IMU wing leveler
Currently I am working on my first UAV so I am only a beginner. I understand the basics of IMUs and some of the challenges but do not yet fully understand the code. It occurred to me that we all wanted good IMUs in order to get rid of thermopiles and make everything nice and neat but what about people who just want to replace their old FMA copilot with something nice and neat? Has anyone made an IMU device you can just stick in line with your current RC gear and have it keep your wings level. Something you only have to read the manual once on, you know, so the average RC guy could understand and use it.
Replies to This Discussion
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ModeratorPermalink Reply by Chris Anderson on -
We haven't released such a product yet, but you're right that there is a clearly a place for it. I expect that this community will create one in the next few months. It is, as you say, an obvious choice and pretty easy to do. The only catch is that it would probably require a new PCB to integrate the RC (ArduPilot) and ArduIMU components into one small package, with an enclosure.
We should be able to duplicate all the functionality (and more) of the FMA CoPilot II for about half the price, with the additional advantages of being smaller and self contained (and open source!)
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Permalink Reply by Jason on -
Most of the guys in my local RC club don't use any of the electronics we use here, but a compact affordable wing leveler that fits entirely inside their old Cub and helps them fly more relaxed, they would absolutely buy it.
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ModeratorPermalink Reply by Gary Mortimer on -
Well just adding gyros can help in that regard sort of!
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ModeratorPermalink Reply by Krzysztof Bosak on -
This is impossible as the knowledge of absolute pitch angle in most IMUs is not stable during acceleration, so it cannot work with out good airspeed sensor. You end with a very complex system that has order of magnitude higher manufacturing costs (both tuning, calibrating and sensors).
There was a German system for this purpose that ended around 1000EUR+, don't remember now.
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ModeratorPermalink Reply by Chris Anderson on -
That sounds like a challenge to me ;-) (The word "impossible" is highly incentivizing!)
The fact that a German system cost 1,000 Euros is all the more encouraging. That tells me that it can be done now, open source, for a tenth that.
We're already running just such a system with ArduIMU+, which Doug Weibel is flying with the 2.51 alpha. I don't know if he's using the airspeed sensor, but even if so I doubt he's got it as part of the attitude estimation.
What say we start a project here to prove Krzys wrong? Maybe make it a T3 contest: IMU-based stabilization without an airspeed sensor. We'll productize the winner, and share the revenues.
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ModeratorPermalink Reply by Krzysztof Bosak on -
In order to REPLACE FMA you need to produce the solution that replicates the functionality, vibration resistance, absolute angle precision of 2deg, total plug-n-play functionality at some 50USD OR ELSE it it will not be the replacing solution.
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ModeratorPermalink Reply by Chris Anderson on -
Whatever. We'll just do it and you can figure out the semantics. Let the markeplace sort it out.
BTW, if you'll read the thread you'll see we're talking about competing with the FMA CoPilot II ($170).
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ModeratorPermalink Reply by Krzysztof Bosak on -
Do you realize that making this is MORE demanding than making IMU driven autopilot?
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Permalink Reply by Brian Wolfe on -
I have yet to close the loop around my airspeed sensor on the PICPilot and Bill's UDev board doesn't even have one, so we know it can and is being done.
Krzys is right that an error will build during acceleration but if the loop gains are chosen with care and we use the assumption that that the accelerations will be relativily short lived then the errors can be kept down to an acceptable level.
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ModeratorPermalink Reply by Chris Anderson on -
Good point. I'm using the UAVDevBoard in stabilization mode, too. Works great. No airspeed sensor. Flown it on three platforms, in a range of performance windows from fast to slow. So not only CAN we prove Krzy wrong, we already have ;-)
The only difference between that and a wing leveler is that it has GPS, but that's just for yaw correction. Not necessary for a wing leveler.
Krzy, have you tried the UAVDevBoard? The DCM is awesome at dealing with the problems you describe. Which is why we use the DCM on ArduIMU+, too.
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Permalink Reply by Brian Wolfe on
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I suppose to put Krzys' concerns to the test we would need to do a bunch of tight loops, turns and rolls then see how long it would take the errors to settle out and the aircraft to come back to level. I can't say I've tried that except when I was in the process of crashing, so there may be some issues that require a little code trickery to work through.
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Permalink Reply by MechatronicsLLC on -
Hi all,
We had originally thrown this module together: Analog Node
To use with our CHIMU AHRS to do what you are talking about. Its a little pricey for this group (the unit uses magnetometers instead of GPS for yaw) and no open source, but...
Here is a picture of it next to an FMA module:
As for the speed question, we allow an analog input of voltage to accomodate corrections for this if you have a pitot tube speed measurement signal available. However, both experience (see Chris's reply) and the math say that you can get by without speed input if you aren't planning on threading any needles :)
The quick math version is that your attitude estimate will be corrupted by centripetal acceleration if not accounted for. Centripetal acceleration is dominated by your forward velocity, and it only couples into Y and Z accelerometer errors via yaw and pitch rates (respectively). Since (typically) body z and y velocities for a plane are centered about zero, your y accelerometer will see just your yaw rate* body velocity as an error.
So quick, high rate motion could produce high errors (for a constant speed), but those typically don't last long, and if they do then the pilot is probably pretty good and doesn't need a wing leveler. Further mitigating the error build up is what Brian said basically, which is that your autopilot / IMU filter incorporates your accelerometer measurements thru its filter gains anyway, so its a minimized effect.
Sustained slower motion (say, a long loiter), could be worse as it lets a constant build up in the error states occur. If you are doing a 20 meter radius loiter at a speed of 10 m/s for, say 1 or 2 minutes (depending on your IMU filter), you'll still only build up 30 degrees of attitude error durng that time (say one minute), which the pilot should be able to handle, especially if he recognizes the possibilty.
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