A robot comprising: a base； an articulated arm extending distally of the base and including two arm members coupled by a joint； a motor； a gearbox having an input shaft coupled to an output of the motor and an output shaft configured to drive relative motion of the arm members about the joint； a position sensor configured to sense relative position of the arm members about the joint； and a control system coupled to the arm configured to drive the motor, the control system being arranged to perform a calibration operation to estimate torque loss in the gearbox by the steps of (i) estimating the inertia of the portion of the arm distal of the joint for motion about the joint； (ii) applying a determined drive power to the motor； (iii) receiving from the position sensor position data indicating the motion of the arm in response to the applied drive power； and (iv) estimating the torque loss in the gearbox in dependence on the estimated inertia, the determined drive power and the position data.
