it's interesting to see someone has the same struggle as I do..
Since One month I have been trying to do the same thing as you did. Just a bit more ambitious though, We wanted to also do a 3D mapping, a radioactivity detection, on top of obstacle avoidance with control out of line of sight. Now we are sitting in the competition, with not even a quadcopter that is able to stay at same place. I feel terrible.
First off I must tell you, you might consider switching hardware. Yes the idea and description of Erle looks amazing --- This is the reason I chose it too. But the community is rather small and many things are not thoroughly tested, but simply transported from the ardupilot webpage.
To my knowledge, there has been no report of successful usage of px4flow on Erle brain board, however, there has been multiple bug report. Nobody has shown position control or velocity estimation so far. I doubt it works on Erle brain at all.
Erle is a tool to start discovering robotics, but not if you want to test the boundary of existing technologies, at least for now. I am considering switching to pixhawk 2 or other hardware and ardupilot combination for my future development, and I am tempted to suggest you the same. Check ardupilot supported hardware page. For more active community, go to discuss.ardupilot.com.
Reason? pixhawk 2 is a more established hardware with more active in-depth users. It doesn't natively support ROS, but object avoidance feature is supported from 3.5 ardupilot version, that has been tested to work on pixhawk 2. http://ardupilot.org/dev/docs/code-overview-object-avoidance.html If all you want to do is object avoidance, this would be sufficient, with some additional sensory like tera tower or other laser or proximity based sensors.
@patrick can definitely tell you more about this, or anything in general. He has been amazingly helpful.
For sure we can have a skype call. Drop me a mail at firstname.lastname@example.org. I will gladly share what I know.
Robotics is not yet plug-and-play, but full of black boxes and invisible magic. So let's struggle our path through.