System and method for navigating a robotic lawnmower into a docketing position

1. A robotic lawnmower system comprising a charging station and a robotic work tool, the charging station comprising a signal generator to which a navigation signal cable is to be connected, the signal generator being configured to transmit a signal through the navigation signal cable, and the robotic working tool comprising:

a propulsion system;

a sensor configured to sense field values of magnetic fields generated by the signal in the navigation signal cable; and

a controller configured to

determine that the robotic lawnmower is in a docking position, and in response thereto

record the field values in the docking position by sensing field values of the signal in the docking position and storing the field values of the sensed signal as a stored field value; and

control the propulsion system to move out of said docking position;

and to control the propulsion system to enter into said docking position by

sensing a current magnetic field value;

comparing the current magnetic field value to the stored field value; and

determining how to navigate the robotic lawnmower based on the comparison and navigating accordingly.

2. The robotic lawnmower system according to claim 1 , wherein the controller is further configured to record a plurality of magnetic field values while exiting the charging station.

3. The robotic lawnmower system according to claim 1 , wherein the navigation signal cable is a guide cable configured to extend into a work area of the robotic lawnmower for guiding the robotic lawnmower to the docking station.

4. The robotic lawnmower system according to claim 3 , wherein the controller is further configured to record magnetic field values also from an F field and/or an N field.

5. The robotic lawnmower system according to claim 3 , wherein the controller is further configured to record field values also from a magnetic field emitted by a control signal being transmitted through the boundary cable.

6. The robotic lawnmower system according to claim 1 , wherein the controller is further configured to stop for recording the field values.

7. The robotic lawnmower system according to claim 1 , wherein the controller is further configured to record said field values while moving.

8. The robotic lawnmower system according to claim 7 , wherein the controller is further configured to record said field values while moving at a reduced speed relative a standard operating speed.

9. The robotic lawnmower system according to claim 1 , wherein the controller is further configured to determine that the robotic lawnmower is in a docking position by determining that there is contact between a charging connector comprised in the robotic lawnmower and a charging connector comprised in the charging station.

10. The robotic lawnmower system according to claim 1 , wherein the controller is further configured to record an amplitude and a polarity of the current magnetic field as a magnetic field value.

11. The robotic lawnmower system according to claim 1 , wherein the guide signal comprises synchronization information, and wherein the controller is further configured to record the synchronization information.

12. The robotic lawnmower system according to claim 1 , wherein the robotic lawnmower is configured to filter the sensed signal in time, frequency, and/or code domain in a filtering arrangement locking on the generated signal.

13. The robotic lawnmower system according to claim 1 , wherein recording the field values of the sensed signal comprises associating a plurality of field values with respective robot positions, and storing the associations in a memory.

14. The robotic lawnmower system according to claim 1 , wherein determining how to navigate the robotic lawnmower comprises determining a set of consecutive magnetic field values for the robotic lawnmower to follow.

15. A method for use in a robotic lawnmower system comprising a charging station and a robotic work tool, the charging station comprising a signal generator to which a navigation signal cable is to be connected, the signal generator being configured to transmit a signal through the navigation signal cable, and the robotic working tool comprising:

a propulsion system;

a sensor configured to sense field values of magnetic fields generated by the signal in the navigation signal cable; and

the method comprising:

determining that the robotic lawnmower is in a docking position, and in response thereto

recording the field values in the docking position by sensing field values of the signal in the docking position and storing the field values of the sensed signal as a stored magnetic field value; and

controlling the propulsion system to move out of said docking position; and

controlling the propulsion system to enter into said docking position by

sensing a current magnetic field value;

comparing the current magnetic field value to the stored magnetic field value; and

determining how to navigate the robotic lawnmower based on the comparison and navigating accordingly.

16. A non-transitory computer readable storage medium encoded with instructions that, when executed on a controller, perform the method according to claim 15 .