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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 361 records · Page 20

Fault-tolerant control of large space structures using the stable factorization approach

Large space structures are characterized by the following features: they are in general infinite-dimensional systems, and have large numbers of undamped or lightly damped poles. Any attempt to apply linear control theory to large space structures must therefore take into account these features. Phase I consisted of an attempt to apply the recently developed Stable Factorization (SF) design philosophy to problems of large space structures, with particular attention to the aspects of robustness and fault tolerance. The final report on the Phase I effort consists of four sections, each devoted to one task. The first three sections report theoretical results, while the last consists of a design example. Significant results were obtained in all four tasks of the project. More specifically, an innovative approach to order reduction was obtained, stabilizing controller structures for plants with an infinite number of unstable poles were determined under some conditions, conditions for simultaneous stabilizability of an infinite number of plants were explored, and a fault tolerance controller design that stabilizes a flexible structure model was obtained which is robust against one failure condition.

Razavi, H. C.↗

The use of automatic programming techniques for fault tolerant computing systems

It is conjectured that the production of software for ultra-reliable computing systems such as required by Space Station, aircraft, nuclear power plants and the like will require a high degree of automation as well as fault tolerance. In this paper, the relationship between automatic programming techniques and fault tolerant computing systems is explored. Initial efforts in the automatic synthesis of code from assertions to be used for error detection as well as the automatic generation of assertions and test cases from abstract data type specifications is outlined. Speculation on the ability to generate truly diverse designs capable of recovery from errors by exploring alternate paths in the program synthesis tree is discussed. Some initial thoughts on the use of knowledge based systems for the global detection of abnormal behavior using expectations and the goal-directed reconfiguration of resources to meet critical mission objectives are given. One of the sources of information for these systems would be the knowledge captured during the automatic programming process.

Wild, C.↗

Reconfiguration algorithms for tree architectures using sub-tree oriented fault tolerance

An approach to reconfiguration in tree architectures has been developed in which redundant processors are allocated at the leaves. The scheme is called sub-tree oriented fault tolerances (SOFT) and is capable of tolerating both link failures as well as multiple processor failures. In this paper, the SOFT scheme is examined from the perspective of reconfigurability. Specific algorithms are presented for reconfiguration.

Lowrie, M. B.↗

On the implementation and use of Ada on fault-tolerant distributed systems

Due to a lack of distribution semantics and failure semantics, Ada was found to be inadequte for programming distributed systems in which process failure has to be tolerated. It is shown, however, that it is possible to define adequate distribution and failure semantics for Ada so that the language can be used to program fault-tolerant distributed systems. It is noted that although the new semantics require no additions or changes to Ada's syntax, an extensive support system is necessary to implement them.

Knight, John C.↗

A survey and comparison of fault-tolerant multistage interconnection networks

Intrinsically fault-tolerant multistage interconnection networks (MINs) are surveyed, categorizing the MINs by the amount of hardware modifications they use to provide redundancy. The diversity of such MINs and the scope of fault-tolerance techniques are examined. A hypothetical MIN with ideal engineering characteristics is defined, and the surveyed MINs are compared to this standard.

Adams, George B., III↗

Reliability modeling of fault-tolerant computer based systems

Digital fault-tolerant computer-based systems have become commonplace in military and commercial avionics. These systems hold the promise of increased availability, reliability, and maintainability over conventional analog-based systems through the application of replicated digital computers arranged in fault-tolerant configurations. Three tightly coupled factors of paramount importance, ultimately determining the viability of these systems, are reliability, safety, and profitability. Reliability, the major driver affects virtually every aspect of design, packaging, and field operations, and eventually produces profit for commercial applications or increased national security. However, the utilization of digital computer systems makes the task of producing credible reliability assessment a formidable one for the reliability engineer. The root of the problem lies in the digital computer's unique adaptability to changing requirements, computational power, and ability to test itself efficiently. Addressed here are the nuances of modeling the reliability of systems with large state sizes, in the Markov sense, which result from systems based on replicated redundant hardware and to discuss the modeling of factors which can reduce reliability without concomitant depletion of hardware. Advanced fault-handling models are described and methods of acquiring and measuring parameters for these models are delineated.

Bavuso, Salvatore J.↗

Experiments in fault tolerant software reliability

The reliability of voting was evaluated in a fault-tolerant software system for small output spaces. The effectiveness of the back-to-back testing process was investigated. Version 3.0 of the RSDIMU-ATS, a semi-automated test bed for certification testing of RSDIMU software, was prepared and distributed. Software reliability estimation methods based on non-random sampling are being studied. The investigation of existing fault-tolerance models was continued and formulation of new models was initiated.

Mcallister, David F.↗

Reliability of voting in fault-tolerant software systems for small output spaces

Under a voting strategy in a fault-tolerant software system there is a difference between correctness and agreement. An independent N-version programming reliability model is proposed for treating small output spaces which distinguishes between correctness and agreement. System reliability is investigated using analytical relationships and simulation. A consensus majority voting strategy is proposed and its performance is analyzed and compared with other voting strategies. Consensus majority strategy automatically adapts the voting to different component reliability and output space cardinality characteristics. It is shown that absolute majority voting strategy provides a lower bound on the reliability provided by the consensus majority, and 2-of-n voting strategy an upper bound. If r is the cardinality of the output space it is proved the 1/r is a lower bound on the average reliability of fault-tolerant system components below which the system reliability begins to deteriorate as more versions are added.

Mcallister, David F.↗

Rule-based fault-tolerant flight control

Fault tolerance has always been a desirable characteristic of aircraft. The ability to withstand unexpected changes in aircraft configuration has a direct impact on the ability to complete a mission effectively and safely. The possible synergistic effects of combining techniques of modern control theory, statistical hypothesis testing, and artificial intelligence in the attempt to provide failure accommodation for aircraft are investigated. This effort has resulted in the definition of a theory for rule based control and a system for development of such a rule based controller. Although presented here in response to the goal of aircraft fault tolerance, the rule based control technique is applicable to a wide range of complex control problems.

Handelman, Dave↗

Fuel containment, lightning protection and damage tolerance in large composite primary aircraft structures

The damage-tolerance characteristics of high strain-to-failure graphite fibers and toughened resins were evaluated. Test results show that conventional fuel tank sealing techniques are applicable to composite structures. Techniques were developed to prevent fuel leaks due to low-energy impact damage. For wing panels subjected to swept stroke lightning strikes, a surface protection of graphite/aluminum wire fabric and a fastener treatment proved effective in eliminating internal sparking and reducing structural damage. The technology features developed were incorporated and demonstrated in a test panel designed to meet the strength, stiffness, and damage tolerance requirements of a large commercial transport aircraft. The panel test results exceeded design requirements for all test conditions. Wing surfaces constructed with composites offer large weight savings if design allowable strains for compression can be increased from current levels.

Griffin, Charles F.↗

Brain cholinergic involvement during the rapid development of tolerance to morphine

The effect of repeated administration of morphine on the activities of the cholinergic enzymes, choline acetyltransferase (ChAT) and acetylcholinesterase (AChE), in specific brain regions were studied in rats treated with 10 mg/kg morphine for one or two days. Repeated administration of morphine was associated with a decline in the degree of analgesia produced and with a significant increase of AChE activity of the medulla oblongata. A single injection of morphine resulted in a significant decline in ChAT activity in the hypothalamus, cerebellum, and medulla oblongata regions. After two consecutive injections, no decline in ChAT was observed in these regions, while in the cerebral cortex the second administration elicited a significant decline. The results suggest that the development of tolerance to morphine may be mediated through changes in ChAT activity and lend support to the involvement of the central cholinergic system in narcotic tolerance.

Wahba, Z. Z.↗

Gigarad-tolerant power switches and memory elements

A new class of silicon devices operating without p-n junctions has been studied for radiation hardness. The electric field in these devices is uniform over most of the device, thus causing high-voltage breakdown to increase with electrode spacing. Deep acceptor levels are created in the silicon by diffusing gold atoms into the lattice, or by electron radiation. These acceptor levels, near the center of the band gap, trap out electrons so that the low resistivity of the n-type doped wafer can be raised to intrinsic resistivity. Switching between a high-resistivity state at low currents and a low-resistivity state at high currents occurs at a definite threshold voltage, exhibiting characteristics similar to silicon-controlled rectifiers. Compensating 0.1 ohm-meter (10 ohm-cm) silicon wafers requires electron fluxes of 10 to the 23rd electrons per square meter (10 to the 19th electrons per square centimeter). Experiments demonstrating gamma tolerance to 1 Gigarad(Si) are described. Calculations for maximum tolerable neutron fluence are shown, and a phenomenological explanation for these results is presented.

Joyner, W. T.↗

Design of an optical PPM communication link in the presence of component tolerances

A systematic approach is described for estimating the performance of an optical direct detection pulse position modulation (PPM) communication link in the presence of parameter tolerances. This approach was incorporated into the JPL optical link analysis program to provide a useful tool for optical link design. Given a set of system parameters and their tolerance specifications, the program will calculate the nominal performance margin and its standard deviation. Through use of these values, the optical link can be designed to perform adequately even under adverse operating conditions.

Chen, C.-C.↗

Damage tolerant composite wing panels for transport aircraft

Commercial aircraft advanced composite wing surface panels were tested for durability and damage tolerance. The wing of a fuel-efficient, 200-passenger airplane for 1990 delivery was sized using grahite-epoxy materials. The damage tolerance program was structured to allow a systematic progression from material evaluations to the optimized large panel verification tests. The program included coupon testing to evaluate toughened material systems, static and fatigue tests of compression coupons with varying amounts of impact damage, element tests of three-stiffener panels to evaluate upper wing panel design concepts, and the wing structure damage environment was studied. A series of technology demonstration tests of large compression panels is performed. A repair investigation is included in the final large panel test.

Smith, Peter J.↗

Predeployment validation of fault-tolerant systems through software-implemented fault insertion

Fault injection-based automated testing (FIAT) environment, which can be used to experimentally characterize and evaluate distributed realtime systems under fault-free and faulted conditions is described. A survey is presented of validation methodologies. The need for fault insertion based on validation methodologies is demonstrated. The origins and models of faults, and motivation for the FIAT concept are reviewed. FIAT employs a validation methodology which builds confidence in the system through first providing a baseline of fault-free performance data and then characterizing the behavior of the system with faults present. Fault insertion is accomplished through software and allows faults or the manifestation of faults to be inserted by either seeding faults into memory or triggering error detection mechanisms. FIAT is capable of emulating a variety of fault-tolerant strategies and architectures, can monitor system activity, and can automatically orchestrate experiments involving insertion of faults. There is a common system interface which allows ease of use to decrease experiment development and run time. Fault models chosen for experiments on FIAT have generated system responses which parallel those observed in real systems under faulty conditions. These capabilities are shown by two example experiments each using a different fault-tolerance strategy.

Czeck, Edward W.↗

General linear codes for fault-tolerant matrix operations on processor arrays

Various checksum codes have been suggested for fault-tolerant matrix computations on processor arrays. Use of these codes is limited due to potential roundoff and overflow errors. Numerical errors may also be misconstrued as errors due to physical faults in the system. In this a set of linear codes is identified which can be used for fault-tolerant matrix operations such as matrix addition, multiplication, transposition, and LU-decomposition, with minimum numerical error. Encoding schemes are given for some of the example codes which fall under the general set of codes. With the help of experiments, a rule of thumb for the selection of a particular code for a given application is derived.

Nair, V. S. S.↗

Human tolerance to space flight

Medical studies of astronauts and cosmonauts before, during, and after space missions have identified several effects of weightlessness and other factors that influence the ability of humans to tolerate space flight. Weightlessness effects include space motion sickness, cardiovascular abnormalities, reduction in immune system function, loss of red blood cells, loss of bone mass, and muscle atrophy. Extravehicular activity (EVA) increases the likelihood that decompression sickness may occur. Radiation also gives reason for concern about health of crewmembers, and psychological factors are important on long-term flights. Countermeasures that have been used include sensory preadaptation, prebreathing and use of various air mixtures for EVA, loading with water and electrolytes, exercise, use of pharmacological agents and special diets, and psychological support. It appears that humans can tolerate and recover satisfactorily from at least one year of space flight, but a number of conditions must be further ameliorated before long-duration missions can be considered routine.

Huntoon, C. L.↗

Investigation of the applicability of a functional programming model to fault-tolerant parallel processing for knowledge-based systems

In a fault-tolerant parallel computer, a functional programming model can facilitate distributed checkpointing, error recovery, load balancing, and graceful degradation. Such a model has been implemented on the Draper Fault-Tolerant Parallel Processor (FTPP). When used in conjunction with the FTPP's fault detection and masking capabilities, this implementation results in a graceful degradation of system performance after faults. Three graceful degradation algorithms have been implemented and are presented. A user interface has been implemented which requires minimal cognitive overhead by the application programmer, masking such complexities as the system's redundancy, distributed nature, variable complement of processing resources, load balancing, fault occurrence and recovery. This user interface is described and its use demonstrated. The applicability of the functional programming style to the Activation Framework, a paradigm for intelligent systems, is then briefly described.

Harper, Richard↗