Abstract
THE PROBLEM Front wheel driven vehicles with high torque powertrains often suffer from steering disturbances due to the influence of drive torque on the steering system – so called torque steer.
This problem has been known since the introduction of front wheel driven vehicles in the early seventies and is reasonably well understood. With the expansion of front wheel drive to larger carlines, increasing engine power and the trend to bigger wheels with lower aspect ratio tires the torque steer problem is becoming more predominant.
THE OPPORTUNITY During recent years hydraulic power steering assistance is being replaced more and more by Electric Power Assisted Steering (EPAS). This gives the opportunity to provide steering forces that are completely independent of steering wheel torque applied by the driver.
With this advantage, disturbances can easily be rejected if they can be quantified in direction and magnitude. This is carried out by fully automated control functions based on the inputs from sensors already existing in the vehicle.
THE SOLUTION To compensate for torque steer using an EPAS torque overlay, disturbance magnitude and direction must be quantified.
Several contributors inside the vehicle, such as king pin offset, driveshaft rear view angles and driveshaft stiffness balance in combination with differential gear friction, are well known. The behaviour of these internal contributors can be predicted with sufficient quality to estimate torque steer influences during high power accelerations. Using simulation and estimation of these internal contributors torque steer could be compensated under optimum conditions on a dry and flat surface.
Next to these internal factors, torque steer is mainly influenced by the road/tire contact patch. Changes in friction or undulations in the road, as seen on typical roads, can in no way be calculated, predicted or foreseen. High driving torques in these natural conditions will again result in undesired steering influences.
The resulting steering disturbance can be identified if the nominal steering wheel torque for the current driving situation is known. Therefore a model based approach has been developed to derive nominal steering torque with respect to the driving situation. In the case of a probable torque steer event the difference between this nominal steering wheel torque and the torque being applied by the driver can be calculated and used to eliminate, or strongly reduce torque steer effects.
Keywords: Torque Steer, EPAS, Controls, Disturbance Rejection