If I am doing the arithmetic correctly, I need an 8:1 reduction gesrbox to turn a 1 rpm motor down to 1 rev/day. It looks funny to write this as 1 rpd.
UPDATE: I think the answer is this motor Model: 16HM13-0404S
Nema 16 Bipolar 0.9deg 18Ncm(25.5oz.in) 0.4A 39x39x34mm 4 Wires. It is a 0 9 degree step angle. If I turn it through a 10:1 worm gear reduction, that is 5.4 arc minutes per step or a step every 1.1 seconds. This is not a continuous motion but close enough that I will likely not see it with the powers that I will be using. I suppose that I could look for something with a smaller step angle.
Anaheim Automation sells a combined microstepper controller and motor with .225 degree per step resolution. $111 is a little pricey compared to the cheap certainly Chinese ones but one step every .02 seconds. This should be so smooth as to be invisible.
UPDATE: Thanks for noticing that was a 10:1 reducer. This is a 40:1 reducer. A .225 degree step angle with that would give .005625 degree steps to RA axis. This is fine enough. I need a driver to control the stepping. Ideally, one with a fine vernier adjustment.
Your arithmetic is correct: 8 x 180 = 1,440. But the link is to an "Industrial Robot 10:1 Worm Wheel Bronze Worm Gear + Metal Worm Set", not 180:1. BTW, I note that the gear is bronze, but the worm is just "metal".
ReplyDeleteLooking through Amazon, I find worm gear sets in 10:1, 20:1, 30:1, 40:1, 50:1, 60:1, and 120:1. The difficulty is that to get to 1440:1, one needs some combination that equals 2 x 2 x 2 x 3 x 2 x 2 x 3 x 5. One must have 3 twice and 5 once, and there doesn't seem to be any set that has 3 but not 5.
Maybe... start with a 10 rpm motor, then run through 120:1 twice.
A big question will be how accurate is is the motor speed.
I would be careful to ensure that whatever solution you use allows you to adjust the motor speed in case the math doesn't perfectly match reality.
ReplyDeleteYou can find high reduction ratio (30-100 ) gearboxes for stepper motors pretty easily. They appear to fall into two main categories with pros and cons:
- Planetary gearboxes:
-- Cheaper (~$50 with motor)
-- More readily available
-- Reduction ratio is often not a nice round number (27:1, 99.05:1, etc)
-- Significantly higher backlash (usually measured in degrees)
- Harmonic drive (strain-wave) gearboxes
-- More expensive (~$110 without motor)
-- Reduction ratio is usually a tidy integer value (30:1, 50:1, 80:1, 100:1, etc)
-- Significantly lower backlash (usually measured in arc-minutes)
I would personally look at a 80:1 or 100:1 strain-wave gearbox on a NEMA 17 stepper motor.
The other question is how you are going to connect the drive to your polar axis. I would NOT trust the bearings on the gearbox to support the axis, so you will need some way of connecting it to the existing shaft. You probably also want some way of disconnecting it easily so that you can 'slip' the adjustment to quickly line up on an object.
I would probably look at a toothed belt before a gear drive in the interest of reduced backlash. It is also easier to adjust belt tension than to get gear mesh dead-on...
A gearbox like https://www.amazon.com/PGFUN-Harmonic-Reducer-Strain-Gearbox/dp/B0CY2YN74P combined with a standard 200-step motor would need to advance 1 full step every 4.32 seconds. Considering there are controllers that can handle microsteps (anywhere from 1/8 to 1/128), that should let you set up a "nudge-a-bit, wait-a-bit" cycle.
Merry Christmas!
Second the recommendation of a timing belt for final drive. I would also suggest looking for a microstepping controller for the motor (virtually all CNC stepper controllers do microstepping). The question is interfacing it to whatever you want to have run the whole thing. Dunno if you're going to try to work out some PC interface, or roll your own Arduino device.
DeleteI am trying to avoid rolling an Arduino. I even am considering a synchronous DC motor.
DeleteThat stepper motor is actually just a standard 200 step per revolution motor with a microstepping capable controller ([200 'physical' steps] * [8 'virtual' steps per physical step] = 1600).
ReplyDeleteThere are stepper motor controllers providing up to 128 micro-steps (128 'virtual' steps per physical step). If you combine that with a low-backlash gearbox (strain wave), you can arrive a a VERY good resolution ([200 steps per revolution] * [30:1 gear reduction] * [128 micro-steps per physical step] = 768,000 'steps' per revolution).
This works out to about 16 micro-steps every 1.8 seconds, allowing a pretty good resolution for trial-and-error tuning. Cost looks to be well under $200.
Gearbox: https://www.amazon.com/PGFUN-Harmonic-Reducer-Strain-Gearbox/dp/B0CRRTR157
Motor: https://www.amazon.com/STEPPERONLINE-Stepper-Bipolar-Connector-compatible/dp/B00PNEQKC0
Controller: https://www.amazon.com/STEPPERONLINE-Stepper-Controller-5-30VDC-Communication/dp/B0D6BFMLKH
Does that controller have to be programmed? The description suggests that youi can set all that from the panel.
Delete