Hey, I had a little brainstorm last night after watching the DJI Wookong video. It's a sweet little system, but I wondered if AC2 could do that? So I put a battery on the end of one arm and it couldn't hold level. It came close but 3-4° error meant that it couldn't fly perfectly still. So during the ride in to work I thought of a way to rework the controller so it replicates my hand on the stick rather the the PI>PI rate controller we have now.
I've never been happy with the Outer I term. It never really seemed to be very effective. I started thinking that my hand is putting a constant bias to the stick to get the perfect stability, so I could easily replicate that with the outer loop's I-term. I re-routed the Integral to directly sum with the rate controller output. I flew it tonight and it does work. I can hold perfectly level with a 22003S battery dangling from the arm. I think this is exactly what is needed for better Hexa/Octa redundancy.
Enclosed is the patch that works with 2.0.54. Just replace attitude.pde with the linked file. Then set your STB_PIT_I and STB_RLL_I to .1, and your limit to something high like 30° or 40°
Let me know how it goes for you and please let me know how you might improve it.