Showing 11 results for Moetakef-Imani
Seyed Ali Hashemian, Behnam Moetakef-Imani,
Volume 14, Issue 12 (3-2015)
Abstract
In a mechanical assembly, errors arising from part manufacturing or assembly process may cause significant variation in final assembly with respect to the ideal model and affect the quality and performance of product. In sheet metal products due to high order of compliancy of components, errors generated during assembly process are as important as parts’ manufacturing tolerances. Therefore, it is crucial to have a comprehensive model in order to analyze the assembly process of these structures and represent the relationship between part tolerances and final assembly errors. However, it should be noted that assembly processes are often complex and nonlinear in nature. In sheet metal structures, the most important factor that makes the assembly process nonlinear is contact interaction between mating parts during assembly. If this factor is disregarded and the assembly process is only represented based on linear force-displacement relationship, the model will result in part penetration and a remarkable difference between theoretical and experimental results will occur. Another important feature in sheet metal tolerance analysis is the surface continuity of components which makes the deformation of the neighboring points of a plate correlated. This paper aims to present a new methodology for tolerance analysis of compliant sheet metal assemblies in which a nonlinear finite element analysis is integrated with improved sensitivity-free probability analysis in order to account ...
Seyed Farhad Hosseini, Behnam Moetakef-Imani, Saeid Hadidi Moud,
Volume 14, Issue 13 (First Special Issue 2015)
Abstract
The need for complex surfaces in CAD motivates researchers for methods which can produce smooth and visually pleasing surfaces. In this research, a new method is presented for creating compatible cross-sectional curves for surface fitting to certain sections or lofting. In this method, the distribution of sections' data points along with basis knot vectors are improved in order to reach a desired smooth surface. In compatibility process, the section curves' degrees and their knot vectors must be set equal before implementing lofting process. Based on proposed algorithm, in this research, the constructed smooth and faired surfaces can be used in many engineering applications such as reverse engineering, biomedical engineering, quality control, etc. The main focus of the method is improvement of data points' distributions and their assigned parameters in a way that by a few iterations, data points' distribution are improved in order to reach a common knot vector for all cross-sectional curves. The method is implemented on some benchmarking examples and its efficiency are confirmed. In addition, the amount of final data points' deviation from the initial section curve is analyzed using the vigorous Hausdorff method. It is worth mentioning that the quality of obtained final surface is visually pleasing. In order to quantitatively confirm that the proposed method will result in smooth and fair surfaces, MVS is used. Finally the application of the method in modeling the root joint zone of a wind turbine blade is presented.
Hassan Ghorashi, Behnam Moetakef-Imani,
Volume 14, Issue 15 (Third Special Issue 2015)
Abstract
Because of high accuracy and low weight-to-force ratio, servo hydraulic systems are widely used in various branches of industry. Simultaneous improvement of accuracy and time response are among ever increasing needs for these systems. Rapid movement commands to hydraulic actuator excite attached mechanical components and consequently produce undesired vibrations. Recommended solution to overcome the above mentioned problem is to design and implement advanced controller which takes into consideration the high frequency uncertainties. In this research a two-degree-of–freedom (2DOF) position controller has been design and implemented for undesirable vibration regulation and robust performance achievement on a servo hydraulic table. In this regard various elements of the system are modeled and then the servo hydraulic table nominal system and uncertainty are identified using grey-box method. The 2DOF robust controller is designed using general H∞ framework and analyzed by structured singular value, Mu. The feedback block of controller is used to reduce the effect of uncertainty, measurement noises and reject disturbances, whereas the forward controller shapes the command signals to improve the performance. The designed controller has been implemented on the servo hydraulic test rig in order to track sine and trapezoid position command signals. It has been observed the controller has a more accurate performance and faster time response than the common robust controller with just one feedback block. Extensive experimental results of the developed controller indicate robust performance and acceptable response to disturbance and measurement noise rejection in the defined uncertainty range.
Arash Hatami, Behnam Moetakef-Imani,
Volume 16, Issue 11 (1-2017)
Abstract
The attenuation of mechanical load is one of the most effective approaches in wind turbine components cost reduction, and improving the control system reduces mechanical loads with minimum effort. In modern wind turbines, electrically-excited synchronous generators are mostly applied in direct-drive structure. In current research, generator field voltage along with the blade pitch angle is employed for tower load reduction in a novel multivariable-adaptive control structure. The controller is designed based on the extracted model with aerodynamic, vibratory and electrical interactions. The centralized multivariable structure is chosen to simultaneously reduce rotor speed fluctuations and tower vibrations. Since the nonlinear wind turbine model is complex, the controller is designed via optimization process. The nonlinear aerodynamic behavior of blades influences the closed-loop performance in different operating condition; therefore controller is adapted to the condition by employing gain-scheduling method. The effects of signal noise, digital control and higher-order dynamics of electrical system might defect the closed-loop stability. The designed controller is implemented on a wind turbine simulator which includes the before-mentioned effects. By comparing the performance of the multivariable adaptive controller with a two input-one output multivariable controller, it is proven that the mechanical loads acting on tower have been greatly decreased.
Mohsen Fallah, Behnam Moetakef-Imani,
Volume 16, Issue 12 (2-2017)
Abstract
The present article deals with analytical modeling of boring bar dynamics as well as identification of unknown parameters for the dynamic model. Experimental modal analysis is utilized to measure the Frequency Response Functions (FRFs) of cutting tool. Using the analytical methods of modal analysis theory, dynamic parameters of boring bar (i.e. natural frequencies, damping ratios and modeshapes) are extracted from curve fitting of experimental FRFs. A new physical configuration is proposed, in order to accurately estimate the dynamic response of boring bar in time/frequency domains. In the proposed dynamic model, boring bar is modeled as an Euler-Bernoulli beam with flexible support and tip mass. The mechanical properties (i.e. modulus of elasticity and density) are considered to be constant along beam length. The flexibility of boring bar's clamping interface is modeled by linear translational/torsional spring elements. Particle Swarm Optimization (PSO) is utilized to identify the unknown parameters of dynamic model. The parameters include translational/rotational clamping stiffness and dimensionless correction factors for boring bar's diameter/tip mass. These parameters directly control the mass/stiffness distribution of proposed dynamic model. The FRFs obtained from updated model of boring bar are compared with experimental FRFs. It is shown that, by optimal selection of unknown parameters, boring bar FRFs can be accurately calculated at any point along its length. Hence, by incorporating the dynamic model of passive/active actuator into the proposed dynamic model, the stability lobes of dampened boring bars can be predicted.
Pooria Naeemi Amini, Behnam Moetakef-Imani,
Volume 17, Issue 8 (10-2017)
Abstract
Boring operations due to the large length to diameter ratio and the high flexibility of tool are prone to self-excited (chatter) vibration. This vibration may cause poor surface quality, low dimensional accuracy and tool breakage. In practice, chatter is the main limitation on production rate. The main reason of chatter phenomenon is the dynamic interaction between cutting process and structure of machine tool. By increasing the length of the cutting tool, the vibration tendency in the tool’s structure increases. Improving dynamic stiffness of the tool is the most effective solution for decreasing vibration and increasing chatter stability. For increasing the stability of the tool in long overhang boring operations, passive and active vibration control has been proposed and implemented. In active control methods, vibrations can be effectively damped over a various cutting conditions. The aim of this research is to enhance chatter stability of an industrial boring bar by increasing the dynamic stiffness. A VCA actuator is used for active vibration control. The designed setup can effectively suppress undesirable vibrations in the radial direction. First, modal parameters of the boring bar are determined by experimental modal analysis. Then, the transfer function of the actuator-tool setup is identified with the sweep frequency excitation. In the following, the direct velocity feedback is successfully implemented in the vibration control loop. The results of cutting tests indicate that the actuator has a great performance in suppressing vibrations and increasing the dynamic stiffness. Hence, the developed method can significantly increase chatter stability of boring operations.
Pooria Naeemi Amini, Behnam Moetakef-Imani,
Volume 18, Issue 8 (12-2018)
Abstract
One of the most important constraints on manufacturing productivity is the machining vibrations. This vibrations may cause increase in machining costs, lower accuracy of products and decrease tool life. The effective solution for increasing cutting process stability and vibration suppression is to improve structural dynamic stiffness. There has been presented different techniques for enhancing dynamic stiffness of structures using passive and active vibration control methods. Although passive vibration control methods are always stable, they exhibit limited performance. In active control methods, vibrations can be effectively damped over a various conditions. The aim of this research is to enhance the dynamic stiffness of an industrial boring bar by using active damping. Cutting process mainly exposed to parameter perturbations and unknown external disturbances, therefore, designing an active vibration control system for cutting process is a challenging problem. In this research an extended state observer based control strategy was proposed that can overcome these uncertainties. The proposed strategy was implemented into an active vibration control system for a boring bar. Moreover, the direct velocity feedback is successfully implemented in the vibration control loop. The results of impact tests indicate that the control algorithms have a great performance in suppressing vibrations and increasing the structural dynamic stiffness. Voltage impact results show that ADRC controller spends less control effort than direct velocity feedback controller.
M. Fallah, B. Moetakef-Imani,
Volume 19, Issue 8 (August 2019)
Abstract
In this paper, a novel dynamic model is proposed for an actively damped boring bar equipped with electromagnetic actuator. The dynamic models of actuator and boring bar are obtained by using the suggested systematic identification approach, which is based upon the fundamental tools and techniques of system identification theory. The electro-mechanical system or the forward path is consisted of 3 basic components, i.e. linear power amplifier, electrodynamic shaker, and boring bar structure. In this paper, the dynamic models of forward path’s sub-systems are simultaneously identified. The component-based identification approach has led to a remarkable finding about the source of nonlinearity in the dynamic model of forward path. According to the presented experimental observations, it has been concluded that electromagnetic actuator can be modeled as a linear dynamic system, while the boring bar structure exhibits nonlinear behavior, since the prediction accuracy of boring bar dynamic model is drastically reduced by changing the amplitude of excitation. As a result, a new parameter varying dynamic model is presented for describing the dynamic behavior of forward path in terms of both frequency and excitation level. The proposed dynamic model has a predefined representation with the least possible mathematical order. It can anticipate the time domain response of forward path due to chirp excitation with 88% accuracy. In addition, during the validation stage, the proposed model forecasts the dynamic response of system due to Gaussian white noise excitation with remarkable accuracy. Moreover, the dynamic model of electromagnetic actuator can predict the dynamic force signature of actuator with 85% accuracy.
S. Haji Zahedi , B. Moetakef-Imani ,
Volume 20, Issue 3 (March 2020)
Abstract
With the advancement of the manufacturing processes and the continuing need for increasingly precise assemblies, consideration of dimensional and geometric tolerances has been of great importance in tolerance analysis of mechanical assemblies. Therefore, in recent decades, several methods have been developed and implemented for calculating the influences of geometric errors of components on the final performance of the assembly. One of the proposed methods for tolerance analysis is the Direct Linearization Method (DLM). However, DLM has significant advantages in dimensional tolerance analysis, due to simplifications used in this technique, it does not have the ability to solve assemblies including free form profiles. In this research, a new method has been proposed to consider the complex profiles in the process of DLM. In the proposed combination method, rational Bezier curves have been used to define component profiles such as elliptical profiles, cams, edge joints, and non-circular profiles that have a complex error variation. Then, by using principles of DLM and rational Bezier equations, the developed algorithm is successfully accomplished. In this way, we can not only use significant advantages of DLM in dimensional tolerance analysis but also it is possible to solve assemblies including a component with complex profiles without any simplification. The developed hybrid approach has been presented in detail by solving an example of assembly tolerance analysis. Finally, validation has been performed and the accuracy of the proposed approach was confirmed using Monte Carlo simulation.
M. Fallah, B. Moetakef-Imani,
Volume 20, Issue 4 (April 2020)
Abstract
In this paper, a new active vibration control system has been proposed for the elimination of boring bar chatter in the internal turning process. The system is composed of a boring bar equipped with electromagnetic actuator and accelerometer, as well as a novel adaptive control algorithm that is widely used in the field of active noise control. The controller is known as feedback FxNLMS and is composed of two finite impulse response adaptive filters. One of the filters is known as a model filter, which predicts the dynamic model of actuator-boring bar assembly. The other is known as the control filter and anticipates the inverse model of forwarding path dynamics. The weight vector of the adaptive filter is adjusted by using the normalized least mean square algorithm. Firstly, the impact test is conducted in the presence of an adaptive controller. It is observed that the magnitude of the dominant mode on the forward path’s frequency response function is drastically suppressed by 36 dBs. Secondly, the internal turning tests are conducted on Aluminum alloy 6063-T6, to investigate the performance of the adaptive controller for the purpose of chatter mitigation. Due to the optimal performance of the adaptive controller, the dominant magnitude of the boring bar’s power spectral density is successfully attenuated up to 68 dBs, and the critical limiting depth of cut is increased by 10 folds. Also, the roughness of the machined surface is remarkably improved by 8 folds compared to the control-off cutting test. Moreover, the actuator cost is considerably reduced by 3 folds in comparison to the optimal constant-gain integral controller.
Ali Pordel, Mohammad Kazemi Nasrabadi, Behnam Moetakef-Imani,
Volume 21, Issue 6 (June 2021)
Abstract
Although there have been several research work published in the field of simulating and predicting the surface roughness of machining processes, most of them are limited to turning and milling operations. A few number of studies concerning the internal turning processes is very limited. Furthermore, the existing publications in this field have implemented statistical approaches which not only clearly lack in generality, but also require a huge amount of experiments. In the current research, the simulation of surface roughness has been investigated by using kinematics and dynamics of the process. Despite the numerous applications of this approach in turning operations, this approach has not applied in the internal turning processes. In order to implement the proposed approach, firstly the insert nose profile of the tool has been measured. Then, the surface profile consisting the periodical component of feed marks has been constructed. In the next step, excessive amount of vibrations imposed by the long boring bar have been measured by an accelerometer, which are then converted to displacements and added to the periodical component of the roughness profile. Results obtained from internal turning experiments show that the developed simulation approach has a maximum error of 19.3% in estimating roughness parameters which can be considered as a reasonably accurate results due to the complicated nature of surface roughness.
