[amsat-bb] Re: Non-mechanical Azimuth/Elevation Feedback Advice
normanlizeth at gmail.com
Wed Aug 7 08:14:15 PDT 2013
Check out K3NG's Arduino rotator control project. It incorporates the
proper sensors and should, with the proper interface hardware, drive your
I'm in the process of a build myself.
On Wed, Aug 7, 2013 at 9:58 AM, Zach Leffke <zleffke at vt.edu> wrote:
> Hello Everyone,
> I recently came into possession of a Pelco PT170-24P
> tracking pan tilt pedestal designed to support large security cameras. My
> intent with this new acquisition is to repurpose it as a "low cost" (got it
> on Ebay for ~$75 + S&H) alternative for an antenna tracking pedestal for
> amateur satellites. It uses 24VAC induction motors to move the azimuth and
> elevation assemblies, pretty much just like the G5500s that I'm sure so
> of us are familiar with. It definitely cannot support the same amount of
> weight as the G5500, but I'm looking to construct a small, portable
> satellite ground station node and this thing is plenty beefy enough to
> handle a couple of Arrow style antennas. Here is the problem, it provides
> absolutely no feedback.
> My question to the group is does anyone know of a non-mechanical method for
> getting relatively accurate feedback for azimuth and elevation. I'm
> for an all electronic means that I can mount somewhere outside of the
> pedestal assembly (like perhaps on the cross-boom) that will be able to
> provide measurement of the az/el (or pan/tilt, or yaw/pitch, whatever you
> want to call it) position. I'm using an arduino microcontroller for the
> tracking controller. Originally I intended to find a way to mount
> potentiometers in inside the unit and simply use the ADCs on the arduino to
> read the position feedback voltage from the pots, however, there is barely
> enough space to mount an elevation feedback pot inside the unit, and there
> is virtually no space for an azimuth feedback pot. Hence I'm looking for a
> non-mechanical method.
> My first thoughts for the elevation feedback was to use the old
> potentiometer plus nice heavy weight method mounted out on the boom. This
> idea doesn't appeal to me very much as other factors can now affect the
> position feedback (such as high winds). I then thought of something along
> the lines of an accelerometer. I also tossed around the idea of a 2-axis
> gyro for both Az/El. My issue is I have limited experience working with
> these types of sensors, and was hoping to get advice from everyone in this
> group. I know for example that the gyro will provide rate of motion around
> an axis and thus I have to integrate over time to get the actual position.
> This becomes cumbersome because now I have to keep track of time in the
> Arduino while executing movement commands (certainly do-able, just more
> complicated than reading an ADC voltage). Additionally, I believe these
> devices suffer from drift and require frequent calibration (although there
> may be a scheme of starting from a known position, say at one of the limit
> switch contact points, for each pass that might work). I also toyed with
> the idea of an electric compass for azimuth feedback, but I'm worried about
> distortion of the magnetic field near the pedestal due to the AC induction
> motors or when the antennas are radiating. In theory the motors are housed
> inside the metal pedestal enclosure and thus are shielded from the outside
> world, but I can just see it now, nice steady feedback when the pedestal is
> stopped and as soon as I execute a motion command the azimuth feedback
> starts dancing all over the place. Since the motion stop command is based
> on achieving the target position, system instability is sure to occur.
> if I solve the AC motor EMI problem, I still worry that when transmitting
> the fields could potentially be distorted if near the antenna (remember my
> goal is a compact design) and taint the position feedback.
> Any ideas from the group would be greatly appreciated. I'm looking for a
> "sparkfun" type solution here and if anyone has experience working with
> accelerometers, gyros, electric compasses, etc. I would love your advice on
> which might be the way to go for the position feedback. If you think I've
> hit on a good idea above and should go with it please let me know. Again
> I'm using an Arduino, so analog voltage feedback, I2C, SPI, and UART serial
> are all on the table for communicating with the sensors to get the feedback
> Thanks in advance!
> Zach, KJ4QLP
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