Finally gonna do something about dome automation

[KathyNS]

I sent off for a high-torque stepper motor, an Arduino, and a few other bits to replace the DC motor that drives my dome. They arrived in today's mail. It will take a bit of work to put it all together and write the driver for it, but hopefully soon I will be able to send the dome to a specified azimuth reliably.

The motor is smaller than I expected, but I did the math for the torque requirements and it should be able to handle the dome just fine. It has a big planetary gearbox on the end of the motor. As long as I ramp up the speed to avoid sudden starts and stops, I will be well within the torque limits. I calculate it will be about 49,000 steps for a full revolution, so I should be able to get 26 arcsecond pointing accuracy if I needed to. I'll happily settle for the nearest degree.

It comes not a moment too soon. Last night's imaging session was just too damned cold!

[Hillrat]

Cool! Hope you've taken into account the enormous snowbanks that manage to sit on seemingly impossible places down your way!!!

Was just out getting set up for tonight! If there was any breeze I'd say to heck with it. -25C, calm and clear. Plenty of humidity too!

[ghswen]

This might help with your dome driver:

https://github.com/ejholmes/Arduino-...Dome/Driver.cs

and another:

https://github.com/mahanmawadj/arduino-dome-driver

[CamelHat]

Good move!

Be careful about using the Arduino standard stepper library. I used it, and it reduced the torque by quite a bit compared to a simple driver written according to the stepper motor's specification. This was a while ago so I don't know if it is still around. Just in case.

I recommend defining a time line to step against, not to wait a delta and then another delta because those errors accumulate.

You can use C++ but I never found the need for that, the C-like .ino language works fine and is easy to use.

I presume you got an Arduino power shield to drive it because the amps provided by a 5 V USB port won't be enough.


Good luck!

[KathyNS]

Thanks, Gordon, I will check out those resources.

Henk, that little "Stepper Motor Driver Carrier" board in the envelope will handle the Arduino-to-motor interface. It takes a logic input plus DC power, and drives this motor directly.

[KathyNS]

Well, I am learning to program an Arduino. I thought I had seen the last of the C language years ago. No such luck: the Arduino uses a C-like language. I'm trying to remember its syntax peculiarities. Getting there. I think I have an idea how I want to structure the program.

[CamelHat]

Well, I am learning to program an Arduino. I thought I had seen the last of the C language years ago. No such luck: the Arduino uses a C-like language. I'm trying to remember its syntax peculiarities. Getting there. I think I have an idea how I want to structure the program.

Check out the examples, they are under one of the menus. You can browse the source code. That's the easiest way to get back on track.

[KathyNS]

Thanks, yes, that's what I have been doing.

I heard that the C developers had a bet going for how ugly a language they could develop and still have people buy it.

[KathyNS]

Progress report. I am nearing completion on the programming. It's a lot more asynchronous than anything I've done in years, and a hard slog to put it together, but it is getting there.

I was able to find a nice function for computing step times that will give a linear acceleration. Acceleration is easy to calculate when you are working with speeds, but much harder and not at all intuitive when you are working with step times. To get a linear acceleration, the step times have to increase logarithmically. The function works very well, according to the spreadsheet I set up, and I was able to code it into the Arduino program.

I will be accelerating the dome to its full 1 RPM speed over one second. I used a spring scale to measure the force required to rotate the dome, and found that 5 pounds would produce a steady rotation speed. 10 pounds produced quite a brisk acceleration, more than I need. The one second acceleration time I coded should take about 6 pounds, well within the torque range of this motor.

All my stop functions use a 1-second deceleration, including the emergency stop. Limiting the acceleration and deceleration rates is important, not only to avoid overstressing the mechanical components, but also to avoid missing steps. If the motion of the dome does not match the number of step pulses sent to the motor, because I tried to accelerate too fast, the software will lose track of the azimuth. Which is the problem I had with the old DC motor drive, and the reason I am doing this stepper motor version. So being able to program the acceleration and deceleration is critical to the system's operation.

So far, I have been working just with software and the Arduino. Hardware assembly will have to wait until warmer weather, when I can get out to the dome and do some wiring. I am optimistic that I will have a working drive system before too long.

Then, I'll have to modify the ASCOM driver to work with the Arduino instead of running the DC motor for dome azimuth. I will keep the shutter subsystem already in place, since it "ain't broke".

[CamelHat]

Thanks for the update, glad it is starting to work. Yes for stepper systems you want to avoid missing steps because that's the only thing you can go by.

Just curious, there has to be some logic that determines where to move the dome opening in function of the object coordinates. Is that piece of logic done by the Arduino or is it part of the ASCOM driver?