A Concept for Using Road Wetness Information in an All-Wheel-Drive Control

Abstract

This paper presents a concept for using road wetness information in an all-wheel-drive (AWD) control that distributes drive torques in the longitudinal direction. Driving on wet roads requires special attention. Not only does the road surface friction coefficient decrease, but driving dynamics targets must be adjusted to prevent vehicle instability under wet conditions. As an exemplary application, the otherwise generic control concept is implemented on an AWD vehicle with a torque-on-demand transfer case. Therefore, the AWD topology of a drive train with a torque-on-demand transfer case is analysed in advance in terms of occurring torques and rotational speeds. In the following, the vehicle dynamics goals for driving in wet road conditions are described—divided into primary and secondary goals. Starting from a state-of-the art AWD control, an adaptive control strategy is derived by superimposing a wetness coordination unit. With the knowledge of occurring road wetness, this unit adapts newly introduced parameters in order to meet the target driving behaviour under wet conditions. Lastly, the derived AWD control is implemented into a 14-DOF, non-linear vehicle model in Matlab/Simulink, which is used as a virtual plant. The performance of the developed concept is assessed by the driving maneuver “Power On Cornering“ (PON), which means an acceleration out of steady-state circular motion. As its essential benefit, the AWD control enables a maximum spread between driving stability, agility and traction under combined dynamics when using wetness information. The newly introduced wetness coordination unit uses only a few additional and physically interpretable key parameters for this purpose, without significantly increasing the controller complexity.

Estimation of Road Wetness from a Passenger Car

“…Tests conducted in a laboratory have shown that the coefficient of friction between a rubber slider and a road surface can decrease rapidly in the presence of a water film as thin as 0.2 mm [1]. As it is well known that a vehicle's braking performance and stability depend on the available tire/road friction [2], this result highlights the need to estimate the road wetness. Mainly, when the water depth is so low-for instance, under a drizzle or after a precipitation-the road has an apparent safe aspect and the driver is tempted to adopt inappropriate maneuvers such as driving fast.…”