Controlling a joint in a robotic arm

A method of controlling the configuration of a joint in a surgical robot, the joint being driven by a drivetrain which transfers power from a drive source to the joint, the method comprising:

• receiving a drive source configuration signal indicating a configuration of the drive source;
• receiving a joint configuration signal indicating a configuration of the joint;
• determining a configuration error from a difference in configurations indicated by the drive source configuration signal and the joint configuration signal;
• determining a configuration error limit from a measured joint torque of the joint and a joint stiffness of the joint;
• comparing the determined configuration error and the determined configuration error limit;
• and
• outputting a fault signal where the determined configuration error equals or exceeds the determined configuration error limit.

A control system for controlling the configuration of a joint in a surgical robot, the joint being driven by a drivetrain which transfers power from a drive source to the joint, the control system comprising:

• a joint controller coupled to the joint;
• and
• a robot controller coupled to the surgical robot;
• the control system being configured to:
• receive a drive source configuration signal indicating a configuration of the drive source;
• receive a joint configuration signal indicating a configuration of the joint;
• determine a configuration error from a difference in configurations indicated by the drive source configuration signal and the joint configuration signal;
• output a first fault signal where the determined configuration error equals or exceeds a first configuration error limit determined from a measured joint torque of the joint and a joint stiffness of the joint, the first configuration error limit being associated with one of the joint controller and the robot arm controller;
• and
• output a second fault signal where the determined configuration error equals or exceeds a predetermined second configuration error limit, the second configuration error limit being associated with the other of the joint controller and the robot arm controller.