Mooring winch and a method for controlling a cable of a mooring winch

1. A mooring winch comprising:

a winding drum configured to wind a mooring rope;

a brake;

an AC motor configured to drive the winding drum;

a frequency conversion unit configured to supply electrical power to the AC motor; and

a control unit configured to control the frequency conversion unit based on an indicator for tension of the mooring rope,

wherein the control unit is configured to set a reference value of rotational speed of the AC motor to a predetermined value, release the brake of the mooring winch, drive the AC motor in one direction for a predetermined time interval, define a first value of a torque of the AC motor, drive the AC motor in an opposite direction for the predetermined interval, define a second value of the torque of the AC motor, and compute a torque estimate using the first and second values of the torque.

2. A mooring winch according to claim 1 , comprising:

a sensor configured to measure the tension of the rope,

wherein the control unit is configured to receive the measured tension of the rope as an input thereto.

3. A mooring winch according to claim 1 , wherein the control unit is configured to:

compute a flux space vector for modelling a stator flux of the AC motor; and

compute the first and second values of the torque based on the flux space vector and a space vector of stator currents of the AC motor.

4. A mooring winch according to claim 1 , comprising:

a speed measuring device configured to measure the speed of the motor,

wherein the control unit is configured to receive the measured speed as an input thereto.

5. A mooring winch according to claim 1 , wherein the control unit is configured to:

make the AC motor wind the mooring rope in as a response to a situation in which the torque estimate goes below a first pre-determined limit value; and

make the AC motor wind the mooring rope out as a response to a situation in which the torque estimate exceeds a second pre-determined limit value, the second pre-determined limit value being greater than the first predetermined limit value.

6. A mooring winch according to claim 1 , wherein the control unit is configured to:

make the AC motor wind the mooring rope in as a response to a situation in which a first pre-determined delay has elapsed after the torque estimate goes below a first pre-determined limit value; and

make the AC motor wind the mooring rope out as a response to a situation in which a second pre-determined delay has elapsed after the torque estimate exceeds a second pre-determined limit value, the second pre-determined limit value being greater than the first pre-determined limit value.

7. A mooring winch according to claim 1 , wherein the control unit is configured to carry out the following successive phases for accomplishing a periodical mooring operation:

conditional phase A: controlling the AC motor to wind the mooring rope in as a response to a situation in which the computed torque estimate is lower than a first limit value;

conditional phase B: controlling the AC motor to wind the mooring rope out as a response to a situation in which the computed torque estimate is higher than a second limit value; and

phase C: waiting a predetermined time interval, re-computing the torque estimate, and continuing from the conditional phase A.

8. A method for controlling the mooring rope tension of a mooring winch, wherein the mooring winch includes a brake, a winding drum for winding a mooring rope, an AC motor configured to drive the winding drum, and a frequency conversion unit configured to supply electrical power to the AC motor, wherein the method comprises:

controlling the frequency conversion unit based on an indicator for tension of the mooring rope;

setting a reference value of rotational speed of the AC motor to a predetermined value;

releasing the brake of the mooring winch;

driving the AC motor in one direction for a predetermined time interval;

defining a first value of a torque of the AC motor;

driving the AC motor in an opposite direction for another predetermined time interval;

defining a second value of the torque of the motor; and

computing a torque estimate using the first and second values of the torque.

9. A method according to claim 8 , comprising:

computing a flux space vector for modelling a stator flux of the AC motor; and

computing the first and second values of the torque based on the flux space vector and a space vector of stator currents of the AC motor.

10. A method according to claim 8 , comprising:

controlling the AC motor to wind the mooring rope in as a response to a situation in which the torque estimate goes below a first pre-determined limit value; and

controlling the AC motor to wind the mooring rope out as a response to a situation in which the torque estimate exceeds a second pre-determined limit value, the second pre-determined limit value being greater than the first predetermined limit value.

11. A method according to claim 8 , comprising:

controlling the AC motor to wind the mooring rope in as a response to a situation in which a first pre-determined delay has elapsed after the torque estimate goes below a first pre-determined limit value; and

controlling the AC motor to wind the mooring rope out as a response to a situation in which a second pre-determined delay has elapsed after the torque estimate exceeds a second pre-determined limit value, the second predetermined limit value being greater than the first pre-determined limit value.

12. A method according to claim 8 , wherein the method comprises the following successive phases for accomplishing a periodical mooring operation:

conditional phase A: controlling the AC motor to wind the mooring rope in as a response to a situation in which the computed torque estimate is lower than a first limit value;

conditional phase B: controlling the AC motor to wind the mooring rope out as a response to a situation in which the computed torque estimate is higher than a second limit value; and

phase C: waiting a predetermined time interval, re-computing the torque estimate and continuing from the conditional phase A.

13. A non-transitory computer-readable recording medium having a computer program recorded thereon that causes a processor of a computing device to control the mooring rope tension of a mooring winch, wherein the mooring winch includes a break, a winding drum for winding a mooring rope, an AC motor configured to drive the winding drum, and a frequency conversion unit configured to supply electrical power to the AC motor, wherein the computer program causes the processor of the computing device to execute operations comprising:

controlling the frequency conversion unit based on an indicator for tension of the mooring rope;

setting a reference value of rotational speed of the AC motor to a predetermined value;

releasing the brake of the mooring winch;

driving the AC motor in one direction for a predetermined time interval;

defining a first value of a torque of the AC motor;

driving the AC motor in an opposite direction for the predetermined interval;

defining a second value of the torque of the AC motor; and

computing a torque estimate using the first and second values of the torque.

14. A non-transitory computer-readable recording medium according to claim 13 , wherein the program causes the processor of the computing device to execute operations comprising:

computing a flux space vector for modelling a stator flux of the AC motor; and

computing the first and second values of the torque based on the flux space vector and a space vector of stator currents of the AC motor.

15. A non-transitory computer-readable recording medium according to claim 13 , wherein the program causes the processor of the computing device to execute operations comprising:

controlling the AC motor to wind the mooring rope in as a response to a situation in which the torque estimate goes below a first pre-determined limit value; and

controlling the AC motor to wind the mooring rope out as a response to a situation in which the torque estimate exceeds a second pre-determined limit value, the second pre-determined limit value being greater than the first predetermined limit value.

16. A non-transitory computer-readable recording medium according to claim 13 , wherein the program causes the processor of the computing device to execute operations comprising:

controlling the AC motor to wind the mooring rope in as a response to a situation in which a first pre-determined delay has elapsed after the torque estimate goes below a first pre-determined limit value; and

controlling the AC motor to wind the mooring rope out as a response to a situation in which a second pre-determined delay has elapsed after the torque estimate exceeds a second pre-determined limit value, the second predetermined limit value being greater than the first pre-determined limit value.

17. A non-transitory computer-readable recording medium according to claim 13 , wherein the program causes the processor of the computing device to execute the following successive phases for accomplishing a periodical mooring operation:

conditional phase A: controlling the AC motor to wind the mooring rope in as a response to a situation in which the computed torque estimate is lower than a first limit value;

conditional phase B: controlling the AC motor to wind the mooring rope out as a response to a situation in which the computed torque estimate is higher than a second limit value; and

phase C: waiting a predetermined time interval, re-computing the torque estimate and continuing from the conditional phase A.