Practical Implementation of Attitude-Control Algorithms for an Underactuated Satellite

The challenging problem of controlling the attitude of satellites subject to actuator failures has been the subject of increased attention in recent years. The problem of controlling the attitude of a satellite on all three axes with two reaction wheels is addressed in this paper. This system is controllable in a zero-momentum mode. Three-axis attitude stability is proven by imposing a singular quaternion feedback law to the angular velocity trajectories.Two approaches are proposed and compared to achieve three-axis control: The first one does not require angular velocity measurements and is based on the assumption of a perfect zero momentum, while the second approach consists of tracking the desired angular velocity trajectories. The full-state feedback is a nonlinear singular controller. In-orbit tests of the first approach provide an unprecedented practical proof of three-axis stability with two control torques. The angular velocity tracking approach is shown to be less efficient using the nonlinear singular controller. However, when inverse optimization theory is applied to enhance the nonlinear singular controller, the angular velocity tracking approach is shown to be the most efficient. The resulting switched inverse optimal controller allows for a significant enhancement of settling time, for a prescribed level of the integrated torque

Techno-economic analysis of production of octane booster components derived from lignin

In this study, a comprehensive process for production of an environmentally friendly octane booster (acetophenone) from lignin is presented, along with a detailed techno-economic analysis. Recognizing that much of the prior research on octane boosters has been confined to experimental lab-level investigations, this study develops comprehensive process design to unravel the intricacies of large-scale acetophenone production. The acetophenone production process involves catalytic hydrogenolysis, which also yields phenol as a valuable side product. Based on the process flow diagram, mass and energy balances were developed, revealing significantly improved yields and purity of acetophenone compared to industry standards, reaching 0.74 kg acetophenone per kg of lignin and 99 wt%. In the techno-economic analysis, calculations involving fixed capital investment (FCI), operating costs, and working capital were conducted based on a feed of 100 kg/h of dry lignin. The results indicate FCI at 2.72 million USD, operating costs at 1.09 million USD per year, and working capital at 0.57 million USD. Assuming a 20-year operational lifespan, the payback period is estimated at 6.09 years, as depicted by the cumulative cash flow diagram. Moreover, techno-economic analysis demonstrates a net present value (NPV) of 3.24 million USD at a 10% discount rate, an internal rate of return (IRR) of 22.73%, and a return on investment (ROI) of 34.39%. These positive outcomes underscore the robust profitability of the proposed acetophenone production plant derived from lignin. Additionally, a sensitivity analysis on the IRR indicates that increasing the production capacity could further enhance profitability, reaffirming the feasibility of the plant’s operation. Crucially, this study highlights the potential for sustainable and economically viable production of acetophenone, offering an environmentally friendly alternative to toxic octane boosters and advancing the development of sustainable fuel additives. Graphical Abstract: [Figure not available: see fulltext.

Dynamic replication strategies in data grid systems: A survey

In data grid systems, data replication aims to increase availability, fault tolerance, load balancing and scalability while reducing bandwidth consumption, and job execution time. Several classification schemes for data replication were proposed in the literature, (i) static vs. dynamic, (ii) centralized vs. decentralized, (iii) push vs. pull, and (iv) objective function based. Dynamic data replication is a form of data replication that is performed with respect to the changing conditions of the grid environment. In this paper, we present a survey of recent dynamic data replication strategies. We study and classify these strategies by taking the target data grid architecture as the sole classifier. We discuss the key points of the studied strategies and provide feature comparison of them according to important metrics. Furthermore, the impact of data grid architecture on dynamic replication performance is investigated in a simulation study. Finally, some important issues and open research problems in the area are pointed out

Comparison of Salivary Secretory IgA in Caries-free Children and Children with Severe Early Childhood Caries

Abstract: Background & Aims: Sever early childhood caries (SECC) is a multifactor common disease that unfortunately there have been a lot of unknown points about it. This study was performed to compare the levels of total S-IgA in children with and without SECC. Methods: In this cross-sectional study 44 children aged 6-24 months were divided into two groups of with SECC (n=20) and without SECC (n=24). The two groups were compared for the concentration of S-IgA assessed with Nephlometric method. The obtained data were analyzed by using t-test and through SPSS 15 software. Results: Mean saliva concentration of S-IgA in children with SECC (18.30 mg/dl) was higher than that in children without SECC (9.46 mg/dl) and this difference was statistically Significant (P<0.05). Conclusion: According to this study, the hypothesis of lower levels of S-IgA in children with SECC might be rejected. Keywords: Immunoglobulins, Saliva, Dental caries » XM

Rheological behaviour of NiO/YSZ slurries for drying-free casting

We have recently developed a new drying-free casting method for shape forming of ceramics, in which a polymerizable solvent such as furfuryl alcohol (FA) is used to disperse the ceramic powders, and then polymerized into a poly (furfuryl alcohol) (PFA) binder during the casting process. Compared with conventional casting processes, this method offers more flexibility in controlling the microstructure of ceramics and eliminates defects that are likely to arise from the drying step. Controlling the rheological behaviour of ceramic slurries is an important step in the casting process to achieve improved microstructural properties. In this study, the viscosity of slurries comprising NiO/YSZ (yttria stabilized zirconia) powders and FA as solvent was investigated over a wide range of shear rates (1 s−1–1000 s−1) and at different volumetric solid concentrations (from 20% to 70%), while the effects of dispersant were quantified by adding different amounts of PVP (polyvinyl pyrrolidone) between 1 wt.% and 4 wt.%. The minimum viscosity of NiO/YSZ/FA slurries could be achieved with 2 wt.% dispersant. The experimental data relating the changes in viscosity to the volumetric solid concentration were modeled using different viscosity models, with Liu's equation showing the best fit for NiO/YSZ/FA slurries with and without dispersant. The slurries showed a highly pseudoplastic behaviour without dispersant, while adding PVP as dispersant effectively modified the rheological behaviour toward Newtonian fluids

Continuous Catalyst/Wax Separation Method

An improved Method for the separation of catalyst particles and the wax product from the output slurry of a Fischer-Tropsch bubble column reactor comprising the contact of a hydrocarbon solvent from a cyclic solvent stream with the slurry, wherein the solvent is a hydrocarbon fraction which is pressurized and heated to its supercritical state and the temperature and the pressure of the solvent at the supercritical state are similar to those of the F-T reactor. After the separating of the catalyst from the hydrocarbon mixture of the solvent and the slurry in a catalyst separation section the hydrocarbon solvent and the wax product are separated, whereby the recovered solvent phase is lead to the cyclic solvent stream; which is used after re-pressurizing and re-heating in a supercritical solvent supply module to recycling the hydrocarbon solvent for the contact step. A system for carrying out the method is also disclosed

Evaluation of fluoride uptake in the enamel of primary teeth after using different concentrations of a green tea extract: An in-vitro study

It is believed that extracts of green tea increase fluoride uptake in deciduous enamel and the aim of this in vitro study was to evaluate the fluoride uptake in the enamel of primary teeth after using different concentrations of an extract of green tea. Seventy-five enamel samples were prepared from deciduous canine teeth of children in Kerman, Iran, and randomly divided into five groups of 15: Group 1 (G1): no treatment (negative control); Groups 2 (G2), 3 (G3), 4 (G4), and 5 (G5) were treated with green tea extract in concentrations of 0.2, 1, 2, and 10, respectively. After the surface treatment, all the samples were submitted to an acid challenge consisting of daily immersion for 5 days in a demineralizing (3 hr) and a mineralizing solution (21 hr). The fluoride concentrations were then calculated with a potentiometer. One-way ANOVA was used for statistical analyses (p<0.05). There was a significant difference between the fluoride uptake in the control and all other groups except for the G2 group (0.2 green tea extract) (p<0.05). There was no statistically significant difference between the fluoride uptake in the enamel in the G2 (0.2 green tea extract and the G3 (1 green tea extract) groups. The findings reveal that green tea can increase the amount of fluoride in the enamel of the primary teeth. © 2020, International Society for Fluoride Research. All rights reserved

Solar evaporation enhancement using floating light-absorbing magnetic particles

We have demonstrated a new strategy for enhancing solar evaporation by using floating light-absorbing materials. Floating Fe3O4/C magnetic particles with an average size of 500 nm were synthesized by carbonization of poly(furfuryl alcohol) (PFA) incorporated with Fe3O4 nanoparticles. The Fe3O4/C particles had a BET surface area of 429 m2 g−1, and a density of 1.44 g cm−3. Because of their hydrophobicity and a bulk packing density of 0.53 g cm−3, Fe3O4/C particles were floatable on water. Our results indicated that these Fe3O4/C particles enhanced the water evaporation rate by as much as a factor of 2.3 in the solar evaporation of 3.5% salt water. In addition, Fe3O4/C particles were easily recycled using a magnet, and stable after being recycled three times. Our work provides a low-cost and highly effective way for accelerating solar evaporation for industrial applications such as solar desalination, salt production, brine management and wastewater treatment