Slip handling and ground friction estimation for robots

Abstract

A method of mitigating slip conditions and estimating ground friction for a robot having a plurality of feet includes receiving a first coefficient of friction corresponding to a ground surface. The method also includes determining whether one of the plurality of feet is in contact with the ground surface, and when a first foot of the plurality feet is in contact with the ground surface, setting a second coefficient of friction associated with the first foot equal to the first coefficient of friction. The method also includes determining a measured velocity of the first foot relative to the ground surface, and adjusting the second coefficient of friction of the first foot based on the measured velocity of the foot. One of the plurality of feet of the robot applies a force on the ground surface based on the adjusted second coefficient of friction.

What is claimed is: 
     
         1 . A method of ground friction estimation for a robot, the method comprising:
 estimating, by data processing hardware of the robot, a first coefficient of friction for a ground surface supporting the robot;   determining, by the data processing hardware, that a distal end of a first leg of the robot is in contact with the ground surface;   selecting, by the data processing hardware, from a plurality of stance states, a current stance state for the distal end of the first leg;   setting, by the data processing hardware, a second coefficient of friction associated with the distal end of the first leg based on the first coefficient of friction and the current stance state;   determining, by the data processing hardware, a measured velocity of the distal end relative to the ground surface;   adjusting, by the data processing hardware, the second coefficient of friction based on the measured velocity of the distal end; and   applying, by the data processing hardware, a force to the ground surface by the distal end of the first leg based on the adjusted second coefficient of friction.   
     
     
         2 . The method of  claim 1 , wherein adjusting the second coefficient of friction based on the measured velocity comprises comparing the measured velocity of the distal end to a threshold velocity. 
     
     
         3 . The method of  claim 2 , wherein adjusting the second coefficient of friction based on the measured velocity further comprises decreasing the second coefficient of friction when the measured velocity of the distal end is less than the threshold velocity. 
     
     
         4 . The method of  claim 3 , wherein adjusting the second coefficient of friction based on the measured velocity further comprises calculating, by the data processing hardware, a negative friction adjustment rate based on the measured velocity of the distal end of the first leg. 
     
     
         5 . The method of  claim 2 , wherein adjusting the second coefficient of friction based on the measured velocity further comprises increasing the second coefficient of friction when the measured velocity of the distal end is greater than the threshold velocity. 
     
     
         6 . The method of  claim 1 , wherein the plurality of stance states comprises three or more stance states including a nominal stance state, a difficult stance state, and a slipping stance state. 
     
     
         7 . The method of  claim 1 , wherein adjusting the second coefficient of friction comprises choosing, by the data processing hardware, an amount of adjustment based on the measured velocity of the distal end and the current stance state. 
     
     
         8 . The method of  claim 7 , wherein adjusting the second coefficient of friction comprises limiting, by the data processing hardware, a decrease of the second coefficient of friction when the current stance state is the nominal stance state. 
     
     
         9 . The method of  claim 8 , further comprising limiting, by the data processing hardware, the decrease of the second coefficient of friction by a limiting constant. 
     
     
         10 . The method of  claim 1 , wherein the plurality of stance states comprises a nominal stance state and a difficult stance state, wherein selecting the current stance state comprises initially setting the current stance state to the nominal stance state. 
     
     
         11 . The method of  claim 10 , wherein selecting the current stance state further comprises transitioning the current stance state from the nominal stance state to the difficult stance state based on determining, by the data processing hardware, a vertical position error in a first direction relative to the ground surface and a shear positional error in a second direction relative to the ground surface. 
     
     
         12 . The method of  claim 11 , wherein the plurality of stance states further comprises a slipping stance state, wherein selecting the current stance state further comprises transitioning the current stance state from the difficult stance state to the slipping stance state based on comparing, by the data processing hardware, the measured velocity of the distal end to a threshold velocity. 
     
     
         13 . A robot comprising:
 a body and a plurality of legs attached to the body, the plurality of legs including a first leg having a distal end;   data processing hardware; and   memory hardware in communication with the data processing hardware, the memory hardware storing instructions that when executed on the data processing hardware cause the data processing hardware to perform operations comprising:
 estimating a first coefficient of friction for a ground surface supporting the robot; 
 determining that the distal end of the first leg is in contact with the ground surface; 
 selecting, from a plurality of stance states, a current stance state for the distal end of the first leg; 
 setting a second coefficient of friction associated with the distal end of the first leg based on the first coefficient of friction and the current stance state; 
 determining a measured velocity of the distal end relative to the ground surface; 
 adjusting the second coefficient of friction based on the measured velocity of the distal end; and 
 applying a force to the ground surface by the distal end of the first leg based on the adjusted second coefficient of friction. 
   
     
     
         14 . The robot of  claim 13 , wherein adjusting the second coefficient of friction based on the measured velocity comprises comparing the measured velocity of the distal end to a threshold velocity. 
     
     
         15 . The robot of  claim 14 , wherein adjusting the second coefficient of friction based on the measured velocity further comprises decreasing the second coefficient of friction when the measured velocity of the distal end is less than the threshold velocity. 
     
     
         16 . The robot of  claim 15 , wherein adjusting the second coefficient of friction based on the measured velocity further comprises calculating, by the data processing hardware, a negative friction adjustment rate based on the measured velocity of the distal end of the first leg. 
     
     
         17 . The robot of  claim 14 , wherein adjusting the second coefficient of friction based on the measured velocity further comprises increasing the second coefficient of friction when the measured velocity of the distal end is greater than the threshold velocity. 
     
     
         18 . The robot of  claim 13 , wherein the plurality of stance states comprises three or more stance states including a nominal stance state, a difficult stance state, and a slipping stance state. 
     
     
         19 . The robot of  claim 13 , wherein adjusting the second coefficient of friction comprises choosing, by the data processing hardware, an amount of adjustment based on the measured velocity of the distal end and the current stance state. 
     
     
         20 . The robot of  claim 19 , wherein adjusting the second coefficient of friction comprises limiting, by the data processing hardware, a decrease of the second coefficient of friction when the current stance state is the nominal stance state. 
     
     
         21 . The robot of  claim 13 , wherein the plurality of stance states comprises a nominal stance state and a difficult stance state, wherein selecting the current stance state comprises initially setting the current stance state to the nominal stance state. 
     
     
         22 . The robot of  claim 21 , wherein selecting the current stance state further comprises transitioning the current stance state from the nominal stance state to the difficult stance state based on determining, by the data processing hardware, a vertical position error in a first direction relative to the ground surface and a shear positional error in a second direction relative to the ground surface. 
     
     
         23 . The robot of  claim 22 , wherein the plurality of stance states further comprises a slipping stance state, wherein selecting the current stance state further comprises transitioning the current stance state from the difficult stance state to the slipping stance state based on comparing, by the data processing hardware, the measured velocity of the distal end to a threshold velocity.