What do this satellite thruster, plastic tool, and micro mechanical switch have in common?
Well they all contain components that bend, so-called compliant mechanisms
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Now about a month ago I was giving a talk in Utah
hence the suit and that's where I met this guy, -Larry Howell professor of mechanical engineering.
So it's always been considered to be bad to have
flexibility in your in your machines. Well we've tried to take that that thing
that everybody hates, that is trying to avoid and say how can we use flexibility
to our advantage? how can we use that to do cool stuff?
Now Professor Howell literally wrote the book on compliant mechanisms -that's the
most cited book -but he's pretty nonchalant about his work
just watch how he introduces this mechanism he developed to prevent nuclear weapons
from going off accidentally:
actually in safing and arming of nuclear weapons.
And so if... -What? -Yeah
And so if you want.... -Hang on, hang on hang on
What-ing nuclear weapons? -Safing and arming
Safing and arming -yeah so if there's anything in
the world that you want to be safe it's not going to accidentally go off
I feel like this is - it doesn't even need saying but yes nuclear weapons obviously you don't
want them to go off. What I don't understand how this is gonna keep the
nuclear weapons safe.
Now I want to come back to this device and explain how it
works once we understand why compliant mechanisms are best suited to this task
[that's cool] So let's start with something basic.
Probably the first
compliant mechanism I ever designed was this thing. What it is is a compliant
mechanism that is a gripper so you can put something in there and it will get
actually a really high force. I can put that in there and and it breaks the chalk
What have you put your finger in there and squeeze it? You would scream in
pain, would you like to try? -I would like I would actually like to feel the force
OK, you need to squeeze it yourself though or it's... -Really?
well all right, I'll squeeze until you scream in pain
Aaahh hahaha
That very quickly got incredibly painful it felt like having my finger like in a in a vice.
That looks suspiciously like vice grips but now with these flexible
components where the hinges are.
What I learned in my visit with Professor Howell
is that compliant mechanisms have a number of advantages over traditional
mechanisms but I thought he needed kind of a clever pithy way to remember all of
these advantages. So I came up with the eighth P's of compliant mechanisms and
the first of those is Part count. Compliant mechanisms have reduced part
count because they have these bendy parts instead of having things like
hinges and bearings and separate springs. This gripper is just a single piece of
plastic but achieves a similar result to the much more complicated vice grips.
Like how much does it amplify the force? This will get about thirty to one so I
could get for one pound force in, get thirty pounds out. That's pretty good
It seems like that would be super cheap -and really inexpensive so this we just made
here in our shop but you can imagine also injection molding that
- that would cost like cents -yep this would cost cents
the other thing is because of its shape
you could extrude it and then just chop them off and that would be cool.
So the simple design allows
different production processes to be used which lowers the price these
switches for example achieve in one piece of plastic what is normally done
with springs, hinges and many rigid plastic pieces
also a good fidget device
how long can these last?
-we've had these in our fatigue testing machine. We've been
able to go over a million cycles without failure
What have we got there?
All right, Derek I've got a quiz -uh oh
quiz for you okay, I'm gonna -elephant I'm gonna
Very good!
okay I'm gonna push on elephant's rump this direction okay? I'm gonna hold this
and that little dot right there, is that dot when I push on it, is it gonna go
left, right, up or down?
Um...
I just you know what I wanted to guess without even thinking about it?
Yeah, please do.
I'm gonna say like up and in -okay -and I kind of feel like that because like
that would be a logical way for an elephant to hold its trunk -okay
but also because like
if this is all going over then I feel like this is gonna kind of extend there and
that's gonna get pushed up in there. - ah, good thinking
well I don't know is that
good thinking? that's well it's thinking at least so... this is designed
so that when you push on that it actually just rotates in space it
doesn't move at all. -I knew you were gonna pull some sort of trick
it's a trick question!
now since I was fooled by it I had to try it out on my friend the
physics girl. That's so trippy. That is so cool! I don't understand - what?!
it's modeled after the mechanisms you use in
wind tunnels where you want to have say a model that's that's attached here but
you move it and all you want to do is is control its its angle and move it around
in a wind tunnel. don't displace it but be able to change the angle.
Devices like this demonstrate that compliant mechanisms are capable of
producing very precise motion, which I personally found pretty counterintuitive
because these objects are made up of flexible parts
but maybe that shouldn't
be surprising because compliant mechanisms don't suffer from backlash
for one thing.
So backlash occurs when you have a hinge which is basically just
a pin in a hole and it's moving in one direction and now if at some point the
motion reverses it doesn't happen instantaneously because there's some
give in the hinge.
This also causes wear and requires lubricant and that is why
compliant mechanisms have better performance than their traditional counterparts.
This one though is my favorite. That is is one of my favorites too.
It's just so pleasing, right?
Ahhh, that sound is so satisfying.
This actually, believe it or not was inspired when we were doing things at the microscopic
level, where we're building compliant mechanisms
on chips. We had to be able to make these compliant mechanisms out of silicon,
which is as brittle as glass. -mm hmm
And if you're trying to make something like
this out of glass, right? it's it's crazy hard but that also means once we figured
out the design we could make it in a material even like PLA which is also you
know not the ideal compliant mechanism material.
So you can get on our website
and get the material... and get the files to make this yourself I'll put a link in
the description ya- that also has a nice feel and I snap to it has a really nice
snap I like when it comes out, it's like 'gunk' you know like there's something
about that that's really it's very pleasing.
So these things actually move?
oh yeah, yeah yeah -I need to see this
okay all right we'll do it
were those etched on there? - yeah those are etched and so just using the same
process as used to make computer chips.
So another advantage of compliant
mechanisms is that they can be made with significantly smaller proportions
because they take advantage of production processes like photo-lithography
And we have motion that we want at the microscopic level -that's brilliant.
Plus since they simplify design compliant mechanisms are much more
portable meaning lightweight which makes them perfect for space applications.
This here is something we did with NASA making a hinge that could replace
bearings for say deploying solar panels. This is titanium, 3d printed titanium but
what's freaky about it is you get that motion which people expect but there's a
piece of titanium that can bend plus minus 90 degrees, 180 degree deflection
that is solid titanium. - That is one piece of titanium that is 3d printed
There's no alloy, nothing to make it flexible. -yep, this is yeah and even freakier than this
is this guy right there. So that looks like a crazy beast but every part in
there has a purpose. All these flexible beams
here are the two inputs and again we did this with NASA for a thruster application
where you can put a thruster right there and now with our two motor inputs we can
direct that thruster in any direction. That titanium device moves that, you
notice that's just all bending and then there's no pinch points for the fuel
lines or electrical lines coming in.
Here, this single piece of titanium allows you
to use one thruster in place of two.
Okay, that is a clutch, so the idea is if you
spin it up really fast because it's flexible this outer part will actually
start coming outwards and then if there's a drum around it it'll it'll
contact with that drum and spin that thing -oh so this like kind of oh that
kind of comes out like so -when it gets spinning really fast and then you're you essentially
engage this this outer drum so this is like the way that a chainsaw would work
or something like that because you get it spinning fast enough and then it
engages the chain and then it turns it over
-centrifugal force -yeah wow that's cool so
So here this is made in plastic so that it you know you can see it but in reality
it's gotta be a lot stiffer so here it is made in steel -What?
So hang on, you're saying
that that thing, which is made of steel yup
You spin it up to a certain speed
and then it expands and engages a drum that is around it
-yep so idle with no
motion but then at a certain speed that are what we designed it for it will
speed up to that rpm
You speed it up and it engages -Yup
I had no idea like I have
learned something today
So let's come back to the safing and
arming device for nuclear weapons.
Its purpose is to ensure that no random
vibrations say from an earthquake inadvertently disable safeties and arm
the nuclear weapon.
Now one of the requirements was that this device be
made as small as possible.
They had made those as small as they possibly could
using traditional methods even using things like what the
Swiss watch manufacturers were using.
With compliant mechanisms they produced
a device out of hardened stainless steel where some components were the size of a
human hair.
This is high-speed video, here the device
is operating at 72 Hertz meaning this little hole makes two complete
revolutions each second.
The way it's meant to work is an arming laser shines
on the rotor wheel and when the proper input is given to the system the wheel
rotates a notch.
If all the proper inputs are given then the hole lines up with
the laser beam and crazy things happen from there.
So it is essential that this
device's performance is perfectly predictable even if it sits unused in a
silo for decades.
So are these now being used on nuclear weapons?
You know, it turns out they don't tell us what they do with their nuclear weapons and so we
design them, we made prototypes we tested them and then it goes what they call
behind the fence.
And... where it's all classified and, you know we
don't know what happened, so...
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