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Underactuated Rotor for Simple Micro Air Vehicles

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Re: Underactuated Rotor for Simple Micro Air Vehicles

#61

Post by jzwinck »

swah wrote: post #57
jzwinck wrote: post #51

Earlier quoted context omitted.

Look at the hinge picture. See how the two hinge pins are parallel? Now imagine the blades turning 180 degrees. The hinge pins will now be at the "opposite" angle to before, despite that the blades are symmetrical so identical at 180 degrees to 0 degrees. So at 0 degrees, increasing torque will, say, increase pitch of the "right-hand" blade while decreasing pitch of the "left-hand" one. But at 180 degrees it will be…

I got that part, I just didn't get the "informational control problems" comment.

In an old-school helcopter you have, say, one main rotor with a vertical shaft, and one tail rotor with a horizontal shaft. Each of them has complex control mechanisms implemented in hardware: the main rotor has swash plates, and so does the tail rotor (or it has variable speed). The pilot controls the system in a direct and even crude way: a side stick adjusts blade pitch, sometimes via a simple cable linkage. Throttle is kept quite stable much of the time.

In this newfangled bird, we use some combination of GPS sensors, an input which is presumably a touchscreen (with all the associated latency, plus wireless), and a speed controller which has to adjust multiple times per revolution of the rotors. For some information on why this is tricky, start here: http://en.wikipedia.org/wiki/Nyquist_frequency

Re: Underactuated Rotor for Simple Micro Air Vehicles

#63

Post by lsaferite »

woah wrote: post #42
TaylorAlexander wrote: post #37

Earlier quoted context omitted.

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 coar…

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

You can give the parts a vapor bath, but you lose some of the exactness. The longer you do it the smoother the part but the more it loses it's dimensions. You are basically melting the plastic with fumes.

http://hackaday.com/2013/02/26/giving-3d-printed-parts-a-shi...

Re: Underactuated Rotor for Simple Micro Air Vehicles

#64

Post by ansible »

sebastianavina wrote: post #60
marvin wrote: post #15

Earlier quoted context omitted.

Electric motors remove a lot of the complexity in powered-lift flying machines. As soon as your power/endurance requirements demand a combustion engine, you also have to manage the complexity of a piston engine or turbine (jet) engine. This means either including a crankshaft and optionally a system of gears to route the rotational power along the axes you want the rotation in, or ducting the output of a jet turbine…

or you can generate electricity with a combustion engine a move two electric motors... like the railroads

I don't think a hybrid like what you suggest is practical.

Locomotives need lots and lots of torque at zero and low speeds. This would otherwise necessitate a huge transmission. Plus, the weight penalty for a train locomotive isn't nearly as severe as it is for any kind of flying craft.

Other commenters have mentioned a hybrid system, with a relatively small electric motor that generates just the instantaneous torque changes needed for the control system, but the majority of the power is supplied via direct mechanical linkage as in today's helicopters. That might be viable, but I'm not sure. The weight and complexity penalty for the old control scheme isn't so bad when scaled up, but is really bad when scaled down.

So a hybrid scheme (like in a Prius) might be practical, by my estimation.

Re: Underactuated Rotor for Simple Micro Air Vehicles

#66

Post by kordless »

swah wrote: post #57
jzwinck wrote: post #51

Earlier quoted context omitted.

Look at the hinge picture. See how the two hinge pins are parallel? Now imagine the blades turning 180 degrees. The hinge pins will now be at the "opposite" angle to before, despite that the blades are symmetrical so identical at 180 degrees to 0 degrees. So at 0 degrees, increasing torque will, say, increase pitch of the "right-hand" blade while decreasing pitch of the "left-hand" one. But at 180 degrees it will be…

I got that part, I just didn't get the "informational control problems" comment.

The information is the position of the rotor, pulled from the motor sensors. The control is the pulsing of the motor's torque at the right position to increase lift in a particular area of the rotor's arc. Informational control simply means you are pulling information out of the system to better drive the system itself.

Re: Underactuated Rotor for Simple Micro Air Vehicles

#67

Post by Vendan »

durkie wrote: post #43
woah wrote: post #42

Earlier quoted context omitted.

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

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... 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.

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.... (I'm actually a member of Fablocker, so I'd seen those squirrels before)

Re: Underactuated Rotor for Simple Micro Air Vehicles

#68

Post by robotresearcher »

mhb wrote: post #50
mrfusion wrote: post #46

Earlier quoted context omitted.

I'm still not getting it. Wouldn't the pitch of the two sides be the same so how would that be useful? How do you control the pitch of two blades with a single motor?

There are two motors.

The second motor and rotor provides vertical thrust and torque compensation only. It spins the opposite direction to the upper rotor. It does not steer the vehicle.

Re: Underactuated Rotor for Simple Micro Air Vehicles

#69

Post by phkahler »

jahnu wrote: post #62
zan2434 wrote: post #5

This is so clever! Turning mechanical control problems into informational control problems is critical to the ubiquity of micro air vehicles.

How do they determine blade position?

>> How do they determine blade position?

I assume you mean the motor rotation angle. It's probably not a brush motor, but a 3-phase synchronous motor. That means instead of having a commutator to apply the voltage to the coils, a computer switches some FETs to apply the voltage. In the simplest case, the rotor is assumed to follow the applied voltage waveforms. In a more typical system, the rotor position is estimated from the applied phase voltages and measured currents (usually via PLL or a sliding mode observer).

If you mean "how do they determine the blade pitch?" then the answer is that they don't need to. The controller - weather that's a computer with inertial sensors, or a person watching it - will just manipulate the amount of torque variation until it gets the response that it wants from the copter. Much like you don't actually need feedback of your cars gas pedal position so long as you have vehicle speed feedback.

Re: Underactuated Rotor for Simple Micro Air Vehicles

#70

Post by logfromblammo »

mrfusion wrote: post #46
Cushman wrote: post #24

Earlier quoted context omitted.

"The linkage to change the pitch of this rotating blade is way too complex! Can we simplify it somehow?" "How about we just add a simple device that associates the pitch of the blade with the torque, and let a computer figure out how to spin the motor to get the pitch we want? No linkage!" Yeah, that is dang clever.

I'm still not getting it. Wouldn't the pitch of the two sides be the same so how would that be useful? How do you control the pitch of two blades with a single motor?

The hinge that connects each blade to the rotor is angled with the same pitch, but on opposite sides of the rotor, like this:

------/ || /------

When the rotor applies more torque than the current angular momentum of the blades, the blades lag behind the central hub, pushing them the same direction against their hinges. On one side, that increases the blade pitch. On the opposite side, it decreases the blade pitch. The motor could also apply a slight braking force, to pitch the blades in the opposite direction.

The flight computer determines the correct timing at which to apply more oomph to the drive motor, which translates to the same effect as a mechanical swashplate due to the angled pins. The total energy imparted to the blades over a single rotation remains the same as with a motor operating at constant torque.

One disadvantage is that you will need a motor that is capable of applying more maximum torque, because you won't be running it at 100% all the time, but with a sinusoidal power level that will go both above and below that steady level. The other disadvantage is complexity in the computer controlling the motor. The advantage is mechanical simplicity.

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