On the Use of Adaptive/Integral Actions for Six-Degrees-of-Freedom Control of Autonomous Underwater Vehicles

In this paper, the control of autonomous underwater vehicles (AUVs) in six-degrees-of-freedom (6-DOFs) is analyzed in a comparison study among several controllers. At steady state, the vehicle needs to compensate for two dynamic effects, the ocean current and the restoring forces; the appropriateness of the adaptive/integral action designed with respect to the persistent effects is discussed. Moreover, for each controller, an adaptive/integral proportional derivative (PD) plus gravity compensation-like version is derived and eventually modified so as to achieve null steady-state error under modeling uncertainty and presence of ocean current. Numerical simulations are presented to better illustrate the controllers' behavior.     

Tracking control for underwater vehicle-manipulator systems withvelocity estimation

In this paper, the problem of tracking a desired motion trajectory for an underwater vehicle-manipulator system without using direct velocity feedback is addressed. For this purpose, an observer is adopted to provide estimation of the system's velocity needed by a tracking control law. The combined controller-observer scheme is designed so as to achieve exponential convergence to zero of both motion tracking and estimation errors. In order to avoid representation singularities of the orientation, unit quaternions are used to express the vehicle attitude. Implementation issues are also considered and simplified control laws are suggested, aimed at suitably trading off tracking performance against reduced computational load. Simulation case studies are carried out to show the effectiveness of the proposed controller-observer algorithm. The obtained performance is compared to that achieved with a control scheme in which the velocity is reconstructed via numerical differentiation of position measurements. The results confirm that the chattering on the control commands is significantly reduced when the controller-observer strategy is adopted in lieu of raw numerical differentiation; this leads to lower energy consumption at the actuators and increases their lifetime     

Torsional instability in structures

Torsional instability under harmonic excitation was detected in structures with cubic softening member stiffness. A variational procedure was introduced to perform the analysis for the general problem formulation. Single-storey structures with zero eccentricity were studied first; previous findings were checked and corrected, and the behaviour was examined in greater detail. Two-storey structures with large eccentricity were also examined, and further regions of torsional instability were detected. Time history analysis was then employed to quantify the behaviour of various typical structures. An unbounded growth in rotation or column displacement was not observed. Rather, the instability appeared to be related to the jump in response which exists in single degree-of-freedom softening oscillators subjected to harmonic excitation. The practical significance of torsional instability is therefore rather weak, although prudent practice would recommend its avoidance.     

Spatial Variability of Trends in the Rainfall-Runoff Relationship: A Mesoscale Study in the Mosel Basin

The spatial variability of observed trends in rainfall structure over the last 5 decades and its effects on the spatial variability of maximum daily water levels in the Grand-Duchy of Luxembourg (Europe) were investigated in 9 sub-basins of the Mosel river. Over the past 25 years, an increase in westerly atmospheric circulation types during winter months has caused an increase in winter rainfall totals, duration and intensity. More specifically, the spatial variability of trends having affected winter rainfall totals, duration and intensity have led to spatially varying positive trends in maximum daily water levels. Observed trends in rainfall characteristics and maximum daily water levels during winter show closely linked spatial patterns that are strongly related to the topography of the study area.     

Petrogenesis of the Eocene and Mio–Pliocene alkaline basaltic magmatism in Meseta Chile Chico, southern Patagonia, Chile: Evidence for the participation of two slab windows

The Meseta Chile Chico (MCC, 46.4°S) is the westernmost exposure of Eocene (lower basaltic sequence, LBS; 55–40 Ma, K–Ar ages) and Mio–Pliocene (upper basaltic sequence, UBS; 16–4 Ma, K–Ar ages) flood basalt volcanism in Patagonia. The MCC is located south of the Lago General Carrera-Buenos Aires (LGCBA), southeast from the present day Chile Triple Junction (CTJ), east of the actual volcanic gap between Southern South Volcanic Zone and Austral Volcanic Zone (SSVZ and AVZ, respectively) and just above the inferred location of the South Chile Ridge segment subducted at 6 Ma (SCR-1). Erupted products consist of mainly ne-normative olivine basalt with minor hy-normative tholeiites basalt, trachybasalt and basanite. MCC lavas are alkaline (42.7–53.1 wt.% SiO₂, 3–8 wt.% Na₂O+K₂O) and relatively primitive (Ni: 133–360 ppm, Cr: 161–193 ppm, Co: 35–72 ppm, 4–16.5 MgO wt.%). They have a marked OIB-like signature, as shown by their isotopic compositions (⁸⁷Sr/⁸⁶Sr_o=0.70311–0.70414 and εNd=+4.7–+5.1) and their incompatible trace elements ratios (Ba/La=10–20, La/Nb=0.46–1.09, Ce/Pb=15.52–27.5, Sr/La<25), reflecting deep mantle origin. UBS-primitive lavas have characteristics similar to those of the Eocene LBS basalts, while UBS-intermediate lavas show geochemical imprints (La/Nb>1, Sr/La>25, low Ce/Pb, Nb/U) compatible with contamination by arc/slab-derived and/or crustal components. We propose that the genesis and extrusion of magmas is related to the opening of two slab windows due to the subduction of two active ridge segments beneath Patagonia during Eocene and Mio–Pliocene.     

Epiclastic deposits and ‘horseshoe-shaped’ calderas in Tahiti (Society Islands) and Ua Huka (Marquesas Archipelago), French Polynesia

Occurrences of debris avalanche deposits newly identified in Tahiti (Society Islands) and Ua Huka (Marquesas Archipelago) are described and interpreted here. In both islands, the breccias are located within horseshoe-shaped residual calderas. In Tahiti, the epiclastic formations, up to 500 m thick, lie on the floor of the central depression and in the valley of the northwards running Papenoo River. In Ua Huka, the breccias crop out within a depression limited by a semicircular crest in four bays along the southern coast. Their thickness is ca. 100 m. A few clasts collected in the Tahitian breccias and some rocks forming their substratum have been dated (K–Ar datings) and analysed (major and trace elements, Sr–Nd isotopes) for this study. Using these data, we show that the debris avalanche(s) occurred in Tahiti Nui at the end of the growth of the shield volcano (between 570 000 and 390 000 years ago), maybe in consequence of the emplacement of the plutonic body which occupies the central part of the caldera. In Ua Huka, the collapse took place nearly 3 Ma ago, between the construction of the shield volcano and that of the inner one. The southwards orientation of the caldera, like that of the neighbouring island Nuku Hiva, might reflect a preferential direction of weakness in the substratum of the central Marquesas.     

Geochemistry and residence time estimation of groundwater from the upper aquifer of the Chihuahua desert (Comarca Lagunera, Northern Mexico)

178 groundwater and surface waters have been sampled from April to September 1994 in an endoreic basin located in the N of Mexico (Comarca Lagunera). In this area, groundwater has been exploited over the past century mainly for irrigation and cattle supply. Recent intensive pumping has caused the lowering of the water table at a rate of 1 m a⁻¹ Chemical analyses have been performed on all collected samples and 37 of them have been selected for isotopic measurements (¹⁸O,²H,¹³C and¹⁴C). Water stable isotope contents (¹⁸O,²H) show an increasing evaporation of the groundwater towards the Nazas river. They also indicate that the recharge occurs from the Nazas river and from the mountains surrounding the depression (Sierra Madre Occidental). Water presents a large spatial variability of the chemical facies (SO₄Ca, SO₄ClNa, HCO₃-Ca and HCO₃-Na) which is in relation with (i) their interaction with the geological formations of the basin (carbonates, gypsum and various silicates) and (ii) evaporation. This evaporation occurs in the upper part of the unsaturated zone during infiltration especially for the groundwater sampled near the Nazas river. The¹⁴C activity varies between 110.4 (± 1.1) and 4.0 (±0.2) pmc. The¹³C contents of the total dissolved inorganic C (TDIC) range between −11.0 and −3.6‰. The calculated¹³C contents of the CO₂ in equilibrium with the TDIC, varying between −18.4%0 to −10.9% indicate two origins of C in solution: the carbonate matrix (δ¹³C= +0.9‰) and the soil CO₂ (δ¹³C from −27.7‰ to −21.7‰ for the cultivated areas). Mean residence times have been determined after correction of the initial activities for dead C from the rock matrix. The mean residence times confirm a modern recharge of the groundwater from the Nazas and indicate the presence of palaeowaters in the northern and southern parts of the basin (up to 30 ka BP).     

Reconstitution des apports en éléments traces métalliques et bilan de leur migration dans un Luvisol sableux soumis à 100 ans d'irrigation massive par des eaux usées brutes

Raw wastewaters were massively spread on sandy luvisols near Paris from 1899 to 2002, leading to high trace metals (TM) pollution of soils. Mass balance calculations were performed on a soil profile to assess vertical migration of TM. The contamination was estimated by subtracting the natural pedo-geochemical background of the horizons. TM inputs were estimated using Cr as an invariant. It is shown that Pb and Cr remained in the surface horizon, while Ni, Cd, Cu and Zn migrated downward, being more or less trapped depending on the physicochemical properties of the horizons. To cite this article: C. Dère et al., C. R. Geoscience 338 (2006).     

Apport des isotopes stables dans l'estimation des altitudes de recharge de sources thermales du MarocRecharge altitude estimation of thermal springs using stable isotopes in Morocco

The recharge altitude estimation of thermal springs from northern and eastern Morocco using ¹⁸O and ²H contents requires the definition of regional isotopic altitudinal gradients (?0.25‰ for 100 m for the Rif and ?0.30‰ for the East) and the calculation of residence time using ¹⁴C. The altitudes of emergence vary widely between 170 and 1040 m under the altitude of the recharge areas. The ¹⁸O and ²H compositions of palaeowaters ($\text{>10}\text{000}$ yr BP) indicate two effects, altitude and palaeoclimate. To cite this article: A. Winckel et al., C. R. Geoscience 334 (2002) 469–474.