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Rotary Delta Robot Design Considerations

Posted by MaxMKA 
Rotary Delta Robot Design Considerations
September 12, 2017 06:34AM
I am working on designing a delta robot. Not the regular Rockstock-style linear delta, but a rotary delta.
It will be used for picking up small electronic components, so it doesn't need the accuracy of a regular 3d printer. Around +-1mm would be sufficient.
A very similar project is the firepick delta: [delta.firepick.org]
The robot should have a workspace diameter of at least 40 cm. This would mean a 50% increase in size compared to the Firepick.
I already built a smaller prototype using hobby servos, so I don't see the software as that much of a problem: [www.youtube.com]
Hobby servos are too weak for a robot this size, so I am thinking about what motors I could use.
One option are regular nema 17 or 23 stepper motors combined with wheels and timing belts for reduced speed and higher precision.
Geared stepper motors would reduce the part count but I am not sure about backlash. Often it's around 1° for regular gears and 0.25° for precision gears, which is still a lot compared to the belt drive.
Do you have experience with this kind of robot and know which kind of motor is best in this case?
Re: Rotary Delta Robot Design Considerations
September 12, 2017 06:59AM
Looks like a perfect use case for worm gear stepper gearboxes like Flex3Drive or Zesty Nimble.
Linear actuators with lead screws might be another good option.
The arms are printed, so it would be easy to add a herringbone gear. Just like good old GregsWade extruders.

Re: Rotary Delta Robot Design Considerations
September 12, 2017 12:01PM
A worm gear sounds interesting, I haven't seen this on any delta robot so far.
I'm not so sure about gears in general. It would be difficult to get it completely backlash-free and there might be more wear on the printed gears than on a belt-driven system.
Re: Rotary Delta Robot Design Considerations
September 13, 2017 08:07AM
In a low speed application gravity will probably preload the arms, so backlash can be ignored. That being the case, pretty much any thing will work fairly well if you can get enough reduction. I'd be inclined towards belts because they perform well and usable pulleys are easy to print, whereas if you don't hit a gears dimensions bang-on their performance will be highly degraded.

Direct use of geared steppers might also be an option but you might have problems transferring torque from the motor shaft to a printed arm without stripping the hole.
Re: Rotary Delta Robot Design Considerations
September 14, 2017 10:01AM
On your prototype

What servos?
What controller?
What firmware?
What software?

confused smiley
Re: Rotary Delta Robot Design Considerations
September 14, 2017 11:01AM
The servos are probably the cheapest you can get from Hobbyking: [hobbyking.com]
Before Arduinos were widely available I had bought a Pololu Maestro 6-channel servo controller, which is now used for this project out of convenience.
The maestro receives commands from a Processing(.org) Script on my pc. It does not much more than take the xy-position if the mouse and send the calculated servo angles to the maestro.
The kinematics code is copy/pasted from the trossen robotics forum (Which is basically the code source for every delta project): [forums.trossenrobotics.com]
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