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

Greg D ko6th.greg at gmail.com
Wed Aug 7 20:11:18 PDT 2013

Yes, indeed the fine art of pulse counting works very well.  My rotor 
controller does the same with the "clicker" kind of rotors, where the 
contacts were intended to drive a solenoid ratchet switch mechanism for 
positioning.  Instead of the Arduino, I used a Basic Stamp (this is 
pre-Arduino heritage), but the pulses are 6-10 degrees apart.  500 per 
360 degrees would be too fast for this little squirrel.

Good luck with your projects.

Greg  KO6TH

Ing. Pavel Milanes Costa wrote:
> 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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