Nader Vahdati

Mathematical Model of Drum-type MR Brakes using Herschel-Bulkley Shear Model:

Most of the commercially available magnetorheological (MR) fluids are only tested up to 1200 1/s shear rates but with no magnetic field. Data are rarely available at high shear rates with magnetic field applied. In most of the applications where MR fluids are used, such as MR rotary brakes or MR translational dampers, the shear rates can be in the order of thousands and in some applications, the shear rates could be in the order of ten thousands (1/s) and higher. At these high shear rates, most MR fluids will be shear thinning and Bingham model will be inappropriate to use. The focus of this study is on the mathematical modeling of a drum-type MR rotary brake using the Herschel-Bulkey model.

Transient Dynamics of Semiactive Suspensions with Hybrid Control

The effect of hybrid control technique on transient dynamics of semiactive vehicle suspensions is studied analytically, using a quarter-car model. Hybrid semiactive control refers to a technique that mathematically combines the commonly used skyhook and groundhook control methods. The transient analysis includes an evaluation of the effect of hybrid control on different aspects of the suspension dynamics, vehicle body response, and axle dynamics, due to a step input at the wheel. The damper force–velocity trajectories, vehicle body and axle response, and vehicle body jerk are among the metrics that are examined as part of this analysis. The results of the study show that several tradeoffs exist between the transient response of the vehicle body and the axle. The tradeoffs can be adjusted through changing the hybrid control gain, which shifts the emphasis on skyhook and groundhook controls. The results further indicate that, in balance, hybrid control can be more effective in controlling suspension dynamics than either skyhook or groundhook control.

A dual-reciprocity boundary element method for a class of elliptic boundary value problems for non-homogeneous anisotropic media

A dual-reciprocity boundary element method is proposed for the numerical solution of a two-dimensional boundary value problem (BVP) governed by an elliptic partial differential equation with variable coefficients. The BVP under consideration has applications in a wide range of engineering problems of practical interest, such as in the calculation of antiplane stresses or temperature in non-homogeneous anisotropic media. The proposed numerical method is applied to solve specific test problems.

Hybrid sliding mode control of semi-active suspension systems

In order to design a controller which can take both ride comfort and road holding into consideration, a hybrid model reference sliding mode controller (HMRSMC) is proposed. The controller includes two separate model reference sliding mode controllers (MRSMC). One of the controllers is designed so as to force the plant to follow the ideal Sky-hook model and the other is to force the plant to follow the ideal Ground-hook model; then the outputs of these two controllers are linearly combined and applied to the plant as the input. Also, since the designed controller requires a knowledge of the terrain input, this input is approximated by the unsprung mass displacement. Finally, in the simulation section of this study, the effect of the relative ratio between the two MRSMCs and the knowledge of the terrain on the performance of the controller is numerically investigated for both steady-state and transient cases.

MR-fluid yield surface determination in disc-type MR rotary brakes

Magneto-rheological (MR) fluids are currently attracting a great deal of attention because of their unique rheological behavior. Many devices have been designed using MR fluids, and of potential interest here are disc-type MR rotary brakes. The plug flow region in MR devices is defined as the region where the fluid is not flowing. The plug flow region plays an important role in design and analysis of MR devices. In MR dampers, the damping coefficient is a function of the plug thickness. In MR valves, the plug thickness is used to control the flow rate through, and the pressure drop across, the MR valve. A MR clutch is performing at the highest efficiency when the entire MR gap is the plug region. For an MR rotary brake, the highest restraining torque is obtained when the entire gap is the plug region as far as there are no wall slip effects. In this paper, using the Bercovier and Engelman constitutive model, the MR fluid flow in disc-type MR brakes is modeled to determine the plug flow region. The resulting system of equations is solved numerically. It is shown that the existence of a plug flow region in the brake will affect the control torque ratio. Better estimation of the plug flow region results in better estimation of the viscous torque.

Single Pumper Semi-Active Fluid Mount

Passive fluid mounts have been in use in the automotive and aerospace applications for the purpose of cabin noise and vibration reduction since 1940s. Cabin noise and vibration isolation is provided at a frequency coined “notch frequency”. The design location of the notch frequency depends on the application, but with most applications, it is designed to coincide with the longest period of constant speed. To obtain greater cabin noise and vibration reduction at any desired frequency, the notch frequency needs to be as close to that desired frequency as possible. Unfortunately, due to tolerances on all the fluid mount dimensions, material property variations, and variation in elastomer molding processes, the notch frequency never ends up at the right location on the first manufacturing pass. Since none of the passive fluid mount parameters are controllable, the only way to tune the mount is to redesign the mount by changing fluid, changing inertia track length or diameter, or changing rubber stiffness. This trial and error manufacturing process is very costly. To reduce the fluid mount notch frequency tuning cycle time, a new fluid mount design is proposed. In this paper, the new design concept, and its mathematical model and simulation results will be presented.© 2003 ASME

ANALYTICAL RANDOM VIBRATION ANALYSIS OF BOUNDARY-EXCITED THIN RECTANGULAR PLATES

Fatigue life, stability and performance of majority of the structures and systems depend significantly on dynamic loadings applied on them. In many engineering cases, the dynamic loading is random vibration and the structure is a plate-like system. Examples could be printed circuit boards or jet impingement cooling systems subjected to random vibrations in harsh military environments. In this study, the response of thin rectangular plates to random boundary excitation is analytically formulated and analyzed. In the presented method, closed-form mode shapes are used and some of the assumptions in previous studies are eliminated; hence it is simpler and reduces the computational load. In addition, the effects of different boundary conditions, modal damping and excitation frequency range on dynamic random response of the system are studied. The results show that increasing both the modal damping ratio and the excitation frequency range will decrease the root mean square acceleration and the maximum deflection of the plate.

Design and Analysis of Shock and Random Vibration Isolation of Operating Hard Disk Drive in Harsh Environment

An effective vibration isolation system is important for hard disk drives (HDD) used in a harsh mechanical environment. This paper describes how to design, simulate, test and evaluate vibration isolation systems for operating HDD subjected to severe shock and random vibrations based on military specifications MIL-STD-810E. The well-defined evaluation criteria proposed in this paper can be used to effectively assess the performance of HDD vibration isolation system. Design concepts on how to achieve satisfactory shock and vibration isolation for HDD are described. The concepts are tested and further enhanced by the two design case studies presented here. It is shown that an effective vibration isolation system, that will allow a HDD to operate well when subjected to severe shock and random vibration, is feasible.

Variable Volumetric Stiffness Fluid Mount Design

Passive fluid mounts are commonly used in the automotive and aerospace applications to isolate the cabin from the engine noise and vibration. Due to manufacturing and material variabilities, no two identical fluid mount designs act the same. So, fluid mounts are tuned one by one before it is shipped out to customers. In some cases, for a batch of fluid mounts manufactured at the same time, one is tuned and the rest is set to the same settings. In some cases they are shipped as is with its notch frequency not being in its most optimum location. Since none of the passive fluid mount parameters are controllable, the only way to tune the mount is to redesign the mount by changing fluid, changing inertia track length or diameter, or changing rubber stiffness. This trial and error manufacturing process is very costly. To reduce the fluid mount notch frequency tuning cycle time, a new fluid mount design is proposed. In this new fluid mount design, the notch frequency can be easily modified without the need for any redesigns. In this paper, the new design concept, and its mathematical model and simulation results will be presented.

A new hydraulic engine mount design without the peak frequency

Hydraulic engine mounts are generally applied to the aerospace and automotive applications for the purpose of cabin noise and vibration reduction. By careful selection of mount design parameters, at a certain frequency called the notch frequency, the dynamic stiffness will be smaller than the static stiffness. Following the notch frequency there will be a frequency, called the peak frequency, where the dynamic stiffness is much higher than the static stiffness. Increase in the dynamic stiffness above the static stiffness is not desirable because of the increase in transmitted force to the airframe or the chassis. Here in this paper, a new hydraulic engine mount design is proposed that uses two working fluids. This new design has two notch frequencies and two peak frequencies. Under special conditions, the peak frequency can be eliminated. As a result, one can obtain a hydraulic engine mount design with only one notch frequency but no peak frequency, by using a controllable fluid or a high viscosity fluid as one of the working fluids. The new hydraulic engine mount design, its mathematical model, and some discussions on the simulation results are presented.