[Cushman]

"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.

[mrfusion]

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?

[jzwinck]

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 the opposite. This has the presumably beneficial effect of allowing the craft to climb by simply increasing rotor speed steadily (it will "wobble" a bit but in a spiral fashion which will let it climb without too much inefficiency). Put another way, this means that increasing torque at 0 degrees but decreasing it at 180 degrees allows an asymmetrical pitch to be maintained at a rotational speed which can be seen as constant (over the long term).

In other words, your intuition falls down because the mechanism is not as symmetrical as your brain wants it to be at first glance. Symmetry is an intuitive and attractive for mechanical systems, but it is actually limiting in many cases, and this is a great example.

[swah]

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

[jzwinck]

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

[kordless]

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.

[swah]

Thank you!

[logfromblammo]

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.

[lnanek2]

Didn't you read the article and watch the video? There is a sinusoidal signal added (i.e. up and down) that matches the rotation of the blades and controls the hinges on the blades. So the signal can be high when the blade is on one side of the vehicle then low when the blade is on the other side. Or vice-a-versa if the control system wants to go the other way. Both blades tilt the desired direction when they are on the desired side of the vehicle. The video is quite good and shows the blades hinging different directions on different sides when the signal is applied.

[mhb]

There are two motors.

[robotresearcher]

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.