I decided that I wanted to graduate from a thermopile based autopilot to a full IMU based system and the UAV DevBoard looked like it was going to scratch my itch. In the end, I wound up with it on my Multiplex Twister, an elapor foam electric ducted fan rc aircraft. Here's what I've done -
One of my motivations starts with the fact that I have flown an FPV Telemaster with an Ardupilot quite succesfully but use it primarily as a failsafe. I did, however, test the waypoint system to confirm it worked. I did have to modify the gains for the aircraft but all in all, I was completely satisfied - that is until the several mornings back in the spring when we had a heavy overcast cloud ceiling where the sky temperature was equal to the ground temperature and the thermopiles couldn't see enough difference to work. (Now that summer is here in full swing, that doesn't look like I'll have that problem again for a while)
Once I realized that the red version of the UAV DevBoard came in at a much lower price than the original, I took the plunge. I followed all the instructions, acquired the recommended programmer, and decided the first airplane was going to be my basically stock Easy Star with the only modification being the addition of ailerons. I compiled and programmed the board with the AileronAssist firmware and was pretty amazed by the results when just holding the aircraft in my hands. The DCM algorithm seems to work like a champ. I shook the aircraft pretty agressively, rolled it all around, and then left it propped up against the wall for about fifteen minutes and then came back and leveled it out, and all of the surfaces returned to their trimmed position. Of course, this still doesn't simulate the turning forces on the aircraft so I was going to have to hold my opinion until I actually flew.
Just looking at the control surface deflections, I decided I needed more and updated both the roll and pitch gains. Now it was time to fly. Since I was mainly interested in the airplane's behavior in the "stability augmentation" mode I didn't fully test the RTL capability. So when I engaged the system in flight, I quickly discovered that putting a stability augmentation system on an aircraft that is already pretty darned stable turned out to be pretty boring. I could tell it was working, but it wasn't like it had to work too hard to keep a stable airplane level.
I then realized that the UAV DevBoard would fit into my Twister with little effort. That would be a much more satisfying endeavor. So I did it, and flew it. When I flipped the switch on, I quickly realized that the gain for the Easy Star was too much for the Twister (since I didn't change it). The airplane started roll rocking with the bank angles getting larger and larger (and the roll rate going back and forth was pretty high) so I quickly turned the system off and brought the airplane in. Hey, at least this was exciting! I was out of daylight so there was no time to change the gain and get back in the air.
I knocked the roll gain down (back to the original default value) and when I did get a chance to get back in the air, the wings locked solid when the UAV DevBoard was engaged. I then made what too me was an almost comical discovery - I could only bank the airplane about 30 to 45 degrees (with full lateral stick deflection on the transmitter) and when I added elevator, the airplane would tend to climb while it turned very slowly. I hadn't really thought about the difference in how you turn something like a Telemaster or Easy Star versus a Twister. The Twister moves quickly enough so that I tend to bank 90 degrees and pull on the elevator to get it to come back around. Only banking 30 to 45 degrees makes it difficult to keep in the flight area. I found it entertaining to take an airplane, that although relatively easy to fly is very responsive, and damp out its performance so that it feels sluggish. Since it is a relatively fast airplane, you can't spend much time exploring the control response before it turns into a dot in the distance. So I did a couple of passes with the system engaged, but it was only on long enough to confirm that I didn't have the patience for it to turn around and come back. I wrapped up as the sun set again and landed safely.
So - I guess the next step is for me to better decide what I really want the system to do - I'm afraid if I just change the proportional roll gain back up so that it will allow a 90 degree bank angle, I'll get the roll oscillations again. I'll have to think about whether or not a PI control loop will cut it and if not, will a PID system solve the problem.
Not knowing when to leave well enough alone, I'd like to be able to add some telemetry and an airpseed sensor so those bring up a question. My understanding is that the spare dsPIC pins that are available are digital and do not support any A/D conversions. If that is true, and if I want to integrate other sensors, I'll need to have another processor do the A/D conversion and send that data to the RX pin of the unused/debugging USART. And when I'm ready to send telemetry, I can use the transmit pin on that USART. That sounds great except...
I'm not sure how to make the second USART work. It appears to be set up in the AileronAssist software to write debug data, but if I hook it up to an FTDI cable and monitor the serial port, I get nothing. When I put an oscilloscope on the Tx pin I get nothing. Is there something I'm missing?
Regardless, this hobby sure provides me with the ability to tinker, tinker, and tinker some more, which is exactly what I enjoy.