Abstract

Articulated freight vehicles transmit high levels of whole-body ride vibration to the driver and high magnitudes of dynamic tire forces to the pavements. The high levels of ride vibration and tire forces are attributed to the excessive sizes and weights of these vehicles. The driver health and safety risks posed by ride vibrations, and the significant tire induced road damage caused by heavy vehicles have prompted a growing demand for design of driver- and road-friendly vehicles. Different active and passive suspension systems are thus analyzed to enhance the performance characteristics of articulated freight vehicles. The investigation is carried out in five phases: (i) development of a representative dynamic model of the vehicle; (ii) passive suspension design and optimization; (iii) ideal active suspension design; (iv) limited-state active suspension design; and (v) assesment of tire dynamic forces transmitted to the pavement. An articulated freight vehicle is characterized by an inplane nine degrees-of-freedom (DOF) dynamic system model. An analytical characterization of the randomly irregular road surface is presented and the time delays between the consecutive wheel inputs are incorporated using Pade approximation. The validity of the analytical vehicle model is asserted by comparing its response characteristics with the road measured data. A technique, based on covariance analysis, is employed to perform the multi-parameter sensitivity analysis and to design an "optimum" passive suspension. A performance index comprising ride quality, cargo safety, suspension rattle space and dynamic tire forces is formulated to derive the "optimum" suspension design. The effects of varying the suspension properties on the frequency response characteristics are investigated to further verify the conclusions drawn from the covariance analysis. Linear Quadratic Gaussian (LQG) control technique is employed to design an ideal fail-safe active suspension scheme based on full-state feedback. Passive damping and stiffness elements are incorporated in the active suspension system to yield a fail-safe configuration with minimal power requirement. The performance characteristics of the ideal active suspension are compared to those of the "optimum" passive suspension to determine their potential performance benefits. In view of the excessive hardware and signal processing requirements, high cost and poor reliability of a full-state feedback active suspension design, a thorough analysis of a more realistic active suspension scheme, based on H$\sb2$ synthesis, is undertaken. Two suspension schemes based upon limited-state measurements are investigated. The performance characteristics of the propose suspension designs are compared to those of the full-state active suspension and the "optimum" passive suspension systems. Finally, a comprehensive analysis is undertaken to assess the effects of the passive and various active suspension schemes on the dynamic tire forces. It is concluded that the reduced state active suspension systems yield performance characteristics comparable to those of an ideal active suspension.