Real-time measuring method for rotating speed of tail shaft in speed reduction process of large steam ship

Abstract

The invention relates to a method for measuring the rotating speed of a tail shaft in real time in the speed reduction process of a large steam ship, belonging to the technical field of ship rotating speed measurement, aiming at solving the problem of delay in the speed reduction measurement of the rotating speed of the tail shaft, comprising the following steps of: comparing a main engine rotating speed Nz (t) with a main engine running state threshold value c, if so, judging whether a quick-closing valve is in a closed state; if yes, the difference value of the main engine rotating speed and the tail shaft rotating speed serves as an input signal of a program block, an output signal of the program block is K (t), whether K (t) = 1 is judged, if yes, the tail shaft rotating speed Nw (t) serves as the real-time output tail shaft rotating speed Nx (t), if not, whether the tail shaft rotating speed Nw (t) is larger than 0 is judged, and if yes, the tail shaft rotating speed Nw (t) is compared with a speed reduction process delay threshold value d; and if yes, taking the main engine rotating speed Nz (t) as the real-time output tail shaft rotating speed Nx (t). The device is used for measuring the rotating speed of the ship tail shaft in real time.

Description

Technical Field

[0001] The invention relates to a real-time measurement method for the stern shaft rotation speed of a large steam ship during the speed reduction process, and belongs to the technical field of ship rotation speed measurement. Background Art

[0002] The ship's stern shaft speed refers to the speed of the ship's propeller. The electrical signal is sent through the stern shaft speed sensor and transmitted to the ship's monitoring system through the analog-to-digital conversion device. The accuracy and reliability of the stern shaft speed are of great significance to ensure the safe and efficient operation of the ship. In addition, the monitoring of the stern shaft speed is also an important basis for ship maintenance and fault diagnosis. By monitoring the stern shaft speed, potential problems and risks can be discovered in time and corresponding maintenance can be carried out, thereby avoiding greater failures and accidents. Therefore, it is of great significance to obtain accurate stern shaft speed in real time.

[0003] However, in the actual operation of steam ships, the digital-to-analog conversion equipment usually transmits the electrical signal sent by the speed sensor in real time, but when the electrical signal of the speed sensor drops from 5 to 0, a delay judgment will be made, that is, the digital-to-analog conversion equipment will send the value of 0 to the monitoring system only after receiving a signal of 0 for a period of time from the first time the value is 0. During this period, when the received value is less than 5, a filter delay value of 5-0 will be output to ensure the accuracy of the measurement. Therefore, there is a delay in the stern shaft speed during the speed reduction process from 5rpm to 0rpm. Among them: the main engine speed is the value of the turbine speed converted into the stern shaft speed according to the gear ratio. The main engine speed and the stern shaft speed are theoretically in a 1:1 relationship, but they are measured by two sets of sensors, so there is a certain error.

[0004] Therefore, how to solve the delay problem in the stern shaft speed reduction measurement is an urgent problem to ensure the safe and efficient operation of the ship. Summary of the invention

[0005] The invention aims to solve the problem of time delay in measuring the stern shaft speed reduction, and provides a real-time measurement method for the stern shaft speed during the speed reduction process of a large steam ship.

[0006] The present invention provides a method for real-time measurement of the stern shaft speed during the deceleration process of a large steam ship, comprising:

[0007] S1, real-time measurement of the stern shaft speed N at the current time t w (t) and main engine speed N z (t);

[0008] S2, set the main engine speed to N z (t) is compared with the host operation status threshold c to determine |N z (t) |<c is true, if yes, execute S3, otherwise execute S6;

[0009] S3, determine whether the quick-closing valve is in a closed state, if yes, execute S4, otherwise execute S6;

[0010] S4, taking the difference between the main engine speed and the stern shaft speed as the input signal of the program block, the output signal of the program block is K(t), judging whether K(t)=1 is established, if yes, executing S5, otherwise executing S6;

[0011] S5, stern shaft speed N w (t) is used as the real-time output stern shaft speed N x (t), complete real-time measurement;

[0012] S6. Determine the stern shaft speed N w (t) is greater than 0, if N w (t)>0, execute S7, otherwise return to execute S5;

[0013] S7, change the stern shaft speed to N w (t) is compared with the threshold d of the speed reduction process delay to determine N w (t)≤d is true, if so, execute S8, otherwise execute S5;

[0014] S8, host speed N z (t) is used as the real-time output stern shaft speed N x (t), complete the real-time measurement.

[0015] Preferably, the difference between the main engine speed and the stern shaft speed is used as the input signal of the program block, and the output signal of the program block is K(t). The specific method includes:

[0016] S2-1, real-time measurement of the stern shaft speed N at the current time t w (t), stern shaft speed N at time t-1 w (t-1) and the main engine speed N at time t z (t);

[0017] S2-2, judging whether the stern shaft speed is in the process of deceleration, that is, judging whether |Nw(t)|-|Nw(t-1)|<0 is established, if yes, executing S2-5, otherwise executing S2-3;

[0018] S2-3, set the main engine speed to N z (t) and stern shaft speed N w (t), and compare the difference with the upper limit a of the hysteresis to determine whether |Nw(t)|-|Nz(t)|≥a holds. If so, execute S2-4; otherwise, execute S2-7;

[0019] S2-4, set K(t) to: K(t)=1;

[0020] S2-5, set the initial signal K(0) of K(t) = 0, and determine whether the output signal K(t-1) = 1 of the difference between the main engine speed and the stern shaft speed at time t-1 is established. If yes, execute S2-6, otherwise execute S2-7;

[0021] S2-6, set the main engine speed to N z (t) and stern shaft speed N w (t), and compare the difference with the lower limit b of hysteresis to determine whether |Nw(t)|-|Nz(t)|≤b is true. If so, execute S2-7; otherwise, execute S2-4;

[0022] S2-7. Set K(t) to: K(t)=0.

[0023] Preferably, the value of the hysteresis upper limit a in S2-3 is: greater than the difference between the main engine speed and the stern shaft speed when the shaft is towed.

[0024] Preferably, the value of the lower limit b of the hysteresis lag in S2-6 is: when it is greater than the split shaft, the speed difference between the main engine speed and the stern shaft speed in the main engine cranking state, and takes a minimum value.

[0025] Preferably, the value of the host operation status threshold c in S2 is: a minimum value that can determine whether the host is in an operation state.

[0026] Preferably, the value of the threshold value d for the delay of the speed reduction process in S7 is: the speed value at which the delay begins to occur during the speed reduction process of the stern shaft.

[0027] The invention has the advantages of: a real-time measurement method for the stern shaft speed during the deceleration process of a large steam ship, which does not require replacement of any equipment, and only solves the problem of the time delay in the speed reduction measurement of the stern shaft speed of a large steam ship through an algorithm, which can greatly save costs. The stern shaft speed value is still retained in the non-delay range of the stern shaft speed measurement, and the stern shaft speed and the main engine speed can be saved at the same time, ensuring the reliability of the ship. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a flowchart of a method for real-time measurement of the stern shaft speed during the deceleration process of a large steam ship according to the present invention;

[0029] Figure 2 It is a flowchart for obtaining the output signal K(t) of the difference between the main engine speed and the stern shaft speed. DETAILED DESCRIPTION

[0030] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0032] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, but they are not intended to limit the present invention.

[0033] Embodiment 1:

[0034] Combine the following Figure 1 and Figure 2 The present embodiment is described as follows. The present embodiment is a method for real-time measurement of the stern shaft speed during the deceleration process of a large steam ship, which comprises:

[0035] S1, real-time measurement of the stern shaft speed N at the current time t w (t) and main engine speed N z (t);

[0036] S2, set the main engine speed to N z (t) is compared with the host operation status threshold c to determine |N z (t) |<c is true, if yes, execute S3, otherwise execute S6;

[0037] S3, determine whether the quick-closing valve is in a closed state, if yes, execute S4, otherwise execute S6;

[0038] S4, taking the difference between the main engine speed and the stern shaft speed as the input signal of the program block, the output signal of the program block is K(t), judging whether K(t)=1 is established, if yes, executing S5, otherwise executing S6;

[0039] S5, stern shaft speed N w (t) is used as the real-time output stern shaft speed N x (t), complete real-time measurement;

[0040] S6. Determine the stern shaft speed N w (t) is greater than 0, if N w (t)>0, execute S7, otherwise return to execute S5;

[0041] S7, change the stern shaft speed to N w (t) is compared with the threshold d of the speed reduction process delay to determine N w(t)≤d is true, if so, execute S8, otherwise execute S5;

[0042] S8, host speed N z (t) is used as the real-time output stern shaft speed N x (t), complete the real-time measurement.

[0043] Furthermore, the difference between the main engine speed and the stern shaft speed is used as the input signal of the program block, and the output signal of the program block is K(t). The specific method includes:

[0044] S2-1, real-time measurement of the stern shaft speed N at the current time t w (t), stern shaft speed N at time t-1 w (t-1) and the main engine speed N at time t z (t);

[0045] S2-2, judging whether the stern shaft speed is in the process of deceleration, that is, judging whether |Nw(t)|-|Nw(t-1)|<0 is established, if yes, executing S2-5, otherwise executing S2-3;

[0046] S2-3, set the main engine speed to N z (t) and stern shaft speed N w (t), and compare the difference with the upper limit a of the hysteresis to determine whether |Nw(t)|-|Nz(t)|≥a holds. If so, execute S2-4; otherwise, execute S2-7;

[0047] S2-4, set K(t) to: K(t)=1;

[0048] S2-5, set the initial signal K(0) of K(t) = 0, and determine whether the output signal K(t-1) = 1 of the difference between the main engine speed and the stern shaft speed at time t-1 is established. If yes, execute S2-6, otherwise execute S2-7;

[0049] S2-6, set the main engine speed to N z (t) and stern shaft speed N w (t), and compare the difference with the lower limit b of the hysteresis to determine whether |Nw(t)|-|Nz(t)|≤b is true. If so, execute S2-7; otherwise, execute S2-4;

[0050] S2-7. Set K(t) to: K(t)=0.

[0051] Furthermore, the value of the hysteresis upper limit a in S2-3 is: greater than the difference between the main engine speed and the stern shaft speed when the shaft is towed.

[0052] Furthermore, the value of the lower limit b of the hysteresis lag in S2-6 is: when it is greater than the split shaft, the speed difference between the main engine speed and the stern shaft speed when the main engine is in the cranking state, and takes a minimum value.

[0053] Furthermore, the value of the host operation status threshold c in S2 is: a minimum value that can determine whether the host is in an operation state.

[0054] Furthermore, the value of the threshold value d of the delay in the speed reduction process in S7 is: the speed value at which the delay begins to occur in the process of speed reduction of the stern shaft.

[0055] In the present invention, since the steam turbine of the steam ship needs to pass through the reducer and then connect to the stern shaft, at the same time, the speed of the steam turbine is much higher than the stern shaft speed, and the main engine speed enters the delay judgment interval only when it is below 0.5rpm, which meets the error allowable range. Therefore, the stern shaft speed can be replaced during the stern shaft speed delay. Therefore, the present invention proposes a method for solving the delay in the speed reduction measurement of the stern shaft speed of large steam ships, so as to solve the problem of delay in the speed reduction measurement of the stern shaft speed. The specific process is as follows:

[0056] Step 1: Obtain the ship's stern shaft speed N in real time w (t),N w (t-1), where t represents the current time, t = 1, 2, 3...;

[0057] Step 2: Determine whether the stern shaft speed is in the process of increasing speed, that is, |N w (t)|-|N w (t-1)|>0 is true, if true, continue to step 3, otherwise, continue to step 5;

[0058] Step 3: Determine whether the difference between the main engine speed and the stern shaft speed exceeds a rpm, that is, ||N w (t)|-|N z (t) Whether ||≥a holds true. If so, proceed to step 4. Otherwise, proceed to step 7.

[0059] Step 4: Take K(t) = 1 and continue with step 8;

[0060] Step 5: Determine whether K(t-1)=1 holds, set K(0)=0, if so, proceed to step 6, otherwise, proceed to step 7;

[0061] Step 6: Determine whether the difference between the main engine speed and the stern shaft speed does not exceed b rpm, that is, ||N w (t)|-|N z (t) Whether ||≤b holds. If so, proceed to step 7. Otherwise, proceed to step 4.

[0062] Step 7: Take K(t) = 0 and continue with step 8;

[0063] Step 8: Get the ship's main engine speed N in real time z (t);

[0064] Step 9: Determine whether the host speed is less than c rpm, that is, |N z (t) |<c whether it is true, if so, continue with step 10, otherwise, continue with step 13;

[0065] Step 10: Determine whether the quick-closing valve is in a closed state. If yes, proceed to step 11; otherwise, proceed to step 13;

[0066] Step 11: Determine whether K(t)=1 holds. If so, proceed to step 12. Otherwise, proceed to step 13.

[0067] Step 12: Output the stern shaft speed as the actual stern shaft speed, that is, N x (t) = N w (t), end step.

[0068] Step 13: Determine whether the stern shaft speed is greater than 0, that is, |N w (t) |>0, if yes, continue to step 14, otherwise, continue to step 12;

[0069] Step 14: Determine whether the stern shaft speed does not exceed d rpm, i.e. |N w (t) |≤d is true, if true, continue to step 15, otherwise, continue to step 12;

[0070] Step 15: The output stern shaft speed is the actual main engine speed, that is, N x (t) = N z (t), end step.

[0071] Furthermore, in step 3, the value of a should be greater than the difference between the main engine speed and the stern shaft speed when the shaft is towed, so as to distinguish between the towed shaft and the split shaft.

[0072] Furthermore, in step 6, the value of b should be as small as possible, but should be greater than the speed difference between the main engine speed and the stern shaft speed when the main engine is in a cranking state when the shaft is split;

[0073] Furthermore, in step nine, the value of c should be able to determine whether the host is in operation;

[0074] Furthermore, in step fourteen, the value of d is the speed value at which the delay begins to occur during the process of reducing the stern shaft speed.

[0075] In the present invention, according to Figure 1 , the output stern shaft speed is the actual stern shaft speed, that is, N x (t) = Nw (t) needs to meet three conditions, which are also the conditions for judging whether the output stern shaft speed should be the actual stern shaft speed when the split shaft state is met. One of the conditions is that K(t) = 1, and K(t) is:

[0076] The input signal is: the difference between the main engine speed and the stern shaft speed, that is, ||N w (t)|-|N z (t)||, the upper limit of hysteresis is: a, the lower limit of hysteresis is: b, and the output is K(t).

[0077] When the stern shaft speed is in the rising state, that is, |N w (t)|-|N w (t-1)|>0, determine whether the difference between the main engine speed and the stern shaft speed exceeds a rpm, that is, ||N w (t)|-|N z (t)||≥a whether it holds true, if so, K(t)=1, otherwise K(t)=0;

[0078] When the stern shaft speed is in a decreasing state, it is determined whether K(t-1)=1 is true at the previous moment, that is, whether the state described in (1) has occurred. If not, K(t)=0. If true, it is determined whether the difference between the main engine speed and the stern shaft speed does not exceed b rpm, that is, ||N w (t)|-|N z (t)||≤b is true, if so, K(t)=0, otherwise K(t)=1.

[0079] Take K(0)=0.

[0080] In the present invention, the ship status is divided into four situations, namely, no-load, with shaft, towing shaft, and split shaft. In each situation, there is a speed-up and speed-down process. In the no-load and with shaft situations, the quick-closing valve is opened, and in the towing shaft and split shaft situations, the quick-closing valve is closed. In the no-load situation, the stern shaft speed is 0rpm, and in the split shaft situation, the main engine speed is 0rpm or 0.2rpm. Only in the speed-down process, when the stern shaft speed is less than 5rpm, there is an obvious delay phenomenon. In other situations, there are very small errors in the main engine speed and the stern shaft speed. This example selects some key point data for illustration.

[0081] Explanation of the value: a is 5, b is 0.5, c is 1, and d is 5. 1. No-load condition:

[0082] 1. Speed-up process:

[0083] <![CDATA[Stern shaft speed N w (t) / rpm]]> <![CDATA[Main engine speed N z (t) / rpm]]> Output judgment <![CDATA[Output stern shaft speed N x (t) / rpm]]> 0 1 <![CDATA[N x (t)=N w (t)]]> 0 0 6 <![CDATA[N x (t)=N w (t)]]> 0

[0084] 2. Speed ​​reduction process:

[0085] <![CDATA[Stern shaft speed N w (t) / rpm]]> <![CDATA[Main engine speed N z (t) / rpm]]> Output judgment <![CDATA[Output stern shaft speed N x (t) / rpm]]> 0 6 <![CDATA[N x (t)=N w (t)]]> 0 0 1 <![CDATA[N x (t)=N w (t)]]> 0

[0086] 2. Belt shaft situation

[0087] 1. Speed-up process:

[0088] <![CDATA[Stern shaft speed N w (t) / rpm]]> <![CDATA[Main engine speed N z (t) / rpm]]> Output judgment <![CDATA[Output stern shaft speed N x (t) / rpm]]> 3.5 3.8 <![CDATA[N x (t)=N z (t)]]> 3.8 6.4 6.6 <![CDATA[N x (t)=N w (t)]]> 6.4

[0089] 2. Speed ​​reduction process:

[0090] <![CDATA[Stern shaft speed N w (t) / rpm]]> <![CDATA[Main engine speed N z (t) / rpm]]> Output judgment <![CDATA[Output the stern shaft speed N x (t) / rpm]]> 6.4 6.6 <![CDATA[N x (t)=N w (t)]]> 6.4 5 4.8 <![CDATA[N x (t)=N z (t)]]> 4.8 4.2 2.2 <![CDATA[N x (t)=N z (t)]]> 2.2

[0091] 3. Drag shaft situation

[0092] 1. Speed-up process:

[0093] <![CDATA[Stern shaft speed N w (t) / rpm]]> <![CDATA[Main engine speed N z (t) / rpm]]> Output judgment <![CDATA[Output stern shaft speed N x (t) / rpm]]> 3.5 3.8 <![CDATA[N x (t) = N z (t)]]> 3.8 6.4 6.6 <![CDATA[N x (t)=N w (t)]]> 6.4

[0094] 2. Speed ​​reduction process:

[0095] <![CDATA[Stern shaft speed N w (t) / rpm]]> <![CDATA[Main engine speed N z (t) / rpm]]> Output judgment <![CDATA[Output stern shaft speed N x (t) / rpm]]> 6.4 6.6 <![CDATA[N x (t)=N w (t)]]> 6.4 5 4.8 <![CDATA[N x (t)=N z (t)]]> 4.8 4.2 2.2 <![CDATA[N x (t)=N z (t)]]> 2.2

[0096] 4. Split Axis

[0097] When the main engine speed N z When (t) = 0:

[0098] 1. Speed-up process:

[0099] <![CDATA[Stern shaft speed N w (t) / rpm]]> <![CDATA[Main engine speed N z (t) / rpm]]> Output judgment <![CDATA[Output stern shaft speed N x (t) / rpm]]> 3.5 0 <![CDATA[N x (t)=N z (t)]]> 0 5 0 <![CDATA[N x (t)=N w (t)]]> 5 6.4 0 <![CDATA[N x (t)=N w (t)]]> 6.4

[0100] 2. Speed ​​reduction process:

[0101] <![CDATA[Stern shaft speed N w (t) / rpm]]> <![CDATA[Main engine speed N z (t) / rpm]]> Output judgment <![CDATA[Output stern shaft speed N x (t) / rpm]]> 6.4 0 <![CDATA[N x (t)=N w (t)]]> 6.4 5 0 <![CDATA[N x (t)=N w (t)]]> 5 3.5 0 <![CDATA[N x (t)=N w (t)]]> 3.5 0.5 0 <![CDATA[N x (t)=N z (t)]]> 0 0 0 <![CDATA[N x (t)=N w (t)]]> 0

[0102] When the main engine speed N z When (t) = 0.2:

[0103] 1. Speed-up process:

[0104] <![CDATA[Stern shaft speed N w (t) / rpm]]> <![CDATA[Main engine speed N z (t) / rpm]]> Output judgment <![CDATA[Output the stern shaft speed N x (t) / rpm]]> 3.5 0 <![CDATA[N x (t)=N z (t)]]> 0 5.2 0 <![CDATA[N x (t)=N w (t)]]> 5.2 6.4 0 <![CDATA[N x (t)=N w (t)]]> 6.4

[0105] 2. Speed ​​reduction process:

[0106] <![CDATA[Stern shaft speed N w (t) / rpm]]> <![CDATA[Main engine speed N z (t) / rpm]]> Output judgment <![CDATA[Output stern shaft speed N x (t) / rpm]]> 6.4 0.2 <![CDATA[N x (t)=N w (t)]]> 6.4 5.2 0.2 <![CDATA[N x (t)=N w (t)]]> 5 3.5 0.2 <![CDATA[N x (t)=N w (t)]]> 3.5 0.7 0.2 <![CDATA[N x (t)=N z (t)]]> 0.2 0 0.2 <![CDATA[N x (t)=N w (t)]]> 0

[0107] During the speed increase process, when the stern shaft speed does not exceed 5rpm (corresponding to the main engine speed of 0rpm) or 5.2rpm (corresponding to the main engine speed of 0.2rpm), the output stern shaft speed always follows the main engine speed. Because the speed changes from static to running state, it needs to overcome static friction, so it will instantly rise by more than 10rpm. Therefore, this method is reasonable and feasible;

[0108] During the deceleration process, before the stern shaft speed drops to 0.5rpm (corresponding to the main engine speed of 0rpm) or 0.7rpm (corresponding to the main engine speed of 0.2rpm), the output stern shaft speed always follows the stern shaft speed. Because there is no main engine braking function in the split shaft situation, the stern shaft speed slowly decreases by inertia and is less affected by the delay. Therefore, this method is reasonable and feasible.

[0109] During the deceleration process, when the stern shaft speed drops to 0-0.5rpm (corresponding to the main engine speed of 0rpm) or 0-0.7rpm (corresponding to the main engine speed of 0.2rpm), the output stern shaft speed will follow the main engine speed, the error meets the design requirements and the duration is less than 1S, which meets the use requirements; when the stern shaft speed is 0, the output stern shaft speed follows the stern shaft speed, so this method is reasonable and feasible.

[0110] Although the present invention is described herein with reference to specific embodiments, it should be understood that these embodiments are merely examples of the principles and applications of the present invention. It should therefore be understood that many modifications may be made to the exemplary embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims. It should be understood that the various dependent claims and features described herein may be combined in a manner different from that described in the original claims. It should also be understood that features described in conjunction with individual embodiments may be used in other described embodiments.

Claims

1. A method for real-time measurement of the stern shaft speed during the deceleration of a large steam ship, characterized in that it include: S1, real-time measurement of the stern shaft speed N at the current time t w (t) and main engine speed N z (t); S2, set the main engine speed to N z (t) is compared with the host operation status threshold c to determine |N z (t) |<c is true, if yes, execute S3, otherwise execute S6; S3, determine whether the quick closing valve is in the closed state, if yes, execute S4, otherwise execute S6; S4, taking the difference between the main engine speed and the stern shaft speed as the input signal of the program block, the output signal of the program block is K(t), judging whether K(t)=1 is established, if yes, executing S5, otherwise executing S6; S5, stern shaft speed N w (t) is used as the real-time output stern shaft speed N x (t), complete real-time measurement; S6. Determine the stern shaft speed N w (t) is greater than 0, if N w (t)>0, execute S7, otherwise return to execute S5; S7, change the stern shaft speed to N w (t) is compared with the threshold d of the speed reduction process delay to determine N w (t)≤d is true, if so, execute S8, otherwise execute S5; S8, host speed N z (t) is used as the real-time output stern shaft speed N x (t), complete the real-time measurement.

2. The method for real-time measurement of the stern shaft speed during the deceleration process of a large steam ship according to claim 1, characterized in that: The difference between the main engine speed and the stern shaft speed is used as the input signal of the program block, and the output signal of the program block is K(t). The specific method includes: S2-1, real-time measurement of the stern shaft speed N at the current time t w (t), stern shaft speed N at time t-1 w (t-1) and the main engine speed N at time t z (t); S2-2, judging whether the stern shaft speed is in the process of deceleration, that is, judging whether |Nw(t)|-|Nw(t-1)|<0 is established, if yes, executing S2-5, otherwise executing S2-3; S2-3, set the main engine speed to N z (t) and stern shaft speed N w (t), and compare the difference with the upper limit a of the hysteresis to determine whether |Nw(t)|-|Nz(t)|≥a holds. If so, execute S2-4; otherwise, execute S2-7; S2-4, set K(t) to: K(t)=1; S2-5, set the initial signal K(0) of K(t) = 0, and determine whether the output signal K(t-1) = 1 of the difference between the main engine speed and the stern shaft speed at time t-1 is established. If yes, execute S2-6, otherwise execute S2-7; S2-6, set the main engine speed to N z (t) and stern shaft speed N w (t), and compare the difference with the lower limit b of hysteresis to determine whether |Nw(t)|-|Nz(t)|≤b is true. If so, execute S2-7; otherwise, execute S2-4; S2-7. Set K(t) to: K(t)=0.

3. The method for real-time measurement of the stern shaft speed during the deceleration process of a large steam ship according to claim 2, characterized in that: The value of the hysteresis upper limit a described in S2-3 is: greater than the difference between the main engine speed and the stern shaft speed when the shaft is towed.

4. The method for real-time measurement of the stern shaft speed during the deceleration process of a large steam ship according to claim 2, characterized in that: The value of the lower limit b of the hysteresis lag described in S2-6 is: when it is greater than the split shaft, the speed difference between the main engine speed and the stern shaft speed when the main engine is in the cranking state, and it takes the minimum value.

5. The method for real-time measurement of the stern shaft speed during the deceleration process of a large steam ship according to claim 1, characterized in that: The value of the host operation status threshold c in S2 is: a minimum value that can determine whether the host is in operation status.

6. The method for real-time measurement of the stern shaft speed during the deceleration process of a large steam ship according to claim 1, characterized in that: The value of the delay threshold d of the speed reduction process in S7 is: the speed value at which the delay begins to occur during the speed reduction process of the stern shaft.

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