Experimental frequency-domain identification and cross-coupling analysis of a twin-rotor MIMO system

Abstract Experimental frequency-domain identification and cross-coupling analysis of the Quanser Aero 2 twin-rotor multi-input multi-output (MIMO) system are presented. The study experimentally characterizes the coupled pitch–yaw dynamics using nonparametric frequency response estimation under broadband multisine excitation. Independent excitation signals covering approximately 0.1–5 Hz were applied to the pitch and yaw actuators, and the resulting input–output data were analyzed using spectral density, frequency response, and coherence measures. The complete MIMO frequency response matrix, including both direct and cross-coupled channels, was experimentally identified. High coherence values of 0.95–0.98 were obtained for the principal channels within the dominant operating region, indicating reliable frequency-response estimates. The identified responses demonstrate bidirectional pitch–yaw interaction, with cross-coupling levels remaining within approximately 20–30 dB of the corresponding direct channels over portions of the low-frequency operating range. The coupling strength varies with frequency and excitation direction, indicating asymmetric interaction between the two axes. Independent validation using pseudo-random binary sequence (PRBS) excitation showed good agreement with the multisine-based frequency responses within the dominant operating region, supporting the robustness of the identified dynamics. The results provide an experimentally validated characterization of coupled twin-rotor dynamics and quantify the limitations of simplified decoupled representations for subsequent multivariable control and decoupling analysis.