Tag Archives: measurement

Clever Quadcopters Play Nicely Together

On the off chance you haven’t seen it yet, the UPenn GRASP Lab has just released yet another impressive video of their performance quadcopters.  This time they’ve got a new “nano” version that’s smaller, lighter, and capable of flying in formation.  It’s like synchronized swimming, but with more buzz:

Now as usual, the internet comments on this latest quadrotor development have largely consisted of “WOW!” and “Good heavens, they’ll kill us all!”  But as for me, once I’d retracted my dropped jaw, I started trying to figure out how it all worked.  Unfortunately, I haven’t found any real documentation on this project beyond the videos posted by PhD candidate Daniel Mellinger.  I just might have to send him an email…

But never fear, I have at least discovered how they’re tracking the quadcopters.  Did you notice those camera-like devices mounted along the walls?  And those funky red ring-lights surrounding their lenses?  Well those are VICON motion capture cameras:

VICON T-Series Cameras

These devices by themselves are quite impressive.  Much like traditional video cameras, each of these units contains a sensor with a certain number of megapixels.  However, VICON sensors are designed for fast frame rates (up to 2000fps), high resolution (up to 16MP, more than seven times the resolution of 1080p HD Video), and sensitivity to the red/infrared light emitted by their ring light strobes.  Why red light?  Well, these aren’t your typical video cameras.  Their purpose isn’t to capture a full-color image, it’s to capture points of light coming from passive reflectors.  In fact, what each camera sees looks like a star map of sorts.  Once multiple cameras are setup and calibrated, sophisticated software can measure and track the position of each reflector in real time.

So in the video above, it appears that each quadcopter has at least two reflectors attached to its top surface.  The perimeter cameras can then measure the position and orientation of each unit and relay that information to some kind of controlling computer.  What I still don’t know is how the software distinguishes between each quadrotor.  Perhaps VICON has reflectors which can be distinguised by the precise wavelength of light at which they reflect?  Or maybe the computer is just smart enough to know that the same set of points represents a certain unit from one frame to the next?

I’m also wondering how each quadcopter is controlled.  Some type of ZigBee wireless link perhaps?  And does the main computer handle everything?  I suspect there must be a certain amount of control embedded in each unit.  Perhaps they’ve all got accelerometers and MEMS gyros keeping them straight and level.  Still a lot of unanswered questions.  But in the meantime, enjoy this video on a radically different use of the VICON technology:

Measuring Telluric Currents – First Trial

Way back in November of last year (2010), I wrote a short little article on telluric currents, their history, and related applications.  Now, in case you’re unfamiliar with this topic (as I was prior to November of last year), here’s the executive summary: telluric, or earth currents, are electrical currents which travel through ground or water, primarily near the surface of the earth.  They may be naturally occurring (due to changes in the earth’s magnetic field via solar wind), or man-made (e.g, from mineral exploration).

Well towards the end of my previous post, I expressed a desire to try and measure these currents.  Unfortunately at that time, it was winter, and I was in the process of relocating to Iowa.  But now that I’ve settled in here, and the ground has finally thawed, I’ve gone out and performed a quick first measurement.  Here’s the procedure I followed:

  1. Obtain two 36″ lengths of standard rebar and 100′ of insulated 14 AWG copper wire (solid core works, but I used stranded for better contact with the rebar).
  2. Sand/file any rust from the surface of the rebar (to reduce contact resistance).
  3. Strip about two inches of insulation from the ends of the wire, then fray these ends and wrap them tightly around one end of each piece of rebar.  Cover these attachment areas with electrical tape.
  4. Cut the wire, which is now linking the two pieces of rebar, at any point (this is where the multimeter will be inserted).
  5. Space the two lengths of rebar as far apart horizontally as possible, then drive them into the ground as deeply as possible (in my case, this was about 20″).  For my first test, I configured the two so that they would point north to south based on the map shown here and my location in Iowa).  In other words, if I were to stand at the southern-most length of rebar, facing the other rod, I would be facing north.  They were separated by the 100′ of wire.
  6. Measure both current (short circuit) and voltage (open circuit).
  7. Finally, if anyone should question what the @#$% you’re doing pounding rebar into the ground, simple employ this catch-all excuse: “solar flare protection.

So, without further ado, I give you my results (in low-quality cellphone pic format):

Telluric Current - Well, it is measurable...

If you can’t quite make out that reading, my apologies.  The meter indicates 0.55mA (DC).  Yea, not too incredible, I know.  I also measured the voltage between the two rebar rods, but at just 105mV, it’s not terribly impressive either.  So, at best, we’ve got 14.5uW of power to play with – barely enough to run a digital watch (please see this excellent page on Thevenin equivalent circuits and the maximum power theorem for details on how that number was calculated).

Overall, these results are a little disappointing, both in quality and in quantity.  I had hoped to reconfigure my rods a couple of times so as to measure the current’s heading as well.  Unfortunately, for this test I picked a slightly wooded area that also happened to be teeming with mosquitoes.  I’ll do a lot of things for the sake of science, but serving as a meal for blood-sucking insects isn’t one of those things.

In the future, I’d like to leave the rebar in place for a while longer – say, 24 hours – and record data continuously during that time.  I’ve read in a number of sources that telluric currents tend to vary over the course of a day.  So, when I did my test this morning (8:30AM CST), I may have been measuring things as a low point.  The only trouble with capturing data for such an extended length of time is that I’ll need to find a more controllable location, and I’ll need to figure out how to log the data automatically.  I’ve got a few development boards I can probably re-purpose for that though…

So, in summary, for round two of testing I shall make the following changes:

  • Take measurements with the rods configured along different compass headings.
  • Log data for a consecutive period of at least 24 hours.

If anyone has other suggestions, please leave a comment.  Stay tuned for more.  Thanks!