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

In this paper, the performance of a four degree of freedom parallel manipulator with Schönflies motion is evaluated from kinematic and dynamic points of view. Its low inertia makes it a suitable choice for pick-and-place applications, which demand high velocity and acceleration. So, the dynamic characteristics of the robot are of high importance. Moreover, parallel robots suffer from a small workspace, on which their singularities put additional constrain. Hence, this paper studies the kinematic and dynamic behavior of the robot in-depth to give a clear perspective to path planning and its applications. To perform kinematic analysis, constraint equations are derived based on the geometric method, and then Jacobian matrices are determined via velocity analysis. By considering the constraint equations and joint limits, reachable workspace is determined applying point-to-point search algorithm and singularities are identified by the inverse and direct Jacobian matrices. For dynamic modeling, Euler-Lagrange formulation is applied and both kinematic and dynamic models are verified by the results obtained from mechanism simulation in ADAMS software. Furthermore, for evaluation of the robot performance, pressure angles are employed to show the equality of motion/force transmission, and dynamic indices based on joint space inertia matrix are applied to illustrate its dynamic behavior.

In this paper, the vibration reduction of a small-scale horizontal axis wind turbine blade is investigated using the shunt damping method by considering the coupling between edgewise and flapwise vibrations. First, the nonlinear differential equations governing the blade dynamics with shunt damper are derived using the Lagrange method. Then, by performing the sensitivity analysis and selecting the appropriate cost function and constraints, the shunt damper parameters are optimized using the genetic algorithm method for a real blade. It should be noted that in this study, the wind force applied to the blade is considered sinusoidal with variable frequency at four different speeds. After solving the governing dynamic equations, to evaluate the effectiveness of the mentioned method in reducing vibrations, the obtained results in this study are compared with the corresponding results of employing the optimized tuned mass damper for suppression of edgewise vibrations of the blade. Results show that the tuned mass damper and shunt damping method have good effects on reducing vibrations. Despite that, the tuned mass damper effect on vibration reduction at high wind speeds is greater than the shunt damping method, at low wind speeds, the shunt damping has a greater effect on reducing vibrations.

Most of the continuum robots have flexible backbones that are deformed under the internal and external loads and a considerable amount of potential energy may be stored in the backbone. Hence, the continuum robots are exposed to instability issues such as snap-through. The snap-through instability occurs when, with changes in the applied forces, the robot reaches the boundary of its stable region and then moves toward a stable configuration in an uncontrolled manner. Snap-through instability is harmful to the continuum robots and its prediction is important for the design and control of the robot. However, most of the studies focused on design, kinematics, and dynamics of the continuum robots and there are limited studies worked on stability analysis of these robots. In this paper, the stability analysis of the cable-driven continuum robots is investigated. For this, the static equilibrium configurations of the robot are firstly determined under the internal and external loadings. Then, the stiffness matrix of the robot is obtained and the robot stability and snap-through condition are evaluated. The accuracy of the static equations of the robot is verified using the experimental results and the possibility of snap-through occurrence is modeled through simulations. Besides, the effects of the external loads, robot configuration in space, and cross-section of the backbone on the workspace and snap-through occurrence are studied.

In this paper, mathematical and 3D modeling of a three-legged robotic arm capable of moving objects in rough terrain is first presented. Then, considering the noise and environment disturbances, a suitable control method is proposed. Controlling this robot because of its nonlinear dynamics and the presence of disturbances and environmental effects is a very important and complex issue. Therefore, the controller should be able to set the robot in the right position as quickly as possible and eliminate the effect of environmental disturbances and noise on the system response. Accordingly, in this paper, a Double Hyperbolic Sliding Mode Control based on Unscented Kalman Filter is developed for a three-legged mobile manipulator and system stability is proved by Lyapunov theory. In the proposed controller design, while considering the disturbance term in the dynamic model of the system, an Unscented Kalman Filter is used to reduce the noise effect, which improves the robustness of the system under severe conditions. Finally, the performance of the proposed controller is compared with the inverse dynamic controller and the integral sliding mode control on the robotic system. The results show faster operation speed and accuracy in the system response.

Nowadays, scientific and technological advances have created the ability to replace prosthetic legs with amputated limbs, which the design of a suitable controller is still being discussed by researchers. Therefore, according to the importance of the subject, in this paper, a combination of a nonlinear optimal control method based on the state-dependent Riccati equation approach with the integral state control technique is proposed for an active prosthetic leg for transfemoral amputees. The main objective of this paper is to optimize the energy consumption of the robot/prosthesis system and desirable tracking of the vertical displacement in hip and thigh and knee angles. Also, due to the robustness properties of the suggested controller is investigated sensitivity analysis against ±30% parametric uncertainty and compared with robust adaptive impedance control. The performance of the controller is assessed for both point-to-point motion and tracking modes by considering the saturation bounds of control signals. Finally, the simulation results show a decrease in control effort, desirable performance in tracking, and relatively good robustness in the presence of parametric uncertainty and constant disturbance. Numerical results indicate a significant reduction in energy consumption and total cost in this method compared to the robust adaptive impedance control.

This paper intends to stabilize the flight of an avian-scale flapping robot with articulated. Modeling has been performed using Multibody dynamics, considering a tail.  The equations of motion have been derived from Lagrange equations. Kampf mechanism, inspired by the birds, is used to drive the inner and outer wings with a phase shift. The aerodynamic model has been obtained from applying the blade element theory to the wings divided into twelve elements, considering the inner and outer wing distinction. The aerodynamic forces emerging from the movement of wing elements, in terms of flapping frequency and flight speed, are determined separately. Regarding the flight path angle and effective angle of attack, aerodynamic forces of the entire wings have been achieved in horizontal and vertical axes. The coupling of aerodynamic and dynamic completes the nonlinear time-periodic equations. Due to the impact of the fuselage pitch angle on the flight altitude, the cascade control was used to control fuselage and tail pitch angles in inner loops and altitude in the outer one. Proportional-derivative-integral control has been used to control the performance of the loops, the coefficients of which have been optimally designed

This paper deals with the control design and internal and string stability analysis of heterogeneous traffic flows with bi-directional communication topology under random data loss, time-varying communication delay, and actuator lag. A third-order linear model is employed to describe the longitudinal dynamics of each vehicle and the constant spacing policy is employed to adjust the inter-vehicle spacing. In the practical implementation of vehicular networks, due to the high amount of different exchanged information between vehicles and infrastructures, data loss and communication delay are unavoidable effects that may cause adverse effects on the closed-loop performance. Moreover, the actuator lag is an inherent characteristic of the engine which causes delay in implementing the control commands. Therefore, all these issues are considered in system modeling and stability analysis, simultaneously. A linear control protocol using the relative position and velocity measurements with respect to predecessor and subsequent vehicles is introduced for each following vehicle. The Lyapunov-Krassovskii theorem is employed to derive the necessary conditions on control parameters assuring internal stability. Afterward, by performing the error propagation analysis in the frequency domain, sufficient conditions on control parameters assuring string stability are obtained. Finally, several simulation results are provided to show the effectiveness of the presented algorithm.

The vibration of axially graded Rayleigh and Euler-Bernoulli micro-beams under a moving load on Pasternak foundation is studied numerically and analytically. Accurate mathematical modeling is acquired to analyze the effect of various parameters such as longitudinal gradient parameter of material, whirling inertia factor, the stiffness of Pasternak foundation, and strain gradient parameter on the critical velocity, and cancellation mechanism and the maximum amplitude of vibrations. Natural frequencies are obtained and compared with available results in the technical literature. Closed-form expressions are extracted for dynamic magnification coefficient and maximum amplitude of free vibration. The changes in material characteristics of the system have inverse influences on the amplitude of free and forced vibrations for lower and higher values of the critical gradient parameters. It is concluded that in comparison with homogenous Euler-Bernoulli beams, in the axially graded Rayleigh micro-beams surrounded by shear Pasternak foundation. It can be controlled the cancellation and maximum free vibration phenomenon, by choosing the accurate values of gradient parameter, whirling inertia coefficient, the stiffness of foundation, and strain gradient parameter. Also, the results of the present study can be used as a criterion for the optimal design of heterogeneous structures under the moving loads.

Condition monitoring of mechanical systems, such as structures and rotating machines is always a major challenge. This paper is presented a new method for damage detection of real mechanical systems in presence of the uncertainties such as modeling errors, measurement errors, varying loading conditions, and environmental noises based on a simulated model and real healthy state. In this method, data of a real healthy system is used to updating the parameters of the simulated model. Some parts of the signals that are not related to the nature of the system are removed using the complete ensemble empirical mode decomposition method. A deep convolutional network is designed to learn damage-sensitive features from raw frequency data of simulated model and real healthy state. Raw frequency data is extracted from vibration signals using the power spectral density method. In order to train the proposed deep network, raw frequency data of the simulated model andreal healthy state are used. Then, raw frequency data of the real model are used to test the proposed deep network. The proposed method is validated using an experimental beam structure. The results show that using the proposed algorithm for identification and damage detection of the beam-like structure has more accuracy with respect to the other comparative methods

Vibration control of satellite antenna is the main concern to good quality data transmission and reduction of mechanical disturbance in attitude maneuvers. This paper is devoted to mathematical modeling and vibration control of cube-sat antenna. To do this aim, piezoelectric sensor and actuator are utilized and mathematical model of antenna by considering piezoelectric actuator as input parameter and antenna tip deflection as the output parameter. By performing experimental tests, system unknown parameters as damping ratio and natural frequency are obtained based on FFT analysis and the least square method. To control the antenna vibration, its mathematical model is obtained by considering piezoelectric voltage as an input and antenna tip deflection as an output. Herein, due to limitation on the power subsystem, it is not possible to apply continuous voltages and only 100V voltage is available which complicates the control task. Three different control algorithms are proposed for antenna control and compared together. The results show that the proposed control strategies are efficient and can reduce the control time from 10 to about 1 second. The appearing parameters in the selected control algorithm are optimized using genetic algorithm. The presented results in this paper are useful for the design and control of antenna and also for the accurate design of satellite control subsystem.

In this research, an approach based on the isogeometric method is developed to study the free vibration behavior of functionally graded carbon nanotubes reinforced composite skew folded plates. In this method, non-uniform rational B-splines basis functions are used for approximation of the geometry as well as the displacement field. The plates are reinforced by single-walled carbon nanotubes which are assumed to be graded through the thickness direction with different distribution patterns. The effective mechanical properties of composite skew folded plates are captured by the modified rule of mixtures approach. Modeling of the skew folded plate is accomplished by two non-uniform rational B-splines patches which is one of the strengths of the research. The equations of motion of each patch are derived based on classical plate theory and then are discretized using non-uniform rational B-splines basis functions. The final form of the discretized equations is generated after the transformation of the element matrices of each patch and then applying the continuity conditions along the boundary of the patches with the aid of the bending strip method. Afterward, several numerical examples are provided to prove the accuracy and reliability of the proposed formulation. The results exhibit that the present approach can precisely predict the natural frequencies of skew folded plates with a low computational cost. Eventually, a set of new results are presented for different geometrical and material parameters of the skew folded plate.

In industries where manufacturing and assembly operations are to be carried out with a high degree of precision on a micro scale, precise control and movement of components on a micro scale are desperately needed. Integrated microgripper mechanisms are used for this purpose. In this paper, a compliant-based microgripper is designed using multi-objective topology optimization method and the final form of the mechanism is prepared for manufacturing using a new optimal structural boundary modification algorithm. Usually, the optimization faces some problems in the designing step of the structure topology, such as node to node joining rather than the correct joining of the elements, as well as staircase boundaries due to the analysis of the problem with the finite element method. To overcome these drawbacks, in this paper, the curve fitting method is used to minimize the sum of squared errors in the boundary profile of the structure; meanwhile, the optimized objective functions of the structure are improved and better results are obtained. Finally, the performance results of the microgripper are confirmed using the comparison between numerical simulations and empirical tests.

Aerospace structure reliability is analyzed to increase the availability and decrease the stochastic failures of the system. A degradation-based modeling method is an effective approach for reliability assessment. Degradation models are usually developed based on degradation data or understandings of physics behind the degradation processes of products or systems. Stochastic models such as the Wiener process are one of the powerful tools in this field, especially the analysis of damage expansion and fatigue crack growth. This study presents a survey of degradation modeling approaches with consideration of random effects frequently used in engineering programs. Firstly, Wiener processes are used to model the degradation process of the product, which considers measurement errors simultaneously with random effects. Moreover, the closed-form expressions of some reliability quantities such as the probability density function are derived. Then, the maximum likelihood estimation method based on the expectation-maximation algorithm is presented to estimate the unknown parameters in the degradation models. Finally, a practical case study of fatigue crack growth using proposed models is provided and compared with the basic Gamma process to demonstrate the superiority and effectiveness of the Wiener process. It is shown that the Wiener process model estimates fatigue crack growth path better than the Gamma model and by adding the measurement error parameter to the model, its accuracy is increased.

The possibility of workpiece deformation after or during machining due to residual stresses is of crucial importance in precise components. These stresses are induced mainly due to plastic deformation or heat generation during the metal cutting process. Therefore, the magnitude of machining residual stresses is affected by mechanical and thermal stresses. Mechanical stresses depend on the cutting forces and thermal stresses originate from the magnitude of heat generation during cutting action. Therefore, it is expected that machining processes with lower cutting forces and cutting temperatures, will induce lower machining residual stresses as well. Plasma assisted machining is a process that uses a heat source to increase workpiece local temperature and thereby decrease the strength of the material which is to be removed; therefore lower values of cutting forces, temperatures, and residual stresses are expected. In this research work, the effects of undeformed chip temperature, cutting speed, and feed have been investigated on the machining induced residual stresses in the plasma-assisted orthogonal turning of AISI 4140. According to the achieved results, undeformed chip temperature is the most effective parameter on machining residual stresses and by increasing this parameter from 75 to 220˚C, machining induced surface residual stresses became more compressive averagely by 85.30%.

This paper deals with the time-dependent bending behavior of a rectangular viscoelastic plate based on the two-variable refined plate theory using the fractional calculus approach. The plate is fully simply-supported and is subjected to uniformly-distributed loading and the three-parameter merchant model is used for simulation of viscoelastic behavior. The time-domain governing equations are converted into frequency-domain ones using the Laplace transform and then, these equations are solved by the Navier method. The viscoelastic plate response is obtained using the elastic-viscoelastic correspondence principle so that the response of an elastic equivalent problem is extended into the viscoelastic problem. The results of this study, including plate deflection, and in-plane and transverse strains are compared with the results of the elastic model and the standard merchant model where the comparison of obtained results with the reference ones shows that the proposed approach has good accuracy. Also, the variation of deflection through the plate thickness and the effect of aspect ratio on the results are studied. This study shows that the proposed fractional model has the ability to simulation of both elastic and viscose effects simultaneously which is more compatible with the nature of viscoelastic materials.

In the current study, an experimental study and modeling of the penetration behavior of single-layered and multi-layered targets made of either aluminum alloy or mild steel or a combination of these materials impacted by a spherical projectile were introduced. For conducting 66 experiments, eight different layering configurations consist of monolithic aluminum and steel plates with the thickness of 2 mm and 3mm, double-layered aluminum and steel plates with a total thickness of 2 mm, triple-layered aluminum, and steel plates with the total thickness of 3 mm, and triple mixed layered plates of Aluminum-Steel-Aluminum and Steel-Aluminum-Steel configurations with the total thickness of 3 mm were considered under various impact velocities of 42 to 158 m/s. The impact velocity and maximum permanent deflection of specimens were measured in all experiments. In the numerical modeling section, the group method of data handling neural network was used to present a mathematical model based on dimensionless numbers to predict the maximum permanent deflection of monolithic and multi-layered metallic plates under the rigid projectile impact. To increase the prediction capability of the proposed neural network for this process, the experimental data were divided into two training and prediction sets. The results showed that good agreement between the proposed model and the corresponding experimental results is obtained and 94% of data points are within the ±10% error range.

In this paper, the static analysis of hyperelastic plates under uniform and sinusoidal distributed loading is investigated. Right Cauchy-Green deformation tensor and Lagrange strains are used to derive the nonlinear strain relations. Also, the first-order shear deformation plate theory is considered. For the first time, the governing equations of hyperelastic plates using the neo-Hookean strain energy function are derived. The Lagrange equation is utilized to implement the variational method on potential energy function. The governing nonlinear differential equations are discretized using the meshless collocation method and radial basis functions. The thin plate spline basis function is applied for deriving shape functions of the meshless method. The results are compared to the results of the finite element method. The static analysis is investigated on hyperelastic plates for uniform and sinusoidal loading and various thicknesses. Additionally, the effect of thickness is studied on the deflection of the hyperelastic plates. The results show an acceptable accuracy for static analysis of hyperelastic plates under uniform and sinusoidal loading; also, the stress contour is the same in both methods. Consequently, the meshless collocation method using the thin-plate spline basis function is an adequate method for analyzing FSDT hyperelastic plates due to no integration and imposing boundary conditions directly.

A small-scaled device for ambient energy harvesting is a high voltage cantilever nanogenerator with interdigitated electrodes carrying a tip mass that acts upon the strain induced in the top piezoelectric layer. In this device, more strain gradient over the length, more electric potential in adjacent electrodes depending on the vibration mode shape at which voltage cancelation may occur. In this work, changing the distance between the electrodes proportional to the inverse of strain function, the induced voltage in all the electrodes are equalized that prevents the voltage cancelation. The Euler-Bernoulli beam model is used for the problem and the governing time-dependent equation is derived based on the energy method. Then, the 4th order Runge-Kutta method is used to solve it from which the output voltage is derived for base excitation. The results show that it is possible to increase the voltage by 36% for optimal electrical load by this procedure and for 40% for open circuit conditions. The system coupling is also increased by 10%. Moreover, the results show that the smaller size of electrodes, the higher the output voltage. Whereas, increasing the number of electrodes makes the voltage reduce in contrast with the electric current.

In this study, global sensitivity analysis of nanomachining parameters by using a dynamic scanning thermal microscope is investigated. Thus, the cross-section of a nanomachined sample by sweeping with different tip radius on the vibrational response of the system at different speeds and temperatures are simulated. It is shown that by increasing temperature, the depth of nanomachining decreases, and by increasing tip radius, the depth of nanomachining increases. Also, it is declared that the final quality of the nanomachining decreases by increasing speed traveling. Then, the Sobol indices for the mean depth and surface finish of the nanomachined sample are studied. It is shown that traveling speed is not affected the mean depth of nanomachining in its physical range and so the effects of the probe traveling speed and interaction between parameters are negligible. It is declared that the effect of interaction between temperature, traveling speed and tip radius is important on the final surface finish of the sample, however, the most important parameter is still the temperature difference. Then, the total indices and Sobol indices are compared. It is stated that the total indices are significantly higher than the Sobol indices for the final surface of ​​the nanomachining. For the mean depth of the nanomachining the total indices and Sobol indices for temperature and tip radius are approximately equal and the effect of probe traveling speed is negligible.