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
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 ...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.
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.
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!
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.)
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
Just wanted to add, that is a cool project on hack-a-day, but once you start looking at precision motion, or variable loads (eg. moving a robot arm), you start to become keenly aware of the physical imperfections of your motor and gear-box, and start having to think of things like cogging in the motor, or torque-ripple in the gearbox output and all that. The drive signals on those direct drive robots are NOT going to look like the perfect "3 sine-waves 120 deg out of phase" you see on wikipedia, and you need some damn good control software, and sensing of motor state to get good motion.
I'm not trying to say it's impossible, or to knock anyones project or sounds argumentative. It's been done, as you pointed out! Just wanted to point out that while in theory it's much nicer to be able to do direct torque control on the motors, the headaches to become apparent in practice. (I still love the idea of direct drive :-D )
You seem to know stuff about actuation systems; are there any that function like muscles? E.g. strands of wire hooked up across pulleys that repel each other as current flows to increase tension and hence move a part?
To function like a muscle, you want an actuator that acts like a spring with controllable zero point and spring constant. That allows you to adjust both position and stiffness. You also get spring-type energy recovery, which matters for legged systems. (Humans recover 60%-70% of running energy elastically. Cheetahs, 90%.)
A double-ended pneumatic cylinder will do this. Two motors pulling on opposed springs will do this. A series elastic actuator can fake it by active control. You don't get the energy recovery that way, though. There have been systems with wires and pulleys to get muscle-like actuation, but they're bulky.[1]
The series elastic actuator is a position actuator driving a stiff spring, with positional and force sensing at both ends of the spring. If a load starts to compress the spring, the control system frantically runs the actuator to unload the spring before it compresses much. This gives the illusion of a less stiff spring. The spring stiffness simulated can be adjusted in the control system.
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.
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.
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.
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.
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.
As an interesting aside, anyone who is interested in this stuff should check out the Omega Tau podcast(http://omegataupodcast.net). In a recent episode(#247), he went in depth with Dr. Hoffman about his research and the uses of BMG materials.
As another interesting aside on these gears -- besides the very cool materials in these particular gears, the technology behind "strain-wave" / "harmonic drive" gears used in a lot of precision robots is very very cool. Parts of the gear set are actually flexible yet you get very smooth* motion with negligible backlash out of them!
* - (Compared to more typical gears, and the cogging you get out of them. There are other issues with these gears the controls guys have to deal with when you start going for really high precision motion, but I don't want to seem like I'm knocking them as they do solve a lot of problems in a really clever way!)
Why do robots rely so much on gears? There is no gear in the animal kingdom, and yet their limbs are vastly more energy efficient and agile, aren't they?
Muscles/limbs are only 'vastly' more efficient if you consider they have large numbers of nano scale support systems constantly rebuilding them. Since we don't have nanobots, gears will be better for machines. Also, nature didn't naturally develop a axle.
Robots have bearings and simple, efficient rotary electric motors. A machinists lathe is a fantastic piece of technology. We're really good at building things that rotate.
Nature uses cellular-scale nanotechnology to build millions of microscopic linear motors. We can build linear electric motors by essentially unrolling the stator, but they're on a much larger scale.
We get better results with servomotors, either installed with some gear reduction at a pivot joint, or hooked to a ballscrew linear actuator.
Is that an inherent property of muscle-like mechanisms, or just several billion years' worth of environmental bacteria evolved to consume living tissue if not constantly opposed by a immune system?
That's an inherent property of all living material. All tisssue is on the edge of thermodynamic breakdown i.e. Unlike manufactured material, resting state for biology is death).
Electric motors can't generate the torque to drive something like a knee joint, without being very large and heavy.
Motor torque is limited by the current you can run through copper wire, times the magnetic field you can generate with permanent magnets. The best motors are only about 2 Nm / kg (continuous torque / motor weight). A knee joint requires torque around (weight of robot/2 * length of tibia) in order to squat, say 100 nM for a human-sized robot. Direct-drive motors would weigh 50 kg each! So in practice, you use a 20:1 gearbox and 2.5 kg motors.
Superconductors could totally change that some day.
Alas, there is no muscle in the robot kingdom! A lot of interesting work has been done recently on direct drive or minimally geared legged robots, however - check out Ghost Robotics for an example. [https://www.ghostrobotics.io/]
There are no muscles on mars either though. NASA has a different set of constraints than nature (one of them being human lifespans of time to make a workable solution).
Agility varies quite a lot. For example, a pick-and-place robot's arm has more agility than a grasshopper's hind leg, as that leg is pretty limited in its movement.
>“Mass producing strain wave gears using BMGs may have a major impact on the consumer robotics market,” Hofmann said. “This is especially true for humanoid robots, where gears in the joints can be very expensive but are required to prevent shaking arms. The performance at low temperatures for JPL spacecraft and rovers seems to be a happy added benefit.”
Highly unlikely. Teflon (PTFE) can be injection moulded and is self lubricating and probably cheaper although it still costs and arm and a nut. Also the flex spline is one of the cheapest parts of a harmonic drive- the elliptical bearing is more expensive.
There are also other reasons not to use harmonic drives. Their efficiency usually isn't as good as advertized and is often worse than other gearsets.
Shouldn't this be "Glassy Metal Gears", instead of the opposite? They say they heat it up to get rid of the crystals, then somehow cool it without allowing them to form, leading to a metal in an amorphous solid form like that glass takes on when it cools.
since there are robotics people in here, would someone mind linking me to videos or blogs about tearing down/talking about the motors and drivers used in cutting edge robotic arms such as those used in the automotive industry?
