Procedure for the Characterization of Friction in Automobile Power Steering Systems

In developing a nonlinear steering system model for vehicle simulation, it was determined that proper inclusion of system friction is necessary to correctly predict steering wheel torque response in on-center driving using simulation models. A method to characterize the inherent friction behavior for a given steering gear has been developed and performed on two types of power steering gears: a recirculating ball gear and a rack-and-pinion gear.

During this research it was discovered that levels of static and dynamic friction can differ widely for these two types. Therefore this characterization procedure provides a method to ascertain both static and dynamic friction levels. The results from these tests show that friction levels can depend on steering gear input shaft position, steering gear input angular velocity and steering gear loading conditions.

A method to represent these findings is established so that friction characteristics can be properly included in steering system models from parallel research efforts. Static breakout levels were found to be dependent mainly on steering gear input shaft position and direction of travel, whereas dynamic friction levels were found to be dependent on these factors as well as on the anglar velocity. These dynamic friction levels can be predicted based on information about the static friction levels and a velocity correction factor that has been established in this study.