Journal of Aerospace Mechanics
Year:2010 | Volume:6 | Issue:3 (21) (DYNAMICS, VIBRATIONS AND CONTROL)|Papers Count:8

In this paper, it is tried to design an altitude hold mode autopilot of unmanned aerial vehicles (UAV's).This autopilot has practical importance for VAV's due to flying in the vicinity of terrain to implement missions by terrain following maneuvers. This case presents an interesting challenge due to the non-minimal phase characteristics, non-linearities, and uncertainties of altitude to elevator relation. Unlike the conventional methods (in which the altitude autopilot is designed with three loops, so that altitude, pitch rate, and pitch angle variables are measured), a single loop scheme is proposed so that only altitude is measured. This single loop scheme is simple, but it is not robust with respect to the uncertainties. The fuzzy logic control is proposed to overcome parametric uncertainties and unmodeled dynamics (due to the complexity and expenses in aerospace industries, these uncertainties are usually available in modeling).Our 6DOF non-linear simulation shows that it is possible to achieve a simple and robust autopilot by fuzzy logic control.

Wind tunnel tests with good flow conditions have great roles in the design and optimization of models of flying vehicles. It is crucial to obtain test section flow parameters, such as velocity and pressure distribution, as well as uniformity along with the direction of the flow in the calibration tests. This process is very expensive and time consuming due to large Mach number range, especially for relatively large sizes of wind tunnel test section. To overcome these problems, the calibration tests were done in the specified sections and Mach number and the results were extracted for other sections and Mach numbers by soft computing. With real time monitoring of wind tunnel data, based on GRNN and the network training, the related data in other sections and Mach numbers were estimated.It was shown that the GRNN is more capable with respect to classical methods for estimating non-linear functions, such as least square method. Cost, time, and wind tunnel calibration tests were decreased extensively using this intelligent technique.

In the present work, study of the free vibration of thin cylindrical shells of a functionally gradient material (FGM) with ring support composed of stainless steel and nickel is presented. The effects of boundary conditions and ring support on the natural frequencies of the shell are studied. The cylindrical shells have ring supports of arbitrarily location. The study is carried out using third shear deformation shell theory and analysis is carried out using Hamilton's principle. The governing equations of motion, the results of frequency characteristics, the influence of ring support position, and the influence of boundary conditions are presented. The results are validated with those available in the literature.

This article investigates and analyzes the snake-like robot by applying Lagrange method to establish the equations of motion. Based on these equations, the generalized coordinates are the absolute angles of bodies and center of mass of the snake robot. The friction force was considered as the Coulomb model and the contacting surface was based on the rectangle shape. This model allows more realistic result compared to viscous friction model. Although the robot' s relative angles are specified as functions of time, the robot's equations of motion were derived such that the torque of actuators were not directly inserted into the equations and the relative angles were the system's input. Based on the serpenoid curve and parametric control through this curve (which influences the whole movement of the robot), the robot will be controlled in such a way that it can travel in a prescribed path. Accordingly, the robot could travel weil through the prescribed curvilinear and linear paths. By utilizing this method, the robot was navigated from a point outside the design path towards it.

In this paper, free vibration behavior of a cracked cantilever beam is investigated. The lateral vibration of the cracked beam in its first mode is modeled as a SDOF system with an equivalent mass, stiffness, and damping. In this model, it is assumed that the equivalent stiffness of the system is varying as a harmonic function due to the opening and closing of the crack during vibration. The main purpose of this investigation is to derive an analytical relation to describe the effects of the system characteristics and the crack parameters on super harmonic components of the response spectrum. To this end, the method of multiple scales is employed to solve the governing differential equation of motion. The results show that the respor.se is composed of two parts, one of which (the main part) is the response of a system with the mean equivalent stiffness of the systems corresponding to the closed crack and the open crack cases and the second part is the first order correction term, which reflects the effect of opening and closing the crack on the vibration response. In fact, the correction term consists of the superharmonic components of the spectrum. Results have been validated by numerical methods and experimental tests.

In this paper two non-linear control strategies were employed to design the controller for vehicle active suspension systems, considering the non-linear dynamics of the hydraulic actuator. At first, a controller is designed using Lyapunov's method in which the square of the vertical acceleration was chosen as the Lyapunov function.Simulation results indicate that this controller is successful in reducing the vertical acceleration to improve the ride comfort, but it cannot control the stability and steer ability of the vehicle. Meanwhile, the designed controller is not robust enough to system parameter perturbations. Therefore, the sliding mode control as a robust non-linear control method was adopted as an alternative. In this method, the sliding surfaces were selected in a way that the non-linear system tracks a "sky-hook" model, which has desirable behavior. In order to improve the characteristics of the reference model in terms of ride comfort and steer ability, considering the practical constraints of the suspension system, an optimal LQR controller was designed for the sky-hook model. Simulation results show that the controlled system tracks the behavior of this new reference model well. Also, the sliding mode controller is robust to parameter perturbations.

In this paper, forced vibration analysis of simply supported sandwich panel with composite layers and functionally graded material as core material is investigated using first shear deformation theory (FSDT) for face-sheet and three dimensional elasticity polynomials with unknown coefficients as Frostig method for its core. Hamilton's principle is applied to derive a set of fully coupled dynamic equations of motion. The core is assumed to have variable material properties only in the thickness direction. The forced vibration analysis has been performed using the New mark integration method for investigating the vibration characteristics and finding the response of rectangular panels under dynamic loads. The governing equations have been solved numerically using MATLAB software. Results have been verified with similar previous work. Finally, the central amplitude of sandwich panel on frequency parameter of excitation and the variation of the central deflection of sandwich panel as a function of time, subjected to different types of external force and the effects of geometry, material properties, number of layer, and different angle in composite layers are investigated. These numerical results have been then compared either with the available experimental ones or with some available theoretical results.