The Falcon DS1 - BeagleBone Blue Drone (Part 1)

Flight Controller

Figure 1. BeagleBone Blue Flight Controller

This project will document the construction of a self built drone using the BeagleBone Blue as a flight controller.

An objective of the build was that the drone should be modular, open sourced hardware and software and include an API to allow mission control.

My preference would be to use a board that I already have experience with (e.g. Arduino or Raspberry Pi) as a flight controller.

Unfortunately even though ArduCopter firmware was originally designed for the Arduino, the consensus seems to be that it doesn’t have enough grunt these days (ref: https://dojofordrones.com/drone-flight-controller/).

I like the Navio2 & Raspberry Pi, as the hardware platform for the flight controller. The only problem is that it is expensive ($299 for the HAT alone). Also the Navio2 is not open source hardware.

The PXFmini is a pHAT designed for the Raspberry Pi Zero/Raspberry Pi Zero W but is also compatible with other models from the Raspberry Pi family. It was more affordable (approx. $74) but is now discontinued. The good news is that it is open source hardware and the schematics are available. You could develop your own HAT or pHAT based on this design. It would be great to get hold of one but I haven’t been able to track it down. An issue with this design is that all the soldering is surface mount, which is a pain without the right gear (and great eye sight).

I haven’t used the BeagleBone, but the BeagleBone Blue looks like a pretty good option. It is relatively cheap at $130 delivered and includes most of the sensors you need on the board. The exception is a GPS (the recommended module is the u-blox M8N GPS - $60 from Hobby King). There is an ArduCopter build available for the BeagleBone.

Based on the above I am going to try out the BeagleBone for my initial prototype. The build documentation is very sketchy, so there is going to be a bit of trial and error.

Air Frame

Figure 2. Power Distribution Board on frame.

For my prototype I propose using a Martian II 220mm frame, because that is what BeagleBone suggest in their half documented project. Half documented is probably generous, apparently they never actually got this design flying, nevertheless there is nothing like a challenge to get the juices flowing!

There is a 3D printable case, designed for the BeagleBone for use on a drone, available from Thingiverse (Figure 3).

Figure 3. BeagleBone drone mounting case.

There are plenty of videos documenting how to build the Martian II air frame and its various clones so I wont go into that in detail, except where you need to do something different to utilise the BeagleBone. This happens pretty early in the build.

Figure 4. Airframe with front and motor numbers.

The first thing that you will notice is that the yellow XT60 socket mounted on the power distribution board (Figure 2) prevents you from using the 3D printed case in Figure 3. So we need to do some modifications.

It is a good idea to mark your air frame with the forward direction and motor numbers (Figure 4). This will assist with layout and help ensure that you mount boards in the correct locations.

Technology Stack

The proposed technology stack is shown in Figure 5. We now need to work out the specifics, there wont be a camera in the initial build, in fact my first objective is just to get things working on the bench. I will then be in a better position to work out where the various modules can fit on the airframe. 

Figure 5. Falcon Drone Technology Stack.

Power Distribution

Figure 6. Turnigy 1300mAh 2S 20C Lipo Pack

All of the drones power comes from our 2S 1300 mAh Lithium Polymer battery (Figure 6). The specifications for our battery are:

Minimum Capacity: 1300mAh
Configuration: 2S1P / 7.4v / 2Cell
Constant Discharge: 20C
Peak Discharge (10sec): 30C
Pack Weight: 81g
Pack Size: 73 x 35 x 17mm
Charge Plug: JST-XH
Discharge plug: XT60

At 3.7V per cell our 2S battery will deliver a nominal 7.4V from the contacts on the power distribution board. The 2S indicates that we have two cells in series so the sum of the two cells voltage is the output voltage. As an aside, cells in parallel add to the batteries capacity (i.e. mAh) not voltage.

Figure 7. BeagleBone connected to LiPo

The BeagleBone Blue includes a 2-cell LiPo battery charger with balancing, and an LED state-of-charge monitor. The state of charge LED's are shown in Figure 7. They are just above the 3-pin JST XH battery charging connector.

One of our considerations when mounting the BeagleBone is that we will need access to the JST XH connector in order to get power off the micro-controller. We will also need access to the XT60 in order to power down the distribution board. Ideally the battery needs to be less than 50mm from the BeagleBone or we will need to extend the charging cables.

Figure 8. Racerstar Quad 20A ESC.

The other modules we will need to power are:

• The Racerstar BLHeli_S 20A Quad Electronic Speed Control - shown in Figure 8 (connects to the battery, receiver and the four motors). It doesn't have a BEC (Battery Elimination Circuit) to provide a regulated supply but the BeagleBone does.
• The UBLOX Micro M8N GPS Compass Module - shown in Figure 9. This module expects a regulated 5V on VCC but control voltages are 3.3V (see Figure 10). The BeagleBone supplies 3.3VDC and 5VDC power output via 4 pin JST connector.
• The FrSKY X8R 2.4Ghz SBUS Receiver. The receiver will also need to be connected to the regulated 5VDC bus.

Figure 9. The UBLOX Micro M8N GPS Compass Module.

The overall power distribution block diagram is shown in Figure 11. The 3 wire LiPo charging cable will connect directly to the BeagleBone as mentioned above. We will then have two power buses, 7.4VDC which powers the ESC and 5VDC to power the GPS and Receiver modules.

Figure 10. GPS Compass Module connections.

When building robots I have had problems when there is a common supply for the motors and the micro-controller. The current surge when starting the motor can bring down the voltage to the micro-controller and cause it to reboot. It may not be an issue with the drone since the motors are always on. The bench test will demonstrate whether this is going to be a problem.

In part 2 we will cover the control wiring required for our drone.

Figure 11. Falcon DS1 Power Distribution.