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

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

Spatial control of a smart beam

This study presents the design and implementation of a spatial H(infinity) controller for the active vibration control of a smart beam. The smart beam was modeled by assumed-modes method that results in a model including large number of resonant modes. The order of the model was reduced by direct model truncation and the model correction technique was applied to compensate the effect of the contribution of the out of range modes to the dynamics of the system. Additionally, spatial identification of the beam was performed, by comparing the analytical and experimental system models, in order to determine the modal damping ratios of the smart beam. Then, the spatial H(infinity) controller was designed and implemented to suppress the first two flexural vibrations of the smart beam

Smart structures and their applications on active vibration control: Studies in the Department of Aerospace Engineering, METU

This work presents the theoretical and experimental studies conducted in Aerospace Engineering Department of Middle East Technical University on smart structures with particular attention given to the structural modelling characteristics and active suppression of in-vacuo vibrations. The smart structures considered in these analyses are finite and flat aluminium cantilever beam-like (called as smart beam) and plate-like (called as smart fin) structures with surface bonded lead-zirconate-titanate patches. Finite element models of smart beam and smart fin are obtained. Then the experimental studies regarding open loop behaviour of the structures are performed by using strain gauges and laser displacement sensor to determine the system models. Further studies are carried out to obtain H(infinity) and mu-synthesis controllers which are intended to be used in the suppression of free and forced vibrations of the smart structures. It is observed that satisfactory attenuation levels are achieved and robust performance of the systems in the presence of uncertainties is ensured. In that respect a comparative study involving H(infinity) and sliding mode controls is also conducted. Recently, the studies involving aerodynamic loading are also gathering pace

All-trans retinoic acid prevents cisplatin-induced nephrotoxicity in rats

The aim of this study is to investigate the effects of all-trans retinoic acid (ATRA) use on cisplatin (CP)-induced nephrotoxicty. Twenty-eight rats were randomly divided into four groups. The rats in the control group were injected a single dose of 1ml/kg saline intra-peritoneally (IP) during 10days. The rats in the ATRA group were injected a single dose of ATRA during 10days. The rats in the ATRA+CP group were injected a single dose of CP on the fourth day of the 10days of ATRA treatment. The rats in the CP group were injected a single dose of CP on the fourth day of 10days without administering a treatment. After treatment, the groups were compared with regard to total antioxidant status (TAS), total oxidant status (TOS), and oxidative stress index (OSI) levels in renal tissue and renal histopathology. The serum creatinine and urea values were statistically significantly higher in the CP group compared to the other groups. The serum creatinine and urea values were statistically significantly lower in the ATRA+CP group when compared to the CP group. Although the TOS and OSI levels were found to be lower in the ATRA+CP group compared to the CP group, the difference was not statistically significant. Administration of ATRA together with CP was observed to reduce the histopathologic destruction in the kidney and lead to mild tubular degeneration, vacuolization, and necrosis (57.1% grade 1; 28.6% grade2, and 14.3% grade 3 necrosis). The results of the present study have revealed that ATRA administration ameliorates CP-induced nephrotoxicity; however, further studies are required to identify this issue before clinical application

Protective Effect of All-Trans Retinoic Acid in Cisplatin-Induced Testicular Damage in Rats

Purpose: To investigate the effects of all-trans retinoic acid (ATRA) in cisplatin (CP)-induced testicular damage in rats. Materials and Methods: Twenty-eight male Wistar rats were divided into four groups: Control, ATRA alone, ATRA+CP, and CP alone. Body weight, testicular weight, sperm count, sperm motility, percentage of abnormal sperm, total antioxidant status (TAS), total oxidant status (TOS), oxidative stress index (OSI) in testicular tissue, and testicular histopathology were compared among groups. Results: The sperm count and motility significantly decreased and the percentage of abnormal sperm significantly increased in the CP group compared to the control and ATRA groups. CP+ATRA administration significantly increased the sperm count and motility, but reduced the abnormal sperm count. CP administration significantly increased TOS and OSI compared to the control group and the other groups. Administering CP+ATRA significantly decreased TOS and the OSI in testicular tissue and reduced spermatogenesis, but increased the Johnsen score. Conclusions: The destructive effects of CP treatment on testicular tissue and spermatogenesis were reduced by administering ATRA