Author Topic: Disc launcher attachment  (Read 766 times)

Offline klugesmith

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Re: Disc launcher attachment
« Reply #20 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.
« Last Edit: November 27, 2019, 02:53:42 AM by klugesmith »

Offline MRMILSTAR

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Re: Disc launcher attachment
« Reply #21 on: November 27, 2019, 05:06:19 AM »
That's some nice simulation work.
Steve White
Cedar Rapids, Iowa
Retired electrical engineer

Online davekni

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Re: Disc launcher attachment
« Reply #22 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.
David Knierim

Offline klugesmith

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Re: Disc launcher attachment
« Reply #23 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. 
« Last Edit: November 27, 2019, 08:53:50 AM by klugesmith »

Offline Uspring

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Re: Disc launcher attachment
« Reply #24 on: November 27, 2019, 01:11:26 PM »
klugesmith wrote:
Quote
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.

Offline hammertone

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Re: Disc launcher attachment
« Reply #25 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
« Last Edit: November 28, 2019, 02:13:56 PM by hammertone »

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Re: Disc launcher attachment
« Reply #25 on: November 28, 2019, 11:21:36 AM »

 


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