  Operational Modal Analysis (OMA) is a powerful tool, widely used in the
fields of structural identification and health monitoring, and certainly
eligible for identifying the real in-operation behaviour of vehicle systems.
Several attempts can be found in the literature, for which the usage of
algorithms based on the classical OMA formulation has been strained for the
identification of passenger cars and industrial trucks. The interest is mainly
focused on the assessment of suspension behaviour and, thus, on the
identification of the so-called vehicle rigid body modes. But issues arise when
the operational identification of a vehicle system is performed, basically
related to the nature of the loads induced by the roughness of rolling
profiles. The forces exerted on the wheels, in fact, depending on their
location, are affected by time and/or spatial correlation, and, more over, do
not fit the form of white noise sequences. Thus, the nature of the excitation
strongly violate the hypotheses on which the formulation of classical OMA modal
model relies, leading to pronounced modelling errors and, in turn, to poorly
estimated modal parameters. In this paper, we develop a specialised modal
model, that we refer to as the Track-Vehicle Interaction Modal Model, able to
incorporate the character of road/rail inputs acting on vehicles during
operation. Since in this novel modal model the relationship between vehicle
system outputs and modal parameters is given explicitly, the development of new
specific curve fitting techniques, in the time-lag or frequency domain, is now
possible, making available simple and cost-effective tools for vehicle
operational identification. More over, a second, but not less important outcome
of the proposed modal model is the usage of the resulting techniques for the
indirect characterisation of rolling surface roughness, that can be used to
improve comfort and safety.
