Cascaded PID and MPC control for heterogeneous multirotor unmanned aerial systems
Creators: name: Hoshu, Ayaz
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400702
subject: Automation engineering
classificationCode: 400706
subject: Field robotics

description: Due to the recent technological advancements, multirotor unmanned aerial systems (UAS) have enjoyed much popularity over the past couple of decades in the multitude of military as well as civilian applications, due to their numerous extra-ordinary characteristics. Paramountcy of these UAVs was also realized during the peaks of COVID-19 pandemic. However, despite of the numerous characteristics, multirotors have been termed as less energy efficient providing less endurance as compared to fixed-wing and helicopter counterparts. As power needed for a rotorcraft to hover has the inverse relation with the rotor disc, therefore, for a provided diameter footprint, a single large rotor of helicopter energetically outperforms the numerous small rotors of the multirotor, within a same boundary.

This thesis presents a heterogeneous configuration of the multirotor unmanned aerial system (UAS) that features the combined characteristics of both helicopter and quadrotor in a single multirotor design, featuring the energy efficiency and endurance of a helicopter, keeping the mechanical simplicity, control and maneuverability of a quadrotor. By employing a single large central rotor to get the majority of the lift and multiple small arm canted rotors for control. Designing a stable and robust control system for such a dynamically complex multirotor configuration remains the main challenge as the studies previously carried out on these designs have successfully demonstrated energy efficiency but at the cost of degraded attitude control.

Cascaded control approach is proposed to provide the control solution for this heterogeneous multirotor as the cascade control system provides an effective solution to the system with complex dynamics and non-linearities. PID controllers are used in the cascaded structure. Model-based technique and the automatic tuning based controller design approach have been employed to design the PID controllers. Auto-tuning approach has the superiority over the other approach as the controller parameters are tuned through the experimentally acquired data using relay feedback experiments and system identification, incorporating all of the actual dynamics of the complex configuration, including sensors, actuators, and large rotor gyroscopics.

Considering the benefits of the model predictive controller of featuring multivariable system framework and ability to handle constraints, MPC is also employed in the cascade control structure. System identification is performed to experimentally identify the transfer functions for the heterogeneous multirotor. MPCs have been then designed through identified transfer functions of multirotor as SISO as well as MIMO system. Designed cascade control systems are experimentally implemented over multirotor prototype through a custom designed flight controller. The experimental analysis of the designed controllers demonstrates the stable and robust control performance along with disturbance mitigation capabilities.