Journal of Advanced Materials in Engineering (Esteghlal)

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This paper presents a robust controller design methodology for a class of linear uncertain multivariable systems with hard time-domain constraints on their outputs and control signals in response to step disturbance inputs. In this approach, the m×m MIMO system is replaced by m2 SISO systems and then, using the QFT technique, desirable controllers are synthesized. The final controller will be diagonal and since its entries are designed separately with suitable bandwidths, an economic design can be achieved. The application of this new method will be demonstrated through an example.

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In this paper, a new algorithm for system identification based on frequency response is presented. In this method, given a set of magnitudes and phases of the system transfer function in a set of discrete frequencies, a system of linear equations is derived which has a unique and exact solution for the coefficients of the transfer function provided that the data is noise-free and the degrees of the numerator and denominator are selected correctly. If the data is corrupted with (bounded) noise, then the answer is no longer unique and an acceptable transfer function is one that has a frequency response with a noise bound that covers the noisy data. To find one of these acceptable results, a new performance index is defined as “the least squares distance in the coefficient space”. By minimizing this index, an initial transfer function is obtained which passes optimally through the noisy data. Then, using the so-called dynamic programming technique, the noise is reduced in such a way that at each step the resulting transfer function is pushed toward one of the acceptable noise free systems. An illustrative example shows the effectiveness of the proposed algorithm.

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In this paper, dynamic and stability analysis of a flexible cam-follower system is investigated. Equation of motion is derived considering flexibility of the follower and camshaft. Viscous and Coulomb frictions are considered in the rocker arm pivot. The normalized equation of motion of the system is a 2nd- order differential equation with periodic coefficients. Floquet theory is employed to study parametric stability of the system. Stability diagrams are presented and the effects of varying cam profiles and motion events on the stability of the system are compared. Results show that viscous and Coulomb frictions stabilize the motion of the system

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In this paper, the phase formation process of Mn_2.5Ge samples, prepared by mechanical alloying of Mn and Ge metal powders and annealing, has been studied. Results showed that in the milled samples the stable phase is Mn₁₁Ge₈ compound with orthorhombic structure and Pnam space group. The value of saturation magnetization increases by increasing milling time from 0.2 up to 1.95 (Am²Kg^-1). The remanece of the samples increases by increasing the milling time while the coercivity decreases. Annealing of 15-hour milled sample results in disappearance of Mn and Ge and the formation of new phases of Mn₃Ge, Mn₅Ge₂, Mn₅Ge₃ and Mn_2.3Ge. Mn₃Ge is the main phase with Do₂₂ tetragonal structure and I4/mmm space group which is stable and dominant. The enhancement of saturation magnetization in the annealed sample is related to the formation of three new magnetic phases and the increase of coercivity is due to the presence of Mn₃Ge compound with tetragonal structure. Studies were replicated on samples made by arc melting method to compare the results and to investigate the effect of the preparation method on phase formation and structural and magnetic properties of the materials. In these samples the saturation value was in range of 0.2 up to 1.95 (Am²Kg^-1) depending on preparation methods. Rietveld refinement shows that Mn_2.3Ge sample prepared from arc melted under 620^oC anealing is single phase. Magnetic analysis of this sample show a saturation magnetization of 5.252(Am²Kg^-1) and 0.005 T coercive field.