Stability Analysis during Active Tire Excitation for Friction Estimation

Abstract: Road accidents have been a persistent cause of death worldwide, and claim millions of lives everyyear. Recent developments in the active safety systems like Electronic Stability Control (ESC) havehelped in reducing these numbers quite signicantly over the years. However, a major challenge forthese systems is to know the friction coecient between the tire and the road, as this value limits theamount of force the tires can generate. Knowledge of the coecient of friction can be used to adaptthe driving style, thereby avoiding interventions by stability control at the limit, making vehiclessafer. However, it is a major challenge within the automotive industry to estimate the coecientof friction accurately, and with sucient availability, as that requires high levels of tire utilization,such that the tire is forced to reach the non-linear range of operation. Such events are very rarein everyday driving, and requires a system induced active excitation of the tires. One such methodthat has been proposed earlier, to carry out an active tire excitation, is by using a simultaneouspropulsive and brake force on front and the rear the axles. However, applying an equal magnitudeof propulsive and brake force results in a force neutral situation at the vehicle level, which forcesthe velocity to be constant, overriding driver acceleration requests. Thus, an active tire excitationmethod was proposed by Volvo Cars, which is able to apply an unequal propulsive and brake forceto the front and the rear axle, such that the driver's acceleration demand can be met, during frictionestimation. However, such an excitation can be dangerous to carry out, if it leads to instability ofthe vehicle.Several methods have been developed to analyze and quantify stability of a vehicle, but detailedanalysis about the stability under forced excitation, for friction estimation, is very rare. This thesiswork investigates the lateral stability of a vehicle undergoing an active tire excitation for frictionestimation. The objective is to understand which vehicle and tire models can be used to quantifythe lateral stability of a vehicle under forced excitation, and how phase portrait methods can beused to develop a stability monitor that is able to indicate the lateral stability of the vehicle undera forced excitation.The results of using a stability monitor during active tire excitation clearly show that it is able toindicate when the vehicle becomes unstable and looses control. It also shows that for slow speedsteady-state maneuvers and straight line maneuvers, the stability monitor does not indicate instability.A comparison between phase portrait based and conventional side-slip based stability monitorsshow the eectiveness and generality of the phase portrait based monitor, which is able to detectinstability earlier than the conventional side-slip based method.