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NASA NTRS1975

The growth of deactivated layers on CsI(Na) scintillating crystals

An effective and sensitive measurement of the depth of a deactivated or dead layer can be obtained from the relative attenuation of the 22.162 KeV and 87.9 KeV X-rays emitted by Cd 109. The alpha-particles emitted by Am 241 are also useful in measuring dead layers less than 25 microns. The properties and temporal development of dead layers are discussed in detail. The rate of growth of a deal layer is closely related to the ambient humidity, and the damage to the crystal is irreversible by any known process. The dead layer can be minimized by polishing all crystal surfaces and by keeping the crystal in a vacuum or a dry atmosphere. Since a dead layer seriously inhibits the response of a crystal to X-rays of energies below approximately 20 keV, CsI(Na) detectors should not be used at these energies unless precautions are taken to ensure that no dead layer forms.

Goodman, N. B.↗

NASA NTRS1990

Remote Manipulator System (RMS)-based Controls-Structures Interaction (CSI) flight experiment feasibility study

The feasibility of an experiment which will provide an on-orbit validation of Controls-Structures Interaction (CSI) technology, was investigated. The experiment will demonstrate the on-orbit characterization and flexible-body control of large flexible structure dynamics using the shuttle Remote Manipulator System (RMS) with an attached payload as a test article. By utilizing existing hardware as well as establishing integration, operation and safety algorithms, techniques and procedures, the experiment will minimize the costs and risks of implementing a flight experiment. The experiment will also offer spin-off enhancement to both the Shuttle RMS (SRMS) and the Space Station RMS (SSRMS).

Demeo, Martha E.↗

NASA NTRS1989

NASA CSI suspension methods overview

New suspension techniques will be necesary for ground testing the flexible spacecraft anticipated in NASA's future space activity. The most complex spacecraft involve nonlinear maneuvering (i.e., large angle slewing) with articulating substructures such as remote manipulating systems. The NASA control-structure interaction (CSI) ground test method team has begun researching and developing methodology to suspend the future class of spacecraft. This overview describes the work completed thus far. The research objective and technical approach will be presented first. Second, will be a suspension device overview followed by an assessment of existing hardware. Two different mechanical zero-spring-rate mechanisms will be compared for optimal performance. Next, will be a description of how existing hardware can be evolved to meet more general suspension requirements. A comparison of suspending articulating structures overhead vs underneath will follow. After a few experimental results from the zero-spring-rate mechanism/air suspension cart will be concluding remarks and future work.

Woodard, Stanley E.↗

NASA NTRS1989

CSI technology validation on an LSS ground experiment facility

The test bed developed at JPL for experimental evaluation of new technologies for the control of large flexible space structures is described. The experiment consists of a flexible spacecraft dynamic simulator, sensors, actuators, a microcomputer, and an advanced programming environment. The test bed has been operational for over a year, and thus far nine experiments were completed or are currently in progress. Several of these experiments were reported at the 1987 CSI conference, and several recent ones are documented in this paper, including high order adaptive control, non-parametric system identification, and mu-synthesis robust control. An aggressive program of experiments is planned for the forseeable future.

Wang, S. J.↗

NASA NTRS1989

Algorithmic considerations of integrated design for CSI on a hypercube architecture

An approach is presented to the integrated design problem for actively controlled large, flexible mechanical systems for which Control Structure Interaction (CSI) problems are of concern. The two coupled design problems were identified as the optimal Structural Design problem the optimal Controller Design problem. These two problems can be addressed within a decision making loop that would consider each separately, and then sequentially analyze the effects of one on the other. Embedded in such a loop would be the simulation and coordination tasks as part of the decision tools required in a total (software) package. All of the above are compute-intensive tasks. In any such task, possible decompositions and gains due to the inherent parallelism have to be exploited. The problems under consideration, as applied to large flexible mechanical structures, are particularly suited to be mapped onto multicomputer systems in a hypercube topology.

Oezguener, UE.↗

NASA NTRS1990

High-pressure phase transition and equation of state of CsI

Stuructural properties of CsI at high pressure have been investigated by the use of synchrotron X-ray-diffraction techniques with the diamond-anvil cell at 300 K. The results indicate that the material undergoes a continuous distortion from B2 to an hcp-like phase, which is stable to at least 302 GPa. The intermediate orthorhombic phase is geometrically different from previous assignments and about 10 percent denser. With the new structure, the p-V results of static pressure experiments, shock-wave studies, and theoretical calculations are now in good agreement.

Mao, H. K.↗

NASA NTRS1991

Experimental demonstration of a classical approach for flexible space structure control: NASA CSI testbeds

The results of active control experiments performed for the Mini-Mast truss structure are presented. The primary research objectives were: (1) to develop active structural control concepts and/or techniques; (2) to verify the concept of robust non-minimum-phase compensation for a certain class of non-colocated structural control problems through ground experiments; (3) to verify a 'dipole' concept for persistent disturbance rejection control of flexible structures; and (4) to identify CSI (Control Structure Interaction) issues and areas of emphasis for the next generation of large flexible spacecraft. The classical SISO (Single Input and Single Output) control design approach was employed.

Bond, Wie↗

NASA NTRS1993

CSI related dynamics and control issues in space robotics

The research addressed includes: (1) CSI issues in space robotics; (2) control of elastic payloads, which includes 1-DOF example, and 3-DOF harmonic drive arm with elastic beam; and (3) control of large space arms with elastic links, which includes testbed description, modeling, and experimental implementation of colocated PD and end-point tip position controllers.

Schmitz, Eric↗

NASA NTRS1993

A synopsis of test results and knowledge gained from the Phase-0 CSI evolutionary model

The Phase-0 CSI Evolutionary Model (CEM) is a testbed for the study of space platform global line-of-sight (LOS) pointing. Now that the tests have been completed, a summary of hardware and closed-loop test experiences is necessary to insure a timely dissemination of the knowledge gained. The testbed is described and modeling experiences are presented followed by a summary of the research performed by various investigators. Some early lessons on implementing the closed-loop controllers are described with particular emphasis on real-time computing requirements. A summary of closed-loop studies and a synopsis of test results are presented. Plans for evolving the CEM from phase 0 to phases 1 and 2 are also described. Subsequently, a summary of knowledge gained from the design and testing of the Phase-0 CEM is made.

Belvin, W. Keith↗

NASA NTRS1992

Active strut placement using integer programming for the CSI Revolutionary Model

A method for determining the most effective locations for active struts on large space structures is developed and tested on the NASA CSI Evolutionary Model. Depending on the choice of weighting factors, the method can be used to maximize the maximum modal damping ratio or decay rate, or to maximize a mission-oriented measure of performance. Placement of 8, 16, and 32 active struts out of 1507 candidate truss elements is demonstrated. Preliminary estimates of damping enhancement are reported pending refined structural models and dynamic test results. The method can handle complicated FEM models wih a large number of truss elements and many target modes. It can treat each mode equally or it can emphasize the importance of selected modes. The method can eliminate some combinations of actuator locations based on topological constraints.

Padula, Sharon L.↗

NASA NTRS1992

Line-of-sight control of the CSI evolutionary model - mu control

The design, analysis, and simulation of a control law that guarantees a fine pointing performance under external disturbances and model uncertainties are investigated. The authors examine the achievable performance and modeling errors for the control-structure interaction (CSI) evolutionary model structure and highlight the importance of accurate knowledge of structural frequencies and damping values for very lightly damped structures. Numerical results indicate that a limited but significant line-of-sight pointing performance robustness can be attained in spite of modeled structured and unstructured uncertainties in the system. The simulation results reported demonstrate typical performance levels, types of uncertainties and their magnitudes, and design tradeoffs for a realistic laboratory model of a large flexible space structure.

Lim, K. B.↗

NASA NTRS1993

Integrated design and experimental validation of the CSI evolutionary structure

An experimental validation of the optimization-based integrated design methodology performed under the Controls-Structures Interaction (CSI) program for a class of flexible spacecraft is reviewed. The studies have been performed using an integrated design software tool which is under development at the NASA-Langley Research Center. It is analytically and experimentally demonstrated that integrated controls-structures design can yield designs which are substantially superior to those obtained through the traditional sequential approach.

Maghami, P. G.↗

NASA NTRS1992

Joint Langley Research Center/Jet Propulsion Laboratory CSI experiment

This paper describes a joint Control Structure Interaction (CSI) experiment in which Jet Propulsion Laboratory (JPL) damping devices were incorporated into the Langley Research Center (LaRC) Phase 0 Testbed. The goals of the effort were twofold: (1) test the effectiveness of the JPL structural damping methods in a new structure and (2) assess the feasibility of combining JPL local control methods with the LaRC multiple input multiple output global control methods. Six dampers (2 piezoelectric active members, 4 viscous dampers), placed in three different regions of the structure, produced up to 26 dB attenuation in target modes. The combined control strategy in which the JPL damping methods contributed local control action and the LaRC control scheme provided global control action, produced and overall control scheme with increased stability margins and improved performance. This paper presents an overview of the technologies contributed from the two centers, the strategies used to combine them, and results demonstrating the success of the damping and cooperative control efforts.

Neat, Gregory W.↗

NASA NTRS1994

Integrated active and passive control design methodology for the LaRC CSI evolutionary model

A general design methodology to integrate active control with passive damping was demonstrated on the NASA LaRC CSI Evolutionary Model (CEM), a ground testbed for future large, flexible spacecraft. Vibration suppression controllers designed for Line-of Sight (LOS) minimization were successfully implemented on the CEM. A frequency-shaped H2 methodology was developed, allowing the designer to specify the roll-off of the MIMO compensator. A closed loop bandwidth of 4 Hz, including the six rigid body modes and the first three dominant elastic modes of the CEM was achieved. Good agreement was demonstrated between experimental data and analytical predictions for the closed loop frequency response and random tests. Using the Modal Strain Energy (MSE) method, a passive damping treatment consisting of 60 viscoelastically damped struts was designed, fabricated and implemented on the CEM. Damping levels for the targeted modes were more than an order of magnitude larger than for the undamped structure. Using measured loss and stiffness data for the individual damped struts, analytical predictions of the damping levels were very close to the experimental values in the (1-10) Hz frequency range where the open loop model matched the experimental data. An integrated active/passive controller was successfully implemented on the CEM and was evaluated against an active-only controller. A two-fold increase in the effective control bandwidth and further reductions of 30 percent to 50 percent in the LOS RMS outputs were achieved compared to an active-only controller. Superior performance was also obtained compared to a High-Authority/Low-Authority (HAC/LAC) controller.

Voth, Christopher T.↗

NASA NTRS1994

LQG optimal compensator transfer function for the NASA LaRC CSI Evolutionary Model

Following the general form for LQG optimal compensators for flexible structures with collocated rate sensors we develop an explicit compensator transfer function for the NASA LaRC CSI Evolutionary model in the form: psi(i omega) = g i omega B(sub u)(sup *)(-M(sub b)omega(exp 2) + T(i omega) + i gamma omega B(sub u)Bu(sub u)(sup *))(exp -1)B(sub u) where T(i omega) is a 48 x 48 positive definite matrix whose derivation is the main result of this report. The undamped mode frequencies can be expressed in terms of T(i omega) as the zeros of Det (-omega(exp 2)M(sub b) + T(i omega)) while 'clamped-clamped' modes of the structure (with all nodes clamped) are the poles.

Balakrishnan, A. V.↗

NASA NTRS2015

CSI 2264: Characterizing Young Stars in NGC 2264 With Short-Duration Periodic Flux Dips in Their Light Curves

We identify nine young stellar objects (YSOs) in the NGC 2264 star-forming region with optical CoRoT light curves exhibiting short-duration, shallow, periodic flux dips. All of these stars have infrared (IR) excesses that are consistent with their having inner disk walls near the Keplerian corotation radius. The repeating photometric dips have FWHM generally less than one day, depths almost always less than 15%, and periods (3 < P < 11 days) consistent with dust near the Keplerian co-rotation period. The flux dips vary considerably in their depth from epoch to epoch, but usually persist for several weeks and, in two cases, were present in data collected on successive years. For several of these stars, we also measure the photospheric rotation period and find that the rotation and dip periods are the same, as predicted by standard \disk-locking" models. We attribute these flux dips to clumps of material in or near the inner disk wall, passing through our line of sight to the stellar photosphere. In some cases, these dips are also present in simultaneous Spitzer IRAC light curves at 3.6 and 4.5 microns. We characterize the properties of these dips, and compare the stars with light curves exhibiting this behavior to other classes of YSO in NGC 2264. A number of physical mechanisms could locally increase the dust scale height near the inner disk wall, and we discuss several of those mechanisms; the most plausible mechanisms are either a disk warp due to interaction with the stellar magnetic field or dust entrained in funnel- ow accretion columns arising near the inner disk wall.

CSI 2264↗

NASA NTRS1989

CSI sensing and control: Analytical and experimental results

Recent work on structural identification and large-angle maneuvers with vibration suppression was presented. The recent work has sought to balance structural and controls analysis activities by involving the analysts directly in the validation and experimental aspects of the research. Some new sensing, actuation, system identification, and control concepts were successfully implemented. An overview of these results is given.

Junkins, J. L.↗

NASA NTRS1989

Issues in CSI analysis for large scale systems

Future spacecraft such as the International Space Station result in flexible models with hundreds, or perhaps thousands of modes in a frequency range where the potential for control/structure interaction exists. This provides the analyst with a formidable model reduction problem at both the component and the system level. Approaches to normal modes as a structural representation, applicability of alternate structural representations and algorithms for selecting important modal degrees of freedom at both the component and the system level are discussed. Practical implementation of these techniques on a large scale model of the Space Station are presented.

Blelloch, Paul↗