Phase-advanced attitude sensing and control for fixed-wing micro aerial vehicles in turbulence
Creators: name: Mohamed, Abdulghani
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400103
subject: Aircraft performance and flight control systems
classificationCode: 400105
subject: Flight dynamics
classificationCode: 401201
subject: Aerodynamics (excl. hypersonic aerodynamics)

description: The scale of fixed-wing Micro Aerial Vehicles (MAVs) lend them to many unique applications. These applications often require low speed flights close to the ground, in the vicinity of large obstacles and in the wake of buildings. A particular challenge for MAVs is attitude control in the presence of high turbulence.<br><br>Such flights pose a challenging operational environment for MAVs, and in particular, ensuring sufficient attitude control in the presence of significant turbulence. Low-level flight in the atmospheric boundary layer without sufficient attitude control is hazardous, mainly due to the high levels of turbulence intensity close to the ground. MAV accidents have occurred due to the lack of a reliable attitude control system in turbulent conditions as reported in the literature.<br><br>Challenges associated with flight control of fixed-wing MAVs operating in complex environments are significantly different to any larger scale vehicle. The scale of MAVs makes them particularly sensitive to atmospheric disturbances thus limiting their operation. A review of the literature revealed that rolling inputs from turbulence were the most challenging whereby conventional inertial-based attitude control systems lack the responsiveness for roll control in high turbulence environments.<br><br>The solution might lie with flying animals, which have adapted to flight within turbulence. The literature survey identified bio-inspired phase-advanced sensors as a promising sensory solution for complementing current reactive attitude sensors. The development of a novel bio-inspired phase-advanced sensor and associated control system, which can sense the flow disturbances before an attitude perturbation, is the focus of this research.<br><br>The development of such a system required an in-depth understanding of the features of the disturbing phenomena; turbulence. Correlation studies were conducted between the oncoming turbulence and wing-surface pressure variations. It was found that the highest correlation exists between upstream flow pitch angle variation and the wing-surface pressure fluctuations. However, due to the insufficient time-forward advantage, surface pressure sensing was not used for attitude control.<br><br>A second sensing approach was explored to cater for the control system’s time-lags. Multi-hole pressure probes were embedded in the wings of the MAV to sense flow pitch angle and magnitude variation upstream of the wing. The sensors provide an estimate of the disturbing turbulence. This approach caters for the time-lags of the system providing sufficient time to counteract the gust before it results in an inertial response. Statistical analysis was used to assess the disturbance rejection performance of the phase-advanced sensory system, which was benchmarked against a conventional inertial-based sensory system in a range of turbulence conditions. Unconstrained but controlled test flights were conducted inside the turbulence environment of two wind-tunnels, in addition to outdoor flight testing in the atmosphere. These three different turbulence conditions enabled testing of a wide range of turbulence spectra believed to be most detrimental to the MAV. A significant improvement in disturbance rejection performance was observed in relation to conventional inertial-based sensory systems. It can be concluded that sensory systems providing time-forward estimates of turbulence can complement conventional inertial-based sensors to improve the attitude stability performance.