Abstract

Vehicle suspensions impose conflicting design requirements to satisfy the performance goals related to ride, handling and road holding. Semi-active damping suspensions, with their low cost and low power requirement, have been extensively investigated to achieve better compromises among different performance measures. The magneto-rheological (MR) fluid dampers offer superior potential to achieve rapid variations in the damping force and thus the wide bandwidth. The MR dampers, however, exhibit strong nonlinearities associated with force saturation and hysteresis, which affect the force-tracking performance of the controller in an adverse manner. This dissertation research focuses on characterization and modeling of the hysteretic force-velocity ( F-v ) characteristics of a MR-fluid damper, and analyses of different semi-active controller syntheses to achieve improved multi-objective vehicle suspension performance. The force-limiting and hysteresis properties of a prototype MR-damper are characterized in the laboratory as functions of applied magnetic field, and response and excitation variables. An asymmetric force generation algorithm is formulated and integrated into the command current circuit to achieve asymmetric force in compression and rebound from the symmetric damper hardware. The measured data are used to identify the low-speed pre-yield, post-yield, force-limiting and hysteretic force-velocity characteristics in both symmetric as well as asymmetric damping modes. A generalized analytical model of the MR-damper is developed using symmetric and asymmetric sigmoid functions. The validity of the proposed model is demonstrated under wide ranges of control current and excitations. A number of control syntheses are formulated to achieve semi-active modulation in drive current of the MR-damper, including four different on-off and "skyhook"-based hi-lo, and "inverse-model"-based hi-lo and sliding-mode controllers. Continuous modulation (CM) and asymmetric damping force generation (ADFG) algorithms are proposed and integrated within the control policies to minimize switching transients in the symmetric and asymmetric modes