Can't get sonar working right

I’ve installed a Maxbotix MB1240 on my 330. When testing in Mission Planner, it works great. In-flight altitude hold when flying low is inconsistent. Sometimes I can fly over an obstacle or put my hand under it and the quad will rise. Sometimes not. But even when it does, it doesn’t lower to its original altitude.

Looking through the logs, it appears there is a ton of noise on the sonar. (At least I think “SAlt” is Sonar Altitude - documentation on the wiki says “SonAlt” but I can’t find that).

[attachment=0]Sonar log.PNG[/attachment]

I’ve done a number of things to try to eliminate this noise. I’ve made the noise filtering cable - shielded cable (it has both a foil shield and a braided shield around that) with the capacitor and resistor. I’ve made a small cone around the sensor and covered it with a thin layer of cotton (sort of like a microphone wind cover). I’ve completely covered everything except the end of my handmade cone. Noise remains.

Vibrations are minimal with AccX and AccY very seldom spiking beyond 1 or -1 on the graph.

With all of that, bench testing works great. As I move the quad around manually pointing it at objects of various distances away, it reports the correct measurement. But I just can’t seem to eliminate that noise.

Any help?

Hobby King F330 frame with DJI Flamewheel legs
Q-Brain 25amp ESC
Suppo 2212-10 motors
7043 props
Telemetry radio, GPS and compass on a mast.

Can someone confirm that “SAlt” is the item in the log for the sonar altitude? So I at least know for sure I’m looking at the right thing? I’m running Arducopter 3.2.1 and Mission Planner 1.3.31. I’ve searched the wiki and the documentation, but it is severely lacking.

I’ve wrapped everything near the sonar with tin foil. I ran the cable to the sonar through a ferrite core. Here’s my latest graph (again, barometer in red, sonar (if that’s what “SAlt” is) in green).

[attachment=0]Sonar log 2.PNG[/attachment]