MakerBot + OpenSCAD

8 11 2011

When I started this project 3 years ago, one of the first things I did was build a reprap. Sadly the state of the art  just wasn’t there yet. I assembled the reprap, but it never worked well enough to print.

Since then I’ve been following the impressive progress of the reprap and then MakerBot. All along I’ve been using 3D printing services like shapeways, with a 2 week turn around. That’s a painfully long time to wait only to discover your design doesn’t work.

So I have purchased the latest Makerbot Thing-O-Matic with MK7 extruder. It should arrive in 4 weeks. Can’t wait!

Along the way I discovered OpenSCAD, an open source solid modeling program for programmers. I like it. It’s easier than the ruby + BRL-CAD I’ve been using. Additionally there is broad community support.

I’m rebuilding my polywell models in OpenSCAD now. I’ve gotten this far:

Discrepancy Between Circuit Simulation and Reality

29 09 2011

Previously we modeled the polywell coils and power supply in SPICE.

Today I returned to that model.

All resistance values in the simulation are based on real world measurements with the exception of coil inductance (code).

Starting with an estimate for coil inductance of 0.1 mH the discharge current looks like this:

The simulation’s peak of 1.5 kA is nowhere near the 2.3 kA we are getting in the real world:

OK. Maybe the value for coil inductance is off?

I played around with the value for coil inductance but the simulation would not match reality.

As a control I replaced the simulated inductor with a 1 mΩ resistor (code). Looks like this:

The simulation predicts ~1.8 kA but in reality we see 2.3kA!

Where does this discrepancy come from?



UPDATE: Reader Andrew solved the mystery:

You could try changing the ON resistance of your switch/SCR to something a bit lower than 100mOhms

.model MySwitch SW(Ron=.1 Roff=1Meg Vt=3 Vh=0)

I can’t see the part number of your SCR but 2mOhms would seem reasonable.

I didn’t notice that rather high resistance lurking in the SCR model.

Now the simulation matches reality very closely with 0.06mH coil inductance (code):

Good work Andrew and the rest of the internet brain!

Circuit Modeling with SPICE

2 09 2011

Last week Raymond Rogers made a SPICE model of my coil circuit. Extremely helpful and awesome, thanks Ray!

SPICE is a general-purpose open source analog electronic circuit simulator.

I’ve been trying to get started with SPICE for a while now, but the steep learning curve prevented much progress. So to have a working example of a circuit I’m familiar with is so very useful.

Now we can run virtual experiment on the coil circuit and see how much current we get. Pretty damn cool!

Here are some example input values and resulting current graph:

Capacitance: 15 mF, 450V

Coil resistance: 180 mΩ

Coil inductance:  0.1mH






In this diagram the vertical axis is the voltage of a 1mΩ ammeter resistor, so 1V = 1KA.

I encourage anyone who knows splice to run this code, make changes and share results.

Also a shout out to jstults for his python script for inductance modeling.


Computationally Intractable (or maybe not?)

26 03 2011

In Bussard’s 2006 Google tech talk  Should Google Go Nuclear? he talks about computer modeling of his reactor. He concludes that computer modeling is unfeasible. Beyond a handful of particles in the model, the computation slows down to the point of useless.

Now there may be a new approach to this type of problem.

Professor Charbel Farhat, chair of the Aeronautics and Astronautics Department at Stanford’s School of Engineering, and David Amsallem, an engineering research associate who worked on his PhD thesis with Farhat, have been studying and trying to solve aeroelastic flutter for years. Computers help, but only to a point.

Essentially it’s a story of the unfeasible made feasible by mathematical inovation:

How have Farhat and Amsallem succeeded where others have come up short? The answer sounds suitably complex: interpolation on manifolds. What it means, in essence, is approximating unknowns based on known information. The two engineers devised a system of mathematical approximations that break down complex, computationally demanding equations into smaller, more manageable parts. In mathematics, this is known as “reducing.” Reducing allows them to make some very educated guesses, very quickly.

I wonder if this technique could be applied to computer modeling of the Bussard reactor?

I suppose in our case we would be looking FOR the flutter, not trying to avoid it.



Another good article:


8 04 2010

A friend in academia is loaning me his copy of Labview 2009 for Mac / Win. Great news. I got it installed on the mac. I could not install it on the PC, as the PC lacks a DVD drive (CD only). I’m scrounging up an external drive.

We got the last of the parts for the coil power supply today too:

From left to right: Wire would resistors, isolation transformers, and mechanical relays.

Also got some grounding upgrades from mcmaster:

Remote Control

14 11 2009

Now that our Fusor seems to be working, I must operate it from a safe distance. Last night I did the first successful remote run. I connected an iSight to the Fusor mac (G4 running OS X Tiger). Then I used VNC to remote control the Fusor mac from my laptop:


I made headway with the command line program to control the fusor and record data in mysql via ruby. It’s currently setup to record vacuum chamber pressure, voltage and current according to the Glassman, and the effective flow rate of the mass flow controller. Currently I can enter commands to turn the high voltage on/off and set the flowrate of the mass flow controller. Next I want to control the voltage and current on the Glassman.

I also got a geiger counter:


It takes two D batteries. There is a BNC connector for headphones. I have a BNC connector on order with mcmaster. In the meantime I improvised a connection to some computer speakers to test it out. Seems to be working. It picks up the expected background radiation producing that erie clicking sound. You can definitely hear an uptick in the clicks when I run the Fusor.

I’m working towards producing a comprehensive mapping of this device’s performance envelope using computer control to search the parameter space and record the results.

This is all so fun and exciting.

Joule Heating and Ampère’s force law

30 11 2008

I’ve completed a first attempt at coding calculations for Joule Heating and Ampère’s force law. Joule Heating tells us how hot the copper coil will get when we pass a currant through it. Ampère’s force law tells us the mechanical force the coils exert on the chassis. No idea if these calculations are correct, going to review them with someone who knows better.  A shout out to ruby-units, this rubygem makes working with physical units very easy. Here is the source code for the calculations

Here are the calculations for the current dimentions:

  • outside_radius: 242.487113059643 mm
  • wraps: 225
  • torus_midplane_radius: 192.693468865964 mm
  • donut_exterier_radius: 107.94 mm
  • torus_radius: 84.14 mm
  • donut_hole_radius: 60.34 mm
  • torus_tube_radius: 23.8 mm
  • torus_tube_hollow_radius: 18.802 mm
  • joint_radius: 16.66 mm
  • joint_negative_radius: 5.831 mm
  • torus_tube_wall_thickness: 4.998 mm


Joule Heating

  • drive_amps: 2000 A
  • coil_length: 116507 mm
  • specific_heat_of_copper: 24.44 J/mol*degK
  • atomic_weight_of_copper: 63.546 g/mol
  • coil_weight_in_moles: 53.9277 mol
  • coil_weight: 3426.89 g
  • wire_resistance: 5.21096e-06 Ohm/mm
  • coil_resistance: 0.607114 Ohm
  • joule_heating: 1842.54 degK


Ampère’s force law. I simplified the model. I’m calculating the force between two coils at a distance equal to the torus_midplane_radius.

  • magnetic_constant: 1.25664e-06 N/A^2
  • magnetic_force_constant: 2e-07 N/A^2
  • seperation_of_wires: 0.192693 m
  • coil_force_per_meter: 4.15167 N/m
  • coil_force: 483.7 N

Open Source

13 11 2008

Unless specified otherwise, all intellectual property generated by this project will be open sourced under the MIT license.

The decawell source code is available on github.


21 10 2008

First off, I’m all about open source software. Free, unencumbered, agile, fast.

For CAD I’m using BRL-CAD which is based on constructive solid geometry

For glue code I’ll use Ruby or Python. I know ruby I don’t know python.

SAGE for a mathematics package.

I havn’t looked closely at this yet, but here is a list of robots software packages

My laptop is a mac, I use linux on the server side, and will likely use EC2 for cloud computing.


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