[amsat-bb] Re: Non-mechanical feedback follow up

Robert Bruninga bruninga at usna.edu
Fri Aug 9 22:54:56 PDT 2013

Just to clarify, the "time tracking" works just as well for the
elevation rotor.  You can see my balloon chasing 2.4 GHz antenna on my
van here:  http://aprs.org/balloons.html about 1/3rd down the page
(the second balloon mission).  But it used a full Yaesu AZ/EL system
so I did have feedback.

My controller had two serial inputs, one was for the GPS in the van
and the other was to a TNC to receive the APRS posits from the
balloon.  THen the controler does the math so the antenna remains
pointed at the balloon (or UAV or what have you)...  (though you
cannot stop very long or you lose heading info).

On Fri, Aug 9, 2013 at 8:56 AM, Zach Leffke <zleffke at vt.edu> wrote:
> Hello again everyone,
>                 I have received a couple of emails off list asking about
> details of how I'm planning to use the magnetometer for the azimuth
> feedback.  Instead of individually replying I'm placing my response here so
> everyone gets the same info (skip to the end of this long winded email for
> the details).  Before I get into that I want to address Bob's (WB4APR) note
> about the constant rate of these motors.  Basically, I like the idea because
> of its simplicity.  That's something I hadn't considered, but will look
> into.  My logic there is run my system through a few dozen trials to really
> lock down the turn rate baseline.  Then all I have to do is keep track of
> the amount of time and in which direction I'm telling the motors to turn,
> and voila I should know the azimuth/elevation.  As Bob mentioned with the
> beamwidth of these antennas that should get me close enough.
> If I were only planning to use the system for tracking LEO birds at VHF/UHF
> frequencies, I would probably just go with that.  However, I was slightly
> understating my goals when I initially brought up this issue.  In addition
> to LEO birds, I plan to also use this system for UAV tracking and High
> Altitude Balloon Tracking.  I am an EE graduate student at Virginia Tech and
> each semester I help out with an Undergraduate Lab with Aerospace and EE
> students that design and fly a high altitude balloon (generally with APRS
> tracking and some kind of camera/sensor suite payload).  One of the
> professor in charge's (Dr. Dennis Sweeney, WA4LPR, some folks might know him
> for his filter design work among many other things) goals is to up the data
> rate in order to attempt a high speed real time data link.  So in my own
> time I want to use this tracking pedestal for LEO tracking (and for my
> Master's Thesis prototype), but for this HAB project, the elevation can get
> quite high depending on the geometry of our chase scenario, and beamwidths
> can get quite small depending on the frequency we finally choose.  In the
> end it might not matter much, but when we get to that point I want to at
> least have the OPTION for tight control over the pointing.  This whole thing
> will likely get vehicle mounted in the bed of my pickup, and though I
> haven't completely thought out the details yet, My gut is telling me the use
> of a magnetometer for azimuth feedback could have a slight advantage because
> as the vehicle is moving the system could automatically compensate for the
> turning of the vehicle, where as a potentiometer type solution would not.
> With position updates from the HAB and a local GPS in the chase vehicle,
> calculation of Az/El to the target is pretty straight forward.  Combine that
> with magnetometer feedback about the antennas' azimuth and elevation
> feedback from an accelerometer (which should also be somewhat "self
> compensating", say if the vehicle were moving up or down a hill), and I
> should be able to come up with a fairly sexy tracking pedestal that
> automatically compensates for the vehicle's motion to keep the antennas "on
> target."
> All that being said, here is my plan for the magnetometer implementation:
> I don't really have the details locked down yet, but I can give the overall
> plans.  First I plan to use the HMC5883L from sparkfun.  This device is
> $14.95 + Shipping and handling.  It is a 3 axis magnetometer.  When dealing
> with magnetometers it is important to understand whether or not your device
> is tilt compensated.  This particular device is NOT tilt compensated.  What
> that means is that it must be kept oriented parallel with the Earth (within
> say maybe + or - ten degrees or so).  Assuming it remains oriented properly,
> the device will output (over I2C) the magnetic field strength in the X, Y,
> and Z directions.  The Z direction is essentially pointed straight up at
> zenith (and should remain near zero since it is orthogonal to the earth's
> magnetic field).  Also assuming no other strong magnetic fields exist near
> the sensor, it will detect Earth's magnetic field (the sensor just returns
> the highest magnetic field for a particular direction, so if other magnetic
> fields exist near the sensor, it can ruin the readings).  In order to get
> the bearing, or azimuth, information one simply has to use the atan2
> function with the Y and X axes, here is the code snippet from some examples
> online:  heading = atan2(scaled.YAxis, scaled.XAxis).  The rest is just
> calibration and making sure the heading makes sense (i.e. if the heading is
> a negative number add 360 to get a 0 - 360 output).  One issue too is that
> most satellite tracking programs return the TRUE heading or azimuth
> (reference to the North Pole) to the spacecraft.  This magnetometer with
> produce a MAGNETIC azimuth (referenced to the magnetic pole).  Thus one must
> compensate for the magnetic declination of the area of operation.  The
> declination can be found online for each person's individual location and
> can be hard coded into the code to adjust the magnetic readout to a true
> readout.
> Since this device is not tilt compensated I plan to mount it on my azimuth
> base on a short boom that sticks straight out in front of the antenna
> assembly.  It will be center mounted on a non-conductive extension (maybe
> two or three feet in front of the azimuth assembly).  My logic here is to
> try to get the sensor as far away from the AC induction motors and other
> metals as I can.  The AC motors will produce a strong magnetic field and if
> the sensor is near them, it will distort the readings.
> Thanks again to everyone that provided feedback for me.  Below are a few
> links that should help out those who want more details.  Keep in mind too
> that there is a plethora of information out there on these devices
> (tutorials and example code) and with the links below and some skillful
> Googling, anyone can found out everything they need to make one of these
> work.
> Sparkfun Link:  https://www.sparkfun.com/products/10530
> Bildr Tutorial:  http://bildr.org/2012/02/hmc5883l_arduino/
> K3NG's Arduino rotator control project:
> http://blog.radioartisan.com/yaesu-rotator-computer-serial-interface/
> (Thanks to Lizeth, N3YKF for pointing me in this direction)
> ---------------
> -Zach, KJ4QLP
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