فصلنامه مکانیک هوافضا
سال هفدهم شماره 2 (پیاپی 64، تابستان 1400)

The conventional methods for designing autopilots deal conservatively with uncertainties such that the autopilot should be slowed down to the extent in which the system can maintain its desired performance in the presence of a certain uncertainty. However, there will be no guarantee of good performance in the presence of all uncertainties while the capabilities of the system will not be used properly. In the proposed method, using the extended state observer, the lumped uncertainties along with the system’s states will be estimated, and using feedback linearization approach, along with the nonlinear terms will be removed from the control loop. What remains is a simple linear system that is easier to control. In this method, uncertainties is treated without conservatism and consequently performance is improved in different conditions. Finally, for comparing the results of proposed method with linear autopilot a 1000 times Mont Carlo running with specific condition has been used and is shown that the proposed method is better than the conventional method in performance.

Parallel robots are of great interests to many advanced industries due to their lots of advantages. But in the spherical parallel robotic arms, addition to shortcomings of is their limited workspace, using of high power actuators made complexity their dynamics and control. Some applications of these types of robots require large work space and high load capacity on a nonstationary base and during moving. In this paper, a new spherical manipulator with 3PUS/S structure and three rotational movements are suggested and its kinematics and dynamics are studied for special applications. Unlimited rotation about Z axis and most of the load bearing by central joint, have made structure of this robot different from others kinematically and dynamically. In this study,  after extracting the kinematics relations of the mentioned arm giving analytical answers for position, velocity and acceleration, the dynamics of the robot with the base mobile platform is investigated using the principle of virtual work. For checking the accuracy of kinematic and dynamic relationships, results verified by Adams software simulation which Acceptable matching of the results confirms the accuracy of the equations. Unlimited rotation about Z axis, the fixed grooves of actuators motion and the reduction of the dynamic effect due to their mass, low effect of the dynamics of robot links, bearing the static part of the load by the central spherical joint and very noticeable reduction of the required motor power are the main advantages of the introduced mechanism and has made it very attractive for special applications.

In this study, the vibration and stability analysis of functionally graded (FG) doubly curved panels reinforced with nano-graphene platelets (NGPLs) embedded by piezoelectric layers under the influence of aeroelastic force are investigated. Multilayer sandwich doubly curved panel makes of porous materials reinforced with nano-graphene platelets are considered. The NGPLs are assumed to be uniform and non-uniform distribution, as well as three types of targeted porosity distribution that change continuously along the thickness in each layer. This structure is always under influence of external fluid flow and modeled by the piston theory. Based on the higher-order shear deformation theory, the governing equations of motion of doubly curved panels reinforced with NGPL are obtained using extended Hamilton’s principle. Galerkin method is employed to transform the partial differential equations of motion into the ordinary equations. The effect of advanced materials including various patterns of porosity distribution, porosity coefficients, distribution or dispersion of nano graphene platelets, NGPL weight fraction, as well as the effect of structural geometric dimensions on vibrations and dynamic instabilities of structures are studied.

Time-varying motion consensus and formation control of leader-follower unmanned aerial vehicle multi-agent system (UAV-MAS) is studied, in this paper. It is assumed that the number of agents changes during the motion. According to the variations in number of agents, the desired formation would also change in the form of various regular polygons in such a way that, whose sides, which are the relative distances of the neighboring followers from each other, are kept constant. A two-loop controller is provided, in which, the internal loop uses the feedback linearization control law to linearize the dynamics of the agents. In the outer loop, an integral state feedback controller is employed as a new consensus formation controller based on the relative position and velocity vectors of neighboring agents. The followers do not know the path of the leader and only maintain the formation and track the leader by maintaining their relative distance with their neighbors. Controller coefficients are determined offline by solving the algebraic Riccati equation and based on the graph theory. Therefore, no online tuning is needed.  Closed loop stability of system under the proposed control law would be investigated via the Lyapunov stability theorem and various simulations are performed to illustrate the capability of the proposed controller.

In this paper, the experimental, numerical and semi-analytical study of the deflection of a blunt projectile after hitting the perforated plate has been done. In this regard, for experimental study, AISI 52100 steel projectiles and 1045 AISI perforated plates with 3 hole diameters of 5, 7 and 9 mm have been used, and for numerical modeling, Abacus finite element software has been used. Then, after comparing the numerical and experimental results and confirming the accuracy of the presented numerical model, according to the laboratory limitations, the projectile impact is numerically evaluated with different overlap, and 3 different projectile velocities. And 40 type of impacting modes are designed and modeled. According to the obtained data, the use of optimization algorithm based on the teaching-learning, which is an evolutionary optimization algorithm, and coding in MATLAB software, semi-analytical relations to obtain the rate of projectile deviation after Impact on the perforated plate for each value of hole diameter, projectile diameter, projectile velocity and the amount of overlap is obtained. Finally, a comparison is made between the experimental, numerical and equation results. And this indicates a good match between the results.

In this paper, experimental and regression analysis of the effective parameters in the free and die forming of circular metallic plates using gas mixture detonation were investigated. In the experimental section, steel, aluminum, and brass plates in different thicknesses were used. To better evaluate the experimental results, the condition of the same areal density was used to compare the results of steel, aluminum, and brass plates under the same loading conditions. In the regression analysis section, the Design-Expert software package and response surface methodology were used. The effect of parameters measured in experimental tests including the pressure of Oxygen, the pressure of Acetylene, plate thickness and die apex angle on the deflection of circular plates were investigated simultaneously. To find a significant model, the confidence level of 95% was considered in the analysis. Experimental results showed that using a die with an apex angle of 60° leads to the decrease of the maximum permanent deflection. On the other hand, using aluminum and brass plates in the die forming resulted in a more reduction of permanent deflection in comparison with free forming. The results of the regression analysis show that the predicted values of the model are in good agreement with the experimental data and the presented model is suitable. Optimal conditions for the minimum deflection of the circular plates under explosive loading were also presented. The pressure of Oxygen and the die apex angle has the highest effect and the least effect on the forming of circular metallic plates.

In this study, the shape of the kinetic penetrator nose is optimized with the aim of achieving maximum penetration. For optimization, Lagrange analytical optimization method and genetic evolutionary algorithm, several types of different nose-generating functions, two different objective functions, and shape coefficient and penetration depth have been used. Comparing the shape and depth of penetration of the optimized noses, it is observed that there is a good agreement between the results of the optimization in different cases. In analytical optimization, the objective function is to optimize the shape of the nose and the Lagrange optimization method is used. In numerical optimization, two different objective functions of penetration depth and nose shape coefficient as well as three types of nose generating functions have been used for optimization. The proximity of the optimization results in all the mentioned methods shows the high accuracy of the optimizations performed. In this paper, it is shown that the nose shape coefficient is a suitable objective function to optimize the nose of kinetic penetrators in order to achieve the maximum penetration depth. One of the characteristics that should be considered in optimizing the shape of the nose is the ratio of the stem radius to the length of the nose (τ). In this study, the ratio (τ) in optimization by different methods is equal to 0.3. After optimizing and obtaining the shape of the projectile nose, the penetration depth of the projectile at different speeds was calculated and compared with the penetration depth of the oyster noses with a ratio (τ) equal to 0.3. It can be seen that the penetration depth of the optimized noses is significantly greater than the penetration depth of the ips in different collision velocities.

Currently, electronic components are very thin as well as fragile and their assembly on PCB boards is highly sensitive. The components may be damaged and broken in response to an excessive force. Therefore, it is necessary to use a mechanism with force control ability. A mechanism with a large member (macro) for long movements and a small member (micro) for short and delicate movements, is selected to provide accurate force control as well as high speed in installation. The macro member is rapidly approaches the surface of the PCB with the position control and then the micro member initiate its movement. As the micro member contacts the surface of the PCB, the position control is switched to force control and enables the installation of the device with controlled force. At the first attempt, PID controller is used. Thereafter, the fuzzy controller is selected to control the system. The fuzzy controller has a better performance and reduces the disturbances during installation. The relation between maximum contact force and contact time with velocity is also investigated and optimal velocity is determined.