A robot comprises a base 20, an articulated arm 21-28 extending distally of the base 20 and including two arm members 24, 25 coupled by a joint 33, a motor 51, a gearbox 53 having an input shaft 52 coupled to an output of the motor 51 and an output shaft 55 arranged for driving relative motion of the arm members 24, 25 about the joint 33, a position sensor 60 for sensing relative position of the arm members 24, 25 about the joint 33 and a control system coupled to the arm for driving the motor, the control system being arranged to perform a calibration operation to estimate torque loss in the gearbox 53 by the steps of (i) estimating the inertia of the portion of the arm 25 distal of the joint 33 for motion about the joint 33 (ii) applying a determined drive power to the motor 51 (iii) receiving from the position sensor 60 position data indicating the motion of the arm in response to the applied drive power and (iv) estimating the torque loss in the gearbox 53 in dependence on the estimated inertia, the determined drive power and the position data. Joint 33 may be a revolute joint or a linear joint and the arm may comprise one or more further joints 30-37. A torque sensor 59 may sense torque applied between the output shaft of the gearbox and arm member 25. The robot may be a surgical robot with a surgical instrument 29 attached at the distal end 28 of the arm 21-28.
