Looking for the right motor/whatever to concroll a rotating section

[Youkai]

I'm back to working on the arm cannon build again. This time version 2. I'd like to try and have a rotating section. See this youtube video for example:

I need to see if there's a decent way to control the rotation. Here are some notes about the parameters of the build:

• Most of the project is 3D printed
• There is an internal core which is the static part where the motor/whatever is mounted.
• The rotating section will be attached to the core via some bearings.
• The rotating section will be symmetrical  so there shouldn't be any weight imbalance issues.
• The rotating section will be light (maybe a pound or two)
• Other than inertia the section will rotate freely. There will be some paths that a lever arm will follow to cause some movement in the rotating section but that should produce minimal resistance.
• I can print some gears or whatever to interface with the motor if ready/made items are not available.
• The rotating section needs to either be able to rotate 360 degrees and stop when a lever switch is triggered OR rotate at least 90 degrees and then rotate back. In the second scenario I could run it "forward" for some set amount of time then run it backwards until the switch is tripped if that makes things easier, in this case the exact degrees of rotation is not super critical as long as I can play with the amount of time to get the rotation to hit within 10% of my desired rotation. I don't have a strong preference over which of these two options are used. I would pick the one that's simpler/easier to implement.
• I need the rotating section to hold it's position when it's not actively rotating. This can be done using the motor if it's capable of that or it can be some other external break.

Does anybody have some good ideas for how I could implement this? If I did the full rotation I assume a small DC motor with some kind of gear would work; then I would need to figure out a breaking mechanism for when the motor is off to keep it from rotating freely. If I did the forward and back possibly a stepper motor or continuous rotation servo?

Any advice/suggestions would be appreciated. Thank you!

[jwet]

I saw you got no responses so I'll try, maybe someone will be motivated by my dumb answer.

It would seem pretty clean to have a sun type gear on the inner diameter of the rotating part and a pinion gear on a stepper motor.  If you don't have room for the motor inside, you could make a right hand drive by putting the gear on the "back" of the rotating sleeve and have a pinion engage with it.  Pancake motors would make sense and could be hidden in the surface bumps.  Stepper have detent torque to keep things from rotating.

Good luck,

[Youkai]

Ok thanks for the help. I haven't worked with Stepper motors before so I don't know how complicated they are. I'll need a fairly small motor to fit in the space.

If it helps: the inner core where the motor will be mounted is a 4-inch diameter tube. I can use the whole of the interior of the tube as free space. I'll print a track into the outside of the tube for the bearings to slot into.

I'll do some research about using stepper motors with Arduino. I think that might be a good option if it's easy enough to use and I can find a motor the size that works for me. I'll probably want a switch somewhere so I can detect "home" but using a servo should make it easy to rotate some desired amount of degrees then return to the start.

I'd still appreciate any other suggestions if there are other options to consider. Thank you.