فصلنامه مکانیک هوافضا
سال شانزدهم شماره 4 (پیاپی 62، زمستان 1399)

Magnetic levitation systems (Maglev) are widely used in various industries. The open loop Maglev system is highly nonlinear and unstable. Therefore, designing a simple, but effective controller for such a system is a challenging issue. In this paper, a fractional order predictive functional controller (FPFC) is proposed for the control of the magnetic levitation system based on its linearized unstable model. At first, the unstable plant is decomposed into two stable models. Then, using these two stable models and employing fractional order cost function, the PFC controller is designed. Because of more degrees of freedom and its flexibility of fractional order calculus, the proposed fractional PFC would improve the performance of closed loop systems, noticeably. Robust stability of closed-loop systems has also been studied considering the uncertainties and model mismatches via the small gain theorem. Simulation results show good performance of the proposed controller in nominal and perturbed conditions. Based on provided simulations, via the proposed controller, overshoot has been omitted and performance indices have been improved more than 50% with respect to the first and second orderof freedom integer/fractional order PID controllers, designed for this system, in the literature.

In this study, the solid particle erosion of Al 7075-T6 and Ti-6Al-4V alloys, as two typical alloys in an aircraft structure, under multiple particle impact is investigated using finite element modeling. The erosive behavior of these alloys has been simulated as a micro scale impact model based on Johnson-Cook constitutive equations using the representative volume element technique. Erosive behavior is usually described by the ratio of the eroded material of the alloy surface to the mass of the eroding particles which is called the erosion rate. In this study, the results of the finite element model are validated by comparison with the results of typical erosion models. Then, the two most effective factors on erosive behavior, impacting particles’ velocity and particles’ impact angle, are to be investigated. Results show that there is an exponential relation between the particles’ velocity and the erosion rate. According to the results, maximum erosion rates of Ti-6Al-4V and Al 7075-T6 have been recorded at the impact angles of 40 and 30 degrees, respectively. It is shown that Ti-6Al-4V is more erosion-resistant than Al 7075-T6.

A fuel cell is an electro-chemical tool capable of converting chemical energy into electrical energy. The high operating temperature of the solid oxide fuel cell (SOFC), (between 700oC to 1000oC), causes thermal stress which is the origin of crack initiation and propagation. Thermal stress causes gas escape, structure variability and SOFC operation cessation before its lifetime. The purpose of the current paper is to present a method that predicts the thermal stress distribution and forecasts the beginning of fissure or crack occurrences in an anisotropic porous electrode of the planar SOFC. The governing coupled non-linear differential equations of heat transfer, fluid flow, mass transfer, mass continuity, and momentum are solved numerically. A code based on computational fluid dynamics (CFD), computational structural mechanics and finite element method (FEM) is developed and utilized. The results show that the highest thermal stress occurs at the lower corners of anode and the upper corners of cathode. The cathode’s thickness at the left side increases by 1.5% and the concentrated temperature and thus the fissure occurs between the top and bottom left corners of the cathode.

In this research, laboratory investigations and numerical studies are performed on flat and curved steel sheets under the influence of impact caused by the free fall of weights, with and without reinforcers of different transections. In this study, a flat sheet (infinite curvature) and a curved sheet with the curvature radius of 110 mm and a specific level of impact energy (a free fall height) are used for st.12 steel sheets of 1 mm thickness with the dimensions of 220 * 200 mm. The material and thickness of the energy absorbers are similar as the original sheet, and rectangular, cylindrical, half cylindrical, sinusoidal and triangular transection shapes, are considered. In the experimental method, the pickup acceleration is measured by the accelerometer sensor and the post-blow-out plate deformation is measured. The evaluated parameters include the amount of impact acceleration, the rate of permanent deformation, and the amount of energy absorption by the sheet. The Abaqus finite element software is used for numerical modeling. The results show that the curvature reduces the acceleration of the impact and increases the steady deformation and the energy absorption. The reinforcers in general, reduce the amount of crush length, but tape reinforcers reduce also the energy absorption of these sheets compared to plain sheets. Regarding the variety of the reinforcers, the cylindrical element performs better, because it has less impact acceleration and more energy absorption than orthogonal reinforcers, however, there is an increase in the amount of deformation of these sheets.

In this paper, an integrated controller including the anti-lock braking system along with an active suspension is designed for a quarter car model, on the basis of the improved sliding mode control algorithm. Dynamical model of the active suspension along with the anti-lock braking system are derived using the Newton-Euler equations. Also, in order to model the friction between the tire and the road surface, the Pacejka model is used. The application of the control algorithm for the ABS yields a higher safety rate for the vehicles. On the other hand, the active suspension system beside the anti-lock braking system can improve the car safety and passenger comfort. To increase the friction between tire and the road surface, the active suspension system increases the normal force over the tire. Consequently, the performance of the braking system is improved with decreasing the stopping distance. In order to design the hybrid control system, the sliding mode control (SMC) system and proportional-integral-derivative (PID) controller are merged. In the PID-SMC strategy, the sliding surface is defined based on the PID algorithm. This novel control algorithm can decrease the chattering phenomenon besides shortening the stopping distance that yields the reduction of energy consumption.

In the present research work, the reduction of peel stress in the adhesively bonded single-lap joints has been studied. The distributions of normal and shear stresses generated in the adhesive joint were obtained using the two-dimensional elasticity theory, as well as the stress-strain and strain-displacement relationships. Minimization of the peel stress was performed using the bees algorithm, during which the process variables included the adhesive and adherends thicknesses.  The composite joint was loaded by a tensile force while the adherends and adhesive layers were considered to behave as isotropic materials with linear elastic properties. The results showed that based on an optimum thickness ratio of 2.54, the magnitude of the peel stress can be reduced by 36%. As Young's moduli ratio, and consequently, the asymmetric adhesion bonding increased, the maximum amount of peeling stress decreased. Also, the increase in Young's modulus of the bottom layer led to the disruption of stress distribution at the interface of the softer adherend (top layer) and the adhesive layer, while this effect was almost absent at the other interface.

In this paper, a non-dimensional analysis approach has been used to propose two empirical equations based on dimensionless numbers to predict the maximum transverse permanent deflection-thickness ratio of single-layered quadrangular plates under uniformly and locally distributed dynamic loading. In the presented dimensionless numbers, the effect of plate geometry, the impulse of the applied load, the mechanical properties of the plate, the strain-rate sensitivity, and the loading radius have been considered. To validate the empirical models, eight series of conducted experiments and 267 data points in the state of the art over the past forty years have been used. The obtained results show good agreement between the model prediction results and the experimental values so that in the total of 117 experimental data for uniform loading, 94% (110 data) and 98% (115 data) of the data points were distributed in the ±10% and ±20% error range, respectively. Besides, in the total of 150 experimental data for localized loading, 83% (124 data) and 97% (146 data) of data points were distributed in these two ranges, respectively. The results also showed that assuming the material constants as variables based on the plate thickness, in the Cooper-Symonds constitutive equation improves the prediction accuracy of the empirical models so that only 6% and 12% of the data for uniform and localized loading respectively, are not within the 10% error range.

In this article, controlling the end-effector forces is accomplished using a command system with six degrees of freedom. The command system can use a joystick implemented on a parallel mechanism. The follower robot’s end-effector is controlled by the command system and is capable of traversing the reference path with the specified force. To demonstrate this, several paths on different planes are considered and the robot’s end-effector is seen to traverse each assigned path by applying a constant force. It is also possible to identify obstacles and avoid them with this system. The control system is designed so that while approaching an obstacle, the robot’s end-effector does not collide with it. Hence, a general-purpose robot can be navigated with better performance and it can be used as a surgical robot, a robot for carrying sensitive materials or other applications. In such operations, to adjust the contact forces after a collision of the end-effector with the surface, this control system produces the appropriate control feedback. This feedback gives the operator a sense of the forces at the end-effector position and increases the operator manipulation and maneuvering skill.