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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 793 records · Page 44
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Experimental study of robustness in adaptive control for large flexible structures

An experimental study is performed to investigate the robustness of model reference adaptive control for the large flexible structures control application. The main nonidealities of concern are unmodeled dynamics, input saturation, and time delay effects (here, actuator and sensor dynamics are lumped into the last item for convenience). This study focuses on the robustness with respect to input saturation and time delay effects, since robustness to unmodeled dynamics is inherent to the basic algorithm and has been demonstrated experimentally elsewhere.

Ih, Che-Hang Charles↗
NASA NTRS1991

Experiments In Multivariable Adaptive Control Of A Flexible Structure

Report describes experiments in use of six-input/six-output multivariable adaptive control system to suppress vibrations in complicated flexible structure. Represents significant upgrade relative to two-input/two-output system, yielding significant improvement in spatial controllability and effective damping of largest set of modes. Structure exhibits low modal frequencies and complicated dynamics like those of large structures in outer space. Conclusions drawn from research relevant to such applications as active suppression of wind and earthquake vibrations in tall buildings and other large terrestrial structures.

Ih, Che-Hang C.↗
NASA NTRS1991

Damping Of Vibrations In Graphite/Epoxy Structures

Report describes study of vibration-damping properties, in air and vacuum, of graphite/epoxy composite structural members joined by adhesives. Determines vibration-damping effects of specimens representative of members and joints of truss, particularly after gases and moisture trapped during manufacture released in vacuum. Damping ratios measured under free and forced vibrations and before and after outgassing in vacuum. Effects of end-support conditions and high and low temperatures on damping also measured.

Crocker, Malcolm J.↗
NASA NTRS1991

Large Angle Transient Dynamics (LATDYN) user's manual

A computer code for modeling the large angle transient dynamics (LATDYN) of structures was developed to investigate techniques for analyzing flexible deformation and control/structure interaction problems associated with large angular motions of spacecraft. This type of analysis is beyond the routine capability of conventional analytical tools without simplifying assumptions. In some instances, the motion may be sufficiently slow and the spacecraft (or component) sufficiently rigid to simplify analyses of dynamics and controls by making pseudo-static and/or rigid body assumptions. The LATDYN introduces a new approach to the problem by combining finite element structural analysis, multi-body dynamics, and control system analysis in a single tool. It includes a type of finite element that can deform and rotate through large angles at the same time, and which can be connected to other finite elements either rigidly or through mechanical joints. The LATDYN also provides symbolic capabilities for modeling control systems which are interfaced directly with the finite element structural model. Thus, the nonlinear equations representing the structural model are integrated along with the equations representing sensors, processing, and controls as a coupled system.

Abrahamson, A. Louis↗
NASA NTRS1991

A finite element approach for the dynamic analysis of joint-dominated structures

A finite element method to model dynamic structural systems undergoing large rotations is presented. The dynamic systems are composed of rigid joint bodies and flexible beam elements. The configurations of these systems are subject to change due to the relative motion in the joints among interconnected elastic beams. A body fixed reference is defined for each joint body to describe the joint body's displacements. Using the finite element method and the kinematic relations between each flexible element and its corotational reference, the total displacement field of an element, which contains gross rigid as well as elastic effects, can be derived in terms of the translational and rotational displacements of the two end nodes. If one end of an element is hinged to a joint body, the joint body's displacements and the hinge degree of freedom at the end are used to represent the nodal displacements. This results in a highly coupled system of differential equations written in terms of hinge degrees of freedom as well as the rotational and translational displacements of joint bodies and element nodes.

Chang, Che-Wei↗
NASA NTRS1990

Modeling and stability of segmented reflector telescopes - A decentralized approach

The decentralization of a segmented reflector telescope based on a finite-element model of its structure is considered. The decentralization of the system at the panel level is considered. Each panel is originally treated as an isolated subsystem so that the controller design is performed independently at the local level, and then applied to the composite system for stability analysis. The panel-level control laws were designed by means of pole placement using local output feedback. Simulation results show a better 1000:1 vibration attenuation in panel position when compared to the open-loop system. It is shown that the overall closed-loop system is exponentially stable provided that certain conditions are met. The advantage to the decentralized approach is that the design is performed in terms of the low-dimensionality subsystems, thus drastically reducing the design computational complexities.

Ryaciotaki-Boussalis, Helen A.↗
NASA NTRS1992

Computing Large-Angle Transients In Structures

LATDYN is computer code for calculating large-angle transient dynamics of flexible articulating structures and mechanisms including joints about which members rotate through large angles. Extends and brings together some of aspects of finite-element analysis of structures, dynamics of multiple bodies, and analysis of control systems. Combines significant portions of their distinct capabilities into one analysis software tool. Software consists of three FORTRAN computer programs: LATDYN program, Preprocessor, and Postprocessor. Written in FORTRAN 77.

Housner, Jerrold M.↗
NASA NTRS1992

Modeling For Stabilization Of Segmented Telescope Reflector

Report discusses mathematical modeling of vibrations and active control to suppress vibrations of large, segmented telescope reflector. Vibrating structure treated as collection of substructures, each initially treated separately. Vibrations suppressed considerably, in comparison with open-loop vibration-attenuation mechanism.

Ih, Che-Hang C.↗
NASA NTRS1992

Dynamics And Control Of Flexible Manipulator

Nonlinear equations solved with linear approximations applicable during small increments of time. Report presents theoretical study of selected aspects of dynamics and control of robotic manipulator that includes multiple flexible links. Addresses problem of control primarily from perspective of inverse-dynamics problem.

Gawronski, Wodek K.↗
NASA NTRS1991

Dynamic performance modeling and stability analysis of a segmented reflector telescope

The problem of vibration suppression in segmented reflector telescopes is considered. The decomposition of the structure into smaller components is discussed, and control laws for vibration suppression and conditions for stability at the local and global levels are presented. The states of the reflector segments are mapped into ray displacements on the detector plane.

Ryaciotaki-Boussalis, Helen A.↗
NASA NTRS1992

Finite element approach for transient analysis of multibody systems

A three-dimensional, finite element based formulation for the transient dynamics of constrained multibody systems with trusslike configurations is presented. A convected coordinate system is used to define the rigid-body motion of individual elements in the system. Deformation of each element is defined relative to its convected coordinate system. The formulation is oriented toward joint-dominated structures. Through a series of sequential transformations, the joint degree of freedom is built into the equations of motion of the element to reduce geometric constraints. Based on the derivation, a general-purpose code has been developed. Two examples are presented to illustrate the application of the code.

Wu, Shih-Chin↗
NASA NTRS1993

Robustness In Suppressing Vibrations With Adaptive Control

Report describes theoretical and experimental study of robustness of adaptive model-reference control scheme designed to suppress vibrations in large, flexible structure. Model can be mathematical model or collection of models approximating dynamics of structure, vibration sensors, and vibration-suppressing actuators.

Ih, Che-Hang C.↗
NASA NTRS1991

Results of calibrations of the NOAA-11 AVHRR made by reference to calibrated SPOT imagery at White Sands, N.M

The calibration method reported here makes use of the reflectances of several large, uniform areas determined from calibrated and atmospherically corrected SPOT Haute Resolution Visible (HRV) scenes of White Sands, New Mexico. These reflectances were used to predict the radiances in the first two channels of the NOAA-11 Advanced Very High Resolution Radiometer (AVHRR). The digital counts in the AVHRR image corresponding to these known reflectance areas were determined by the use of two image registration techniques. The plots of digital counts versus pixel radiance provided the calibration gains and offsets for the AVHRR. A reduction in the gains of 4 and 13 percent in channels 1 and 2 respectively was found during the period 1988-11-19 to 1990-6-21. An error budget is presented for the method and is extended to the case of cross-calibrating sensors on the same orbital platform in the Earth Observing System (EOS) era.

Nianzeng, Che↗
NASA NTRS1992

Grasp synthesis for planar and solid objects

This paper presents an analysis of the mechanics for multifingered grasps of planar and solid objects. Squeezing and frictional effects between the fingers and the grasped objects is fully visualized through our approach. An algorithm for qualitively choosing the grasp points is developed based on the mechanics of grasping. It is shown further that our method can be easily extended for the soft-fingered grasp model where the torsional moments along the contact normals can be transmitted through the grasp points.

Chen, Yu-Che↗
NASA NTRS1993

Grasp synthesis for planar and solid objects

An analysis of the mechanics for multifingered grasps of planar and solid objects is presented. A method that is intuitive and computationally efficient is proposed. The search for finger grasp positions is combined with finger (manipulation and squeezing) for calculations in a single method. Physically, the squeezing and frictional effects between the fingers and the grasped objects are fully visualized through this approach. Mathematically, the complexity of finger force calculations are reduced when this scheme is compared with previously available schemes. The efficiency of the scheme is illustrated. On the basis of the analysis of grasp mechanics, an algorithm for quantitatively choosing the grasp points is proposed to ensure stable grasps.

Chen, Yu-Che↗
NASA NTRS1992

Visualization of redundancy resolution for kinematically redundant robots through the Jacobian null space

We present a unified formulation for the inverse kinematics of redundant arms, based on a special formulation of the null space of the Jacobian. By extending (appropriately re-scaling) previously used null space parameterizations, we obtain, in a unified fashion, the manipulability measure, the null space projector, and particular solutions for the joint velocities. We obtain the minimum norm pseudo-inverse solution as a projection from any particular solution, and the method provides an intuitive visualization of the self-motion. The result is a computationally efficient, consistent approach to computing redundant robot inverse kinematics.

Chen, Yu-Che↗
NASA NTRS1994

Pulley With Active Antifriction Actuator And Control

Torque actuator and associated control system minimizes effective friction of rotary bearing. Motor exerts compensating torque in response to feedback from external optical sensor. Compensation torque nearly cancels frictional torque of shaft bearings. Also useful in reducing bearing friction in gyro-scopes, galvanometers, torquemeters, accelerometers, earth-motion detectors, and balances.

Ih, Che-Hang C.↗