Slip avoidance
An example method may include i) determining, by a robot having at least one foot, a representation of a coefficient of friction between the foot and a ground surface; ii) determining, by the robot, a representation of a gradient of the ground surface; iii) based on the determined representations of the coefficient of friction and the gradient, determining a threshold orientation for a target ground reaction force on the foot of the robot during a step; iv) determining the target ground reaction force, where the target ground reaction force comprises a magnitude and an orientation; v) determining an adjusted ground reaction force by adjusting the orientation of the target ground reaction force to be within the determined threshold orientation; and vi) causing the foot of the robot to apply a force on the ground surface equal to and opposing the adjusted ground reaction force during the step.
1. A method comprising:
determining, by a processor of a robot having a foot, a representation of a coefficient of friction between the foot and a surface;
determining, by the processor, a cone centered on a normal force of the surface and pointed perpendicular to the surface, wherein a radius of the cone is based on the coefficient of friction;
determining, by the processor, a target ground reaction force to be exerted by the surface on the foot, wherein the target ground reaction force includes a magnitude and an orientation, and wherein the orientation is within the cone; and
causing, by the processor, the foot to apply a force on the surface, wherein the force is based on the target ground reaction force.
2. The method of claim 1 , wherein the representation of the coefficient of friction comprises a first representation of the coefficient of friction and a second representation of the coefficient of friction, wherein the first representation corresponds to movement of the foot on the surface in a forward direction, and wherein the second representation corresponds to movement of the foot on the surface in a lateral direction.
3. The method of claim 2 , wherein the second representation is smaller than the first representation.
4. The method of claim 1 , wherein the foot applies the force on the surface during a footstep of the robot.
5. The method of claim 4 , wherein the radius of the cone is a first radius during an initial stage of the footstep, wherein the radius of the cone is a second radius during a final stage of the footstep, and wherein the second radius is greater than the first radius.
6. The method of claim 5 , further comprising:
continuously and smoothly increasing the radius of the cone from the first radius to the second radius throughout the footstep.
7. The method of claim 5 , further comprising:
increasing the radius of the cone from the first radius to the second radius so that the cone has the second radius halfway through the footstep.
8. The method of claim 5 , further comprising:
adjusting the target ground reaction force to have an updated orientation during the final stage of the footstep, wherein the updated orientation is within the cone when the cone has the second radius.
9. The method of claim 1 , further comprising:
detecting, by the processor, an indication of a slip by a second foot of the robot;
based on detecting the indication of the slip, adjusting, by the processor, the cone to have a second radius, wherein the second radius is less than the first radius;
determining, by the processor, a second target ground reaction force to be exerted by the surface on the foot, wherein the second target ground reaction force includes a second magnitude and a second orientation, and wherein the orientation is within the cone as defined by the second radius; and
causing, by the processor, the foot to apply a second force on the surface, wherein the second force is based on the second target ground reaction force.
10. An article of manufacture including a non-transitory computer-readable medium, having stored thereon program instructions that, upon execution by a processor of a robot, cause the robot to perform operations comprising:
determining a representation of a coefficient of friction between a foot of the robot and a surface;
determining a cone centered on a normal force of the surface and pointed perpendicular to the surface, wherein a radius of the cone is based on the coefficient of friction;
determining a target ground reaction force to be exerted by the surface on the foot, wherein the target ground reaction force includes a magnitude and an orientation, and wherein the orientation is within the cone; and
causing the foot to apply a force on the surface, wherein the force is based on the target ground reaction force.
11. The article of manufacture of claim 10 , wherein the representation of the coefficient of friction comprises a first representation of the coefficient of friction and a second representation of the coefficient of friction, wherein the first representation corresponds to movement of the foot on the surface in a forward direction, and wherein the second representation corresponds to movement of the foot on the surface in a lateral direction.
12. The article of manufacture of claim 11 , wherein the second representation is smaller than the first representation.
13. The article of manufacture of claim 10 , wherein the foot applies the force on the surface during a footstep of the robot.
14. The article of manufacture of claim 13 , wherein the radius of the cone is a first radius during an initial stage of the footstep, wherein the radius of the cone is a second radius during a final stage of the footstep, and wherein the second radius is greater than the first radius.
15. The article of manufacture of claim 14 , the operations further comprising:
continuously and smoothly increasing the radius of the cone from the first radius to the second radius throughout the footstep.
16. The article of manufacture of claim 14 , the operations further comprising:
increasing the radius of the cone from the first radius to the second radius so that the cone has the second radius halfway through the footstep.
17. The article of manufacture of claim 14 , the operations further comprising:
adjusting the target ground reaction force to have an updated orientation during the final stage of the footstep, wherein the updated orientation is within the cone when the cone has the second radius.
18. The article of manufacture of claim 10 , the operations further comprising:
detecting an indication of a slip by a second foot of the robot;
based on detecting the indication of the slip, adjusting the cone to have a second radius, wherein the second radius is less than the first radius;
determining a second target ground reaction force to be exerted by the surface on the foot, wherein the second target ground reaction force includes a second magnitude and a second orientation, and wherein the orientation is within the cone as defined by the second radius; and
causing the foot to apply a second force on the surface, wherein the second force is based on the second target ground reaction force.
19. A robot comprising:
a foot;
a processor;
memory; and
program instructions, stored in the memory, that upon execution by the processor cause the robot to perform operations comprising:
determining a representation of a coefficient of friction between the foot and a surface;
determining a cone centered on a normal force of the surface and pointed perpendicular to the surface, wherein a radius of the cone is based on the coefficient of friction;
determining a target ground reaction force to be exerted by the surface on the foot, wherein the target ground reaction force includes a magnitude and an orientation, and wherein the orientation is within the cone; and
causing the foot to apply a force on the surface, wherein the force is based on the target ground reaction force.
20. The robot of claim 19 , further comprising:
a second foot, wherein the operations further comprise:
detecting an indication of a slip by the second foot;
based on detecting the indication of the slip, adjusting the cone to have a second radius, wherein the second radius is less than the first radius;
determining a second target ground reaction force to be exerted by the surface on the foot, wherein the second target ground reaction force includes a second magnitude and a second orientation, and wherein the orientation is within the cone as defined by the second radius; and
causing the foot to apply a second force on the surface, wherein the second force is based on the second target ground reaction force.