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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