Length measuring device and control method thereof

1. A length measuring device, comprising:

a line to which a hook part is attached at one end;

a case having an opening formed therein;

a rotating part to be rotated according to extraction or retraction of the line out of or into the case through the opening;

a rotation sensing part which senses a rotation of the rotating part; and

a control part which converts an amount of rotation of the rotating part into a length measurement value and resets the converted length measurement value to a predetermined value when the length measuring device is in a first state.

2. The length measuring device of claim 1 , further comprising a hook sensing part which senses the hook part,

wherein the control part determines that the length measuring device is in the first state when the hook part is sensed by the hook sensing part.

3. The length measuring device of claim 2 , wherein the hook sensing part comprises a physical switch to be pressed by the hook part when the hook part is hooked in the opening.

4. The length measuring device of claim 2 , wherein the hook sensing part comprises a magnetic sensor which senses a strength of a magnetic field, and

wherein the hook part is made of a material having magnetism or a magnet is attached thereto.

5. The length measuring device of claim 4 , wherein the control part determines that the length measuring device is in the first state when the strength of the magnetic field sensed by the magnetic sensor is equal to or greater than a reference magnetic field strength.

6. The length measuring device of claim 5 , wherein the control part determines that the length measuring device is in the first state when the strength of the magnetic field sensed by the magnetic sensor is equal to or greater than a reference magnetic field strength and the converted length measurement value is equal to, or less than a predetermined reference.

7. The length measuring device of claim 5 , wherein the control part sets the reference magnetic field strength based on the strength of the magnetic field measured by the magnetic sensor upon power on of the length measuring device.

8. The length measuring device of claim 1 , wherein, when the rotating part is rotated in a retraction direction of the line with a faster speed than the reference rotation speed, the control part determines that the length measuring device is in the first state.

9. The length measuring device of claim 1 , wherein, when the rotating part is rotated in a retraction direction of the line with a faster speed than the reference rotation speed, and then the rotating part remains in the stop state for the reference time or longer, the control part determines that the length measuring device is in the first state.

10. The length measuring device of claim 1 , wherein, when the rotating part is rotated in a retraction direction of the line, and then the converted length measurement value is equal to, or less than a predetermined reference, the control part determines that the length measuring device is in the first state.

11. The length measuring device of claim 1 , further comprising an inertial sensor part which senses a movement of the length measuring device,

wherein the control part determines that the length measuring device is in the first state when the movement of the length measuring device meets a predetermined condition.

12. A control method of a length measuring device including a rotating part that is rotated according to extraction or retraction of the line out of or into a case through an opening, the control method comprising:

sensing an amount of rotation of the rotating part;

converting the amount of rotation of the rotating part into a length measurement value;

determining whether the length measuring device is in a first state or not; and

resetting the converted length measurement value to a predetermined value, when the length measuring device is in the first state.

13. The control method of claim 12 , comprising determining that the length measuring device is in the first state when a hook part attached to one end of the line is sensed by a hook sensing part.

14. The control method of claim 13 , comprising determining that the length measuring device is in the first state when a strength of a magnetic field sensed by the hook sensing part is equal to, or greater than a reference magnetic field strength.

15. The control method of claim 14 , comprising determining that the length measuring device is in the first state when the strength of the magnetic field sensed by the hook sensing part is equal to, or greater than the reference magnetic field strength and the converted length measurement value is equal to, or less than a predetermined reference.

16. The control method of claim 15 , comprising setting the reference magnetic field strength based on the strength of the magnetic field measured by the hook sensing part upon power on of the length measuring device.

17. The control method of claim 12 , comprising determining that the length measuring device is in the first state when the rotating part is rotated in a retraction direction of the line with a faster speed than the reference rotation speed.

18. The control method of claim 12 , comprising determining that the length measuring device is in the first state when the rotating part is rotated in a retraction direction of the line with a faster speed than the reference rotation speed and then the rotating part remains in the stop state for the reference time or longer.

19. The control method of claim 12 , comprising determining that the length measuring device is in the first state when the rotating part is rotated in a retraction direction of the line and then the converted length measurement value is equal to, or less than a predetermined reference.

20. The control method of claim 12 , comprising determining that the length measuring device is in the first state when a movement of the length measuring device meets a predetermined condition.