Department of Mechanical Engineering

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https://hdl.handle.net/10919/23261

The Virginia Tech Mechanical Engineering Department serves its students, alumni, the Commonwealth of Virginia, and the nation through a variety of academic, research and service activities. Our missions are to: holistically educate our students for professional leadership as creative problem-solvers in a diverse society, conduct advanced research for societal advancement, train graduate students for scholarly inquiry, and engage with alumni, industry, government, and community partners through outreach activities. In order to produce engineers prepared for success across a range of career paths, our academic program integrates training in engineering principles, critical thinking, hands-on projects, open-ended problem solving, and the essential skills of teamwork, communication, and ethics.

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Achieving anti-roll bar effect through air management in commercial vehicle pneumatic suspensions
Chen, Yang; Peterson, Andrew W.; Ahmadian, Mehdi (Taylor & Francis, 2019-12-02)
This paper introduces the concept of managing air in commercial vehicle suspensions for reducing body roll. A conventional pneumatic suspension is re-designed to include higher-flow air hoses and dual levelling valves for improving the dynamic response of the suspension to the body roll, which commonly happens at relatively low frequencies. The improved air management allows air to get from the air tank to the airsprings quicker, and also changes the side-to-side suspension air pressure such that the suspension forces can more readily level the vehicle body, much in the same manner as an anti-roll bar (ARB). The results of a multi-domain simulation study in AMESim and TruckSim indicate that the proposed suspension configuration is capable of providing balanced airflow to the truck’s drive-axle suspensions, resulting in balanced suspension forces in response to single lane change and steady-state cornering steering maneuvers. The simulation results further indicate that a truck equipped with the reconfigured suspension experiences a uniform dynamic load sharing, smoother body motion (less roll angle), and improved handling and stability during steering maneuvers commonly occurring in commercial trucks during their intended use.
Comparative Analysis of Emergency Evasive Steering for Long Combination Vehicles
Chen, Yang; Zhang, Zichen; Ahmadian, Mehdi (SAE International, 2020-10-10)
This study provides a simulation-based comparative analysis of the distance and time needed for long combination vehicles (LCVs) - namely, A-doubles with 28-, 33-, and 48-ft trailers - to safely exercise an emergency, evasive steering maneuver such as required for obstacle avoidance. The results are also compared with conventional tractor-semitrailers with a single 53-ft trailer. A multi-body dynamic model for each vehicle combination is developed in TruckSim® with an attempt to assess the last point to steer (LPTS) and evasive time (ET) at various highway speeds under both dry and wet road conditions. The results indicate that the minimum avoidance distance and time required for the 28-ft doubles vary from 206 ft (60 mph) to 312 ft (80 mph) and 2.3 s to 2.6 s, respectively. The required LPTS represents a 6% to 31% increase when compared with 53-ft semitrucks. When driving below 76 mph on a dry road and below 75 mph on a wet road, the 28-ft doubles exhibit LPTS and ET that are larger than 33-ft doubles. In addition, the 33-ft doubles exhibit larger LPTS and ET than 48-ft doubles for the highway speeds considered. This is mainly attributed to the longer trailer wheelbase that causes smaller rear trailer amplifications. At speeds higher than 76 mph on dry roads and 75 mph on wet roads, however, an opposite trend is observed. As the trailer length increases, the distance and time needed to safely avoid an obstacle also increase. A comparison between dry and wet road conditions is also conducted, with the results indicating that more time and distance would be needed for obstacle avoidance on wet roads.
Countering the Destabilizing Effects of Shifted Loads through Pneumatic Suspension Design
Chen, Yang; Ahmadian, Mehdi (SAE International, 2019-11-08)
This article proposes a novel approach to reduce the destabilizing impacts of the shifted loads of heavy trucks (due to improper loading or liquid slosh) by pneumatic suspension design. In this regard, the pneumatically balanced suspension with dual leveling valves is introduced, and its potential for the improvement of the body imbalance due to the shifted load is determined. The analysis is based on a multi-domain model that couples the suspension fluid dynamics, shifted-load impacts, and tractor-semitrailer dynamics. Truck dynamics is simulated using TruckSim, which is integrated with the pneumatic suspension model developed in AMESim. This yields a reasonable prediction of the effect of the suspension airflow dynamics on vehicle dynamics. Moreover, the ability of the pneumatic suspension to counteract the effects of two general shifted loads - static (rigid cargo) and dynamic (liquid) - is studied. The simulation results indicate that the dual-leveling-valve suspension results in a reduction in roll angle and roll rate of the vehicle body for both static and dynamic load-shifting cases, as compared to the conventional single-leveling-valve suspension. Suppression of the liquid sloshing behavior is obtained by the truck with the dual-leveling-valve suspension. Furthermore, the co-simulation platform established in the study is useful for efficient and accurate analyses of the coupled shifted load-pneumatic suspension-vehicle system dynamics.
Design and Implementation of a Redress System for Roller Rig
Jones, Dwight W.; Ahmadian, Mehdi (Virginia Tech, 2022-12-16)
The Virginia Tech – Federal Railroad Administration Roller Rig at the Center for Vehicle Systems and Safety is a state of the art facility used to evaluate the wheel-rail contact mechanics and dynamics of railway vehicles and track. The Roller Rig consists mainly of an upper wheel and a lower roller that serve to simulate a railway vehicle and track respectively. Upon the completion of testing using the Roller Rig, a section of roughness is created on the surface of the wheel as a product of the two surfaces coming into contact with each other. In order to ensure data accuracy between different tests, the rough surface of the wheel must be smoothed down in between different tests and experiments performed on the Roller Rig. Initially, this process of smoothing the wheel was done completely manually, with the operator fixing a piece of sandpaper around a plastic tube and physically holding it contact while the wheel spins. Quite obviously, this manual process introduces issues of consistency and safety of the user, considering the Roller Rig is a device capable of outputting several hundred pounds of force. Therefore, in order to solve the problems that the manual process introduced, a project that converted the entirely manual process to that of a semi-automatic process of redressing. Therefore, in the Spring of 2021, the first version of the Roller Rig Redresser was constructed and reported on. However, due to the problems that the first version had, it was unable to be used for redressing on a reliable and consistent basis. Therefore, over the Fall 2022 semester, a second version of the Roller Rig Redresser was constructed and reported on, focusing on directly improving the shortcomings of the manual process and the aforementioned first redresser version.
Design and Operational Assessment of a Railroad Track Robot for Railcar Undercarriage Condition Inspection
Kasch, James; Ahmadian, Mehdi (MDPI, 2024-07-10)
The operational effectiveness of a railroad track robot that is designed for railcar undercarriage inspection is provided. Beyond describing the robot’s design details and onboard imaging system, the paper analyzes the recorded video images and offers design improvements to increase their clarity. The robot is designed to be deployed trackside, traverse over the rails, and then maneuver in between the rails beneath a stopped train in a siding or a railyard. The under-carriage conditions are documented by onboard video cameras for automated or manual postprocessing. The intent is to inspect the components that are not visible to the conductor or train inspector during a walk-along inspection of a stationary train. An assessment of the existing design, followed by modification and validation, is presented. The results from a prototype unit developed by the Railway Technologies Laboratory at Virginia Tech (RTL) indicate that with proper positioning of off-the-shelf imaging systems such as cameras manufactured by GoPro® in San Mateo, CA, USA and appropriate lighting, it is possible to capture videos that are sufficiently clear for manual (by a railroad engineer), semi-automated, or fully automated (using Artificial Intelligence or Machine Learning methods) inspections of rolling stock undercarriages. Additionally, improvements to the control, mobility, and reliability of the system are documented, although reliability throughout operation and the ability to consistently climb out of the track bed remain points of future investigation.
Detailed Modeling of Pneumatic Braking in Long Combination Vehicles
Zhang, Zichen; Sun, Nan; Chen, Yang; Ahmadian, Mehdi (SAE International, 2021-08-23)
A detailed model for pneumatic S-cam drum brake systems is developed and integrated into a multibody dynamic model for a 33-ft A-double long combination vehicle (LCV). The model, developed in TruckSim®, is used to study the dynamics of LCVs during straight-line braking at various speeds. It includes the response delay in braking that occurs from the time of application to when the brakes are applied at the drum for all axles. Additionally, the model incorporates an accurate characterization of brake torque versus chamber pressure at different speeds, along with the anti-lock brake system (ABS) dynamics, to yield an accurate prediction of the vehicle's deceleration during braking. The modeling results are compared with test results at speeds ranging from 20 mph to 65 mph on dry pavement. A close match between the model's prediction and test results is observed. The model is then used to perform a parametric study that evaluates braking distance and time for different pavement coefficients of friction (μp) at various speeds. The results indicate a distinct nonlinear relationship between μp and braking dynamics. At various μp, stopping time increases linearly with speed, as perhaps expected. Stopping distance, however, increases nonlinearly for a larger μp and linearly for a smaller μp versus speed. At a given speed, stopping time increases nonlinearly with a reduced μp, whereas stopping distance increases relatively linearly with a reduced μp.
Enhancing Autonomous Vehicle Navigation with a Clothoid-Based Lateral Controller
Shaju, Aashish; Southward, Steve; Ahmadian, Mehdi (MDPI, 2024-02-22)
This study introduces an advanced lateral control strategy for autonomous vehicles using a clothoid-based approach integrated with an adaptive lookahead mechanism. The primary focus is on enhancing lateral stability and path-tracking accuracy through the application of Euler spirals for smooth curvature transitions, thereby reducing passenger discomfort and the risk of vehicle rollover. An innovative aspect of our work is the adaptive adjustment of lookahead distance based on real-time vehicle dynamics and road geometry, which ensures optimal path following under varying conditions. A quasi-feedback control algorithm constructs optimal clothoids at each time step, generating the appropriate steering input. A lead filter compensates for the vehicle’s lateral dynamics lag, improving control responsiveness and stability. The effectiveness of the proposed controller is validated through a comprehensive co-simulation using TruckSim^® and Simulink^®, demonstrating significant improvements in lateral control performance across diverse driving scenarios. Future directions include scaling the controller for higher-speed applications and further optimization to minimize off-track errors, particularly for articulated vehicles.
Experimental Evaluation of Effect of Leaves on Railroad Tracks in Loss of Braking
Kumar, Nikhil; Radmehr, Ahmad; Ahmadian, Mehdi (MDPI, 2024-04-29)
This study aims to comprehensively assess the lubrication effect of leaves on wheel–rail contact dynamics using the Virginia Tech-Federal Railroad Administration (VT-FRA) Roller Rig, which closely simulates field conditions with precision and repeatability. Railway operators grapple with the seasonally recurring challenge of leaf contamination, which can cause partial loss of braking and lead to undesired events such as station overruns. Better understanding the adhesion-reducing impact of leaf contamination significantly improves railway engineering practices to counter their effects on train braking and traction. This experimental study evaluates the reduction in traction and braking forces (collectively called “adhesion”) as a function of leaf volume, using two leaf species that commonly grow along U.S. railroad tracks. The test methods rely on the chosen leaves’ transpiration characteristics while ensuring the result’s reproducibility. Leaves were symmetrically positioned on the wheel surface, centered around the mid-rib area within the wear band, and taped on the edges far from the wear band. The critical test parameters (i.e., wheel load, wheel velocity, and percentage creepage) are kept constant among the tests. At the same time, leaf volume is reduced from a maximum amount that covers the entire wheel surface (100% coverage) to no leaves (0%). The latter is used as the baseline. The percentage creepage is kept constant at an exaggerated amount of 2% to accelerate the test time. The results indicate a nonlinear relationship between leaf volume and the loss of braking. Even a small amount of leaf contamination causes a significant reduction in adhesion by as much as 50% compared with no contamination (i.e., baseline). Increasing leaf volume results in contact saturation, beyond which adhesion is not reduced. The minimum adhesion observed in this study is 20% of the maximum adhesion that occurs when no leaf contamination is present.
Extended Viterbi Algorithm for Hidden Markov Process: A Transient/Steady Probabilities Approach
Soltan, Reza A.; Ahmadian, Mehdi (Hikari Ltd., 2012)
In this paper an extended Viterbi algorithm is presented for first-order hidden Markov processes, with the help of a dummy combined state sequence. For this, the Markov switching’s transient probabilities and steady probabilities are studied separately. The algorithm gives a maximum likelihood estimate for the state sequence of a hidden Markov process. Comparing with the standard Viterbi algorithm, this method gives a higher maximum likelihood, and also picks up the state switching earlier, which is particularly important for the out of sample applications. The theory of this method is discussed in this paper and then a sample of a series of experiment is presented to illustrate the theory. A quantitative comparison is also given between this method and the standard Viterbi algorithm.
Failure mode and effects analysis of dual levelling valve airspring suspensions on truck dynamics
Chen, Yang; Hou, Yunbo; Peterson, Andrew W.; Ahmadian, Mehdi (Taylor & Francis, 2019-04-03)
Failure mode and effects analysis are performed for a dual levelling valve pneumatic suspension to determine the effect of suspension failure on tractor–semi-trailer dynamics, using a detailed model of suspension pneumatics coupled with a truck dynamic model. A key element of failure analysis in suspensions with one or two levelling valves is determining the effect on the vehicle body roll when one or more failures occur. The failure modes considered are mainly the suspension pneumatic components, including clogged levelling valve, bent control rod, disabled lever arm, and punctured or leaking connectors and pipes. The pneumatic suspension is modelled in AMESim, with critical parameters established through component testing. Upon validating the AMESim component model experimentally, the pneumatic suspension model is integrated into TruckSim for studying the consequences of suspension failure on truck dynamics. The simulation results indicate that the second levelling valve in a dual-valve arrangement brings a certain amount of failure redundancy to the system, in the sense that when one side fails, the other side can compensate for the failure. Equipping the trailer with dual levelling valves brings an additional stabilising effect to the vehicle in the event of tractor suspension failure.
In-Motion, Non-Contact Detection of Ties and Ballasts on Railroad Tracks
Mirzaei, S. Morteza; Radmehr, Ahmad; Holton, Carvel; Ahmadian, Mehdi (MDPI, 2024-09-30)
This study aims to develop a robust and efficient system to identify ties and ballasts in motion using a variety of non-contact sensors mounted on a robotic rail cart. The sensors include distance LiDAR sensors and inductive proximity sensors for ferrous materials to collect data while traversing railroad tracks. Many existing tie/ballast health monitoring devices cannot be mounted on Hyrail vehicles for in-motion inspection due to their inability to filter out unwanted targets (i.e., ties or ballasts). The system studied here addresses that limitation by exploring several approaches based on distance LiDAR sensors. The first approach is based on calculating the running standard deviation of the measured distance from LiDAR sensors to tie or ballast surfaces. The second approach uses machine learning (ML) methods that combine two primary algorithms (Logistic Regression and Decision Tree) and three preprocessing methods (six models in total). The results indicate that the optimal configuration for non-contact, in-motion differentiation of ties and ballasts is integrating two distance LiDAR sensors with a Decision Tree model. This configuration provides rapid, accurate, and robust tie/ballast differentiation. The study also facilitates further sensor and inspection research and development in railroad track maintenance.
Isolation Properties of Low-Profile Magnetorheological Fluid Mounts
Ahmadian, Mehdi; Southern, Brian M. (MDPI, 2021-04-19)
This study evaluates the stiffness and damping characteristics of low-profile magnetorheological (MR) fluid mounts (MRFM) to provide a better understanding of the vibration improvements offered by such mounts, as compared with conventional elastomeric mounts. It also aims at assessing how much of the mount’s performance is due to the MR fluid and how much is due to the elastomer and steel insert that is used in MRFM. The study includes the design, analysis, fabrication, and testing of a unique class of MRFM that is suitable for the isolation of sensitive machinery and sensors. The MR fluid is compressed (squeezed) in response to dynamic force applied to the mount. The test results are compared with conventional elastomeric (rubber) mounts of the same configuration as MRFM, to highlight the changes in stiffness and damping characteristics for frequencies ranging from 1 to 35 Hz. With no current supplied, the MRFM has a slightly higher stiffness and nearly the same damping as a conventional rubber mount. The slight increase in MRFM stiffness is attributed to the MR fluid’s compressive stiffness, which is higher than the rubber. When current is supplied to the MRFM, the stiffness and damping increase significantly at lower frequencies and taper off to nearly the same level as the rubber mount at higher frequencies. Both the stiffness and damping are directly proportional to the supplied current. At the maximum current of 2 A, the MRFM has 200% higher stiffness and 700% higher damping than the rubber mount. The significantly higher damping and stiffness and the tapering off to nearly the same level as the rubber mount is quite interesting and intriguing. It indicates that MRFM delivers high damping and stiffness when needed, while significantly tapering them off when high damping and stiffness are not desirable.
Mechanical Design & Fabrication of Redressing System for the VT-FRA Roller Rig
Jones, Dwight W.; Ahmadian, Mehdi; Molzon, Michael (Virginia Tech, 2022-01-31)
At the Center for Vehicle Systems and Safety at Virginia Tech, the Virginia Tech – Federal Railroad Administration Roller Rig is a state of the art facility used in evaluating the wheel-rail mechanics and dynamics of railway vehicle and track. Consisting of an upper wheel, a lower roller, and multiple actuators/servos, the Roller Rig is fitted with sensors that measure different parameters from force, rotation speed, and more. However, while the Roller Rig is a fitting device for a multitude of scientific and engineering studies related to railway vehicles, wheel-rail interaction while the Roller Rig is in operation, creates a rough wear band that is visible both to the naked eye, and on the micron level with the use of laser scanners. With the presence of this wear band, before the start of each experiment, a process known as redressing must be performed to smooth the surface of the wheel. This process is necessary to promote data precision and consistency throughout experiments. Previously, this process was done entirely manually, with the individual wrapping various grades of sandpaper around a common PVC pipe and applying pressure while the Roller Rig’s wheel spins. This is done with a total of 5 grades of sandpaper, varying from 40 grit up to 220 grit. As one can imagine, this process is incredibly labor intensive and presents many concerns related to safety, worker productivity, and overall precision and consistency of the redressing process. This paper will look at the development of a device to perform this redressing process in a semi-automatic, hands-off manner. Through initial brainstorming and concept generation to fabrication methods and manufacturing, this paper looks at the different the Roller Rig Redresser project throughout the entire engineering design process.
Nonlinear vibrations of microcantilevers subjected to tip-sample interactions: Theory and experiment
Delnavaz, Aidin; Mahmoodi, S. Nima; Jalili, Nader; Ahmadian, Mehdi; Zohoor, Hassan (American Institute of Physics, 2009-12-01)
Improvement of microcantilever-based sensors and actuators chiefly depends on their modeling accuracy. Atomic force microscopy (AFM) is the most widespread application of microcantilever beam as a sensor, which is usually influenced by the tip-sample interaction force. Along this line of reasoning, vibration of AFM microcantilever probe is analyzed in this paper, along with analytical and experimental investigation of the influence of the sample interaction force on the microcantilever vibration. Nonlinear integropartial equation of microcantilever vibration subject to the tip-sample interaction is then derived and multiple time scales method is utilized to estimate the tip amplitude while it is vibrating near the sample. A set of experiments is performed using a commercial AFM for both resonance and nonresonance modes, and the results are compared with the theoretical results. Hysteresis, instability and amplitude drop can be identified in the experimental curves inside the particle attraction domain. They are likely related to the interaction force between the tip and sample as well as the ever-present water layer during the experiments. A fair agreement is observed between the theoretical simulations and experimental findings, which obviously demonstrates the effectiveness and applicability of the developed model.
A Novel Double-Piston Magnetorheological Damper for Space Truss Structures Vibration Suppression
Wang, Qiang; Ahmadian, Mehdi; Chen, Zhaobo (Hindawi, 2014-07-22)
The design, fabrication, and testing of a new double-piston MR damper for space applications are discussed. The design concept for the damper is described in detail. The electromagnetic analysis of the design and the fabrication of the MR damper are also presented. The design analysis shows that the damper meets the weight and size requirements for being included in a space truss structure. The prototype design is tested in a damper dynamometer. The test results show that the damper can provide nearly 80 N of damping force at its maximum velocity and current. The test results also show that the seal drag could contribute significantly to the damping forces. Additionally, the test results indicate that both the work by the damper and damping force increase rapidly with increasing current at lower currents and taper off at higher currents as the damper starts to saturate. The damper force versus velocity plots show hysteresis in both pre- and postyield regions and asymmetric forces in jounce and rebound. A model is proposed for representing the force-displacement, force-velocity, and asymmetric forces observed in test results. A comparison of the modeling results and test data indicates that the model accurately represents the force characteristics of the damper.
PID-Based Longitudinal Control of Platooning Trucks
Shaju, Aashish; Southward, Steve; Ahmadian, Mehdi (MDPI, 2023-12-05)
This article focuses on the development and assessment of a PID-based computationally cost-efficient longitudinal control algorithm for platooning trucks. The study employs a linear controller with a nested architecture, wherein the inner loop regulates relative velocities while the outer loop governs inter-vehicle distances within platoon vehicles. The design of the proposed PID controller entails a comprehensive focus on system identification, particularly emphasizing actuation dynamics. The simulation framework used in this study has been established through the integration of TruckSim^® and Simulink^®, resulting in a co-simulation environment. Simulink^® serves as the platform for control action implementation, while TruckSim^® simulates the vehicle’s dynamic behavior, thereby closely replicating real world conditions. The significant effort in fine-tuning the PID controller is described in detail, including the system identification of the linearized longitudinal dynamic model of the truck. The implementation is followed by an extensive series of simulation tests, systematically evaluating the controller’s performance, stability, and robustness. The results verify the effectiveness of the proposed controller in various leading truck operational scenarios. Furthermore, the controller’s robustness to large fluctuations in road grade and payload weight, which is commonly experienced in commercial vehicles, is evaluated. The simulation results indicate the controller’s ability to compensate for changes in both road grade and payload. Additionally, an initial assessment of the controller’s efficiency is conducted by comparing the commanded control efforts (total torque on wheels) along with the total fuel consumed. This initial analysis suggests that the controller exhibits minimal aggressive tendencies.
Pneumatically Balanced Heavy Truck Air Suspensions for Improved Roll Stability
Chen, Yang; Ahmadian, Mehdi; Peterson, Andrew (SAE International, 2015-01-01)
This study provides a simulation evaluation of the effect of maintaining balanced airflow, both statically and dynamically, in heavy truck air suspensions on vehicle roll stability. The model includes a multi-domain evaluation of the truck multi-body dynamics combined with detailed pneumatic dynamics of drive-axle air suspensions. The analysis is performed based on a detailed model of the suspension's pneumatics, from the main reservoir to the airsprings, of a new generation of air suspensions with two leveling valves and air hoses and fittings that are intended to increase the dynamic bandwidth of the pneumatic suspensions. The suspension pneumatics are designed such that they are able to better respond to body motion in real time. Specifically, this study aims to better understand the airflow dynamics and how they couple with the vehicle dynamics. The pneumatic model is coupled with a roll-plane model of the truck to evaluate the effect of the suspension pneumatic dynamics on the body roll, as well as the force transmission to the sprung mass. The results of the study show that maintaining a balanced airflow through the suspension improves the dynamic responsiveness of the suspension to steering, causing less body roll.
Quantifying the effect of roadway, driver, vehicle, and location characteristics on the frequency of longitudinal and lateral accelerations
Ali, Gibran; McLaughlin, Shane B.; Ahmadian, Mehdi (Pergamon-Elsevier, 2021-10-01)
The purpose of this study is to understand and quantify the simultaneous effects of roadway speed category, driver age, driver gender, vehicle class, and location on the rates of longitudinal and lateral acceleration epochs. The rate of usual as well as harsh acceleration epochs are used to extract insights on driving risk and driver comfort preferences. However, an analysis of acceleration rates at multiple thresholds incorporating various effects while using a large-scale and diverse dataset is missing. This analysis will fill this research gap. Data from the 2nd Strategic Highway Research Program Naturalistic Driving Study (SHRP2 NDS) was used for this analysis. The rate of occurrence of acceleration epochs was modeled using negative binomial distribution based generalized linear mixed effect models. Roadway speed category, driver age, driver gender, vehicle class, and location were used as the fixed effects and the driver identifier was used as the random effect. Incidence rate ratios were then calculated to compare subcategories of each fixed effect. Roadway speed category has the strongest effect on longitudinal and lateral accelerations of all magnitudes. Acceleration epoch rates consistently decrease as the roadway speed category increases. The difference in the rates depends on the threshold and is up to three orders of magnitude. Driver age is another significant factor with clear trends for longitudinal and lateral acceleration epochs. Younger and older drivers experience higher rates of longitudinal accelerations and decelerations. However, the rate of lateral accelerations consistently decreases with age. Vehicle class also has a significant effect on the rate of harsh accelerations with minivans consistently experiencing lower rates.
Rail Surface Measurement And Multi-Scale Modeling Of Wheel-Rail Contact
Alemi, Mohammad Mehdi; Taheri, Saied; Ahmadian, Mehdi (ASME, 2016-01-01)
In railroad industries, one of the most important concepts is the ability to model and estimate the friction between the rail and the wheels. Overall, creating a general friction model is a challenging task because friction is influenced by different factors such as surface metrology, properties of materials in contact, surface contamination, flash temperature, normal load, sliding velocity, surface deformation, inter-surface adhesion, etc. Moreover, increase in the number of interfering factors in the process would add to the complexity of the friction model. Therefore, reliable prediction of the friction both theoretically and empirically is sensitive to how the model parameters are measured. Due to both safety and energy concerns, any attempts towards a better understanding of wheel/rail contact are considered important for the railroad industry. In this study, surface characteristics of four rail surfaces were measured at 20 microns over a rectangular area using a portable Nanovea Jr25 optical surface profilometer and the results were studied using various statistical procedures and Fractal theory. Furthermore, a 2D rectangular area was measured in this study because 1D height profile doesn’t capture all the necessary statistical properties of the surface. For surface roughness characterization, the 3D parameters such as root-mean-square (RMS) height, skewness, kurtosis and other important parameters were obtained according to ISO 25178 standard. To verify the statistical results and fractal analysis, a British Pendulum Skid Resistance Tester was used to measure the average sliding coefficients of friction based on several experiments over a 5 cm contact length for all four rail sections. The results supported this fact that the rail surface with lower fractal dimension number has the lower friction. In effect, the larger fractal dimension number simply would add more microtexture features to the contact surface which potentially increases the friction. This paper will discuss the results and the next steps towards a better understanding of the friction potential between the wheels and the track.
A Review on Eigenstructure Assignment Methods and Orthogonal Eigenstructure Control of Structural Vibrations
Rastgaar, Mohammad; Ahmadian, Mehdi; Southward, Steve (Hindawi, 2009-01-01)
This paper provides a state-of-the-art review of eigenstructure assignment methods for vibration cancellation. Eigenstructure assignment techniques have been widely used during the past three decades for vibration suppression in structures, especially in large space structures. These methods work similar to mode localization in which global vibrations are managed such that they remain localized within the structure. Such localization would help reducing vibrations more effectively than other methods of vibration cancellation, by virtue of confining the vibrations close to the source of disturbance. The common objective of different methods of eigenstructure assignment is to provide controller design freedom beyond pole placement, and define appropriate shapes for the eigenvectors of the systems. These methods; however, offer a large and complex design space of options that can often overwhelm the control designer. Recent developments in orthogonal eigenstructure control offers a significant simplification of the design task while allowing some experience-based design freedom. The majority of the papers from the past three decades in structural vibration cancellation using eigenstructure assignment methods are reviewed, along with recent studies that introduce new developments in eigenstructure assignment techniques.

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