It seems to be a consensus from what I have read on forums that the Alt parameter (calculated from IMU acceleromters) and the BarAlt (Barometer reading) should match closely on a well set up machine. so far, the most likely culprit seems to be vibration.
Changing the InAV_TC parameter from default of 5 to 1 apparently will the altitude control loop twards the barometer (the correct alt data). This indicates the source of my problem is in downward swing of the ALT parameter. The control loop is responding as it should, and trys to increase the altitude based on the ALT information.
So I am going to run a few test and try to reduce the vibrations as much as possible. Running out of ideas.
Try two layers of vibration tape under the PixHawk
Change main rotor bades
Change tail rotor blades
inspect all shafts and bearings.
anything suspect will be changed.
Any Ideas or past knowledge will be greatly appreicated.
Remember than more damping foam is not necessary better, the mount may become to stiff or loose.
Also, check that the cables do not touch frame near AP, the cables can conduct vibrations well.
I did what you said and gave the wireing some more slack (service loop) also, I had a few that could have transmitted some vibration.
First suscessfull ALT_Hold and Loiter this morning before work. That was it, vibrations, vibrations, vibrations It was locked it very well. I was shocked with I hit the switch and it stayed put.
I doubbled up the Zeal tape under the PixHawk
New set of 695mm Z blades
Replaced the tail rotor blades with new 105mm Z’s
I have never seen Z vibs that bad. Usually it is the y that is the highest with a heli. You have something mechanically wrong in my opinion. Can you see it shaking up and down when it is in flight!!!
Those Zaxis vibes are a little high, but it should not cause a problem I don’t think. I’ve seen much much much worse. The usual target for Z-axis is +/-5 m/s/s, and he seems to have about 2, with only occaisional spikes to 5.
- not unusual on heli.