Robotic joint control

1. A method for limiting joint velocity of a plurality of joints of a surgical robotic system, the surgical robotic system comprising a robot having a base and an arm extending from the base to an attachment for an instrument, the arm comprising a plurality of joints whereby the configuration of the arm can be altered, the method comprising:

obtaining joint states for a first group of k joints of the arm, where k>1; and

for each of the k joints:

determining from the obtained joint state a permitted range of motion for that joint;

deriving, using the permitted range of motion, a joint velocity limit for that joint;

selecting the minimum joint velocity limit of the k joints to be a common joint velocity limit used to limit each of the k joints individually;

calculating drive signals for driving the k joints to actuate at least one of the k joints wherein the velocity of each of the k joints is limited using the common joint velocity limit; and

controlling the arm to actuate at least one of the k joints in response to the drive signals.

2. A method according to claim 1 , in which the first group of k joints comprises k joints proximal to the base of the arm, and the common joint velocity limit limits the positional velocity of the kth joint along orthogonal directions in Cartesian space.

3. A method according to claim 1 , in which the first group of k joints comprises k joints proximal to the base of the arm, and the k joints enable a position of a (k+m)th joint to be uniquely determined, where m>0.

4. A method according to claim 1 , in which the common joint velocity limit limits the positional velocity of a (k+m)th joint along orthogonal directions in Cartesian space.

5. A method according to claim 1 , in which the velocity of each of the k joints individually is limited to the common joint velocity limit.

6. A method according to claim 1 , in which the arm comprises n joints, where n>k, and the method comprises:

obtaining joint states for a second group of (n−k) joints of the arm;

for each of the (n−k) joints:

determining from the obtained joint state a permitted range of motion for that joint;

deriving, using the permitted range of motion, a joint velocity limit for that joint;

selecting the minimum joint velocity limit of the (n−k) joints to be a further common joint velocity limit for each of the (n−k) joints individually; and

calculating drive signals for driving the (n−k) joints wherein the velocity of each of the (n−k) joints is limited to the further common joint velocity limit.

7. A method according to claim 1 , in which obtaining joint states for one or both of the first group of k joints and the second group of (n−k) joints comprises obtaining joint angles.

8. A method according to claim 1 , in which determining the permitted range of motion of a joint comprises determining a closest angular distance to a joint angular limit for that joint.

9. A method according to claim 8 , in which deriving the joint velocity limit comprises using a maximum deceleration for the respective joint and the determined closest angular distance to the joint angular limit.

10. A method according to claim 6 , in which the further common joint velocity limit limits the angular velocity of a (k+m)th joint, where m>0.

11. A method according to claim 8 , in which at least one of the joint angular limit for a joint and the maximum deceleration for a joint comprises a predetermined value and/or is determined from a physical characteristic of the joint, and/or is user-definable.

12. A method according to claim 8 , in which where the determined closest angular distance to the joint angular limit exceeds a threshold angular distance, the joint velocity limit for that joint comprises a predetermined joint velocity limit value.

13. A method according to claim 1 , in which deriving the joint velocity limit comprises translating joint angular positions and/or velocities into positions and/or velocities, respectively, in Cartesian space.

14. A method according to claim 13 , in which the translating comprises determining a Jacobian matrix.

15. A method according to claim 14 , in which the translating comprises using the determined Jacobian matrix to derive an inverse matrix.

16. A method according to claim 15 , in which the translating comprises determining a Euclidean norm in respect of each row of the inverse matrix.

17. A method according to claim 1 , comprising providing feedback to a user of the surgical robotic system based on a commanded joint velocity for a joint exceeding the common joint velocity limit or further common joint velocity limit for that joint.

18. A method according to claim 17 , in which the surgical robotic system comprises an input controller manipulatable by a user thereby to alter the configuration of the arm, and the method comprises providing haptic feedback via the input controller.

19. A joint velocity limiting system for limiting joint velocity of a plurality of joints of a surgical robotic system, the surgical robotic system comprising a robot having a base and an arm extending from the base to an attachment for an instrument, the arm comprising a plurality of joints whereby the configuration of the arm can be altered, the joint velocity limiting system being configured to:

obtain joint states for a first group of k joints of the arm, where k>1; and

for each of the k joints:

determine from the obtained joint state a permitted range of motion for that joint;

derive, using the permitted range of motion, a joint velocity limit for that joint;

select the minimum joint velocity limit of the k joints to be a common joint velocity limit used to limit each of the k joints individually;

calculate drive signals for driving the k joints to actuate at least one of the k joints wherein the velocity of each of the k joints is limited using the common joint velocity limit; and

control the arm to actuate at least one of the k joints in response to the drive signals.

20. A non-transitory computer readable storage medium having stored thereon computer readable instructions that, when executed at a computer system, cause the computer system to perform a method for limiting joint velocity of a plurality of joints of a surgical robotic system, the surgical robotic system comprising a robot having a base and an arm extending from the base to an attachment for an instrument, the arm comprising a plurality of joints whereby the configuration of the arm can be altered, the method comprising:

obtaining joint states for a first group of k joints of the arm, where k>1; and

for each of the k joints:

determining from the obtained joint state a permitted range of motion for that joint;

deriving, using the permitted range of motion, a joint velocity limit for that joint;

selecting the minimum joint velocity limit of the k joints to be a common joint velocity limit used to limit each of the k joints individually;

calculating drive signals for driving the k joints to actuate at least one of the k joints wherein the velocity of each of the k joints is limited using the common joint velocity limit; and

controlling the arm to actuate at least one of the k joints in response to the drive signals.