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This thread is privately moderated by Jack Crossfire, who may elect to delete unwanted replies.
Dec 27, 2006, 05:05 AM
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Autopilot research


Trying to learn about this stuff is like trying to figure out witholding allowances for maximum 401k contributors. The main problem in copter autopilots is knowing the orientation of the copter. The current state of autopilot seems to be:

1) Robbie Helicommand: uses downward video to hold position. Only low altitude due to requirement for high resolution ground features.

2) Micropilot 2128 Heli: $6000 Uses GPS and inertia. 1oz

3) CARVEC: "Low drift MEMS" Another inertia system.

4) FMA Copilot: IR. $70 Can't hold position. Uses only absolute roll & pitch but not gimbal rates. Videos show it has enough information to maintain attitude but quickly flies away without manual control to maintain position.

5) rotomotion.com: They use 2 accelerometers to detect movement and 2 gyros to detect rate of gimballing motion.

People are loath to give out any details of their autopilot algorithms.

GPS is too slow and inaccurate to hover a helicopter. One $50 GPS unit claims a 1Hz measurement within only 7 meters. Most of the work is definitely by inertial measurement.

Most autopilots use a combination of accelerometers and gyros. This allows the computer to differentiate translational movement from gimballing movement. This alone seems to be enough information for a helicopter to hold a position.

The autopilot algorithm involves plugging the sensor output into specific slots of a matrix, transforming the matrix by a complicated, fixed formula, and plugging resulting matrix slots into the servo positions. It also takes some boundary tests.

Developing the matrix formula, implementing the matrix operations on the lightest computer possible, and determining the constants is where the effort goes.

Since the gyros drift over time, it's not clear how they compensate for the drift. Maybe the accelerometers have the extra sauce to keep those gyros in line.

http://www.sparkfun.com currently has the best deals on inertial measurement parts, including the famous Invensense gyroscopes. A 5 axis inertial measurement unit can be bought for $100. It's not the $2 that Invensense was claiming but it's drastically cheaper than last year.

http://www.rotomotion.com/downloads/SR20_R34.mov

is an impressive video of an intertial navigation system flying a helicopter. It sounds like their GPS is just used for tagging images while the flying is purely inertial.
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Dec 27, 2006, 02:27 PM
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GeekGod's Avatar
Funny that you mention this. I am starting up a project that will do just these types of things.

Currently I am researching AVR programming using a http://www.atmel.com/dyn/products/pr...+&part_id=2004

But the much quicker route would be to use: http://www.acroname.com/robotics/inf.../rcrcvr-1.html

I will be starting a thread once I figure out which direction I am going to go. I am leaning towards the AVR side since the chip is only 3$!!
Dec 27, 2006, 07:30 PM
What can I crash today?
GeekGod's Avatar
Here is also a great article about Gumstix + a DGPS + a couple other goodies. Everything that is needed to build on top of.

http://www.logicalgenetics.com/showa...566&page_num=0
Dec 27, 2006, 08:38 PM
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The microprocessor side of it is easy. It's a 40Mhz PIC18F because that's all I can afford. The unknown is how much information it needs to fly and how heavy it needs to be. Another nugget of information said the combination of 2 gyros and 2 accelerometers didn't detect translation. They only detected attitude. With every email, the gyros seemed more and more redundant, with only accelerometers required to hover.

Differential GPS may work out after all. The literature only states that using 2 GPS units with one as a reference allows very accurate measurements. There's nothing on comparing 2 results from a single GPS unit over time to precisely determine relative movement of the single unit, so that's a $50 unknown.

The first order is now just a 2 axis accelerometer for the purpose of maintaining level attitude. If that looks promising, the next order will be a GPS unit.


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