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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Thanks, this should help the guys move forward.

The problem, mostly is that when fault finding poeple generally tend to change more than one variable in the equasion at once, so its hard to determine what changed what....and why.....just tryong to get people to start simply and work up, NOT go for the most complicated questions before starting at teh beginning (im terribly guilty of this too, always taking the path of most resistence! :))

excellent work Krysztof.

Mike.
No time to really work on the problem, but after reading your posts I am convinced the IMU is the problem.

So my conclusion is:
- Using lower gains the pitch problem becomes manageable combined with slower throttle changes.
- Let the autopilot control throttle to make sure ground speed never approach zero.
Hi John,

Just a thought here, couldn't we scale the gain based on the change in throttle input over time? For example, if we set a threshold for any 20ms time period of X where "X" is a value determined to cause an undesirable response to acceleration, then if that threshold is exceeded the gain is scaled to dampen the effect. Just a thought I wouldn't even begin to be able to suggest how to code it.

Nathaniel
It should work if you exclude external factors. The main problem for me is that I live by the coast in Norway, and get lots of currents straight from the North sea. So I pretty much have to fly in winds that you normally would not fly in, or else I would hardly get to fly any at all. The problem with changing winds and gusts in particular is that for the accelerometer they read like hitting a wall. And with the IMU lifting the nose as a result of deceleration things only get worse.

A possible solution I can think of would be to look at the pitch gyro and factor in possible drift over time so that you have an approximated reference angle to compare with the accelerometer corrected pitch (and then reset the reference angle when you have good IMU conditions). But I don't like this approach since it would be a band aid for something the IMU should already be doing.
John,

The wind is our friend, it makes us better pilots! I love flying my Sig Somethin' Extra in a good 20MPH wind, it's like flying a kite against the wind, and holding on to a missile with the wind.......landing is a bit of a challenge though, once you get into ground effect it's like the plane gets sucked back to mother Earth!

I see what you mean by external factors, scaling the gain based on throttle wouldn't do much for wind drift, or in your case, being literally thrown down wind!

Keep us posted on any efforts you make to battle the wind in the willows, in the mean time have fun with it!

Nathaniel

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