The Falcon DS1 - BeagleBone Blue Drone (Part 3)

Construction

The hardest part of the build is coming up with a bracket to mount the BeagleBone on the Martian II air frame. In part 1 of these tutorials, I mentioned that there was an existing bracket design available. This doesn't fit for a number of reasons, including clashes with the XT60 socket, the propellers and the top deck of the air frame. But we will come to that...

Start off by putting the frame together. There is a very thorough video of how to do this on YouTube. They are using some different components (including the flight controller), but it does a great job of showing what goes where.

As shown in Figure 20, I labelled the front of the drone and motor number locations using electrical tape. This helps keep you oriented.

Figure 20. Martian II Frame Construction.

Once you have the arms attached to the bottom frame and the Power Distribution Board (PDB) mounted, you can add the quad 20A Electronic Speed Controller (ESC). The RaceStar ESC that I bought has mounting holes which line up nicely with the PDB. I assume that this isn't a coincidence but I admit that I fluked it. Figure 21, provides an overview of what the bottom layer of the drone will look like when we are finished.

Figure 21. Bottom layer Component layout

Figure 22, illustrates the Quad ESC in place above the PDB. Now comes the tricky part (for me, you will find it easy since I supply the bracket design), mounting the BeagleBone. Referring to Figure 21, you can see that there is only 10 mm clearance between the PDB and the propellers, so our bracket can't overhang much. In addition the XT60 socket and associated cables must remain accessible with the BeagleBone in place. If we are going to stick with a stock Martian II frame, the available distance between the top and bottom layers is 30 mm and with the ESC in place we have already used 20 mm of that. Finally, we want to keep the weight centred as much as possible.

Figure 22. Bottom layer components in place.

In the end, I wasn't able to satisfy all of these constraints. The BeagleBone is just too big, it requires 17mm even if you didn't have to make connections to the pin headers (which you do). In addition, to provide access to the XT-60 the BeagleBone needs to be offest from the centre line of the drone.

Typically with the Martian II build, the LiPo battery is attached to the top frame. Instead, I have placed it between the ESC and BeagleBone. As illustrated in Figure 23, it took 5 attempts but I eventually came up with a design that met most of the constraints.

Figure 23. BeagleBone mounting plate with LiPo.

The STL file for the mounting plate is available for download from Thingiverse.

Figure 24. BeagleBone Blue 3D Printed Mounting Plate.

Wire up the power and control cabling as per the schematics in Parts 1 and 2. As our build is a bit different to the standard Martian II, I will provide a few supplementary notes and guidelines based on my experience.

Mounting the Motors

We need to mount the four brushless motors to work out the correct wire length to the ESC. My RacerStar motors came with two sets of M3 screws, 8mm and 6mm. The thickness of the frame arms is 4mm, so use the 6mm screws to ensure they don't short out the motor coils. For now you can just do them up finger tight but eventually you will want to apply loctite to ensure that the motors don't fall off mid flight (bad).

The RacerStar ESC can be programmed using the BLHeliSuite software on PC. With that we can reverse the motor direction but you need compatible Flight Controller software. For our purposes, it is probably simpler to just swap two wires if the motor is going in the wrong direction.

My motors indicate their default rotational direction (clockwise or anticlockwise) by arrows on the top of the motor and different coloured propeller  nuts. The black nuts rotate clockwise and the red nuts anticlockwise. To minimise the amount of work, you should mount the motors in the configuration shown in Figure 25.

Figure 25. Required Motor Directions (X Configuration)

Once you have mounted the motors you can then wire them to the Quad ESC. While you are doing this, also solder the ESC battery connection to the appropriate pads (+ and -) on the Power Distribution Board and a connector to allow you to plug power into the flight controller.

The Falcon DS1 - BeagleBone Blue Drone (Part 2)

Control Wiring

The BeagleBone uses mostly JST connectors. We need the following:

• Four (4) of the 2-pin 1.5 JST ZH female connectors, with attached 150mm 28AWG wires, for the motors,
• Eight (8) of the 4-pin 1.0 JST SH female connectors, with attached 150mm 28AWG wires, for the encoders as well as the UART, I2C, CAN, and PWR busses, and
• Four (4) of the 6-pin 1.0 JST SH female connectors, with attached 150mm 28AWG wires, for the SPI, GPS, GPIO, and ADC busses.

These connectors are small but they are also fiddly to crimp and a bit delicate. Nevertheless this is what we are stuck with. Despite a LOT of searching, I was unable to find a complete diagram which indicated all of the correct connections for using the BeagleBone Blue as a flight controller for ArduCopter. The best I was able to find was the Imfatant version, which he provides as part of his description regarding how to get ArduCopter onto the BeagleBone. I have reproduced his image below in Figure 12.

Figure 12. BeagleBone Blue Connections (ref: https://github.com/imfatant/test)

I ended up doing my own drawing, to ensure I knew what connected to what. I needed this in order to work out the best layout on the airframe and what cables and connectors are required.

Figure 13. BeagleBone Blue Control Wiring.

Note that the front of the drone is towards the top of the diagram in Figure 13. The motor numbers match up with the nearest motor outputs on the QUAD ESC. The control wires for each motor (S1, S2, S3 and S4) then go to the corresponding servo/ESC connections on the BeagleBone Blue.

Now that we have the connections worked out we can load up some software and start testing our rig.

Loading ArduCopter on the BeagleBone Blue

The most comprehensive guide to loading the ArduCopter software onto the BeagleBone Blue is found at the Imfatant Github repository. There is also a summarised version of these instructions on the Mirkix Github repository (Mirko Denecke's port of ArduPilot).

Figure 14. Checking the BBB Connection to the Mac.

I don't intend to reproduce what has already been written in these guides but I will point out areas where things have changed or if I had difficulties.

PART 1 - Preparation

1. The first issue I found in the Imfatant Guide is that the recommended console image is no longer available at  https://rcn-ee.net/rootfs/bb.org/testing/. I used the latest version that I could find: https://rcn-ee.net/rootfs/bb.org/testing/2019-06-30/stretch-console/bone-debian-9.9-console-armhf-2019-06-30-1gb.img.xz Download this and flash it to a microSD card using Etcher (or equivalent). I used a Toshiba 16GB class 10 card so that I would have plenty of room and speed.
2. I inserted the SD card (it only goes in one way) and booted up the BeagleBone Blue (BBB from here on), by connecting it to my Mac via the micro USB connection. As suggested in the instructions I tried "ssh debian@192.168.7.2" but the operation timed out. It appears that there are two drivers that you have to download and install for this to work on the Mac. 
3. Download the two drivers: Network: https://beagleboard.org/static/Drivers/MacOSX/RNDIS/HoRNDIS.pkg and Serial: https://beagleboard.org/static/Drivers/MacOSX/FTDI/EnergiaFTDIDrivers2.2.18.pkg and then install them. The Serial driver may ask you to reboot your computer (it did for me).  The Beaglebone Blue creates a network connection and emulates an Ethernet adapter. Your computer will receive IP addresses 192.168.7.1 and 192.168.6.1. The Beaglebone Blue has IP addresses 192.168.7.2 and 192.168.6.2. You can check whether this has worked by looking at Networks in system preferences (ref: Figure 14).
4. You should now be able to ssh in ("ssh debian@192.168.7.2") without difficulty. Password is "temppwd" (see Figure 15 to view a successful first log in).  If you receive a “REMOTE HOST IDENTIFICATION HAS CHANGED” error, you have probably previously connected to a different computer on 192.168.7.2 (like another BBB). Remove the old ECDSA key with: "ssh-keygen -R 192.168.7.2".
5. Next thing you need to do is get WiFi up on the BBB. Follow the Imfatant instructions for this but I found that I had to power the device down and reboot before the changes would take effect. Make sure you "sudo shutdown -h now" before powering down. While you have the power off, CAREFULLY turn the antennas around so that they face off the board. They swivel so don't just try and bend them up and over. You can unplug and replug the antennas if you prefer. Once you power back up, you should have the central green LED lit to indicate a WiFi network connection (ref: Figure 16). 
6. To get the WiFi IP address of your BBB, I usually use "hostname -I" or "ip addr show wlan0". Alternatively, you can just have a look on your router. Terminate your usb connection and ssh back in using the WiFi IP address. The password is the same. Verify that you have an internet connection using "ping -c 3 google.com".
7. The remainder of the Imfatant Part 1 Preparation instructions went smoothly for me.

Figure 15. Logged into the BBB via USB.

Figure 16. Green WiFi LED on indicating a connection.

Part 2 - Installing ArduPilot

Follow the Imfatant instructions for setting up the ArduPilot environment configuration file, /etc/default/ardupilot.

The switch -A in the ardupilot config file maps ArduPilot's "Console" serial port (SERIAL0, default 115200) to a target IP address and port number of one's choosing. For example, this allows MAVLink data to be transmitted over WiFi for test purposes. This data stream is auto-sensed by ground control station software like Mission Planner (Windows) and QGroundControl (Windows, OS X, Linux, iOS and Android).

To get the IP address of your Mac, from the Apple menu pull down “System Preferences” and click on the “Network” preference pane. The IP address for your WiFi connection will be shown on the right (see Figure 17).

Figure 17. Getting your Mac IP address for WiFi.

For Windows, press the Windows Start key to open the Start screen. Type cmd and press Enter to launch the command prompt. Type ipconfig at the command prompt to check the network card settings. See Figure 18, you want the IPv4 address.

Figure 18. Getting IP address for Windows

The next tricky bit is getting the latest ArduCopter executable, built specifically for the BBBlue's Arm architecture, onto the BBB. The Mirko Denecke link no longer works but the Imfatant one will allow you to download arducopter v3.6.

Once you have the file downloaded, there are a few different ways you can copy it to the BeagleBone. You can use SCP, just copy it onto a USB drive and then plug that into the BBB, or you can use something like FileZilla. I took the last approach. Depending on permissions, this may be as easy as dragging the file across (Figure 19).

To check the current permissions on the file type, "ls -ld /usr/bin/ardupilot". My initial result was:

drwxr-xr-x 2 root root 4096 Aug  6 08:29 /usr/bin/ardupilot

Which is a problem because the directory's owner and group is root and we have FTP'ed in as debian. In the string displayed above, the first character indicates if we are looking at a directory (d), file (-) or link (l). After that, the folder and file access rights are divided into three categories:

xxxyyyzzz
xxx are access rights for the owner
yyy are access rights for the owner's group
zzz are the access rights for everyone else

The access rights symbols will indicate whether you can read (r), write (w), execute (x) or are missing a right(-). Based on our results, the only person who can write a file into this directory is the owner (i.e. root). We can use chmod to fix this. In terminal type: "sudo chmod o+rwx /usr/bin/ardupilot" The "o" switch applies this change to all users and we are adding (+rwx) read, write and execute rights for this directory. Check with "ls -ld /usr/bin/ardupilot". You should get something like:

drwxr-xrwx 2 root root 4096 Aug  6 08:29 /usr/bin/ardupilot

You should now be able to drag across the arducopter executable into the /usr/bin/ardupilot directory as shown in figure 19. Username and password is the same as for ssh. Use port 22.

Figure 19. Copying the arducopter executable to the BBB using FileZilla.

After this we "sudo systemctl enable arducopter.service" and rebooted the BBB. Job done! You can check that arducopter is enabled with "systemctl is-enabled arducopter".

In Part 3 we will start putting everything together.

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.