Controlling roll for a device in a computer-assisted medical system

1. A method of controlling roll for a device in a computer-assisted medical system, the method comprising:

specifying a reference frame that corresponds to a reference orientation for the device, the reference frame including a reference yaw axis, a reference pitch axis, and a reference roll axis;

accessing values for a device frame that corresponds to an orientation of the device, the device frame including a device yaw axis, a device pitch axis, and a device roll axis;

determining a roll-angle offset about the device roll axis, the roll-angle offset characterizing an angular difference about the device roll axis between the device frame and a roll-axis-alignment rotation of the reference frame, and the roll-axis-alignment rotation corresponding to a rotation about a combination of the reference yaw axis and the reference pitch axis to align the reference roll axis with the device roll axis; and

controlling roll for the device by controlling the roll-angle offset about the device roll axis.

2. The method of claim 1 , wherein controlling the roll-angle offset includes rotating the device about the device roll axis to maintain a specified roll-angle offset.

3. The method of claim 1 , wherein controlling the roll-angle offset includes rotating the device about the device roll axis by an amount based on a comparison between the determined roll-angle offset at a given time and a specified roll-angle offset.

4. The method of claim 1 , further comprising:

specifying a remote center of motion (RCM) at a given location of the device, the RCM corresponding to an origin of the device frame; and

maintaining the RCM at a given location in the reference frame while controlling roll for the device by controlling the roll-angle offset about the device roll axis.

5. The method of claim 1 , further comprising;

determining a first orientation of the device from the accessed values for the device frame at a first time, the first orientation corresponding to a first roll-angle offset about the device roll axis;

specifying a second orientation corresponding to a second roll-angle offset about the device roll axis; and

controlling the device from the first orientation to the second orientation by controlling the roll-angle offset about the device roll axis from the first roll-angle offset to the second roll-angle offset in combination with operations for controlling the device about the device pitch axis and device roll axis from the first orientation to the second orientation.

6. The method of claim 1 , wherein the reference frame is a first reference frame that corresponds to a first position and orientation for the device, and the method further comprises:

disengaging the control of the roll-angle offset with respect to the first reference frame at a first time;

after a movement of the device to a second position and orientation for the device at a second time that is after the first time,

specifying a second reference frame from accessed values for the device frame at the second position and orientation for the device; and

controlling the roll-angle offset with respect to the second reference frame at a third time that is after the second time.

7. A computer-assisted medical system comprising at least one computer configured to perform operations for computer-implemented modules including:

a reference-frame module configured to specify a reference frame that corresponds to a reference orientation for a device, the reference frame including a reference yaw axis, a reference pitch axis, and a reference roll axis;

a device-frame module configured to access values for a device frame that corresponds to an orientation of the device, the device frame including a device yaw axis, a device pitch axis, and a device roll axis;

a roll-angle module configured to determine a roll-angle offset about the device roll axis, the roll-angle offset characterizing an angular difference about the device roll axis between the device frame and a roll-axis-alignment rotation of the reference frame, and the roll-axis-alignment rotation corresponding to a rotation about a combination of the reference yaw axis and the reference pitch axis to align the reference roll axis with the device roll axis; and

a roll-control module configured to control roll for the device by controlling the roll-angle offset about the device roll axis.

8. The system of claim 7 , wherein controlling the roll-angle offset includes rotating the device about the device roll axis to maintain a specified roll-angle offset.

9. The system of claim 7 , wherein controlling the roll-angle offset includes rotating the device about the device roll axis by an amount corresponding to a difference between the determined roll-angle offset at a given time and a specified roll-angle offset.

10. The system of claim 7 , wherein controlling the roll-angle offset includes rotating the device about the device roll axis by an amount based on a comparison between the determined roll-angle offset at a given time and a specified roll-angle offset.

11. The system of claim 7 , wherein:

a difference between the device frame and the reference frame includes a yaw-angle offset relative to the reference yaw axis and a pitch-angle offset relative to the reference pitch axis; and

the roll-axis-alignment rotation corresponds to a rotation for the yaw-angle offset about the reference yaw axis, a rotation for the pitch-angle offset about the reference pitch axis, and no rotation about the reference roll axis.

12. The system of claim 7 , wherein the roll-control module is further configured to perform operations comprising:

specifying a remote center of motion (RCM) at a given location of the device, the RCM corresponding to an origin of the device frame; and

maintaining the RCM at a given location in the reference frame while controlling roll for the device by controlling the roll-angle offset about the device roll axis.

13. The system of claim 7 , wherein the roll-control module is further configured to perform operations comprising:

determining a first orientation of the device from the accessed values for the device frame at a first time, the first orientation corresponding to a first roll-angle offset about the device roll axis;

specifying a second orientation corresponding to a second roll-angle offset about the device roll axis; and

controlling the device from the first orientation the second orientation by controlling the roll-angle offset about the device roll axis from the first roll-angle offset to the second roll-angle offset in combination with operations for controlling the device about the device pitch axis and device roll axis from the first orientation to the second orientation.

14. The system of claim 7 , wherein the roll-control module is further configured to perform operations comprising:

accessing values for the device frame at an initial position and orientation for the device;

specifying the reference frame from the accessed values for the device frame at the initial position and orientation for the device, the reference frame being specified to include athc reference position and the reference orientation for the device; and

specifying a roll-angle offset corresponding to the initial position and orientation for the device, the device being controlled about the device roll axis to maintain the specified roll-angle offset.

15. The system of claim 7 , wherein the reference frame is a first reference frame that corresponds to a first position and orientation for the device, and the roll-control module is further configured to perform operations comprising:

disengaging the control of the roll-angle offset with respect to the first reference frame at a first time;

accessing values for the device frame at a second position and orientation for the device at a second time that is after the first time;

specifying a second reference frame from the accessed values for the device frame at the second position and orientation for the device; and

controlling the roll-angle offset with respect to the second reference frame at a third time that is after the second time.

16. The system of claim 7 , wherein:

controlling roll for the device includes transmitting at least one command to at least one actuator for rotating the device about the device roll axis; and

wherein the device is a medical device that includes a spar for mounting a surgical tool, the spar having a long axis that corresponds to the device roll axis.

17. The system of claim 7 , wherein the roll-angle offset is controlled independently of control for pitch about the device pitch axis and yaw about the device yaw axis.

18. A computer-readable medium that stores a computer program for controlling roll for a device in a computer-assisted medical system, the computer program including computer-program instructions that, when executed by at least one computer, cause the at least one computer to perform operations comprising:

specifying a reference frame that corresponds to a reference orientation for the device, the reference frame including a reference yaw axis, a reference pitch axis, and a reference roll axis;

accessing values for a device frame that corresponds to an orientation of the device, the device frame including a device yaw axis, a device pitch axis, and a device roll axis;

determining a roll-angle offset about the device roll axis, the roll-angle offset characterizing an angular difference about the device roll axis between the device frame and a roll-axis-alignment rotation of the reference frame, and the roll-axis-alignment rotation corresponding to a rotation about a combination of the reference yaw axis and the reference pitch axis to align the reference roll axis with the device roll axis; and

controlling roll for the device by controlling the roll-angle offset about the device roll axis.

19. The computer-readable medium of claim 18 , wherein controlling the roll-angle offset includes:

rotating the device about the device roll axis to maintain a specified roll-angle offset; or

rotating the device about the device roll axis by an amount corresponding to a difference between the determined roll-angle offset at a given time and a specified roll-angle offset; or

rotating the device about the device roll axis by an amount based on a comparison between the determined roll-angle offset at a given time and a specified roll-angle offset.

20. The computer-readable medium of claim 18 , wherein the operations further comprise:

specifying a remote center of motion (RCM) at a given location of the device, the RCM corresponding to an origin of the device frame; and

maintaining the RCM at a given location in the reference frame while controlling roll for the device by controlling the roll-angle offset about the device roll axis.