Implementations of a four-level mechanical architecture for fault-tolerant robots

This paper describes a fault tolerant mechanical architecture with four levels devised and implemented in concert with NASA (Tesar, D. & Sreevijayan, D., Four-level fault tolerance in manipulator design for space operations. In First Int. Symp. Measurement and Control in Robotics (ISMCR '90), Houston, Texas, 20–22 June 1990.) Subsequent work has clarified and revised the architecture. The four levels proceed from fault tolerance at the actuator level, to fault tolerance via in-parallel chains, to fault tolerance using serial kinematic redundancy, and finally to the fault tolerance multiple arm systems provide. This is a subsumptive architecture because each successive layer can incorporate the fault tolerance provided by all layers beneath. For instance a serially-redundant robot can incorporate dual fault-tolerant actuators. Redundant systems provide the fault tolerance, but the guiding principle of this architecture is that functional redundancies actively increase the performance of the system. Redundancies do not simply remain dormant until needed. This paper includes specific examples of hardware and/or software implementation at all four levels.     

Change in Refractive Index of a Borosilicate Glass Fiber Annealed Below the Glass Transformation Temperature

Glass fibers pulled from multihole bushings can have a slight difference in thermal history that causes a distribution in the refractive index that can be narrowed by annealing the fibers. The kinetics for the initial stage change in refractive index for fiber annealed between 300° and 500°C are best described by a second-order reaction with an activation energy of 120° 17 kj/mol. The improved uniformity in refractive index for annealed fibers is indicated by a decrease in the half-height width of the optical transmission versus temperature curve for glass fiber immersed in a liquid. The standard deviation in the refractive index of glass fibers with a bimodal distribution in diameter decreases from 8 × 10^-4≤0.0002 to 4 × 10^-4 0.0002 after the fibers are annealed at 400°C for 1 h.     

Glass Formation and Properties in the System Calcia-Gallia-Silica

The glass-formation region of the calcia-gallia-silica system was determined. The glasses within this region were measured to have a density of 3 to 4 g/cm³, a refractive index of 1.6 to 1.73, an Abbe number between 35 and 58, a thermal expansion coefficient of 6.5 × 10⁻⁷/°C to 11.5 × 10⁻⁷/°C, softening temperatures between 730° and 790°C, and a Vickers microhardness of 5.2 to 7.3 GPa. Crystalline phases were identified along the glass-formation boundary. Infrared transmission spectra were used to explain glass structures and their effect on glass properties. The results suggest that the role of calcia in the glass structure is similar to that for calcia in calcium aluminosilicate glasses.     

Color and Selected Properties of PbO─BiO1.5─GaO1.5 Glasses

The density; molar volume; thermal expansion coefficient; dissolution rate in water, HC1, and NaOH; glass transition and crystallization temperatures; and the absorption edge in the ultraviolet-visible and infrared were measured for PbO─BiO1.5─GaO1.5 glasses. The range of compositions investigated was x PbO (100 − (x + y) )BiO_1.5. yGaO_1.5 for x between 20 and 60 cat% and y of 20, 25, 30, and 35 cat%. The glass-forming tendency increased with increased GaO_1.5 and decreased with increased PbO or BiO_1.5. The compositional dependence of these properties was consistent with the weight, size, charge, and bond strength of the cations. The Ga^{3 +} ions in these glasses are believed to act primarily as network-forming cations, whereas the majority of the Bi³⁺ and Pb²⁺ ions behave as network-modifying cations. It is suggested that a small friction of the lead ions are present as Pb⁴⁺. Depending upon melting conditions, these glasses ranged in color from brown to yellow. Various attempts, including containerless melting, were made to obtain colorless glasses, but no conditions were found which totally eliminated the color. The least color (pale yellow) was obtained when the glasses were melted in an air or nitrogen atmosphere in an alumina or gold crucible.     

Aging behavior of optically transparent poly (methyl methacrylate) composites

The thermal aging between 25 and 115°C of hot pressed glass fiber reinforced poly (methy1 methacrylate)(PMMA) transparent composites was studied as a function of the temperature and time of hot pressing. Thermal aging at near the T_g of the PMMA matrix caused dimensional changes and a reduction in optical transmission and clarity. The reduction in transmission was attributed to gas bubbles that formed in the matrix, which may be due to the evaporation of residual MMA monomer or low T_g (∼ 75°C) polymer in the composites during aging. Thermal cycling between 25 and 100°C by eliminating low T_g of the PMMA matrix establishes the upper temperature limit to which the composite can be exposed without seriously damaging its optical properties.     

Early Stages of Calcium Phosphate Formation on Bioactive Borosilicate Glass in Aqueous Phosphate Solution

The conversion of bioactive glass to a calcium phosphate material, typically hydroxyapatite (HA), by solution–precipitation reactions in aqueous phosphate solution, has been commonly reported. This paper describes the structural and compositional characteristics of the calcium phosphate material formed during the early-stage conversion (<5 h) of a borosilicate glass (designated H12) in aqueous phosphate solution. Disks of H12 glass were reacted with 0.25 M K₂HPO₄ solution with a starting pH=7.0 at 37°C. The structure and composition of the product layer were characterized using thin film X-ray diffraction, scanning electron microscopy, and Fourier transform infrared reflectance spectroscopy. For reaction times <5 h, the results indicated the formation of brushite (CaHPO₄·2H₂O) with coarse, plate-like crystals. In comparison, the formation of HA with small needle-like crystals was found at later times. This early-stage formation of brushite has not been reported in previous studies of converting bioactive silicate and borate glasses in aqueous phosphate solution.     

Isobaric thermal expansivities of mixtures ofm-cresol and quinoline from 0.1 to 400 MPa at 303 to 503 K

Isobaric thermal expansivities, α _p (p, T), of five binary mixtures ofm-cresol with quinoline (0.1499, 0.2998, 0.5005, 0.6325, and 0.8501 mol fraction ofm-cresol) were measured in a pressure-controlled scanning calorimeter over the pressure range from just above the saturation vapor pressures to 400 MPa, and at 303.15, 353.15, 403.15, 453.15, and 503.15 K. Molecular association ofm-cresol with itself and ofm-cresol with quinoline exerts large effects on the pressure and temperature behavior of α _p isotherms. The extent of association changes significantly with conditions in all except the 2∶1 mixture as demonstrated by the crossing of isotherms at lower pressures as the temperature increases. In the 2∶1m-cresol quinoline mixture the extent of association is not perturbed significantly by temperature change and the mixture behaves like a simple liquid, exhibiting a unique crossing point of α _p isotherms.