Problem maintaining pitch when accelerating / decelerating.

I just finished up some initial flight tests using the ArduIMU+ V2 in my Skywalker airframe.
This was my first flight tests using IMU stabilization insteed of thermopile sensors. After some PID adjustment I have nice and stable stabilization of the airplane. But... Whenever I accelerate quickly the plane goes into a dive. Or if I decelerate it pulls up. The plane is trimmed and motor mounting angle is good, so it is not that. It does not seem to be a problem with vibrations. I have mounted the IMU using vibration pads, and if I slowly accelerate the IMU maintain the correct pitch at any throttle setting. Same with deceleration. But punch the throttle and the plane goes into a steep dive. Or sharply pulls up if I let go of the throttle quickly.

My setup is the ArduIMU+ V2 with unmodified v1.7 firmware and the ublox gps.
I am currently focusing on stabilization so the only autopilot logic in use, are PID loops for elevator and ailerons using roll and pitch from the IMU as input.

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John,

The simple navigation PID control loops we are using will not be able to handle the case where ground speed approaches zero. We do a gain weighting/de-weighting based on tailwind/headwind but it will work over a limited % range from nominal speed.

You need to either (1) Fly at a faster airspeed, (2) Fly in less wind, or (3) rewrite the navigation algorithm to handle this extreme case. We did not anticipate anyone trying to navigate waypoints with zero ground speed.
Maybe you shouldn't get rid of the pitot/static system and rely on GPS GS only?!?
Doug,

I haven't seen any posts lately related to the shield V2 and problems using it with the IMU. Do you know if there has been any progress identifying the problem (I.E. is it hardware or software)?
Nathaniel - by happy coincidence I just finally today got a blue version of the shield. Could you message me and remind me what the problem was...
Doug,

Basically the problem is that when you add the shield between the AP and IMU you lose GPS data in the serial stream. There are a couple of threads that have the details here and here. I look forward to hearing what you have to say!

Thanks for looking into it

Nathaniel
Is it possible to calculate the linear acceleration/deceleration by using the SOG info from the GPS and substract it from the DCM acceleration vector? Will the GPS update rate and latency simply too slow for this purpose?
Yes, the gps latency is too slow.
Nathaniel, Dr. Black and everybody -

Please see my reply here - http://diydrones.com/forum/topics/imu-v2-to-ardupilot?commentId=705...
I will probably get jumped all over for this comment, aswere dealing with the symptoms, not the desease here.

but....Occams Razor 'the simplest solution is probably the best'

With our 'Rubber Johny' (please dont ask! :)) (an old twin star, covered in glue and tape that has been used as a test bed for sub systems on most continents instead of UAS) this problem was evident, when my CG was where its supposed to be, but with the gains tuned, and MOST importantly moving the CG forward by 1inch, this problem has gone completely.

It makes the aircraft stoogy for RC flight, but its not used for that, were not trying to do rolling circles with over a key nailed to it, in Automode, its straight as a die, no oscillations.no lossof height, with only a 10-15 foot vario on alt...

if you cant move from inside the box, think from outside it...


just a simple workaround, with very predictable results (also tested in other airframes to prove the theory with exactly the same conclusion.....works)

mike.
Your IMU is simply endangered by transient stalls, which is followed by acceleration along an axis not perfectly colinear to the fuselage.
You can induce those events after:
-too dynamic/deep turn
-ending climb too abruptly (while cutting throttle too much)
-encountering wind shear above trees or in strong thermals

No easy single solution to counter all those scenarios, and the
COG placement only hides the problem,
LOL.......not an un-expected responce.

accurate as always......

i just wasnt going to go into the full physics of what is occuring during that transitions of flight by the causality and reaction.

(I wasnt suggesting that making the aircraft way to nose heavy was the solution by any means!)

Mike.
Just wanted to let you know when to hunt for the issue. All is about IMU tuning and extra math.

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