A method of relative position keeping control of a dynamically positioned vessel

Abstract

The application relates to the field of ship motion control and discloses a relative position keeping control method for a dynamically positioned ship, which comprises the following steps: determining the coordinates of a relative position keeping point in a ship coordinate system, moving a rotation center to the point and keeping steady state by using an absolute position measuring sensor; simultaneously enabling the absolute and relative position measuring sensors, calibrating the position coordinates of a relative position measuring target in a geodetic coordinate system according to pose information; then switching to using only the relative sensor, combining the calibrated coordinates to calculate real-time measurement coordinates of a ship body and target coordinates for positioning; generating a guide pose through a guide strategy containing acceleration and deceleration sections, calculating required force, and driving various actuators. The application effectively solves the position jump problem during switching of heterogeneous sensors, realizes smooth control of position and heading under a non-ship body rotation center, reduces power grid load fluctuation, and significantly improves the relative operation safety of a dynamically positioned ship.

Description

Technical Field

[0001] This invention relates to the field of ship motion control technology, and in particular to a method for maintaining the relative position of a dynamically positioned ship. Background Technology

[0002] The main function of a dynamic positioning system is to achieve automatic control of the longitudinal, lateral, and heading degrees of freedom of a ship or offshore platform, so that the ship or offshore platform can maintain the set position, heading, and trajectory.

[0003] Generally, dynamic positioning systems use an absolute position reference system as the sensor for position measurement and control. However, in certain special operational scenarios on dynamically positioned vessels, a relative position reference system is required for position measurement and control, maintaining a relative distance from the target. Since the two position reference systems use different measurement coordinate systems, significant position control deviations must be avoided when switching between sensors, while also ensuring interface consistency at the control algorithm level.

[0004] On the other hand, in relative positioning control, the center of rotation is not amidships, increasing the difficulty of control. If the control law is designed to directly use the difference between the control target and the measured state for feedback control, it will cause a series of problems such as excessive control force, grid load exceeding limits, and control oscillations. A guidance strategy needs to be designed for smooth control. Because relative positioning control involves simultaneous heading and position control, the coupling between heading guidance and position guidance must be considered.

[0005] In practical applications, there is an urgent need for a control scheme that can overcome the above-mentioned technical problems. Summary of the Invention

[0006] The present invention aims to at least solve one of the technical problems existing in the related art. To this end, the present invention provides a method for maintaining the relative position of a dynamically positioned vessel.

[0007] A method for maintaining the relative position of a dynamically positioned vessel, comprising the following steps: S1, obtain the coordinates of the relative position holding point in the ship coordinate system, switch the control rotation center of the dynamic positioning control system from the midships to the relative position holding point, and use the absolute position measurement sensor to maintain the ship's position; S2, simultaneously enable the absolute position measurement sensor and the relative position measurement sensor on the ship, and calculate the position coordinates of the relative position measurement target in the geodetic coordinate system based on the measurement values ​​of the absolute position measurement sensor, the measurement values ​​of the relative position measurement sensor, the ship attitude information, and the coordinates of the relative position holding point in the ship coordinate system. S3, turn off the absolute position measurement sensor, and use the real-time measurement value of the relative position measurement sensor, combined with the position coordinates of the relative position measurement target obtained in S2, to calculate the real-time measurement coordinates of the ship midship in the geodetic coordinate system and the coordinates of the ship midship positioning target in the geodetic coordinate system. S4. Based on the real-time measured coordinates of the ship amidships in the geodetic coordinate system and the positioning target coordinates of the ship amidships in the geodetic coordinate system, a guiding position and guiding speed are generated. Based on the guiding position and guiding speed, the control demand force is calculated to drive the ship's propulsion actuator to maintain the relative position.

[0008] Furthermore, the absolute position measurement sensor includes the BeiDou Navigation Satellite System or the Global Positioning System (GPS); the relative position measurement sensor includes a laser ranging device, a microwave ranging device, or a tension cable ranging device.

[0009] Further, in S2, the real-time position coordinates of the ship's midships in the geodetic coordinate system are obtained based on the measured values ​​of the absolute position measurement sensor, the position of the antenna of the absolute position measurement sensor relative to the midships, and the ship's attitude information.

[0010] Furthermore, in S2, the real-time position coordinates of the relative position holding point in the geodetic coordinate system are obtained based on the real-time position coordinates of the ship amidships in the geodetic coordinate system, the coordinates of the relative position holding point in the hull coordinate system, and the ship attitude information.

[0011] Furthermore, the position coordinates of the relative position measurement target in the geodetic coordinate system are the difference between the real-time position coordinates of the relative position holding point in the geodetic coordinate system and the measurement value of the relative position measurement sensor.

[0012] Furthermore, before S4, it also includes: using a filtering algorithm to filter the real-time measured coordinates of the ship amidships in the geodetic coordinate system, filtering out high-frequency motion components and sensor noise, and obtaining the optimal estimated state of the ship in the longitudinal, transverse and bow directions; The filtering algorithm used is either extended Kalman filtering or unscented Kalman filtering.

[0013] Furthermore, in S4, a guidance position and guidance velocity are generated through a guidance strategy, which includes a heading guidance strategy. The process is as follows: based on the preset heading acceleration and heading deceleration, the heading guidance process is divided into an acceleration phase, a constant speed phase, and a deceleration phase; it is determined in real time whether the remaining heading distance meets the deceleration requirements. If it does, guidance continues; if it does not, it enters the deceleration phase until the guidance angular velocity becomes zero.

[0014] Further, in S4, a guidance position and a guidance speed are generated through a guidance strategy. The guidance strategy includes a position guidance strategy, the process of which is as follows: calculate the distance between the current midship positioning target coordinates in the geodetic coordinate system and the previous midship positioning target coordinates; determine whether the remaining guidance distance can complete the deceleration according to the preset position deceleration, and execute acceleration guidance, uniform speed guidance or deceleration guidance according to the determination result, and output the guidance position and the guidance speed.

[0015] Furthermore, in S4, a proportional-integral-derivative control algorithm is used to obtain the control demand force based on the deviation between the guide position and the real-time measured coordinates of the ship amidships in the geodetic coordinate system, and the deviation between the guide speed and the real-time estimated speed of the ship.

[0016] Furthermore, prior to S1, the method includes: enabling the absolute position measurement sensor to maintain its initial position, and triggering the switching of the control rotation center after detecting that the ship has entered the measurement range of the relative position measurement sensor.

[0017] The above-described one or more technical solutions in the embodiments of the present invention have at least one of the following technical effects: 1. By performing online calibration of the measurement target during the parallel phase of dual sensors, the logical unification of coordinate systems of different measurement systems was achieved, ensuring a smooth and seamless transition from absolute positioning to relative positioning and eliminating the position jump problem caused by coordinate inconsistency.

[0018] 2. This invention allows the control rotation center to be moved to a designated point outside the midships, and with the design of position and heading guidance strategies, it effectively solves the problems of excessive control force, overload of the power grid and system oscillation that may be caused by direct feedback control, and achieves stable control under non-center rotation conditions.

[0019] 3. By combining the Kalman filter algorithm with a specific "acceleration-uniform speed-deceleration" guidance logic, the ship can move to the target position quickly and smoothly. The control process is responsive and effectively avoids high-frequency noise interference, improving the response accuracy and smoothness of the control system.

[0020] 4. By unifying the transformation of multiple coordinate systems, the algorithm design at the control level is simplified, and the operational safety and system stability of ships are significantly improved in complex relative positioning operation scenarios.

[0021] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0023] Figure 1 This is a flowchart of the method of the present invention; Figure 2 It is a time-history curve of the rotation center change; Figure 3 This is a time-series curve of the position sensor's enable state; Figure 4 It is a time-history curve of the target enable state in relative position; Figure 5 It is a time-history curve of the rotation center position in three stages: absolute position sensor positioning, relative position sensor positioning, and relative position sensor displacement. Figure 6 It is a time-history curve of the bow position in the relative position coordinate system, which is based on the positioning of the absolute position sensor, the positioning of the relative position sensor, and the displacement of the relative position sensor. Figure 7 It is a time-history graph of the ship's midship displacement to the north, east, bow, longitudinal velocity, lateral velocity, and bow angular velocity. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention. The following embodiments are used to illustrate this invention but cannot be used to limit the scope of this invention.

[0025] Figure 1 The implementation process of the method of the present invention is shown.

[0026] This invention provides a method for maintaining the relative position of a dynamically positioned vessel, the specific steps of which include: S1, obtain the coordinates of the relative position holding point in the ship's coordinate system, switch the control rotation center of the dynamic positioning control system from midships to the relative position holding point, and use an absolute position measurement sensor to maintain the ship's position. Position holding is performed using an absolute position measurement sensor. Includes the following steps: Absolute position measurement sensors that enable dynamic positioning control systems, such as BeiDou and GPS; Switch to fixed-point positioning mode while maintaining the coordinates near the relative position; The position setting using the fixed-point positioning mode enables the ship to maintain coordinate maneuvering towards the relative position and then maintain its position after entering the measurement range of the relative position measurement sensor.

[0027] Switch the rotation center of the ship's fixed-point positioning to the designated relative position holding point; Includes the following steps: Determine the coordinates of point C, which maintains its relative position in the ship's coordinate system. The ship's coordinate system has the ship's center of gravity as the origin, the x-axis pointing towards the bow, the y-axis pointing towards the starboard side, and the z-axis pointing towards the bottom. The absolute position measurement sensor is used to maintain the position of the relative position holding point in the ship's coordinate system.

[0028] S2, simultaneously enable the absolute position measurement sensor and the relative position measurement sensor on the ship, and calculate the position coordinates of the relative position measurement target in the geodetic coordinate system based on the measurement values ​​of the absolute position measurement sensor, the measurement values ​​of the relative position measurement sensor, the ship's attitude information, and the coordinates of the relative position holding point in the ship's coordinate system. Switch the position measurement sensor to the relative position measurement sensor; Includes the following steps: Simultaneously enabling the absolute position measurement sensor and the relative position measurement sensor of the dynamic positioning control system, the types of relative position measurement sensors include laser ranging, microwave ranging, tension cable ranging devices, etc. Based on the relative position measurement (X, Y), the coordinates of the relative position measurement coordinate system, and the coordinates of the relative position holding point C in the ship's coordinate system ( The position A of the absolute position measurement sensor antenna relative to the midships in the ship's coordinate system. Absolute position measurement in geodetic coordinate system Based on the ship's attitude information, the position of the relative position measurement target T in the geodetic coordinate system is calculated. Details are as follows: In the geodetic coordinate system, the x-axis is positive for north, the y-axis is positive for east, and the z-axis is positive for the earth.

[0029] The origin of the relative position measurement coordinate system is the relative position measurement target T. The x-axis is positive north, the y-axis is positive east, and the z-axis is positive ground. The relative position measurement (X, Y, Z) represents the position offset of the ship's relative position holding point relative to the measurement target.

[0030] 1) Based on the absolute position measurements in the geodetic coordinate system, the sensor antenna positions in the ship's coordinate system, and the ship's attitude information, the midship position O in the geodetic coordinate system is calculated. ).

[0031] Let M be the attitude transformation matrix from the ship's coordinate system to the geodetic coordinate system: (1) c represents the cosine operation, and s represents the sinine operation. For the heading angle, For pitch angle, This is the roll angle.

[0032] The position offset of the absolute position measurement sensor antenna relative to the midship position in geodetic coordinates is denoted as . : (2) The result here It is a three-dimensional vector, that is .

[0033] : Indicates the offset of the absolute position measurement sensor antenna relative to the midship position in the geodetic coordinate system in the north-north direction.

[0034] : This indicates the offset of the antenna relative to the midships in the eastward direction in the geodetic coordinate system.

[0035] : This indicates the vertical upward offset of the antenna relative to the midships in the geodetic coordinate system.

[0036] The coordinates of the ship's midship position O in the geodetic coordinate system are calculated as follows: (3) 2) Based on the position of the ship amidships in the geodetic coordinate system and the relative position of point C in the hull coordinate system, maintain the coordinates of point C. Based on the ship's attitude information, the coordinates of point C, which maintains its relative position in the geodetic coordinate system, are calculated. ).

[0037] The position offset of the relative position holding point in geodetic coordinates relative to the midship position is denoted as : (4) The result here It is a three-dimensional vector .

[0038] : Indicates the point where the relative position is maintained. Relative to the midship position The northward offset in the geodetic coordinate system.

[0039] : Indicates the point where the relative position is maintained. Relative to the midship position The position offset eastward in the geodetic coordinate system.

[0040] : Indicates the position offset of this point relative to the midships in the geodetic coordinate system in the ground direction.

[0041] The coordinates of point C, whose relative position is maintained in the geodetic coordinate system, are calculated as follows: (5) 3) The coordinates of the target T in the relative position measurement system are calculated as follows: (6) S3, turn off the absolute position measurement sensor, use the real-time measurement value of the relative position measurement sensor, and combine it with the position coordinates of the relative position measurement target obtained in S2 to calculate the real-time measurement coordinates of the ship midship in the geodetic coordinate system and the positioning target coordinates of the ship midship in the geodetic coordinate system; turn off the enable state of the absolute position measurement sensor of the dynamic positioning control system, and only enable the relative position measurement sensor.

[0042] Shift to the target position according to the relative position command and maintain the relative position; Includes the following steps: The relative position in the relative position measurement coordinate system is maintained by preserving the coordinates of the command point S. ) and heading instructions Send to the dynamic positioning control system; 1) The coordinates of the command point S, which maintains the relative position in the geodetic coordinate system, are: (7) 2) The coordinates of the target midships in the geodetic coordinate system were calculated. ): (8) 3) Based on the real-time relative position measurement (X, Y) and the coordinates of the relative position measurement target T in the geodetic coordinate system, calculate the real-time measurement of the relative position holding point C in the geodetic coordinate system. The calculation is as follows: (9) 4) Calculate the real-time coordinates of the ship amidships in the geodetic coordinate system. ) (10) in, , To be based on real-time heading angle Pitch angle Roll angle The attitude transformation matrix from the ship's coordinate system to the geodetic coordinate system is calculated as shown in formula (1).

[0043] The result here It is a three-dimensional vector .

[0044] : Indicates the point where the relative position is maintained in real time. Relative to the midship position The northward offset in the geodetic coordinate system.

[0045] : Indicates the point where the relative position is maintained in real time. Relative to the midship position The position offset eastward in the geodetic coordinate system.

[0046] : Indicates the real-time position offset of this point relative to the midships in the geodetic coordinate system in the ground direction.

[0047] S4. Based on the real-time measured coordinates of the ship amidships in the geodetic coordinate system and the positioning target coordinates of the ship amidships in the geodetic coordinate system, a guiding position and guiding speed are generated. Based on the guiding position and guiding speed, the control demand force is calculated to drive the ship's propulsion actuator to maintain the relative position.

[0048] Design the control law and calculate the longitudinal thrust, lateral thrust, and bow moment required to maintain the relative position.

[0049] 1) Use filtering algorithms such as EKF (Extended Kalman Filter) and UKF (Unscented Kalman Filter) to estimate the ship's motion state and filter out measurement information. , and By analyzing the high-frequency motion components and measurement sensor noise, the optimal estimates of the ship's midship position, bow direction, longitudinal and transverse velocities, and bow angular velocity in the geodetic coordinate system are obtained. The estimated midship position is denoted as (…). The heading estimate is denoted as... The longitudinal velocity estimate is denoted as u, the lateral velocity estimate is denoted as v, and the bow angular velocity estimate is denoted as rot.

[0050] 2) Position and heading guidance strategy design The bow guidance strategy is as follows: a. Determine the direction of bow motion .

[0051] Will Transform to range If the result is positive, then... This indicates that the bow movement is clockwise; if the result is negative... This indicates that the bow movement is counterclockwise. Indicates the previous heading instruction. This is the current heading instruction.

[0052] b. Guide the bow angular velocity according to the pre-embedded bow acceleration rot_acc and bow deceleration rot_dcc. Bow guidance The guidance phase is divided into three types: acceleration, constant speed, and deceleration. The initial guidance phase is defined as acceleration, where the guiding angular velocity at the previous moment is... Defined as 0.

[0053] Acceleration phase guidance: , where dt is the control period.

[0054] in: : Current heading steering angular velocity; : The heading guide angular velocity at the previous moment; The current heading angle; : The heading angle at the previous moment; dt: Control period (time step).

[0055] After the acceleration phase is completed, determine whether the remaining heading distance is sufficient to complete deceleration according to the pre-embedded rot_dcc. The determination method is as follows: in, : The estimated time required to decelerate to 0 at the current angular velocity; : Current heading steering angular velocity; : Pre-embedded (pre-set) bow deceleration constant; The remaining heading angle required to complete the deceleration phase at the current angular velocity.

[0056] like If the deceleration is deemed complete, the process proceeds to determine whether to enter the constant speed phase; otherwise, the deceleration is deemed impossible, and the guiding phase is defined as deceleration.

[0057] The method for determining whether a uniform speed segment has been entered is: if The guidance phase is defined as uniform velocity, and rot_set sets the upper limit of the heading angular velocity.

[0058] Guided uniform speed segment: After the constant speed guidance phase is completed, determine whether the remaining heading distance can be reduced according to the pre-embedded rot_dcc, using the same method as above. If reduction can be achieved, continue with the constant speed guidance phase; otherwise, switch to the deceleration guidance phase.

[0059] Deceleration phase guidance: ,when Change to 0 or Exceeding This indicates the end of the guidance.

[0060] in: : Current heading steering angular velocity; : The heading guide angular velocity at the previous moment; The current heading angle; : The heading angle at the previous moment; dt: Control period (time step).

[0061] 3) The location-guided strategy is as follows: a. Calculate the current midship positioning target ( ) and the previous target located amidships ( The distance (dis) and orientation (azi) of the previous time step, and the guiding distance. Previous moment guiding speed Defined as 0.

[0062] The calculation method is as follows: b. Determine whether the remaining guiding distance is sufficient to complete the deceleration according to the pre-embedded deceleration dis_dcc. The determination method is as follows: like If so, it is determined that deceleration can be completed, and further judgment is made on whether to guide the vehicle to accelerate or maintain a constant speed. If If the speed is too fast, an accelerated guidance strategy is adopted; otherwise, a constant speed guidance strategy is adopted. To set a speed limit.

[0063] like Then, a deceleration guidance strategy will be adopted.

[0064] c. Provide speed and position guidance.

[0065] Accelerated bootstrapping strategy: This is the current guiding distance. This refers to the pre-embedded acceleration.

[0066] Uniform speed guidance strategy: .

[0067] Deceleration guidance strategy: ,when Change to 0 or Exceeding This indicates the end of the guidance.

[0068] in: The current guiding speed; The guiding speed at the previous moment; : Pre-embedded (pre-set) position acceleration; : The distance currently guided; The distance already guided in the previous moment; dt: Control period; : Preset position deceleration constant.

[0069] d. Calculate the midships guiding position Longitudinal guiding speed Lateral guidance speed .

[0070] in: : The current northward guidance position of the ship amidships in the geodetic coordinate system.

[0071] : The current eastward guiding position of the ship amidships in the geodetic coordinate system.

[0072] The previous midship positioning target's northward coordinates in the geodetic coordinate system.

[0073] The previous midship positioning target's eastward coordinates in the geodetic coordinate system.

[0074] Distance has been guided so far.

[0075] azi: The azimuth of the target currently positioned amidships relative to the previous target.

[0076] : Longitudinal guidance speed in the ship's coordinate system.

[0077] : Lateral guidance speed in the ship's coordinate system.

[0078] Current boot speed.

[0079] Current heading angle.

[0080] 4) Calculation of position deviation The longitudinal positional deviation is denoted as err_surge, and the lateral positional deviation is denoted as err_sway. The calculations are as follows: (11) in: Longitudinal position deviation of the vessel; : Deviation in the lateral position of the vessel; : Northward guidance position amidships; The optimal estimate of the ship's northward position amidships in the geodetic coordinate system output by the filtering algorithm. Midships guide the ship eastward; The optimal estimate of the ship's eastward position amidships in the geodetic coordinate system output by the filtering algorithm; Current heading angle.

[0081] 5) Bow deviation calculation The heading deviation, denoted as err_hdg, is calculated as follows: (12) in: : Deviation in heading of the vessel; Current heading angle; : The optimal estimate of the ship's heading angle output by the filtering algorithm.

[0082] 6) Control force calculation The control requirements for the ship's longitudinal, transverse, and bow directions are denoted as frc_dmnd_x, frc_dmnd_y, and frc_dmnd_n, respectively. The PID control law is designed to control the ship's position and bow direction, using the following formula: (13) In the formula, , , , , , , , , For longitudinal scaling, longitudinal integral, and longitudinal differential coefficients; for lateral scaling, lateral integral, and lateral differential coefficients; and for bow scaling, bow integral, and bow differential coefficients. These are longitudinal deviation, lateral deviation, and heading deviation, respectively. These are the longitudinal guiding velocity, the lateral guiding velocity, and the guiding bow angular velocity, respectively. u, v, rot: These are the optimal estimates of the ship's longitudinal velocity, lateral velocity, and heading angular velocity, respectively, output by the filtering algorithm.

[0083] Based on the required force and torque, the thrust distribution algorithm is used to solve the problem and then the thruster commands are sent to each propulsion actuator.

[0084] Figure 2 The diagram shows a time-series curve of the rotation center change according to an embodiment of the present invention.

[0085] Figure 3 The specific changes in the position sensor enable state time-history curve according to an embodiment of the present invention are shown.

[0086] Figure 4 The diagram shows a time-series curve of the relative position target enable state according to an embodiment of the present invention.

[0087] Figure 5 The illustration shows the time-series changes in the rotation center position during three stages: absolute position sensor positioning, relative position sensor positioning, and relative position sensor displacement, according to an embodiment of the present invention.

[0088] Figure 6 This illustration shows the time-series changes in the bow position in a relative position coordinate system, including absolute position sensor positioning, relative position sensor positioning, and relative position sensor displacement, according to an embodiment of the present invention.

[0089] Figure 7 The present invention illustrates the time-history changes of the midship northward displacement, eastward displacement, bow direction, longitudinal velocity, lateral velocity, and bow angular velocity of a ship according to an embodiment of the present invention.

[0090] By comparing the changes in the above curves, it can be seen that after the rotation center is switched or the positioning sensor used is switched from absolute to relative during the positioning process of the present invention, the position target and measurement of the control system change accordingly, but the position control deviation does not jump. The present invention decouples the guidance and control of position and heading, and the position and heading control are smooth and the control accuracy is high during the movement to the relative position target point.

[0091] The relative position holding control method for dynamically positioned ships proposed in this invention calibrates the origin of the coordinate system of the relative position measurement system based on the measurement information of the absolute position measurement system during the dynamic positioning position sensor enabling stage. This enables the conversion of the measurement information of the relative position measurement system to the absolute position measurement coordinate system during subsequent independent use of the relative position measurement system.

[0092] The method involves coordinate transformations of the midships, sensor antenna, relative position holding point, and target under the geodetic coordinate system, ship coordinate system, and relative position measurement coordinate system. It ensures coordinate system uniformity under various operating conditions, including absolute switching to the relative position reference system, relative switching back to the absolute position reference system, and simultaneous data fusion using two types of position reference systems. This simplifies control algorithm design and achieves smooth, abrupt position control.

[0093] The position and heading guidance strategy designed in this invention, when the rotation center is not amidships, can achieve smooth control of position and heading.

[0094] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for maintaining the relative position of a dynamically positioned vessel, characterized by the following steps: include: S1, obtain the coordinates of the relative position holding point in the ship coordinate system, switch the control rotation center of the dynamic positioning control system from the midships to the relative position holding point, and use the absolute position measurement sensor to maintain the ship's position; S2, simultaneously enable the absolute position measurement sensor and the relative position measurement sensor on the ship, and calculate the position coordinates of the relative position measurement target in the geodetic coordinate system based on the measurement values ​​of the absolute position measurement sensor, the measurement values ​​of the relative position measurement sensor, the ship attitude information, and the coordinates of the relative position holding point in the ship coordinate system. S3, turn off the absolute position measurement sensor, and use the real-time measurement value of the relative position measurement sensor, combined with the position coordinates of the relative position measurement target obtained in S2, to calculate the real-time measurement coordinates of the ship midship in the geodetic coordinate system and the coordinates of the ship midship positioning target in the geodetic coordinate system. S4. Based on the real-time measured coordinates of the ship amidships in the geodetic coordinate system and the positioning target coordinates of the ship amidships in the geodetic coordinate system, a guiding position and guiding speed are generated. Based on the guiding position and guiding speed, the control demand force is calculated to drive the ship's propulsion actuator to maintain the relative position.

2. The relative position holding control method for a dynamically positioned vessel according to claim 1, characterized in that, The absolute position measurement sensor includes the BeiDou Navigation Satellite System or the Global Positioning System (GPS); the relative position measurement sensor includes a laser ranging device, a microwave ranging device, or a tension cable ranging device.

3. The relative position holding control method for dynamically positioned vessels according to claim 1, characterized in that, In S2, the real-time position coordinates of the ship's midships in the geodetic coordinate system are obtained based on the measured values ​​of the absolute position measurement sensor, the position of the antenna of the absolute position measurement sensor relative to the midships, and the ship's attitude information.

4. The relative position holding control method for a dynamically positioned vessel according to claim 3, characterized in that, In S2, the real-time position coordinates of the relative position holding point in the geodetic coordinate system are obtained based on the real-time position coordinates of the midships in the geodetic coordinate system, the coordinates of the relative position holding point in the hull coordinate system, and the ship attitude information.

5. The relative position holding control method for a dynamically positioned vessel according to claim 4, characterized in that, The position coordinates of the target in the geodetic coordinate system are the difference between the real-time position coordinates of the relative position holding point in the geodetic coordinate system and the measured value of the relative position measuring sensor.

6. The relative position holding control method for a dynamically positioned vessel according to claim 1, characterized in that, Before S4, it also includes: using a filtering algorithm to filter the real-time measured coordinates of the ship amidships in the geodetic coordinate system, filtering out high-frequency motion components and sensor noise, and obtaining the optimal estimated state of the ship in the longitudinal, transverse and bow directions. The filtering algorithm used is either extended Kalman filtering or unscented Kalman filtering.

7. The relative position holding control method for a dynamically positioned vessel according to claim 1, characterized in that, In S4, a guidance position and guidance speed are generated through a guidance strategy, which includes a heading guidance strategy. The process is as follows: based on the preset heading acceleration and heading deceleration, the heading guidance process is divided into an acceleration phase, a constant speed phase, and a deceleration phase; it is determined in real time whether the remaining heading distance meets the deceleration requirements. If it does, guidance continues; if it does not, it enters the deceleration phase until the guidance angular velocity becomes zero.

8. The relative position holding control method for a dynamically positioned vessel according to claim 1, characterized in that, In S4, a guidance position and guidance speed are generated through a guidance strategy. The guidance strategy includes a position guidance strategy, the process of which is as follows: calculate the distance between the current midship positioning target coordinates in the geodetic coordinate system and the previous midship positioning target coordinates; determine whether the remaining guidance distance can complete the deceleration according to the preset position deceleration, and execute acceleration guidance, uniform speed guidance or deceleration guidance according to the determination result, and output the guidance position and the guidance speed.

9. The relative position holding control method for a dynamically positioned vessel according to claim 1, characterized in that, In S4, a proportional-integral-derivative control algorithm is used to obtain the control demand force based on the deviation between the guide position and the real-time measured coordinates of the ship amidships in the geodetic coordinate system, and the deviation between the guide speed and the real-time estimated speed of the ship.

10. The relative position holding control method for a dynamically positioned vessel according to claim 1, characterized in that, Before S1, the method further includes: enabling the absolute position measurement sensor to maintain its initial position, and triggering the switching of the control rotation center after detecting that the ship has entered the measurement range of the relative position measurement sensor.

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