11:00
Nonlinear control
Chair: Vincenzo Vaccaro
11:00
30 mins
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Dynamic analysis of nonlinear aeroservoelasticity by a modified frequency-time domain method
Peihan Wang, Zhigang Wu, Chao Yang
Abstract: Dynamic analysis of nonlinear aeroservoelastic systems has been a subject of concern for decades. A modified frequency–time domain method is competent in analyzing nonlinear aeroelastic systems, with the capability of addressing various nonlinearities and initial conditions. An extension of this method is presented to allow nonlinear responses of closed-loop systems with freeplay and actuator nonlinearities. Aeroservoelastic systems can be reconstructed by extracting nonlinear elements as pseudo forces in the nonlinear feedback loops in the time domain, whereas the original feedback loops are also introduced via the convolution integral. Hence, nonlinear responses with various nonlinearities and initial conditions can be obtained by the proposed method. Numerical results are provided for a three-degree-of-freedom airfoil section with freeplay and actuator nonlinearities, which is augmented to an aeroservoelastic system. Compared with the Runge–Kutta algorithm, the feasibility and accuracy of the proposed method can be validated. As an alternative to time-marching approaches, the modified frequency–time domain method initiates a novel process to address various nonlinearities and initial conditions in nonlinear aeroservoelastic systems.
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11:30
30 mins
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Nonlinear Incremental Robust Model Predictive Control for a Nonlinear Aeroservoelastic System
Tingyu Zhang, Xuerui Wang
Abstract: This paper presents a novel approach to suppress gust-induced limit cycle oscillations (LCOs). The proposed method integrates incremental nonlinear dynamic inversion (INDI) and robust nonlinear model predictive control (NMPC) with tightened constraints. The INDI method estimates and actively rejects gusts, resulting in a reduced disturbance residue. The upper bound of the disturbance residue can be estimated either online or offline and is used to bound the maximum state deviation caused by uncompensated disturbances, thereby imposing tightened constraints for the NMPC scheme to improve the robustness of constraint satisfaction and stabilize the system. Simulation results on a 2-D nonlinear aeroservoelastic wing demonstrate that the proposed method stabilizes the nonlinear system and reduces the disturbed peak plunge and pitch motions by up to 20.69% and 33.70%, respectively. Additionally, the method mitigates the conservativeness of the robust NMPC, with an 81.67% reduction in the offline estimated upper bound of the disturbance residue. The online estimation of the disturbance residue captures its peak value while further relaxing the tightened constraint set when disturbance effects are small. The proposed control scheme effectively suppresses gust-induced LCO motions and reduces the conservativeness of the tightened constraint sets used in the robust NMPC.
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12:00
30 mins
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Nonlinear analysis of combined rate and acceleration limits effect on actuator performance
Luca Marino, Xuerui Wang, Jurij Sodja
Abstract: In modern aircraft design, electro-mechanical actuators are increasingly being considered as an alternative to conventional, hydraulic actuation systems for flight control surfaces. While offering advantages in terms of weight reduction and increased efficiency, these actuators are also characterised by a higher sensitivity to nonlinear effects. Actuator models can strongly affect the effectiveness of control function such as gust load alleviation or flutter suppression, it is essential to correctly understand, model and identify nonlinearities in the actuator response, as well as to integrate nonlinear actuator models into aeroservoelastic models.
This contribution explores the nonlinear effects of rate and acceleration limits in actuation systems, focusing on the actuator steady-state response to sinusoidal input and the effectiveness of closed-loop control systems. The saturation regimes determined by rate and acceleration limits are investigated, and analytical formulations are derived for the nonlinear actuator response and the boundaries of these regimes within the two-dimensional parameter space defined by non-dimensional rate and acceleration limits. Describing functions for each regime are determined in a closed form, establishing the relationship between actuator input and output in the frequency domain. Combined rate and acceleration limits are found to induce a low-pass filter behaviour in the actuator, with a -40 dB/decade roll-off, and can lead to nonsmooth phase dependence on frequency. The describing functions of combined rate and acceleration limits are applied to the analysis of an aeroservoelastic wing model developed for gust load alleviation (GLA) purposes. The effect of the actuator limits is investigated by evaluating the onset point of the nonlinear behaviour and an equivalent describing function for the entire actuator-plant-control feedback loop. The resulting findings illustrate that rate and acceleration limits can substantially affect the performance of closed-loop systems, leading to phenomena such as jump resonances when partial-to-full saturation regime transitions occur, and thereby constraining the effective frequency range of the GLA control systems.
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