مجله مهندسی مکانیک امیرکبیر
سال پنجاه و سوم شماره 4 (تیر 1400)

This paper investigates the point-to-point motion of the end effector of a 2-DOF parallel robot with minimum torque consumption. The presented method improves the dynamic performance of the robot. This method compensates the inertia force, gravity, Coriolis and the centrifugal terms of the system. The design parameters and optimal trajectory of the robot are simultaneously obtained for a predefined point-to-point motion. Two adjustable counterweights are attached to each active link. The mass of the counterweights and the installation angle of them are considered as design parameters. The optimal trajectory of the robot is obtained by the third-order spline interpolation. Minimum-effort is the objective function of the problem. The numbers cup optimization method is used to find optimum values of the design variables of trajectory and design parameters of the robot. The simulation results show that the objective function has been approximately reached zero value. An experimental robot was developed to verify the simulation results and illustrate the efficiency of the proposed approach. With adjusting the design parameters of the robot, the servo-actuators are operated in position control mode. The experimental outputs show that the objective function has been reduced by about 90% compared to the typical form of the robot.

In recent decades, quadrotors are considered, because of the special missions and reducing the cost of flight operation. In this paper, three flight missions are defined to the quadrotor for shooting the special area. Attitude control of quadrotor is analyzed on basis of state-dependent Riccati equation. In the first mission, an experimental sample is taken in order to find the Euler angles for the implementation of routes. The sample quadrotor is on basis of the proportional–derivative controller. For this purpose, results of simulation base on proportional–derivative controller are conducted and the results are validated by state-dependent Riccati equation controller method. In the second and the third missions, the quadrotor is given maneuver by state-dependent Riccati equation method and flies in more complex routes such as square and round to cover more surface. Considering external wind fieldis the important parameter for the mention missions. The feasibility of these missions related to quadrotor stability and guaranteed security in wind field, for this purpose, the influence of force and moment of the wind field is applied to equations of motion of quadrotor. Dynamic performance of quadrotor is investigated for proportional–derivative, linear quadratic regulator and state-dependent Riccati equation methods encountering wind field.

In this paper deep neural controller is evaluated in self-driving car application which is one of the most important and critical among human-in-the-loop cyber physical systems. To this aim, the modern controller is compared with two classic controllers, i.e. proportional–integral–derivative and model predictive control for both quantitative and qualitative parameters. The parameters reflect three main challenges: (i) design-time challenges like dependency to the model and design parameters, (ii) implementation challenges including ease of implementation and computation workload, and (iii) run-time challenges and parameters covering performance in terms of speed, accuracy, control cost and effort, kinematic energy and vehicle depreciation. The main objective of our work is to present comparison and concrete metrics for designers to compare modern and traditional controllers. A framework for design, implementation and evaluation is presented. An end-to-end controller, constituting six convolution layers and four fully connected layers, is evaluated as the modern controller. The controller learns human driving behaviors and is used to drive the vehicle autonomously. Our results show that despite the main advantages of the controller i.e. being model free and also trainable, in terms of important metrics, this controller exhibits acceptable performance in comparison with proportional–integral–derivative and model predictive controllers.

In this paper, the nonlinear vibration of a rotating blade with varying rotating speeds is investigated. The rotating blade is considered as a rotating cantilever Euler-Bernoulli beam without geometric nonlinearity. The angular velocity is assumed as a constant value which is fluctuated with small amplitude. The nonlinear partial differential equations of the rotating cantilevered beam are derived in three-dimensional using Hamilton's principle. Then, the Galerkin discretization method is applied to the nonlinear partial differential equations to obtain three nonlinear ordinary differential equations. The method of multiple scales is utilized to derive six first-order ordinary differential equations to describe the time variation of amplitudes and phases of interacting modes. The stability and bifurcation of fixed points are obtained by using the eigenvalues of the Jacobian matrix of the modulation equations. Numerical results demonstrated that near the primary resonance and internal resonance the fixed points lose the stability through the saddle node bifurcation. Moreover, the transfer energy among the modes and jump in amplitude of modes occur in frequency response at the different cases of internal resonance.

In this paper, free and forced vibrations of a cylindrical sandwich shell with orthogonal stiffeners using high-order theory were analyzed. The sandwich structure is consists of two orthotropic composite face sheets and a flexible foam core and longitudinal and environmental stiffeners. The face sheets and core are perfectly glued together and there is no relative displacement between them at the interfaces. To analyze this shell under simply supported boundary conditions, the face sheets’ displacements are based on Kant's third-order theory and in the core, the second Frostig’s model is used. The Rayleigh-Ritz method to solve free vibration and the assumed modes method to analyze forced vibration were used. In the forced vibrations section, sinusoidal loading is applied uniformly and radially to the shell. The vibrations of the stiffened sandwich shell are simulated with Abaqus software. Finally, the effect of various parameters as length to radius ratio, thickness to radius ratio, core thickness to total thickness, structure and material of face sheets and core on the vibration response of the structure has been investigated. To validate, the results of free and forced vibrations obtained from the present analysis were compared with the results of other references and Abaqus software.

In this paper, a new method for removing the noise from the vibration signals acquired from the rotating machinery for its condition monitoring is presented. Firstly, each signal is decomposed into its modes using the empirical mode decomposition method. Then for distinguishing the noisy modes from the noise-free modes, the similarity measure between the probability density function of the raw signal and its modes is calculated. Finally, the noise-dominate modes are denoised by the improved thresholding function, and the denoised signal is reconstructed. In this study, the proposed method is implemented for denoising the simulated signal and real data corresponding to different bearing conditions. Finally, the kurtosis and the envelope spectrum of the denoised signal are calculated for detecting the fault presence and its type. The results show that the proposed technique can improve the quality of the reconstructed signals so that the sensitivity of the kurtosis factor to the presence of the defect in the inner and outer rings is increased. Also, the defects frequencies appear in the spectrum of the signals denoised, and the fault type can easily be detected. The results indicate that the proposed denoising technique is superior to the conventional empirical mode decomposition-based denoising method.

The vibration transmitted to helicopter aircrew is the main risk factor for their health. In this paper, a seat suspension based on a negative stiffness structure is proposed to improve the vibration environment for the aircrew. The main advantage of the proposed seat suspension is mitigation of vibration transmitted to occupant in the same time keeping the system payload capacity. Hereafter deriving the dynamic model of the proposed system, the occupant model is attached to achieve an integrated occupant-seat-suspension model. Next, the design procedure of suspension parameters is presented to reduce the vibration transmission. In order to reach realistic results, the simulations are performed using the measured data on Bell-412 helicopter cabin floor. Then, the level of vibrations transmitted to seat and pilot body parts are evaluated using ISO-2631 and common criteria. The results show the performance of system based on negative stiffness structure is good in terms of vibration reduction so that root mean square and vibration dose value of vertical vibration for pilot’s body parts are mitigated about 40% in comparison with cabin floor vibration. Also, according to ISO-2631, comfort level is upgraded from uncomfortable to a little uncomfortable which represents promotion of ride quality and improvement of vibration environment for the pilot. Furthermore, the results indicate that no frequency modulation happens in the vibration transfer path from the cabin floor to the pilot’s head.

In this paper, chaos control in the vehicle during passaging of intermittent roughness has been investigated using a fuzzy fast terminal sliding mode control method. For this purpose, the nonlinear half model for the vehicle is considered due to the nonlinear behavior of the springs and dampers used in the suspension system and tires. Initially, the dynamical equations of motion are derived using the Newton-Euler laws and then are solved using the fourth-order Runge-Kutta method. To analyze the chaotic dynamics, the nonlinear dynamic system is studied by specific techniques for identifying the chaotic behaviors such as frequency response diagrams, bifurcation diagrams, frequency spectra, phase plane trajectories, Poincare¢ section and max Lyapunov exponent. Therefore, using these methods, the chaotic zones along with the critical values in order to excite chaos based on the input force of the road surface are depicted on the uncontrolled model. Consequently, to eliminate this chaotic behavior, the control signals in the active suspension system are generated using the novel fuzzy fast terminal sliding mode control algorithm. According to the simulation results of the feedback system, the unwanted vibrations in the suspension system can be stabilized at a proper time via the efficient fuzzy fast terminal sliding mode controller besides the rejection of the irregular chaotic behaviors.

Early fault detection of the rolling element bearings has a very important role in increasing the reliability of rotating machines.It leads to better decision-making for maintenance activities.  In recent decades, the shock pulse method has been developed to detect faults in the early stage of rolling element bearings degradation. In this paper, the accuracy of the remaining useful life estimation using extracted features from vibration signals and that from the shock pulse method are compared. In this regard, a set of accelerated life tests on rolling element bearings were designed and performed. Both shock pulse signals and vibration signals of the under-test rolling element bearings were recorded. Then two models based on feed-forward neural-network are developed to predict the remaining useful life of rolling element bearings. In the first model, only extracted features from vibration signals are fed for remaining useful life prediction. In the second model, the extracted features from shock pulse method are fed too. The results show that using shock pulse method-based features improves the accuracy of remaining useful life estimation. Also, using the health indicators extracted from vibration analysis and shock pulse method leads to a better estimating of the degradation behavior.

In this study, natural frequency of single- and double-walled boron-nitride nanotubes under physical adsorption of Flavin Mononucleotide molecules are investigated employing the molecular dynamics simulations in vacuum and aqueous environments. The effects of different boundary conditions and geometrical parameters on the natural frequency have been explored. According to the results, the physical adsorption of polymers reduces the natural frequency of boron-nitride nanotubes which is considerable in the case of boron-nitride nanotubes with fully clamped boundary conditions. Moreover, it has been observed that the frequency shift for clamped-free boundary condition in an aqueous environment due to change in mode-shape which is the result of van der Waals interaction with environment, is positive. Also, it is observed that frequency shift of single-walled boron-nitride nanotubes with smaller aspect ratios is higher than that of single-walled boron-nitride nanotubes with higher aspect ratios and double-walled boron-nitride nanotubes. Considering the aqueous environments, frequency shift considerably increases, whereas the slope of variation with the weight percentage decreases. The result of this study can be used as the benchmark for further studies in nanoelectromechanical systems to design more efficient molecular recognition nanobiosensors in aqueous environments.

Thermal stresses caused by temperature changes, along with high angular velocities in industrial rotating disks will reduce the strength of the disk material. Therefore, analysis of rotating disks under thermal-mechanical loads and estimation of the elastic limit angular velocity have particular importance as a criterion of the initiation of plastic deformation. In this paper, analytical modeling for thermoelastic analysis of functionally graded rotating disks is performed by considering the variations of all the geometric and mechanical properties of the rotating disk in a radial direction. The homotopy perturbation method is used as an analytical method to solve equations. The results are verified by the finite difference method and the data in the references. Numerical analysis is performed to investigate the influence of thickness parameter, thermal loading type and boundary conditions on the limit angular velocity and the radius of initiation of the plastic deformation. The Tamura-Tomota-Ozawa model is used to calculate yield stress at different radius of functionally graded disk. Finally, it is shown that by defining the appropriate temperature gradient on the outer surface as a boundary condition, the level of thermal stresses can be controlled and reduced up to 20% compared to the constant thermal boundary condition.

In this research, buckling of polymer matrix composite plates containing unidirectional glass fibers under in-plane pressure load is investigated. Moreover, the effect of adding carbon nanotubes as reinforcement into the matrix material on the critical buckling load of composite plates is studied. Using hand lay-up method, rectangular glass/epoxy composite plates with and without carbon nanotubes were fabricated. 0.25, 0.5 and 1.0 weight percent of carbon nanotubes were added to the epoxy resin and composite plates were subjected to the longitudinal in-plane pressure load. The plates were tested under fix ended boundary conditions utilizing Instron hydraulic universal testing machine. Based on the results, adding 0.5 weight percent of carbon nanotubes has the most influence on the critical buckling load of the plates. Results show that adding 0.5 weight percent of carbon nanotubes into the matrix material increases the critical buckling load of the carbon nanotubes /glass/epoxy plate more than two times with respect to that of glass/epoxy composite plate. Furthermore, when the carbon nanotubes weight percent reaches 1 %, because of carbon nanotubes agglomeration, carbon nanotubes are not dispersed well into the epoxy resin and the critical buckling load of the composite plate decreases. To validate the results, buckling analysis of glass/epoxy composite plate was done in Abaqus finite element software. Finite element models confirm the experimental results obtained.

Although X-ray imaging is a precise method for measuring vertebral curvature, the radiation dose that patient receives may be detrimental for the body. The purpose of this study is to introduce a non-invasive method based on surface data acquisition to determine vertebral curvatures in three-dimensional space. In this method, infrared depth-sensing cameras are used to generate a 3D point cloud from the patient's back surface. To analyze the topographic map obtained from the back surface, first of all, the anatomical landmarks are determined. These landmarks are necessary for transferring the point cloud data into the frontal plane and make the results free of small setup errors of the sensor. Then, the central position of each vertebra is estimated and the vertebral curvatures are calculated by Cobb's angle method. A review of similar past studies and our case study results demonstrate that estimation of vertebral curvatures from back topographic map is possible. Accordingly, can be said, this non-invasive, inexpensive and portable method with acceptable results can be used in clinics and orthopedic centers for monitoring and screening of scoliosis patients.

This paper dealing with an experimental study, numerical simulation, and mathematical modeling of the dynamic response of single-layered and double-layered circular plates under localized impulsive loading. In the experimental section, six experiments were conducted on double-layered configurations made of copper-aluminum plates (2mm+2mm) under different loading conditions. ABAQUS commercial software via a user-defined FORTRAN subroutine was used to develop a numerical model. The Johnson-Cook thermo-viscoplastic constitutive law, as well as the Johnson-Cook damage model, was used in the model. The numerical simulations were verified using the conducted experiments in this study and those in the literature. It has been shown that simulations correctly reproduce the deformation mechanisms of structures during the loading process. Furthermore, a rigorous parametric study was conducted on double-layered metallic plates made of five different layering configurations including aluminum-aluminum, steel-steel, steel-aluminum, copper-aluminum, and copper-steel plates. Two and four different plate radiuses and charge diameters were considered. After conducting the parametric study, an empirical model based on new dimensionless numbers was developed to predict the maximum transverse deflection of plates. Very good overall agreement was obtained between the results of numerical simulations and empirical predictions.

In this article, the sensitivity analysis of the displacement rate effect under tensile loading on the crack growth behavior in the carbon-epoxy composite has been investigated. For this objective, the tensile test was performed on double cantilever beam samples, according to the ASTM-D5528 standard under displacement-controlled loading and under displacement rates of 0.05, 0.5, 5 and 50 mm/min. Then, the regression analysis was done on outputs of experimental data by the Minitab software. For this objective, the sensitivity analysis was performed on three fracture characteristics of composite specimens. These fracture mechanics parameters were the critical energy release rate (with three different fracture energy methods), the maximum failure force and the initial crack tip opening displacement by the use of the digital image correlation technique. Results showed that the variation of the displacement rate affected the maximum failure force and the energy release rate and therefore, these values were sensitive to changes in the displacement rate. However, the maximum value of the initial crack tip opening displacement of the composite was dependent on the logarithmic displacement rate under tensile loading. Finally, the changing trend of fracture mechanics characteristics in the composite was increasing by increasing the displacement rate.

In this study, the thermo-elastic behavior of an aluminum die cast die during the casting process is analyzed. The cyclic symmetry nature of the die geometry offers a simplified reduced model for the finite element simulation. It is assumed that the temperature distribution on the molten metal affected boundaries follows a smooth function variation while the free surfaces of the die experience a convection heat transfer type. Considering a particular boundary condition, the results of the current study conform to the available analytical solutions. A detailed sensitivity analysis is conducted to highlight the effects of die preheat, groove corner radius of curvature and the thermal barrier coatings. The results indicate that the die preheating before the casting process can significantly decrease the stress level within the system. Also, an increase in the radius of curvature for the groove corners may result in a 25% reduction of the von Mises stress around these crack susceptible zones. Application of hard chromium or a silicon nitride thermal barrier coating considerably increases the thermal shock strength of the die such that a 0.1mm thick hard chromium coating can decrease the maximum von Mises stress about 49% with respect to the non-coated specimen. Especially for a functionally graded coating type, the reduction in maximum von Mises stress is around 70% compared to the non-coated specimen.

Determination of machining forces in order to calculate the required power and torque for cutting and select the right tools, equipment, and cutting parameters (feed rate, cutting depth, cutting speed) for machining the desired geometry and material prior to the process is significant. The analysis of machining forces is necessary to determine the forces to reduce the cost of performing multiple empirical experiments. In this study, the components of, , and  milling forces with two cutting edges with main adjustment angle were predicted by the mechanistic modeling method by calculating cutting and edge force coefficients. Also, for the first time, the cutting section of the workpiece was designed in order to eliminate the calculating error of the round corner of inserts.To avoid the interaction of the parameters, simultaneous change of cutting speed with feed rate was avoided and 8 experiments with different feed rates but with the same cutting speed were performed. A comparison of the modeled forces curve with the results obtained from the dynamometer shows acceptable agreement.As the feed rate increases, the difference between the predictive and experimental force decreases relative to the increase of force.

The thermal cycles involved in friction stir welding process cause softening in the joint of heat-treatable aluminum alloys. To overcome this limitation, submerged friction stir welding process has been developed. In this study, firstly, the butt joints were produced from Al7075-T6 alloy using friction stir welding process. To this end, the response surface methodology was selected as the experiment design technique. So, the factors such as tool rotational speed, tool feed rate, tool shoulder diameter, and tool tilt angle were identified as the input variables. Then, a statistical analysis of variables affecting the tensile strength of joints was performed. Afterward, the butt joints were produced using submerged friction stir welding process based on the optimal values of tool feed rate and tool tilt angle. The obtained results from analysis of variance and regression analysis of experimental data confirmed the accuracy of regression equations. Furthermore, it is shown that the linear, interactional and quadratic terms of the tool rotational speed and tool shoulder diameter are effective on the ultimate tensile strength of the underwater welded joints. Also, the optimal condition of input variables was determined using the desirability method. In addition, the optimal condition has been confirmed by implementing the verification test.

In this paper, the effective factors in the welding process and its effects such as residual stresses in the peripheral welding process of a pipe have been investigated using Abaqus software. The analysis was performed in two stages, thermal and mechanical. First, in thermal analysis, the temperature distribution of the welding process was obtained, and then the obtained temperature field was used as a load in mechanical analysis to obtain the distribution of residual stresses and deformations. The effect of different parameters such as welding speed, heat input, and sequence of welding path on residual stress values ​​and deformations were investigated. Results showed that with a 50% increase in electrode speed, tensile and compressive stresses on the outer surface of the pipe showed an increase of 43.47 and 15.15%, respectively. Also, a 20% increase in heat input led to a reduction of 2.6 and 18.18% in tensile and compressive stresses on the outer surface of the pipe. Also in this paper, an optimal path for peripheral welding of pipes is presented.

The canopy as a clear cockpit protector is one of the strategic polymer parts in the aviation industry. Conventional forming processes aren't cost-effective for individual production of transparent polymer canopies due to high energy consumption and high costs of machinery, equipment, and tools. In this paper, rapid prototyping of geometry similar to integral canopies is investigated using an incremental forming process of transparent polycarbonate sheets on a laboratory scale. Polycarbonate sheets with suitable mechanical, thermal, chemical, and optical properties are used in the manufacturing of the latest integral canopies. In single point incremental forming experiments, effect of tool rotational velocity on apparent transparency and effect of toolpath strategy on geometric accuracy were investigated. Three toolpath strategies including raster, spiral from outside, and spiral from inside were applied. Post-forming heating at about 55°C for 30 min resulted in a 50% reduction in spring back by releasing process-induced residual stresses. The use of a non-rotating tool as well as a mechanical-chemical surface polishing improved the final finishing and transparency of samples. Additionally, the amount of deviation for the raster strategy in both depth and radial directions was less than 1 mm, which was within the allowable range of process window of single point incremental forming.