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The corrosion resistance of two-phase (fcc-hcp) Co-27Cr-5Mo-0.05C alloys produced by isothermal aging at 800 ЊC was studied using potentiostatic polarization tests in Ringer's solution. Critical pitting potentials were estimated from the potentiostatic polarization curves and were found superior to that exhibited by the conventional ASTM-F75 cast alloy used for the manufacture of orthopedic implants. Formation of suitable distributions of hcp embryos (incoherent twin boundaries and stacking faults) prior to and during the early stages of aging required for isothermal fcc-hcp transformation led to a relative reduction of the corrosion resistance of two-phase alloys. However, once the transformation proceeded rapidly, between 4 and 8 hours of aging, the elimination of lattice defects caused a reduction of the dissolution rates and the breakdown potential became nearly independent of the relative amounts of fcc and hcp phases present in the microstructure. This behavior was due to the uniform chemical composition of the two-phase alloys. Concurrent work has shown that the hardness and yield strength of a 50 pct hcp alloy are increased by at least 30 pct without undue ductility losses. Therefore, the results of the present article suggest that these materials are excellent candidates for the manufacture of orthopedic implant devices requiring higher strength than provided by conventional ASTM-F75 materials.

This work proposes three robust mechanisms based on the MIT rule and the sliding-mode techniques. These robust mechanisms have to tune the gains of an adaptive Proportional-Derivative controller to steer a quadrotor in a predefined trajectory. The adaptive structure is a model reference adaptive control (MRAC). The robust mechanisms proposed to achieve the control objective (trajectory tracking) are MIT rule, MIT rule with sliding mode (MIT-SM), MIT rule with twisting (MIT-Twisting), and MIT rule with high order sliding mode (MIT-HOSM).

Parallel robots are nowadays used in many high-precision tasks. The dynamics of parallel robots is naturally more complex than the dynamics of serial robots, due to their kinematic structure composed by closed chains. In addition, their current high-precision applications demand the innovation of more effective and robust motion controllers. This has motivated researchers to propose novel and more robust controllers that can perform the motion control tasks of these manipulators. In this article, a two-loop proportional–proportional integral controller for trajectory tracking control of parallel robots is proposed. In the proposed scheme, the gains of the proportional integral control loop are constant, while the gains of the proportional control loop are online tuned by a novel self-organizing fuzzy algorithm. This algorithm generates a performance index of the overall controller based on the past and the current tracking error. Such a performance index is then used to modify some parameters of fuzzy membership functions, which are part of a fuzzy inference engine. This fuzzy engine receives, in turn, the tracking error as input and produces an increment (positive or negative) to the current gain. The stability analysis of the closed-loop system of the proposed controller applied to the model of a parallel manipulator is carried on, which results in the uniform ultimate boundedness of the solutions of the closed-loop system. Moreover, the stability analysis developed for proportional–proportional integral variable gains schemes is valid not only when using a self-organizing fuzzy algorithm for gain-tuning but also with other gain-tuning algorithms, only providing that the produced gains meet the criterion for boundedness of the solutions. Furthermore, the superior performance of the proposed controller is validated by numerical simulations of its application to the model of a planar three-degree-of-freedom parallel robot. The results of numerical simulations of a proportional integral derivative controller and a fuzzy-tuned proportional derivative controller applied to the model of the robot are also obtained for comparison purposes.

Our contribution is devoted to a further theoretic development of the attractive ellipsoid method (AEM). We consider dynamic models given by nonlinear ordinary differential equations in the presence of bounded disturbances. The resulting robustness analysis of the closed-loop system incorporates the celebrated Clarke invariancy concept (an analytic extension of the celebrated Lyapunov methodology). We finally obtain a new general geometric characterization of the AEM-based approach to the robust systems design. Moreover, we also discuss the corresponding numerical aspects of the proposed theoretical extensions of the method. The theoretic results obtained in this contribution are finally illustrated by a practically oriented computational example. KEYWORDSattractive ellipsoid method, nonlinear systems, robust control 1418

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