امیرکبیر
سال:1386 | دوره:18 | شماره:ب - 66 (گروه مهندسی مکانیک، صنایع، هوافضا) |تعداد مقالات:14

The poly-ethylene model of a hexapod table with six degrees of freedom has been developed by the authors on the basis of Stewart-Gough mechanism. A conventional CNC milling machine can be converted into a six axis machine by installing the hexapod table in place of the table of the machine. The axial forces of the struts and their changes have been investigated with the table center situated at its most critical position .The diagrams illustrating these changes can be employed to decide the workspace involving minimum axial forces in the struts for specific machining conditions and placement of the spherical and universal joints. The analytical results have been verified with the experiments carried out on the model.

The majority of developed models in supply chain management are limited to planning at most two of three objectives that are production, distribution, and inventory. Since designing and specially solving a general model to optimize these three objectives all together is too complex, in this research we will try to find a near optimal solution using an integrated model made of three sub-models. Solving the integrated model needs each sub-model solved separately and by considering a sequence. Therefore an analytical approach including start point, the sequence of sub-models and termination criterion developed. Because deterministic condition restricts model's applicability, sub-models designed in stochastic condition with Poisson distribution that depends on arrival of customer. Finally, some numerical researches used to check the model validity.

In most statistical quality control (SQC) applications, quality of a process or product is characterized by a univariate quality characteristic or a vector of quality characteristics, which is controlled by a univariate or multivariate quality control chart, respectively. However, in many practical situations, the quality of a process or product is characterized by a relationship between two or more variables. This relationship, which is referred to as profile can be linear or nonlinear in nature. So far, several methods have been proposed for monitoring linear profiles in phase H. In this paper, two other methods are proposed for improving the performance of linear profiles in phase H. Average run length criterion is used as a vehicle to evaluate the performance of the proposed methods.

This paper applies the Markov models, optimal control theory, eigen vectors and eigen values concepts to propose a methodology to analyze the transient behavior of Exponential queuing systems. We propose a procedure to calculate the transient probability, the average queue length and the duration of the system to reach the steady state. The method is implemented through an algorithm which is developed and tested in MATLAB software environment. The new method enjoys a stronger mathematical foundation and more flexibility to analyze the transient behavior of Exponential queuing systems.

A new mathematical model of the dressing of grinding wheels by diamond disk dresser is developed. It predicts the components of dressing forces between the diamond grits of a rotating disk dresser and the grits of an alumina grinding wheel. This model is based on the fracture of grits and the contact forces between dresser and grits. It considers the kinematical influences and, in particular, speed ratio of the dressing process. Dressing forces are measured experimentally and are compared with theoretical calculation. Theoretical results closely match with the experimental findings. The variation of dressing forces versus speed ratio and dressing feed rate are presented at three depth of dressing. At each case, the grinding forces were measured experimentally and the effect of changing dressing parameters on the grinding performance is analyzed.

The theoretical development of the direct adaptive tracking control architecture using neural networks is presented. Emphasis is placed on utilization of neural networks in a flight control architecture based on feedback linearization of the aircraft dynamics. Neural networks are used to represent the nonlinear inverse transformation needed for feedback linearization. Neural networks capable of on-line learning are required to compensate for inversion error, which may arise from imperfect modeling, approximate inversion, or sudden change in aircraft dynamics. A stable weights adjustment rule for on-line neural network is training derived.Under mild assumptions on the nonlinearities representing the inversion error, the adaptation algorithm ensures that all of the signals in the loop are uniformly bounded and that the weights of the on-line neural network parameters tend to constant values. The main objective of the control design is to demonstrate adaptation to aerodynamic uncertainty in the form of both unmodeled parameter variations and unmodeled dynamics not present in the nominal inverting control design. Finally, nonlinear six-degree-of-freedom simulation results for an F-18 aircraft model are presented to demonstrate the effectiveness of the proposed control law.

An improved method for solving the full viscous shock layer (FVSL) equations for hypersonic flows at low and high altitudes over long slender bodies is presented. This approach can solve both subsonic and supersonic regions of the shock layer without a starting solution for the shock shape and the global iterations are limited only to the nose region. In the nose region (where the subsonic or elliptic flow fields prevails), shock shape is specified from an algebraic expression and corrected through global passes through that region. The shock shape is computed as part of the solution beyond the nose region and requires only a single global pass. Under the high Reynolds number (at low altitudes), the Cebeci-Smith (CS) turbulent model has been analyzed with the present numerical technique for application to long slender bodies. The surface-slip and the currently corrected shock-slip boundary conditions are employed to account for the low-density effects. The method of solution is a spatial-marching, implicit, finite-difference technique, which includes coupling of the normal momentum and continuity equations and uses simple relation instead of Vigneron pressure condition in the subsonic nose region for instability. The results are compared with experimental data and other solutions. The comparisons have been shown to yield accurate results.