Gear packaging for robotic joints

1. A robot arm comprising a first arm segment having a first longitudinal axis and a second arm segment having a second longitudinal axis, the first and second arm segments coupled to each other by a first revolute joint having a first rotation axis which is perpendicular to the first longitudinal axis and a second revolute joint having a second rotation axis which is perpendicular to the second longitudinal axis and intersects and is non-parallel to the first rotation axis, and a joint mechanism configured to articulate the first arm segment relative to the second arm segment about the first and second rotation axes, the joint mechanism comprising:

a first driven gear disposed about an axle coincident with the first rotation axis;

a second driven gear disposed about the second rotation axis and fast with the second arm segment of the robot arm;

a first drive gear disposed about a third rotation axis and configured to drive the first driven gear to rotate about the first rotation axis, the first drive gear being arranged to engage the first driven gear;

a second drive gear disposed about a fourth rotation axis and configured to drive the second driven gear to rotate about the second rotation axis;

an intermediary gear arrangement arranged to engage the second drive gear and the second driven gear, whereby rotation of the intermediary gear arrangement relative to the first arm segment can be driven, the intermediary gear arrangement comprising a first intermediate gear arranged to engage the second drive gear, and a second intermediate gear arranged to engage the second driven gear, the second intermediate gear intersecting a projection of a profile of the second drive gear in the direction of the fourth rotation axis.

2. A robot arm as claimed in claim 1 , wherein the first rotation axis is transverse to the second rotation axis.

3. A robot arm as claimed in claim 1 , wherein the first drive gear is attached to a first drive shaft extending along the first arm segment and/or the second drive gear is attached to a second drive shaft extending along the first arm segment.

4. A robot arm as claimed in claim 1 , wherein the first drive gear is attached to a first drive shaft extending along the first arm segment and the second drive gear is attached to a second drive shaft extending along the first arm segment, and the first and second drive shafts lie on one side of a plane on which the axle lies.

5. A robot arm as claimed in claim 1 , wherein the first driven gear is a ring gear.

6. A robot arm as claimed in claim 5 , wherein the first driven gear is a hypoid gear.

7. A robot arm as claimed in claim 1 , wherein the intermediary gear arrangement is mounted on the axle, the axle being mounted at each of its ends to the first arm segment.

8. A robot arm as claimed in claim 1 , wherein the first intermediate gear is fast with the second intermediate gear.

9. A robot arm as claimed in claim 1 , wherein the first driven gear is a sector.

10. A robot arm as claimed in claim 9 , wherein an operative arc of the sector is less than or equal to 50 degrees.

11. A robot arm as claimed in claim 1 , wherein the first intermediate gear is a ring gear.

12. A robot arm as claimed in claim 1 , wherein the first intermediate gear is a hypoid gear.

13. A robot arm as claimed in claim 1 , wherein the first drive gear and first driven gear form a first hypoid gear set, and the second drive gear and first intermediate gear form a second hypoid gear set, the first and second hypoid gear sets having an equal skew offset.

14. A robot arm as claimed in claim 1 , wherein the second intermediate gear and the second driven gear are bevel gears.

15. A robot arm as claimed in claim 1 , wherein the joint mechanism further comprises a carrier attached to the first arm segment by the first revolute joint and attached to the second arm segment by the second revolute joint, the carrier being fast with the first driven gear.

16. A robot arm as claimed in claim 1 , wherein the first and second revolute joints are wrist joints of the robotic arm.

17. A robot arm as claimed in claim 1 , wherein the robot arm is a surgical robot arm, and the robot arm comprises a tool or tool attachment fast with the second arm segment about the first and second rotation axes.

18. A robot arm as claimed in claim 1 , wherein the robot arm comprises a third arm segment adjacent the second arm segment and located on the opposite side of the second arm segment to the joint mechanism, and a third revolute joint that attaches the third arm segment to the second arm segment, wherein the third arm segment comprises a tool or a tool attachment.

19. A robot arm as claimed in claim 1 , further comprising a control unit arranged to respond to command signals commanding motion of the robot arm by driving the first and second drive shafts to rotate, the control unit being configured to, when the robot arm is commanded to articulate about the first axis without articulating about the second axis, drive the first shaft to rotate to cause articulation about the first axis and also drive the second shaft to rotate in such a way as to negate parasitic articulation about the second axis.

20. A robot arm as claimed in claim 1 , wherein the third rotation axis and the first rotation axis are non-intersecting.

21. A robot arm as claimed in claim 1 , wherein the first intermediate gear is a sector.

22. A robot arm as claimed in claim 18 , wherein the third arm segment has a third longitudinal axis and the third revolute joint has a fifth rotation axis which is colinear with the third longitudinal axis.