Well, to work in a spin stabilized mode, Marcy 2 would have to spin a
lot faster than a 3 channel coaxial copter can. Without spin
stabilization, a $40 Syma S107 or a $25 Sky Invader didn't matter
because either way, we needed another gyro to know heading. A 150
second flight time doesn't demand a very accurate gyro.
Mass produced Marcy 2's would use the cheapest single axis gyro
available, but suddenly a static proof bag of new gyros happened to
fall off a truck. It's the ITG3200, another crazy small 0.5mm Invensense QFN.
Decided to go ahead with it, to evaluate it for another 3 DOF IMU. Have 5 years made
any difference with Invense gyros?
Unfortunately, it immediately showed a
large dead band, a convenient solution to the same old gremlin
of gyro 0 offsets drifting. It automatically 0 centers itself, but it has to
rotate a large, minimum amount to detect anything. You can rotate it
slowly enough, handheld, to go 90 deg without sensing anything.
It's intended for hand held motion sensing & gesture user interfaces,
not a heading hold gyro. The deadband gets a lot
bigger as the lowpass bandwidth gets lower.
Direct analog pins would make this chip a lot more useful, but obviously
doing the samping on the die eliminated a lot of parts
that would be required for analog pins.
Through the IDG300, ADXRS150, LISY300, & ITG3200, gyros have gotten cheaper, but
they still use the same mechanism they did 5 years ago. They can stack multiple gyros,
sample more bits, but the mechanism hasn't gotten any better.
UPDATE:
It wasn't a deadband as much as a mismatch between the lower & upper 8 bits. It took a lot of rotation to get above 0. Then the result was smooth from 0 - 255. Then, it took a lot of rotation to get above 255.
It turned out the chip doesn't work in I2C burst mode. You have to read the analog results 1 byte at a time.
MARCY 1
Read over the monocopter paper again, 2 years later, & figured out 3
factors affecting stability:
Wing pressure needs to go through the CG of the balance beam.
Coning angle needs to be minimal.
Balance beam needs maximum inertia.
The main advantage of a monocopter is having the most wing in the
fastest air for the least weight, but this results in a high coning
angle. An ideal monocopter would have to be much bigger, have an IMU &
use servos for active stabilization. The ideal, small vehicle is better
off eating the cost of a 2nd wing.
So the 2 flexing wings ended up pressing the fuselage onto the takeoff
rod & jamming it down. Another problem was twisting of the angle of
attack to a level position.
It didn't have these problems in monocopter form, but we've also reduced
the wingspan & increased the RPM.
There's still hope for the lighter frame, if the design works & frees up
enough money for CF. Time to dig out a tried & true brick frame. It
was abandonned when we figured the monocopter was more optimum, but the
rigidity of balsa is really needed.
That definitely seemed to be more stable than the monocopter, but led us back to the actuator problem.
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