فصلنامه مکانیک هوافضا
سال هفتم شماره 1 (پیاپی 23، بهار 1390)

This paper deals with dynamic modeling and path planning of a spherical mobile robot. Spherical robot is the prototype of mobile robot used in unknown environments. In this paper, it has five degrees of freedom such that its driver mechanism is 2DOF pendulum moved with two motors. This spherical robot is a non-holonomic and non-linear system. First, the kinematic of the spherical robot is analyzed and next its equations of motion are derived,using the Kane method. For path planning, inverse dynamic of the robot on straight and on circular trajectories are solved. For this, new iterative method, using genetic algorithm for path planning in motion on circular path is used.Finally, the optimal path for achieving minimum power and time of motion are obtained, using multi-objective genetic algorithm. Good agreements were observed between the results of this method and previous computer simulations.

In this paper, dynamic stability of two free-end beam subject to a non-conservative following force has been investigated. In the first step, transverse vibration of beam structure in free-free mode with/without end force by modal analysis and FEM was analyzed and mode shapes and natural frequencies were obtained. Then, dynamic equations of the vibration with using the Galerkin method were solved. It can be seen that with increasing the axial force frequency of transverse the vibration of the beam decreased. Also, flutter phenomenon in the states of axial force much higher than buckling force is observed. Furthermore, in case of non-uniform beam, in free-free mode both flutter and divergence are occurred. It can be concluded that, due to low thrust weight ratio in nowadays flying objects, the effect of flutter on dynamic stability of structure itself is not significant. However, the effect of joint in beam significantly reduces the critical force of flutter.

In this paper, estimation problem is considered as a conditional optimization problem and then is solved using both differential evolution (DE) and particle swarm optimization (PSO).The proposed filter searches stochastically along the state space and obtains the best estimation at any time. The performance of the proposed filter, based on differential evolution (DE) and particle swarm optimization (PSO), is compared with extended Kalman filter (EKF) and with particle filter in different conditions for a non-linear system. Simulation results show that the performance of evolution filters in different conditions is superior to extended Kalman and particle filters.

In this paper a new viewpoint to orbital dynamics simulation is introduced. The novel proposed method is based on the manner by which the orbital elements change in the presence of disturbances. Satellite orbit dynamics is often expressed by Cowll and Enke methods. These methods include solving a set of second-order non-linear differential equations, with all disturbance force deviation effects in the Cartesian space. These forms of dynamics require converting the Cartesian components to orbital elements so that these element changes can be obtained. Here,Gauss’s method has been used to convert the dynamics model into six first-order non-linear equations whereby orbital elements can be directly extracted from these equations. Then, “Corrected Disturbance Viewpoint” is defined for low-altitude orbits. In this viewpoint, J2 oscillatory effect is assumed as a part of orbit dynamics. Other effects such as higher order terms of oblateness (J3-J6), drag aerodynamic force, third-body, radiation pressure and J2 longterm effect have been considered as disturbance factors in the dynamics. The main advantages of this proposedviewpoint in comparison with the previously suggested methods are: a) separation capability between oscillatory and long-term effects in disturbances, b) more concise and accurate calculations, c) reducing fuel consumption in the orbit correction process, and d) simplicity plus feasibility of control methods. The performed simulation results support the aforementioned advantages.

In this paper an optimal control-based approach is proposed for designing three-dimensional trajectory for unmanned aerial vehicles (UAV’s). In this approach, by defining a cost function, based on the flight time and the avoiding terrain and threat objectives, the safe trajectory is planned so that UAV avoids obstacles and threatened zone, as an optimal procedure. Besides, digital elevation map (DEM) is interpolated by using spline surfaces. This leads to reduction of digital data and further smoothing the trajectory. The algorithm is presented in terms of constant speed and energy assumptions. For constant speed situation, the heading and flight path angle rate constraints is satisfied implicitly. In contrast with constant speed assumption, constant energy assumption beside the mentioned constraints leads to satisfying speed and altitude limitations. Finally, this approach is recommended for trajectory planning of autonomous vehicles in presence of threatened zones.

In this paper, a PI guidance law with finite time convergence is proposed to nullify the LOS rate in finite time. With the new guidance law, the line-of-sight angular rate will converge to zero or a small neighborhood of zero before the final time of the guidance process. Furthermore, the proposed PI guidance law is extended to the first order pursuit dynamics which can guarantee the stability of the guidance loop and finite time convergence of the LOS rate. In this case, high derivatives of the LOS rate are appeared in the guidance law. Simulation results show the effectiveness and robustness of the proposed guidance law against maneuvering target as compared with finite time PN and true proportional navigation guidance law.

In this article free vibration of a cylindrical shell with internal pressure made of a functionally gradient material (FGM) composed of stainless steel and nickel is studied. The objective is to study the influence of internal pressure on the frequency characteristics of the FGM shell. The boundary condition of this shell is simply supported. The properties are graded in the thickness direction according to a volume fraction power-law distribution. The analysis is carried out with strain-displacement relations from Love’s shell theory and the eigenvalue governing equation is obtained using Rayleigh-Ritz method. After solving the equation, the results were verified by comparing the obtained frequencies with the frequencies of the other literatures and also with the results from ABAQUS.