Hack of the Whenever I Get Around to It

November 21, 2008

Simulation and Analysis of Maglev

Filed under: Uncategorized — Chris Merck @ 7:16 pm

One of the project objectives for SKIL is to create a simulation of the experiment being run. We were supposed to use LabVIEW, but LabVIEW gives me indigestion, so I used MIT/GNU Scheme instead. The simulation is supposed to model the feedback loop which is formed by the levitation apparatus and the electronics controlling it. Essentially the simulation takes as input a “control function” and outputs the expected behavior of the levitation.

So I threw together some scheme code to simulate the motion of a magnetic object in a changing magnetic field, which is determined by the current of the simulated electromagnet, which is in turn controlled by the changing voltage applied. That applied voltage is the output from the “control function”. The input to the control function is the elevation of the object.

Below is shown the first screenshot I have of simulation output. On the left is the numerical output of the various parameters (unlabeled) and on the right is the elevation vs. time plot of the object. The control function that was used was quite naive: it set the voltage to minimum or maximum when the object was “too high” or “too low” respectively. You can see that the naive control function does not stabilize vibrations properly, so the oscillations get out of control:

showing the effect of a poor control function

showing the effect of a poor control function

I eventually added gnuplot output for fancier graphs and devised a better control function. A screenshot of a testing this better control function is shown here:


The simulation depicted above only handles one dimension of motion, but there are 3 in the real levitator. So, I put together a theoretical model of the levitator in 2 dimensions (the third dimension is symmetrical and so it is not shown). The result was the following “potential curves”. These curves are analogous to terrain that an object is rolling over. The object will tend to move along the valleys and settle in the pits (lower energy regions).

a potential energy curve which takes into account gravity, permanent magnetism, and the stablizing effect of the control function

a potential energy curve which takes into account gravity, preeminent magnetism, and the stabilizing effect of the control function

a potential energy curve for two electromangets spaced a few inches apart, in which one can see two "potential wells"

a potential energy curve for two electromangets spaced a few inches apart, in which one can see two

Lastly, Kevin put together an analysis of Barry’s “anti-gravity relay” circuit:

Kevin's Analysis

More details of the simulation and analysis can be found in the project writeup.

November 16, 2008

Multi-Dimentional Attractive Magnetic Levitation – The Conception

Filed under: Uncategorized — Chris Merck @ 9:24 am

Each semester at Stevens we physics majors are required to work on a so-called “SKIL” project. For this project there is no assigned project, instead we students select a project we feel is both educational and possible to make some headway on during the course of a semester. Last semester on the first day of class we all sat down and discussed our project ideas – everyone was to present one or more ideas, and we were going to argue the relative merits of them and select 3 or 4 viable ideas and choose teams to work on them. I entered the room that morning with a few ideas in my head, amoung them a low-cost usb-powered EEG, improvements to the RepRap project, and something else I have since forgotten. But, during the class I remembered a cool project I had seen on the internet a while back which involved using an actively regulated electromagnet to suspend small metal objects in mid-air. The project had the aesthetic appeal of a good SKIL project, but it lacked the nessisary complexity and origionality. So, I doodled a little imaginative doodle involving a horizontal array of coils suspending and moving the metalic object beneith them, with the ability to control not only the vertical but also the horizontal position of that object. The idea was a hit, and we formed a team of 6. Six may sound like a small number, but this was half of the physics majors in my graduating class, all working on one project. The team size was unpresidented and presented a real organizational challenge. Nevertheless we stuck with the project and now, looking back, I am quite pleased that we did.

The first step was to do some brainstorming. I apt-got myself a copy of FreeMind and made the following mind-map (click for enlargement):


After the brainstorming we starting attacking the project from many angles at the same time. This worked for several reasons: 1) the project split into fairly independant sub-projects, 2) we had 6 people, 3) we talked as a group breifly before each lab and decided on a plan of action for that week and a tentative plan for the following week, but after 20min or so we split into at least 2 independant sub-teams and most importantly in my opinion 4) what we learned in persuing one end was applicable towards another – since we were designing and constructing simultaneously, we were able to figure out what was and wasn’t going to work in reality even while drawing up a plan. In the end we spent approximately 500 man hours on the project (6 people * 4-hour weekly lab * 15 weeks + some people became sort of obsessed and moonlighted many a long night = ~500 hrs).

Here’s a photograph of the device in action:

Levitating a Screw Driver

Levitating a Screw Driver

November 15, 2008

Polarizing Optoacustic Mixer

Filed under: Uncategorized — Chris Merck @ 2:12 am


To mix two audio signals using amplitude modulated lasers as the transition medium and a polarizing filter for the selection of the mixing ratio.


Two low-cost red laser pointers which normally use 3VDC power supplies are modified with an audio transformer so that for each laser the supply voltage varies about 3VDC proportionally to an audio signal. The fluctuating supply voltages result in fluctuating beam brightnesses. The beams are made colinear with a 50/50 beamsplitter and the lasers are rotated along their beam axes such that the polarization of the light due to either laser in the collinear beam is orthogonalized. The beam then passes through a hand-held polarizing filter which selects out a linear combination of the orthagonal polarizations and so also a linear combination of the audio signals those polarizations encode. The beam post-polarizer is then incident on a (passive) phototranstor attached to a high-gain (741-based) pre-amp and a boom-box, so that the mixed audio signal may be heard in real-time.


an annotated beam’s-eye view
(note Steve’s Cell-Detecting Helical Antenna of Death in back left)

Steve hand-mixing some beats

a top-view of the transmitter in the dark

a beam’s-eye view in the dark
(You have to imagine the otherworldly blend of Moondog and eurotrance emanating from the boom-box.)

the development and testing team
(Raj, Langley, Merck, Englehardt, Steve)

Video Demonstration:

DJ Raj demos the POM


Each of the transmitting lasers requires a modulation circuit like the following. Note that you could use many other methods to modulate the laser, but I have found the transformer to be very effective for audio:

the wiring diagram for a single-laser transmitter

the wiring diagram for a single-laser transmitter (click to make bigger)

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