
Metallic Glass Gears for NASA Robots - tribonet
http://www.tribonet.org/metallic-glass-gears-for-nasa-robots/
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olivierlacan
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)

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Animats
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.

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TFortunato
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!

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foobarian
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?

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Animats
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.

[1]
[http://www.mdpi.com/2076-0825/3/2/124/pdf](http://www.mdpi.com/2076-0825/3/2/124/pdf)

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mhb_eng
As an interesting aside, anyone who is interested in this stuff should check
out the Omega Tau
podcast([http://omegataupodcast.net](http://omegataupodcast.net)). In a recent
episode(#247), he went in depth with Dr. Hoffman about his research and the
uses of BMG materials.

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TFortunato
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!

[http://www.harmonicdrive.net/technology](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](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!)

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dom0
> Metallic Glass Gears Make for Graceful Robots Space Gears Harmonic drive
> Bulk metallic glass, a metal alloy, doesn’t get brittle in extreme cold.

What kind of sentence is that?

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grondilu
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?

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pixl97
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.

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vacri
Muscles/limbs also don't power-down for long periods. They're in need of
constant maintenance (energy, food; waste removed) or they rot.

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PhasmaFelis
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?

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infinite8s
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).

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hwillis
>“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.

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NoodleIncident
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.

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reportingsjr
Glassy metal is another name for metallic glass/amorphous metal.

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bronz
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?

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neotriple
Hey there!

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=bzRh672peNk)

[https://www.youtube.com/watch?v=EfmjhfN8D-Q](https://www.youtube.com/watch?v=EfmjhfN8D-Q)

There's a video on the Kuka robot arm.

