Decentralized formation control with connectivity maintenance and collision avoidance under limited and intermittent sensing

A decentralized switched controller is developed for dynamic agents to perform global formation configuration convergence while maintaining network connectivity and avoiding collision within agents and between stationary obstacles, using only local feedback under limited and intermittent sensing. Due to the intermittent sensing, constant position feedback may not be available for agents all the time. Intermittent sensing can also lead to a disconnected network or collisions between agents. Using a navigation function framework, a decentralized switched controller is developed to navigate the agents to the desired positions while ensuring network maintenance and collision avoidance.Comment: 8 pages, 2 figures, submitted to ACC 201

Stationary Cycling Induced by Switched Functional Electrical Stimulation Control

Functional electrical stimulation (FES) is used to activate the dysfunctional lower limb muscles of individuals with neuromuscular disorders to produce cycling as a means of exercise and rehabilitation. However, FES-cycling is still metabolically inefficient and yields low power output at the cycle crank compared to able-bodied cycling. Previous literature suggests that these problems are symptomatic of poor muscle control and non-physiological muscle fiber recruitment. The latter is a known problem with FES in general, and the former motivates investigation of better control methods for FES-cycling.In this paper, a stimulation pattern for quadriceps femoris-only FES-cycling is derived based on the effectiveness of knee joint torque in producing forward pedaling. In addition, a switched sliding-mode controller is designed for the uncertain, nonlinear cycle-rider system with autonomous state-dependent switching. The switched controller yields ultimately bounded tracking of a desired trajectory in the presence of an unknown, time-varying, bounded disturbance, provided a reverse dwell-time condition is satisfied by appropriate choice of the control gains and a sufficient desired cadence. Stability is derived through Lyapunov methods for switched systems, and experimental results demonstrate the performance of the switched control system under typical cycling conditions.Comment: 8 pages, 3 figures, submitted to ACC 201

Half-Megasecond Chandra Spectral Imaging of the Hot Circumgalactic Nebula around Quasar Mrk 231

A deep 400-ksec ACIS-S observation of the nearest quasar known, Mrk 231, is combined with archival 120-ksec data obtained with the same instrument and setup to carry out the first ever spatially resolved spectral analysis of a hot X-ray emitting circumgalactic nebula around a quasar. The 65 x 50 kpc X-ray nebula shares no resemblance with the tidal debris seen at optical wavelengths. One notable exception is the small tidal arc 3.5 kpc south of the nucleus where excess soft X-ray continuum emission and Si XIII 1.8 keV line emission are detected, consistent with star formation and its associated alpha-element enhancement, respectively. An X-ray shadow is also detected at the location of the 15-kpc northern tidal tail. The hard X-ray continuum emission within 6 kpc of the center is consistent with being due entirely to the bright central AGN. The soft X-ray spectrum of the outer (>6 kpc) portion of the nebula is best described as the sum of two thermal components with T~3 and ~8 million K and spatially uniform super-solar alpha element abundances, relative to iron. This result implies enhanced star formation activity over ~10^8 yrs accompanied with redistribution of the metals on large scale. The low-temperature thermal component is not present within 6 kpc of the nucleus, suggesting extra heating in this region from the circumnuclear starburst, the central quasar, or the wide-angle quasar-driven outflow identified from optical IFU spectroscopy on a scale of >3 kpc. Significant azimuthal variations in the soft X-ray intensity are detected in the inner region where the outflow is present. The soft X-ray emission is weaker in the western quadrant, coincident with a deficit of Halpha and some of the largest columns of neutral gas outflowing from the nucleus. Shocks created by the interaction of the wind with the ambient ISM may heat the gas to high temperatures at this location. (abridged)Comment: 43 pages, 11 figures, accepted for publication in the Astrophysical Journa

Simulation of a Bioreactor with an Improved Fermentation Kinetics – Fluid Flow Model

Ethanolic fermentation experiments were carried out using a stirred tank equipped with a Rushton turbine. The data were used to estimate kinetic parameters based on a newly developed kinetics model originated from Herbert’s microbial kinetics model. This newly developed model took into account the effects of aeration rate (AR) and stirrer speed (SS). Experiment data i.e. glucose, ethanol and biomass concentrations obtained from different experiment sets were used for kinetics prediction. Assuming a perfectly-stirred condition, the kinetic parameters were initially estimated through solving Herbert’s model equations. These estimated kinetic parameters were then incorporated in a Computational Fluid Dynamics (CFD) model but the simulation results did not agree well with the experiment findings. Based on the proposed CFD model, the kinetic parameters were corrected. The correction factors were expressed as functions of AR and SS. This analysis highlighted the need to estimate kinetic parameters based on CFD simulation because it is able to account for the spatial variation in a reactor. A sensitivity analysis of the kinetic parameters using the coupled CFD-fermentation kinetic model was carried out to further understand the influence of each set of kinetic parameters on the model prediction. It was found that the sensitivities of the kinetic parameters varied with the concentrations of glucose, ethanol and biomass

Soil carbon varies between different organic and conventional management schemes in arable agriculture

The effects of organic versus conventional farming systems on changes in soil organic carbon (SOC) has long been debated. The effects of such comparisons may depend considerably on the design of the respective systems and climate and soil conditions under which they are performed. Here, we compare a range of arable organic and conventional crop systems at three sites (Jyndevad, Foulum and Flakkebjerg) in Denmark through long-term experiments initiated in 1997. The experimental treatments in the organic farming systems included use of whole-year green manure crops, catch crops and animal manure (as cattle, pig or digested slurry). Data on plant residues and animal manure were used to estimate C inputs to the soil. This was compared with measured changes in topsoil (0–25 cm) SOC content over 4–8 years. During 1997–2004, green manure, catch crops and animal manure enhanced estimated C input by 0.9, 1.0 and 0.7 Mg C ha−1 yr−1 respectively, across all locations. Based on measured SOC changes, green manure enhanced SOC by 0.4 Mg C ha−1 yr−1 and catch crops by 0.2 Mg C ha−1 yr−1, while animal manure by insignificantly 0.1 Mg C ha−1 yr−1. After 2005, advantages of using green manure (grass-clover) on SOC change disappeared, because cuttings of the grass-clover was removed whereas before 2005 they were mulched in the field, albeit there was still a small extra estimated C input of 0.2 Mg C ha−1 yr−1. An estimated higher C input of 0.7 Mg C ha−1 yr−1 with catch crops did not result in significant increase in measured topsoil SOC. From 2005–2008, the first 4 years of comparison between organic and conventional farming at all three sites, organic farming with animal manure had 0.3 Mg C ha−1 yr−1 higher estimated C input, but SOC measurements showed that conventional farming accumulated 0.4 Mg C ha−1 yr−1 more SOC than organic farming. At Foulum from 2005 to 2012, organic farming with animal manure had 0.7 Mg C ha−1 yr−1 more input, and topsoil SOC measurements showed a higher accumulation of 0.4 Mg C ha−1 yr−1 in organic compared with conventional farming. Regressions of changes in topsoil SOC against estimated C inputs showed that 10–20% of C inputs were retained in topsoil SOC over the experimental period. There was no clear indication that belowground C input contributed more to SOC than aboveground C inputs. Despite consistently higher estimated C inputs in organic versus conventional systems, we were not able to detect consistent differences in measured SOC between the systems

Benchmarking explanatory models for inertia forecasting using public data of the nordic area

This paper investigates the performance of a day-ahead explanatory model for inertia forecasting based on field data in the Nordic system, which achieves a 43% reduction in mean absolute percentage error (MAPE) against a state-of-the-art time-series forecast model. The generalizability of the explanatory model is verified by its consistent performance on Nordic and Great Britain datasets. Also, it appears that a long duration of training data is not required to obtain accurate results with this model, but taking a more spatially granular approach reduces the MAPE by 3.6%. Finally, two further model enhancements are studied considering the specific features in Nordic system: (i) a monthly interaction variable applied to the day-ahead national demand forecast feature, reducing the MAPE by up to 18%; and (ii) a feature based on the inertia from hydropower, although this has a negligible impact. The field dataset used for benchmarking is also made publicly available