
Underactuated Rotor for Simple Micro Air Vehicles - command_tab
http://modlabupenn.org/underactuated-rotor/
======
zan2434
This is so clever! Turning mechanical control problems into informational
control problems is critical to the ubiquity of micro air vehicles.

~~~
fasteddie31003
There is a lot of potential in solving mechanical problems with information
systems, rather than using complicated mechanical solutions. For instance,
hydraulic automatic transmissions and differential.

~~~
lumpypua
The mechanical solutions are always really interesting and clever, but digital
systems are much more flexible.

Really cool video on the mechanical ignition timing control in a delorean to
control emissions. Explanation of ignition advance at the end:
[https://www.youtube.com/watch?v=ge1GwepqtK0](https://www.youtube.com/watch?v=ge1GwepqtK0)

------
command_tab
This thing is so nimble in the air yet has a fraction of the complexity of a
regular helicopter. I wonder if this method will scale up to "full size"?

~~~
wiredfool
Probably not, for the same reason that a quadcopter doesn't really scale to
full size. The inertia effects of larger rotors make changing the speed of the
rotors within a rotation much harder.

With full sized helis, it takes a long time to spin up the rotors to speed
before the pitch is changed to take off.

~~~
TaylorAlexander
Well the critical difference here is that changing blade speed is not needed
or desired. They just need to change blade torque, which would apparently
immediately cause a change in blade pitch.

I don't see a specific reason why this wouldn't scale. I do wonder what
changes in load do to the system though. If it can't handle changing loads
without messing up the blade dynamics, it would only work for fixed payload
systems like camera platforms.

~~~
erobbins
It won't scale because of inertia. Rapidly changing the speed/torque of a
large combustion engine is nearly impossible.

It's a great solution for reducing mechanical complexity in mini/micro sized
UAVs, though.

~~~
TaylorAlexander
You can use an electric motor. We can easily make extremely high power
electric motors (see: CNC machines and Diesel trains), and power storage will
continue to improve.

The added cost of batteries at even today's prices could be worth it for the
decreased mechanical complexity of the rest.

If all we need to do is change the torque of an electric motor, that can be
done essentially instantaneously even at high power levels.

~~~
lambda
> power storage will continue to improve

Power storage may continue to improve, but it will take a long time before you
hit the energy density of fossil fuels; the difference between batteries and
fossil fuels is something like a factor of 10 (actually greater than that, but
electric motors are more efficient so let's say 10 for the sake of argument),
and it's improving much more slowly than Moore's Law, doubling maybe once
every 10 years. That means if those trends continue (and there's no guarantee
they will), you're looking at 30-40 years before the energy density allows
electric helicopters to be competitive with fossil fuel powered.

For cars, energy density isn't quite as important, as the weight of the car is
not the dominant factor in its efficiency (it does have an effect, but the
aerodynamics, engine efficiency, transmission efficiency, tires, etc. all make
a big difference too). But for a helicopter, every pound you add to the
batteries is another pound you have to lift, so energy density of your power
source is quite important.

So, if trends hold on battery technologies, it will come about eventually; but
I would put money more on the decades timeline than the years timeline.

------
mhandley
This is such a neat idea, but I wonder if it suffers from vibration problems.
Because the blades are mounted on pivots, whenever you've commanding
differential pitch, the high angle-of-attack blade will incur more drag and so
lag slightly more than the low angle-of-attack blade. Thus the blades won't be
exactly opposite each other anymore, creating vibration. Perhaps the blades
are spinning so fast this is not a big deal? Probably would be if you scaled
up though.

~~~
MadManE
Vibration only becomes a problem when the scales of the drag/lag and
rotational frequency are similar. My gut feeling is that this will only happen
when your lift-to-drag ratio is close to 1, so not in any meaningful case.

------
emmanueloga_
Reminds me of PWM [1] on old PC speakers. Many old computers (e.g. of the 286
era) had a speaker that was only able to generate square tones. By controlling
the start and duration of the pulses, programmers could generate sounds that
were a lot more rich than what you would expect from "plain" square waves [2].

In a way this is similar: they start with two actuators that seem very limited
in what they can accomplish: you can only speed them up and down. By
modulating the speed of both motors in sync, they are able to control that
cheap rotor mechanism they came up with, achieving 6DoF [3]

1: [http://en.wikipedia.org/wiki/Pulse-
width_modulation](http://en.wikipedia.org/wiki/Pulse-width_modulation)

2:
[https://www.youtube.com/watch?v=FSe3ysBrXq4](https://www.youtube.com/watch?v=FSe3ysBrXq4)

3:
[http://en.wikipedia.org/wiki/Six_degrees_of_freedom](http://en.wikipedia.org/wiki/Six_degrees_of_freedom)

------
alimoeeny
The whole thing looks great and all. But I specially liked their last sentence
that said "for civilian needs".

------
DickingAround
I wonder if this will cause some parts of the motor to heat up more; since the
blade is 1:1 rotations with the motor then driving more torque (power) during
certain phases of the cycle will put more watts on certain coils.

It's a cool idea. Human size helicopters would love to avoid all that blade-
pitch complexity. :)

~~~
theoh
Couldn't that be addressed (if a large enough effect to be a problem) by just
having the body of the vehicle spin at a low rate, like less that 1Hz. With
appropriate sensors this wouldn't be a problem[1]. More to the point, there
shouldn't be any need for sustained constant "cyclic" input anyway, in normal
maneuvering.

[1] an axially-symmetric imaging system could compensate easily, e.g. a
panoramic system.

------
joosters
Very clever! I hadn't fully understood the mechanical complexities involved in
a standard single-rotor helicopter. I just thought that they were hard to fly
manually (which made me very confused to see all the computer-controlled
quadrocopters - why didn't they use just one rotor?)

~~~
TaylorAlexander
Well the issue isn't just that they're hard to fly or that they're
mechanically complex - the issue is that they're both. So you crash often
(when flying models) and crashes are expensive. This makes it a very expensive
hobby.

The issue when flying is that they balance as if they are on a ball. You need
to constantly adjust the controls to stop from "falling off the ball". Go the
wrong direction and you speed up the rate at which it falls. When it is facing
away from you, that's not very hard. When it is facing towards you everything
is backwards. When you are turning, the correct direction to stay balanced is
constantly changing.

Automated systems can do this automatically for you though, so a drone-like
autopilot in a standard helicopter could make them as easy to fly as
quadcopters. Traditional helicopters are still mechanically complex though.
This system looks to fix that. And this would be more efficient than a
multirotor.

------
ww520
At first viewing of the video I didn't understand why the body doesn't spin
around with one rotor and a pair of blades. That was some magic! Then looked
at the picture again and saw another rotor underneath. Pretty neat to have two
rotors counteracting each other.

~~~
vutekst
This always bothered me about Babylon 5 - how/why did the spine of the station
remain stationary while the bulk of the station rotated?

------
aosmith
Now if we could just start printing these parts at home...

~~~
brk
Seriously, why? Injection molding has far lower cost, higher quality, and
you're talking about things that are very cheap and easy to ship. What does 3D
printing bring to this?

~~~
TaylorAlexander
Injection molding is on the order of 10,000x more expensive if you only need a
single part.

I'm currently on a quest to design highly functional robots that can be made
with just a 3D printer and a minimum of external parts - so far only bearings,
motors, drive belts, batteries and electronics are the non-printed parts
needed. I make everything so that it fits together by interlocking or with
minimal use of some coarse printed fasteners.

Since nothing needs to be bought from the store that won't be useful if the
design changes, it is trivial to iterate and build a newer version.

My hope is that with developers worldwide working on improvements, injection
molded versions of the parts would become obsolete before the mold was even
cut.

When it comes to manufacturing, 3D printing is a whole different ballgame. No
other manufacturing technique can make so many complex parts without human
intervention. This means it opens up all new possibilities for how we
manufacture things - like continuous iteration of shipping hardware.

~~~
woah
Seems to me that the worst part of 3d printed stuff is "the grain" is there
any way to fix that?

~~~
durkie
i was about to propose some sort of annealing process like they do with
metals, and it seems like someone has already beaten me to it:
[http://3dprint.com/3388/study-how-to-make-3d-prints-
stronger...](http://3dprint.com/3388/study-how-to-make-3d-prints-stronger/)

as a side note, this looks like kind of a cool way to smooth pla prints, for
which up till now there wasn't really a great option.

~~~
Vendan
that is a horrible article, considering the picture they are using is ripped
off of a reprap blog on smoothing abs prints using acetone vapors.
[http://blog.reprap.org/2013/02/vapor-treating-abs-rp-
parts.h...](http://blog.reprap.org/2013/02/vapor-treating-abs-rp-parts.html).
(I'm actually a member of Fablocker, so I'd seen those squirrels before)

~~~
durkie
well shit. we tried putting some big pla prints in the oven last night and
they definitely just started warping and caving.

~~~
Vendan
on that note though, from my experiments, acetone vapor bath does seem to
strengthen abs prints some, cause it melts the surface together. Not terribly
big effect, but it actually helps a lot for low infill parts and stuff.

------
lotsofmangos
This is really cool. Would be interesting to see a configuration with two of
them front and back, or a conventional tailrotor version.

------
paulftw
Curious to see whether this system is robust enough for unpredictable outdoor
breezes.

