System Identification of an Octocopter in Hover Using Full-Harmonic Orthogonal Multisine Inputs

A new method for multi-input flight maneuver design is presented. The method consists of simultaneously injecting “full-harmonic” orthogonal multisine signals into the flight control system. Orthogonality is achieved by repeating flight maneuvers with changing multisine polarities. The multisines can each contain the same frequency content, which can simplify frequency response estimation and can allow for a single flight maneuver designed using typical orthogonal multisine methods to be split into several shorter maneuvers, while maintaining the same frequency resolution and minimum frequency. Additionally, an input allocation scheme is presented that augments the multisines to size the vehicle response amplitude about a specific degree of freedom. To demonstrate the developed approach, flight testing was conducted with a small octocopter aircraft. The input allocation scheme was utilized successfully to increase excitation about the yaw axis, which often lacks control power for multirotor aircraft. Individual motor speeds and electrical power consumption measurements were recorded during flight and used to model the power and motor speed dynamics in addition to the rigid-body flight dynamics. The identified models are shown to predict the response of the vehicle and motor subsystems accurately. The models are parameterized primarily by individual rotor thrust and torque coefficients, which makes them suitable for analysis of both the aircraft flight dynamics and individual rotor aerodynamics. Furthermore, the results demonstrate that, for the small octocopter studied, the vehicle flight dynamics can be modeled accurately in near-hover conditions by neglecting rotor hub moments, variations in thrust and torque coefficients, gyroscopic moments in roll and pitch, and aerodynamic interaction effects.