Simple Distributed Flight controller network


Over the last few years of flying FPV I have flown one piece foamie flying wings of around 1 meter span pretty much exclusively.The main advantage of this type of aircraft is that the electronics can be distributed anywhere within the wing. The plane is stored assembled, therefore there is no need to assemble at the field. Servicing and accessing components is easy.

My main interest lies in soaring though. A 1 meter flying wing foamie loaded with electronics and sensors just doesnt soar very well!

The problem with FPVing a large soaring model is the issue of where to put the electronics and how to deal with all the sensor connections. From my flying wing experience, I know that for best rf range, it is optimal to separate the various components,in particular the GPS, RC receiver and video transmitter.  I also have bad memories of my first wing and tail FPV model, an EasyStar, where I put everything in the fuselage. I remember hours of fiddling with wire and connectors in the tiny fuselage. Each change of the flight-battery involved pulling wiring out and moving various components and connectors, which is when I decided to build flying wings in future. 

Recently I decided I really want a soarer type model but one in which I can place arbitrary electronics in the wings.

I bought a 12 foot Thermal soarer ( Hobbyking Aether ) and resolved to solve once and for all the problem of maintenance and assembly at the field.

I wanted to use a similar layout as I had come to use in my flying wings, putting various components in the wings, since I know it works well from an RF perpective and is easy for maintenance ( assuming you dont mind ironing solarfilm :))

I decided to use a distributed system, broken into subsystems with links where the aircraft needs to be physically disassembled for transport. Each part is to be connected with one connector. For the Aether that means wings to pod and tailboom to pod connections. (I made the Aether boom detachable so that it can be disassembled from the main fuselage pod for transport, but that now means making the elevator and rudder connections easily dismantleable)

  • In the port wing I provisionally put the Rc Receiver and airspeed sensor. There are also 2 port flapperon servos.
  • In the starboard wing I provisionally put the GPS, magnetometer and the 2 starboard flapperon servos

In the heading diagram the red edges are where there will need to be connectors. There is one connector for each wing to the fuselage pod conection and 1 connector for the tailboom, The black edges dont need to be disassembled at the field since they are all in the Pod .

For the tail "link" I decided to just run the Elevator and rudder servos direct from the flight controller so they dont need anything fancy, however, for the wings I figured I would make a "sub-hub" with a serial link to the main FC in the Pod.

The requirements on the sub hub were

  1. to provide bi-directional comms between the FC and sensors and servos.
  2. to provide power to the servos.

I looked at 3 comms choices. CAN, USB , UART. In each case the physical cable consists of 4 wires, Power, Ground and 2 signal wires.

USB would be interesting with the FC as the host . Might work well with a Raspberry PI FC in particular
CAN also looks interesting but decided to avoid it for now.
Both of these I should certainly investigate for the future, but for V1 I decided to use UARTs.
On the physically short links ( under 400 cm) a full duplex per wing baud rate of 1 M bit per sec should be achieveable with little overhead The sub hub can be attached to a PC for testing ( also true of the USB option). CAN seeems overall a bit heavyweight and a bus topology doesnt seem that great. (I think a star works better for a plane)

For  power to each sub-hub on this version,I opted to specify a minimum of 7 V and a maximum of 20 V at max 10 watts per link. That means the system can be powered direct from 2S to 5S flight batteries and each wing has 2 amps at 5V for powering the servos.

The actual sub-hub board has a mcu and a switching regulator on board to provide servo outputs, and various connectors to which can be attached some combination of a serial device, an I2C device, Rc Receiver and analog inputs, for the various devices that need to be connected.

With that in mind I am currently designing the sub-hub board.

Views: 1306


3D Robotics
Comment by Chris Anderson on March 19, 2016 at 7:58am

Interesting. Which FC are you going to use?


Developer
Comment by Andy Little on March 19, 2016 at 11:23am

Currently I am looking to use a Pixracer or a Raspberry Pi based controller, but the requirements look to be modest, since much low level work is offloaded from the FC.  The main requirement of the scheme is a FC with 2 spare serial ports. (For Raspberry Pi, it would be interesting to try USB instead.) One of  the serial ports can be the ex GPS port and, if available, one can reuse the ex SBUS port (if SBUS has TX and RC connected which I believe is the case for the Pixracer)

Another point is that a larger plane reacts quite slowly, so it should be possible to meet the requirements using a flight controller with a low speed/low power processor. (This project is targetted at a Thermal soarer after all so low power consumption is of interest)

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