Grip force control in a surgical robotic system

A method of controlling grip force 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, the arm comprising a plurality of joints whereby the configuration of the arm can be altered, the instrument having an end effector comprising a plurality of jaws and the drive assembly being for transferring drive to the plurality of jaws of the instrument to drive the jaws relative to each other in a closing direction for gripping an object between the jaws and in an opening direction for releasing a gripped object, the method comprising:

• controlling movement of the jaws according to a demanded spread between the jaws;
• determining an occurrence of a gripping event as the jaws are driven relative to each other in the closing direction so as to apply a grip force between the jaws;
• determining a demanded spread between the jaws on determining the occurrence of the gripping event;
• setting the determined demanded spread as a minimum spread limit for the jaws;
• and
• controlling movement of the jaws using the minimum spread limit so as to control grip force applied by the jaws.

A grip force controller for controlling grip force 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, the arm comprising a plurality of joints whereby the configuration of the arm can be altered, the instrument having an end effector comprising a plurality of jaws and the drive assembly being for transferring drive to the plurality of jaws of the instrument to drive the jaws relative to each other in a closing direction for gripping an object between the jaws and in an opening direction for releasing a gripped object, the grip force controller being configured to:

• control a force applied between the jaws according to a demanded force;
• determine an occurrence of a gripping event as the jaws are driven relative to each other in the closing direction so as to apply a grip force between the jaws;
• determine an indication of a demanded force between the jaws on determining the occurrence of the gripping event;
• set the determined demanded force as a maximum demanded force limit for the jaws;
• and
• control the force applied between the jaws using the maximum demanded force limit so as to control grip force applied by the jaws.