Connecting a Raspberry Pi Zero W to the Hovergames Drone Kit

In my submission for the NXP Hovergames 2 competition, I struggled to find resources that made the process of connecting a Raspberry Pi to the RDDRONE-FMUK66 clear (I wasted a lot of time on it). So this project is my effort to communicate what worked, and some of what didn't work, when connecting. I used a raspberry pi zero w, but I would imagine that this would work on most of the later models of pi's. This process will probably also work for connecting to a different flight controller, such as a Pixhawk.

I've also added a brief section on setting up an ad hoc network on the raspberry pi.

Initial Setup

Begin by flashing the FMU with the PX4 firmware image. This is described very well, for the Hovergames Drone Kit in the NXP gitbook. It is also useful to set up either a Linux virtual environment, or use a Linux machine, and follow the gitbook to install PX4 development tools and build PX4. You'll also want to install a simulator, like jMav sim. This is all out of scope for this guide, but I found it simplest to just use NXP's preconfigured virtual machine.

Next write the Raspbian OS image to an SD card and enable ssh. Make sure you are able to connect.

Now, fire up your FMU and connect through QGroundcontrol. In Settings > Parameters, set MAV_2_CONFIG to TELEM 2.

Physical Connections

Power:

To start with, there needs to be a method for power the raspberry pi off of the drone battery. I used the DROK 90010 5V DC buck converter, with the input side connected to the battery output via the drone's power distribution board, and the output connected to the raspberry pi. I soldered jumper wires to pins 4 (5V in) and 6 (Gnd), which could then be connected with WAGOs or wire nuts to the buck converter. You could also just solder the converter's output directly to the GPIO pins.

Serial Connection:

I opted to connect to the telem2 port on the FMU via one of the raspberry pi's UARTs. To do so, take a JST-GH 4 pin connector (with wires, cut and stripped on the non-connector end), and solder the correct FMU serial pins to pins 8 (Tx), 10 (Rx), and 9 (Gnd) on the raspberry pi (Tx on the pi goes to Rx on the FMU, and vice versa). Refer to the NXP gitbook for pinouts of the telem2 port.

• Pin 1 is VCC,
• Pin 2 is Tx
• Pin3 is Rx
• Pin 4 is Gnd

When looking top down at the FMU, with the PWM ports pointed toward you, pin 1 (VCC) is on the left side of the telem2 port. Don't connect to the VCC pin on the FMU - the pi will not get sufficient power from the FMU, and with the buck converter connected, connecting the VCC pin could fry something. It is probably best to cut the wire that is connected to pin 1 (VCC).

Software and MAVSDK-python

First disable Linux serial console and disable bluetooth by adding

dtoverlay=disable-bt

to the end of /boot/config.txt. Then run

sudo systemctl disable hciuart

MAVSDK-python

You no longer need to install mavsdk-server separately, or build everything manually. All you need to do is run:

pip3 install --user --upgrade mavsdk

Then any time you call await drone.connect use the following for your connection line (note the reference in the system address):

await drone.connect(system_address="serial:///dev/serial0:921600")

Make sure your connectors are attached, power up the drone, and try connecting using any of the mavsdk-python examples (probably the ones that only arm the drone, or read telemetry data.

--Brief Note on USB connection--

I didn't try it, but refer to this github issue, where the following code is referenced for connecting through USB:

await drone.connect(system_address="serial:///dev/ttyUSB0:921600")

Potential Issues

Connecting

I ran into problems, when connecting to the drone via mavsdk-python, with the following block of code (included in many of the examples):

 async for state in drone.core.connection_state():
        if state.is_connected:
            print(f"Drone discovered with UUID: {state.uuid}")
            break

I don't really have an explanation for it, but this very often created an error on connection. After removing it and just having

print("Waiting for drone to connect...")
await drone.connect(system_address="serial:///dev/serial0:921600")

seemed to work well, and I had no issues.

The simulator is not the same as the real thing!

Be careful of takeoff speeds, altitudes, landing speeds, etc. In reality, the drone did not behave exactly as expected. Trusting it to do some maneuvers, such as landing, without first tweaking the code slowly and having a good method to take over, can lead to crashes

--Ad Hoc Network--

It may be useful, as it was for me, to set up an ad hoc network on the raspberry pi. This will allow you to connect to it remotely when in the field. Keep in mind that once the ad hoc network is up and running, the pi will not have internet access, and it is difficult to escape to return to normal interfacing without being able to connect to the ad hoc network - so make sure your computer can! Otherwise, you may have to re-write the Raspbian OS. Do the following to set it up.

cd /etc/network

Back-up the original interfaces file

sudo cp interfaces interfaces.orig

create a new interfaces.adhoc file

sudo cp interfaces interfaces.adhoc

open it with a text editor

sudo nano interfaces.adhoc

and replace the code with the following:

# interfaces(5) file used by ifup(8) and ifdown(8)

# Please note that this file is written to be used with dhcpcd
# For static IP, consult /etc/dhcpcd.conf and 'man dhcpcd.conf'

# Include files from /etc/network/interfaces.d:
source-directory /etc/network/interfaces.d

auto lo
iface lo inet loopback

iface eth0 inet dhcp

auto wlan0
iface wlan0 inet static
  address 10.2.1.1
  netmask 255.255.255.0
  wireless-channel 1
  wireless-essid PiAdHocNetwork
  wireless-mode ad-hoc

To enable the ad hoc network, run

sudo cp interfaces.adhoc interfaces

and also run

sudo systemctl stop dhcpcd.service

Reboot. After that, you should be able to connect to the adhoc network and ssh in like normal. To return to normal wifi connectivity, do the following:

sudo cp interfaces.orig interfaces

and run

sudo systemctl start dhcpcd.service

then reboot.