Drive mechanisms for robot arms
A robot arm comprising a joint mechanism for articulating one limb of the arm relative to another limb of the arm about two non-parallel rotation axes, the mechanism comprising: an intermediate carrier attached to a first one of the limbs by a first revolute joint having a first rotation axis and to a second one of the limbs by a second revolute joint having a second rotation axis; a first drive gear disposed about the first rotation axis, the first drive gear being fast with the carrier; a second drive gear disposed about the second rotation axis, the second drive gear being fast with the second one of the limbs; a first drive shaft for driving the first drive gear to rotate about the first rotation axis, the first drive shaft extending along the first one of the limbs and having a first shaft gear thereon, the first shaft gear being arranged to engage the first drive gear; a second drive shaft for driving the second drive gear to rotate about the second rotation axis, the second drive shaft extending along the first one of the limbs and having a second shaft gear thereon, the second shaft gear being arranged to engage the second drive gear; the second drive shaft comprising a prismatic joint whereby the length of the shaft can vary in response to motion of the carrier about the first axis.
1. A robot arm comprising a joint mechanism for articulating one limb of the arm relative to another limb of the arm about two non-parallel rotation axes, the mechanism comprising:
an intermediate carrier attached to a first one of the limbs by a first revolute joint having a first rotation axis and to a second one of the limbs by a second revolute joint having a second rotation axis;
a first drive gear disposed about the first rotation axis, the first drive gear being fast with the carrier;
a second drive gear disposed about the second rotation axis, the second drive gear being fast with the second one of the limbs;
a first drive shaft for driving the first drive gear to rotate about the first rotation axis, the first drive shaft extending along the first one of the limbs and having a first shaft gear thereon, the first shaft gear being arranged to engage the first drive gear;
a second drive shaft for driving the second drive gear to rotate about the second rotation axis, the second drive shaft extending along the first one of the limbs and having a second shaft gear thereon, the second shaft gear being arranged to engage the second drive gear;
the second drive shaft comprising a prismatic joint whereby the length of the shaft can vary in response to motion of the carrier about the first axis.
2. A robot arm as claimed in claim 1 , wherein the prismatic joint is a sliding splined coupling.
3. A robot arm as claimed in claim 1 , wherein the second dive shaft comprises a first flexible joint on one side of the prismatic joint and a second flexible joint on the other side of the prismatic joint.
4. A robot arm as claimed in claim 3 , wherein the second drive shaft is connected by a revolute joint to the carrier on the opposite side of the second flexible joint to the prismatic joint.
5. A robot arm as claimed in claim 1 , wherein one or both of the first and second shaft gears is/are worm gears.
6. A robot arm as claimed in claim 1 , wherein one or both of the first drive gears is/are bevel gear(s).
7. A robot arm as claimed in claim 6 , wherein one or both of the first drive gears is/are skew axis gear(s).
8. A robot arm as claimed in claim 1 , wherein the first drive gear is a part-circular gear.
9. A robot arm as claimed in claim 8 , wherein at least part of the second drive gear intersects a circle about the first axis that is coincident with the radially outermost part of the first drive gear.
10. A robot arm as claimed in claim 5 , wherein the first and second axes are orthogonal.
11. A robot arm as claimed in claim 5 , wherein the first and second axes intersect each other.
12. A robot arm as claimed in any preceding claim, further comprising a control unit arranged to respond to command signals commanding rotation 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.
13. A robot arm as claimed in any preceding claim, further comprising a control unit arranged to respond to command signals commanding rotation 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 second rotation axis when the second one of the limbs is articulated with respect to the first one of the limbs about the first rotation axis, drive the second drive shaft to rotate to cause articulation about the second axis in such a way as to provide smooth rotation about the second axis.