Mechanically-timed footsteps for a robotic device

1. A method comprising:

receiving, by a computing device of a robot, a position of a center of mass of the robot and a velocity of the center of mass of the robot moving in a forward direction along a surface, the robot comprising a stance foot in contact with the surface and a swing foot not in contact with the surface;

determining, by the computing device, a capture point based on the received position of the center of mass and the received velocity of the center of mass, the capture point indicating an instantaneous position on the surface that the robot can place the swing foot to arrest a falling motion, the capture point moving along a trajectory from an initial position on the surface to a threshold-target position on the surface;

determining, by the computing device, whether the capture point has reached the threshold-target position; and

when the capture point has reached the threshold-target position:

determining, by the computing device, a target-touchdown position to place the swing foot of the robot in contact with the surface to generate a center of pressure between the swing foot and the surface to translate the trajectory of the capture point in the forward direction; and

causing, by the computing device, the swing foot to contact the surface at the target-touchdown position.

2. The method of claim 1 , wherein the target-touchdown position translates the trajectory of the capture point in the forward direction toward a current center of mass of the robot.

3. The method of claim 1 , wherein the target-touchdown position is located at a position laterally outside of the threshold-target position.

4. The method of claim 1 , further comprising determining, by the computing device, the threshold-target position based on an intersection of a lateral threshold for the capture point and a forward threshold of the capture point.

5. The method of claim 4 , wherein the lateral threshold and the forward threshold are based on a stance width and a stride length of the robot.

6. The method of claim 4 , wherein the lateral threshold and the forward threshold are based on dynamic motion of the robot according to a linear inverted pendulum model, the linear inverted pendulum model indicating a fall rate for the robot as a function of a height of the robot between the swing foot and the position of the center of mass of the robot.

7. The method of claim 4 , wherein:

the forward threshold of the capture point comprises a forward threshold range, the forward threshold range comprising:

a first threshold corresponding to a first nominal forward position of the capture point at a beginning of a swing period for the swing foot; and

a second threshold corresponding to a second nominal forward position of the capture point at an end of the swing period for the swing foot; and

the lateral threshold of the capture point comprises a lateral threshold range, the lateral threshold range comprising:

a first lateral threshold corresponding to a first nominal lateral position of the capture point at the beginning of the swing period for swing foot; and

a second lateral threshold corresponding to a second nominal lateral position of the capture point at the end of the swing period for swing foot.

8. The method of claim 1 , wherein causing the swing foot to contact the surface at the target-touchdown position occurs at a set period of time before the capture point reaches the threshold-target position, the set period of time corresponding to a range of 0-100 milliseconds.

9. The method of claim 1 , further comprising:

determining, by the computing device, that a gait disturbance has resulted in an altered trajectory for the capture point; and

updating, by the computing device, the threshold-target position for the capture point.

10. The method of claim 9 , wherein updating the threshold position for the capture point comprises:

estimating an intersection between the altered trajectory of the capture point and an original trajectory of the capture point; and

defining the intersection as the updated threshold position.

11. The method of claim 1 , further comprising:

determining, by the computing device, that a gait disturbance has resulted in an altered trajectory for the capture point; and

shifting, by the computing device, a location of a center of pressure associated with the stance foot to correct the altered trajectory for the capture point towards an original trajectory for the capture point.

12. The method of claim 1 , further comprising:

determining, by the computing device, that a gait disturbance has resulted in an altered trajectory for the capture point; and

adjusting, by the computing device, the position of the center of mass of the robot to correct the altered trajectory for the capture point towards an original trajectory for the capture point.

13. A robot comprising:

a body comprising a first leg and a second leg, the first leg comprising a stance foot in contact with a surface, the second leg comprising a swing foot not in contact with the surface;

a control system in communication with the stance foot and the swing foot, the control system comprising at least one processor configured to perform operations comprising:

receiving a position of a center of mass of the robot and a velocity of the center of mass of the robot moving in a forward direction along the surface;

determining a capture point based on the received position of the center of mass and the received velocity of the center of mass, the capture point indicating an instantaneous position on the surface that the robot can place the swing foot to arrest a falling motion, the capture point moving along a trajectory from an initial position on the surface to a threshold-target position on the surface;

determining whether the capture point has reached the threshold-target position; and

when the capture point has reached the threshold-target position:

determining a target-touchdown position to place the swing foot of the robot in contact with the surface to generate a center of pressure between the swing foot and the surface to translate the trajectory of the capture point in the forward direction; and

causing the swing foot to contact the surface at the target-touchdown position.

14. The robot of claim 13 , wherein the target-touchdown position translates the trajectory of the capture point in the forward direction toward a current center of mass of the robot.

15. The robot of claim 13 , wherein the target-touchdown position is located at a position laterally outside of the threshold-target position.

16. The robot of claim 13 , further comprising determining the threshold-target position based on an intersection of a lateral threshold for the capture point and a forward threshold of the capture point.

17. The robot of claim 16 , wherein the lateral threshold and the forward threshold are based on a stance width and a stride length of the robot.

18. The robot of claim 16 , wherein the lateral threshold and the forward threshold are based on dynamic motion of the robot according to a linear inverted pendulum model, the linear inverted pendulum model indicating a fall rate for the robot as a function of a height of the robot between the swing foot and the position of the center of mass of the robot.

19. The robot of claim 16 , wherein:

the forward threshold of the capture point comprises a forward threshold range, the forward threshold range comprising:

a first threshold corresponding to a first nominal forward position of the capture point at a beginning of a swing period for the swing foot; and

a second threshold corresponding to a second nominal forward position of the capture point at an end of the swing period for the swing foot; and

the lateral threshold of the capture point comprises a lateral threshold range, the lateral threshold range comprising:

a first lateral threshold corresponding to a first nominal lateral position of the capture point at the beginning of the swing period for swing foot; and

a second lateral threshold corresponding to a second nominal lateral position of the capture point at the end of the swing period for swing foot.

20. The robot of claim 13 , wherein the causing the swing foot to contact the surface at the target-touchdown position occurs at a set period of time before the capture point reaches the threshold-target position, the set period of time corresponding to a range of 0-100 milliseconds.

21. The robot of claim 13 , wherein the operations further comprise:

determining that a gait disturbance has resulted in an altered trajectory for the capture point; and

updating the threshold-target position for the capture point.

22. The robot of claim 21 , wherein updating the threshold position for the capture point comprises:

estimating an intersection between the altered trajectory of the capture point and an original trajectory of the capture point; and

defining the intersection as the updated threshold position.

23. The robot of claim 13 , wherein the operations further comprise:

determining that a gait disturbance has resulted in an altered trajectory for the capture point; and

shifting a location of a center of pressure associated with the stance leg to correct the altered trajectory for the capture point towards an original trajectory for the capture point.

24. The robot of claim 13 , wherein the operations further comprise:

determining that a gait disturbance has resulted in an altered trajectory for the capture point; and

adjusting the position of the center of mass of the robot to correct the altered trajectory for the capture point towards an original trajectory for the capture point.