Detecting collisions of robot arms

A method of determining collisions between a first surgical robot and a second surgical robot of a surgical robotic system, each surgical robot having a base, and an arm extending from the base to an attachment configured to attach an instrument, the arm comprising a plurality of joints whereby the configuration of the arm can be altered, for each joint the surgical robot comprising a driver configured to drive the joint to move, and a position sensor and torque sensor configured to sense the joint, the surgical robotic system further comprising a control unit configured to receive inputs from the position sensors and torque sensors and configured to control the drivers in dependence on those received inputs, the method comprising:

• determining the gravitational torques on the joints of the arms of the first and second surgical robots in the arm configurations indicated from the inputs from the position sensors;
• from the inputs from the torque sensors and the determined gravitational torques, determining residual torques on the joints of the arms of the first and second surgical robots in the indicated arm configurations;
• calculating one or more candidate force for each arm which when applied to that arm would cause the determined residual torques;
• and
• determining a collision if one or more candidate force on the arm of the first surgical robot balances an opposing candidate force on the arm of the second surgical robot.