* The accelerometer data is very noisy. I'm not sure he's using proper anti-aliasing filters.
* The X&Y accel data is certainly not constantly zero as the "theory" would have it. What I think is happening is this: as the quad is commanded to rotate, one motor increases its RPM, resulting in much more noise in the X&Y channels (but not as much in the Z axis for some reason).
* Having said that, after analyzing the spectrum of the data via FFT tools, I am seeing a significant low frequency component that is present in both the gyro signal and the X&Y accel signals, which could mean that the accels are responding to the tilt (but it could mean other things, like both signals are responding to some other stimulus, or aliasing from the motor, or just that the "noise" from the faster-turning rotors is correlated with the tilt.
* I do plan on running the data through a low-pass filter, to see if cleaning up the "noise" shows me anything.
Also, I emailed my question to Dr. Robert Mahony (whom Bill Premerlani credits with the basic concept used in the DCM algorithm). Dr Mahony wrote back "This one twists my brain also. Firstly - the complementary filters work extremely well - so there must be an explanation." Interesting (and somewhat disheartening) to see that even the experts are scratching their heads! But he then goes on to say that, on average, the thrust vector has to match the gravity vector, as long as the vehicle is mostly hovering. So I think he's acknowledging that the accels are only reading the thrust vector.
Replies
I was able to get a quad owner (Mikro - http://aeroquad.info/bin/view) to give me some data (http://www.rcgroups.com/forums/showpost.php?p=12890721&postcoun...). I have yet to complete the data analysis, but here are a few findings from what I've looked at so far:
* The accelerometer data is very noisy. I'm not sure he's using proper anti-aliasing filters.
* The X&Y accel data is certainly not constantly zero as the "theory" would have it. What I think is happening is this: as the quad is commanded to rotate, one motor increases its RPM, resulting in much more noise in the X&Y channels (but not as much in the Z axis for some reason).
* Having said that, after analyzing the spectrum of the data via FFT tools, I am seeing a significant low frequency component that is present in both the gyro signal and the X&Y accel signals, which could mean that the accels are responding to the tilt (but it could mean other things, like both signals are responding to some other stimulus, or aliasing from the motor, or just that the "noise" from the faster-turning rotors is correlated with the tilt.
* I do plan on running the data through a low-pass filter, to see if cleaning up the "noise" shows me anything.
Also, I emailed my question to Dr. Robert Mahony (whom Bill Premerlani credits with the basic concept used in the DCM algorithm). Dr Mahony wrote back "This one twists my brain also. Firstly - the complementary filters work extremely well - so there must be an explanation." Interesting (and somewhat disheartening) to see that even the experts are scratching their heads! But he then goes on to say that, on average, the thrust vector has to match the gravity vector, as long as the vehicle is mostly hovering. So I think he's acknowledging that the accels are only reading the thrust vector.
- Roy