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.