4 ms·

Had never seen a harmonic drive (mentioned in the article) in action, this video rendering showing how it functions is fascinating: https://www.youtube.com/watc

Had never seen a harmonic drive (mentioned in the article) in action, this video rendering showing how it functions is fascinating: https://www.youtube.com/watch?v=bzRh672peNk https://www.youtube.com/watch?v=bzRh672peNk

falcolas 9y ago
I'm curious how this has inherent "zero backlash", since the typical cause of backlash is imprecision in the tooth profiles - and it seems like you could run into this same issue with the harmonic drive. That said, very cool. Being able to blow mold such a gear train would be awesome for consumer robotics, especially considering how challenging it would be to create such a flexible gear with normal alloys.
- jpm_sd 9y ago
  The flex-spline is a spring that is constantly under compression, so there is no "rattle" between teeth. There is one downside to this inherent springiness of harmonic drives, which is some play at the end effector. I worked on a robot arm once that had 6DOF of harmonic drives and it was quite tricky to tune the controller for precise placement of tools.
  - falcolas 9y ago
    Even if it was under compression against the outside ring, if the teeth were smaller than those of the outside ring, a force could cause them to rattle. That said, if the difference in tooth counts is big enough, and the number of engaged teeth is high enough, you might get engagement from the teeth at the edges of the engaged surface, even if the inner teeth are a bit out of spec. This would cause some odd wear profiles in the teeth over time, though.
    - TFortunato 9y ago
      If I recall correctly (Not an ME - I'm more of an electrical / software side of the robot guy) -- between the flexibility of the spline gear giving a tight engagement, and the high preload / friction at the gear interface, you end up just backdriving the input if you try to jerk around the output shaft. They backlash may not be truly 0 (ask a knowledgable Mech E.), but they are definitely much better on that front than any other gearset with the same reduction I've ever got my hands on.
  - tlb 9y ago
    Yes, and also that it's single-stage to get 50x reduction. With conventional gears you'd need 3 stages, with the backlash of the first being multiplied by the gear ratio of the other two.
- tsomctl 9y ago
  Backlash is also intentionally introduced to allow proper lubrication. For example, on the ring and pinion of an automobile differential.
deleted 9y ago
[deleted]
Animats 9y ago
Harmonic drives were once popular for robot arms, because they provide huge gear reductions in a small space. But they're not back-drivable at all - overload one and you break gear teeth. As motor control has improved, the need for huge gear reductions has decreased, and harmonic drives have fallen out of favor in industrial robots.
- TFortunato 9y ago
  Perhaps for some larger robots, but I can say that Universal Robots UR5s and Rethink Robotics Sawyers use them, so not completely out of favor :-) Also the sets made by Harmonic Drive (the company) are definitely designed to be back-drivable. Your conment made me smile, because I remember that I heard the same thing about these gears. My first week at Rethink I asked my boss about it while looking on in slight horror at him moving around an unpowered robot while he was explaining the system to me -- I was very concerned we were about to break something!
  - Animats 9y ago
    Do you know how much of a gear reduction the Sawyer uses? If it's harmonic and back-drivable, it can't be that large. This is all much easier with three-phase brushless motors. You can hold the motor rotor at a specific angle under computer control.[1] With DC motors, you could just exert a torque, and it was hard to maintain tight position control, especially if the load changed. So with DC motors, robots needed more gearing down. The usual arrangement was a small, fast motor and a high-reduction geartrain. Adept Robotics was the first to go in the opposite direction with a direct-drive motor, a pancake-like thing almost a foot across. (Robotics is full of routine engineering headaches like that. Many have been solved in recent years. Motor control used to be a much larger headache, and good motor controllers used to be a lot more expensive.) [1] http://hackaday.com/2015/04/20/driving-a-brushless-dc-motor-sloooooooowly/ http://hackaday.com/2015/04/20/driving-a-brushless-dc-motor-...
    - TFortunato 9y ago
      Haha -- yes indeed about the engineering headaches! (Although as I'm sure you know...the more I learn about any field of engineering, the more I learn about just how-many trade-offs there are in building something!) As for gear ratio -- depends on which joint, but Harmonic Drive's standard gearboxes come in ranges of 50:1 to 320:1, and I'd say they were all within that range. As far as direct drive motors go, all I can say is that I was very aware of them, and very interested in looking at it. (I was on the embedded systems team doing motor control), but by the time I joined, the architecture had already been decided and robot's were being built. That said, there are certain other difficulties there, both with the control electronics for larger motors, and with the relative cost of the motors, especially since Rethink is going for the goal of reducing robot costs. I can't say they won't go that way some day. I think at the very least, it's worth exploring, but I'm not longer there so I don't have much say in that! Other fun stuff -- Even though we used a harmonic drive, it was coupled to what is basically a torsion spring between it and the output. This arrangement is called a series-elastic actuator, and it allows you to do some neat stuff with rapid torque sensing, both from the control and safety side of things. It's also a lot of fun for the control engineers trying to control something sitting at the end of a spring :-P
ramses0 9y ago
I had a tough time understanding the purpose of one of these gears as the operation of it is quite strange. https://www.youtube.com/watch?v=kcdj-b49TW8 https://www.youtube.com/watch?v=kcdj-b49TW8 ...this lego version made it a little more understandable. Roughly: it's a motor turning an ellipse, and the ellipse is pushing against a flexible "sock" made of metal with geared teeth. All this is contained _inside_ a fixed gear. As the motor turns, the ellipse turns, and causes the geared teeth to push and squeeze against the fixed container. This gearing motion is designed to take ~100rpm's down to ~1rpm (very high _reduction_ ratios)... as the outer container has ~100 teeth and the inner has ~99 ... every one lap around the large gear only advances the small inner gear a single notch. So... high motor speed is converted to low motor speed. The fixed containing gear never moves (it's fixed), and you rely on the flexibility of the inner metal flexible "sock" in order to turn another shaft which is giving the output power. In the lego model you see using red "nubs" for the fixed outer gear, yellow "windows" to catch the "nubs", and then the yellow "splines" simulate the flexibility of the metal / sock component. As the lego model turns you see where the motor driver is turning the ellipse. The basketball-picker-upper is effectively attached to the "sock" directly and that is the output work. A lot of the animations are really hard to understand (compared to physically seeing it working) because the gearing ratio is so low... hard to see the output movement over time compared to the lego on.
- jpm_sd 9y ago
  There's quite a nice animation on the wikipedia page: https://en.wikipedia.org/wiki/Harmonic_drive https://en.wikipedia.org/wiki/Harmonic_drive

falcolas 9y ago

I'm curious how this has inherent "zero backlash", since the typical cause of backlash is imprecision in the tooth profiles - and it seems like you could run into this same issue with the harmonic drive. That said, very cool. Being able to blow mold such a gear train would be awesome for consumer robotics, especially considering how challenging it would be to create such a flexible gear with normal alloys.

jpm_sd 9y ago
The flex-spline is a spring that is constantly under compression, so there is no "rattle" between teeth. There is one downside to this inherent springiness of harmonic drives, which is some play at the end effector. I worked on a robot arm once that had 6DOF of harmonic drives and it was quite tricky to tune the controller for precise placement of tools.
- falcolas 9y ago
  Even if it was under compression against the outside ring, if the teeth were smaller than those of the outside ring, a force could cause them to rattle. That said, if the difference in tooth counts is big enough, and the number of engaged teeth is high enough, you might get engagement from the teeth at the edges of the engaged surface, even if the inner teeth are a bit out of spec. This would cause some odd wear profiles in the teeth over time, though.
  - TFortunato 9y ago
    If I recall correctly (Not an ME - I'm more of an electrical / software side of the robot guy) -- between the flexibility of the spline gear giving a tight engagement, and the high preload / friction at the gear interface, you end up just backdriving the input if you try to jerk around the output shaft. They backlash may not be truly 0 (ask a knowledgable Mech E.), but they are definitely much better on that front than any other gearset with the same reduction I've ever got my hands on.
- tlb 9y ago
  Yes, and also that it's single-stage to get 50x reduction. With conventional gears you'd need 3 stages, with the backlash of the first being multiplied by the gear ratio of the other two.
tsomctl 9y ago
Backlash is also intentionally introduced to allow proper lubrication. For example, on the ring and pinion of an automobile differential.

deleted 9y ago

[deleted]

Animats 9y ago

Harmonic drives were once popular for robot arms, because they provide huge gear reductions in a small space. But they're not back-drivable at all - overload one and you break gear teeth. As motor control has improved, the need for huge gear reductions has decreased, and harmonic drives have fallen out of favor in industrial robots.

TFortunato 9y ago
Perhaps for some larger robots, but I can say that Universal Robots UR5s and Rethink Robotics Sawyers use them, so not completely out of favor :-) Also the sets made by Harmonic Drive (the company) are definitely designed to be back-drivable. Your conment made me smile, because I remember that I heard the same thing about these gears. My first week at Rethink I asked my boss about it while looking on in slight horror at him moving around an unpowered robot while he was explaining the system to me -- I was very concerned we were about to break something!
- Animats 9y ago
  Do you know how much of a gear reduction the Sawyer uses? If it's harmonic and back-drivable, it can't be that large. This is all much easier with three-phase brushless motors. You can hold the motor rotor at a specific angle under computer control.[1] With DC motors, you could just exert a torque, and it was hard to maintain tight position control, especially if the load changed. So with DC motors, robots needed more gearing down. The usual arrangement was a small, fast motor and a high-reduction geartrain. Adept Robotics was the first to go in the opposite direction with a direct-drive motor, a pancake-like thing almost a foot across. (Robotics is full of routine engineering headaches like that. Many have been solved in recent years. Motor control used to be a much larger headache, and good motor controllers used to be a lot more expensive.) [1] http://hackaday.com/2015/04/20/driving-a-brushless-dc-motor-sloooooooowly/ http://hackaday.com/2015/04/20/driving-a-brushless-dc-motor-...
  - TFortunato 9y ago
    Haha -- yes indeed about the engineering headaches! (Although as I'm sure you know...the more I learn about any field of engineering, the more I learn about just how-many trade-offs there are in building something!) As for gear ratio -- depends on which joint, but Harmonic Drive's standard gearboxes come in ranges of 50:1 to 320:1, and I'd say they were all within that range. As far as direct drive motors go, all I can say is that I was very aware of them, and very interested in looking at it. (I was on the embedded systems team doing motor control), but by the time I joined, the architecture had already been decided and robot's were being built. That said, there are certain other difficulties there, both with the control electronics for larger motors, and with the relative cost of the motors, especially since Rethink is going for the goal of reducing robot costs. I can't say they won't go that way some day. I think at the very least, it's worth exploring, but I'm not longer there so I don't have much say in that! Other fun stuff -- Even though we used a harmonic drive, it was coupled to what is basically a torsion spring between it and the output. This arrangement is called a series-elastic actuator, and it allows you to do some neat stuff with rapid torque sensing, both from the control and safety side of things. It's also a lot of fun for the control engineers trying to control something sitting at the end of a spring :-P

ramses0 9y ago

I had a tough time understanding the purpose of one of these gears as the operation of it is quite strange. https://www.youtube.com/watch?v=kcdj-b49TW8 https://www.youtube.com/watch?v=kcdj-b49TW8 ...this lego version made it a little more understandable. Roughly: it's a motor turning an ellipse, and the ellipse is pushing against a flexible "sock" made of metal with geared teeth. All this is contained _inside_ a fixed gear. As the motor turns, the ellipse turns, and causes the geared teeth to push and squeeze against the fixed container. This gearing motion is designed to take ~100rpm's down to ~1rpm (very high _reduction_ ratios)... as the outer container has ~100 teeth and the inner has ~99 ... every one lap around the large gear only advances the small inner gear a single notch. So... high motor speed is converted to low motor speed. The fixed containing gear never moves (it's fixed), and you rely on the flexibility of the inner metal flexible "sock" in order to turn another shaft which is giving the output power. In the lego model you see using red "nubs" for the fixed outer gear, yellow "windows" to catch the "nubs", and then the yellow "splines" simulate the flexibility of the metal / sock component. As the lego model turns you see where the motor driver is turning the ellipse. The basketball-picker-upper is effectively attached to the "sock" directly and that is the output work. A lot of the animations are really hard to understand (compared to physically seeing it working) because the gearing ratio is so low... hard to see the output movement over time compared to the lego on.

jpm_sd 9y ago
There's quite a nice animation on the wikipedia page: https://en.wikipedia.org/wiki/Harmonic_drive https://en.wikipedia.org/wiki/Harmonic_drive