Surgical robot driving mechanism

A surgical robot comprising a surgical robot arm and a surgical instrument. The surgical robot arm terminates at its distal end in a drive assembly comprising a first drive interface element. The surgical instrument comprises a shaft, an articulation attached to the distal end of the shaft, and a driving mechanism at the proximal end of the shaft. The articulation is for articulating an end effector, the articulation driveable by at least a first driving element and a second driving element. The driving mechanism comprises a first instrument interface element to which the first driving element is connected and a second instrument interface element to which the second driving element is connected. The first drive interface element engages both the first and second instrument interface elements such that: when the first drive interface element moves linearly, both the first and second instrument interface elements move in the same linear direction thereby driving both the first and second driving elements in the same direction; and when the first drive interface element rotates, the first and second instrument interface elements move reciprocally in opposing linear directions thereby driving the first and second driving elements in opposing directions.