Flutter Suppression Control Laws Research Articles

Active flutter suppression of nonlinear fin-actuator system via inverse system and immersion and invariance method

Structural nonlinearities such as freeplay will affect the stability and even flight safety of the fin-actuator system. There is a lack of a practical method for designing Active Flutter Suppression (AFS) control laws for nonlinear fin-actuator systems. A design method for the AFS controller of the nonlinear all-movable fin-electromechanical actuator system is established by combining the inverse system and the Immersion and Invariance (I&I) theory. First, the composite control law combining the inverse system principle and internal model control is used to offset the nonlinearity and dynamics of the actuator, so that its driving torque can follow the ideal signal. Then, the ideal torque of the actuator is designed employing the I&I theory. The unfavorable oscillation of the fin is suppressed by making the output torque of the actuator track the ideal signal. The simulation results reveal that the proposed AFS method can increase the flutter speed of the nonlinear fin-actuator system with freeplay, and a set of controller parameters is also applicable for wider freeplay within a certain range. The power required for the actuator does not exceed the power that can be provided by the commonly used aviation actuator. This method can also resist a certain level of noise and external disturbance.

Wind Tunnel System for Active Flutter Suppression Research: Overview and Insights

This paper describes the development of a new state-of-the-art large wind tunnel model for active flutter suppression studies as well as the supporting techniques used in tests focused on the effects of uncertainty. Design guidelines and the resulting aeroelastic characteristics of the model are covered together with representative test results. Those would allow other researchers working in this area to develop control laws for the new model and evaluate them. A number of important lessons and insight are reported regarding the design of the model, the level of success of commonly used mathematical modeling techniques to capture its behavior, sources of analysis/test correlation discrepancies, multifunction utilization of control surfaces for both system identification and flutter suppression, active flutter suppression testing safety, and techniques for estimating the robustness of closed-loop active aeroservoelastic systems by tests. The new system has made it possible to repeatedly push the actively controlled model, using various flutter suppression control laws (safely), to the actual flutter limit in tests numerous times. This capability is just one of the new experimental capabilities that the new system brings to the aeroelastic active control community.

Aeroservoelastic Test of the Subsonic Ultra-Green Aircraft Research Truss-Braced Wing Model

The Subsonic Ultra Green Aircraft Research Truss-Braced Wing aeroservoelastic wind-tunnel test was conducted in the NASA Langley Transonic Dynamics Tunnel. The primary goals of the test were to identify the open-loop flutter boundary and then demonstrate flutter suppression. A secondary goal was to demonstrate gust load alleviation. Open-loop flutter and limit cycle oscillation onset boundaries were identified for a range of Mach numbers and various angles of attack. Two sets of control laws were designed for the model, and both sets of control laws were successful in suppressing flutter. Control laws optimized for gust load alleviation were not designed; however, the flutter suppression control laws were assessed using the Transonic Dynamics Tunnnel airstream oscillation system. This paper describes the experimental apparatus, procedures, and results of the truss-braced wing wind-tunnel model test. Acquired system identification data used to generate aeroservoelastic models are also discussed.

Active flutter suppression control law design method based on balanced proper orthogonal decomposition reduced order model

Active control for nonlinear aeroelastic structures is an attractive innovative technology. The design of classic active flutter controllers has often been based on low-fidelity and low-accuracy linear aerodynamic models. Multi-physics high-fidelity reduced order model (ROM) was used to design active control laws. In order to provide a lower-order model for controllers design, a balanced proper orthogonal decomposition ROM (POD-BT/ROM) was investigated. A state-space aeroservoelastic model and the active flutter suppression control law design method based on POD-BT/ROM were proposed. The effectiveness of the proposed method was then demonstrated by NACA 0012 airfoil, AGARD 445.6 wing and the Goland wing+ aeroelastic model.

Wing flutter suppression enhancement using a well-suited active control model

State-space theory is employed here to model a new active wing flutter suppression control. In this paper, the design of a flutter suppression control law, for the NASA Benchmark Active Control Technology wing, is proposed through a single input-single output controller and unsteady aerodynamics is modelled using the Theodorsen's theory. Wing dynamic model is obtained by combining the aeroelastic equations of motion with the actuator model presented. Open-loop dynamic behaviour is examined for a single feedback variable that combines pitch and plunge accelerations. Here, a new formulation of control law, based on classical control techniques and featuring two feedback closed-loops, is successfully employed to reach superior stability robustness with respect to other control laws designed with classical flutter suppression schemes having only one feedback closed-loop. The process allows the stability of the aeroelastic system for flight speeds that exceed the open-loop flutter velocity.

Robust Multivariable Flutter Suppression for Benchmark Active Control Technology Wind-Tunnel Model

The Benchmark Active Controls Technology (BACT) project is part of NASA Langley Research Center''s Benchmark Models Program for studying transonic aeroelastic phenomena. In January of 1996 the BACT wind-tunnel model was used to successfully demonstrate the application of robust multivariable control design methods (H infinity and mu-synthesis) to flutter suppression. This paper addresses the design and experimental evaluation of robust multivariable flutter suppression control laws with particular attention paid to the degree to which stability and performance robustness was achieved.

Active Control of Wind-Tunnel Model Aeroelastic Response Using Neural Networks

Under a joint research and development effort conducted by NASA and The Boeing Company (formerly McDonnell Douglas), three neural-network-based control systems were developed and tested. The control systems were experimentally evaluated using a transonic wind-tunnel model in the NASA Langley Research Center Transonic Dynamics Tunnel. One system used a neural network to schedule flutter suppression control laws, another employed a neural network in a predictive control scheme, and the third employed a neural network in an inverse model control scheme. All three of these control schemes successfully suppressed flutter to or near the limits of the testing apparatus and represent the first experimental applications of neural networks to flutter suppression. The findings of this project are summarized.

Transonic Flutter Suppression Control Law Design and Wind-Tunnel Test Results

Fluttersuppressioncontrollawdesignandwind-tunneltestresultsintransonice owforaNACA0012wingmodel, under the benchmark active control technology program at NASA Langley Research Center, will be presented. Two control law design processes using classical and minimax techniques are described. Design considerations for improving the multivariable system robustness are outlined. The classical control law was digitally implemented and tested in the NASA Langley Transonic Dynamics Tunnel. In wind-tunnel tests in air and heavy gas medium, the closed-loop e utter dynamic pressure was increased by over 50% up to the wind-tunnel upper limit. The active e utter suppression system also provided signie cant robustness, relative to gain and phase perturbations, even in the presence of transonic shocks and e ow separation.

Flutter suppression control law design and testing for the active flexible wing

Design of a control law for simultaneously suppressing the symmetric and antisymmetric flutter modes of a sting-mounted, fixed-in-roll aeroelastic wind-tunnel model is described. The flutter suppression control law was designed using linear quadratic Gaussian theory, and involved control law order reduction, a gain root-locus study, and use of previous experimental results. A 23% increase in the open-loop flutter dynamic pressure was demonstrated during the wind-tunnel test. Rapid roll maneuvers at 11% above the symmetric flutter boundary were also performed when the model was in a free-to-roll configuration.

On-line performance evaluation of multiloop digital control systems

A real-time controller, developed to implement flutter suppression and rolling maneuver load alleviation control laws digitally, was tested on an aeroelastic wind-tunnel model. A controller-per formance-evaluation (CPE) methodology to evaluate various multi-input/multi-output digital control systems on line using sampled digital data was also developed and tested. Modern signal processing methods were used to generate appropriate transfer matrices of the control system from the sampled time-domain data of the whole system (controller and plant). Matrix procedures were used to calculate singular values of return-difference matrices at the plant input and output points to evaluate the performance of the control system. The CPE procedures confirmed satisfactory performance of stabilizing control laws and effectively identified potentially destabilizing ones.

Frequency domain synthesis of a robust flutter suppression control law

Computer-aided conventional synthesis techniques are employed to obtain a robust active-flutter-suppression control law. The relatively high dynamic order of such problems are dealt with effectively with a computer-aided approach, while computer graphics allows conventional graphical techniques to be utilized. Key design information is displayed for variations in flight conditions such that a simple control law is obtained that is robust over variations in the flight condition. Through visualization of the pole and zero migration with dynamic pressure, for example, control synthesis, evaluation of effects of unsteady aerodynamics and model reduction were performed almost by inspection. A candidate control law is analytically shown to compare very favorably to several others taken from the literature.

Reduced-Order Optimal Feedback Control Law Synthesis for Flutter Suppression

A method of synthesizing reduced-order optimal feedback control laws for a high-order system is developed. A nonlinear programming algorithm is employed to search for the control law design variables that minimize a performance index defined by a weighted sum of mean-square steady-state responses and control inputs. An analogy with the linear quadratic Gaussian solution is utilized to select a set of design variables and their initial values. An input-noise adjustment procedure is used in the design algorithm to improve the stability margins of the system. The method is applied to the synthesis of an active flutter-suppression control law for an aeroelastic wind tunnel wing model. The reduced-order control law is compared with the corresponding full-order control law. The study indicates that by using the present algorithm, near optimal low-order flutter suppression control laws with good stability margins can be synthesized.

Control Law Synthesis for Active Flutter Suppression Using Optimal Control Theory

This paper describes a study investigating the use of optimal control theory for the synthesis of an active flutter-suppression control law. For an example design application, a high-aspect-ratio cantilever wind-tunnel wing model is considered. The structural dynamics are represented by analytically computed natural frequencies and mode shapes. The three-dimensional unsteady aerodynamic forces for oscillatory motion are computed employing the doublet-lattice technique. With the aid of finite-order approximating functions for representing the aerodynamic forces in the time domain, the equations are written in the standard state vector form. Linear optimal control theory is then applied to find particular sets of gain values which minimize a quadratic cost function of the states and controls. These control laws are shown to increase the flutter dynamic pressure by at least 50% at Mach numbers 0.7 and 0.9. The closed-loop system's control surface activity in a gust environment is also examined.