Showing papers by "Jet Propulsion Laboratory published in 1988"

Experimental and simulation studies of hard contact in force reflecting teleoperation

Abstract: Experiments and simulations of a single-axis force-reflecting teleoperation system have been conducted to investigate the problem of contacting a hard environment and maintaining a controlled force in teleoperation in which position is fed forward from the hand controller (master) to the manipulator (slave), and force is fed back to the human operator through motors in the master. The simulations, using an electrical circuit model, reproduce the behavior of the real system, including effects of human operator biomechanics. It is shown that human operator properties, which vary as a result of different types of grasp of the handle, affect the stability of the system in the hard-contact task. The effect of a heavier grasp on the handle is equivalent to increased hand-controlled velocity damping in terms of the systems stability in the contact task, but control system damping sufficient to guarantee stable contact results in perceptible sluggishness of the control handle's response in free motion. These results suggest that human operator biomechanics must be taken into account to guarantee stable and ergonomic performance of advanced teleoperators. >

The dust distribution within the inner coma of comet P/Halley 1982i: encounter by Giotto's impact detectors.

Abstract: Analysis of the data from Giotto’s Dust Impact Detection System experiment (DIDSY) is presented These data represent measurement of the size of dust grains incident on the Giotto dust shield along its trajectory through the coma of comet P/Halley on 1986 March 13/14 First detection occurred at some 287000 km distance from the nucleus on the inbound leg; the majority of the DIDSY subsystems remained operational after closest approach (604 km) yielding the last detection at about 202000 km from the nucleus

On the K-Winners-Take-All Network

Abstract: We present and rigorously analyze a generalization of the Winner-Take-All Network: the K-Winners-Take-All Network. This network identifies the K largest of a set of N real numbers. The network model used is the continuous Hopfield model.

Measured line positions and strengths of methane between 5500 and 6180 cm(-1).

Abstract: The positions and strengths of more than 2000 methane absorption lines stronger than 1.0 x 10(-4) cm(-2) atm(-1) have been measured in the 5500-6180-cm(-1) spectral region. The measurements were made using spectra recorded at room temperature (296 K) with a resolution of 0.01 cm(-1). The spectra were obtained using the Fourier transform spectrometer at the McMath solar telescope at the Kitt Peak National Observatory. The 2nu(3) band strength of (12)CH(4) was determined to be 1.641 cm(-2) atm(-1); a Herman-Wallis-type correction was also determined to be (1 + 0.0298 m + 0.000536 M(2)).

An Experimental Analysis of Unanimity in Public Goods Provision Mechanisms

Abstract: The paper reports on an experimental investigation of four methods of allocating public goods. The two basic processes studied are direct contribution and a public goods auction process. Both of these processes are studied with and without an additional unanimity feature. The results suggest that the auction process outperforms direct contribution. The effect of unanimity is to decrease the efficiency of both processes. Much of the paper is focused on an analysis of these results.

Ion temperature and flow profiles in comet P/Halley's close environment

Abstract: The HIS instrument of the ion mass spectrometer on board the Giotto spacecraft identified the contact surface at 4800 km distance from the comet nucleus. This boundary is clearly seen by a drastic drop in the temperatures of different ion species from about 2000 K outside to values as low as 300 K inside. Also, inside the contact surface an outflow speed of > 1 km s −1 was measured, in contrast to a value around zero right outside. We discuss how these numbers might be affected by a potential charge-up of the spacecraft. Outside the contact surface, the ion temperature rises gradually with increasing distance. Between 9000 and 10 000 km distance the ion density increases by a factor of 4. This “ion pile-up” is not yet explained uniquely. Between 25 000 and 28 000 km distance there is again a rather abrupt jump to significantly higher temperatures and higher outflow speeds.

Simplified procedure for correcting both errors and erasures of Reed-Solomon code using Euclidean algorithm

Abstract: It is well known that the Euclidean algorithm or its equivalent, continued fractions, can be used to find the error locator polynomial and the error evaluator polynomial in Berlekamp's key equation that is needed to decode a Reed-Solomon (RS) code. In the paper, a simplified procedure is developed and proved to correct erasures as well as errors by replacing the initial condition of the Euclidean algorithm by the erasure locator polynomial and the Forney syndrome polynomial. By this means, the errata locator polynomial and the errata evaluator polynomial can be obtained simultaneously and simply, by the Euclidean algorithm only. With this improved technique, the complexity of time-domain Reed-Solomon decoders for correcting both errors and erasures is reduced substantially from previous approaches. As a consequence, decoders for correcting both errors and erasures of RS codes can be made more modular, regular, simple, and naturally suitable for both VLSI and software implementation. An example illustrating this modified decoding procedure is given for a (15, 9) RS code.

Fiber optic frequency transfer link

Abstract: A stabilized fiber-optic reference frequency distribution system has been fabricated and tested. The distribution system is designed to transmit a 100-MHz reference signal, which is generated by a hydrogen maser frequency standard, over a distance of 22 km. The goal is to maintain a stability of 1 part in 10/sup 17/ for 1000-s averaging times. The stabilizer circuitry reduced delay variations that result from the effect environmental changes, such as temperature, on the fiber. The distribution system was designed using a computer simulation program that allows the delays, bandwidths, gains and damping factors to be varied for optimum performance. A breadboard version of the stabilizer was fabricated and tested in an environmentally controlled test chamber. In preliminary tests, the stabilizer reduced phase variations caused by temperature changes of 20 degrees C by as much as 45 times. >

Finite-state codes

Abstract: A class of codes called finite-state (FS) codes is defined and investigated. The codes, which generalize both block and convolutional codes, are defined by their encoders, which are finite-state machines with parallel inputs and outputs. A family of upper bounds on the free distance of a given FS code is derived. A general construction for FS codes is given, and it is shown that in many cases the FS codes constructed in this way have a free distance that is the largest possible. Catastrophic error propagation (CEP) for FS codes is also discussed. It is found that to avoid CEP one must solve the graph-theoretic problem of finding a uniquely decodable edge labeling of the state diagram. >

Kali: An Environment for the Programming and Control of Cooperative Manipulators

Abstract: The paper describes the design of a controller for cooperative robots designed at McGill University in a collaborative effort with the Jet Propulsion Laboratory. The first part of the paper discusses the background and motivation for multiple arm control. Then, a set of programming primitives, which permit a programmer to specify cooperative tasks are described. Motion primitives specify asynchronous motions, master/slave motions, and cooperative motions. In the context of cooperative robots, trajectory generation issues are discussed and our implementation briefly described. The relations between programming and control in the case of multiple robot are examined. Finally, the paper describes the allocation of various tasks among a multiprocessor computer.

The design of an onboard digital Doppler processor for a spaceborne scatterometer

Abstract: A digital Doppler processor, which will permit the Doppler center frequency of the measurement cell bandwidths to be adjusted to compensate for the effects of the Earth's rotation, will be used in the next NASA spaceborne scatterometer known as NSCAT. The authors describe the design and genesis of the NSCAT digital Doppler processor and discuss the performance tradeoff issues that were evaluated during the design phase. In this FFT (fast Fourier transform)-based technique, computation of the adjustment to the cell center frequencies will be done onboard using an approximate expression for the Doppler shift of the cell center versus orbit time. This technique also permits modification of the parameters used to locate the radar-backscatter-coefficient measurements cells by ground command in response to orbit changes. >

Relationships between the average offset and the stress drop for two- and three-dimensional faults

Abstract: Finite-element results for C , the nondimensional shape factor which relates stress drop to fault offset, are presented for rectangular strike-slip and two dimensional strike-slip and dip-slip faults at various depths of burial. All faults were oriented vertically except for a two-dimensional fault which was dipped at 10°. The finite-element meshes were constructed to limit the error in C to 1 per cent, as demonstrated by comparison with the analytical solution for a circular fault in an infinite region. Attainment of such accuracy results in large, finite-element matrix equations which were solved by a fast iterative technique: the multi-grid method. For the rectangular faults, significant differences were seen in values of C between the finite-element results and some previously reported solutions which were obtained by a different numerical technique.

Robustness Analysis based on Passivity

Abstract: A new technique of robust stability analysis for multivariable control systems is introduced in this paper. Existing tools in the literature are mostly based on small gain type of conditions (such as the H ? -norm, ? measure etc.). In contrast, the approach presented here is motivated by passivity theory. The natural energy Lyapunov function associated with a passive system allows robustness to be analyzed with Lyapunov stability theory. Consequently nonlinear and time varying perturbations can be considered within the same framework. For unstructured uncertainties, the robustness margin is characterized in terms of an index related to the Hermitian part of a transfer function. If the uncertainties contain additional diagonal structure, stability margins are obtained for variations of each uncertain block in both positive and negative direction. Applications of this robustness analysis technique are illustrated in several examples.

Stable, high quantum efficiency silicon photodiodes for vacuum-UV applications

Abstract: Silicon photodiodes have been developed by defect-free phosphorus diffusion having practically no carrier recombination at the SiSiO2 interface or in the front diffused region. The quantum efficiency of these photodiodes was found to be around 120 percent at 100 nm. Unlike the previously tested silicon photodiodes, the developed photodiodes exhibit extremely stable quantum efficiency over extended periods of time. The possibility of using these photodiodes as vacuum ultraviolet detector standards is being currently investigated.

LIMEX'87: The Labrador Ice Margin Experiment, March 1987–A pilot experiment in anticipation of RADARSAT and ERS 1 data

Abstract: The first field program of the international Labrador Ice Margin Experiment (LIMEX) took place in the Grand Banks region off the east coast of Newfoundland, Canada, during the last 2 weeks of March 1987. LIMEX was conceived as a series of studies to be conducted at 2-year intervals to address a number of objectives related to the dynamics of the Labrador Sea ice cover, the surveillance of ice and oceanic conditions with microwave instruments on aircraft and satellites, the roles of oceanic and atmospheric processes in influencing ice conditions, and the development of air-sea-ice models to predict the behavior of the ice extent, compactness, and motion. The first study in the series, LIMEX'87, was a pilot project to examine air-sea-ice interactions and the remote sensing of sea ice conditions and characteristics. It is the first study of the marginal ice zone in this region. The LIMEX'87 study area, shown in Figure 1, was chosen because of its economic importance and the nature of its ice cover. The most important oil exploration site on the Canadian east coast, Hibernia, lies within the range of excursion of the ice extent. The experiment was timed to coincide with the period of maximum ice extent and the onset of retreat.

A relationship between radar backscatter and aerodynamic roughness: Preliminary results

Abstract: The ability of the wind to move particles and the flux of windblown sand are both dependent on the topographic roughness of the surface, as measured by the aerodynamic roughness, z(0). For most surfaces, topographic roughness controls many of the characteristics of the radar return, and the magnitude of the radar backscatter can be regarded as a measure of the surface roughness at or near the wavelength scale. Radar backscatter data may therefore be useful in obtaining a value of aerodynamic roughness which can be used to assess aeolian sediment transport via remote sensing. In this study, calibrated L(HH), C(HH) and Ku(VV) radar data were used to derive characteristic backscatter coefficients sigma(0) for three lava flow units, an alluvial fan, and a playa surface. Preliminary analyses show that values of sigma(0) and z(0) both increase with topographic roughness and that there is a good correlation between the two coefficients. This correlation suggests that it may be possible to assess aerodynamic roughness directly from radar data.

An Information Theoretic Approach to Rule-Based Connectionist Expert Systems

Abstract: We discuss in this paper architectures for executing probabilistic rule-bases in a parallel manner, using as a theoretical basis recently introduced information-theoretic models. We will begin by describing our (non-neural) learning algorithm and theory of quantitative rule modelling, followed by a discussion on the exact nature of two particular models. Finally we work through an example of our approach, going from database to rules to inference network, and compare the network's performance with the theoretical limits for specific problems.

Autonomous Control Systems: Architecture and Fundamental Issues

Abstract: Autonomous control systems must perform well under significant uncertainties in the plant and the environment for extended periods of time and they must be able to compensate for system failures without external intervention. Such control systems evolve from conventional control systems and their development requires interdisciplinary research. A hierarchical functional autonomous control architecture is introduced here and its functions are described in detail. The fundamental issues in autonomous control system modelling and analysis are discussed.

Atmospheric backscatter vertical profiles at 9.2 and 10.6 μm: a comparative study

Abstract: The paper reports a series of atmospheric aerosol backscatter measurements at two widely spaced CO2 laser wavelengths: 9.25 and 10.6 microns. Comparisons are made between backscatter coefficient profiles at these two wavelengths up to 20-km altitude. Measurements such as those reported here can be used to assess the feasibility of coherent CO2 lidar for wind measurements, and they also provide a partial test of backscatter model predictions.

Contact formations and design constraints: a new basis for the automatic generation of robot programs

Abstract: In order to achieve an ultimate goal of automatically generating assembly programs for robots from design information, it is necessary that one be able to devise part-mating strategies that will work in spite of sensor, control and manufacturing errors. In general, this is almost certainly unachievable. However, if appropriate design and motion constraints relating nominal and error parameters (of the system) are enforced, significant progress can be made. As the first step in our approach, we introduce a concept of contact formations to describe contacts among parts in a system, aiming at reducing the dimensionality of assembly verification. We also describe a technique for identifying contact formations in spite of system errors. Next, we develop a replanning strategy together with design and motion constraints sufficient to guarantee the success of the strategy for certain insertion tasks. The constraints are reasonable in the sense that they do not impose unrealistic conditions on typical designs. Simulation results uphold the theoretical derivations and show empirically that the theoretical constraints can be relaxed somewhat with excellent results still obtained.

Superposed ridges of the Hesperia planum area on Mars

Abstract: Mare ridges of the Hesperia Planum area form linear, reticular and circular structures. The main factors effective in mare ridge formation have been (i) a large areal, or maybe even global, shortening and compression, (ii) major crustal tectonics, and (iii) the moderation of tectonic movements by the megaregolith discontinuity layer(s) between surface lavas and the bedrock leaving the compressional thrust to dominate over other fault movements in surface tectonics.

Compact non-thermal radio emission from B-peculiar stars

Abstract: Some stars hotter than 10,000 K show propensity for unusual surface abundances and excessive magnetic fields. These peculiar stars, Ap and Bp in the spectral nomenclature, show unusually prominent absorption lines of heavier elements. Rapid rotation and strong magnetic fields are revealed by line shapes and Zeeman splitting. VLBI is here used to directly probe the source size and brightness temperature of weak radio emission recently discovered from two isolated Bp stars, sigma Orionis E and HD37017. The emitting zone for each star is no more than 6 stellar diameters in extent, reflecting brightness temperatures of more than one billion K. Such high surface brightness resembles gyrosynchrotron radiation from mildly relativistic electrons trapped in the strong magnetic fields surrounding these stars. Compact radio radiation from these two stars presents new opportunities for probing the physical environments of early-type stars and for precise radio astrometry.