CONTROLLING A SURGICAL INSTRUMENT

A control system for controlling manipulation of a surgical instrument in response to manipulation of a remote surgeon input device. The surgical instrument comprises opposable first and second end effector elements connected to a shaft by an articulated coupling. The control system: transforms commands from the surgeon input device to alter the opening angle between the first and second end effector elements according to a first control relationship to drive signals to drive the first and second end effector elements to rotate; receives sensed forces applied to the first and second end effector elements, and compares the sensed forces to a threshold force; and upon determining that the threshold force has been exceeded, transforms subsequent commands from the surgeon input device to alter the opening angle between the first and second end effector elements according to a second control relationship to drive signals to drive the first and second end effector elements to rotate, wherein the second control relationship is different to the first control relationship.