Improving the PX4 software-in-the-loop multirotor UAV simulator accuracy

Abstract

Recent developments have enabled the mass production of cheap, high-performance multirotors. As a result of this, the multirotor has found a large variety of different uses. Testing new algorithms using real-life flights is costly in terms of time and potentially in terms of materials in the case of a crash. Simulators have quickly gained prominence as a tool in algorithm development. The accuracy of these simulators is vital to ensure the step from simulated flights to real-life testing is as small as possible. Current simulator models are held back due to a lack of properly identified multirotor coefficients. Often, the coefficients from a different multirotor are used, even if the new multirotor is vastly different in shape and size. Additionally, most researchers do not have a sufficient overview of which parts of the simulator are most important in providing an accurate simulator environment.

This research will give an overview of the different parts that influence the simulator results. The simulation of the motor controllers and rotor aerodynamics is identified as a major contributor to inaccuracies. The coefficients used in these two components will be found by performing a system identification process of the NXP HoverGames quadcopter. This identification process entails determining the mass, dimensions, inertia, motor response and thrust, torque, rotor drag and rolling moment coefficients. Experiments will be performed to find the correct coefficient values and to evaluate the impact that each effect has on the simulator's performance. A focus is put on ease of repeatability, such that other researchers can use the same process for their specific multirotor.

The theoretical background behind each effect in the motor controller and rotor aerodynamics models is discussed. This theoretical knowledge is used to perform experiments to find the coefficients for each effect and validate them. As a result, new coefficients for the motor controller simulator, thrust, torque and rotor drag were found. Additionally, this process showed that the rolling moment is much smaller than the errors currently existing in the model and this effect has, therefore, been ignored. The motor controller simulation is identified as the largest cause of inaccuracies in the current model. This simulation maps the motor commands to the output rotational velocity of the rotor and simulates the time response of this system. The formulas currently used for this do not provide a sufficient model of real-life behaviour. An analysis is done to gain a better understanding of what variables influence motor behaviour.

Finally, the new coefficients are put to the test in a variety of different trajectories to compare the trajectory tracking performance with the old model. This comparison consists of two parts. In the first part, a qualitative analysis is used to understand the difference in simulator behaviour. In the second part, a set of metrics are defined to quantify the difference in accuracy. This comparison process is used to prove that the new model provides significant improvements compared to the old coefficients, especially in the case of faster, more aggressive manoeuvres.