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.

Ballistic metrics on the cheap

Once upon a time I had nice home-made ballistic timer. You can see it here on the right. It ran on 5V. Once upon a time I plugged it into a 14V power supply. I no longer own a nice ballistic timer.

What I do own is a half-assed ballistic timer and a triggering oscilloscope. Here you can see the parallax USB oscilloscope. They're about $100 and probably one of the best buys a home electrical engineer can make.



I've got it hooked up to some infrared sensors (shown in the next pic). When the beam is broken the sensor makes a pulse that the scope can pick up. Setting up the scope every time, however, is kind of a bitch. There's a lot of clicking and getting things just right. When you're tuning an electromagnetic rifle, you'll need to take many shots in succession and this will slow you down.



My solution was to hack a microcontroller into doing it for me instead. The microcontroller also watches the sensors but it resets every time it sees a new round go through so there's not re-setting. And instead of having a nifty, highly accurate but click-drag heavy way of finding the distances between the pulses the microcontroller just counts off in binary on the lights that come mounted on it by default. All lights on means it's ready and waiting for a shot.

I didn't try real hard to calibrate it but the pulses come out to about 1ms per binary digit. When the gun is firing at full speed I'll have to reduce the wait time.

Here's the code:

#include

void wait(int a, int b)
{
int i,j;
for(i = 0; i < a; i++)
for(j = 0; j < b; j++)
}

void count()
{
int time;
TRISC = 0xFF;
TRISD = 0x00;

PORTD = 0xFF;
//Check portc 0 going to high
//Check portc 1 going to high
//Show elapsed time

while(1==1)
{
if( (PORTC & 0b00000001) > 0)
{
PORTD = 0b11001100;
time = 0;
while( (PORTC & 0b00000010) == 0)
{
wait(0x01,0x2F);
time++;
}
PORTD = time;
wait(0xFF,0xFF);
}
}
}

void main()
{
//testPortC();
count();
}

Quick initial results

I fired at 250V (of 450V max), using 2 of the accelerator gates (later to be 13, but they'll be of different dimensions so hard to compare that now). The projectile was clocked at 20m/s. More specifically, it took 7.6ms to pass a distance of 15cm. (I'll post later about how I do ballistic measurement.)

I was using some relatively beefy SCRs in a Super-247 package but I toasts them heartily anywhere beyond 200Vs. Instead I switched to a bolt-style SCR that's been doing fine. Oddly they have exactly the same characteristics on their datasheets. Clearly one of them is lying. Too bad too, cuz the one I toasted was only $6 and the other one is $26. It'll need more than one of two of these things before the project is done.

Bolt style: http://ixdev.ixys.com/DataSheet/L106.pdf
247 style: http://www.vishay.com/docs/93712/93712.pdf

I makes me feel like less of a man

I just did a small test to show that it works not just firing one of the magnets but actually using two of them. There's still a lot of work to do but this shows that the prototype wiring is all correct (or at least close enough).

This shot was taken at 100V or about 1/8th power. It also only used two of the magnets. In the final version there will be 12 of them. That comes out to about 2% power. In reality I probably won't get the power out of later ones that I might want so call it about 5% power.

Also, finally got a working credit card again so the buying continues:
* T-slot nuts for the milling machine
* Helmet cam so I don't have to use the cell-phone
* An ass ton of electronics from digikey (or I will later tonight after I debug one last thing)

High power analog is a little bitch and always has been

I've set it up to be able to fire on two of the magnets as a test.

It does indeed charge and fire. That said, the electromagnetic disturbance caused by just the charging system seems to be able to set off the SCRs that fire the magnets whenever it charges above 130V (the electromagnetic disturbance from the charger gets more agressive as the voltage increases. Indeed, you can hear it once it passes about 100V). The reason I get this pre-firing is that the magnets are set off by an infared beam sensor. When the round breaks the beam, the sensor gives a signal to the firing circuit. It seems that the disturbance is enough to cause bumps in the signal and thus fire the magnet before it's supposed to. At one point, when it fired while charging, the charger seems to have been able to toast one of the firing circuits.

My solution right now is to put a computer (microcontroller) between all the sensors and the firing circuits. The computer will be able to see when the signal is bouncing and discount it. I could also use a capacitor-resistor filter for this but that would provide less flexibility in the long run.

The computer will have it's own 5V power supply and need not share one with the charger. This should also help reduce jitters.

The duct-tape approach

Instead of buying a lathe... I got a cheap file. I cut the bolt and mounted it in the chuck. It spins. I press. Takes longer than you'd think, but it sure works.

I have no idea what I'm doing.

It turns out I have no idea how to operate a milling machine.

My goal here is making ammunition for the rifle out of bolts. Specifically, I want to put a point on the chunk of bolt that I'm going to be shooting. To accomplish this, I intend to mount the bolt vertically and then used a metal filing bit that already has an angle to it. All I'll need to do is rotate the milling head around the bolt to make it pointed.

In the end, it sure beats doing it by hand with a file but it's clearly going to take a while. Additionally, it's pretty clear I have no idea how to use the mounting set. In this situation I've set it up to work well forward and backwards but it can slide to the left and right.

I need to mount the milling machine to the table next time I do this because I intend to tilt the head to the side and just use a normal milling bit to cut away at it from and angle.

This job totally demands a lathe. Like the one job I can imagine I'll ever have to do that demands a lathe instead of a milling machine...

Getting messy

Machine kept falling over, so I added a brace. Not fancy, the but the theme of recent construction is 'get it done'.

Been working mostly on getting the overall wiring diagrams done. There will be a succession of 12 electromagnets, each will need identical wiring but I don't want to have any point that has a breakout of 6 different wires times 12 magnets. That would be a disaster. Instead I'm going for a daisy chaining design where each plugs into the next one. Hopefully that won't mean too many failure points and power lost through connections but we'll see.



Even with that simplification, there's a need for a central junction to which many different systems can connect. Last time I did this, I just made a specialized connector. It worked but that was a mistake from a recycle-and-modify-the-parts-later standpoint. I still don't like the junction connector but next time I'll remember to make room for it somewhere on the device itself instead of just having it hang out up in the front of the rifle as it'll likely be doing.

Forget all that: Too complex

For V1 I won't be using the aluminium main bar as stated earlier. I believed at one time I could just print off and throw brackets at it but it looks like some of them will require drilling. That could be as many as 8 holes per electromagnet (of which there are 12) and even that at odd angles. Also, the cost of the ABS plastic for the 3d-printer won't be zero.

Instead, I'm going to use a steel bar wherever possible.

The bars to hold racks of components are a bit long but I'll cut them later. It looks like I can squeeze all the major stuff (big ass battery included) into just the top-half of the space I have.