What is the gearing for the thriftybot swerve modules?
From page 6 of the assembly guide, this is what comes with the kit -
5.25:1 for your drive wheels
5.33 x planetary output for your azimuth
What does it mean by planetary output?
I’m sorry, I’m with the coding part of my team, so I don’t really know what this means. All I know is that I needed the gearing for the characterization of the robot.
Thrifty Swerve allows the team to choose which planetary gearbox & gear ratio that they want to use.
In this example, 1720 used the REV Ultraplanetary with a 4:1 & 3:1 stage, giving us a 12:1 overall reduction for the planetary gearbox output -
Take your input motor’s RPM (in this case with the REV NEO550 motor, 11,000 rpm) and divide it by 5.33 x your planetary output reduction. So in 1720’s case, we’d just take 11000 / (5.33 x 12) and get roughly 172 rpm for the module’s overall rotation speed free speed.
Ask your team which gear reduction they used for their planetary gearbox and then you should have all the information you need.
@Ryan_Dognaux Ultraplanetary gear reductions are not clean integer numbers like Versaplanetaries are. If someone is using the motor’s internal encoder for tracking steering it is important to use the actual ratio and not the rounded nominal value.
Today I Learned. That’s pretty important information, I guess I just assumed the sticker on the stage was the ratio.
So in my example you’d be at 10.5:1 and your overall module rotation RPM is more like 197.
So does this apply to versa planetary gears as well?
No. The VPs are at good, clean integers.
For those interested in why this is: for a planetary that has a fixed ring gear, input sun gear, and output carrier, the ratio is equal to ( sun + ring ) / sun
VPs all have a 72t ring gear.
3:1 has a 36t sun gear, and ( 36 + 72 ) / 36 = 3
4:1 --> 24t sun
5:1 --> 18t sun
7:1 --> 12t sun
9:1 --> 9t sun
10:1 --> 8t sun
UPs all have a 55t ring gear.
3:1 has a 29t sun, and ( 29 + 55 ) / 29 = 2.89655…
4:1 --> 21t sun
5:1 --> 13t sun
FWIW, that rotation speed seems a little low. Our SDS modules are at ~500rpm free speed for rotation. (6380 / 12.8)
You’re definitely on the really fast end at 500rpm.
Azimuth ratio and azimuth RPM is used because its easy to talk about and visualize in our heads. There is slowly developing consensus of appropriate gear ratio versus motor used.
I personally am more interested in seeing a graph of the module doing a step change of 90 degrees and the following info on the graph:
Setpoint vs. time
Position vs. time
Motor current vs. time
Time resolution set to the fastest as you can run it so the graph isn’t choppy.
This tells us a lot about how healthy and happy the whole azimuth mechanism is during a maximum step change. If you gear too fast the motor never gets happy and is pulling more current than needed to get the job done and it really isn’t as fast as one thought. If geared too slow then you’re just taking too much time.
I’m curious why you’d need anything that fast. You have to make at most a quarter-rotation to go in any direction, yeah?
I’d love to see a video of that in action. I think yours seems high, but that doesn’t mean that a Falcon or a NEO couldn’t handle it. Have you tried running it with a fully weighted robot as well?
The table we list is what we found works well with a fully weighted Thrifty Swerve. But that doesn’t mean teams shouldn’t experiment with RPM, especially this season when robots are likely to be lighter & smaller. Just remember that when robots weigh 120 lbs + bumpers & battery again that you may need to revisit this.
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