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Alt/Az Antenna Rotator from Pan/Tilt Camera Mount

Turn a COTS, Heavy duty pan/tilt camera mount into a functional satellite tracking Alt/Az mount for use with Hamlib

jrsphotoJrsphoto
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I've been looking at the variety of DIY antenna rotator projects lately, with in interest in trying my hand at satellite tracking. Most, use 3D printed parts to a varying degree, some almost exclusively. The one I was most interested in was Satnogs, which took first place here on HackaDay in 2014. I love this design but lets be honest, its a lot of work. Lots of extrusion to cut, and cut accurately, lots of 3d printed gears, and lots of work just to get to the point where I could start to build it. There had to be another solution.

I started thinking about those heavy duty, outdoor pan/tilt camera mounts. Those big ones you see on poles and on buildings. This all lead to a lengthy google session, trying to get some insight as to how these are made and what kinds of loads they can carry, and what models might work for me. The model I had zeroed in on could handle around 10kg! More than enough for my needs. But, will it blend?

The Plan

The idea is to take this once, pan/tilt camera mount, and turn it into a alt/az mount for tracking satellites, or just about any other celestial body for that matter.  Sounds simple enough, and I think it might be fairly straight forward to accomplish

Phase 1 - Out with the old

So for my needs, The only thing I need is the drive assembly.   I can remove the motors, electronics PCB, the wiring, and connectors on the bottom as I'll be replacing them with my own.  You might think you can keep the BNC connector but is a 75 ohm connector and for my needs, that will need to be a 50 ohm connector.  This leaves is with a pretty blank slate to build from.

Phase 2 - In with the new

Seeing that we remove the motors, were going to need something to replace them.  I'll replace the Hurst AC 30:1 gear motors with the Hurst 4004-010 DC stepper motors with 30:1 gear-head.  It has the added benefit of being exactly the same mounting pattern and spacing so it saves me from having to design some type of mounting adapter plate.

I will also need electronics to drive the motors and monitor the limit switches, and perhaps some shaft encoding, if I can figure that out.  Lots of ways to do this.  The Satnogs rotator controller would work, and I may end up going that way, but my first plan is to test this with a telescope motion controller, the Onstep miniPCB2.  I've been involved with that project for a long time, and the idea of using modified version of Meade's LX200 command set to control the mount brings with it a lot of options.  But time will tell

Phase 3

I'll call this phase software but I'm not sure how much will need to be written.  Most of this phase will be testing compatibility between Onstep and things like Hamlib's rotctld.  I'm not here yet so I'm open to suggestions.

Data_sheet_enUS_2062615435.pdf

Data sheet for the LTC9420 mount.

Adobe Portable Document Format - 326.41 kB - 02/24/2021 at 23:47

Preview

  • A little more progress

    Jrsphoto • 02/26/2021 at 07:18 • 0 comments

    I spent some time tonight getting the Onstep controller mounted and the motors wired.  I have not connected the alt/az limit switches yet, though they are wired up.  

    I’ve done some minimal testing with hamlib’s rotctld, rotctl, and gpredict, and things look good.  Some issues with slewing speeds given the 0.25 deg/ step of these motors, but this was expected.  How I’ll deal with this I’m not exactly sure.   I’ll post a video link tomorrow. 

  • New Motors

    Jrsphoto • 02/25/2021 at 19:13 • 0 comments

    The new motors came in today:

    Old motor on the left.  The new motor is also slightly thinner with the old motor being 54mm deep, and the new one just shy of 46mm.


    And now to mount them:

    I'll work on mounting the Onstep board and connecting the motors next..

  • Motion Control

    Jrsphoto • 02/25/2021 at 03:28 • 0 comments

    I mentioned that I plan on using the Onstep telescope motion control system to handle motion control for this rotator

    Howard, the creator of Onstep, has quite a few models available, this one is the MiniPCB v2. and the processor can either be a Teensy 3.2 or 4.  I've got a Teensy 4 in there as that's what I have.   He also has a lot of options for stepper motor drivers.  Just change the code to suit.

    Limit switches are connected to the 3.5mm mini stereo jack on the bottom right.

    What does this get me?  A few things.  One is, its use of an advanced version of Mead's LX200 protocol.  That means  theory at least, I could track satellites with something like kStars& ekos.  Lots of power in those two systems.


  • Gear ratios and tracking

    Jrsphoto • 02/25/2021 at 01:27 • 0 comments

    Tracking objects in space requires very specific tracking-rates, depending on the object your interested in tracking.  lunar, solar, and the Sidereal rate for stars.  For satellites, things get interesting fast, and each one is different, and requires (for the most part) knowing its orbital elements for a specific point in time.  The call this data, a Two Line Element Set or TLE.

    For satellites, things move pretty quick and I need to have some idea of what tracking or slew rates this mount will be capable of, or all of this is for nothing.


    The stepper motors I'm using are 1440 steps/rev, and have a 30:1 gear reduction.  So this makes things interesting if I would like to have a 5° / second tracking rate:

    I have my doubts that these motors will handle 1250 RPM!


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Jrsphoto wrote 02/25/2021 at 18:40 • point

Hi James,  Yep, I've seen that.  There are several derivatives of that design now.  Its a great design and would work amazingly well as a camera platform,  but not much in the way of  load capability, even if one took the time (and expense)  and printed with PETG or nylon.  I also really like the idea of repurposing otherwise discarded things, into new and functional rolls.

  Are you sure? yes | no

James Finch wrote 02/25/2021 at 08:56 • point

Guessing you've seen already... though figured I'd note this Thingiverse DLSR Gimbal thing if you or anyone finds useful in case they can't find an old PT mount.  https://www.thingiverse.com/thing:4316563   I printed out the gears and the rest only in a few layers so to use as templates for cutting out with another plastic (most likely) material.  This can be remixed/designed to suite the user requirements. 

  Are you sure? yes | no

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