Identification of flexible structures for robust control

Documentation is provided of the authors' experience with modeling and identification of an experimental flexible structure for the purpose of control design, with the primary aim being to motivate some important research directions in this area. A multi-input/multi-output (MIMO) model of the structure is generated using the finite element method. This model is inadequate for control design, due to its large variation from the experimental data. Chebyshev polynomials are employed to fit the data with single-input/multi-output (SIMO) transfer function models. Combining these SIMO models leads to a MIMO model with more modes than the original finite element model. To find a physically motivated model, an ad hoc model reduction technique which uses a priori knowledge of the structure is developed. The ad hoc approach is compared with balanced realization model reduction to determine its benefits. Descriptions of the errors between the model and experimental data are formulated for robust control design. Plots of select transfer function models and experimental data are included

High performance, accelerometer-based control of the Mini-MAST structure at Langley Research Center

Many large space system concepts will require active vibration control to satisfy critical performance requirements such as line of sight pointing accuracy and constraints on rms surface roughness. In order for these concepts to become operational, it is imperative that the benefits of active vibration control be shown to be practical in ground based experiments. The results of an experiment shows the successful application of the Maximum Entropy/Optimal Projection control design methodology to active vibration control for a flexible structure. The testbed is the Mini-Mast structure at NASA-Langley and has features dynamically traceable to future space systems. To maximize traceability to real flight systems, the controllers were designed and implemented using sensors (four accelerometers and one rate gyro) that are actually mounted to the structure. Ground mounted displacement sensors that could greatly ease the control design task were available but were used only for performance evaluation. The use of the accelerometers increased the potential of destabilizing the system due to spillover effects and motivated the use of precompensation strategy to achieve sufficient compensator roll-off

NASA/MSFC ground experiment for large space structure control verification

Marshall Space Flight Center has developed a facility in which closed loop control of Large Space Structures (LSS) can be demonstrated and verified. The main objective of the facility is to verify LSS control system techniques so that on orbit performance can be ensured. The facility consists of an LSS test article which is connected to a payload mounting system that provides control torque commands. It is attached to a base excitation system which will simulate disturbances most likely to occur for Orbiter and DOD payloads. A control computer will contain the calibration software, the reference system, the alignment procedures, the telemetry software, and the control algorithms. The total system will be suspended in such a fashion that LSS test article has the characteristics common to all LSS

Robustness and performance tradeoffs in control design for flexible structures

The design of control laws for the Caltech flexible structure experiment using a nominal design model with varying levels of uncertainty is considered. A brief overview of the structured singular value (µ) H∞ control design, and µ-synthesis design techniques is presented. Tradeoffs associated with uncertainty modeling of flexible structures are discussed. A series of controllers are synthesized based on different uncertainty descriptions. It is shown that an improper selection of nominal and uncertainty models may lead to unstable or poor-performing controllers on the actual system. In contrast, if descriptions of uncertainty are overly conservative, performance of the closed-loop system may be severely limited. Experimental results on control laws synthesized for different uncertainty levels on the Caltech structure are presented

Identification for Robust Control of Flexible Structures

An accurate multivariable transfer function model of an experimental structure is required for research involving robust control of flexible structures. Initially, a multi-input/multi-output model of the structure is generated using the finite element method. This model was insufficient due to its variation from the experimental data. Therefore, Chebyshev polynomials are employed to fit the data with a single-input/multi-output transfer function models. Combining these lead to a multivariable model with more modes than the original finite element model. To find a physically motivated model, as ad hoc model reduction technique which uses a priori knowledge of the structure is developed. The ad hoc approach is compared with balanced realisation model reduction to determine its benefits. Plots of select transfer function models and experimental data are included

The dynamics and control of large flexible space structures, 8

A development of the in plane open loop rotational equations of motion for the proposed Spacecraft Control Laboratory Experiment (SCOLE) in orbit configuration is presented based on an Eulerian formulation. The mast is considered to be a flexible beam connected to the (rigid) shuttle and the reflector. Frequencies and mode shapes are obtained for the mast vibrational appendage modes (assumed to be decoupled) for different boundary conditions based on continuum approaches and also preliminary results are obtained using a finite element representation of the mast reflector system. The linearized rotational in plane equation is characterized by periodic coefficients and open loop system stability can be examined with an application of the Floquet theorem. Numerical results are presented to illustrate the potential instability associated with actuator time delays even for delays which represent only a small fraction of the natural period of oscillation of the modes contained in the open loop model of the system. When plant and measurement noise effects are added to the previously designed deterministic model of the hoop column system, it is seen that both the system transient and steady state performance are degraded. Mission requirements can be satisfied by appropriate assignment of cost function weighting elements and changes in the ratio of plant noise to measurement noise

A Hybrid Reduced-Order Model for the Aeroelastic Analysis of Flexible Subsonic Wings—A Parametric Assessment

A hybrid reduced-order model for the aeroelastic analysis of flexible subsonic wings with arbitrary planform is presented within a generalised quasi-analytical formulation, where a slender beam is considered as the linear structural dynamics model. A modified strip theory is proposed for modelling the unsteady aerodynamics of the wing in incompressible flow, where thin aerofoil theory is corrected by a higher-fidelity model in order to account for three-dimensional effects on both distribution and deficiency of the sectional air load. Given a unit angle of attack, approximate expressions for the lift decay and build-up are then adopted within a linear framework, where the two effects are separately calculated and later combined. Finally, a modal approach is employed to write the generalised equations of motion in state-space form. Numerical results were obtained and critically discussed for the aeroelastic stability analysis of a uniform rectangular wing, with respect to the relevant aerodynamic and structural parameters. The proposed hybrid model provides sound theoretical insights and is well suited as an efficient parametric reduced-order aeroelastic tool for the preliminary multidisciplinary design and optimisation of flexible wings in the subsonic regime

A preliminary study on passive and active flutter suppression concepts for aeronautical components

The scope of this work is to study computationally both passive and active flutter suppression characteristics of a cantilever cork agglomerate core sandwich with CFRP facings and an aluminum plate, the latter through the application of piezoelectric patches, respectively. Recently, cork agglomerates have been gaining an increasing interest from the aerospace industry due to their good thermal and acoustic insulation capabilities. In addition, cork based materials intrinsically have excellent vibration suppression properties, which suggest that the combination of cork with high performance composites (such as CFRPs) may lead to high specific strength materials with improved damping characteristics suitable for flutter prevention. Sandwich specimens were modeled using commercially available software ANSYS® and a demo version of ZAERO® software for the determination of the flutter speed and related frequencies. ANSYS® piezoelectric modeling and transient analysis capabilities were used for the active vibration study. Specimen aspect ratio and thickness were chosen as a function of wind tunnel maximum speed for further experimental tests. Results were compared with conventional CFRP and aluminum plates. It was demonstrated that a cork agglomerate core sandwich with CFRP facings can act as a natural flutter suppresser which allows the reduction of the wing weight for a given flight envelope and that the application of piezoelectric actuators is a valuable aeroelastic control concept. An increase of about 20% in flutter speed was achieved using actuated piezoelectric devices. The main goal remains in investigating higher strain smart materials and control strategies, since these improvements are only possible in small structures.O objectivo deste trabalho é o estudo computacional de soluções de supressão de flutter, passiva e activa, através de uma sandwich com núcleo de aglomerado de cortiça e faces de carbonoepoxy e de uma placa de alumínio, esta última através de actuadores piezoeléctricos, respectivamente. Recentemente, os aglomerados de cortiça têm ganho um interesse crescente por parte da indústria aeronáutica devido às suas propriedades de isolamento térmico e acústico. Além disso, os materiais à base de cortiça têm intrinsecamente excelentes propriedades antivibráticas, o que sugere que a sua combinação com materiais de alto desempenho (como o carbono-epoxy) pode levar a materiais de resistência específica elevada e com características de amortecimento melhoradas, adequados à prevenção do flutter. A sandwich foi modelada usando o software de elementos finitos ANSYS® e uma versão de demonstração do ZAERO® para a determinação da velocidade de flutter e respectiva frequência. Por sua vez, as capacidades de modelação piezoeléctrica e transiente do ANSYS® foram usadas para o estudo do controlo de vibração activa. A razão de aspecto das placas foi escolhida em função da velocidade máxima do túnel de vento, para posteriores testes experimentais. Os resultados foram comparados com placas de alumínio e carbono-epoxy convencionais. Foi demonstrado que a sandwich com núcleo de aglomerado de cortiça pode actuar como um supressor natural de flutter que permite uma redução do peso da estrutura para um dado envelope de voo. No que concerne ao controlo activo, a aplicação de actuadores piezoeléctricos é um conceito de controlo aeroelástico valioso que permitiu, neste estudo, um aumento de 20% na velocidade de flutter. No entanto, o principal objectivo permanece em investigar estratégias de controlo e materiais de características piezoeléctricas com capacidade de induzir maiores extensões a custo de uma menor potência

Fourth NASA Workshop on Computational Control of Flexible Aerospace Systems, part 2

A collection of papers presented at the Fourth NASA Workshop on Computational Control of Flexible Aerospace Systems is given. The papers address modeling, systems identification, and control of flexible aircraft, spacecraft and robotic systems