Advances in helicopter vibration control methods time-periodic reduced order modeling and H2/H1 controller design

This paper presents the implementation of recent developments in system theory within a novel framework to enhance the vibration control of helicopters. Particular focus is given to the vibration control of helicopters flying in a forward flight regime, where the sys- tem exhibits time-periodic behavior. The objective of this framework is to provide high performance controllers that can satisfy stability and design performance criteria when implemented in high-fidelity computer simulations or in real time experiments. The frame- work emphasizes the integration of state-of-the art coupled Computational Fluid Dynamics (CFD) /Computational Structural Dynamics (CSD) analysis in the controller design pro- cess to obtain accurate reduced-order aeroelastic models of the helicopter rotor system. Design of time-periodic H2 and H ∞ controllers are proposed owing to their rigorous sta- bility formulation based on Floquet-Lyapunov theory, and advantages over time-lifted con- trollers. Within this framework, the time-periodic system models in state-space form were identified using robust subspace model identification method. The time-periodic H2 and H∞ synthesis problem was solved using both Linear Matrix Inequality and periodic Ric- cati based formulations. The controllers performance were validated using the high-fidelity aeroelastic simulations. The computational efficiency of using these advanced methods, and the necessity of using the novel framework were demonstrated by implementing an actively controlled ap strategy for vibration suppression of helicopters

Advances in helicopter vibration control methods time-periodic reduced order modeling and H2/H∞ controller design

This paper presents the implementation of recent developments in system theory within a novel framework to enhance the vibration control of helicopters. Particular focus is given to the vibration control of helicopters flying in a forward flight regime, where the system exhibits time-periodic behavior. The objective of this framework is to provide high performance controllers that can satisfy stability and design performance criteria when implemented in high-fidelity computer simulations or in real time experiments. The framework emphasizes the integration of state-of-the art coupled Computational Fluid Dynamics (CFD) /Computational Structural Dynamics (CSD) analysis in the controller design process to obtain accurate reduced-order aeroelastic models of the helicopter rotor system. Design of time-periodic H2 and H∞ controllers are proposed owing to their rigorous stability formulation based on Floquet-Lyapunov theory, and advantages over time-lifted controllers. Within this framework, the time-periodic system models in state-space form were identified using robust subspace model identification method. The time-periodic H2 and H∞ synthesis problem was solved using both Linear Matrix Inequality and periodic Riccati based formulations. The controllers performance were validated using the high-fidelity aeroelastic simulations. The computational effciency of using these advanced methods, and the necessity of using the novel framework were demonstrated by implementing an actively controlled flap strategy for vibration suppression of helicopters

Vibration suppression of an elastic beam via sliding mode control

This paper presents experimental results of sliding mode control (SMC) technique applied to an elastic beam. The aim of the controller is to suppress first two vibration modes of the beam. Mathematical model of the beam is a finite dimensional model obtained from the Bernoulli-Euler beam equation. As the system states are to be available in order to design the SMC, an observer has been designed to obtain the states of the system by measuring tip deflection of the beam. By using observed states of the finite dimensional model, SMC is designed and applied to the elastic beam giving thoroughly suppressed vibration modes

Active vibration suppression of a flexible beam via sliding mode and H ∞ control

n this study, sliding mode and H. control techniques are applied to a flexible beam in order to suppress some of the vibration modes. The beam is a clamped-free flexible structure having piezoelectric (PZT) patches as actuators and a laser displacement sensor for measuring the tip point deflection. The beam is modeled in two different ways for each control algorithm. To implement sliding mode control (SMC), Euler-Bernoulli beam model is used and a finite dimensional LTI model is formed by using assumed mode method. As the SMC requires state measurement, an observer is designed to estimate the states from the measured tip deflection. In order to implement H-infinity control algorithm, the model of the flexible beam, which is an approximate transfer function, is constructed by using system identification technique. The experimental results of designed SMC and H-infinity control algorithms are presented

Active vibration control of a smart fin

This paper summarizes the design and wind tunnel experimental verifications of robust H 1e controllers for active vibration suppression of a dynamically scaled F-18 vertical smart fin. The smart fin consists of a cantilevered aluminium plate structure with surface bonded piezoelectric (Lead-Zirconate-Titanete, PZT) patches, Integrated Circuit Piezoelectric (ICP) type accelerometers and strain gauges. For H 1e controller design, the transfer function of the fin was first estimated outside the wind tunnel. Then, experiments were carried out to determine the aeroelastic characteristics of the smart fin at free flow and vortical (i.e. buffet) flow conditions. Variable air speeds and Angle of Orientations (AoO) were considered in both flow conditions. Significant shifts in vibration frequencies and the damping ratios were observed at the various values of airspeed and AoO. Taking into account these variations, the H 1e controllers were designed to suppress the fin's buffeting response at the first and second bending and first torsional modes. A second set of wind tunnel experiments was conducted to verify the performance of the designed H 1e controllers at various flow scenarios. Successful vibration suppression levels were obtained within the desired frequency intervals.Peer reviewed: YesNRC publication: Ye

Active control of smart fin model for aircraft buffeting load alleviation applications

Following the program to test a hybrid actuation system for high-agility aircraft buffeting load alleviation on the full-scale F/A-18 vertical fin structure, an investigation has been performed to understand the aerodynamic effects of high-speed vortical flows on the dynamic characteristics of vertical fin structures. Extensive wind-tunnel tests have been conducted on a scaled model fin integrated with piezoelectric actuators and accelerometers to measure the afttip vibration responses under various freestream and vortical airflow conditions. Test results demonstrated that the airflow induced considerable damping to the fin structure, which generally increased with airflow speed as well as the vertical fin angle of attack relative to the airflow direction. Moreover, it was observed that at the angle of attack of 10 deg, the high-speed airflow introduced large deflection to the smart fin structure and caused significant frequency shift to the vibration modes due to nonlinear geometrical coupling of bending and torsional modes. These aerodynamic effects may adversely affect the performance and robustness of the closed-loop control laws developed based on vertical fin dynamic model identified without considering the varying aerodynamic effects. To explore this problem, the structured singular values synthesis technique was adopted to develop robust control law using smart fin model identified without aerodynamic excitations, and the aerodynamic effects on the fin structure were assumed as smart fin parametric and dynamic uncertainties. The effectiveness and robust performance of the control law was demonstrated through extensive closed-loop wind-tunnel tests using various airflow conditions. This provided a verified control law design strategy for future flight tests of the full-scale aircraft buffeting load alleviation system.NRC publication: Ye