Robot robust path tracking

A new discontinuous robust control law for a rigid manipulator with bounded parameter uncertainties to track a desired trajectory, is presented. Global exponential stability is proved by the use of a natural Lyapunov function based on a transformation of the manipulator's differential equation due to Slotine and Li [2]. Convergence is to a sliding mode along which the tracking error is reduced at an arbitrary exponential rate. It is also shown how adaptation of bounds on uncertainties, and parameter adaptation, can be incorporated. For a continuous approximation of the discontinuous control law,practical stability (essentially, global uniform ultimate boundedness) of the tracking error is proved. Simulation results for the continuous control law exhibit excellent robust tracking.     

The controlling effect of 0.05% hydrogen sulfide gaseous atmosphere on the accelerated fatigue failure of coated MM-002 nickel-base superalloy at 650 °C

A systematic study was carried out aimed at evaluating the environmental effects of H₂S containing environment on the creep-fatigue failure of coated undirectionally solidified MM-002 nickel-base superalloy at high temperature. The cyclic loading conditions were constant and consisted of creep tension and plastic compression following the CP-mode of the strain range partitioning method. The results obtained have shown that although the pack cementation coating used showed adequate resistance under the combination of cyclic loading and oxidizing environments, this resistance was not evident under the H₂S containing atmosphere. In fact, it was found that even an amount as little as 0.05% H₂S had an extremely detrimental effect on the creep-fatigue life of the coated superalloy at 650 °C. The crack initiation and propagation in the H₂S environment was controlled by an aggressive sulphidation attack ahead of the crack tip. This was demonstrated by the formation of low melting eutectic sulfides at the plastic zone ahead of the crack tip, causing an accelerated transdendritic cracking and premature failure.     

Randomization in individual choice behavior.

There is ample evidence that people cannot generate random series when instructed to do so. Rather, they produce sequences with too few symmetries and long runs and too many alternations among events. The authors propose a psychological theory to account for these findings, which assumes that subjects generate nonrandom sequences that locally represent theoretical random series subject to a constraint on their short-term memory. Closed-form expressions are then derived for the major statistics that have been used to test for deviations from randomness. Results from 3 experiments with 2 and 3 equiprobable alternatives support the model on both the individual and group levels. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Corrosion fatigue of extruded magnesium alloys

Corrosion fatigue tests were carried out on extruded AZ31 (3% Al, 1% Zn, 0.3% Mn, Mg—the rest), AM50 (5% Al, 0.4% Mn, Mg—the rest) and ZK60 (5% Zn, 0.5% Zr, Mg—the rest) Mg alloys in air, NaCl-based and borate solutions. N_sol/N_air ratios (the relative fatigue life) were used for the analysis of the corrosion fatigue behavior of Mg alloys in various environments, where N_sol and N_air are the numbers of cycles to failure in the solution and in air, respectively. Extruded ZK60 alloy reveals very high fatigue and corrosion fatigue properties in comparison with other alloys. However, it has the lowest relative fatigue life (N_sol/N_air 10⁻³–10⁻²) or the highest sensitivity to the action of NaCl-based solutions in comparison with that of AM50 and AZ31 alloys (N_sol/N_air 10⁻²–10⁻¹). Under the same stress, the corrosion fatigue life of extruded alloys is significantly longer than that of die-cast alloys (N_sol for extruded AM50 in NaCl is two to three times longer than that of die-cast AM50).     

Provision of step-level public goods: Effects of inequality in resources.

Twelve groups of 5 players each participated in a multiple-trial social dilemma game in which each player, i, receives a possibly different monetary endowment, ei, and then chooses independently and anonymously whether to contribute it to a monetary public good. The collective good, r, is supplied to all of the group members if the sum of contributions is equal to or larger than a prespecified and known threshold; it is not supplied otherwise. It was found (a) that the proportion of contributions decreased with experience, (b) that players contributed more often, the higher their endowment, but (c) that players with lower endowments were perceived to be more likely to contribute than were players with higher endowments. Group data provided strong support to a model in which the decision to contribute or not depends on ei and r, altruism, group size, and the probability of being critical to the group outcome. (PsycINFO Database Record (c) 2010 APA, all rights reserved)