Rewiring: It's not sexy

It's not sexy, but sometimes you have to do it: Re-wiring.

I posted earlier about a main terminal block that would connect all the different parts of the system. That one served me well as a prototype but it wasn't the right size or had the right wire lengths to mount anywhere. Instead of trying to replace the connections one by one, I just made another main terminal block and mounted it in as a replacement.

Now, once I get the brackets to mount the firing circuits there will be nothing that prevents me from actually picking it up and toting it around.

Printed circuits?

I was recently referred to this company pcbsingles.com that make 1-off circuit boards for low prices (~$35). They're only 1-sided but it turned out I didn't two sides. If it works, I'll save myself a lot of wiring wrapping.

Notice that I've actually added a line of drill points along the edge of each circuit. I'll use these as guides to cut the boards apart.

Firing on two magnets

I've got all the kinks worked out of the second magnetic accelerator. The video is firing at essentially full power with two of thirteen gates. The ballistic timer reads 36 m/s, up from the 25 m/s with just a single magnet. If you do the math, it shows that the projectile picked up double the power it had before (~1.55J to ~3.1)

This linear increase in the energy makes sense given that the magnets are applying a constant force on the projectile. The projectile moves in that force and acquires energy regardless of how quickly it moves in that force. Of course there's the complicating factor here that our force actually varies with time (the magnets are only on for a moment) so I was prepared for some non-linear interaction.

The fact that the increase is linear so far is quite optimistic about the performance of the device when we have all the magnets online. Perhaps they will all just add and not decrease in performance as the projectile is moving faster.

Otherwise, I'm still waiting on the 3d-printer to arrive before I can print the brackets to hold the circuits. I'll also be trying my hand at writing circuit boards for printing. This will ultimately be less work than hand wiring wrapping 11 more such accelerating gates.

Firing results and problems with noise

First: Firing results.

I've got sensors for the device now and thus can test speed again. I was firing from just one magnet at 400V. The capacitor stores 1,800uF so the device was carrying 0.5*0.0018*400^2 = 144J.

Slug weight	Velocity	Energy	Efficiency
4.8 g	25.7 m/s	1.59 J	1.1 %
8.4 g	22.5 m/s	2.13 J	1.5 %
11.6 g	18.0 m/s	1.88 J	1.3 %

If all the gates worked at the same efficiency as the 8.4g slug (which they won't) the device will contain 28J of energy and reach 82 m/s. By comparison a 0.22 fired from a rifle will have between 100 and 300J of energy and will generally be doing more than 300 m/s. This slug will probably have better penetration, however, given that it'll be a pointed chunk of hard steel rather than a soft and dull lead slug.

In other news, getting the second gate working was a bit of a challenge. The charger is giving me a bit of trouble in that is makes so much electromagnetic noise that it creates jitters in the optical sensor of the second gate. I've tried adding a low pass filter but for some reason it's actually causing the optical sensor to just read low. Instead, I'm simply making the charge switch also force the sensor into the non-firing position. I've not yet completed this work but I'll have that soon.

A circuit so hot it might as well be wearing a wet t-shirt.

Parts arrived today. This is the layout of the new firing circuit. There's all the components for amplifying and filtering the firing signal, allowing power into the cap, giving the visual indicator for the cap, and sporting the connectors for everything on the this magnet. Notice that I'm also leaving extra room on these boards in case I need to cram extra stuff I'm not expecting in there. I'll be making 13 of these.

You can also see in the background a test setup for the sensor. This circuit must also be built but is far simpler.

Down below, you can see the very simple firing circuit. I've been going out of my way to get this system as absolutely minimally simple as possible.

New circuit holder is ready

This bracket will get bolted in underneath each of the magnet holders. It should contain both the basic circuits and the large bolt-style SCR that I'll be using.

The code for this one is actually quite dynamic so I didn't bother to measure and plug in the real values. I'll do all that when the 3D printer gets here and I can also investigate the strength of the plastic.

The code:
circuitLength = 60;

circuitWidth = 45;

circuitDepth = 3;

circuitLip = 4;

bedToBoltCenter = 20; //Distance between the bottom of the barrel holder and the mounting bolt for each barrel section

bedWidth = 50; //How wide the bar that holds the barrel is.

boltDiameter = 8;

wallThickness = 3;

a = 10;

//X is along the length of the circuit

//y is along the barrel length

//z is up

module circuitHolder()

{

difference()

{

union() //Added parts

{

translate([circuitLength/2+ wallThickness/2,0,0])

cube(size=[wallThickness+circuitLength, wallThickness*2 + circuitWidth, wallThickness*2 + circuitDepth], center=true);

rotate([0,-90,0]) //bolt

cylinder(h=30,r=(boltDiameter/2+2*wallThickness),center=true);

translate([bedWidth/2,0,-bedToBoltCenter/2-circuitDepth/2])

cube(size=[bedWidth,circuitWidth,bedToBoltCenter-circuitDepth/2],center=true);

}

union() //Removed parts

{

translate([circuitLength/2 + wallThickness + 1,0,0])

cube(size=[circuitLength+1, circuitWidth, circuitDepth], center = true);

translate([circuitLength/2 + wallThickness + 1,0,0])

cube(size=[circuitLength+1, circuitWidth - 2*circuitLip, circuitDepth+wallThickness*2+100],center= true);

rotate([0,-90,0]) //Bolt

cylinder(h=30,r=boltDiameter/2,center=true);

translate([-50,0,0])

cube(size=[100,100,100],center=true);

}

}

}

circuitHolder();

Hot shit: A test shot at full power with one of thirteen magnets

This is the first test shot on the new mounted barrel for the MA-12. I charge up to 400Vs (full power) but I haven't been tuning for speed because I can't do ballistics measurements. Specifically, I don't yet have the new larger sensors that can fit over this barrel (the old ones are about 2mm too small).

You'll notice there are a lot of magnets that haven't been hooked up and aren't running. Parts to arrive later this week.