POWERING A SURGICAL ROBOT ARM

A control system for a surgical robotic system, the surgical robotic system comprising a remote surgeon console having a surgeon input device, and a surgical robot arm comprising a series of joints extending from a base to a terminal end for attaching to a surgical instrument, the surgical robot arm operable in a full power mode in which the joints of the surgical robot arm are powered by a first power source and a reduced power mode in which the joints of the surgical robot arm are powered by a second power source, he control system configured to: whilst the surgical robot arm is operating in the full power mode, control the surgical robot arm in a surgical mode by converting inputs from the surgeon input device to control signals for moving joints of the surgical robot arm; detect a power failure of the first power source; in response to detecting the power failure, enable the reduced power mode, and control the surgical robot arm in a locked mode by sending control signals to lock joints of the surgical robot arm; whilst in the reduced power mode, detect a cessation of the power failure; and in response to detecting the cessation of the power failure, disable the reduced power mode, re-enable the full power mode, and control the surgical robot arm in the surgical mode.