FIELD OF THE INVENTION
This invention relates to calibrating an instrument interface of an instrument in a surgical robotic system.
5 The calibration uses usage data indicative of usage of a joint of the instrument.
BACKGROUND
It is known to use robots for assisting and performing surgery. Figure 1 illustrates a typical surgical robot
100 which consists of a base 108, an arm 102, and an instrument 105. The base supports the robot, and is
10 itself attached rigidly to, for example, the operating theatre floor, the operating theatre ceiling or a trolley.
The arm extends between the base and the instrument. The arm is articulated by means of multiple flexible
joints 103 along its length, which are used to locate the surgical instrument in a desired location relative to
the patient. The surgical instrument is attached to the distal end 104 of the robot arm. The surgical
instrument penetrates the body of the patient 101 at a port 107 so as to access the surgical site. At its distal
15 end, the instrument comprises an end effector 106 for engaging in a medical procedure.
Figure 2 illustrates a typical surgical instrument 200 for performing robotic laparoscopic surgery. The
surgical instrument comprises an instrument interface 201 by means of which the surgical instrument
connects to the robot arm. A shaft 202 extends between the interface 201 and an articulation 203. The
20 articulation 203 terminates in an end effector 204. In figure 2, a pair of serrated jaws are illustrated as the
end effector 204. The articulation 203 permits the end effector 204 to move relative to the shaft 202. It is
desirable for at least two degrees of freedom to be provided to the motion of the end effector 204 by
means of the articulation. The instrument interface is configured to couple to an arm drive assembly of the
robot arm to which the instrument is engageable. When the instrument is engaged with the arm, the
25 instrument interface engages with the drive assembly thereby to transfer drive to the instrument.
Figure 3 illustrates a surgical robot having an arm 300 which extends from a base 301. The arm comprises a
number of rigid limbs 302. The limbs are coupled by revolute joints 303. The most proximal limb 302a is
coupled to the base by a proximal joint 303a. It and the other limbs are coupled in series by further ones of
30 the joints 303. Suitably, a wrist 304 is made up of four individual revolute joints. The wrist 304 couples one
limb (302b) to the most distal limb (302c) of the arm. The most distal limb 302c carries an attachment 305
for a surgical instrument 306. Each joint 303 of the arm has one or more motors 307 which can be operated
to cause rotational motion at the respective joint, and one or more position and/or torque sensors 308
which provide information regarding the current configuration and/or load at that joint. Suitably, the
35 motors are arranged proximally of the joints whose motion they drive, so as to improve weight distribution.
3
For clarity, only some of the motors and sensors are shown in figure 3. The arm may be generally as
described in patent application PCT/GB2014/053523 (WO 2015/132549).
The arm terminates in the attachment 305 for interfacing with the instrument 306. Suitably, the instrument
5 306 takes the form described with respect to figure 2. The instrument has a diameter less than 8mm.
Suitably, the instrument has a 5mm diameter. The instrument may have a diameter which is less than 5mm.
The instrument diameter may be the diameter of the shaft. The instrument diameter may be the diameter
of the profile of the articulation. Suitably, the diameter of the profile of the articulation matches or is
narrower than the diameter of the shaft. The attachment 305 comprises a drive assembly for driving
10 articulation of the instrument. Movable interface elements of the drive assembly mechanically engage
corresponding movable interface elements of the instrument interface in order to transfer drive from the
robot arm to the instrument. One instrument is exchanged for another several times during a typical
operation. Thus, the instrument is attachable to and detachable from the robot arm during the operation.
Features of the drive assembly interface and the instrument interface aid their alignment when brought into
15 engagement with each other, so as to reduce the accuracy with which they need to be aligned by the user.

CLAIMS
1. A method of calibrating an instrument interface of an instrument in a surgical robotic system, the
surgical robotic system comprising a robot having a base and an arm extending from the base to a drive
assembly for engaging with the instrument interface to transfer drive to the instrument, the instrument
5 interface being configured to drive joints of the instrument via driving elements, the method
comprising:
obtaining usage data indicative of usage of a joint of the instrument;
comparing the usage data with one or both of a maximum range of joint movement of the joint
and a model of expected joint movement of the joint;
10 determining, from the comparison, a calibration offset to adjust a control relationship of a
driving element arranged to drive the joint; and
adjusting the control relationship of the driving element using the calibration offset so as to
calibrate the instrument interface.
15 2. A method according to claim 1, in which determining the calibration offset comprises determining,
from the comparison, a bias value, indicative of bias of joint usage away from an expected joint usage;
and determining the calibration offset from the bias value.
3. A method according to claim 1 or claim 2, in which the control relationship of the driving element
20 comprises a positioning of the driving element, and adjusting the control relationship comprises
adjusting the positioning of the driving element using the calibration offset.
4. A method according to any preceding claim, in which the control relationship of the driving element
comprises a configuration of a gearing arranged to drive the driving element, and adjusting the control
25 relationship comprises adjusting the configuration of the gearing using the calibration offset.
5. A method according to claim 4, in which adjusting the control relationship comprises changing from a
first gear ratio to a second gear ratio, moving the driving element, and changing from the second gear
ratio back to the first gear ratio.
30
6. A method according to any preceding claim, in which the control relationship of the driving element
comprises a tension of the driving element, and adjusting the control relationship comprises adjusting
the tension of the driving element using the calibration offset.
35
7. A method according to any preceding claim, in which the method comprises obtaining the usage data
from a memory and/or from the instrument.
8. A method according to any preceding claim, in which the usage data comprises joint angle data of the
5 joint of the instrument.
9. A method according to any of claims 1 to 7, in which the usage data comprises instrument driver
position data of an instrument driver configured to drive the joint of the instrument.
10 10. A method according to any preceding claim, in which the usage data is obtained based on one or
more of: a type of instrument, a procedure to be performed using the instrument, a stage in a
procedure to be performed using the instrument, an action within a procedure to be performed using
the instrument, an ID of a surgical robotic system to which the instrument is attached, and a surgeon ID.
15 11. A method according to any preceding claim, in which comparing the usage data with the maximum
range of joint movement of the joint comprises using at least one of a probability density of instrument
driver positions and/or joint angular positions, a machine learning algorithm, a statistical analysis
technique, and a goodness-of-fit technique.
20 12. A method according to any preceding claim, in which determining the calibration offset comprises
identifying a characteristic instrument driver position and/or joint angle from the comparison of the
usage data with the respective maximum range of instrument driver position and/or joint movement of
the joint, and the method comprises determining the calibration offset from the characteristic
instrument driver position and/or joint angle.