The purpose of this study is to mitigate the compromise between ride comfort and handling of a
small single seat off-road vehicle known as a Baja. This has been achieved by semi-active control of
the suspension system containing controllable magnetorheological (MR) dampers and passive
hydro-pneumatic spring-damper units.
MR fluid is a viscous fluid whose rheological properties depend on the strength of the magnetic
field surrounding the fluid, and typically consists of iron particles suspended in silicone oil. When a
magnetic field is applied to the fluid, the iron particles become aligned and change the effective
viscosity of the fluid. The use of MR fluid in dampers provides variable damping that can be changed
quickly by controlling the intensity of the magnetic field around the fluid. Various benefits associated
with the use of MR dampers have led to their widespread implementation in automotive engineering.
Many studies on conventional vehicles in the existing literature have demonstrated the conflicting
suspension requirements for favourable ride comfort and handling. Generally, soft springs with low
damping are ideal for improved ride comfort, while stiff springs with high damping are required for
enhanced handling. This has resulted in the development of passive suspension systems that provide
either an enhanced ride quality or good drivability, often targeting one at the expense of the other.
The test vehicle used for this study is distinct in many ways with multiple characteristics that are
not commonly observed in the existing literature. For instance, the absence of a differential in the test
vehicle driveline causes drivability issues that are aggravated by increased damping.
The majority of existing MR damper models in the literature are developed for uniform excitation
and re-characterisation of model parameters is required for changes in input conditions. Although
recursive models are more accurate and applicable to a wider range of input conditions, these models
require measured force feedback which may not always be available due to limitations such as packaging constraints. These constraints required the development of alternative MR damper models
that can be used to prescribe the current input to the damper.
In this study parametric, nonparametric and recursive MR damper models have been developed
and evaluated in terms of accuracy, invertibility and applicability to random excitation. The
MR damper is used in parallel with passive damping as a certain amount of passive damping is always
present in suspension systems due to friction and elastomeric parts.
Most of the existing studies on suspension systems have been performed using linear two degree
of freedom vehicle models that are constrained to specific conditions. Usually these models are
implemented without an indication of the ability of these models to accurately represent the vehicles
that these studies are intended for.
For this study, a nonlinear, three-dimensional, 12 degrees of freedom vehicle model has been
developed to represent the test vehicle. This model is validated against experimental results for ride
comfort and handling. The MR damper models are combined with the model of the test vehicle, and
used in ride comfort and handling simulations at various levels of passive damping and control gains
in order to assess the potential impact of suspension control on the ride quality and drivability of the
Simulation results show that lower passive damping levels can significantly improve the ride
comfort as well as the handling characteristics of the test vehicle. Furthermore, it is observed that
additional improvements that may be obtained by the implementation of continuous damping control
may not be justifiable due to the associated cost and complexity.
Dissertation (MEng)--University of Pretoria, 2013.