Detecting collisions in a robotic surgical system

A method of operating a surgical system, the surgical system including a surgical manipulator having a plurality of links and joints and being configured to support and move a surgical instrument for manipulation of an anatomy, and a controller coupled to the surgical manipulator, the method comprising the controller performing the following:

• operating the surgical manipulator for controlling movement of the surgical instrument in a semi-autonomous mode;
• measuring an actual torque for at least one active joint;
• calculating an expected torque for the at least one active joint;
• comparing the actual torque and the expected torque for estimating an external force;
• determining, based on the external force, that the surgical manipulator and/or the surgical instrument has collided with an object;
• and
• in response to determining that the surgical manipulator and/or the surgical instrument has collided with the object, switching operation of the surgical manipulator for enabling movement of the surgical instrument in a manual mode.