Slip detection for robotic locomotion
An example method may include i) determining a first distance between a pair of feet of a robot at a first time, where the pair of feet is in contact with a ground surface; ii) determining a second distance between the pair of feet of the robot at a second time, where the pair of feet remains in contact with the ground surface from the first time to the second time; iii) comparing a difference between the determined first and second distances to a threshold difference; iv) determining that the difference between determined first and second distances exceeds the threshold difference; and v) based on the determination that the difference between the determined first and second distances exceeds the threshold difference, causing the robot to react.
1. A method comprising:
obtaining, by a robot, a sequence of measurements of distances between a pair of feet of the robot, wherein the sequence includes a first measurement taken when the pair of feet are in contact with a surface and in a stance state, wherein the sequence includes a second measurement taken when the pair of feet remain in the stance state from a time of the first measurement, and wherein the first measurement is of a first distance and the second measurement is of a second distance;
determining, by the robot, that a difference between the first distance and the second distance exceeds a threshold difference and that a particular foot of the pair of feet has lost contact with the surface;
causing, by the robot, the particular foot to discontinue ground reaction force control;
causing, by the robot, the particular foot to engage in position control by extending the particular foot toward the surface; and
after the particular foot is in contact with the surface, causing, by the robot, the particular foot to resume ground reaction force control.
2. The method of claim 1 , further comprising generating, by the robot, an indication that at least one of the pair of feet has slipped.
3. The method of claim 1 , further comprising increasing, by the robot, the threshold difference.
4. The method of claim 1 , wherein the second measurement occurs 10-50 milliseconds after the first measurement.
5. The method of claim 1 , wherein obtaining the sequence of measurements of distances between the pair of feet of the robot comprises:
determining a first x-distance, a first y-distance, and a first z-distance between the pair of feet along three respective axes as components of the first distance; and
determining a second x-distance, a second y-distance, and a second z-distance between the pair of feet as components of the second distance.
6. The method of claim 5 , wherein determining that the difference between the first distance and the second distance exceeds the threshold difference comprises:
comparing an x-difference between the first x-distance and the second x-distance to a threshold x-difference;
comparing a y-difference between the first y-distance and the second y-distance to a threshold y-difference;
comparing a z-difference between the first z-distance and the second z-distance to a threshold z-difference; and
determining that the x-difference exceeds the threshold x-difference, the y-difference exceeds the threshold y-difference, or the z-difference exceeds the threshold z-difference.
7. The method of claim 5 , wherein determining that the difference between the first distance and the second distance exceeds the threshold difference comprises determining that one or more of an x-difference, a y-difference, or a z-difference exceeds the threshold difference.
8. A method comprising:
determining, by a robot, a first estimate of a distance traveled by a body of the robot in a time period, wherein the first estimate is relative to a position of a first foot of the robot that is in contact with a surface;
determining, by the robot, a second estimate of the distance traveled by the body of the robot in the time period, wherein the second estimate is based on inertial measurements of the body of the robot;
determining that a difference between the first estimate and the second estimate exceeds a threshold difference and that a second foot is not in contact with the surface;
causing, by the robot, the second foot to discontinue ground reaction force control;
causing, by the robot, the second foot to engage in position control by extending the second foot toward the surface; and
after the second foot is in contact with the surface, causing, by the robot, the second foot to resume ground reaction force control.
9. The method of claim 8 , wherein the first estimate is based on input from a first set of sensors associated with the first foot, and wherein the second estimate is based on input from a second set of sensors associated with the body.
10. The method of claim 9 , wherein the first set of sensors detects a position of the first foot relative to the body, and wherein the second set of sensors detects an acceleration of the body.
11. The method of claim 8 , further comprising generating, by the robot, an indication that the robot has slipped.
12. The method of claim 8 , further comprising increasing, by the robot, the threshold difference.
13. An article of manufacture including a non-transitory computer-readable medium, having stored thereon program instructions that, upon execution by a processor, cause a robot to perform operations comprising:
obtaining a sequence of measurements of distances between a pair of feet of the robot, wherein the sequence includes a first measurement taken when the pair of feet are in contact with a surface and in a stance state, wherein the sequence includes a second measurement taken when the pair of feet remain in the stance state from a time of the first measurement, and wherein the first measurement is of a first distance and the second measurement is of a second distance;
determining that a difference between the first distance and the second distance exceeds a threshold difference and that a particular foot of the pair of feet has lost contact with the surface;
causing the particular foot to discontinue ground reaction force control;
causing the particular foot to engage in position control by extending the particular foot toward the surface; and
after the particular foot is in contact with the surface, causing the particular foot to resume ground reaction force control.
14. The article of manufacture of claim 13 , the operations further comprising increasing the threshold difference.
15. The article of manufacture of claim 13 , wherein the second measurement occurs 10-50 milliseconds after the first measurement.
16. The article of manufacture of claim 13 , wherein obtaining the sequence of measurements of distances between the pair of feet of the robot comprises:
determining a first x-distance, a first y-distance, and a first z-distance between the pair of feet along three respective axes as components of the first distance; and
determining a second x-distance, a second y-distance, and a second z-distance between the pair of feet as components of the second distance.
17. The article of manufacture of claim 16 , wherein determining that the difference between the first distance and the second distance exceeds the threshold difference comprises: comparing an x-difference between the first x-distance and the second x-distance to a threshold x-difference;
comparing a y-difference between the first y-distance and the second y-distance to a threshold y-difference;
comparing a z-difference between the first z-distance and the second z-distance to a threshold z-difference; and
determining that the x-difference exceeds the threshold x-difference, the y-difference exceeds the threshold y-difference, or the z-difference exceeds the threshold z-difference.
18. The article of manufacture of claim 16 , wherein determining that the difference between the first distance and the second distance exceeds the threshold difference comprises determining that one or more of an x-difference, a y-difference, or a z-difference exceeds the threshold difference.