Showing papers by "California Institute of Technology published in 1981"

Hybrid position/force control of manipulators

Abstract: A new conceptually simple approach to controlling compliant motions of a robot manipulator is presented. The 'hybrid' technique described combines force and torque information with positional data to satisfy simultaneous position and force trajectory constraints specified in a convenient task related coordinate system. Analysis, simulation, and experiments are used to evaluate the controller's ability to execute trajectories using feedback from a force sensing wrist and from position sensors found in the manipulator joints. The results show that the method achieves stable, accurate control of force and position trajectories for a variety of test conditions.

Quantum Mechanical Noise in an Interferometer

Abstract: The interferometers now being developed to detect gravitational waves work by measuring the relative positions of widely separated masses. Two fundamental sources of quantum-mechanical noise determine the sensitivity of such an interferometer: (i) fluctuations in number of output photons (photon-counting error) and (ii) fluctuations in radiation pressure on the masses (radiation-pressure error). Because of the low power of available continuous-wave lasers, the sensitivity of currently planned interferometers will be limited by photon-counting error. This paper presents an analysis of the two types of quantum-mechanical noise, and it proposes a new technique---the "squeezed-state" technique---that allows one to decrease the photon-counting error while increasing the radiation-pressure error, or vice versa. The key requirement of the squeezed-state technique is that the state of the light entering the interferometer's normally unused input port must be not the vacuum, as in a standard interferometer, but rather a "squeezed state"---a state whose uncertainties in the two quadrature phases are unequal. Squeezed states can be generated by a variety of nonlinear optical processes, including degenerate parametric amplification.

tRNA punctuation model of RNA processing in human mitochondria

Abstract: A 3'-end proximal segment of most of the putative mRNAs encoded in the heavy strand of HeLa cell mtDNA has been partially sequences and aligned with the DNA sequence. In all cases, the 3'-end nucleotide of the individual mRNA coding sequences has been found to be immediately contiguous to a tRNA gene or another mRNA coding sequence. These and previous results indicate that the heavy (H) strand sequences coding for the rRNA, poly(A)-containing RNA and tRNA species form a continuum extending over almost the entire length of this strand. We propose that the H strand is transcribed into a single polycistronic RNA molecule, which is processed later into mature species by precise endonucleolytic cleavages which occur, in most cases, immediately before and after a tRNA sequence.

Minerals formed by organisms

Abstract: Organisms are capable of forming a diverse array of minerals, some of which cannot be formed inorganically in the biosphere. The initial precipitates may differ from the form in which they are finally stabilized, or during development of the organism one mineral may substitute for another. Biogenic minerals commonly have attributes which distinguish them from their inorganic counterparts. They fulfill important biological functions. They have been formed in ever-increasing amounts during the last 600 million years and have radically altered the character of the biosphere.

Primary structure, gene organization and polypeptide expression of poliovirus RNA

Abstract: The primary structure of the poliovirus genome has been determined. The RNA molecule is 7,433 nucleotides long, polyadenylated at the 3' terminus, and covalently linked to a small protein (VPg) at the 5' terminus. An open reading frame of 2,207 consecutive triplets spans over 89% of the nucleotide sequence and codes for the viral polyprotein NCVPOO. Twelve viral polypeptides have been mapped by amino acid sequence analysis and were found to be proteolytic cleavage products of the polyprotein, cleavages occurring predominantly at Gln-Gly pairs.

Encounter with Saturn: Voyager 1 imaging science results

Abstract: As Voyager 1 flew through the Saturn system it returned photographs revealing many new and surprising characteristics of this complicated community of bodies. Saturn's atmosphere has numerous, low-contrast, discrete cloud features and a pattern of circulation significantly different from that of Jupiter. Titan is shrouded in a haze layer that varies in thickness and appearance. Among the icy satellites there is considerable variety in density, albedo, and surface morphology and substantial evidence for endogenic surface modification. Trends in density and crater characteristics are quite unlike those of the Galilean satellites. Small inner satellites, three of which were discovered in Voyager images, interact gravitationally with one another and with the ring particles in ways not observed elsewhere in the solar system. Saturn's broad A, B, and C rings contain hundreds of "ringlets," and in the densest portion of the B ring there are numerous nonaxisymmetric features. The narrow F ring has three components which, in at least one instance, are kinked and crisscrossed. Two rings are observed beyond the F ring, and material is seen between the C ring and the planet.

The middle temporal visual area in the macaque: Myeloarchitecture, connections, functional properties and topographic organization

Abstract: The location, topographic organization, and function of the middle temporal visual area (MT) in the macaque monkey was studied using anatomical and physiological techniques. MT is a small, elliptically shaped area on the posterior bank of the superior temporal sulcus which can be identified by its direct inputs from striate cortex and by its distinctive pattern of heavy myelination. Its average surface area is 33 mm2, which is less than 3% of the size of striate cortex. It contains a complete, topographically organized representation of the contralateral visual hemifield. There are substantial irregularities in the detailed pattern of topographic organization, however, and the representation is significantly more complex than that found for MT in other primates. Much of MT is devoted to the representation of central visual fields, with the emphasis on central vision being similar to that found in striate cortex. Electrophysiological recordings have confirmed previous reports of a high incidence of direction selective cells in MT. The transition in functional properties, from cells lacking direction selectivity outside MT to direction selective cells within, occurs over a distance of 0.1–0.2 mm or less along the lateral border of MT. Such a transition does not occur along the medial border, however, as the cortex medial to MT contains many cells with strong direction selectivity. Nevertheless, this region differs from MT in its myeloarchitecture, its lack of inputs from striate cortex, and the large size of its receptive fields. These results demonstrate the existence of three distinct visual areas on the posterior bank of the superior temporal sulcus which can be distinguished on the basis of both physiological and anatomical criteria.

On the diffusion of new technology: A game theoretic approach

Abstract: This paper is an attempt at a rigorous (albeit not exceedingly general) analysis of the diffusion of new technology. In particular, consider an industry composed of two firms, each using the current best-practice technology. The firms are assumed to be operating at Nash equilibrium output levels, generating a market price (given demand) and profit allocations. When a cost-reducing innovation is announced, each firm must determine when (if ever) to adopt it, based in part upon the discounted cost of implementing the new technology, and in part upon the behavior of the rival firm. If either firm adopts before the other, it can expect to make substantial profits at the expense of the other firm. On the other hand, the discounted sum of purchase price and adjustment costs may decline with the lengthening of the adjustment period as various quasi-fixed factors become more easily variable. Therefore, although waiting costs the firm more in terms of foregone profits, it may save money on the cost of purchasing the new technology. Thus the firm must weigh the costs and benefits of delaying adoption, as well as take account of its rival's strategic behavior.

IgG antibodies to phosphorylcholine exhibit more diversity than their IgM counterparts.

Abstract: An amino acid sequence analysis of the N-terminal immunoglobulin heavy and light chain variable regions (VH and VL) from 16 hybridoma proteins which bind phosphorylcholine as well as the complete sequence analysis of 9 of these VH regions is presented. There seem to be more VH regions participating in the phosphorylcholine response than can be encoded directly by germ-line VH gene segments. Moreover, the V regions from IgG antibodies are considerably more variable than those from their IgM counterparts. These observations raise the possibility that a somatic mechanism for V region diversification produces greater diversity in IgG than in IgM antibodies.

AFGL atmospheric absorption line parameters compilation: 1982 edition.

Abstract: The latest edition of the AFGL atmospheric absorption line parameters compilation for the seven most active infrared terrestrial absorbers is described. Major modifications to the atlas for this edition include updating of water-vapor parameters from 0 to 4300 per cm, improvements to line positions for carbon dioxide, substantial modifications to the ozone bands in the middle to far infrared, and improvements to the 7- and 2.3-micron bands of methane. The atlas now contains about 181,000 rotation and vibration-rotation transitions between 0 and 17,900 per cm. The sources of the absorption parameters are summarized.

Defect in cyclic AMP phosphodiesterase due to the dunce mutation of learning in Drosophila melanogaster.

Abstract: Cyclic AMP is an intracellular mediator ('second messenger') in the nervous and endocrine control of cellular function, regulating different processes in different cell types. Although evidence is incomplete, it seems that cyclic AMP enhances the calcium-mediated release of neurotransmitter in some neurones. A simple form of memory in the mollusc Aplysia is probably encoded as a cyclic AMP-induced enhancement of neurotransmission at certain synapses of the central nervous system. The possibility that cyclic AMP participates in learning mechanisms may be explored using genetic mutants. For this purpose the fruitfly Drosophila is suitable as it is genetically well characterized and can learn through olfaction, vision or taste. We show here that independent searches for mutations of olfactory learning and of cyclic AMP metabolism, and for mutations causing female infertility have each led to the same gene--the dunce gene. Our evidence indicates that the normal dunce gene may specify a cyclic AMP phosphodiesterase.

Models of the Earth's Core

Abstract: Combined inferences from seismology, high-pressure experiment and theory, geomagnetism, fluid dynamics, and current views of terrestrial planetary evolution lead to models of the earth's core with the following properties. Core formation was contemporaneous with earth accretion; the core is not in chemical equilibrium with the mantle; the outer core is a fluid iron alloy containing significant quantities of lighter elements and is probably almost adiabatic and compositionally uniform; the more iron-rich inner solid core is a consequence of partial freezing of the outer core, and the energy release from this process sustains the earth's magnetic field; and the thermodynamic properties of the core are well constrained by the application of liquid-state theory to seismic and laboratory data.

Effects of drift on the transport of cosmic rays IV. Modulation by a wavy interplanetary current sheet

Abstract: The effect of a wavy interplanetary current sheet on the solar modulation of galactic cosmic rays is studied. A simple model of the heliospheric magnetic field is employed in which there is an Archimedean spiral magnetic field directed in one direction above the current sheet and another direction below. The current sheet itself is taken to be the extension by the solar wind of a surface that, at the sun, is a plane inclined at an angle alpha to the solar equator. Two magnetic sectors will therefore be seen by an observer within an angle alpha of the heliographic equator. A simple approximate model is used to investigate modulation. As the tilt angle alpha is increased from zero, it is found that the cosmic-ray intensity decreases. For reasonable variation of alpha from approximately 10 deg to approximately 30 deg, the change in intensity can be compared to that observed as one goes from solar minimum to solar maximum. It is concluded that changes in the waviness of the interplanetary current sheet can have an important, and perhaps dominant, effect in producing solar modulation of galactic cosmic rays.

Melt segregation from partially molten source regions: The importance of melt density and source region size

Abstract: The compressibility of basic melt at 1 atmosphere is about an order of magnitude higher than that of mantle minerals. Consequently, the density contrast between melt and the principal residual crystals in mantle source regions is expected to decrease with increasing source region depth. The increasingly olivine-normative character of primary melts produced at greater depths is also expected to result in a decrease in this density contrast with increasing source region depth. Once vertical permeability is established by melt generated during partial melting, buoyancy-driven melt percolation can under some circumstances segregate melt from the residual crystals in its source region on a geologically rapid time scale. Limits to this process are provided by cooling of the source region (freezing melt in) and rigidity of the crystalline matrix (mechanically trapping melt). Source region size influences these limits strongly: consequently, small, partially molten diapirs (∼km in diameter) may be able to trap large melt fractions (≳30%), but larger source regions would be unable to do so. The reduction in density contrast with pressure reduces the buoyant force driving melt percolation and provides another limit to melt segregation. Diapirs at depth may thus stably contain large fractions of melt but may decompress and unload their melt during ascent; this effect would be enhanced in small diapirs and may be relevant to the genesis of komatiitic magma. Melt compression may also be a factor in explaining why the very different maximum depths inferred for typical basic melt segregation from source regions on different planets—∼500 km on the moon, ∼250 km on Mars, ∼100 km on earth—correspond to similar pressures (25–35 kbar); at greater pressures, melt may no longer be capable under ordinary conditions of segregating upwards by buoyancy. This may also help to explain why depleted peridotites overlie more fertile peridotites and how deep regions of the mantle are able to remain fertile over geologic time.