch pad.
Title: Re: Disc launcher attachment
Post by: MRMILSTAR on November 21, 2019, 05:23:21 AM
I need to do some experimentation with this configuration. I chose 9 AWG magnet wire because it was the largest double-build magnet wire that I could find. This wire worked successfully for my can crusher. It took 1500 Joules at 5500 volts without breaking or shorting so I assumed it would be up to the task for a disc launcher spiral pancake coil for up to 1500 Joules. I didn't want the coil to burst. 8 turns of this wire is about all that I can get in a 3.5" diameter circle. I might be able to get 9 turns but making that spiral pancake coil was a real chore to do properly. I need to figure out a better way to wind an accurate spiral pancake coil.

My resonant frequency is about 8500 Hz. My initial calculations indicate that most of the energy is in the initial 2 cycles of the damped sinusoid assuming a total circuit resistance of 0.1 ohms. That gives about 0.25 milliseconds for the disc to launch. Maybe this is too short for the disk mass. I don't know yet. If it is too short I may have to use smaller wire to get more turns to lower the frequency to give more time to launch the disk.

I'm going to start with low energy levels and work my way up. The lowest voltage that my spark gap will trigger at is about 2 - 3 KV. 3 KV yields 450 Joules so that is probably where I'll start.

I can't accurately measure the total circuit resistance with my multimeter because it is so low but I think it is around 0.1 ohm which should yield good oscillatory behavior.

Its all unknowns right now. I haven't found much quantitative information about disc launchers so I think I have to discover the optimum design incrementally.
Title: Re: Disc launcher attachment
Post by: klugesmith on November 21, 2019, 07:43:34 AM
Have fun!  I think Dave Kni mentioned a disk launcher in his intro a couple months ago.

If it were my project, I'd start at less than 1 joule.  At what energy does the disk barely rise from the launch pad?
Then the voltages are small enough to observe RLC discharge waveform with any old oscilloscope.
The damped ringing waveform (with no disk on the coil) can give you an accurate value for effective total R, and total L including the interconnections.

When I did that with my can crusher, the charging power supply was a 9 volt battery and 1000 ohm resistor.
Of course spark gap switch needs to be bypassed. 
I used a MOSFET, with gate driven by a 555 on same 9 volt battery supply, triggering a discharge about once every second.
With grounded source, the FET happily conducts with low resistance in both directions during the damped oscillation.
Title: Re: Disc launcher attachment
Post by: Hydron on November 21, 2019, 09:01:08 AM
Because of the cold weather I haven't had a chance to test it yet but it should work great.
The cold should make it work better!
Looking forward to seeing it going.
Title: Re: Disc launcher attachment
Post by: Uspring on November 21, 2019, 03:26:42 PM
Here's a disc launcher simulator as a LTSpice model. You need as input the circuit parameters and also the dependence of the coupling of the coil to the ring as a function of separation. The coupling can be experimentally established by using the inductance when coupled i.e. Lcoupled = Luncoupled * (1 - k^2).
 [ Invalid Attachment ]
Title: Re: Disc launcher attachment
Post by: davekni on November 22, 2019, 05:14:28 AM
Have fun!  I think Dave Kni mentioned a disk launcher in his intro a couple months ago.
My original disk-launching project was years ago, 2003.  Started with coin-shrinking in 2002, then a bit of disk launching (using the same ancient 14uF 20kV oil/paper pulse capacitor).  Disk launching was originally going to be my first HV project.  Found a few of the disk-launching artifacts in the garage:

 [ Invalid Attachment ]

Above is an image of an un-launched 5cm disk punched from sheet aluminum, remaining scraps of what was the 16AWG launching coil, the small plywood board that the coil was taped to (with wire impressions from the launch), and the larger target plywood with trip-wire to measure speed.  The disk folded during the launch and buried itself into the target wood.  Speed was barely over sound, 345m/s.  I think the capacitor was close to fully charged, so about 2800 Joules.

 [ Invalid Attachment ]

This image is of a related test - attempting to form aluminum into a shallow mold.  Didn't work well - perhaps need holes for air to escape.  The coil under the white tape was used once or twice, cracking the tape.  The other smaller coil wasn't used.

Finally, recently (2016), I made a couple small disk launchers (penny launchers).  Here's my video of the first launcher.  The completed battery-powered unit is shown in the final 30 seconds.  The first 6 minutes show simpler examples, starting with a small air-core coil under a penny with a mechanical switch, adding an iron core, changing to a TRIAC switch, etc.  That launcher used 30uF DC link capacitor charged to 550V (slightly above rated 500V), with a core ground from standard line-frequency laminated steel I-core.  I think the coil is 20 or 30 turns of 24AWG magnet wire with epoxy coating, but don't recall for sure.  Could be 27AWG.  Perhaps I'll find notes at some future point.  (The second unit, not shown, uses two 30uF caps in parallel.  It can launch old copper pennies 2~3 meters up, and aluminum disks 4~5 meters.  Haven't measured exactly.)

Title: Re: Disc launcher attachment
Post by: MRMILSTAR on November 22, 2019, 11:28:40 PM
I collected some data today on the disc launcher. I have attached 2 scope screen captures. The first one shows the response of the pulsed power generator with the disc launcher coil attached and with no disc. I charged the capacitor to 15 volts and discharged it through the work coil. The second capture shows the same system with a disc in place. The highlights without the disc in place are:

* Resonant frequency = 6.7 kHz
* Total inductance (including parasitics and ESL) = 5.7 uH
* Total resistance (including parasitics and ESR) = 0.04 ohms
* Duration of initial 3 cycles = 450 us

The highlights with the disc in place are:

* Resonant frequency = 9.1 kHz
* Total inductance (including parasitics and ESL) = 3.05 uH
* Total resistance (including parasitics and ESR) = 0.04 ohms
* Duration of initial 3 cycles = 330 us

The summary of what all this tells me seems to be that the time available for the energy to launch the disc is about 0.4 milliseconds assuming most of the energy is expended in the initial 3 cycles. I will have to find out by experimentation if this is too short.
Title: Re: Disc launcher attachment
Post by: klugesmith on November 23, 2019, 03:18:27 AM
Very nice work there, Steve.  What did you use for a switch?

Prompted by Uspring's remarks: 
Now that you know RLC with 0 and 1 disk, how about a short stack of 2 disks?
Better yet, 1 disk with various thickness spacers between disk and work coil? 
Isn't the lifting force per ampere (squared) proportional to dL/dz?
At what spacing is the oscillation frequency halfway between cases 0 and 1?

The scope pictures show that peak current is higher with disk present.  Shorter time to first zero crossing.  Makes it harder on the capacitor, even if the voltage reversal ratio is a bit smaller.  I guess not very rough on a manly capacitor like yours.

The measured total inductance is large compared to what you'd computed (or measured?) for work coil alone.
I bet the 22 kV capacitor's own ESL is less than 0.1 uH, if it's like mine.
If you attribute 1.7 uH to the interconnecting wires, a way to reduce it is physical compactness and small loop areas.
Keep outgoing and returning wires close together, which unfortunately increases the Lorentz force tending to tear them away from each other.

Now that you know the effective RLC parameters for circuit with 0 disks and 1 disk,
you could take a step I never got to in can crushing days (way back when I was 50).
Get a match using circuit model with disk as a L-R loop, with some finite coupling to the L of work coil. 
Apparently already done or found by Uspring.
I met unexpected resistance (NPI) trying to solve it analytically, and never went and simulated it.
Physics note: The most common engineering alloys of aluminum are more resistive, by around 50%, than pure Al or wiring-grade Al.  But I bet the resistance penalty from high temperature is relatively smaller.

Title: Re: Disc launcher attachment
Post by: MRMILSTAR on November 23, 2019, 05:21:55 AM
Thanks kluge!

My "switch" was merely myself manually using a short wire to bypass the spark gap after charging which is about as simple as I can think of.  The scope was set to trigger on an edge.

Since my capacitor is rated for 22 KV, 20% voltage reversal, and over 150,000 amps peak, charging it to a maximum of 5500 volts (1500 Joules) for the disc launcher will pose no problems with voltage reversal.

I've just been using a simple RLC simulation to play with resonant frequency, resistance, capacitance, inductance, and Q factor. This is actually what I used to figure out the total resistance and inductance based on the other known parameters and those gathered from the scope traces. I suppose the next step would be a more sophisticated SPICE model.

It would be very difficult to make my connections much shorter. I explored this a lot before I built the machine. Shortening a lead in one place makes it longer somewhere else. And then there are the Lorentz forces as you mentioned. I also worry a lot about flash-over at high energy levels if things are made too compact. I recall seeing a Brian Basura video several years ago where he recounts a high-energy flash-over that he had with his coin shrinker which knocked him down and stunned him from 8 feet away. I examined the pictures of his machine and in my opinion he had things too compact which lead to that event.
Title: Re: Disc launcher attachment
Post by: davekni on November 23, 2019, 06:53:40 PM
Great point about flash-over risk.  That's why my coin shrinker is in the far corner of my garage.  Charging and triggering is from inside the house.  It is a compact (and lower energy - only 14uF) system.  When it has flashed over, the bang is much louder than a normal coil shrink.
Title: Re: Disc launcher attachment
Post by: Uspring on November 25, 2019, 03:27:29 PM
klugesmith wrote:
Isn't the lifting force per ampere (squared) proportional to dL/dz?

Yes, it is 1/2 * I^2 *dL/dz
for a perfectly conducting disk. Interestingly, this makes the force only dependent on the energy stored in the coil.

I will have to find out by experimentation if this is too short.

A ballpark estimate of takeoff time is this:
First an estimate of energy acquired by the disk. In the ideal case of no losses and input energy of E0=1500J, the energy E of the disk would be:
E = E0 * (L1-L2)/L1,
where L1 is the coils inductance without the disk and L2 with the disk. This comes out to about 700J.
Assuming a disk mass of 30g, that would be a speed of roughly 200m/s. Say, the acceleration is constant over a 5cm distance, that would imply an average speed of 100m/s, i.e. a travelling time of 500us.
The first assumption, i.e. no losses underestimates the travelling time, the speed of the disk will actually be smaller. The second assumption, i.e. constant acceleration overestimates the travelling time, since the acceleration will be larger initially and then drop off. I expect the effect of losses to be larger, since with ballpark values of around 500us travelling times, the circuit resistances will have eaten a lot of the energy out of the coil.
I recommend wearing the gear you show in your icon.
Title: Re: Disc launcher attachment
Post by: MRMILSTAR on November 25, 2019, 04:18:36 PM
I did a test launch today and the results were impressive to say the least. At 1000 Joules the disc reached a height of about 80 feet. The acceleration was so fast that I couldn't follow it very well. I tried 1250 Joules but the acceleration was so high that it cupped the disc so I am restricting the energy input to 1000 Joules. I don't even want to attempt a 1500 Joule shot.

The best news is that my spiral pancake coil held up with no damage even at 1250 Joules. I would attach a video but the file is too large (35 MB). You can't see much anyway because of the high acceleration but you can hear that distinctive "ping" sound when it launches. I need to make a Youtube channel so that I can post these videos.
Title: Re: Disc launcher attachment
Post by: klugesmith on November 25, 2019, 06:06:16 PM

Here is a question that might deserve answering by experiment, by model, or by talking on a forum.

As a disc elevator, Steve's new toy is about 0.7% efficient, from initially reported numbers.
For 30 g lifted to 80 feet, m g h is about 7 joules, 5 foot-pounds.
(Can you set up under a tree and shoot at squirrels?)

Will it be measurably more efficient at smaller energies, because less is lost to aerodynamic drag?
More efficient still when disc is well coupled for the whole RLC discharge?
Or less efficient because we care about force x distance, not force x time?

Data from a series of energies might help to confirm or refine dynamic models like Uspring has brought here.

Near the low end, 1 joule at same efficiency ought to lift the disc about 1 inch.
Starting with 141 volts, which is much too small for spark gap switch and maybe too large for the casually manipulated bypass wire.
This is a case where one could use an old screwdriver, without being guilty of capacitor abuse.
How 'bout simple attachment to turn mechanically triggered spark gap into a contactor?

[edit] Another easy test, for the field and for modeling, is to launch stacks of 2 or more disks.  Loose, or glued or taped together.  I think they will be strongly attracted to each other by the induced currents.

Title: Re: Disc launcher attachment
Post by: MRMILSTAR on November 25, 2019, 09:05:53 PM
If I wasn't afraid of hurting someone I would design and make a proper aerodynamic projectile that would travel much higher. The problem is that I don't want some pointy object coming down on someone's head at high speed! I don't want to replay the "Lawn Dart" fiasco from many years ago. So even though the discs have terrible aerodynamic properties, they won't hurt anyone falling from altitude.
Title: Re: Disc launcher attachment
Post by: Uspring on November 26, 2019, 01:44:44 PM
Much better than a ballpark estimate is a real simulation. I've attached a LTSPice model modified to MRMILSTARs parameters at 1000J. I'm not sure about the disk mass and the dependence of the coupling on disk height. Out comes a speed of about 55m/s.
 [ Invalid Attachment ]
Comparing the simulation result (55m/s) to my previous estimate (200m/s), there's a big difference. I've traced this to these reasons:

1. I'd assumed an input energy of 1500J instead of 1000J.

2. I hadn't considered, that the energy in the coil is on the average only half that as the peak energy. That's due to the oscillation, sorry, a big blunder.

3. For simplicity I've neglected the losses due to the ESR. A simulation with e.g. a 10 times lower ESR results in more than twice the speed.

55m/s would make the disk rise to about 150m. In vacuum, of course.
Title: Re: Disc launcher attachment
Post by: klugesmith on November 26, 2019, 07:00:04 PM
About disk mass: are the platter sizes standard even between different HDD makers?  The ones right here are just over 0.031" thick, with OD of 3 5/16 inches and ID of 25 mm.  Centigram scale is not far away.

Uspring, what's the basis of your L and R values in electrical model of the disk?

Before reading the latest reply, I could not resist starting a FEMM model, so far just with pancake coil.  Really hope this is not hijacking Steve's thread.  Wanted to investigate how much the current density in Al platter varies with radius.  Anyone else looked into that?  With that knowledge, one could replace the platter with a spiral-wound sense coil of same dimensions, to measure coupling vs Z on the bench.  That would complement Steve's measurement of effective R and L with platter in different places, even just zero and infinity.

As a sanity check, the FEA simulation with coil declared as 8 turns gave inductance of 3.38 uH.  Maximum B is reported as just over 3 teslas with 15000 amperes (my SWAG).

Here's a way to make a strong pancake coil with very good fill factor _and_ ability to make the current density vary with radius.  Start with a plate of copper or aluminum (it doesn't have to be round) and cut a spiral slot with a bandsaw.  The air space from saw kerf could then be reduced by uncoiling the turns slightly, after inserting a thin strip of insulating material.  If the saw blade width is a problem for curved cutting near center of coil, cut spiral with bigger hole in the middle, then tighten the coil from inside out. Or get the cut made by laser or plasma or water jet or EDM. :)
Title: Re: Disc launcher attachment
Post by: MRMILSTAR on November 26, 2019, 08:29:29 PM
Your not hijacking my thread at all. Any information related to this project whether gathered by experiment, simulation, or analysis is valuable for me and others.

Apparently there are 2 different thicknesses of discs that I have. I hadn't noticed until now. Here are the dimensions.

* Thickness: 0.07 inches and 0.05 inches
* Outer diameter: 3.75 inches
* Inner diameter: 0.98 inches

Apparently so-called 3.5 inch HD platters aren't as standardized as I thought. They aren't even 3.5 inches! The outer diameter of my coil is 3.75 inches with an inside diameter of approximately 1 inch. I can't find my scale or I would have supplied the weight. For maximum fidelity in your simulation my coil actually has 7.75 turns.

Those are some interesting ideas for making an accurate spiral. The water jet sounds the most attractive to me. I also have a possibly simpler idea involving the placement of guide nails on a flat wooden winding form after an accurate spiral has been traced onto it.

I had to make a slight modification to my work coil platform. The thrust force was so strong that it would bounce my coil mount up away from the blast box steel sides. It would then fall down into the blast box. To correct that I glued four rare-earth magnets which I salvaged from a dead HD drive to the bottom of the coil mount. These magnets now will hold the mount tightly against the top edges of the steel blast box walls when firing.
Title: Re: Disc launcher attachment
Post by: klugesmith on November 26, 2019, 09:19:59 PM
From picture in your OP, it looks like the steel is far enough away not to magnetically interfere with the launcher.
And the rare earth magnets are far enough away not to be demagnetized by the shot.
How much does the horizontal insulating bar flex during a shot?  Would it help to back it up with a vertical wooden post in the middle?

Hey, to complement the coin and can and disk hammering attachments, how about a magnetizing fixture?
See if you can re-magnetize a rare-earth magnet to be just as strong in the opposite direction, using a durable work coil.
More challenging for show and tell is to get one de-magnetized.  Uh, I guess that can be done nondestructively by heating.
Title: Re: Disc launcher attachment
Post by: MRMILSTAR on November 26, 2019, 11:12:52 PM
Those bolts that you see are made of brass. There is no ferrous material in the launcher. The magnets are mounted at the far ends of the support. I have similar magnets on my can crusher attachment at the same locations and have had no problem with demagnetization.

I don't know how much it is flexing because I can't see it well enough. I can't imagine it is much because the entire mount is made of G10. The coil holder is 0.5 inches thick and the longer supporting bar is 3/8 inch. I can easily add a stiffening bar of G10 if I have to.
Title: Re: Disc launcher attachment
Post by: klugesmith on November 27, 2019, 01:18:46 AM
A bit more play with the finite-element simulation led me to a surprising initial conclusion.  Caught my mistake after starting to write this.

Here's how the wrong idea goes:
"Looks like if work coil is same diameter as the disk, the outside edge of disk is attracted downward at the same time most of the disk is repelled upward !"
Because the vertical component of B field changes sign between the ID and OD of flying disk, while the circulating eddy current has the same sign at all radii.

Flawed thinking, because the axial (propulsive) force depends on the radial component of B field.  The vertical component of B field causes a radial force tending to shrink the disk's ID and, because of the sign change, expand its OD.

This all rests on assumption that FEMM is qualitatively right about the field's orientation.
We see that the direction turns from vertical (call it upward) at the axis, to radial near the outside edge of coil, to downward at larger radii.

In the axisymmetric model are two horizontal lines which are contours for optional charting or integrating of the simulated field. 
The lower contour is 0.025 inches above 0.030 inches above the top of the work coil.
Here's a chart along that contour, from axis to r = 10 cm.

First the normal (axial) component of B, with sign change that does _not_ change the direction of lift.  I need to go back and chart the radial component of B.
Remember the amperes-squared behavior of lifting force?  The magnitude of B is directly proportional to current in the work coil.  It's the vertical component of B that counts for inducing circular voltage and current in the platter.  It's the horizontal component of B, working with the circular current, that produces "useful" motive force. :-)

Second curve is the total flux F crossing the contour inside any given radius. Obtained by integrating the normal component of B over the disk area out to that radius. Spreadsheet agrees with integral tool in the FEMM GUI, that between the platter's ID and OD the total flux amounts to 5 mWb, with average density of about 0.8 T.

Now the induced voltage around a loop at any given radius is dF/dt.  We have to divide that by the length of that loop (2 pi r) to get the E-field, in volts/m; then material conductivity gives us the current density (J-field) in A/m^2.  Third curve in chart is F/(2 pi r), for qualitative view of radial variation in J (where a conducting sheet is present)

A step for next time is to chart the product of J and B_radial, times the disk thickness to get the lifting pressure in pascals, in this hasty un-aimed analytic shot.

Of course to be quantitative we need to know the characteristic time with which I, B, F, and J are varying.  I'm sure Uspring's circuit simulation shows a very significant phase shift between the work coil current and the induced eddy current.  I don't know if the static magnetic field model becomes erroneous when the field energy of induced current becomes substantial, one manifestation being the changes to effective R and L of the work coil circuit.

p.s. re the previous couple of posts, I always agrees that the design is sound.  My previous post saw NO problem with ferrous metal parts or unintentional demagnetizing. 
If the beam of G10 material was jumping out of place after a shot, doesn't that suggest that deflected downward at the moment of launch (by thousandths of an inch), and the rebound made it lose its footing?  As if you had bopped the middle downward with a rubber mallet?  What do they call the skateboard maneuver to jump from street onto sidewalk?

p.p.s. In literature about disc launchers, has anyone tried "external iron" like coilgunners like to talk about?
To increase work coil inductance and field strength for a given current.
Transformer steel might be OK if the laminations are oriented properly.
Ferrite shapes might serve well in places where the augmented flux density isn't too high.

[edit] Good thing I took the day off work! Finished this little FEA experiment by getting the force (pressure) distribution by radius.
New curves in this chart are Bt (the radial field strength) and "J.Bt" which is proportional to upward pressure.
The last curve falls off substantially before the outer edge of the projectile, at r = 1.75 inches.  Especially since the last 1/4 inch of radius has 29% of total disk area.  I bet a work coil with even 1 extra turn on the outside would perform measurably better.
Title: Re: Disc launcher attachment
Post by: MRMILSTAR on November 27, 2019, 05:06:19 AM
That's some nice simulation work.
Title: Re: Disc launcher attachment
Post by: davekni on November 27, 2019, 06:35:13 AM
Agreed, great simulation.  Are you going to expand to dynamic field with the disk present?  Or is that beyond hobby work?  Somewhere years ago I had a time-only simulation based on measured drive coil inductance-reduction vs. position.  No actual geometry.

For fields below saturation, iron is quite helpful.  At least it is on my little penny launchers.  On my higher-power version, I have 10mm x 11mm stack of transformer I laminations for launching 19mm diameter disks (pennies).  Did have trouble with the rebound force.  Initially just clamped the laminations together, backed by a thin rubber pad opposite the coil end for damped support.  The rebound force against the pad slowly worked the center laminations upwards, so the penny no longer fit in its place.
Title: Re: Disc launcher attachment
Post by: klugesmith on November 27, 2019, 07:42:12 AM
Thank you. I guess it had been bottled up & festering since my can crushing days, around 2007, story never told on 4hv before that forum broke.

In all modesty, it's hard to imagine a simpler or easier configuration in FEMM, except a coil section with three corners instead of four. 
Reading the result is not hard if you remember introductory electromagnetics.
Program comes with good tutorials.  Try it, you'll like it.

Create a world with nothing but air and one piece of electric conductor.  Conductor has a size and some number of amperes and turns. 

* New / Magnetics Problem / Axisymmetric
* Point mode.  Insert four points with typed in coordinates (r and z).
* Line mode.  Click on the established points to connect them, forming a rectangle.
* Block mode.  One block tag outside the box, called Air, and one inside the box, called 9_AWG, first set up using Properties/Add Property menu.
* Edit/Create Open Boundary.  Enter size; it sets up magical spherical shells that mimic unbounded space.

Now model is done. Push the Mesh button, then the Run button, then the View Result button.

From the viewing window
* Define a simple contour by connecting two pre-entered points.
* Plot Field Value.  Choices include |B|, B.n, and B.t (wrt the direction of the contour). 
* Plot can be viewed immediately, or saved as tabular data in a text file for spreadsheeting.  Charts in previous post are from spreadsheet with 1 row for each of 101 radius values.

Thanks to Steve (MRMILSTAR) for doing the real work, and inaugurating this subforum with a thread that won't soon be forgotten.
And to Dave for reporting his experiences on the same trail, with video to prove it, and Uspring for bringing tried-and-true circuit simulations.

p.s. FEMM would not accept a non-integral coil turns count, even though N just gets multiplied by the current and then divided by coil area to get J.
In practice, I think fractional turns are just as problematic as irregular space between turns. For example, with 10 kA in the wire, most radial slices have 80 kA in the coil, but in one sector there's only 70 kA.  Maybe you already said that.  In that sector would be less lift, by factor of about 7/8.  Not 7/8 squared, I think, because the induced ampere-turns in platter are the same all the way around. 
Title: Re: Disc launcher attachment
Post by: Uspring on November 27, 2019, 01:11:26 PM
klugesmith wrote:
Uspring, what's the basis of your L and R values in electrical model of the disk?
The model is that of a transformer with a variable coupling and the secondary short circuited. I've neglected secondary resistance, i.e. that being zero. The most important equations from this are the force

F = 1/2 * I^2 * dL/dz

which can be derived from energy conservation and

V = L * dI/dt + I * dL/dt

which can be derived from the equations describing a transformer. V is the voltage across the primary.

To answer your question: Secondary L does not appear in the relevant equations.

The role of secondary resistance can be judged by comparing secondary impedance, i.e. Zsec=2*pi*f*Lsec to secondary resistance Rsec arising from the aluminum resistance around the secondary loop. If Zsec >> Rsec is the case, the secondary can effectively be considered short circuited. By a ballpark estimate I've found this to be true for the frequencies occurring here. Rsec depends on the thickness of the disk. I've taken that as 2mm, which is a bit too much but I still think the short circuit assumption holds. Rsec is also dependent on the skin effect, but that contribution to Rsec also seems small.
It is possible to include secondary resistance in the model. AFAIK only the ratio Rsec/Zsec is needed. I'm not yet convinced of the necessity to add that. Possibly FEMM can shed some light on this.
Title: Re: Disc launcher attachment
Post by: hammertone on November 28, 2019, 11:21:36 AM
I built a launcher quite some time ago, and the coil was made from 1mm x 5mm flat wire, double isolated.

It must have had some 30 turns, starting at 20mm and ending at ~80mm.
This coil was lodged in a fitting cavity routed into 20mm thick HDPE, immersed in epoxy, and finally secured by a lid, 2mm thick paper phenolic, and screwed down. The HDPE was screwed onto a lamination of 50mm thick aeroplane plywood. Standing on 3 acorn nuts, it was very stable. I fired this assembly standing on a concrete tile, 400mm by 400mm by 60mm.

I only have the remains here, because the coil shorted out.
I have no idea how many shots it endured, as it was a job for the electricity museum.

A coil like this is easy to make, and the wire is available as scrap from large distribution transformers, although I managed to get some from the local motor rewind shop.

The coil takes a hard hammering, and I think it is basically a wear-part that should be exchanged on a regular basis. However, it will help if you use the kind of wire that has several layers of nomex spirally wound on it as insulation. Again, this is distribution transformer stuff.

If you want to send your disk high, It has been my experience that the disk should be heavy. I used an alluminum pellet, 20mm thick, 88mm diametre, and it went 20 meters into the air before returning.

4 x 6000µF for 6000µF @ 750V  = 1688J

Cheers, Finn Hammer
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