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Thread: Finally gonna do something about dome automation

  1. #11
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    Default Re: Finally gonna do something about dome automation



    No, I will be doing that in a totally separate program.

    Several years ago, I ran into a side-of-pier incompatibility issue. No matter what software was doing the calculation, they couldn't get the dome azimuth right in the northern hemisphere of the sky. The issue is that the ASCOM definition of the side-of-pier parameter is ambiguous, and lends itself to misinterpretation, especially in the north. With any combination of software, and I tried several, odds are 50-50 that at least one program in the setup will do it wrong. I'd end up with the scope on the east side of the pier but the slot aligned for thee west side, or vice versa.

    I wrote my own dome sync program, worked out all the spherical trigonometry myself, and tested it with several software packages. My dome sync program got it right every time, by totally ignoing the ASCOM side-of-pier parameter and deriving the information another way.

    So, I will be using that dome sync program once I get everything operational. The sync program will, every ten seconds, ask the mount where it is pointing (dec and hour angle). It calculates, based on the pier geometry relative to the dome, what azimuth the dome needs to be at to align the slot with the scope. Then it will simply tell my ASCOM dome driver to go to that azimuth.

    I have found that approach, running my own dome sync software, to work better than taking a chance on someone else's software, even big names like SGP.
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    DSO AP: Orion 200mm f/4 Newtonian Astrograph; ATIK 383L+; EFW2 filter wheel; Astrodon Ha,LRGB filters; KWIQ/QHY5 guide scope; Planetary AP: Celestron C-11; ZWO ASI120MC; Portable: Celestron C-8 on HEQ5 pro; C-90 on wedge; 20x80 binos; Etc: Canon 350D; Various EPs, etc. Obs: 8' Exploradome; iOptron CEM60 (pier);
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    Default Re: Finally gonna do something about dome automation

    Trying not to butt-in on your design... The Pololu A4988 stepper driver will drive max 1.0 Amp per coil (2.0 Amp with lots of cooling). I know you said it was a geared stepper
    but you might want to revisit that particular spec in your design.

    All the best,
    Jack

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    Default Re: Finally gonna do something about dome automation

    I am curious, are you going to figure out the position of the dome by counting Stepper motor steps? Is that accurate enough given missed steps and gear slippage? Maybe better to add encoders to the drive shafts for accuracy.

    What is the max Amps on your Stepper? The DRV8825 driver can do 1.5A (2.2A cooled) per coil in case you need more than 1A.
    AR

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    Default Re: Finally gonna do something about dome automation

    I don't have the torque vs. amps numbers handy, but when I checked out the specs, it appeared that the A4988 would do the job, with cooling. I am programming a gentle 1-second acceleration to 1 RPM in order to keep the torque within the calculated limit, so I shouldn't lose too many steps. With the motor gearing and the dome drive gearing, one step will be about 19 arcseconds of dome rotation. I can afford to lose a fair number of steps per movement and still have the accuracy be significantly better than what I was getting with the DC motor and a rotation counter.
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    DSO AP: Orion 200mm f/4 Newtonian Astrograph; ATIK 383L+; EFW2 filter wheel; Astrodon Ha,LRGB filters; KWIQ/QHY5 guide scope; Planetary AP: Celestron C-11; ZWO ASI120MC; Portable: Celestron C-8 on HEQ5 pro; C-90 on wedge; 20x80 binos; Etc: Canon 350D; Various EPs, etc. Obs: 8' Exploradome; iOptron CEM60 (pier);
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    Default Re: Finally gonna do something about dome automation

    For my RA-DEC geared motors as well as a focuser stepper I have used the Pololu Item #2134 (DRV8834) which has a quiescent drive current of 1.5 Amps per coil. And yes,
    I fully admit that is over kill but I liked the price, footprint and most of all the 1/32 step resolution.

    Now, with all that gearing in your power train that A4988 may well do the job. The A4988 also can use a larger motor voltage than my DRV8834 (35V vs 10.8V), I'm guessing
    you are using a big 12v power supply which would obviate my DRV8834.

    The stiction force of your drive train on that first step is going to be your biggest hurdle. After it gets moving if all your calculations are good you should be as well.

    All the best,
    Jack
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    Default Re: Finally gonna do something about dome automation

    Quote Originally Posted by smalluseful View Post
    For my RA-DEC geared motors as well as a focuser stepper I have used the Pololu Item #2134 (DRV8834) which has a quiescent drive current of 1.5 Amps per coil.
    That should be "(DRV8834) which has a max uncooled drive current of 1.5 Amps per coil"

  9. #17
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    Default Re: Finally gonna do something about dome automation

    Actual wiring and physical testing is going to have to wait for warmer weather. But in the meantime, I have been working on the software.

    I have the Arduino programmed to my satisfaction, as much as can be done without it being hooked up. Right now, it is just flashing the LED instead of turning the motor, but it is doing everything properly.

    I now have a program running on the PC to talk to the Arduino and tell it what to do. This allows me to determine appropriate sequences of events. I am happy with this part now.

    The next step will be to integrate the functions from this test program into the ASCOM driver that I wrote for the old azimuth subsystem. It's been a couple of years since I looked under the hood of that package, so it will take a while to do that.

    It's all a bit like wading through molasses: it's quite a slog, which is why I put the whole project off for a couple of years. However, this is the right way to go. It will be worth it when I am able to program an "all-nighter" in SQL and go to bed, waking up to a disk full of new subs.
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    DSO AP: Orion 200mm f/4 Newtonian Astrograph; ATIK 383L+; EFW2 filter wheel; Astrodon Ha,LRGB filters; KWIQ/QHY5 guide scope; Planetary AP: Celestron C-11; ZWO ASI120MC; Portable: Celestron C-8 on HEQ5 pro; C-90 on wedge; 20x80 binos; Etc: Canon 350D; Various EPs, etc. Obs: 8' Exploradome; iOptron CEM60 (pier);
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