Method for Hill Grade Detection and It's Use in Longitudinal Drive Torque Bias Control with a Controllable AWD System 2008-01-1455

Research pertaining to electronically controllable AWD (All Wheel Drive) systems continues in the automotive industry as a means to further enhance vehicle traction, maneuverability, handling and stability characteristics. Different feed-forward and feedback control approaches have also been explored to enhance AWD system performance and robustness under various vehicle operating conditions. Due to the large variability of vehicle operational conditions, some trade-off usually needs to be made to achieve a balanced overall-AWD system performance, especially traction and maneuverability for both normal flat roads and hill climbing conditions.

The purpose of this research is to develop a hill grade detection method and strategy for application in vehicle front to rear drive torque distribution control to enhance AWD system robustness while climbing low-mu hills. First, an overview of existing vehicle hill climbing related drive torque control approaches are presented. A simple concept to estimate road elevation grade based on measured vehicle wheel speeds and longitudinal acceleration is then proposed. Strategies to achieve accurate and yet robust hill grade estimation under various vehicle operation conditions is explored in detail. Application of the estimated hill grade for enhancing vehicle hill climbing capability using a controllable AWD system in a feed-forward manner is then presented. Representative on-vehicle test results are presented to illustrate the effectiveness of the proposed hill slope estimation and application method.