6 ms·

Why spend so much effort to achieve an "exact" gear ratio? Having more zeros does not equal to being more "precise". Also, I wonder how resistant this mechanis

Why spend so much effort to achieve an "exact" gear ratio? Having more zeros does not equal to being more "precise". Also, I wonder how resistant this mechanism is to wear and fatigue.

skeeter2020 1y ago
>> Having more zeros does not equal to being more "precise" Isn't having more decimal places the exact definition of precision (vs accuracy)?
- munchler 1y ago
  The point is that those decimal places don't have to be zeros. 7.893 is just as precise as 8.001.
  - a3w 1y ago
    Yet for some reason unknown to me, people get annoyed when you tell them „let us meet at 13:37. that is no more accurate than let us meet at 14:00 hours”
    - VMG 1y ago
      14:00 is a Schelling point
    - Mawr 1y ago
      1. It's easier to remember. 2. Do you actually say fourteen-zero-zero or just fourteen? If the latter, that's your answer right there.
  - deleted 1y ago
    [deleted]
sabareesh 1y ago
That was confusing part of this video . May be there are some limitation on the tools he uses to tune
- dvt 1y ago
  I don't think the number of the gear ratio really matters, what matters is that you know what it actually is (since every IK calc depends on said ratio); 8:1 is probably arbitrary and/or looks nice & might simplify some stuff.
  - eichin 1y ago
    It might be a lot easier to check the ratio "by hand" (by counting rotations etc) if it's numerically simple. (IIRC in some earlier videos he noticed that the pulley size ratio wasn't producing the expected movement ratio, because they were built as an obvious 8:1 or 10:1 or something, and didn't match - which led to him figuring out the subtleties of the design - I can easily imagine wanting to preserve that aspect just for debugging, at that point, even if you now have correct math.)
  - mlhpdx 1y ago
    From a coding point of view it’s also nice if all the drives are exactly the same, so each isn’t compensated for separately. But yeah, just a nicety.
michaelt 1y ago
Well, it probably wasn't that much effort. When you're 3D printing you're going to end up printing everything 2-3 times anyway, so why not dial in the ratio while you're at it? And you can't really declare your design is "high precision" and present yourself as someone others should take transmission design advice from if you aimed for a gear ratio of 8 and achieved "somewhere around 7.9 to 8.2"
- LeifCarrotson 1y ago
  It probably doesn't matter so much whether it's 7.913 or 8.186, but it would be important to know the exact value for kinematics. One way to do that is to build an object very accurately, the other is to build inaccurately and then measure the result after the fact. It's also interesting because competing actuators with strain-wave, cycloidal, or planetary gearboxes will state exactly what the ratio is. The actual gear teeth may not be spaced out perfectly around the circumference, but the number of teeth is an integer with an infinite number of zeros.
  - tonyarkles 1y ago
    Yeah, I think one of the nice things about making it a "clean" number (either an integer or a rational with a small integer denominator) is that you can easily validate it without needing high-precision measurement equipment: put a mark on both gears (maybe even embedded in the 3D print), line up the marks, rotate the large gear 1 full rotation, and count the number of rotations the smaller gear makes. Check to see if the marks line up perfectly after those rotations.
    - nullc 1y ago
      His capstan reduction can't go all the way around even once.
      - hinkley 1y ago
        It could though. I don’t think the thought has occurred to him yet. He could make the stack twice as high and go around twice as far. If you moved the worm gear you could go farther, but I don’t know how he would do that with his drive. He could also go with narrower rope, and spread the load over more windings, which would give him more throw.
throwawayffffas 1y ago
I think it's about kinematics, the more precise your gears the better the model fits the real world. That's why pro crews don't use gears and ropes. At high impulses deformations and elasticity throw the kinematics off what's actually happening. Modeling the deformations and the elasticity is a computational no no. Instead what you see is the motors right on the joints. At least that was the case last time I had a look at robotics.
- PaulDavisThe1st 1y ago
  more than 30 years ago I was writing code for a "robotic" device that used motors, directly on the joints. the motors were so sloppy the company wasted a ton of money [0] having me write heuristics to tackle the errors they accumulated over several hours. one of his whole points is that by using dyneema (rope), there's almost no elasticity at all in the capstans. [0] relative to the cost of better motors
- michaelt 1y ago
  > That's why pro crews don't use gears and ropes. [...] Instead what you see is the motors right on the joints. The answer here, as with so many things in robotics, is: It Depends. UR10e robot arm that can lift a 4kg object with a reach of 1m and has sub-1mm repeatability? Strain wave gears in the base and shoulder joints, 100:1 ratio. MIT Mini Cheetah robot dog that can do backflips? 6:1 planetary gearbox. Shadow Hand with 20 degrees of freedom? Tendon driven, with the 20 motors in the forearm to keep the fingers slim. Little dinky Huggingface SO-101? Servo motors, integrating 1:345 gearing with a series of 6 tiny brass gears. Mid-price CNC milling machine, if you call that a robot? Really long ballscrews, driven by stepper motors.
  - Joel_Mckay 1y ago
    In general, for some platforms each gear mechanism adds backlash precision loss, lower energy efficiency, and might not be back driven. >Mid-price CNC milling machine A ball-screw is mostly decorative on small machines... =3
    - 1y ago
ErigmolCt 1y ago
Even small deviations can compound over time in a real-time system
jedimastert 1y ago
> Also, I wonder how resistant this mechanism is to wear and fatigue. He actually discussed this in an earlier video for his initial tests on the capstan drive. He ended up testing the rope he used for around 358 hours (two weeks) on continuous use in the drive itself with very low backlash https://www.aaedmusa.com/projects/capstandrive https://www.aaedmusa.com/projects/capstandrive https://youtube.com/watch?v=MwIBTbumd1Q&t=10m https://youtube.com/watch?v=MwIBTbumd1Q&t=10m
hinkley 1y ago
Because when a real engineer puts 2 and 2 in and gets 3.8 out, it vexes them and they want to at least know why they can’t get 4. He’s trying to make a machine that does what he told it to do, so that he understands what is actually happening.

Why spend so much effort to achieve an "exact" gear ratio? Having more zeros does not equal to being more "precise". Also, I wonder how resistant this mechanism is to wear and fatigue.

skeeter2020 1y ago

>> Having more zeros does not equal to being more "precise" Isn't having more decimal places the exact definition of precision (vs accuracy)?

munchler 1y ago
The point is that those decimal places don't have to be zeros. 7.893 is just as precise as 8.001.
- a3w 1y ago
  Yet for some reason unknown to me, people get annoyed when you tell them „let us meet at 13:37. that is no more accurate than let us meet at 14:00 hours”
  - VMG 1y ago
    14:00 is a Schelling point
  - Mawr 1y ago
    1. It's easier to remember. 2. Do you actually say fourteen-zero-zero or just fourteen? If the latter, that's your answer right there.
- deleted 1y ago
  [deleted]

sabareesh 1y ago

That was confusing part of this video . May be there are some limitation on the tools he uses to tune

dvt 1y ago
I don't think the number of the gear ratio really matters, what matters is that you know what it actually is (since every IK calc depends on said ratio); 8:1 is probably arbitrary and/or looks nice & might simplify some stuff.
- eichin 1y ago
  It might be a lot easier to check the ratio "by hand" (by counting rotations etc) if it's numerically simple. (IIRC in some earlier videos he noticed that the pulley size ratio wasn't producing the expected movement ratio, because they were built as an obvious 8:1 or 10:1 or something, and didn't match - which led to him figuring out the subtleties of the design - I can easily imagine wanting to preserve that aspect just for debugging, at that point, even if you now have correct math.)
- mlhpdx 1y ago
  From a coding point of view it’s also nice if all the drives are exactly the same, so each isn’t compensated for separately. But yeah, just a nicety.

michaelt 1y ago

Well, it probably wasn't that much effort. When you're 3D printing you're going to end up printing everything 2-3 times anyway, so why not dial in the ratio while you're at it? And you can't really declare your design is "high precision" and present yourself as someone others should take transmission design advice from if you aimed for a gear ratio of 8 and achieved "somewhere around 7.9 to 8.2"

LeifCarrotson 1y ago
It probably doesn't matter so much whether it's 7.913 or 8.186, but it would be important to know the exact value for kinematics. One way to do that is to build an object very accurately, the other is to build inaccurately and then measure the result after the fact. It's also interesting because competing actuators with strain-wave, cycloidal, or planetary gearboxes will state exactly what the ratio is. The actual gear teeth may not be spaced out perfectly around the circumference, but the number of teeth is an integer with an infinite number of zeros.
- tonyarkles 1y ago
  Yeah, I think one of the nice things about making it a "clean" number (either an integer or a rational with a small integer denominator) is that you can easily validate it without needing high-precision measurement equipment: put a mark on both gears (maybe even embedded in the 3D print), line up the marks, rotate the large gear 1 full rotation, and count the number of rotations the smaller gear makes. Check to see if the marks line up perfectly after those rotations.
  - nullc 1y ago
    His capstan reduction can't go all the way around even once.
    - hinkley 1y ago
      It could though. I don’t think the thought has occurred to him yet. He could make the stack twice as high and go around twice as far. If you moved the worm gear you could go farther, but I don’t know how he would do that with his drive. He could also go with narrower rope, and spread the load over more windings, which would give him more throw.

throwawayffffas 1y ago

I think it's about kinematics, the more precise your gears the better the model fits the real world. That's why pro crews don't use gears and ropes. At high impulses deformations and elasticity throw the kinematics off what's actually happening. Modeling the deformations and the elasticity is a computational no no. Instead what you see is the motors right on the joints. At least that was the case last time I had a look at robotics.

PaulDavisThe1st 1y ago
more than 30 years ago I was writing code for a "robotic" device that used motors, directly on the joints. the motors were so sloppy the company wasted a ton of money [0] having me write heuristics to tackle the errors they accumulated over several hours. one of his whole points is that by using dyneema (rope), there's almost no elasticity at all in the capstans. [0] relative to the cost of better motors
michaelt 1y ago
> That's why pro crews don't use gears and ropes. [...] Instead what you see is the motors right on the joints. The answer here, as with so many things in robotics, is: It Depends. UR10e robot arm that can lift a 4kg object with a reach of 1m and has sub-1mm repeatability? Strain wave gears in the base and shoulder joints, 100:1 ratio. MIT Mini Cheetah robot dog that can do backflips? 6:1 planetary gearbox. Shadow Hand with 20 degrees of freedom? Tendon driven, with the 20 motors in the forearm to keep the fingers slim. Little dinky Huggingface SO-101? Servo motors, integrating 1:345 gearing with a series of 6 tiny brass gears. Mid-price CNC milling machine, if you call that a robot? Really long ballscrews, driven by stepper motors.
- Joel_Mckay 1y ago
  In general, for some platforms each gear mechanism adds backlash precision loss, lower energy efficiency, and might not be back driven. >Mid-price CNC milling machine A ball-screw is mostly decorative on small machines... =3
  - 1y ago

ErigmolCt 1y ago

Even small deviations can compound over time in a real-time system

jedimastert 1y ago

> Also, I wonder how resistant this mechanism is to wear and fatigue. He actually discussed this in an earlier video for his initial tests on the capstan drive. He ended up testing the rope he used for around 358 hours (two weeks) on continuous use in the drive itself with very low backlash https://www.aaedmusa.com/projects/capstandrive https://www.aaedmusa.com/projects/capstandrive https://youtube.com/watch?v=MwIBTbumd1Q&t=10m https://youtube.com/watch?v=MwIBTbumd1Q&t=10m

hinkley 1y ago

Because when a real engineer puts 2 and 2 in and gets 3.8 out, it vexes them and they want to at least know why they can’t get 4. He’s trying to make a machine that does what he told it to do, so that he understands what is actually happening.