Modares Mechanical Engineering
Year:2019 | Volume:19 | Issue:4 |Papers Count:27

Reducing satellite solar panels temperature, to increase their electrical efficiency is of great importance. In this study, a novel methodology for optimal configuration design of heat pipes in a sun-pointing satellite utilizing the genetic algorithm is presented. The purpose of optimization is the thermal adjustment and design improvement of the solar panels for the satellite that is supposed to orbit in the Low Earth Orbit. Thermal simulations of satellite and solar panels are performed using SINDA/FLUINT and Thermal Desktop software. The computations are validated using experimental measurements of the satellite thermal model in a vacuum chamber and it was shown that the numerical analysis can produce reliable results. Then, applying the constraints of the problem, an optimization algorithm is introduced. This algorithm employs the thermal simulation software for solving the governing equations of the problem and then reports the results. The optimization was performed for the satellite hottest case (β =90° ) and then, conducting an optimization procedure, the optimal configuration of heat pipes is achieved. In the results section, three different configurations, namely no heat pipes, the initial design for configuration of heat pipes and the optimal configuration of heat pipes, are investigated and compared. It was found that the optimal configuration using the genetic algorithm can reduce the temperature of solar arrays by up to 19° C relative to that in which no heat pipe was used. It was observed that an efficiency ratio enhancement is 10. 4% for the solar panels of optimum configuration. The optimization could significantly reduce the temperature of the satellite internal equipment.

Today, sandwich structures are being used in many applications. Understanding the behavior of these structures and their properties is necessary for proper and optimum design. Because of thin face sheet and low stiffness of foam core, sandwich beams are very sensitive when exposed to local loading. Due to their structure, carbon nanotubes (CNT) have excellent mechanical properties, which improve the mechanical properties of the polymer when added to polymer matrix. In this article, the indentation behavior of sandwich structure is studied experimentally and theoretically. ABAQUS software is used for modeling the indentation behavior of sandwich beam. Elastic modulus of epoxy resin reinforced with CNT with different weight fractions is obtained with use of Mori-Tanaka theory and also by fabrication and testing of the composite specimens. Results show that adding CNT up to 0. 3 %wt improve the elastic modulus of composite, while weight fraction of CNT more than 0. 3% decrease the mechanical properties. Finally, the results obtained from the analytical solution and ABAQUS modeling were compared with the results obtained from experimental tests of indentation of sandwich structures. An acceptable agreement was observed between the results.

The sound emission of airplanes has some applications such as localization, classification, and detecting fault. Therefore, investigation of issues, which affects the airplanes sounds, is important. In recent years, pollution of dust in all cities of the Iran shows an increasing trend. In the literature, all variables affecting the sound emission such as temperature, pressure, and relative humidity have been investigated, but there are not any researches about the influence of dust on the atmospheric attenuation coefficient. The experimental tests have been carried out with 3 sensitive microphones, 950m away from the takeoff area of Imam Khomeini international airport for 6 different airplanes, including Airbus 320, 319, 321, Boeing 747, 777, and Embraer 190 at different atmospheric conditions. The air temperature was in the range of 20-40˚ C and the relative humidity was in the range of 2-34%. At first, the experimental setup was validated by available data, considering different temperatures and relative humidities. In this research, a new variable, β , has been introduced to detect the dust effect, which is defined as: the difference between the calculated sound pressure level at no dust and the measured sound pressure level while the dust density is 1μ gr/m3. Airbus 320 has the minimum dust atmospheric attenuation coefficient value (0. 01202db*m3/μ gr) and its maximum is related to the Embraer 190 (0. 0154db*m3/μ gr). Finally, the obtained results show that increasing in dust concentration (PM2. 5 and PM10) leads to increase in atmospheric attenuation coefficient between airplane and microphones area, and the measured sound pressure level decreases.

Many steel structures are damaged due to environmental factors such as accidental loads, exhaustion, rust, and phenomena such as cavitation and time passes. Dams’ bottom outlets are one of the important components of these structures that are subject to numerous hydraulic problems such as cavitation vibration, which causes damage and needs repair. One of the novelties for refining is the use of Carbon Fiber Reinforced Polymer (CFRP). In this paper, the effect of CFRP on gate strengthening under cavitation vibration load and the effect of damage on maximum vibration by using ABAQUS were studied. In order to observe the effects of failure on the maximum vibration of the outlet, two damages were applied to the front or back of the gate. Finally, the damaged gates were reinforced with two layers of CFRP. The outcomes showed that damage resulted in maximum vibration increase and polymer fibers has a significant effect on reducing vibrations and stresses caused by cavitation pressure.

Passive micro-mixers have simpler manufacturing in comparison with active micro-mixers and only require energy for flow pumping. In the present study, non-Newtonian fluids and non-Newtonian power-law fluid’ s mixing behavior in passive micro-mixers have been studied. Simulation has been performed, using computational fluid dynamics commecrical code of Ansys fluent and two different approaches of two-component mixing have investigated. The first approach studies fluid’ s mixing behavior by changing flow behavior index and flow consistency index in 5 different 3D geometries as multiple T-micromixer with aligned and nonaligned inputs in one and two plane, respectively, multiple T-micromixer, double T-micromixer, and T-micromixer, while the second approach studies mixing behavior by changing flow behavior index while flow consistency index is constant in two multiple 3D geometries with non-aligned inputs. In all studies, water was used as Newtonian fluid and carboxymethyl cellulose solution was used as non-Newtonian fluid. The studied range of Reynolds number was 1 to 100. In both approaches, the results for mixing index and pressure drop for powerlaw index according to criterion are reverse of each other; it means that in the first approach, with increasing power-law index, the mixing index increased and the pressure drop decreased and in second approach, this procedure is reversed. But, procedure of non-dimensional fully developed velocities in two approaches investigated is similar in comparison to geometries with non-aligned inputs.

The present study was performed to experimentally investigate the regime of the liquid sheet breakup and the effects of dimensionless numbers on the penetration and trajectory of the liquid sheet in cross flow condition. The shadowgraphy technique was applied to study the tests. In this work, the effect of the non-dimensional numbers (momentum ratio and Weber number) were surveyed on the breakup of the liquid sheet. Also, some equations for the injection trajectory, the length, and the height of the jet were presented based on these nondimensional numbers. The tests were done at atmospheric pressure and temperature, where the Weber number range is from 0. 8 to 12. 5, the variations of the momentum ratio are from 17. 4 to 250, and the changes in the Reynolds number are from 2400 to 10227. Three regimes of jet breakup were observed, defined as column breakup, column-bag breakup, and bag breakup. The Weber number is the most effective parameter in the regime change of the liquid sheet breakup. The results also indicated that the increase in the momentum ratio has a great influence on the depth of penetration of the liquid sheet, but it has a very small effect on the breakup regimes.

In this paper, heat transfer and fluid flow characteristics in a porous coil have been investigated. The characteristic of the boundary layer, distribution of velocity, pressure, and thermal field effects into a porous coil as high heat transfer resource have been analyzed. The developed Brinkman method in fluid flow and power law model of conduction heat transfer coefficient considering porosity and permeability factor is calculated for constant solar heat flux. In order to solve the problem, the COMSOL software based on finite element method with porous medium algorithm is used, using the MUMPS solver. The comparison between variation of normalized temperature at the presented model and experimental data at similar conditions shows an acceptable agreement with an error up to 3%. At constant permeability, decreasing the porosity coefficient, velocity profile is extended due to presence of pores into coil with an accelerated flow, so that the maximum velocity is equal to 2. 5m/s at porosity coefficient of 0. 2. In porous coil, Nusselt number increased, where the greatest difference between porous and the nonporous coil occurs at the beginning of the coil, with a value of 32%, and the smallest difference is 27%. In the porous coil, absorbing solar energy is higher and the heat transfer is improved. However, the amount of pressure drop also increases.

In this paper, a new hybrid intelligent method is presented for detecting the bearing faults in the various rotating speeds. The vibration signals are collected in four conditions, including the normal state, the faulty inner race, the faulty outer race, and the faulty bearing element. Firstly, twenty-two statistical features in the time domain and four frequency features, three Wavelet packet decomposition (WPD), and the first five intrinsic mode functions obtained by the empirical mode decomposition (EMD) are extracted from the original signal; finally, the feature vector for each signal sample has 424 features. However, in the high dimensional feature matrix, there may exist the insensitive features to the presence of defects. Therefore, in this study, the compensation distance evaluation technique (CDET) is used to select the optimal features. Then, the selected features are used as the inputs of the support vector machine (SVM) classifier to diagnose the bearing conditions. In the CDET method, there is a threshold indicator that plays a decisive role in choosing the desired attributes. Also, the SVM method has some parameters that need to be set during the fault detection process. Therefore, the particle swarm optimization (PSO) algorithm is used to determine the optimal threshold in the CDET method and the optimal SVM parameters, so that the prediction error of the bearing conditions and the number of the selected features are minimized. The obtained results demonstrate that the selected features are well able to differentiate between different bearing conditions at various speeds. Comparing the results of this paper with other fault detection methods indicates the ability of the proposed method.

In this paper, the effect of a mannequin location with an integrated respiratory system in a ventilated room on the flow field and particle dispersion was evaluated, using numerical simulations. Dispersion and deposition of particles inside the respiratory system and inside the room have been investigated, using a Lagrangian approach. The respiratory system contains the nasal airway, nasopharynx, oropharynx, and larynx, trachea, which has been generated from CT scan images and installed on a 3D mannequin model. The evaluation conditions varied as standing and sleeping mannequins form in a room that includes 2 input dampers and 4 output dampers. For simulation of the flow field, the ANSYS FLUENT software-version 17. 2 with the 4-equation SST transition turbulence model have been used. Simulations have been performed for 3 different respiration flow rates and 4 different particle diameters. Results indicate higher deposition on the sleeping model rather than standing model due to gravitational effect. The total aspiration of particles inside the respiratory system was less than 0. 4%. In addition, the nasal cavity captures large particles, while for small particles, higher deposition occurs in the lower parts of the respiratory tract.

In this research, the objective is using 4 quadrotors in a group to carry out a certain weighted load. The load is connected by cables to each quadrotor. The equations of motion of the quadrotors are considered completely and without simplification. Unlike other researches, to express the relationship between the load and the quadrotors, the ropes are considered as springs, so they are pulled out and retracted during the mission. Formation control design and path tracking by the group is done by using feedback linearization control. Control protocol design is presented in two structure, centralized, and decentralized. Unlike other papers, in decentralized structure, there is no information communication between the agents to reduce the communication costs. The mission of the group is defined as the quadrotors first pick off the load from the ground and, then, track the desired path to reach the target point. When the load reaches the target point, the quadrotors should put the load on the ground and, then, land themselves. Cutting the cable of one of the quadrotors is applied to the system as a fault and in addition to providing a method to detect its occurrence, the performance of the centralized controller is checked in this situation.

In this paper, the numerical simulation of the diesel and gasoline fuels injection in a constant volume chamber is conducted under the operating conditions of a compression ignition engine with openFoam software. In order to check out the possibility of using gasoline instead of diesel to increase the volumetric efficiency of the compression ignition engine and reduction air pollution, the spray characteristics of the gasoline and diesel under injection pressures of 40 and 80MPa, as well as temperatures of 243, 273 and 313K, is investigated. The simulation results are compared with the experimental data derived from fast imaging techniques. The results show that under the same conditions, the vapor penetration length for the two fuels is approximately equal. Also, due to the lower volatility of the diesel fuel, its liquid penetration length in 40 and 80MPa injection pressure was found to be 7 and 9 mm higher than gasoline, respectively, and high volatility of gasoline leads to enough time to make air and fuel mixtures in compression ignition engine. In addition, the reduction in fuel temperature from 313K to 243K resulted an increase in the penetration of gasoline and diesel liquids by 12 and 10 mm, respectively, and decrease in the evaporation rate, which causes a non-homogeneous mixture and an increase in unburned hydrocarbons and emissions.

Microfluidic chips in the last two decades have had significant advances in the analysis of interfacial tension phenomenon due to their many advantages. To analyze interfacial tension phenomena, droplet flow in microchannels can be used. In this study, water-n-hexane interfacial tension in the presence of surface-active agents was measured, using microfluidic tensiometry. For this purpose, a glass microfluidic flow-focusing junction was fabricated for generating n-hexane droplets within an aqueous phase. The dependence of droplet size on the concentration of surfactants has been investigated. A theoretical equation was developed, considering force balance on the droplet generation in the microfluidic device, giving a relation between the interfacial tension and the generated droplet sizes. By standardizing the microfluidic chips with the aid of a system, whose interfacial tension is known (hexane normal and tween 20 in distilled water), interfacial tension can be measured with measuring the size of produced droplets for other systems that can form droplets in the microchannel. In this study, the microfluidic device and the relation were employed to measure the interfacial tension in the presence of either of sodium dodecyl sulphate (SDS) or Cetyl trimethylammonium bromide (CTAB) surfactants. It was found that the measured interfacial tensions deviate less than 10% compared to those measured with a commercially available ring method.

In sheet metal forming processes, one of the most important limitations relates to the elastic recovery after punch unloading, which usually leads to spring-back phenomenon. Production of precise parts without spring-back controlling is not possible. Hence, the main aim of the present research is to minimize the amount of spring-back as well as to prevent the crack initiation during bending process of Aluminum A1050-H14 sheet. For this purpose, firstly, the sheet metal bending process was numerically simulated in ABAQUS finite element package. Then, the effect of friction coefficient and punch velocity was investigated on elastic recovery and von Mises stress in order to minimize the spring-back as well as to prevent the crack initiation. In this regard, python programming language was utilized. Then, by linking multi-objective genetic algorithm and finite element method in modeFRONTIER software, the optimum values of the process parameters were determined. It should be mentioned that for validation purposes, the simulation results of the present study were compared with the experimental data available in literature, showing a 3. 14% relative error between the numerical and experimental results.

In this paper, we investigate the aeroelastic behavior of double-sweep metal and symmetric composite layup wings. Various strategies have been proposed to suppuration of instability such as using the composite materials in structure. The wing is considered as a cantilever beam with 3 degrees of freedom of bending/ bending/ torsional. For aerodynamic modeling, the quasi-steady and unsteady flow theory in the time domain is used based on the Wagner function and the process of calculation of the flutter velocity is provided in form of software code. Finally, the effect of the ratio of length of the isotropic on the total length of the wing as well as the swept angle of the composite section on the flutter speed have been investigated. The results show that with increasing the length of ratio of the metal part to the total length up to 0. 9, the speed of instability increases and after that position, it decreases. Also, negative sweep angle of the swept part of wing increases the speed of instability than the positive sweep angle of the composite part. The results show the best angle of fiber is 30 degrees in range of negative sweep angle, having the highest speed of instability. Also, studies show that with increasing the negative sweep angle from-90 to 0 degrees at different angles of fiber, the speed of instability decreases till to-20 degrees of sweep angle and, then, increases slightly. With increasing the sweep angle from 0 to 80 degrees at different angles of fiber, the speed of instability decreases.

Conspicuously, pitch angle control strategy has been applied to mitigate the influence of mechanical load and also output power control at above-rated wind speeds. In this paper, a wind turbine is modeled based on simplified two-mass model and an adaptive sliding mode controller (ASMC) is designed based on individual pitch control (IPC) strategy. To do this, the single-blade approach is used and the wind turbine was divided into aerodynamics and mechanical subsystems and governing equations of each subsystem were derived. By designing and applying the ASMC to two-mass model, system behavior is observed and simulated in terms of step and turbulent wind speed inputs. In addition, to verify the validity of the ASMC, the proposed controller is implemented in the FAST environment and the wind speed profiles are generated using TurbSim. In order to analyze the environmental effects on the dynamic behavior of the system, the controller performance is explored in presence of parametric uncertainties. It should be noted that rotor speed tracking error is evaluated and demonstrated through different criteria.

The aim of this paper is designing a decision-making system (DMS) for temperature management of the satellite plates in the presence of actuators faults. The thermal stresses caused by solar radiation pressure perturbations is considered as a threat to the mission of satellites. In this paper, a new mechanism is used, which includes 4 fluidic momentum controller (FMC) actuators for sustaining the situation and performing various satellite missions in a pyramid. In this case, it is assumed that the satellite’ s plates are exposed to solar perturbations, and as a result, various faults have occurred for satellite actuators. To detect and isolate the defect of each actuator, recordable data from satellite and actuators are stored and feature extraction of these data is executed by linear differentiation analysis methods and analysis of the main components. To evaluate these methods, the confidence matrix is used, and the K-nearest neighborhood method is selected as the optimal method. To solve the temperature problem of the plates, the DMS is designed, so that if one of the plates reaches critical temperature, after examining the occurrence of a fault and adopting the appropriate strategy, the plate’ s rotation of the target plate is in the shadow. As a result, the temperature of the plate with the maximum temperature will reduce. The simulation results show that despite the perturbations and actuators’ faults, the designed DMS can manage the temperature of the plates somehow that does not enter the critical point.

In this paper, heat transfer and magnetic fields in a vacuum induction melting furnace have been studied numerically. To solve the coupled equations of thermal and magnetic induction heating, the finite element method has been used. An induction furnace model is simulated using an industrial geometry. The studies indicate that the effect of the geometry of the crucible and the coil on the melting time has not been thoroughly investigated and requires more indepth studies. It is attempted to improve the shape of the induction furnace, so that in less time aluminum is melted in a small scale furnace. The effect of the diameter-to-height ratio of the crucible on the duration of melting has been investigated. By decreasing the diameter-toheight ratio, the temperature reaches melting temperature in a shorter time. The results show that for the diameter-to-height ratio of less than 0. 4, there will not be a significant change at the average temperature. 10% reduction in the distance between the coils leads to an increase in the average temperature of the working material inside the furnace. With considering the constant density of the coil current and the constant induced current in the heated material, the effects of the number of coil turns on the temperature distribution and magnetic flux are investigated. In this way, the accuracy of the model is also checked by induction heating concepts. The effect of frequency on temperature has been investigated in different coil lengths. The results show that an increase of 4 times in the frequency caused an increase of 1. 7 times in the average temperature.

In this paper, a theoretical model is proposed for Leidenfrost droplet evaporation by solving the mass, momentum, and energy conservation equations. This model involves a set of four equations, of which the values of vapor layer thickness, evaporation rate on the lower surface of the drop, the volume of evaporating droplet, and temperature distribution in vapor layer are obtained. This set of equation is solved with Fortran code by the predictor-corrector method. The main unknown value in these equations is the vapor layer thickness, which is predicted in every step of simulation and corrected by the balance of forces that act on the drop. In this study, the upper surface of the drop, where contacts with air and the lower surface of droplet, where contacts with the vapor layer are predicted with high accuracy by solving the Young-Laplace equation. The vapor layer thickness obtained from the proposed model is compared with experimental data and encouraging agreement is observed.

In this study, 1. 5vol. % of SiC nanoparticles was added to AZ31 magnesium alloy via a stir-casting method. Next, the as-cast ingots were extruded at 400° C with the ratio of 3. 78. After extruding, the materials were subjected to multidirectional forging (MDF) at 320° C for 2, 4, 6, and 8 passes. In order to evaluate the mechanical properties of extruded and MDFed materials, shear punch (SPT) and Vickers microhardness tests were applied. The results of these tests showed that hard ceramic nanoparticles improved the shear strength and hardness of the matrix alloy. The shear yield strength, ultimate shear strength, and hardness of extruded alloy were 86. 70 MPa, 119. 43 MPa, and 52. 55 HV, respectively, while in extruded AZ31/SiCp nanocomposite, these values increased by 9. 91%, 5. 48%, and 13. 99%, respectively. It was also observed that nanocomposites processed with multi-directional forging offer better mechanical properties than non-MDFed materials. The results indicated that after the first two passes, there was a significant improvement in the mechanical properties of the nanocomposites, such that the shear yield strength, ultimate shear strength, and hardness were improved in contrast with the extruded state by 27. 12%, 17. 95%, and 16. 03%, respectively. Mechanical properties during the next passes were periodically reduced and increased. Microstructural observations also showed that the average grain size variations were periodic during the increase of MDF passes. After the second pass, the grains were finer than the extruded state, and their size increased in the next two passes. From 4th to 6th pass, the grain size decreased and the smallest grains were obtained in this case, while in the last two passes, the grains grew slightly. Despite the smaller and homogeneous structure created by the 6th and 8th passes, the best mechanical properties were obtained in the second pass, which means, in addition to the microstructural changes, also modifications in the material texture during the MDF process had an impact on mechanical properties.

Magneto-rheological damper is one of the most widely used mechanical equipment, which absorbs mechanical shocks by use of magnetic fluid and electrical coil in its structure. In this paper, for the first time, dissipative particle dynamics as a mesoscopic scale modeling method was used to simulate a magneto-rheological damper and its magnetic fluid. Data from 3 categories including magnetic fluids with brand names 122-EG, 132-DJ, and 140-CG have been used and effect of their physical properties on power of damping force have been investigated. Results of modeling show that by increasing shear rate of fluid, shear stress is first increased and, then, it is applied to a constant value, which results in a greater shear stress by applying a stronger magnetic field. It is also observed that, with increasing both maximum piston velocity and strength of magnetic field, maximum power of damping force increased, which in 140-CG is higher than the other fluids. Results of sensitivity analysis show that weight of magnetic particles and strength of dissipative forces have the greatest effect on damping force, in such a way that by increasing weight of magnetic particles and decreasing the dissipative force of particles, accumulation of magnetic particles decrease, so, increasing quality of damping. It was also found that 122-EG is more suitable than other types of magnetic fluids in forming standard magnetic particle chains, and provides a more favorable viscosity distribution for damping.

The unique characteristics of nanostructures are mainly due to their large surface to volume ratio. One of the most important quantities in investigating the surface properties of materials is the surface energy. Therefore, calculating the surface energy is necessary for the proper understanding of the behavior and properties of nanostructured materials. The present study investigates the size-dependent surface energy of crystalline nanoparticles and nanocavities of aluminum, silver, copper, and iron. For this purpose, spherical nanoparticles and nanocavities with different radiuses are modeled by molecular dynamics simulations and their surface energy is obtained. The simulation results demonstrate that for nanoparticles and nanocavities with sufficiently small radiuses in the range of a few nanometers, the surface energy depends on the size of the nanostructure. For spherical nanoparticles, the surface energy increases with increasing nanoparticle radius, while for the spherical nanocavities, the surface energy decreases by increasing nanocavity radius. Also, the surface energy variation with size is more intense for nanocavities in comparison with nanoparticles. By increasing the radius, the surface energy of nanoparticles and nanocavities approaches to an asymptotical value, which is the surface energy of a crystalline flat surface or the Gibbs surface energy for the crystallographic surface orientation with the maximum surface energy.

New passive double L-shaped micromixers have been investigated based on the split and recombination flow. Numerical study on micromixers was performed in the Reynolds number range of 50 to 200. The three-dimensional Navier-Stokes equations have been used to analyze flow and mixing behavior. Two different configurations from the positioning of L units have been investigated and two solutions have been proposed to improve the mixing index. If two L units are same shaped, aligned on one plate (design 1), the mixing index is low due to inappropriate split and recombination. The placement of two L units of the same shape on a two-plane parallel and non-aligned (design 2) improve the mixing index and increase to over 95% in Reynolds numbers of 100, 150, and 200. The orthogonal solution to the inputs did not affect the pressure drop and only in design 1, the mixing index could exceed 95% in all Reynolds numbers. Unbalanced micromixer solution improves mixing index by increasing pressure drop. The effect of geometric parameter of asymetric width ratio in both designs was studied and design 1 in asymetric width ratio 2. 5 and design 2 in asymetric width ratio 2 and 2. 5 have been completely mixed in all Reynolds numbers. Also, the performance of proposed micromixers was better than L-shaped micromixer due to the split and recombination mechanism. In addition, the mixing index was higher in porposed micromixers compared to the split and recombined micromixers of previous researchers due to the use of L-shaped units.

Nowadays, the need of welding industry’ s to improve weld quality has led to the consideration of robotic welding. The use of articulated industrial robots for welding has many challenges. Because some robots do not have the capability of online error compensating of the seam track. Therefore, in order to remove the welding seam tracking error, the use of an auxiliary mechanism is proposed in this article. This mechanism is a table with 1-degree of freedom (dof), which produces a continuous motion in workpiece under the welding torch. The rotational motion of the motor is transformed into a translational motion of the workpiece by a ball-screw system, where this linear motion compensates the tracking error. Since in the welding process, relative motion accuracy of the workpiece and the welding torch is crucial, proper control of the interface table ensures the weld quality. In this paper, two different methods for controlling the table with 1-dof are studied. In the first method, due to the complexity of friction model of the ball-screw mechanism and the presence of nonlinear terms, this part of the model is considered as an external disturbance, and, then, a PID controller for the linear part is designed. In the second method, known as feedback linearization, a control law is designed for that the tracking error tends to zero by passing time. Throught a comparison between the simulation results, the second control method demonstrates better precision relating the first controller. While the error of PID controller equals to 3 mm and the second controller’ s error does not go beyond 0. 5 mm. At last, the experimental cell used for the robotic welding is introduced to evaluate the mentioned results.

The atomic force microscope (AFM) determines the topography of surfaces in nano scale based on the changes in the exited micro-cantilever’ s dynamic characteristics. Therefore, it is essential to simulate and predict more accurately the dynamic behavior of cantilever beams for use in design and fabrication of AFM. Based on the experimental observations, in contrast to the classic theory, the normalized stiffness of structures is not constant with the reduction of dimensions in micro and nano scales. This change, which can be either softness or stiffness, results in size-dependent behavior, non-classic continuum theories. This paper studies the effect of size on the dynamic behavior of AFM based on modified couple stress theory, and compares the results with those obtained from classic theory. The nonlinear partial differential governing equation of the system is derived, considering intermolecular and hydrodynamic forces, based on the modified couple stress theory. By applying Galerkin projection method, partial differential equations are transformed into ordinary equations and the discrete system is extracted. It is shown that considering size effect leads to enlargement of expected working domain of AFM, and also predicted amplitude and frequency of oscillations decreases and increases, respectively. Moreover, two theories predict different start point of bi-stability region. Solution approach is verified by comparing the results with two degrees-of-freedom model and analogue equations method. Furthermore, effect of hydrodynamic forces of fluid on dynamic behaviour of AFM is investigated.

In this research, the ballistic strength of sandwich structures with aluminum face-sheet and polyurethane foam cores of various densities have been investigated. The effect of graded changes in the density of foam core and arrangement of foamed layers with different densities on the absorption of energy and the ballistic limit of sandwich structures at high velocity (160-300 m/s) under the impact of semi-spherical nosed cylindrical projectiles were investigated. Generally, five different types of panels were designed in dimensions of 100×100 mm2, 6 in each. In total, the prepared samples were 30. Numerical simulations were performed, using Lsdyna software. The results of this study showed that, firstly, there is good agreement between the experimental and simulation results and, secondly, the experimental and simulation results showed that the ballistic limit and energy absorption of sandwich structures of the same mass with the graded foam core in the case a less density foam layer is on the side of the impact for the three-layer panels is, respectively, 5. 5% and 11. 5% higher than the panel with single-layer foam core and average density.