I was not originally intending to have a flight controller in my Fohdy 40″ wing. However the more I thought about it, the more I realized that autonomous Return To Launch (RTL) functionality is a valuable insurance policy for any aircraft that may be flown beyond the confines of the local flying field. I started looking at the options, which can roughly divided into two categories: stabilizers with basic RTH capability and autopilots with more advanced mission planning capabilities.
Stabilizers with Return To Launch
In this category I considered the Arkbird Tiny, Bigaole BGL-6G-AP, Cyclops Storm, and Pitlab Dedalus. I didn’t consider the Arkbird Tiny and BGL-6G-AP to be sufficiently configurable – the only configuration is to adjust the pitch, roll and yaw gains using trimmers. They don’t provide any means to configure of the RTL altitude, which could be disastrous in the mountainous terrain of the KwaZulu-Natal midlands. The Cyclops Storm is an interesting proposition since it is configurable via the OSD and includes a (non-configurable) OSD as a bonus. The Pitlab Dedalus is configurable through a PC application as well as a small button/display interface unit that is supplied with the unit. I really liked it, although it is the most expensive option in this category (about USD 150).
However the big problem with all these stabilizers is that they are designed to fit in long, narrow fuselages, and have to be positioned so the long axis is oriented in the direction of flight. This is a problem in the Fohdy, where the stabilizer or autopilot should fit in the electronics bay behind the battery, where it will be close to the centre of gravity. The available space is 160mm wide but only about 38mm from front to back after taking into account the space required for the planned 5200 mAh battery. There are spars in front of and behind the battery bay, so there’s not much room for expansion.
Available Space in Electronics Bay
I communicated with Pitlabs about this and Piotr Laskowski responded rapidly and helpfully but said that unfortunately it was not possible to allow installation in other orientations. I don’t believe the Cyclops Storm is still being actively developed.
Autopilots with Mission Planning
The full mission planning autopilots that I considered were the Arkbird Autopilot 2.0, Cyclops Tornado, Feiyu Tech FY-41AP Lite, Eagle Tree Micro Vector, mRobotics x2.1, 3DR Pixhawk Mini and mRobotics Pixracer.
Arkbird Autopilot 2.0
This is a full featured autopilot with integrated OSD that runs proprietary software. It includes a current sensor, M8N GPS and airspeed sensor. Unfortunately it can only be oriented lengthwise, and is 60mm long (excluding the connectors that protrude out of the front and the back) so it won’t fit into the available space.
Cyclops Tornado
The Cyclops Tornado was excluded because, like the RTL stabilizers it cannot be rotated and would not fit into the available space, and it’s functionality is quite limited for the price. The others would all do the job adequately, although I wasn’t too comfortable with the FY-41AP Lite’s proprietary software (the Micro Vector software is also proprietary, but is much more capable and widely used).
Eagle Tree Micro Vector
The Micro Vector stands out because it has an OSD, although it is one of the most expensive options ($193 including GPS and power sensor). Although it would fit into the available space, it has to be mounted in a fixed orientation (right side up with arrow facing forwards). In this orientation the receiver input and servo outputs are at the front, and the power and video connections at the back. This would be problematic due to interference with the front of the bay and the battery, and the controller would be facing away from the bay cover which is on the bottom of the wing so the port labels would not be visible through the hatch.
3DR Pixhawk Mini
The 3DR Pixhawk Mini is the least expensive OEM solution ($92 including GPS and PDB). It fits nicely into the available space and it runs PX4, which allows the controller to be rotated. However the Pixhawk Mini does not have any AUX ports, which limits its functionality since the current Wing mixers in PX4 only provide elevon and throttle outputs on the main ports; AUX ports are required for any additional functionality. More importantly, I read a sad tale of a Pixhawk Mini suffering electrical failure supposedly due to an inadvertent short on a power supply input, which suggests that there is insufficient protection on the supply rails and since the schematic diagram for the PixHawk Mini has not as far as I know been made available, I had no way to check this.
mRobotics x2.1
Postscript: see my full review of the x2.1.
This led me to have a look at the schematic diagram for the mRobotics x2.1, which I had originally discounted since at about $120 for the FC and GPS it is a bit more expensive than the Pixhawk Mini. The first thing I noticed was the LTC4415 dual ideal diode which switches between the USB 5V supply and the main power input. Nice, you’re not going to blow the board by accidentally shorting out the USB VBus when it’s powered from the main supply, and the LTC4415 does not have the voltage drop associated with discrete diodes.
Then I saw the multiple independent low noise LP5907 ultra low noise LDO 3V3 regulators for the FMU, IO and Spektrum Satellite, all with their own short-circuit and thermal protection. This means that the failure of one of these subsystems won’t affect the others. For example, if the Spektrum Satellite 3.3V line was shorted to ground then the flight controller would continue to function normally so your RTL would still work and even the Spektrum port would return to normal as soon as the short was removed – I wouldn’t like to try that with other mini flight controllers!
Then I wondered about the 5V line and found a TPS2062 power distribution switch which provides separate, switchable, current limited and short-circuit protected 5V supplies to the RC Receiver and the other external peripherals. This means that if one of the external peripherals (for example, the GPS) fails and shorts the 5V supply to ground, then the flight controller will cut power to the peripherals but the flight controller itself and the RC receiver will continue to operate so the aircraft will remain flyable. Similarly, if the RC receiver fails and shorts the supply, then the flight controller, GPS and other peripherals will continue to operate so the aircraft can return safely to the launch point.
I’ve designed electronics for military applications, and I’ve also sat on design review panels – and I can tell you, these guys really know what they’re doing! What blows me away is that you could save a few dollars by replacing the ideal diode with a discrete Schottky diode, using a single 3.3V regulator for the FMU, IO and Spektrum Satellite and omitting the power distribution switch and, in normal operation, nobody would even notice! Of course if an external fault occurred then the flight controller would fail and the aircraft would be lost. So kudos to mRobotics for sacrificing off their bottom line to give us a fault-tolerant flight controller.
The x2.1 is one of the slimmest boards at only 30.5mm wide and will easily fit into the electronics compartment. It runs PX4 (or Ardupilot) so it can installed in any orientation. It has AUX outputs as well as MAIN outputs, which means I can operate additional devices like my PWM controlled HD camera without having to write a custom mixer. And the servo outputs are on standard 0.1″ pitch headers, so I won’t require the additional breakout board that would have been needed with the Pixhawk Mini.
mRobotics PixRacer
The Pixracer, also from mRobotics, is oriented more towards quads. In order to minimize size it has only a single CPU, unlike other members of the PX4 hardware family (which includes the Pixhawk and x2.1) that typically have separate Flight Management Unit (FMU) and Input/Output (IO) processors. As a consequence it lacks AUX outputs, which would make it more difficult to configure aux channels for an aircraft under PX4 (although the quad mixers remap two aux channels to MAIN5 and MAIN6, the aircraft mixers don’t do this). Due to the small size, various I/Os connect to all four sides of the PCB, which would also make it difficult to fit it into the Fohdy’s electronics bay.
Conclusion
The mRobotics x2.1 is the best choice for this application. It will fit into the electronics bay and does everything I require (and a lot more besides). Although It’s more expensive than my second choice 3DR Pixhawk Mini, it’s well worth it for the fault-tolerant design – the difference is negligible compared to the cost of losing an aircraft.
mRobotics x2.1