Method and system for determining the relation between a robot coordinate system and a local coordinate system located in the working range of the robot

1. A method for determining the relation between a local coordinate system located in the working range of an industrial robot and a robot coordinate system, wherein the method comprises:

attaching a first calibration object in a fixed relation to the robot,

determining the position of the first calibration object in relation to the robot,

locating at least three second calibration objects in the working range of the robot, wherein at least one of the calibration objects is a male calibration object having a protruding part shaped as a sphere, and at least one of the calibration objects is a female calibration object comprising at least two nonparallel, inclining surfaces arranged to receive the sphere so that the sphere is in contact with the surfaces in at least one reference position, wherein the inclination of the surfaces is in the interval of 20-80°,

determining at least one reference position for each of the second calibration objects in the local coordinate system,

A) moving the robot in a compliant way until the sphere is in mechanical contact with said surfaces of the female calibration object,

B) reading the position of the robot when the sphere is in mechanical contact with said surfaces,

repeating the steps A-B for the other second calibration objects, and

calculating the relation between the local coordinate system and the robot coordinate system based on the position of the first calibration object in relation to the robot, the reference positions of the second calibration objects in the local coordinate system, and the positions of the robot when the sphere is in mechanical contact with the surfaces of the second calibration objects.

2. The method according to claim 1 , wherein at least one of the calibration objects comprises three nonparallel, inclining surfaces arranged to receive the sphere so that the sphere is in contact with all three surfaces at the reference position.

3. The method according to claim 1 , wherein the sphere is attached to the robot, the other calibration objects are shaped as grooves with two inclining surfaces, and at least three grooves are located in the robot work area such that their longitudinal directions are non parallel, and the method comprises moving the robot in a compliant way until the sphere has been in mechanical contact with both surfaces on all three grooves and the positions of the robot has been stored for all three grooves, and calculating the relation between the local coordinate system and the robot coordinate system based on the positions of the robot when the sphere is in mechanical contact with the surfaces of said grooves.

4. The method according to claim 1 , wherein the robot is automatically moved by means of force control.

5. The method according to claim 1 , wherein the robot is automatically moved by means of a soft servo.

6. A system for determining the relation between a local coordinate system located in the working range of an industrial robot and a robot coordinate system, wherein the system comprises:

a first calibration object to be attached in a fixed relation to the robot,

three second calibration objects to be positioned in the working range of the robot, wherein at least one of the calibration objects is a male calibration object having a protruding part shaped as a sphere, and at least one of the calibration objects is a female calibration object comprising at least two nonparallel, inclining surfaces arranged to receive the sphere so that the sphere is in contact with the surfaces at a reference position, wherein the inclination of the surfaces is in the interval of 20-80°,

means for automatically moving the robot in a compliant way until the sphere is in mechanical contact with the said surfaces of the female calibration object, and

a computing unit configured to receive and store the position of the robot when the sphere is in mechanical contact with said surfaces, and to calculate the relation between the local coordinate system and the robot coordinate system based on a known relation between the first calibration object and the robot, the reference positions in the local coordinate system, and the position of the robot when the sphere is in mechanical contact with said surfaces.

7. The system according to claim 6 , wherein at least one of the calibration objects comprises three nonparallel, inclining surfaces arranged to receive the sphere so that the sphere is in contact with all three surfaces at the reference position.

8. The system according to claim 7 , wherein the calibration object is shaped as a truncated inner corner of a cube.

9. The system according to claim 6 , wherein the sphere is attached to the robot, the other calibration object is shaped as a groove with two inclining surfaces, and at least three grooves are located in the robot work area such that their longitudinal directions are non parallel.

10. The system according to claim 6 , further comprising a force sensor adapted to measure forces in at least two directions, which force sensor is located between the first calibration object and the robot, and a control unit configured to move the robot in dependence of force measurements from the force sensor.

11. The system according to claim 6 , further comprising a soft servo.

12. The system according to claim 6 , wherein the second calibration objects are located on a work piece to be processed by the robot or on a fixture holding the work piece.

13. The system according to claim 6 , wherein the local coordinate system located in the working range of the industrial robot is a coordinate system of a fixture for holding a work piece to be processed by the robot.

14. The method according to claim 1 , wherein the local coordinate system located in the working range of the industrial robot is a coordinate system of a fixture for holding a work piece to be processed by the robot.