Reducing Kinematic Error

A method for reducing kinematic error in a joint between a distal portion and a proximal portion of a manipulator, the joint having associated to it a motor mounted in one of said portions and coupled to the other one of said portions for driving rotation of the distal portion relative to the proximal portion by a reduction gear, the method comprising:

• providing a kinematic correction generator that is programmable using a parameter vector and outputting, based on said parameter vector and a position input, a kinematic correction, which is a sum of at least one sinusoid having a frequency which is a frequency of rotation of the motor or of a periodic event in the reduction gear occurring at a higher frequency than the rotation of the distal portion, or an integer multiple thereof, and an amplitude and a phase defined by said parameter vector;
• outputting to the motor a first drive control signal specifying a motor speed which is a sum of a standard speed and a first kinematic correction output by the kinematic correction generator based on a first parameter vector;
• extracting, from an evaluation signal received from the joint in response to the first drive control signal, at least one frequency component having the frequency of the at least one sinusoid, and determining a first feedback vector defining a first feedback amplitude and a first feedback phase of said frequency component;
• outputting to the motor a second drive control signal specifying a motor speed which is a sum of the standard speed and a second kinematic correction output by the kinematic correction generator based on a second parameter vector;
• extracting, from an evaluation signal received from the joint in response to the second drive control signal, a frequency component having the frequency of the at least one sinusoid, and determining a second feedback vector defining a second feedback amplitude and a second feedback phase of said frequency component;
• subtracting said first feedback vector from said second feedback vector to obtain a feedback difference vector;
• finding a transformation which transforms said feedback difference vector into a difference between second and first parameter vectors;
• applying the transformation to the second feedback vector to obtain a third parameter vector;
• and
• programming the kinematic correction generator using the third parameter vector.