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

[Ing. Pavel Milanes Costa]

This is the link to the system I described in the earlier email

http://www.cbk.waw.pl/~mstolars/projekty/rotor/index_en.html

73

El 07/08/13 19:01, Ing. Pavel Milanes Costa escribió:
> 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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