M. Ahmadian
Virginia Tech, Mechanical Engineering, Faculty Member
ABSTRACT This paper presents the results of a study to evaluate the effect of semiactive dampers in reducing track dynamic loading at the interface between the rail and moving wheel in rail vehicles. Considering an elastic foundation for... more
ABSTRACT This paper presents the results of a study to evaluate the effect of semiactive dampers in reducing track dynamic loading at the interface between the rail and moving wheel in rail vehicles. Considering an elastic foundation for the track, the study derives a simplified dynamic model for a railcar primary suspension, attached to the wheelset (unsprung mass) and car body (sprung mass). The performance of the system for different semiactive control strategies is presented and compared with conventional passive suspensions. Different system excitations, caused by track irregularities, are considered and the performance of the system is investigated in both time and frequency domains. Simulation results show that displacement-based groundhook control strategies can reduce the dynamic loads caused by the moving wheel in the vertical direction, therefore lessening the track damage caused by high wheel loads.
Research Interests: Engineering and Design
Several control policies of Q-car 2-DOF semiactive system, namely skyhook, groundhook and hybrid controls are presented. Their ride comfort, suspension displacement and road-holding performances are analyzed and compared with passive... more
Several control policies of Q-car 2-DOF semiactive system, namely skyhook, groundhook and hybrid controls are presented. Their ride comfort, suspension displacement and road-holding performances are analyzed and compared with passive system. The analysis covers both transient and steady state responses in time domain and transmissibility response in frequency domain. The results show that the hybrid control policy yields better comfort than a passive suspension, without reducing the road-holding quality or increasing the suspension displacement for typical passenger cars. The hybrid control policy is also shown to be a better compromise between comfort, road-holding and suspension displacement than the skyhook and groundhook control policies.
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In general, a vehicle suspension system can be characterized as a nonlinear dynamic system that is subjected to unknown vibration sources, dependent on road roughness and vehicle speed. In this paper, we will present a... more
In general, a vehicle suspension system can be characterized as a nonlinear dynamic system that is subjected to unknown vibration sources, dependent on road roughness and vehicle speed. In this paper, we will present a nonlinear-model-based adaptive semiactive control algorithm developed for nonlinear systems exposed to broadband non-stationary random vibration sources that are assumed to be unknown or not measurable. If there exist unknown and/or varying parameters of the dynamic system such as mass and stiffness, then the adaptive algorithm can include a recursive least square (RLS) method for on-line system identification. Since the adaptive algorithm is developed for semiactive systems, stability is guaranteed based on the fact that the system is energy conservative. The convergence of the adaptive system, however is not guaranteed, and is investigated through a numerical approach for a specific case. The simulation results for a magneto-rheological seat suspension system with t...
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Investigation of the response time of magnetorheological fluid dampers. [Proceedings of SPIE 5386, 63 (2004)]. Jeong-Hoi Koo, Fernando D. Goncalves, Mehdi Ahmadian. Abstract. The primary purpose of this paper is to investigate ...
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This paper presents a review of the state of the art in magnetorheological technology. As magnetorheological fluids continue to gain attention, the research and development of these fluids must keep pace. Since their invention in 1948, MR... more
This paper presents a review of the state of the art in magnetorheological technology. As magnetorheological fluids continue to gain attention, the research and development of these fluids must keep pace. Since their invention in 1948, MR fluid development has made significant advancements. Throughout this time, MR fluids have been faced with many challenges and today, after many years of development, MR fluid formulations appear to have overcome many of these challenges. The fluids have proven to be commercially viable and well suited for many applications. The intent of this study is to provide a review of the state of the art in MR fluid technologies. To limit the scope, this study will focus on three common concerns associated with MR fluids; namely the strength of the fluid (i.e., the achievable yield stress), the stability of the fluid, and the durability of the fluid. When considering the use of a MR fluid in a particular application, it is these three areas that fall under the greatest scrutiny. The literature is rich in works devoted to addressing these concerns. Furthermore, with the great success of MR fluids and MR fluid devices, a number of the models used to describe the behavior of the fluid are also reviewed; specifically, works related to the visco-plastic modeling of MR fluids are reviewed. Models used to describe the physics behind the formation of the field induced yield stress are also reviewed.
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Dynamic game theory brings together different features that are keys to many situations in control design: optimisation behaviour, the presence of multiple agents/players, enduring consequences of decisions and robustness with respect to... more
Dynamic game theory brings together different features that are keys to many situations in control design: optimisation behaviour, the presence of multiple agents/players, enduring consequences of decisions and robustness with respect to variability in the environment, etc. In the presented methodology, vehicle stability is represented by a cooperative dynamic/difference game such that its two agents (players), namely the driver and the direct yaw controller (DYC), are working together to provide more stability to the vehicle system. While the driver provides the steering wheel control, the DYC control algorithm is obtained by the Nash game theory to ensure optimal performance as well as robustness to disturbances. The common two-degrees-of-freedom vehicle-handling performance model is put into discrete form to develop the game equations of motion. To evaluate the developed control algorithm, CarSim with its built-in nonlinear vehicle model along with the Pacejka tire model is used....
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... SMART MATERIALS AND STRUCTURES Smart Mater. Struct. 15 (2006) 13701384 doi:10.1088/0964-1726/15/5/026 Application of the extended Kalman filter to control of a shape memory alloy arm Mohammad H Elahinia1 and Mehdi Ahmadian2 ...
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This paper will investigate the feasibility of adopting innovative design concepts that could decrease the dependence on bonded insulated joints for railway track monitoring. First, the purpose of insulated joints and their historical... more
This paper will investigate the feasibility of adopting innovative design concepts that could decrease the dependence on bonded insulated joints for railway track monitoring. First, the purpose of insulated joints and their historical usage are examined. Next, designs that utilize track circuits are discussed. This will be followed by a section on acoustic methods, reflectometry, and robotics.
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The objective of this work is to characterize the performance of MR fluid at high shear rates and high velocities. A slit-flow rheometer has been built which allows for high speed testing of MR fluid under varying field strengths. Fluid... more
The objective of this work is to characterize the performance of MR fluid at high shear rates and high velocities. A slit-flow rheometer has been built which allows for high speed testing of MR fluid under varying field strengths. Fluid velocities range from 3.9 m/s to 27 m/s with resulting shear rates ranging from 0.23×105 s -1 to 1.6×105 s -1. In order to evaluate the performance of the fluid, the force required to drive the fluid through the slit is measured and force-velocity characteristics are generated. The zero-field viscosity is found and results indicate that shear thinning is still present even at these high shear rates. Two magnetic field strengths were considered for the field testing (50 kA/m and 100 kA/m). Results indicate that a yield stress is still being developed in the fluid. However, as fluid velocity increases, the yield stress being developed in the fluid decreases. This reduction in the yield stress could be attributed to the dwell time, or the amount of time...
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In order to develop a compact and lightweight controllable damper for space truss structures vibration suppression, a novel double-piston magnetorheological (MR) damper is proposed. Working principle of this damper has been analyzed. One... more
In order to develop a compact and lightweight controllable damper for space truss structures vibration suppression, a novel double-piston magnetorheological (MR) damper is proposed. Working principle of this damper has been analyzed. One prototype damper have been designed and fabricated according to the analysis results. A series of experiments have been performed to get this prototype damper's dynamical properties. Hyperbolic tangent model have been used to describe damper's nonlinear hysteresis. After model optimization using the nonlinear least squares method, the relationship between damper force and drive currents have been acquired under different excitation conditions. Comparison between the reconstructed results and testing data indicates that the optimized model shows enough accuracy to not only present the experimental data, but also forecast the hysteretic properties of this damper.
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A substructural control technique consisting of local and global controllers is proposed for high-order systems. The global controllers are used to minimize the effect of coupling between substructures, whereas the local controllers are... more
A substructural control technique consisting of local and global controllers is proposed for high-order systems. The global controllers are used to minimize the effect of coupling between substructures, whereas the local controllers are employed to accomplish optimal performance and stability. A set of stability conditions based on the properties of the substructures is presented for the global stability of the structure. The advantages of the proposed technique are demonstrated using a structure consisting of two flexible panels and a rigid hub. The examples show that the proposed substructural control technique is more computationally efficient than a centralized control approach, and it can effectively control the structure and maintain stability, with a slight increase in the control inputs.
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... Author(s): S. Nima Mahmoodi; Michael J. Craft; Mehdi Ahmadian. PDF: Member: $18.00, Non-member: $18.00 Hard Copy: Member: $24.00, Non-member: $24.00, Proceedings Vol. 7643. Active and Passive Smart Structures and Integrated Systems... more
... Author(s): S. Nima Mahmoodi; Michael J. Craft; Mehdi Ahmadian. PDF: Member: $18.00, Non-member: $18.00 Hard Copy: Member: $24.00, Non-member: $24.00, Proceedings Vol. 7643. Active and Passive Smart Structures and Integrated Systems 2010, Mehrdad N. Ghasemi ...
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This paper presents two alternative implementations of skyhook control, named “skyhook function” and “no-jerk skyhook,” for reducing the dynamic jerk that is often experienced with conventional skyhook control in semiactive suspension... more
This paper presents two alternative implementations of skyhook control, named “skyhook function” and “no-jerk skyhook,” for reducing the dynamic jerk that is often experienced with conventional skyhook control in semiactive suspension systems. An analysis of the relationship between the absolute velocity of the sprung mass and the relative velocity across the suspension are used to show the damping-force discontinuities that result from the conventional implementation of skyhook control. This analysis shows that at zero crossings of the relative velocity, conventional skyhook introduces a sharp increase (jump) in damping force, which, in turn, causes a jump in sprung-mass acceleration. This acceleration jump, or jerk, causes a significant reduction in isolation benefits that can be offered by skyhook suspensions. The alternative implementations of skyhook control included in this study offer modifications to the formulation of conventional skyhook control such that the damping force...
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ABSTRACT The objective of this work is to highlight and discuss some critical conditions in which the imprecision of atomic force microscopy (AFM) system will be intensified. To precisely address these issues, we developed a complete... more
ABSTRACT The objective of this work is to highlight and discuss some critical conditions in which the imprecision of atomic force microscopy (AFM) system will be intensified. To precisely address these issues, we developed a complete close form solution for a non-linear motion of AFM system subjected to non-linear contact and van der Waals forces. Galerkin method and multiple time scale approach are used to solve the governing non-linear differential equation of the AFM system. A nonlinear frequency-response equation is then obtained as a function of displacement excitation, resonance frequency and associated response amplitude. By simulation of the AFM response under different tip-sample interaction force and distance, it is shown that jump phenomenon takes place due to the non-linear motion of microcantilever at certain operating frequencies. This case can be avoided via operating the AFM in a right frequency region; however the error associated with it must be compensated by a post-processing of collected data.
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... Author(s): Alireza Farjoud; Mehdi Ahmadian. PDF: Member: $18.00, Non-member: $18.00 Hard Copy: Member: $24.00, Non-member: $24.00, Proceedings Vol. 7643. Active and Passive Smart Structures and Integrated Systems 2010, Mehrdad N.... more
... Author(s): Alireza Farjoud; Mehdi Ahmadian. PDF: Member: $18.00, Non-member: $18.00 Hard Copy: Member: $24.00, Non-member: $24.00, Proceedings Vol. 7643. Active and Passive Smart Structures and Integrated Systems 2010, Mehrdad N. Ghasemi-Nejhad, Editors ...
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ABSTRACT This work pertains to laboratory testing of truck cab suspensions for the purpose of improving in-cab ride quality. It describes the testing procedure of a complete truck cab suspension while still being mounted on the vehicle.... more
ABSTRACT This work pertains to laboratory testing of truck cab suspensions for the purpose of improving in-cab ride quality. It describes the testing procedure of a complete truck cab suspension while still being mounted on the vehicle. It allows for testing with minimal amount of resources, limited to two mobile actuators and minimal modifications to the stock vehicle. The actuators can be attached to any axle through a set of modified brake drums and excite the drive axle in a vertical plane. The excitation signal sent to the actuators can be in phase for a heave type motion or out of phase for a roll motion. The chassis shock absorbers are replaced with rigid links to prevent the actuator input from becoming filtered by the primary suspension. This allows the input to reach the cab suspension more directly and the cab to be excited across a broader range of frequencies. The laboratory setup allows for easy calculation of transfer functions, frequency response and other measures vital to the successful development of a better cab suspension without the complications involved in actuating through the primary suspension. The proposed test method makes it easy to quickly revert the cab and the primary suspension to their original configuration and make the vehicle ready for road operation.
ABSTRACT This study presents track alignment and curvature measurement results from a Doppler LIDAR (light detection and ranging) speed measurement system, a non-contact speed and distance measurement system comparable to encoders found... more
ABSTRACT This study presents track alignment and curvature measurement results from a Doppler LIDAR (light detection and ranging) speed measurement system, a non-contact speed and distance measurement system comparable to encoders found on research geometry cars. The system has multiple mounting capabilities with the primary implementations being body-mounted and truck-mounted. Track speed is measured using the individual rails as reference targets, producing two speed signals. Curvature data is obtained from the measured speed differential as the train navigates tangent and curved track. The different dynamic behaviors of the truck and car body influence the motion of the LIDAR system, and thus the results vary depending on the mounting configuration. The curvature, speed, and distance data obtained from the LIDAR system has been compared with results from a geometry car and manual track measurements. The results indicate the LIDAR system has strong potential in serving as a highly precise, non-contact speed, distance, and curvature measurement device suitable for implementation in rail geometry applications.
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The primary purposes of this study are to: (1) establish the vibration characteristics of a typical locomotive cab used in freight locomotives in North America; and (2) evaluate the effect of various structural modifications on the cab... more
The primary purposes of this study are to: (1) establish the vibration characteristics of a typical locomotive cab used in freight locomotives in North America; and (2) evaluate the effect of various structural modifications on the cab interior noise and vibrations. To this end, the structural dynamics of a production locomotive cab is studied in a laboratory environment. The cab
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... Author(s): C. Nagode; M. Ahmadian; S. Taheri. PDF: Member: $18.00, Non-member: $18.00 Hard Copy: Member: $24.00, Non-member: $24.00, Proceedings Vol. 7643. Active and Passive Smart Structures and Integrated Systems 2010, Mehrdad N.... more
... Author(s): C. Nagode; M. Ahmadian; S. Taheri. PDF: Member: $18.00, Non-member: $18.00 Hard Copy: Member: $24.00, Non-member: $24.00, Proceedings Vol. 7643. Active and Passive Smart Structures and Integrated Systems 2010, Mehrdad N. Ghasemi-Nejhad, Editors ...
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... Craft; Mehdi Ahmadian; Alireza Farjoud; William CT Burke; Clement Nagode. PDF: Member: $18.00, Non-member: $18.00 Hard Copy: Member: $24.00, Non-member: $24.00, Proceedings Vol. 7643. Active and Passive Smart Structures and Integrated... more
... Craft; Mehdi Ahmadian; Alireza Farjoud; William CT Burke; Clement Nagode. PDF: Member: $18.00, Non-member: $18.00 Hard Copy: Member: $24.00, Non-member: $24.00, Proceedings Vol. 7643. Active and Passive Smart Structures and Integrated Systems 2010, Mehrdad N ...
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The primary purpose of this study is to evaluate and better understand the isolation properties of semiactive suspensions. Specifically, this study will answer the question regarding why semiactive dampers are able to isolate at... more
The primary purpose of this study is to evaluate and better understand the isolation properties of semiactive suspensions. Specifically, this study will answer the question regarding why semiactive dampers are able to isolate at frequencies well below those for passive dampers, even though they do not add any energy to the system. A single suspension model is used to derive and analytically evaluate the transmissibility properties of passive and semiactive dampers. The results show that for semiactive dampers, the frequency range of isolation and the transmissibility amplitude are functions of ξ, the damping ratio. In contrast, the isolation frequency range for passive dampers is completely independent of ξ. Furthermore, the results show that for sufficiently large ξ, semiactive dampers are able to provide isolation at all frequencies. This feature is useful for many applications, particularly for sensitive machinery that cannot tolerate any overshoot in power- up or power-down, and...
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The main purpose of this study is to identify a suitable control method for semi-active tuned vibration absorbers (TVAs) in structural vibration applications. Four control policies are considered. The semi-active control schemes include... more
The main purpose of this study is to identify a suitable control method for semi-active tuned vibration absorbers (TVAs) in structural vibration applications. Four control policies are considered. The semi-active control schemes include the following: velocity-based, on-off groundhook control (on-off VBG); velocity-based, continuous groundhook control (continuous VBG); displacement-based, on-off groundhook control (on-off DBG); and displacement-based, continuous groundhook control (continuous DBG). A force-excited model that can be representative of many structural systems is adapted as the baseline model for our analysis. Each of the control policies is applied to the baseline model coupled with a TVA. In order to equally evaluate the control policies, the TVA parameters are optimized according to each policy using numerical optimization techniques. The optimal design parameters are obtained based on minimization of peak transmissibility. The performances of each of the optimized cases are then compared along with the equivalent passive model using the peak transmissibility criteria. The results indicate that all of the semi-active peak transmissibilities are lower than those of the passive, implying that the semi-active TVAs are more effective in reducing vibration levels. The results further indicate that on-off DBG performs the best among the considered control polices.