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Summary: This paper presents an Adaptive Terminal Integral Sliding Mode Control (ATISMC) scheme for trajectory tracking problems applied to a differential Wheeled Mobile Robot (WMR). First, a terminal integral sliding variable is designed. Based on the finite‐time concept, an adaptive control law has been developed in which a switching gain is adjusted adaptively by using a novel strategy. This control method aims to deal with unknown bounded disturbances and uncertainties. Moreover, it allows fast convergence of the system states to an equilibrium point. The main features of the proposed ATISMC are its robustness, fast convergence rate, and chattering avoidance. To realize trajectory tracking for WMR, the ATISMC is incorporated into a double closed loop scheme. Stability analysis is performed using the Lyapunov stability theory. Numerical simulations and real‐time experiments demonstrate the feasibility of the proposed controller scheme. A comparison study with the classical ISMC was performed to show the superiority of the developed method. [ABSTRACT FROM AUTHOR]

The long term tightness performance of stuffing-box packings, used in valves, is conditioned by the capacity of its sealing material to maintain a contact pressure to a predetermined minimal threshold value. Due to the creep, this contact pressure decreases with time depending on the creep properties and the stiffness of the housing. Assessing relaxation is a key parameter in determining the tightness performance of a valve stem packing over time. An analytical model based on the packing viscoelastic behavior is developed to assess the contact pressures between the packing material and the stem and the housing and their variation with time. In parallel, an axisymmetric 2D finite element model was build to validate and support the analytical model. The valve stem packing relaxation performance is an important design parameter to consider when selecting compression packings. [DOI: 10.1115/1.3118787] Keywords: packing, stuffing-box, relaxation, analytical modeling, viscoelasticity, FEA

This paper presents a fuzzy adaptive sliding mode controller (FASMC) for electrically driven wheeled mobile robot for trajectory tracking task in the presence of uncertainties and disturbances. First, a finite-time kinematic controller is developed to compute the auxiliary velocity vector. Second, the FASMC, based on the nonlinear dynamic model of the robot and its actuators, is used to guarantee the stability and the convergence of the closed-loop system. Moreover, by employing the advantages of the fuzzy logic systems, the developed controller ensures the robustness of the system against dynamic disturbances and uncertainties, the smoothness of the computing voltage against the chattering phenomenon, and the optimal convergence of the velocity and posture errors. The Lyapunov theory is used to analyse the stability of this algorithm. In order to evaluate the effectiveness of the developed method, numerical simulations are done in the Mahlab/Simulink environment. [ABSTRACT FROM AUTHOR]

O-rings are used in machine devices like the seals components. They are inexpensive, and have simple mounting requirements. In this article, an axisymmetric finite element model is proposed to study the O-ring relaxation during the first day of its installation in the unrestrained axial loading case. The results of the numerical model are compared with those of an analytical approach based on the classical Hertzian theory of the contact. The contact stress profiles and the peak contact stresses are determined versus the time relaxation in order to specify the working conditions thresholds. [ABSTRACT FROM AUTHOR]

The sealing of valve stems is ensured by the traditional systems of packed stuffing boxes. The performance of this type of sealing system, which is also used in rotating equipment, is dependent on the radial contact pressures generated by the packing axial compression. The mechanical behavior of a packing seal is characterized by the transmission ratio of the radial stress over the axial stress known as the lateral pressure coefficient, which is one of the required parameters used to select packing seals. However' the modeling of the packed stuffing box requires the knowledge of other packing seal mechanical characteristics such as compression modulus and Poisson's ratio. In this paper, the mechanical characteristics of packing seals are obtained using a hybrid experimental-numerical procedure. The experimental study is carried out on an instrumented stuffing box packing test bench. The tests results of the experiments are coupled to the ones obtained by a finite element simulation of the test rig to determine the packing seal mechanical characteristics. Two packing types are used: one based on Teflon and the other one based on flexible graphite. In addition, leak rates are measured for different axial compressive stresses and gas pressures in order to estimate the tightness performance of such seals. [ABSTRACT FROM AUTHOR]