The disclosure relates to assessing and responding to wheel slippage and estimating road friction for a road surface. For instance, a vehicle may be controlled in an autonomous driving mode in order to follow a trajectory. A wheel of the vehicle may be determined to be slipping such that the vehicle has limited steering control. In response to determining that the wheel is slipping, steering of one or more wheels may be controlled in order to orient the one or more wheels towards the trajectory in order to allow the vehicle to proceed towards the trajectory when the wheel is no longer slipping. In addition, the road friction may be estimated based on the determination that the wheel is slipping. The vehicle may be controlled in the autonomous driving mode based on the estimated road friction.

An apparatus for estimating a friction coefficient of a road surface is provided. The apparatus includes a current sensor configured to measure a control current value of a rear wheel steering (RWS) motor, a stroke sensor configured to measure a stroke value indicating a movement amount of a rear wheel steering link, and a controller configured to estimate the friction coefficient of the road surface based on the control current value measured by the current sensor and the stroke value measured by the stroke sensor.

In at least some implementations, a method of estimating road friction, includes determining an actual steering load, determining a nominal steering load as a function of vehicle speed, steering angle, and a nominal road friction value, and comparing the actual steering load to the nominal steering load to determine an estimated road friction. In at least some implementations, the nominal steering load is not determined as a function of vehicle yaw, or vehicle lateral acceleration, or vehicle wheel speed compared to vehicle speed, or vehicle tire compliance or road wheel angle.

According to one or more embodiments, a method includes computing, by a steering system, a model rack force value based on a vehicle speed, steering angle, and a road-friction coefficient value. The method further includes determining, by the steering system, a difference between the model rack force value and a load rack force value. The method further includes updating, by the steering system, the road-friction coefficient value using the difference that is determined.

A method for friction coefficient determination on elastically connected subsystems, in which an overall system includes multiple subsystems and at least two subsystems are connected to one another by an elastic connection. The elastic connection has at least one static friction state and a sliding friction state for prescribed external state variables, in which the overall system is excited with a vibration having a variable excitation amplitude at a defined excitation frequency. The excitation amplitude is varied, in which a phase difference between the vibration and a measured reaction torque together with the excitation amplitude are recorded as a function of time, in which no phase difference occurs in the static friction state and a phase difference of 180° occurs in the sliding friction state. In a first step, the excitation amplitude is increased until a transition in the phase difference from 0° to 180° indicates the transition from the static friction state to the sliding friction state.

An AV is described herein. The AV includes a lidar sensor system. The AV additionally includes a computing system that executes a road surface analysis component to determine, based upon lidar sensor data, whether a road surface feature is present on or in a roadway in a travel path of the AV. The AV can be configured to initiate a mitigation maneuver responsive to determining that the road surface feature is present. Performing the mitigation maneuver causes the AV to avoid the road surface feature or decelerate prior to reaching the road surface feature, thereby improving the apparent quality or comfort of the ride to a passenger of the AV.

A steering system for a vehicle, including: a pair of wheel steering devices that respectively steer right and left wheels independently of each other; and a controller configured to control the pair of wheel steering devices, wherein the controller is configured to: determine a standard steering amount of each of the right and left wheels in accordance with a steering request; execute opposite-phase shift steering in which steering amounts of the respective right and left wheels are shifted in mutually opposite directions by respective shift amounts with respect to the standard steering amounts determined respectively for the right and left wheels; and estimate a friction coefficient of a road surface on which the vehicle is running based on steering forces respectively applied to the right and left wheels in the opposite-phase shift steering.

According to one or more embodiments, a method includes computing, by a steering system, a model rack force value based on a vehicle speed, steering angle, and a road-friction coefficient value. The method further includes determining, by the steering system, a difference between the model rack force value and a load rack force value. The method further includes updating, by the steering system, the road-friction coefficient value using the difference that is determined.

Systems and methods for determining whether a vehicle is in an understeer or oversteer situation. The system includes a controller circuit coupled to an IMU and an EPS, and programmed to: calculate, for a steered first axle, an axle-based pneumatic trail for using IMU measurements and EPS signals and estimate a saturation level as a function of a distance between the axle-based pneumatic trail and zero. The system estimates, for an unsteered second axle, an axle lateral force curve with respect to a slip angle of the second axle, and a saturation level as a function of when the axle lateral force curve with respect to the slip angle transitions from positive values to negative values. The saturation level of the first axle and the second axle are integrated. The system determines that the vehicle is in an understeer or oversteer situation as a function of the integrated saturation levels.

A method of maintaining stability of a motor vehicle having a first axle, a second axle, and a steering actuator configured to steer the first axle includes determining localization and heading of the vehicle. The method also includes determining a current side-slip angle of the second axle and setting a maximum side-slip angle of the second axle using the friction coefficient at the vehicle and road surface interface. The method additionally includes predicting when the maximum side-slip angle would be exceeded using the localization, heading, and determined current side-slip angle as inputs to a linear computational model. The method also includes updating the model using the prediction of when the maximum side-slip angle would be exceeded to determine impending instability of the vehicle. Furthermore, the method includes correcting for the impending instability using the updated model and the maximum side-slip angle via modifying a steering angle of the first axle.