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This brief investigates the fixed-time tracking problem for uncertain nonlinear systems with input saturation constraint under the event-triggered scheme. An adaptive fixed-time controller is proposed to drive the tracking error to a residual set in fixed time, which can eliminate the problem of “explosion of complexity”. Furthermore, in order to decrease the communication burden between the controller and the actuator while maintaining the system control performance, we co-design the adaptive controller and the event-triggered strategy, which guarantees that the control input is injected into the actuator only when the predefined event occurs. In addition, the measurement error arisen from event-triggering mechanism is also compensated. Finally, a rigorous mathematical stability proof is given and simulation results are presented to illustrate the effectiveness of the proposed control scheme.

This paper addresses the tracking control problem for uncertain Euler-Lagrange system with time-varying output constraints in an environment containing obstacles. First, a novel log-type attractive potential field is utilized to describe the trajectory tracking task with time-varying constraints, and a bounded artificial potential field is established to describe the obstacle avoidance task. Then, by incorporating the two artificial potential fields (APFs) into the dynamic surface control, a neuro-adaptive tracking control is designed for the uncertain Euler-Lagrange system, which can ensure the system to fulfill the trajectory track task within time-varying limit range while avoiding obstacles. Because the obstacle avoidance task has a higher priority, the proposed control scheme can also guarantee the obstacle avoiding task can be fulfilled first when it is conflicted with the trajectory tracking task. Numerical simulations are provided to demonstrate the efficacy of the control strategy.

This paper investigates the prescribed performance attitude tracking problem for the rigid spacecraft subject to external disturbances and multiple constraints, including input saturation and full-state constraints. By employing the barrier Lyapunov function and an error-shifting transformation, a fixed-time tracking control strategy is devised capable of dealing with the prescribed performance problem under a completely unknown initial tracking condition. In addition, an auxiliary system is also introduced to compensate the adverse effect arising from the input saturation. Finally, the performance of the proposed control schemes is demonstrated by numerical simulations.

This paper investigates the global finite-time attitude tracking problem for the rigid spacecraft subject to inertial uncertainties, external disturbances, actuator faults, and input saturation constraints. The exponential coordinates vector in conjunction with a hysteretic-based jump condition is introduced to overcome the topological obstacles of global stability on the special orthogonal group. A novel nonsingular fixed-time-based sliding mode is designed, which not only avoids the singularity but also guarantees that the convergence time of tracking errors along the sliding surface is independent of the state value. Then, an adaptive fault-tolerant control law is constructed to enforce the system state to reach a neighborhood of the sliding surface in the sense of the fixed-time concept, which can accommodate actuator failures under limited control torque. The total convergence time is independent of the initial conditions information. A rigorous mathematical stability Proof is given. Numerical simulations are finally performed to demonstrate the effectiveness of the proposed finite-time controller.

Purpose The purpose of this paper is to study the influence of alkyl chain length and kind of anions of ionic liquids on the tribological properties with different materials as friction pairs (steel-aluminum, steel-copper and steel-Si3N4 ceramic). Design/methodology/approach Tribological properties were evaluated by an optimol-SRV-IV reciprocation friction tester with a ball-on-block configuration at room temperature and high temperature, respectively. Friction-reducing and anti-wear properties of the ionic liquids for steel/aluminum, steel/copper and steel/ceramic contacts were evaluated on the ball-on-block reciprocating UMT-2MT tribometer. The morphologies of the worn surfaces were observed by a scanning electron microscope. The chemical states of several typical elements on the worn surfaces were examined by X-ray photoelectron spectroscopy. Findings Both the alkyl chain length and kind of anion influence the tribological properties of ionic liquids, especially for the length of alkyl chains. With the increase of alkyl chain length, the load carrying capacity of ionic liquids is improved at both room temperature and high temperature, and the friction reducing and antiwear behaviors are also significantly enhanced. Research limitations/implications The paper presents potentially useful and highly efficient lubricants. Practical implications Owing to their good friction-reducing and wear resistance properties, these ionic liquids are promising candidates for versatile applications. Originality/value This work might provide a promising research direction for design and application of ionic liquids as lubricants.

In this paper, an attitude formation control problem of multi-spacecraft systems is investigated on the special orthogonal group (SO(3)) under the undirected tree communication topology. A novel appointed-time performance function is developed to characterize the user-assigned transient and steady state performance. To online compensate the system uncertainties, an actor-critic neural network architecture is utilized in the control design process. The critic neural network intends to approximate the long-term integral cost function, which can evaluate the consensus performance of the formation system. Based on the exported reinforcement signal, the actor neural network is introduced to generate the feedforward compensation term to cope with the system uncertainties. By utilizing the barrier Lyapunov function, a model-free decentralized control scheme is constructed with an adaptive robustifying term to enhance the disturbance rejection ability, in which the control input for each spacecraft is produced by using only the local relative information from its neighbors. Moreover, an auxiliary system is devised to cope with the input saturation constraint. By using the Lyapunov stability theory, the stability and convergence of the closed-loop system are analyzed. Finally numerical simulations are conducted to illustrate the feasibility and effectiveness of the proposed adaptive actor-critic learning-based control scheme.

In this paper, we investigate the finite-time attitude tracking control problem for a rigid spacecraft on the special orthogonal group ( $SO(3)$ ). The exponential coordinates, derived from the exponential and logarithmic maps of Lie group, are utilized to describe the attitude tracking error on $SO(3)$ almost globally unique except for a zero measure set on which the logarithmic map is double valued. To avoid this issue, a novel nonsingular sliding surface is designed to guarantee that the attitude tracking error will never reach the zero measure set and provide fixed-time stability during the sliding phase. Then, a novel continuous second-order sliding mode control scheme is developed to force the system state to reach the desired sliding surface in finite time. The salient feature of the proposed control scheme is that it significantly suppresses the chattering phenomenon. In addition, to address the lack of angular velocity measurements, a finite-time observer is proposed to recover the unknown angular velocity information in finite time. Then, an observer-based finite-time output feedback control strategy is obtained. Finally, numerical simulations are presented to demonstrate the effectiveness of the proposed control scheme.

In the eutrophic coastal ocean, quick formation of iron (Fe) sulfide is environmentally important to effectively prevent accumulation of dissolved sulfide and its detrimental effects on the benthic ecosystem. In this study, 0.5 N HCl-extractable labile Fe (LFe), acid volatile sulfide, and pyrite in the East China Sea inner shelf sediments were examined to investigate the mechanisms of quick sequestration of dissolved sulfide and potential impacts of frequent algal blooms on the capacity of quick sulfide-buffering in eutrophic coastal areas of the large-sized continental shelf subject to massive terrestrial input. The results indicate that sulfate reduction has been competitively suppressed by dissimilatory Fe reduction due to limited availability of labile organic matter. Dissolved sulfide can be quickly buffered by reaction with LFe and, therefore, is difficult to accumulate to a high level. The quick sulfide-buffering capacity has not become exhausted partly because of the formation of un-sulfidized LFe(II) via dissimilatory reduction of less reactive Fe oxides. It is expected that dissolved sulfide will not pose detrimental effects on the benthic ecosystem in the near future if the current biogeochemical/ecological state remains.

Solid-phase sulfur speciation and stable sulfur isotopic compositions are used to elucidate the formation and burial of pyrite-sulfur (S py ) and organic sulfur (OS) at three selected sites in mud sediments of the East China Sea (ECS) inner shelf, and to infer potential factors influencing the preservation of S py and OS in the sediments. Our results in combination with previous studies show that the overall reactivity of sedimentary organic matter (OM) is low, while OM at the site impacted by frequent algal-bloom events displays somewhat enhanced reactivity. We observed characteristically low contents of acid volatile sulfide (AVS) and S py in the sediments, which can be attributed to low sulfate reduction rate due to high redox potential together with limited availability of labile OM. Several geochemical features, for example, persistent occurrence of S 0 , good coupling among the profiles of AVS, S 0 and S py , and large 34 S py depletion, all suggest that the polysulfide pathway and disproportionation are likely involved in the pyrite formation. Organic sulfur amounts in the sediments are at the lower end of OS contents reported in many other marine sediments around the world. The sources of OS are both biosynthetic and diagenetic, with the biosynthetic OS being the major share (59–73%). In one site studied (C702), enhanced accumulation of OS within the upper layers (14 cm) is believed to be associated with frequent algal-bloom events. Net burial fluxes of S py and OS in the three sites studied range from 0.27 to 0.82 mmol/m 2 /d and from 0.22 to 0.74 mmol/m 2 /d, respectively. Sedimentation rate and algal-bloom events are two important factors influencing the spatial variability of S py and OS burial fluxes in the whole shelf.

This paper investigates the finite-time output feedback stabilization of rigid body attitude dynamics without velocity measurement on the special orthogonal group ( SO(3)). We first transform the stabilization problem on SO(3) into the stabilization issue on its Lie algebra ( so(3)) via the logarithm map. The proposed approach consists of exponential coordinates of SO(3) and the output of a nonlinear filter without the knowledge of the control input. Then the finite-time stability of the resulting closed-loop system is proven based on Lyapunov stability theory and geometric homogeneity technique. Since the set of singularities is zero measure, the proposed velocity-free feedback controller guarantees almost global finite-time stability, which is as strong as the topology of SO(3) can permit. Finally, simulations demonstrate the effectiveness of the proposed approach.

Speciation and reactivity characterization of solid-phase Fe in marine sediments are of significance to understanding its heterogeneous mineralogy and crystallinity, the diagenetic cycling of Fe and its regulating roles on many other elements in sediments. In this study, a combination of sequential and single-step extractions was used for the determination of seven Fe pools in surface sediments of the East China Sea (ECS) continental shelf: (1) carbonate associated Fe (Fe(II) carb ) plus acid volatile sulfide-Fe (Fe(II) AVS ), (2) easily reducible amorphous/poorly crystalline Fe oxides (Fe ox1 ), (3) reducible crystalline Fe oxides (Fe ox2 ), (4) magnetite (Fe mag ), (5) poorly reactive sheet silicate Fe (Fe PRS ), (6) pyrite-Fe (Fe py ), and (7) unreactive silicate Fe (Fe U ). Total Fe (Fe T ) in the sediments is largely determined by terrestrial aluminosilicate particles as indicated by a great similarity of the Fe T with that of the Yangtze River and global riverine particulates. The size of Fe PRS is found to be the largest pool, followed by Fe U , Fe ox2 , Fe mag , Fe(II) AVS+carb , Fe ox1 and Fe py . The large Fe PRS may result from neoformation of Fe-rich clay minerals via reverse weathering and subsequent ageing. The small sizes of Fe(II) AVS+carb and Fe py pools is believed to be the result of low SO 4 reduction due to generally low labile organic matter together with the oxic/suboxic, dynamic environments of the surface sediments. The occurrence of Fe ox1 , Fe ox2 and Fe PRS in the sediments is closely associated with the clay fraction as indicated by a high spatial correlation between the former and the latter. Highly reactive Fe(Fe HR ) in the sediments is comparable to that in global marine sediments, but apparently lower than in the Yangtze River and global riverine particulates due probably to sequestration in the Yangtze Estuary. The ratios of Fe HR /Fe T , Fe PR /Fe T and Fe U /Fe T in the ECS surface sediments consistently show more similarity to those in the Yangtze River particulates than in the global continental margin or deep-sea sediments. The surface sediments maintain a high level of buffering capacity toward sulfidation suggested by a large fraction of highly reactive Fe(III) oxides (Fe(III) HR ) in Fe HR .

Poly(methyl methacrylate) (PMMA) brushes grafted from silicon wafer surface were prepared using a reversible addition fragmentation chain transfer (RAFT) technique. 4-Cyano-4-((thiobenzoyl)sulfanyl)pentanoic succinimide ester, a novel RAFT agent, was immobilized on the silicon wafer surface. RAFT polymerizations were then conducted from the silicon surface to polymerize from the silicon surface. Kinetic studies revealed that the thickness of the PMMA film increased almost linearly with the polymerization time after 2 h, indicating that the chain growth from the surface was a controlled process with a "living" character. The PMMA brushes were characterized using ellipsometry, X-ray photoelectron spectra (XPS), atomic force microscopy (AFM) and contact angle measurements.

Reversible addition–fragmentation chain transfer (RAFT) polymerization was used to prepare polymer brushes grafted onto silicon wafer surface. Novel RAFT agent was prepared and immobilized on the silicon wafer surface. RAFT polymerizations were then conducted from the silicon surface to graft polymer brush to the silicon. Analysis of the polymer brush layers was conducted using ellipsometry, XPS, AFM and contact angle measurements, respectively.

Under the well-controlled experimental conditions, the biological treatability of real municipal landfill leachate with high strength nitrogen and high chemical oxygen demand (COD) concentration using anoxic/anaerobic upflow anaerobic sludge bed (UASB) -sequencing batch reactor (SBR) combined process was conducted in laboratory. The results indicated: stable anoxic/anaerobic UASB-SBR process performance was developed during running continuously for five phase (116 d) when feed COD concentration was range from 1 237.2 mg/L to 12596.8 mg/L, effluent COD concentration was between 108.4 mg/L and 528.26 mg/L, and when the influent ammonia nitrogen (NH4(+) -N) was changed from 155.8 mg/L to 1298.0 mg/L, the effluent NH4(+) -N was varied from 0.12 mg/L to 4.1 mg/L, it achieved high COD and NH4(+) -N removal efficiency. In this present study, it is noted that the anoxic UASB1 has two significant effects: firstly, denitrification reaction of high efficiency was conducted for SBR nitrified effluent recirculated by using the abundant organic matters in the raw leachate as carbon source. Secondly, its removal COD was highly effected by anaerobic biodegradation. The effluent COD of anoxic UASB1 was biodegraded further in the anaerobic UASB2 and aerobic SBR, the maximum organic loading rates (OLR) (as COD) were 13.0, 2.09, 2.14 kg/(m3 x d), respectively. In addition, the correlation between OLR with OLRrem and COD removal efficiency of three reactors was studied, relation between nitrogen loading rate (NLR) with NH4(+) -N removal efficiency of SBR was tested by linear regression analysis, it was found that the OLR of anoxic UASB1, anaerobic UASB2 and aerobic SBR increased linearly with OLRrem. As to SBR, the correlation was significant between NLR (as N) with NLRrem. In addition, the OLR of three reactors shows second order exponential correlation with COD removal efficiency. At last, when the water temperature of SBR ranged from 20.7 degrees C to 10.3 degrees C, and dissolved oxygen was controlled below 1.0 mg/L, the efficiencies of nitrification and denitrification were above 98.5% and 97.7% during the whole experimental running period, it achieved advanced nitrogen removal.