[amsat-bb] Re: Non-mechanical Azimuth/Elevation Feedback Advice

[Ing. Pavel Milanes Costa]

Yes, me to...

I do have a old Japanese tracking camera mounting also with 24 AC motors 
here...  360 degrees azimuth with stop switch on 0 and 360 degrees and 
about 0 to 120 degrees of elevation (0 ~ horizon, 90 ~cenit)

Pretty good construction, a ham here in Havana have one the uses to move 
2 phased yaguis of 5 elements for 2m... the brake system is primitive 
but works fine...

Pointing is by eye and as in your case no means of feedback... recently 
after completing some TODO-IN-THE-FUTURE projects with Jal/JalLib (Just 
Another Language and libs for PIC devices) and pics I get hands on this 
project

A couple o months or so (maybe more) there was a thread on the list 
about this...

I find the arduino project from K3NG, but no arduino here, only 
Microchip PIC (PIC18F4520) so I have to re-invent the wheel...

I was thinking on gray encoding the position with 7 bits, but this is A 
LOT of cables to run..

Then I see on the specs of the arduino a feedback mechanism of pulse 
counting...  and find on the internet a project from a polish? ham that 
uses this mechanism wit a ATMEGA MCU but with a trick... (I have the 
link on my work place... link will be tomorrow)

I explain, he uses several magnets glued on the edge of an intermediate 
gear in the reduction mechanism... then get a magnetic switch (or a hal 
sensing device if you like) close to the edge where the magnets are...

This mechanism do about 500+ pulses for each 360 degrees... pretty neat 
an simple... with only 2 cables...

The only thing is you have to reset the mechanical position of the 
antennas at each power-on to reset the internal count in the 
pic/arduino/atmega MCU...

I'm on the ideas-on-the-boad part of this project for making it 450 
degrees instead of only 360 and how to deal in software with the 0 to 90 
degrees restriction on the elevation part...

Just another idea to the brain storm...

73 de CO7WT.

PS: control will be serial emulating a  RS232B protocol from Yaesu...

El 07/08/13 09:58, Zach Leffke escribió:
> 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 many
> 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 looking
> for an all electronic means that I can mount somewhere outside of the actual
> 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.  Even
> 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
> info.
>
>
>
> Thanks in advance!
>
>
>
> Sincerely,
>
> Zach, KJ4QLP
>
>
>
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