E-42/members/joao-gil/reports/sitRep3.md

SITUATION REPORT 3 (31/10/2022)

How we can introduce a Rocket Model to a Pixhawk-based board

Introduction

This report aims to study the viability of using a Pixhawk board as the central controller of a model rocket. The system must be able to capture relevant data from sensors, send telemetry information to a ground station, store logs in persistent memory, and trigger parachute systems.

---

High-level perspective of a Flight Stack

To understand the requirements of the model rocket system, I'll present an overview of a typical flight system for drones, since the architecture is very similar.

Fig. 1 - System Overview

In the image above, to translate the system to the context of a rocket, the only component that differs is the Drone Hardware. This component represents the elementary drone components, such as the battery, the motors, and the GPS. Thankfully, some of these elementary components are common to both drones and rockets.

Drone Section

Now focusing on the Drone section:

Fig. 2 - Drone Overview

As we can see, above the Drone hardware is the Flight Controller Hardware. This is the responsibility of the Pixhawk Board, which is the object of study. Focusing on the Pixhawk, the sensors and components from the layer below (GPS, telemetry modules, etc.) hook into it, forming an essential hardware hub. This hub is then used by the Software layer to control the drones' basic components.

Regarding the top layer, the most common Flight Controller Software are:

1. Ardupilot
   • Oldest software
   • More examples are using this software
   • Operates under the GPL license, i.e any changes made to the source code are required to be pushed back to the master branch
2. PX4
   • Created based on Ardupilot
   • Feature-wise they seem to equivalent
   • Official documentation available.
   • Operates under the BSD license, i.e changes made to the source code don't need to be updated to the master branch

For our system, any of the two software should work. Based on the official docs, I prefer the PX4 organization.

Simulation

Thankfully, it is possible to test the Flight Controller Software without acquiring the hardware, through STL (Software In The Loop).

Fig. 3 - STL Simulation

Ground Control Station

Now focusing on the Ground Control Station:

Fig. 4 - Ground Control Station

In terms of hardware, the ground station just needs a telemetry module, identical to the USB module represented in the figure, that will communicate with the antenna in the rocket, using the MAVLINK protocol.

In the software layer, resides the user interfaces. The software represented here are essentially GUIs that utilize the MAVLINK protocol. Namely, QGroundControl, APM Planner and Mission Planner. The first 2 are for Linux, while the last one is exclusively for Windows.

Additionally, this layer also contains packages that can control vehicles, with the MAVLINK protocol through a coding language. In other words, these packages allow you to treat Ardupilot and PX4 as an API to control the vehicle. Here we have represented the package DroneKit.

---

PX4 Deep Dive

As mentioned above, PX4 is one of the most popular Flight Controller Software. For that reason, we will explore it in more detail since its supported features are very similar to Ardupilot and the documentation is easier to follow.

Supported Vehicles

Although PX4 supports a high variety of vehicles, model rockets are not part of that list. Nonetheless, I believe that is not an issue since this list is targeted for the usage of the software as a Professional Autopilot, that is, controlling the vehicle's trajectory and moving it as desired. In our use case, that feature isn't required so it should be fine as long as the features mentioned in the Introduction are supported.

System Architecture

The PX4 documentation provides a high-level overview of a typical PX4 system based around a flight controller.

Fig. 5 - PX4 System Architecture

Note that this architecture is similar to what we wish to achieve, as it contains most of the sensors required for the model rocket. The missing pieces in the diagram are the parachute triggering and persistent storage, which are also supported and documented.

Flight Controllers

There are many Pixhawk boards to choose from, such as the Pixhawk 4. The comparison and analysis of the most suitable board should be made in collaboration with the Electrical Engineering Department.

Supported Sensors

It is confirmed that the Pixhawk + PX4 combination supports the sensors and radio systems we are looking for. Anyhow, there are a lot of options to choose from and, once again, contact with the Electrical Engineering Department is needed to reach a consensus.

---

Conclusion and Next Steps

After analyzing this information, Pixhawk seems to be a promising board to introduce in our model rocket. However, the current information isn't enough to reach a conclusion. To do so, we must convert this theoretical information into a practical example. Thankfully, the Software in the Loop solution mentioned in this report provides a way to simulate the Pixhawk and the rocket model. Thus, the next steps involve testing this PX4 and Ardupilot software, to ensure they fit our needs and are capable of adapting to our scenario.

References

1

2

3

Published by João Mesquita