Design and control of an overactuated UAV for application in urban airspace

The future of low altitude urban airspace utilization is poised to be transformed by the increasing dominance of small Unmanned Aerial Vehicles (UAVs). Their application can essentially be categorized as a threefold problem: establishing the regulatory principles for UAV operation, identification of the infrastructure needed to monitor the UAVs and ensure they adhere to the rules, and laying out the minimum performance requirements of the platforms so that they are capable of meeting the flight standards. This work addresses the pressing challenge of enhancing the performance envelope of small UAVs. The primary aim of this research is to develop an innovative overactuated multirotor system that augments UAV maneuverability, fault tolerance and operational safety. A crucial aspect of this study is overcoming the inherent underactuation limitations in standard quadrotors. This challenge is addressed through active propeller tilting, investigated in-depth through extensive real-world flight tests and simulations. The developed quadrotor, aptly named QuadPlus, efficiently combines actuator redundancy and thrust vectoring via propeller rotation in a compact package, compared to its state-of-the-art counterparts. Furthermore, a cascade control methodology based on incremental control allocation is devised to ensure stable attitude and velocity tracking even in the presence of actuator saturation and external disturbances during flight operations. The study showcases the successful realization of the QuadPlus multirotor system with an impressive 12 degrees of freedom. The innovative design allows for independent bi-axial tilting of 115 ° and 180 ° about two perpendicular axes while keeping the mechanical complexity relatively low. Real-world flight tests demonstrate the efficacy of the control methodology, showcasing stable hover and trajectory tracking in a desired hyperplane with accurate attitude tracking. Different test cases demonstrating the capability of QuadPlus to achieve symmetric (20 ° roll and pitch) and asymmetric (20 ° roll, 10 ° pitch and vice versa) attitude with static hover are presented. It is also tested against Level 3 wind conditions of 4m/s and actuator saturation. The extensive bi-axial tilting capability of the system enables precise force production of up to 9N for contact-based tasks, presenting promising potential for inspection-related applications. The research also significantly contributes to ensuring fail-safe UAV operations. A centralized fault-tolerant control (FTC) framework based on nonlinear model predictive control (NMPC) is proposed to address potential propeller failures, effectively mitigating attitude deviations and enhancing overall system control. The FTC framework is validated through intensive simulations, proving its effectiveness in maintaining stable flight even in the presence of failures. The system is successfully tested against a single propeller failure while following an attitude of 30 ° each in roll and pitch. It is also successfully tested for failure recovery in aggressive flights following an 8-shaped trajectory with the attitude varying between ±30 ° for roll and pitch, and ±25 ° for yaw. The corresponding maximum deviations from the commanded trajectory are observed to be within acceptable limits. Additionally, the research introduces a novel approach for on-board state-of-health (SoH) diagnostics of the battery, a critical component of the multirotor system. The energy equilibrium approach enables better management of battery performance, ensuring safe and reliable UAV operations while avoiding rapid performance deterioration. Experimental work is performed to validate the model. However, further analysis onboard QuadPlus is presented as future work as part of realizing the fault tolerance capabilities. It is hypothesized that the integration of the SoH diagnostics into the flight controller will help keep the current drawn within the safe functional limits of the battery. In summary, this research substantially advances the field of UAV technology by presenting a groundbreaking solution to the challenges posed by low altitude urban airspace utilization. The innovative overactuated multirotor system, QuadPlus, coupled with the fault-tolerant control framework and on-board component diagnostics approach, holds immense potential for inspection-related applications and interactions with the physical environment.Doctor of Philosoph

QuadPlus : design, modeling, and receding horizon-based control of a hyperdynamic quadrotor

The maneuverability of standard quadrotors with coplanar propellers is limited by their inherent underactuation. To overcome this challenge, in-flight, active propeller tilting has been widely investigated in the literature. However, biaxial propeller tilting that renders extensive thrust vectoring has not been explored much owing to the ensuing mechanical complexities. Therefore, this paper presents an innovative design capable of achieving independent bi-axial tilting of 100° and 180° about two perpendicular axes while keeping the mechanical complexity relatively low. The developed quadrotor, aptly named QuadPlus, efficiently combines actuator redundancy and propeller rotation in a compact package, compared to its stateof-the-art counterparts. The hyperdynamic QuadPlus with a total of 12-DoFs can control its attitude independent of the position, thus enabling effective maneuvering through narrow spaces. Moreover, a novel cascade approach comprising of a high-level nonlinear model predictive control (NMPC) algorithm is adopted to obtain the optimal actuator configuration for an underdetermined system while dealing with the physical constraints. Also, proportional-integral-derivative controllers are employed at low-level to track attitude references generated by the navigation algorithm. Finally, with the help of realistic Gazebo simulations, the efficacy of the system is demonstrated by tracking complex 3-D trajectories which replicate the motion in a constrained environment. Overall, the obtained results manifest QuadPlus’s capability of achieving independent position and attitude control even with multiple actuator saturation. The Authors envision that the proposed simplistic design would stimulate interest in the community for exploring the benefits offered by bi-axial propeller tilting platforms.Ministry of Education (MOE)Accepted versionThis research is supported by the Ministry of Education, Singapore, under its Academic Research Fund Tier 1 (RG69/20

Framework for measurement of battery state-of-health (resistance) integrating overpotential effects and entropy changes using energy equilibrium

The internal resistance of a battery represents the losses due to heat generation during energy conversion. The state-of-health is used to quantify the increase (degradation) of resistance with usage. However, the current state-of-health analysis merges the total internal resistance into one component. Consequently, the underlying cause of resistance degradation is not understood leading to incorrect estimate of battery health. Therefore, this paper presents a comprehensive framework based on energy equilibrium for the categorization and health analysis of total internal resistance. It is divided into 2 components: one based on irreversible overpotential (includes polarization) effects and a new second resistance component originated from reversible entropy changes. For LiFePO4 cells used in this work, it is observed that the contribution of entropy changes (hitherto unrecognized) to the overall losses increases from 4−10% to more than 40% as state-of-charge reduces. State-of-health of each component is obtained by the determination of its associated degradation factor to quantify the underlying mechanism of resistance degradation. In conclusion, the increase in irreversible resistance is primarily attributed to the permanent loss of active material. Correspondingly, the reversible resistance increase is associated to the formation of concentration gradients in the electrodes due to past load profile and ambient conditions.Maritime and Port Authority of Singapore (MPA)Accepted versionThe research conducted in this paper is funded jointly by ABB Pte Ltd, Singapore and Maritime and Port Authority (MPA) of Singapore

Receding horizon-based fault-tolerant control of QuadPlus : an over-actuated quadrotor

Highly maneuverable, over-actuated aerial robots have gained increasing interest in various inspection applications. However, since these systems carry expensive equipment and must operate in the vicinity of humans, their fail-safe operation is paramount. In this study, we propose a centralized nonlinear model predictive control (NMPC) method to facilitate fault-tolerant control (FTC) of an over-actuated quadrotor against a propeller failure. Thanks to the novel mechanical design, the hyperdynamic quadrotor can independently command and control all 6-degrees-of-freedom (DoFs). Additionally, the underlying reconfigurability feature of the designed NMPC makes it appropriate for normal as well as faulty operations. Moreover, the centralized nature of the control framework fully exploits the actuator redundancy, thereby ensuring complete system control without losing any DoF. The efficacy of the proposed FTC framework is elaborated throughout intensive simulations utilizing a high-fidelity model over two different trajectories. From the sequential failure cases, it is shown that – even with fault detection delay up to 1s – the aerial robot satisfactorily tracks the reference trajectory.Ministry of Education (MOE)Accepted versionThis research is supported by the Ministry of Education, Singapore, under its Academic Research Fund Tier 1 (RG69/20)