Metallic Glass Gears for NASA Robots
tribonet.org
tribonet.org
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.
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!
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-...
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
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 )
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.
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.
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.
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.
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.
http://www.harmonicdrive.net/technology
(EDIT: We commented at same time, so see also olivierlacan's commented video! https://www.youtube.com/watch?v=bzRh672peNk )
* - (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!)
What kind of sentence is that?
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.
http://www.cam.ac.uk/research/news/functioning-mechanical-ge...
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.
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.
If you're interested in the mechanical components of the robot arms, they all probably use harmonic drives.
https://www.youtube.com/watch?v=bzRh672peNk
https://www.youtube.com/watch?v=EfmjhfN8D-Q
There's a video on the Kuka robot arm.