Abstract

Drivers of commercial off-road vehicles are generally exposed to high magnitudes of multi-directional Whole-body vibration (WBV), which predominate in relatively low-frequency range (up to 10 Hz). Low natural frequency suspension seats are widely used to reduce the WBV exposure along the predominant vertical axis. WBV at the driver-seat interface has been associated with potential risk of lower back injuries, discomfort and reduced work efficiency among the occupational vehicle drivers. The vibration attenuation performance of a suspension seat is strongly affected by the kinematic and dynamic properties of the suspension system, visco-elastic properties of the cushion and the seated body biodynamics, apart from the nature of the vehicle vibration. Minimal efforts are done so far to improve the seat suspension performance exposed to different spectral classes of excitation defined in ISO-7096 neglecting the effect of occupant biodynamics. A thorough evaluation of suspension seat and the contribution of components influencing suspension performance is required to solve these problems and take into account the combined influence of suspension dynamics and human body biodynamics.
This dissertation research focuses on improving the vibration isolation performance of vehicle-specific seat suspension by developing occupant-seat suspension model in a Multi-Body Dynamic (MBD) environment which takes into account the suspension kinematics, contribution of suspension component dynamics, elastic motion-limiters and human body biodynamics by identifying vehicle-specific optimal suspension parameters. A kineto-dynamic model was developed in the ADAMS/View MBD environment and the validation of the model is verified by performing extensive laboratory measurements on a selected suspension seat with a test subject with WN input excitation ranging from 0.25-1 m/s2 rms and selected spectral class for EM2.
The parameters of the suspension components such as pneumatic spring connected to auxiliary volume, damper and cushion were identified by characterizing the static and dynamic behavior of these components independently and by formulating suitable mathematical models. The validated model was further simulated to identify the effect of auxiliary volume, seated body mass, ride height on the suspension resonance, rms acceleration and peak relative travel. It is concluded that the suspension responses were sensitive to the auxiliary volume and co-ordinates of the suspension components accounting to the suspension dynamics and overall seat suspension performance. Optimal suspension component parameters were then established by identifying suitable component parameters such as an increase in the auxiliary volume of the pneumatic spring by 12% and damping constants along with their respective co-ordinates to maintain constant suspension seat resonance at 1.27Hz independent of the seated body mass. The suggested model parameters showed improved vibration isolation performance by minimizing SEAT values from 9% to 30.6% depending on the input excitation of spectral classes.
Subsequently, a seat suspension model is formulated to identify the influence of air spring mount and its effective stiffness considering occupant biodynamics. Laboratory measurements are performed with different air spring mount angles to characterize the air spring at different angles. The measured data is used to validate with the developed model and further used to obtain optimal air spring mount angle to minimize the suspension resonant frequency irrespective of the seated body mass.
The contribution of seat cushion has been significant providing comfort to the occupant at various driving conditions. Lack of knowledge of the static and dynamic behavior of visco-elastic properties of seat cushion and the availability of suitable material models in the Finite element environment formulated a scope of research. A novel and preliminary attempt to develop a dynamic Finite Element (FE) model is demonstrated in LS DYNA environment taking into account the dynamic forces experienced by the seat cushion in an operating environment. The validation of the developed FE model was demonstrated by performing laboratory measurements to characterize the cushion behavior at different preloads, excitation frequency and amplitude.
Subsequently, the developed FE seat cushion model was simulated to obtain seat pressure distribution using a dummy and its validation was demonstrated by performing measurements using a single test subject with a pressure mat sensor. The peak pressures forces at contact interface between the occupant and seat cushion under Ischial Tuberosity (IT) are compared. Lowering the peak pressures and increase in the area of pressure distribution can improve comfort of the occupant where prolonged seating in a vibrating environment is inevitable. A multi-layer seat cushion model is suggested to improve the seat pressure distribution in order to minimize the peak pressures and increase the contact area between the occupant and seat cushion.