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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 325 records · Page 18

Near-Mars space

The prevalent attributes of near-Mars space are described: the ambient interplanetary environment, the ionosphere, the upper atmosphere, and more remote regions that are affected by the presence of Mars. The descriptions are based on existing Martian data and/or models constructed from measurements made near Venus. Specific attention is given to the features of solar wind interaction with magnetospheric and ionospheric obstacles. The high-altitude plasma and field environment, the energetic particle environment, the ionosphere environment, and the neutral upper atmosphere environment are described with extensive graphic information, based on existing measurements collected from nine Martian missions. The ionospheric obstacle is assumed to prevail as a mechanism for describing the scenario. Martian perturbation of solar wind is theorized to be of a relatively small order. A distinctive local energetic particle population of planetary origin is shown to result from the direct interaction of solar wind plasma. This phenomenon is considered evidence of the important scavenging of planetary elements from Mars. The absence of a planetary dipole field around Mars, like its low gravity and distance from the sun, is considered important in determining the environment of this earthlike laboratory.

Luhmann, J. G.↗

The magnetic barrier at Venus

Altitude profiles of the Venus magnetic barrier are derived here from a statistical analysis of the Pioneer Venus Orbiter magnetometer data. The outer boundary of the magnetic barrier is then compared with the obstacle expected from gasdynamic models of the bow shock, and the stagnation pressure is compared with that expected from gasdynamic theory. The magnetic barrier is strongest at the subsolar point and weakens as expected with increasing solar zenith angle. The existence of a north-south asymmetry in the barrier strength is also demonstrated. The magnetic barrier is about 200 km thick at the subsolar point and 800 km thick at the terminator. The magnetic barrier transfers most of the solar wind dynamic pressure to the ionosphere via the enhanced magnetic pressure. The convected field gasdynamic model predicts the correct bow shock location if the magnetic barrier is treated as the obstacle.

Zhang, T. L.↗

The solar wind interaction with Mars - Mariner 4, Mars 2, Mars 3, Mars 5, and Phobos 2 observations of bow shock position and shape

An aggregate Mars bow shock data set using Mariner 4, Mars 2, Mars 3, Mars 5, and Phobos 2 observations has been analyzed. The results support the earlier conclusion that the mean distance to the subsolar shock at Mars is nearly 1.5 planetary radii, from which gas dynamic models predict an obstacle altitude of 500 km. The Martian bow shock does not appear to vary significantly in shape or altitude with the phase of the solar cycle. The unusually distant dayside bow shock crossings reported by Mars 2 and 3 also appear in the Phobos 3 observations, suggesting that the dayside obstacle can on rare occasions reach altitudes over 1000 km. The Martian bow shock differs from that of Venus in that its mean altitude is greater, it lacks a strong solar cycle variation, and its location is far more variable, including the occurrence of strong bow shocks over the dayside hemisphere at distances at least as great as the orbit of Phobos 2, i.e., 2.8 Mars radii.

Slavin, J. A.↗

Toward autonomous driving: The CMU Navlab. I - Perception

The Navlab project, which seeks to build an autonomous robot that can operate in a realistic environment with bad weather, bad lighting, and bad or changing roads, is discussed. The perception techniques developed for the Navlab include road-following techniques using color classification and neural nets. These are discussed with reference to three road-following systems, SCARF, YARF, and ALVINN. Three-dimensional perception using three types of terrain representation (obstacle maps, terrain feature maps, and high-resolution maps) is examined. It is noted that perception continues to be an obstacle in developing autonomous vehicles.

Thorpe, Charles↗

Passive range estimation for rotorcraft low-altitude flight

The automation of rotorcraft low-altitude flight presents challenging problems in control, computer vision and image understanding. A critical element in this problem is the ability to detect and locate obstacles, using on-board sensors, and modify the nominal trajectory. This requirement is also necessary for the safe landing of an autonomous lander on Mars. This paper examines some of the issues in the location of objects using a sequence of images from a passive sensor, and describes a Kalman filter approach to estimate the range to obstacles. The Kalman filter is also used to track features in the images leading to a significant reduction of search effort in the feature extraction step of the algorithm. The method can compute range for both straight line and curvilinear motion of the sensor. A laboratory experiment was designed to acquire a sequence of images along with sensor motion parameters under conditions similar to helicopter flight. Range estimation results using this imagery are presented.

Sridhar, B.↗

An alternative to unstructured grids for computing gas dynamic flows around arbitrarily complex two-dimensional bodies

In this paper we describe an approach for dealing with arbitrary complex, two dimensional geometries, the so-called cartesian boundary method. Conceptually, the cartesian boundary method is quite simple. Solid bodies blank out areas of a background, cartesian mesh, and the resultant cut cells are singled out for special attention. However, there are several obstacles that must be overcome in order to achieve a practical scheme. We present a general strategy that overcomes these obstacles, together with some details of our successful conversion of an adaptive mesh algorithm from a body-fitted code to a cartesian boundary code.

Quirk, James J.↗

The use of visual cues for vehicle control and navigation

At least three levels of control are required to operate most vehicles: (1) inner-loop control to counteract the momentary effects of disturbances on vehicle position; (2) intermittent maneuvers to avoid obstacles, and (3) outer-loop control to maintain a planned route. Operators monitor dynamic optical relationships in their immediate surroundings to estimate momentary changes in forward, lateral, and vertical position, rates of change in speed and direction of motion, and distance from obstacles. The process of searching the external scene to find landmarks (for navigation) is intermittent and deliberate, while monitoring and responding to subtle changes in the visual scene (for vehicle control) is relatively continuous and 'automatic'. However, since operators may perform both tasks simultaneously, the dynamic optical cues available for a vehicle control task may be determined by the operator's direction of gaze for wayfinding. An attempt to relate the visual processes involved in vehicle control and wayfinding is presented. The frames of reference and information used by different operators (e.g., automobile drivers, airline pilots, and helicopter pilots) are reviewed with particular emphasis on the special problems encountered by helicopter pilots flying nap of the earth (NOE). The goal of this overview is to describe the context within which different vehicle control tasks are performed and to suggest ways in which the use of visual cues for geographical orientation might influence visually guided control activities.

Hart, Sandra G.↗

Vision-based range estimation using helicopter flight data

Pilot aiding during low-altitude flight depends on the ability to detect and locate obstacles near the helicopter's intended flightpath. Computer-vision-based methods provide one general approach for obstacle detection and range estimation. Several algorithms have been developed for this purpose, but have not been tested with actual flight data. This paper presents results obtained using helicopter flight data with a feature-based range estimation algorithm. A method for recursively estimating range using a Kalman filter with a monocular sequence of images and knowledge of the camera's motion is described. The helicopter flight experiment and four resulting datasets are discussed. Finally the performance of the range estimation algorithm is explored in detail based on comparison of the range estimates with true range measurements collected during the flight experiment.

Smith, Philip N.↗

Perspectives on hypersonic viscous and nonequilibrium flow research

An attempt is made to reflect on current focuses in certain areas of hypersonic flow research by examining recent works and their issues. Aspects of viscous interaction, flow instability, and nonequilibrium aerothermodynamics pertaining to theoretical interest are focused upon. The field is a diverse one, and many exciting works may have either escaped the writer's notice or been abandoned for the sake of space. Students of hypersonic viscous flow must face the transition problems towards the two opposite ends of the Reynolds or Knudsen number range, which represents two regimes where unresolved fluid/gas dynamic problems abound. Central to the hypersonic flow studies is high-temperature physical gas dynamics; here, a number of issues on modelling the intermolecular potentials and inelastic collisions remain the obstacles to quantitative predictions. Research in combustion and scramjet propulsion will certainly be benefitted by advances in turbulent mixing and new computational fluid dynamics (CFD) strategies on multi-scaled complex reactions. Even for the sake of theoretical development, the lack of pertinent experimental data in the right energy and density ranges is believed to be among the major obstacles to progress in aerothermodynamic research for hypersonic flight. To enable laboratory simulation of nonequilibrium effects anticipated for transatmospheric flight, facilities capable of generating high enthalpy flow at density levels higher than in existing laboratories are needed (Hornung 1988). A new free-piston shock tunnel capable of realizing a test-section stagnation temperature of 10(exp 5) at Reynolds number 50 x 10(exp 6)/cm is being completed and preliminary tests has begun (H. Hornung et al. 1992). Another laboratory study worthy of note as well as theoretical support is the nonequilibrium flow experiment of iodine vapor which has low activation energies for vibrational excitation and dissociation, and can be studied in a laboratory with modest resources (Pham-Van-Diep et al. 1992).

Cheng, H. K.↗

Stress versus temperature dependence of activation energies for creep

The activation energy for creep at low stresses and elevated temperatures is associated with lattice diffusion, where the rate controlling mechanism for deformation is dislocation climb. At higher stresses and intermediate temperatures, the rate controlling mechanism changes from dislocation climb to obstacle-controlled dislocation glide. Along with this change in deformation mechanism occurs a change in the activation energy. When the rate controlling mechanism for deformation is obstacle-controlled dislocation glide, it is shown that a temperature-dependent Gibbs free energy does better than a stress-dependent Gibbs free energy in correlating steady-state creep data for both copper and LiF-22mol percent CaF2 hypereutectic salt.

Freed, A. D.↗

Passive stereo range imaging for semi-autonomous land navigation

The paper examines the use of stereo vision (SV) for obstacle detection in semiautonomous land navigation. Feature-based and field-based paradigms for SV are reviewed. The paper presents stochastic models and simple, efficient stereo matching algorithms for the field-based approach and describes a near-real-time vision system using these algorithms. Experimental results illustrate aspects of the stochastic models and lead to the first semiautonomous traversals of natural terrain to use SV for obstacle detection.

Matthies, Larry↗

Self-Damping Sprung Wheel

Self-damping sprung wheel provides shock-absorbing suspension for wheelchair, reducing user's discomfort when traversing rough terrain or obstacles. Pair of self-damping sprung wheels installed in place of conventional large rear wheels of standard wheelchair, which user operates in conventional manner. Rim deflects in vicinity of contact with ground or floor. Includes inner and outer hoops bending when obstacle encountered. Shear deformation of elastomeric hoop between them absorbs energy. Thus, three hoops act together as damping spring. Alternative version of wheel designed for bicycle.

Weddendorf, Bruce↗

Experimental Semiautonomous Vehicle

Semiautonomous rover vehicle serves as testbed for evaluation of navigation and obstacle-avoidance techniques. Designed to traverse variety of terrains. Concepts developed applicable to robots for service in dangerous environments as well as to robots for exploration of remote planets. Called Robby, vehicle 4 m long and 2 m wide, with six 1-m-diameter wheels. Mass of 1,200 kg and surmounts obstacles as large as 1 1/2 m. Optimized for development of machine-vision-based strategies and equipped with complement of vision and direction sensors and image-processing computers. Front and rear cabs steer and roll with respect to centerline of vehicle. Vehicle also pivots about central axle, so wheels comply with almost any terrain.

Wilcox, Brian H.↗

Fuzzy logic control of telerobot manipulators

Telerobot systems for advanced applications will require manipulators with redundant 'degrees of freedom' (DOF) that are capable of adapting manipulator configurations to avoid obstacles while achieving the user specified goal. Conventional methods for control of manipulators (based on solution of the inverse kinematics) cannot be easily extended to these situations. Fuzzy logic control offers a possible solution to these needs. A current research program at SRI developed a fuzzy logic controller for a redundant, 4 DOF, planar manipulator. The manipulator end point trajectory can be specified by either a computer program (robot mode) or by manual input (teleoperator). The approach used expresses end-point error and the location of manipulator joints as fuzzy variables. Joint motions are determined by a fuzzy rule set without requiring solution of the inverse kinematics. Additional rules for sensor data, obstacle avoidance and preferred manipulator configuration, e.g., 'righty' or 'lefty', are easily accommodated. The procedure used to generate the fuzzy rules can be extended to higher DOF systems.

Franke, Ernest A.↗

A neuro-collision avoidance strategy for robot manipulators

The area of collision avoidance and path planning in robotics has received much attention in the research community. Our study centers on a combination of an artificial neural network paradigm with a motion planning strategy that insures safe motion of the Articulated Two-Link Arm with Scissor Hand System relative to an object. Whenever an obstacle is encountered, the arm attempts to slide along the obstacle surface, thereby avoiding collision by means of the local tangent strategy and its artificial neural network implementation. This combination compensates the inverse kinematics of a robot manipulator. Simulation results indicate that a neuro-collision avoidance strategy can be achieved by means of a learning local tangent method.

Onema, Joel P.↗

Controlled motion in an elastic world. Research project: Manipulation strategies for massive space payloads

The flexibility of the drives and structures of controlled motion systems are presented as an obstacle to be overcome in the design of high performance motion systems, particularly manipulator arms. The task and the measure of performance to be applied determine the technology appropriate to overcome this obstacle. Included in the technologies proposed are control algorithms (feedback and feed forward), passive damping enhancement, operational strategies, and structural design. Modeling of the distributed, nonlinear system is difficult, and alternative approaches are discussed. The author presents personal perspectives on the history, status, and future directions in this area.

Book, Wayne J.↗

Distance estimation and collision prediction for on-line robotic motion planning

An efficient method for computing the minimum distance and predicting collisions between moving objects is presented. This problem has been incorporated in the framework of an in-line motion planning algorithm to satisfy collision avoidance between a robot and moving objects modeled as convex polyhedra. In the beginning the deterministic problem, where the information about the objects is assumed to be certain is examined. If instead of the Euclidean norm, L(sub 1) or L(sub infinity) norms are used to represent distance, the problem becomes a linear programming problem. The stochastic problem is formulated, where the uncertainty is induced by sensing and the unknown dynamics of the moving obstacles. Two problems are considered: (1) filtering of the minimum distance between the robot and the moving object, at the present time; and (2) prediction of the minimum distance in the future, in order to predict possible collisions with the moving obstacles and estimate the collision time.

Kyriakopoulos, K. J.↗

Rust transformation/rust compatible primers

Proper surface preparation has been the key to obtain good performance by a surface coating. The major obstacle in preparing a corroded or rusted surface is the complete removal of the contaminants and the corrosion products. Sandblasting has been traditionally used to remove the corrosion products before painting. However, sandblasting can be expensive, may be prohibited by local health regulations and is not applicable in every situation. To get around these obstacles, Industry developed rust converters/rust transformers and rust compatible primers (high solids epoxies). The potential use of these products for military equipment led personnel of the Belvoir Research, Development and Engineering Center (BRDEC) to evaluate the commercially available rust transformers and rust compatible primers. Prior laboratory experience with commercially available rust converters, as well as field studies in Hawaii and Puerto Rico, revealed poor performance, several inherent limitations, and lack of reliability. It was obvious from our studies that the performance of rust converting products was more dependent on the amount and type of rust present, as well as the degree of permeability of the coating, than on the product's ability to form an organometallic complex with the rust. Based on these results, it was decided that the Military should develop their own rust converter formulation and specification. The compound described in the specification is for use on a rusted surface before the application of an organic coating (bituminous compounds, primer or topcoat). These coatings should end the need for sandblasting or the removing of the adherent corrosion products. They also will prepare the surface for the application of the organic coating. Several commercially available rust compatible primers (RCP) were also tested using corroded surfaces. All of the evaluated RCP failed our laboratory tests for primers.

Emeric, Dario A.↗