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

1. A mooring winch comprising:

a winding drum for winding a mooring rope,

an alternating current motor arranged to drive the winding drum,

a frequency conversion unit arranged to supply electrical power to the alternating current motor, and

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

wherein the control unit is arranged to compute a flux space vector for modelling a stator flux of the alternating current motor, to compute a torque estimate on the basis of the flux space vector and a space vector of stator currents of the alternating current motor, and to use the torque estimate as the indicator for the tension of the mooring rope.

2. A mooring winch according to claim 1 , wherein the control unit is arranged to:

set a reference value of rotational speed of the alternating current motor to zero,

release a brake of the mooring winch,

compute a first value of the torque estimate in the situation in which the reference value of the rotational speed has been set to zero and the brake has been released, and

determine whether the mooring rope is to be wound in or out on the basis of the first value of the torque estimate and a pre-determined set value of torque.

3. A mooring winch according to claim 1 , wherein the control unit is arranged to make the alternating current 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 to make the alternating current 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 pre-determined limit value.

4. A mooring winch according to claim 3 , wherein the control unit is arranged to set a reference value of rotational speed of the alternating current motor to zero as a response to a situation in which the torque estimate is within a pre-determined range, the pre-determined range being around a pre-determined set value of torque.

5. A mooring winch according to claim 1 , wherein the control unit is arranged to make the alternating current 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 went below a first pre-determined limit value, and to make the alternating current 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 exceeded a second pre-determined limit value, the second pre-determined limit value being greater than the first pre-determined limit value.

6. A mooring winch according to claim 5 , wherein the control unit is arranged to set a reference value of rotational speed of the alternating current motor to zero as a response to a situation in which a pre-determined delay has elapsed after the torque estimate entered a pre-determined range, the pre-determined range being around a pre-determined set value of torque.

7. A mooring winch according to claim 2 , wherein the control unit is arranged to constitute a speed controller for controlling the rotational speed of the alternating current motor, an output of the speed controller being a target value of torque and the pre-determined set value of torque being an upper limit for the target value of torque.

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

phase A: energizing the alternating current motor so that a reference value of rotational speed of the alternating current motor is zero,

phase B: releasing a brake of the mooring winch,

phase C: computing the torque estimate in the situation in which the reference value of the rotational speed is zero and the brake has been released,

conditional phase D: controlling the alternating current 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 E: controlling the alternating current motor to wind the mooring rope out as a response to a situation in which the computed torque estimate exceeds a second limit value, and

phase F: closing the brake, de-energizing the alternating current motor, waiting for a pre-determined time interval, and continuing from the phase A.

9. A method for controlling mooring rope tension of a mooring winch that comprises a winding drum for winding a mooring rope, an alternating current motor arranged to drive the winding drum, and a frequency conversion unit arranged to supply electrical power to the alternating current motor, the method comprising:

controlling the frequency conversion unit on the basis of an indicator for tension of the mooring rope,

computing a flux space vector for modelling a stator flux of the alternating current motor,

computing a torque estimate on the basis of the flux space vector and a space vector of stator currents of the alternating current motor, and

using the torque estimate as the indicator for the tension of the mooring rope.

10. A method according to claim 9 , wherein the method comprises:

setting a reference value of rotational speed of the alternating current motor to zero,

releasing a brake of the mooring winch,

computing a first value of the torque estimate in the situation in which the reference value of the rotational speed has been set to zero and the brake has been released, and

determining whether the mooring rope is to be wound in or out on the basis of the first value of the torque estimate and a pre-determined set value of torque.

11. A method according to claim 9 , wherein the alternating current motor is controlled 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 the alternating current motor is controlled 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 pre-determined limit value.

12. A method according to claim 11 , wherein a reference value of rotational speed of the alternating current motor is set to zero as a response to a situation in which the torque estimate is within a pre-determined range, the pre-determined range being around a pre-determined set value of torque.

13. A method according to claim 9 , wherein the alternating current motor is controlled 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 went below a first pre-determined limit value, and the alternating current motor is controlled 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 exceeded a second pre-determined limit value, the second pre-determined limit value being greater than the first pre-determined limit value.

14. A method according to claim 9 , wherein the method comprises the following successive phases for accomplishing a periodical mooring operation:

phase A: energizing the alternating current motor so that a reference value of rotational speed of the alternating current motor is zero,

phase B: releasing a brake of the mooring winch,

phase C: computing the torque estimate in the situation in which the reference value of the rotational speed is zero and the brake has been released,

conditional phase D: controlling the alternating current 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 E: controlling the alternating current motor to wind the mooring rope out as a response to a situation in which the computed torque estimate exceeds a second limit value, and

phase F: closing the brake, de-energizing the alternating current motor, waiting for a pre-determined time interval, and continuing from the phase A.

15. A computer readable medium encoded with a computer program for controlling mooring rope tension of a mooring winch that comprises a winding drum for winding a mooring rope, an alternating current motor arranged to drive the winding drum, and a frequency conversion unit arranged to supply electrical power to the alternating current motor, the computer program comprising computer executable instructions for making a programmable processor to:

control the frequency conversion unit on the basis of an indicator for tension of the mooring rope,

compute a flux space vector for modelling a stator flux of the alternating current motor,

compute a torque estimate on the basis of the flux space vector and a space vector of stator currents of the alternating current motor, and

use the torque estimate as the indicator for the tension of the mooring rope.

16. A mooring winch according to claim 4 , wherein the control unit is arranged to constitute a speed controller for controlling the rotational speed of the alternating current motor, an output of the speed controller being a target value of torque and the pre-determined set value of torque being an upper limit for the target value of torque.

17. A mooring winch according to claim 6 , wherein the control unit is arranged to constitute a speed controller for controlling the rotational speed of the alternating current motor, an output of the speed controller being a target value of torque and the pre-determined set value of torque being an upper limit for the target value of torque.