Road friction estimation techniques

Abstract

Techniques are described for estimating road friction between a road and tires of a vehicle. A method includes receiving, from a temperature sensor on a vehicle, a temperature value that indicates a temperature of an environment in which a vehicle is operated, determining a first range of friction values that quantify a friction between a road and tires of a vehicle based on a function of the temperature value and an extent of precipitation in a region that indicate a hazardous driving condition, obtaining, from the first range of friction values, a value that quantifies the friction between the road and the tires of the vehicle, where the value is obtained based on a driving related behavior of the vehicle, and causing the vehicle to operate on the road based on the value obtained from the first range of friction values.

What is claimed is: 
     
         1 . A road friction estimation method, comprising:
 determining a value that quantifies friction between a road and tires of a vehicle in response to determining that the vehicle is decelerating and in response to obtaining a brake pressure value that describes an amount of pressure applied on brakes of the vehicle that triggered an anti-lock braking system (ABS); and   causing the vehicle to operate on the road by sending a first instruction to an engine of the vehicle to reduce a speed of the vehicle in response to determining that the value is less than a pre-determined value.   
     
     
         2 . The method of  claim 1 , wherein the vehicle is determined to be decelerating in response to receiving data from an inertial measurement unit (IMU) device that indicates that the vehicle is decelerating. 
     
     
         3 . The method of  claim 1 , wherein the determining the value that quantifies friction between the road and the tires of the vehicle is performed periodically. 
     
     
         4 . The method of  claim 1 , wherein the causing the vehicle to operate on the road includes sending a second instruction to brakes of the vehicle to decelerate the vehicle at a rate less than a pre-determined rate in response to determining that the value is less than a second pre-determined value. 
     
     
         5 . The method of  claim 1 , wherein the causing the vehicle to operate on the road includes sending a third instruction to a steering motor to steer the vehicle at a rate that is less than a pre-determined steering rate in response to determining that the value is less than a third pre-determined value. 
     
     
         6 . An apparatus for vehicle operation, comprising:
 a processor configured to implement a method, the processor configured to:
 determine a value that quantifies friction between a road and tires of a vehicle in response to determining that the vehicle is decelerating and in response to obtaining a brake pressure value that describes an amount of pressure applied on brakes of the vehicle that triggered an anti-lock braking system (ABS); and 
 cause the vehicle to operate on the road by sending a first instruction to an engine of the vehicle to reduce a speed of the vehicle in response to a determination that the value is less than a pre-determined value. 
   
     
     
         7 . The apparatus of  claim 6 , wherein the value is determined from a first table that includes a plurality of road friction values and a plurality of brake pressure values, and wherein each road friction value corresponds to one brake pressure value. 
     
     
         8 . The apparatus of  claim 7 , wherein at least some of the plurality of road friction values increase as at least some of the plurality of brake pressure values increase. 
     
     
         9 . The apparatus of  claim 6 , wherein the processor is further configured to:
 determine a second value that quantifies friction between the road and the tires of the vehicle in response to determining that the vehicle is accelerating, in response to determining that a traction control system (TCS) is engaged, and based on an engine torque value applied by the engine of the vehicle.   
     
     
         10 . The apparatus of  claim 9 , wherein the second value is determined from a second table that includes a plurality of road friction values and a plurality of engine torque values, and wherein each road friction value corresponds to one engine torque value. 
     
     
         11 . The apparatus of  claim 10 , wherein at least some of the plurality of road friction values increase as at least some of the plurality of engine torque values increase. 
     
     
         12 . The apparatus of  claim 9 , wherein the vehicle is determined to be accelerating in response to data received from an inertial measurement unit (IMU) device that indicates that the vehicle is accelerating. 
     
     
         13 . A non-transitory computer readable program storage medium having code stored thereon, the code, when executed by a processor, causing the processor to implement a method comprising:
 determining a value that quantifies friction between a road and tires of a vehicle in response to determining that the vehicle is decelerating and in response to obtaining a brake pressure value that describes an amount of pressure applied on brakes of the vehicle that triggered an anti-lock braking system (ABS); and   causing the vehicle to operate on the road by sending a first instruction to an engine of the vehicle to reduce a speed of the vehicle in response to determining that the value is less than a pre-determined value.   
     
     
         14 . The non-transitory computer readable program storage medium of  claim 13 , wherein the value is within a range of values that include coefficient values that describe friction between the road and the tires of the vehicle. 
     
     
         15 . The non-transitory computer readable program storage medium of  claim 14 , wherein the range of values are determined in response to determining that an extent of rain or snow in an image exceeds a pre-determined limit, wherein the image is obtained from a camera located on the vehicle. 
     
     
         16 . The non-transitory computer readable program storage medium of  claim 15 , wherein the extent of rain is determined by converting the image to a grayscale image and by counting a number of raindrops in the grayscale image. 
     
     
         17 . The non-transitory computer readable program storage medium of  claim 14 , wherein the range of values is based on a speed setting or a frequency setting of a wiper of the vehicle. 
     
     
         18 . The non-transitory computer readable program storage medium of  claim 17 , wherein the range of values is narrower when the speed setting or the frequency setting of the wiper is higher than when the speed setting or the frequency setting is lower. 
     
     
         19 . The non-transitory computer readable program storage medium of  claim 14 , wherein the range of values is narrowed by obtaining a temperature of an environment in which the vehicle is operating and by determining that the temperature is less than a pre-determined temperature value. 
     
     
         20 . The non-transitory computer readable program storage medium of  claim 19 , wherein the temperature is obtained from a temperature sensor on the vehicle.