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This paper proposes a new evaluation method of analyzing stability and design of a controller for an electric power steering (EPS) system. The main purpose of the EPS system’s control design is to ensure a comfortable driving experience of drivers, which mainly depends on the assist torque map. However, the high level of assist gain and its nonlinearity may cause oscillation, divergence and instability to the steering systems. Therefore, an EPS system needs to have an extra stability controller to eliminate the side effect of assist gain on system stability and attenuate the unpleasant vibration. In this paper, an accurate theoretical model is built and the method for evaluating system quality are suggested. The bench tests and vehicle experiments are carried out to verify the theoretical analysis.

The ride dynamics of typical North-American soil compactors were investigated via analytical and experimental methods. A 12-degrees-of-freedom in-plane ride dynamic model of a single-drum compactor was formulated through integrations of the models of various components such as driver seat, cabin, roller drum and drum isolators, chassis and the tires. The analytical model was formulated for the transit mode of operation at a constant forward speed on undeformable surfaces with the roller vibrator off. Field measurements were conducted to characterize the ride vibration environments during the transit mode of operation. The measured data revealed significant magnitudes of whole-body vibration of the operator-station along the vertical, lateral, pitch and roll-axes. The model results revealed reasonably good agreements with ranges of the measured vibration data.

A coupled human-seat-suspension model is developed upon integrating asymmetric and nonlinear models of the cushion, suspension and elastic end-stops with a three degrees-of-freedom biodynamic model of the occupant. The validity of the model is examined under harmonic and stochastic vibration excitations of different classes of vehicles, using the laboratory measured data. The suspension performance under continuous and shock excitations, assessed in terms of Seat Effective Amplitude Transmissibility (SEAT) and Vibration Dose Value (VDV) ratio, revealed that attenuation of continuous and shock-type excitations pose conflicting design requirements. It is thus proposed to develop suspension design for optimal attenuation of continuous vibration, while the severity of end-stop impacts caused by shock-type excitations be minimized through design of optimal buffers. Two different optimization problems are formulated to minimize the SEAT and VDV ratios.

The human body is most sensitive to low frequency whole body vibrations. Ride vibrations of off-road vehicles, caused primarily by irregular terrains, predominate in the 0.5 - 5 Hz frequency range. A suspension seat offers the simplest means to improve vehicle ride by reducing ride vibrations transmitted to the driver. A computer model of an off-road vehicle suspension seat was developed which can aid the designer in the selection of optimal suspension parameters. A parametric study was performed to determine the frequency response characteristics of the validated suspension model via computer simulation to investigate the influence of suspension parameters on the vibration transmission performance of suspension seats.

Automotive suspensions invariably exhibit asymmetric damping properties in compression and rebound, which is partly attributed to asymmetric damping and in-part to the suspension linkage kinematics together with tire lateral compliance. Although automotive suspensions have invariably employed asymmetric damping, the design guidelines and particular rationale for such asymmetry has not been explicitly defined. The influences of damper asymmetry together with the suspension kinematics and tire lateral compliance on the dynamic responses of a vehicle are investigated analytically under bump and pothole excitations, and the results are interpreted in view of potential design guidance. A quarter-car kineto-dynamic model of the road vehicle employing a double wishbone type suspension comprising a strut with linear spring and multiphase asymmetric damper is formulated for the analyses.

Human-seat interactions are investigated through measurement and analysis of distribution of interface contact force and area under vertical vibration. The time histories of dynamic ischium pressure, effective contact area and contact force on a soft seat revealed significant asymmetry, under large magnitude vibration excitations occurring near the resonant frequency of the human-seat system. The asymmetric response characteristics of the cushion are mostly attributed to the nonlinear force-deflection properties of polyurethane foam materials, contour shape of human buttocks, body-hop motion and cushion bottoming tendencies. The results are utilized to propose a nonlinear and asymmetric seat cushion model incorporating body hop motion and cushion bottoming under vertical vibration. A combined human-seat model is derived upon integrating the proposed cushion model with a bio-dynamic model of the seated occupant.