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At least 631 records · Page 35

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NASA NTRS1992

Vibration attenuation of the NASA Langley evolutionary structure experiment using H(sub infinity) and structured singular value (micron) robust multivariable control techniques

The use is studied of active control to attenuate structural vibrations of the NASA Langley Phase Zero Evolutionary Structure due to external disturbance excitations. H sub infinity and structured singular value (mu) based control techniques are used to analyze and synthesize control laws for the NASA Langley Controls Structures Interaction (CSI) Evolutionary Model (CEM). The CEM structure experiment provides an excellent test bed to address control design issues for large space structures. Specifically, control design for structures with numerous lightly damped, coupled flexible modes, collocated and noncollocated sensors and actuators and stringent performance specifications. The performance objectives are to attenuate the vibration of the structure due to external disturbances, and minimize the actuator control force. The control design problem formulation for the CEM Structure uses a mathematical model developed with finite element techniques. A reduced order state space model for the control design is formulated from the finite element model. It is noted that there are significant variations between the design model and the experimentally derived transfer function data.

Balas, Gary J.↗

DOE OSTI2020

2D-Covalent Organic Frameworks with Interlayer Hydrogen Bonding Oriented through Designed Nonplanarity

We report the synthesis and characterization of a new class of 2D-covalent organic frameworks, called COFamides, whose layers are held together by amide hydrogen bonds. To accomplish this, we have designed monomers with a nonplanar structure that arises from steric crowding, forcing the amide side groups out of plane with the COF sheets orienting the hydrogen bonds between the layers. Here, the presence of these hydrogen bonds provides significant structural stabilization as demonstrated by comparison to control structures that lack hydrogen bonding capability, resulting in lower surface area and crystallinity. We have characterized both azine and imine-linked versions of these COFs, named COFamide-1 and -2, respectively, for their surface areas, pore sizes, and crystallinity. In addition to these more conventional characterization methods, we also used variable temperature infrared spectroscopy methods and van der Waals density functional calculations to directly observe the presence of hydrogen bonding.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Substituent effects on energetics and crystal morphology modulate singlet fission in 9,10-bis(phenylethynyl)anthracenes

Singlet fission (SF) converts a singlet exciton into two triplet excitons in two or more electronically coupled organic chromophores, which may then be used to increase solar cell efficiency. Many known SF chromophores are unsuitable for device applications due to chemical instability or low triplet state energies. The results described here show that efficient SF occurs in derivatives of 9,10-bis(phenylethynyl)anthracene (BPEA), which is a highly robust and tunable chromophore. Fluoro and methoxy substituents at the 4- and 4'-positions of the BPEA phenyl groups control the intermolecular packing in the crystal structure, which alters the interchromophore electronic coupling, while also changing the SF energetics. The lowest excited singlet state (S 1 ) energy of 4,4'-difluoro-BPEA is higher than that of BPEA so that the increased thermodynamic favorability of SF results in a (16 ± 2 ps) -1 SF rate and a 180% ± 16% triplet yield, which is about an order of magnitude faster than BPEA with a comparable triplet yield. By contrast, 4-fluoro-4'-methoxy-BPEA and 4,4'-dimethoxy-BPEA have slower SF rates, (90 ± 20 ps) -1 and (120 ± 10 ps) -1 , and lower triplet yields, (110 ± 4)% and (168 ± 7)%, respectively, than 4,4'-difluoro-BPEA. These differences are attributed to changes in the crystal structure controlling interchromophore electronic coupling as well as SF energetics in these polycrystalline solids.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2021

Grid Forming Inverter: Laboratory-Scale Hardware Test Bed Setup and Weak Grid Operation

In recent times, the concept of grid forming inverters has gained popularity. Grid forming inverters have proven to be a promising alternative to the grid following inverters. In this paper, we present the hardware test bed implementation of grid forming inverter in islanding mode as well as in grid connected mode. The control structure includes inverter-level inner current control and outer voltage control, and plant-level P - f and Q - V droop control. The Q - V droop provides the voltage command, whereas P - f droop provides the frequency command and is also key for the synchronization process. While it is straightforward to set up the test bed for the islanded operation, it is a challenging process to set up the grid-forming inverter in the grid-connected mode. We present the key technologies and demonstrate the start-up process and weak grid operation using the hardware experiment results. Results from EMT model developed in MATLAB/SimPowerSystems are also presented for side-by-side comparison.

Mittal, Ratik↗

NASA NTRS1988

Adaptive control techniques for the SCOLE configuration

Two adaptive controllers designed for the Spacecraft Control Laboratory Experiment (SCOLE) at NASA Langley Research Center are discussed. The first controller is an adaptive model following variable structure controller and the second is an indirect LQ adaptive controller. For each technique, the design of the controller is outlined and simulation results are presented. Plans for future studies are also discussed.

Ossman, K.↗

NASA NTRS1991

Ground support system methodology and architecture

A synergistic approach to systems test and support is explored. A building block architecture provides transportability of data, procedures, and knowledge. The synergistic approach also lowers cost and risk for life cycle of a program. The determination of design errors at the earliest phase reduces cost of vehicle ownership. Distributed scaleable architecture is based on industry standards maximizing transparency and maintainability. Autonomous control structure provides for distributed and segmented systems. Control of interfaces maximizes compatibility and reuse, reducing long term program cost. Intelligent data management architecture also reduces analysis time and cost (automation).

Schoen, P. D.↗

NASA NTRS1998

Sliding Mode Thermal Control System for Space Station Furnace Facility

The decoupled control of the nonlinear, multiinput-multioutput, and highly coupled space station furnace facility (SSFF) thermal control system is addressed. Sliding mode control theory, a subset of variable-structure control theory, is employed to increase the performance, robustness, and reliability of the SSFF's currently designed control system. This paper presents the nonlinear thermal control system description and develops the sliding mode controllers that cause the interconnected subsystems to operate in their local sliding modes, resulting in control system invariance to plant uncertainties and external and interaction disturbances. The desired decoupled flow-rate tracking is achieved by optimization of the local linear sliding mode equations. The controllers are implemented digitally and extensive simulation results are presented to show the flow-rate tracking robustness and invariance to plant uncertainties, nonlinearities, external disturbances, and variations of the system pressure supplied to the controlled subsystems.

Jackson Mark E.↗

NASA NTRS2008

Dynamical Modeling and Control Simulation of a Large Flexible Launch Vehicle

This paper presents dynamical models of a large flexible launch vehicle. A complete set of coupled dynamical models of propulsion, aerodynamics, guidance and control, structural dynamics, fuel sloshing, and thrust vector control dynamics are described. Such dynamical models are used to validate NASA s SAVANT Simulink-based program which is being used for the preliminary flight control systems analysis and design of NASA s Ares-1 Crew Launch Vehicle. SAVANT simulation results for validating the performance and stability of an ascent phase autopilot system of Ares-1 are also presented.

Du, Wei↗

NASA NTRS2012

X-56A Research Opportunities

The X-56A vehicle presents an excellent research opportunity for NASA. The vehicle will be used to prove the concepts for integrated structural control and modeling of extremely lightweight flexible structures.

Bosworth, John T.↗

NASA NTRS2013

SequenceL: Automated Parallel Algorithms Derived from CSP-NT Computational Laws

With the introduction of new parallel architectures like the cell and multicore chips from IBM, Intel, AMD, and ARM, as well as the petascale processing available for highend computing, a larger number of programmers will need to write parallel codes. Adding the parallel control structure to the sequence, selection, and iterative control constructs increases the complexity of code development, which often results in increased development costs and decreased reliability. SequenceL is a high-level programming language that is, a programming language that is closer to a human s way of thinking than to a machine s. Historically, high-level languages have resulted in decreased development costs and increased reliability, at the expense of performance. In recent applications at JSC and in industry, SequenceL has demonstrated the usual advantages of high-level programming in terms of low cost and high reliability. SequenceL programs, however, have run at speeds typically comparable with, and in many cases faster than, their counterparts written in C and C++ when run on single-core processors. Moreover, SequenceL is able to generate parallel executables automatically for multicore hardware, gaining parallel speedups without any extra effort from the programmer beyond what is required to write the sequen tial/singlecore code. A SequenceL-to-C++ translator has been developed that automatically renders readable multithreaded C++ from a combination of a SequenceL program and sample data input. The SequenceL language is based on two fundamental computational laws, Consume-Simplify- Produce (CSP) and Normalize-Trans - pose (NT), which enable it to automate the creation of parallel algorithms from high-level code that has no annotations of parallelism whatsoever. In our anecdotal experience, SequenceL development has been in every case less costly than development of the same algorithm in sequential (that is, single-core, single process) C or C++, and an order of magnitude less costly than development of comparable parallel code. Moreover, SequenceL not only automatically parallelizes the code, but since it is based on CSP-NT, it is provably race free, thus eliminating the largest quality challenge the parallelized software developer faces.

Cooke, Daniel↗

NASA NTRS2016

Historical Contributions to Vertical Flight at the NASA Langley Research Center

The NASA Langley Research Center has had a long and distinguished history in powered lift technology development. This research has formed the foundation of knowledge for the powered lift community worldwide. From aerodynamics to structures, aeromechanics, powered lift, acoustics, materials, stability & control, structural dynamics and human factors, Langley has made significant contributions to the advancement of vertical lift technologies. This research has encompassed basic phenomenological studies through subscale laboratory testing, analytical tool development, applied demonstrations and full scale flight-testing. Since the dedication of Langley in 1920, it has contributed to the understanding, design, analysis, and flight test development of experimental and production V/STOL configurations. This paper will chronicle significant areas of research through the decades from 1920 to 2015 with historical photographs and references.

Hodges, William T.↗

NASA NTRS1981

A controller design approach for large flexible space structures

A controller design approach for large space structures is presented, which consists of a primary attitude controller and a secondary or damping enhancement controller. The secondary controller, which uses several Annular Momentum Control Device (AMCD's), is shown to make the closed loop system asymptotically stable under relatively simple conditions. The primary controller using torque actuators (or AMCD's) and colocated attitude and rate sensors is shown to be stable. It is shown that the same AMCD's can be used for simultaneous actuation of primary and secondary controllers. Numerical results are obtained for a large, thin, completely free plate model.

Joshi, S. M.↗

NASA NTRS1990

Effects of source vs. interactive actuator assumptions on modelling of a smart structure

Embedded or internal actuators in a structure are being utilized to perform vibration attenuation, pointing maneuvers, damage control, and other adaptive tasks. The controls-structures-interaction (CSI) design process depends strongly on the coupled dynamic response of the structure and the actuators. A bond graph approach is used to guide this analytical study to uncover the effects of certain generic actuator types on the closed-loop, initial condition control of a single-mode structural model and an eighteen-state model of an active truss. So-called 'flow' and 'effort' source actuator types were studied and were shown to have significant differences in closed-loop response for ranges of initial conditions and system open-loop natural frequencies.

Robertshaw, Harry H.↗

NASA NTRS1993

Exact methods for modal transient response analysis including feedback control

This paper presents a modal method for the analysis of controlled structural systems that retains the uncoupled nature of the classical transient response analysis of a structure subjected to a prescribed time-varying load. The control force is expanded as a Taylor series that remains on the right side of the equations, and it does not lead to a computational approach that requires coupling between modes on the left side. Retaining a sufficient number of terms in the series produces a solution to the modal equations that is accurate to machine precision. The approach is particularly attractive for large problems in which standard matrix exponential methods become computationally prohibitive. Numerical results are presented to show the accuracy and efficiency of the proposed approach for dynamic feedback compensation of a truss structure with local member modes in the controller bandwidth.

Anderson, Melvin S.↗

NASA NTRS1991

The Langley Research Center CSI phase-0 evolutionary model testbed-design and experimental results

A testbed for the development of Controls Structures Interaction (CSI) technology is described. The design philosophy, capabilities, and early experimental results are presented to introduce some of the ongoing CSI research at NASA-Langley. The testbed, referred to as the Phase 0 version of the CSI Evolutionary model (CEM), is the first stage of model complexity designed to show the benefits of CSI technology and to identify weaknesses in current capabilities. Early closed loop test results have shown non-model based controllers can provide an order of magnitude increase in damping in the first few flexible vibration modes. Model based controllers for higher performance will need to be robust to model uncertainty as verified by System ID tests. Data are presented that show finite element model predictions of frequency differ from those obtained from tests. Plans are also presented for evolution of the CEM to study integrated controller and structure design as well as multiple payload dynamics.

Belvin, W. K.↗

NASA NTRS1989

Uncertainty modelling for the control of an active structure

Two approaches to uncertainty modeling of a precision active truss structure for control design are presented. The first approach is based on the H(infinity) optimal control theory, which is used for robust control design with an additive uncertainty model. It is designed to provide vibration suppression to selected modes of the structure at the accelerometer locations. The uncertainty model can be used as an adjustable design parameter which will enable a study of the trade-offs between performance and robust stability. The second approach is based on mu-synthesis. Although the structured singular value can be used to analyze the robustness of any type of uncertainty, there exist difficulties in synthesis if the uncertainty block contains both real and complex variations. Mu-synthesis techniques do not allow for real-parameter variations in the synthesis procedure. The use of a complex variation description enclosing the real variation is suggested.

Blackwood, G. H.↗

NASA NTRS1990

Robust tracking of a flexible spacecraft subject to induced disturbances and parametric uncertainties

A variable structure control technique combined with an operator spline for bilinear systems is implemented to a fast moving deformable structure for purposes of achieving robust tracking performance. The control torque applied to the structure is based on estimates of the structure parameters and on upper bounds of the model errors. For a rapid rotation of the deformable structure, the elastic response can be modeled by oscillators driven by angular acceleration, where stiffness and damping coefficients are also angular velocity and acceleration dependent. By transforming this 'slew-driven' elastic dynamics into a bilinear form, an operator spline can be constructed, that gives a low order estimate of the induced disturbance. An upper bound between the estimated deformation and the unknown exact deformation is also generated, which can be used where required in the sliding control correction.

Karray, Fakhreddine↗

NASA NTRS1993

Substructure-based control of flexible structures

A decentralized procedure is presented for the design of controllers for flexible structures. Spatially significant components are created which approximate the response of a specific part of the complete structure. For each component, the controller and observer gain matrices which are used in a controller for the complete structure. The proposed method is illustrated on a model of NASA Langley's CSI testbed structure.

Babuska, Vit↗