Real-time monitoring method of blade asynchronous vibration stress based on blade tip timing principle

Abstract

The present invention relates to a real-time monitoring method for the asynchronous vibration stress of blades based on the tip timing principle, which uses finite element analysis to obtain at least six-order modal vibration shapes of blades under prestress, and then uses a tip timing system to obtain the actual vibration displacement of the blade tip when the blade reaches at least one of the tip timing probes, and obtains the relationship between the actual vibration displacement of the blade tip and time, calculates the actual vibration frequency of the blade, the actual vibration amplitude of the blade, and the actual vibration pitch diameter of the blade corresponding to the actual vibration displacement of the blade tip, and determines the actual modal vibration shape of the blade, and then uses the modal vibration shape corresponding to the actual modal vibration shape to determine the maximum stress value and the location of the maximum stress under the asynchronous vibration condition of the blade. The present invention can realize real-time monitoring of blade vibration frequency, vibration amplitude, and circumferential pitch diameter number, which is of great significance to engine blade health early warning, fluid-solid coupling mechanism research, and vibration suppression method research.

Description

Technical Field

[0001] The invention relates to the field of impeller design, and in particular to a blade stress monitoring method. Background Art

[0002] As an important part of the core engine of an aircraft engine, the performance and stability of the impeller largely determine the comprehensive performance and reliability of the aircraft engine. In order to further improve the efficiency of aircraft engines, thrust-to-weight ratio and other comprehensive performance, higher requirements will inevitably be placed on the compression system, namely high pressure ratio, high efficiency, high reliability, long life and lightweight, etc. Driven by these contradictory requirements, the aerodynamic load of the compressor blades continues to increase, and the stress level of the blades is also increasing accordingly; at the same time, the thickness of the blades continues to decrease, and a large amount of lightweight materials are used, which reduces the stiffness of the blades. In addition, the design trend of modern compressor rotors is to design the rotor into an integral blade disk. This design reduces the weight of the compressor but reduces the structural damping at the same time. These aspects will increase the possibility of vibration problems of the blades in complex and harsh flow environments.

[0003] In a broad sense, blade vibration problems can be divided into synchronous vibration and asynchronous vibration according to the frequency of vibration. Synchronous vibration refers to the blade vibration frequency being an integer multiple of the blade rotation frequency, while asynchronous vibration refers to the blade vibration frequency being a non-integer multiple of the blade rotation frequency, which is usually also called non-integer order vibration. For asynchronous vibration, it is also divided into flutter and blade vibration excited by flow instability in a narrow sense. The main difference between these two types of vibration is that the excitation source of blade vibration is different. Flutter is caused by the unsteady aerodynamic force generated by the vibration of the blade itself, which causes the blade to vibrate greatly. The typical characteristics of this vibration are that it usually occurs in low-order modes and low node diameters, and the blade vibration amplitude will show exponential growth in a very short time; while the blade vibration caused by flow instability usually occurs in higher-order modes and higher circumferential node diameters, and the amplitude of the blade shows stable periodic changes.

[0004] In recent years, the problem of blade cracks and even fractures caused by asynchronous vibration of blades has become one of the difficult problems that must be overcome in the development of aircraft engines. In order to effectively prevent fracture accidents caused by blade vibration, the vibration state characteristics of the blades, namely the vibration frequency, vibration amplitude and the number of circumferential pitch diameters of vibration, need to be monitored and warned in real time during the test. Once the amplitude of the corresponding frequency is measured and combined with modal analysis, it can be determined whether the stress exceeds the limit, and then real-time personalized vibration suppression, elimination or avoidance measures can be taken based on the monitoring data. By monitoring and measuring the vibration frequency, amplitude and pitch diameter of all blades under all possible working conditions, and then realizing the measurement of stress levels, it can not only provide accurate input data for blade fatigue life assessment, but also provide technical support for determining the safe operation boundary of aviation compressors.

[0005] At present, the traditional method of measuring blade vibration is mainly to attach strain gauges at different positions on the surface of different blades. Although the measurement method based on strain gauges is direct and the results are accurate and reliable, it can only be carried out on a limited number of blades. In addition, the strain gauges may have low survival rate and poor reusability in high-speed environments. The test cost is high and it is difficult to analyze the vibration node diameter of the blades. At the same time, strain gauge measurement is an invasive measurement method, and its installation may also affect the blade vibration characteristics and flow field characteristics. For the strain gauges on the rotor blades, the output of their measurement data is also difficult. Therefore, the development of non-invasive and low-cost measurement and analysis technology that can simultaneously monitor the vibration characteristics and stress levels of all blades has important practical application value in the development of my country's aero-engines. Summary of the invention

[0006] The purpose of the present invention is to provide a real-time monitoring method for blade asynchronous vibration stress based on the blade tip timing principle, which can realize rapid real-time monitoring and early warning of blades under asynchronous vibration conditions of engine blades, in order to solve the problems of poor real-time performance, limited identified blade vibration characteristic parameters, low identification accuracy and inaccurate stress level monitoring of existing blade tip timing technology.

[0007] To achieve the above object, the technical solution adopted by the present invention is: a real-time monitoring method of blade asynchronous vibration stress based on blade tip timing principle, comprising the following steps:

[0008] Step 1: Use finite element analysis software to mesh the blades to be measured and monitored, and calculate at least six modal vibration modes and natural frequencies of the blades under prestress, as well as the first principal stress and modal displacement corresponding to each modal vibration mode;

[0009] Step 2: The real-time rotation speed of the blade is collected by a speed probe arranged on the circumference of the casing, and one rotation cycle is defined as one rotation of the blade shaft, which triggers a speed pulse signal, and the time difference between two adjacent speed pulse signals is recorded as the time for one rotation of the blade shaft;

[0010] During the time of one rotation of the blade shaft, the speed pulse signal triggered by the blade shaft is divided into N parts according to the time difference between two adjacent times, N being the number of blades, to obtain an ideal pulse signal of the blade tip; then, based on the circumferential position of the speed probe and the adjacent circumferential angles of at least two blade tip timing probes arranged on the circumference of the casing, the time value of the blade reaching the blade tip timing probe when the blade is not vibrating is obtained, which is recorded as the ideal arrival time of the blade tip;

[0011] At the same time, based on the blade tip timing probe, the actual arrival time of the blade tip when the blade reaches the blade tip timing probe within the time when the blade shaft rotates at least twenty times is collected;

[0012] Step 3: According to the difference between the ideal arrival time of the blade tip and the actual arrival time of the blade tip, the actual vibration displacement of the blade tip of the N blades reaching at least one of the blade tip timing probes at the actual arrival time of the blade tip is calculated, and the relationship between the actual vibration displacement of the blade tip and time is obtained, and then the actual vibration frequency of the blade, the actual vibration amplitude of the blade and the actual vibration pitch diameter of the blade corresponding to the actual vibration displacement of the blade tip are calculated, that is, the actual modal vibration shape of the blade is determined;

[0013] Step 4: Use the first principal stress and modal displacement corresponding to the actual modal vibration shape to determine the maximum stress value and the location of the maximum stress under the condition of asynchronous vibration of the blade.

[0014] Further, the actual vibration displacement of the blade tip of the N blades when they reach each blade tip timing probe is calculated, and then the actual vibration displacement of the blade tip is separated to obtain the relationship between the actual vibration displacement of the blade tip of each blade and time;

[0015] Select the actual vibration displacement of the blade tip within at least p rotation periods of any blade, where p is an arbitrary positive integer; perform least square fitting with the natural frequencies corresponding to at least six-order modal vibration shapes, so as to obtain the blade fitting actual vibration amplitude and the actual modal vibration shape order when the blade vibrates and the blade fitting actual vibration frequency corresponding to the order, which are used to correct the blade actual vibration frequency and blade actual vibration amplitude described in step three;

[0016] At the same time, combined with the actual vibration frequency of the blade obtained by the least squares fitting, the relationship between the vibration phase of any blade in p rotation cycles and time is obtained, and then, the N blades are subjected to least squares fitting, that is, the relationship between the vibration phase of the N blades in p cycles and time is obtained, and the vibration phase distribution of the N blades at the same time is extracted to obtain the actual vibration node diameter of the blade, which is used to correct the actual vibration node diameter of the blade described in step four.

[0017] Furthermore, the speed probe is composed of a stator component mounted on the blade casing and a rotor component mounted on the blade shaft. When the stator component and the rotor component are at the same circumferential angle, the speed probe triggers a speed pulse signal.

[0018] The blade tip timing probe is arranged on the casing at any circumferential angle, and when the blade reaches the blade tip timing probe, a blade tip pulse signal is triggered;

[0019] The rotation speed probe, blade tip timing probe and a signal acquisition device for receiving rotation speed pulse signals and blade tip pulse signals constitute a blade tip timing system.

[0020] Furthermore, the specific formula for calculating the vibration displacement of the N blades when they reach a blade tip timing probe in step 3 is:

[0021] ,

[0022] in, represents the ideal arrival time of the blade tip assuming that the blade does not vibrate, Indicates the actual arrival time of the blade tip under vibration conditions, represents pi, Represents the radius value at the blade tip vibration displacement measurement point, Represents the current blade rotation speed in revolutions per second.

[0023] Furthermore, in step 3, the specific calculation steps for calculating the actual vibration frequency of the blade, the actual vibration amplitude of the blade, and the actual vibration pitch diameter of the blade corresponding to the actual vibration displacement of the blade tip are:

[0024] The rotation speed of the blades is measured by the rotation speed probe , the sampling frequency of the blade measured by the blade tip timing probe for:

[0025] ,

[0026] In the formula, is the rotation speed of the blade, in revolutions per second; N is the number of blades; the vibration frequency is obtained by fast Fourier transform of the actual vibration displacement of the blade measured by a single blade tip timing probe ,

[0027] Based on the undersampling effect of the blade tip timing principle and combined with the Nyquist sampling theorem, the blade vibration frequency in the stationary coordinate system is obtained: and the vibration frequency obtained by fast Fourier transform The relationship between them is expressed as:

[0028] ,

[0029] In the formula, is an integer, is the sampling frequency;

[0030] At the same time, the frequency conversion formula in the rotating coordinate system and the stationary coordinate system is:

[0031] ,

[0032] In the formula, is the blade vibration frequency in the stationary coordinate system, is the blade vibration frequency in the rotating coordinate system, is the blade vibration pitch diameter, is the rotation speed of the blade;

[0033] Combining the relationship formula and the frequency conversion formula, we get the conditional formula:

[0034] ,

[0035] Select different integers and blade vibration pitch diameter , when the conditional formula is met, the blade vibration frequency in the rotating coordinate system is obtained , which is the actual vibration frequency of the blade, and the obtained blade vibration pitch diameter , which is the actual vibration pitch diameter of the blade.

[0036] Furthermore, in step 4, the maximum stress value and the position of the maximum stress under the condition of asynchronous vibration of the blade are determined, specifically:

[0037] According to the actual vibration frequency of the blade, the actual modal vibration shape and the position of the maximum stress are determined, and the actual vibration stress of the blade is obtained through the first principal stress and modal displacement corresponding to the actual modal vibration shape, that is, through the following conversion relationship:

[0038] .

[0039] The beneficial effects of the present invention are:

[0040] 1) This method can be used to analyze the actual vibration frequency, actual vibration amplitude, actual vibration pitch diameter, maximum stress value and location of the maximum stress by collecting data within a time span of only 1s, providing a reference for troubleshooting asynchronous vibration faults of engine blades.

[0041] 2) This method is used to realize real-time monitoring and early warning of asynchronous vibration of engine blades, which can provide guarantee for the safe and stable operation of the engine. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 is a schematic diagram of the blade tip timing principle of the present invention;

[0043] Figure 2 It is a schematic diagram of the principle of preliminary processing of data collected by the rotation speed probe and blade tip timing probe of the present invention;

[0044] Figure 3 is a graph showing the change in the rotation frequency of the blade shaft over time measured by the speed probe in a specific example;

[0045] Figure 4is the actual vibration displacement diagram of all blade tips measured by a blade tip timing probe in a specific example;

[0046] Figure 5 is a fast Fourier transform result diagram of the actual vibration displacement of the blade tips of all blades measured by a blade tip timing probe in a specific example;

[0047] Figure 6 It is a graph showing the actual vibration displacement of a blade tip over time in a specific example;

[0048] Figure 7 This is the result diagram of fitting the actual vibration displacement of the blade tip with the fourth-order modal natural frequency in a specific example;

[0049] Figure 8 This is the result diagram of fitting the actual vibration displacement of the blade tip using the fifth-order modal natural frequency in a specific example;

[0050] Fig. 9 This is the result diagram of fitting the actual vibration displacement of the blade tip using the sixth-order modal natural frequency in a specific example;

[0051] Fig.10 It is a graph showing the actual vibration phase of a blade changing with time in a specific example;

[0052] Fig.11 It is the distribution diagram of the actual vibration phase of all blades at a certain moment in a specific instance;

[0053] Fig.12 It is the fifth-order mode vibration shape diagram of the blade in the specific example. DETAILED DESCRIPTION

[0054] The principles and features of the present invention are described below in conjunction with the accompanying drawings. The examples given are only used to explain the present invention and are not used to limit the scope of the present invention.

[0055] In order to achieve the above object, the present invention provides the following specific implementation methods:

[0056] Example 1: Figure 1 , Figure 2 As shown, a real-time monitoring method for blade asynchronous vibration stress based on blade tip timing principle comprises the following steps:

[0057] S01. Using finite element analysis software to mesh the blades to be measured and monitored, and calculating at least six modal vibration modes and natural frequencies of the blades under prestress, as well as the first principal stress and modal displacement corresponding to each modal vibration mode;

[0058] S02, such as Figure 1As shown, the speed probe is composed of a stator component mounted on the blade casing and a rotor component mounted on the blade shaft. When the stator component and the rotor component are at the same circumferential angle, the speed probe triggers a speed pulse signal;

[0059] The blade tip timing probe is arranged on the casing at any circumferential angle, and when the blade reaches the blade tip timing probe, a blade tip pulse signal is triggered;

[0060] The speed probe, blade tip timing probe and a signal acquisition instrument for receiving speed pulse signals and blade tip pulse signals constitute the blade tip timing system.

[0061] Based on the blade tip timing system, the real-time rotation speed of the blade is collected through the speed probe set on the circumference of the casing, and the blade shaft rotates one circle, which is a rotation cycle, then a speed pulse signal is triggered, and the time difference between two adjacent speed pulse signals is recorded as the time for the blade shaft to rotate one circle;

[0062] During the time of one rotation of the blade shaft, the speed pulse signal triggered by the blade shaft is divided into N parts according to the time difference between two adjacent times, N being the number of blades, to obtain an ideal pulse signal of the blade tip; then, based on the circumferential position of the speed probe and the adjacent circumferential angles of at least two blade tip timing probes arranged on the circumference of the casing, the time value of the blade reaching the blade tip timing probe when the blade is not vibrating is obtained, which is recorded as the ideal arrival time of the blade tip;

[0063] At the same time, based on the blade tip timing probe, the actual arrival time of the blade tip when the blade reaches the blade tip timing probe within the time when the blade shaft rotates at least twenty times is collected;

[0064] S03. According to the difference between the ideal arrival time of the blade tip and the actual arrival time of the blade tip, the actual vibration displacement of the blade tip of the N blades arriving at at least one of the blade tip timing probes at the actual arrival time of the blade tip is calculated, and the relationship between the actual vibration displacement of the blade tip and time is obtained, wherein the specific formula for the vibration displacement when the N blades arrive at one blade tip timing probe is:

[0065] ,

[0066] in, represents the ideal arrival time of the blade tip assuming that the blade does not vibrate, Indicates the actual arrival time of the blade tip under vibration conditions, represents pi, Represents the radius value at the blade tip vibration displacement measurement point, Represents the current blade rotation speed, in revolutions per second;

[0067] Furthermore, the specific calculation steps for calculating the actual vibration frequency of the blade, the actual vibration amplitude of the blade, and the actual vibration pitch diameter of the blade corresponding to the actual vibration displacement of the blade tip are as follows:

[0068] The rotation speed of the blades is measured by the rotation speed probe , the sampling frequency of the blade measured by the blade tip timing probe for:

[0069] ,

[0070] In the formula, is the rotation speed of the blade, in revolutions per second; N is the number of blades; the vibration frequency is obtained by fast Fourier transform of the actual vibration displacement of the blade measured by a single blade tip timing probe ,

[0071] Based on the undersampling effect of the blade tip timing principle and combined with the Nyquist sampling theorem, the blade vibration frequency in the stationary coordinate system is obtained: and the vibration frequency obtained by fast Fourier transform The relationship between them is expressed as:

[0072] ,

[0073] In the formula, is an integer, is the sampling frequency;

[0074] At the same time, the frequency conversion formula in the rotating coordinate system and the stationary coordinate system is:

[0075] ,

[0076] In the formula, is the blade vibration frequency in the stationary coordinate system, is the blade vibration frequency in the rotating coordinate system, is the blade vibration pitch diameter, is the rotation speed of the blade;

[0077] Combining the relationship formula and the frequency conversion formula, we get the conditional formula:

[0078] ,

[0079] Select different integers and blade vibration pitch diameter , when the conditional formula is met, the blade vibration frequency in the rotating coordinate system is obtained , which is the actual vibration frequency of the blade, and the obtained blade vibration pitch diameter , which is the actual vibration pitch diameter of the blade.

[0080] S04, selecting the actual vibration displacement of the blade tip within at least p rotation periods of any blade, where p is an arbitrary positive integer; performing least square fitting with the natural frequencies corresponding to at least six-order modal vibration shapes, so as to obtain the blade fitting actual vibration amplitude and the actual modal vibration shape order when the blade vibrates and the blade fitting actual vibration frequency corresponding to the order, which are used to correct the blade actual vibration frequency and blade actual vibration amplitude described in S03;

[0081] S05. Determine the actual modal vibration shape and the position of the maximum stress according to the actual vibration frequency of the blade, and obtain the actual vibration stress of the blade through the first principal stress and modal displacement corresponding to the actual modal vibration shape, that is, through the following conversion relationship:

[0082] .

[0083] S06. Combine the actual vibration frequency of the blade fitting obtained by the least squares fitting to obtain the relationship between the vibration phase of any blade in p rotation periods and time. Then, perform least squares fitting on N blades to obtain the relationship between the vibration phase of N blades in p periods and time. Extract the vibration phase distribution of N blades at the same time to obtain the actual vibration node diameter of the blade fitting, which is used to correct the actual vibration node diameter of the blade described in S05.

[0084] In order to further illustrate the technical scheme and technical effects of the present invention, the following specific examples are provided:

[0085] Specific examples: Figures 3 to 12 For a 1.5-stage fan (inlet adjustable guide vanes, rotor, stator), during the experiment, blade tip timing measurements were taken under certain working conditions (five blade tip timing probes were arranged in the casing). At the same time, in order to verify the conclusion of blade tip timing, strain gauges at the blade tip and dynamic pulsating pressure of the casing were also measured, and it was finally confirmed that the blades had asynchronous vibration. According to the measured blade tip timing data, the method for identifying the asynchronous vibration characteristics of the blade based on the blade tip timing principle includes the following steps:

[0086] (1) Use the commercial finite element software ANSYS Mechanical APDL to mesh the rotor blades and calculate their modal vibration shapes and natural frequencies under prestress. In engineering practice, the first six modes are usually considered, and the blade shaft rotation frequency is approximately Hz, which is one rotation per second The natural frequency, first principal stress and modal displacement of the first six modes are calculated, as shown in the following table, where EO (Engine Order) is a dimensionless parameter obtained by dividing the natural frequency by the rotation frequency of the blade shaft:

[0087]

[0088] (2) According to the embodiment S02, the rotation speed of the blade shaft measured by the rotation speed probe and the ideal pulse signal of the blade tip are extracted by using the time difference of the rotation speed pulse signal of different rotation cycles. The whole experimental process lasts for 1500 rotation cycles. During this process, the rotation frequency of the blade shaft is basically maintained at Nearby, such as Figure 3 As shown, there are some slight fluctuations (reasonable fluctuations) due to force majeure factors such as the environment. At the same time, the actual arrival time of the blade tip collected by the blade tip timing probe is read for subsequent analysis.

[0089] (3) According to the ideal arrival time of the blade tip and the actual arrival time of the blade tip obtained in (2), the method described in Example S03 is used to analyze and obtain the actual vibration displacement of all blades measured by a blade tip timing probe, such as Figure 4 As shown, it can be seen that the actual vibration amplitude of the blade is about 3mm:

[0090] (4) According to the actual vibration displacement of all blades measured at the blade tip timing probe calculated in (3), the actual vibration displacement signal of the blade tip is subjected to a fast Fourier transform using the method described in Example S03. The result is as follows: Figure 5 As shown, the blade vibration frequency obtained by fast Fourier analysis is Hz.

[0091] In combination with the Nyquist sampling theorem in (1) and the possible undersampling effect, the analysis method in Example S03 is used based on the formula:

[0092] ,

[0093] That is, it is preliminarily determined that the blade may have fifth-order mode and four-node diameter vibration, but further confirmation is still needed in subsequent steps using data from multiple blade tip timing probes.

[0094] (5) In the embodiment S04, in order to further verify the actual vibration frequency, actual vibration amplitude and actual vibration pitch diameter of the blade determined in the above S03, the displacements of all blades measured by the timing probes at the blade tips are separated and reintegrated to obtain the displacement of each blade over time. The displacement of one blade over time is as follows: Figure 6 shown.

[0095] (6) Since five blade tip timing probes are arranged on the casing, the actual vibration displacement of the blade tip will be measured five times in one rotation cycle for each blade. Therefore, the actual vibration displacement of the blade tip in three rotation cycles (15 moments) is selected, and the least squares fitting is performed on the natural frequency of each mode. Since the natural frequencies of each mode of the blade are quite different, only the natural frequency of one mode can be accurately fitted for the actual vibration displacement of the blade tip, such as Figure 7-9 are the fitting results of the fourth, fifth and sixth order modal natural frequencies respectively. Figure 7-9 It can be seen that the actual vibration displacement of the blade tip in this example is very consistent with the fitting result of the fifth-order modal natural frequency, that is, Figure 8 , while the fitting results of the fourth and sixth order modes do not conform to the displacement change of the blade, thus further verifying the possibility in (4), that is, determining that the vibration of the blade is the vibration of the fifth order mode.

[0096] According to the graph of the actual vibration displacement of the tip of one of the blades changing with time, that is, Figure 6 It can also be found that the actual vibration amplitude of the blade is about 3mm.

[0097] (7) Based on the analysis in (6), the data of three different blade shaft rotation cycles (15 moments) are fitted. From the fitting results, the change of blade vibration phase over time can be obtained as follows: Fig.10 As shown:

[0098] The same analysis is performed on all blades, and the vibration phases of different blades at one moment are extracted, such as Fig.11 As shown, it can be clearly seen that the pitch diameter of the blade vibration is 4, which verifies the possibility in (4).

[0099] Therefore, the blade vibrates in the fifth-order mode, the actual vibration pitch diameter of the blade is 4, and the actual vibration amplitude of the blade is about 3 mm.

[0100] The first principal stress distribution of the fifth-order mode of the blade is obtained through finite element analysis as follows Fig.12 As shown, and its maximum first principal stress is 0.575×10 12 (Pa), the mass-normalized modal displacement is 7.38m, so the 3mm amplitude is converted to obtain the maximum stress of 233.7Mpa, and the location of the maximum stress is about 80% of the blade height at the leading edge of the blade.

[0101] The above steps can quickly analyze the characteristic parameters and stress levels of the blade asynchronous vibration, realize real-time vibration warning, and provide a reference for studying the fluid-solid coupling mechanism of blade asynchronous vibration and vibration reduction design.

[0102] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A real-time monitoring method for blade asynchronous vibration stress based on blade tip timing principle, characterized in that: The steps include: Step 1: Use finite element analysis software to mesh the blades to be measured and monitored, and calculate at least six modal vibration modes and natural frequencies of the blades under prestress, as well as the first principal stress and modal displacement corresponding to each modal vibration mode; Step 2: The real-time rotation speed of the blade is collected by a speed probe arranged on the circumference of the casing, and one rotation cycle is defined as one rotation of the blade shaft, which triggers a speed pulse signal, and the time difference between two adjacent speed pulse signals is recorded as the time for the blade shaft to rotate one circle; During the time of one rotation of the blade shaft, the speed pulse signal triggered by the blade shaft is divided into N parts according to the time difference between two adjacent times, N being the number of blades, to obtain an ideal pulse signal of the blade tip; then, based on the circumferential position of the speed probe and the adjacent circumferential angles of at least two blade tip timing probes arranged on the circumference of the casing, the time value of the blade reaching the blade tip timing probe when the blade is not vibrating is obtained, which is recorded as the ideal arrival time of the blade tip; At the same time, based on the blade tip timing probe, the actual arrival time of the blade tip when the blade reaches the blade tip timing probe within the time when the blade shaft rotates at least twenty times is collected; Step 3: According to the difference between the ideal arrival time of the blade tip and the actual arrival time of the blade tip, the actual vibration displacement of the blade tip of the N blades reaching at least one of the blade tip timing probes at the actual arrival time of the blade tip is calculated, and the relationship between the actual vibration displacement of the blade tip and time is obtained, and then the actual vibration frequency of the blade, the actual vibration amplitude of the blade and the actual vibration pitch diameter of the blade corresponding to the actual vibration displacement of the blade tip are calculated, that is, the actual modal vibration shape of the blade is determined; Step 4: Use the first principal stress and modal displacement corresponding to the actual modal vibration shape to determine the maximum stress value and the location of the maximum stress under the condition of asynchronous vibration of the blade.

2. The method for real-time monitoring of blade asynchronous vibration stress based on blade tip timing principle according to claim 1, characterized in that: Calculating the actual vibration displacements of the blade tips of the N blades when they reach each blade tip timing probe, and then separating the actual vibration displacements of the blade tips to obtain a variation relationship between the actual vibration displacements of the blade tips of each blade and time; Select the actual vibration displacement of the blade tip within at least p rotation periods of any blade, where p is an arbitrary positive integer; perform least square fitting with the natural frequencies corresponding to at least six-order modal vibration shapes, so as to obtain the blade fitting actual vibration amplitude and the actual modal vibration shape order when the blade vibrates and the blade fitting actual vibration frequency corresponding to the order, which are used to correct the blade actual vibration frequency and blade actual vibration amplitude described in step three; At the same time, combined with the actual vibration frequency of the blade fitted by the least squares fitting, the relationship between the vibration phase of any blade in p rotation cycles and time is obtained, and then, the N blades are subjected to least squares fitting, that is, the relationship between the vibration phase of the N blades in p cycles and time is obtained, and the vibration phase distribution of the N blades at the same time is extracted to obtain the actual vibration node diameter of the blade fitting, which is used to correct the actual vibration node diameter of the blade described in step three.

3. The method for real-time monitoring of blade asynchronous vibration stress based on blade tip timing principle according to claim 1, characterized in that: The speed probe is composed of a stator component mounted on the blade casing and a rotor component mounted on the blade shaft. When the stator component and the rotor component are at the same circumferential angle, the speed probe triggers a speed pulse signal. The blade tip timing probe is arranged on the casing at any circumferential angle, and when the blade reaches the blade tip timing probe, a blade tip pulse signal is triggered; The rotation speed probe, blade tip timing probe and a signal acquisition device for receiving rotation speed pulse signals and blade tip pulse signals constitute a blade tip timing system.

4. The method for real-time monitoring of blade asynchronous vibration stress based on blade tip timing principle according to claim 1, characterized in that: The specific formula for calculating the vibration displacement of the N blades when they reach a blade tip timing probe in step 3 is: d=(t ideal -t real )*(2*pi*r tip )*rps, Among them, t ideal represents the ideal arrival time of the blade tip when the blade is not vibrating, t real represents the actual arrival time of the blade tip under vibration conditions, pi represents pi, r tip Represents the radius value at the blade tip vibration displacement measurement point, and rps represents the current blade rotation speed in revolutions per second.

5. The method for real-time monitoring of blade asynchronous vibration stress based on blade tip timing principle according to claim 1, characterized in that: In step 3, the specific calculation steps for calculating the actual vibration frequency of the blade, the actual vibration amplitude of the blade, and the actual vibration pitch diameter of the blade corresponding to the actual vibration displacement of the blade tip are: According to the rotation speed rps of the blade measured by the rotation speed probe, the sampling frequency f of the blade measured by the blade tip timing probe sample for: f sample =rps*N, Where rps is the rotation speed of the blade, in revolutions per second; N is the number of blades; and the vibration frequency f is obtained by performing a fast Fourier transform on the actual vibration displacement of the blade measured by a single blade tip timing probe. observe , Based on the undersampling effect of the blade tip timing principle and combined with the Nyquist sampling theorem, the blade vibration frequency f in the stationary coordinate system is obtained: stat and the vibration frequency f obtained by fast Fourier transform observe The relationship between them is expressed as: In the formula, n is an integer, f sample is the sampling frequency; At the same time, the frequency conversion formula in the rotating coordinate system and the stationary coordinate system is: f stat =f rot +ND*rps, In the formula, f stat is the blade vibration frequency in the stationary coordinate system, f rot is the blade vibration frequency in the rotating coordinate system, ND is the blade vibration pitch diameter, and rps is the blade rotation speed; Combining the relationship formula and the frequency conversion formula, we get the conditional formula: Select different integers n and blade vibration pitch diameter ND. When the conditional formula is satisfied, the blade vibration frequency f in the rotating coordinate system is obtained. rot , which is the actual vibration frequency of the blade, and the obtained blade vibration node diameter ND, which is the actual vibration node diameter of the blade.

6. The method for real-time monitoring of blade asynchronous vibration stress based on blade tip timing principle according to any one of claims 1 to 5, characterized in that: The maximum stress value and the location of the maximum stress under the condition of asynchronous vibration of the blade in step 4 are determined as follows: According to the actual vibration frequency of the blade, the actual modal vibration shape and the position of the maximum stress are determined, and the actual vibration stress of the blade is obtained through the first principal stress and modal displacement corresponding to the actual modal vibration shape, that is, through the following conversion relationship:

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