Operating a mobile robot

1. A method of operating a mobile robot to traverse a threshold, the method comprising:

detecting a threshold proximate the robot, the robot comprising a drive system having first, second, and third drive elements;

moving the first drive element onto the threshold from a first side;

moving the second drive element onto the threshold to place both the first and second drive elements on the threshold;

moving the first drive element off a second side of the threshold, opposite to the first side of the threshold;

moving the third drive element onto the threshold, placing both the second and third drive elements on the threshold; and

moving both the second and third drive elements off the second side of the threshold.

2. The method of claim 1 , further comprising determining an elevation change associated with the threshold and continuing to traverse the threshold when the elevation change is within a threshold elevation range.

3. The method of claim 2 , wherein the threshold elevation range prevents high centering of the robot.

4. The method of claim 2 , wherein the threshold elevation range is less than a radius of the drive elements.

5. The method of claim 1 , further comprising maneuvering the first and second drive elements to approach a first side of the threshold at least substantially equidistantly before moving the first drive element onto the threshold.

6. The method of claim 1 , further comprising pivoting about the second drive element to move the first drive element onto the threshold.

7. The method of claim 1 , further comprising holding the first drive element stationary while moving the second drive element onto the threshold to place both the first and second drive elements on the threshold.

8. The method of claim 1 , further comprising holding the second drive element stationary while moving the first drive element off the second side of the threshold.

9. The method of claim 1 , further comprising continuing to pivot the robot about the second drive element after moving the first drive element off the second side of the threshold to move the third drive element onto the threshold.

10. The method of claim 1 , further comprising moving both the second and third drive elements off the second side of the threshold at substantially the same time.

11. The method of claim 1 , further comprising moving the first and second drive elements off the threshold at the same time, pulling the third drive element over the threshold.

12. The method of claim 1 , wherein the drive system comprises first, second, and third drive wheels, each drive wheel spaced about a vertical center axis and having a drive direction perpendicular to a radial axis with respect to the vertical center axis.

13. The method of claim 1 , wherein the drive system comprises first, second, and third holonomic drive balls.

14. The method of claim 1 , wherein the drive system comprises first, second, and third drive ball assemblies, each drive ball assembly comprising:

a drive ball;

a transmission roller in continuous contact with the respective drive ball, the transmission roller actively rotating about a first axis and passively rotating about a second axis; and

a drive mechanism rotating the transmission roller to rotate the drive ball.

15. A mobile robot comprising:

a drive system having first, second, and third drive elements; and

a controller in communication with the drive system, upon detecting a threshold proximate the robot, the controller issuing commands to the drive system to:

move the first drive element onto the threshold from a first side;

move the second drive element onto the threshold to place both the first and second drive elements on the threshold;

move the first drive element off a second side of the threshold, opposite to the first side of the threshold;

move the third drive element onto the threshold, placing both the second and third drive elements on the threshold; and

move both the second and third drive elements off the second side of the threshold.

16. The mobile robot of claim 15 , further comprising:

a base supporting the drive system;

a leg extending upward from the base and having a variable height;

a torso supported by the leg, the torso defining a shoulder having a bottom surface overhanging the base; and

a torso imaging sensor disposed on the bottom surface of the torso and pointing downward along a forward drive direction of the drive system, the torso imaging sensor capturing three-dimensional images of a scene about the robot.

17. The mobile robot of claim 16 , wherein the torso imaging sensor is recessed within a body of the torso while maintaining its downward field of view.

18. The mobile robot of claim 15 , further comprising a volumetric point cloud imaging device in communication with the controller and capable of obtaining a point cloud from a volume of space adjacent the robot.

19. The mobile robot of claim 18 , wherein the volumetric point cloud imaging device scans side-to-side with respect to the forward drive direction to increase a lateral field of view of the volumetric point cloud imaging device.

20. The mobile robot of claim 18 , wherein the volumetric point cloud imaging device is positioned at a height of greater than 2 feet above the ground and directed to be capable of obtaining a point cloud from a volume of space that includes a floor plane in a direction of movement of the robot.

21. The mobile robot of claim 15 , further comprising a laser scanner in communication with the controller and having a field of view centered on the forward drive direction and substantially parallel to a work surface supporting the robot.

22. The mobile robot of claim 15 , wherein the controller determines an elevation change associated with the threshold based on a sensor signal of at least one sensor in communication with the controller and issues a command to the drive system to continue to traverse the threshold when the elevation change is within a threshold elevation range.

23. The mobile robot of claim 22 , wherein the threshold elevation range prevents high centering of the robot.

24. The mobile robot of claim 22 , wherein the threshold elevation range is less than a radius of the drive elements.

25. The mobile robot of claim 15 , wherein the controller issues a command to the drive system to maneuver the first and second drive elements to approach a first side of the threshold at least substantially equidistantly before moving the first drive element onto the threshold.

26. The mobile robot of claim 15 , wherein the controller issues a command to the drive system to pivot the robot about the second drive element to move the first drive element onto the threshold.

27. The mobile robot of claim 15 , wherein the controller issues a command to the drive system to hold the first drive element stationary while moving the second drive element onto the threshold to place both the first and second drive elements on the threshold.

28. The mobile robot of claim 15 , wherein the controller issues a command to the drive system to hold the second drive element stationary while moving the first drive element off the second side of the threshold.

29. The mobile robot of claim 15 , wherein the controller issues a command to the drive system to continue to pivot the robot about the second drive element after moving the first drive element off the second side of the threshold to move the third drive element onto the threshold.

30. The mobile robot of claim 15 , wherein the controller issues a command to the drive system to move both the second and third drive elements off the second side of the threshold at substantially the same time.

31. The mobile robot of claim 15 , wherein the controller issues a command to the drive system to move the first and second drive elements off the threshold at the same time, pulling the third drive element over the threshold.

32. The mobile robot of claim 15 , wherein the drive system comprises first, second, and third drive wheels, each drive wheel spaced about a vertical center axis and having a drive direction perpendicular to a radial axis with respect to the vertical center axis.

33. The mobile robot of claim 15 , wherein the drive system comprises first, second, and third drive balls, each drive ball spaced about a vertical center axis.

34. The mobile robot of claim 15 , wherein the holonomic drive system comprises first, second, and third drive ball assemblies, each drive ball assembly comprising:

a drive ball;

a transmission roller in continuous contact with the respective drive ball, the transmission roller actively rotating about a first axis and passively rotating about a second axis; and

a drive mechanism rotating the transmission roller to rotate the drive ball.