On-line measurement device and method for rotor-stator axial clearance based on swept frequency polarization modulation

Abstract

The invention discloses an online measurement device method for the axial clearance between a rotor and a stator based on swept-frequency polarization modulation. The measuring device comprises a secondary modulation optical path unit, a measuring optical path unit, a reference optical path unit, a swept-frequency signal generating unit, and a signal acquisition and processing unit; the secondary modulation optical path unit is connected to the measuring optical path unit and the reference optical path unit, the secondary modulation optical path unit comprises a laser, a first collimator, an isolator, a polarization beam splitter prism, an electro-optical modulator, a quarter wave plate, a second collimator, and a coupler, the laser is connected to the first collimator through a single-mode optical fiber, and the coupler is connected to the second collimator through a polarization-maintaining optical fiber; an isolator, a polarization beam splitter prism, an electro-optical modulator, a quarter wave plate, and a second collimator are sequentially arranged after the first collimator; the swept-frequency signal generating unit is connected to the electro-optical modulator, and the signal acquisition and processing unit is arranged on one side of the polarization beam splitter prism.

Description

Technical Field

[0001] The present invention relates to the field of non-contact distance measurement, and in particular to the field of rotor-stator axial clearance measurement of aircraft engines. Specifically, the present invention relates to an online rotor-stator axial clearance measurement device and method based on frequency-sweeping polarization modulation, and in particular to an online rotor-stator axial clearance measurement method and device that uses a parallel optical path (or common optical path) structure and combines a Fourier transform algorithm to compensate for temperature drift caused by temperature changes during the measurement process. Background Art

[0002] The state parameters of the core components of aircraft engines (rotors, moving blades, etc.) directly affect the operating state, work efficiency and safety performance of the equipment. In particular, the change in the axial clearance between the rotor and the stator is one of the main factors affecting the performance and safety of the equipment. When major equipment is in operation, the interior of the machine is in extremely harsh conditions such as high temperature (above 600°C) and high vibration. In addition, its internal space is narrow and the structure is complex. Traditional measurement methods cannot meet the needs of online dynamic sensing and measurement of the gap. How to achieve high-reliability, high-resolution, and high-precision online dynamic measurement of the gap under narrow space and high temperature conditions is a key scientific and technological problem that needs to be solved urgently.

[0003] Currently, the main measurement methods can be divided into capacitance method, eddy current method, microwave method and optical method.

[0004] The capacitance method has the characteristics of high temperature resistance, fast response speed and good reliability, but its probe size will increase sharply with the increase of measurement range, and due to the temperature characteristics of the medium, the temperature drift of the measurement result is large; the eddy current method has the advantages of resistance to harsh environments such as water vapor and oil pollution, and can measure the gap in a highly polluted environment. It has a simple structure, but because the magnetic core and other structures are not resistant to high temperatures, it is only suitable for normal temperature working environments and rarely used in high temperature applications. In addition, the leakage capacitance, leakage conductance and other parameters of the eddy current sensor are significantly affected by temperature changes, and in severe cases even cause circuit saturation, resulting in measurement failure; the microwave method is not easily affected by the internal working environment of the engine and is suitable for the measurement of tiny gaps in the engine, but the current compensation problem of temperature drift has not been effectively solved. In addition, due to the use of high-frequency microwaves and high-temperature mineral cables, the transmission loss is large, and it is not suitable for use in the narrow space inside major equipment, long distances, and complex lead-out paths.

[0005] Optical measurement methods use lasers to measure axial clearance, which is not easily affected by electromagnetic interference. Optical fiber is used as the transmission medium of laser signals to solve the problem of complex signal extraction paths inside aircraft engines. Optical fiber can also be used to make probes to solve the problem of small size. Compared with electrical measurement methods, optical measurement methods are more suitable for measuring the axial clearance of rotors and stators of aircraft engines. Summary of the invention

[0006] The purpose of the present invention is to overcome the deficiencies in the prior art and provide an online measurement device and method for the rotor-stator axial clearance based on frequency sweep polarization modulation. The measurement method and device are designed with a parallel optical path (or common optical path) structure and combined with a Fourier transform algorithm to compensate for the huge temperature drift caused by the large temperature change inside the aircraft engine, thereby realizing non-contact real-time online accurate measurement of the rotor-stator axial clearance of the aircraft engine under high temperature and space-constrained conditions. The method of the present invention belongs to the category of optics and combines the advantages of optical measurement methods.

[0007] The objective of the present invention is achieved through the following technical solutions:

[0008] An online measurement device for rotor-stator axial clearance based on swept-frequency polarization modulation comprises a secondary modulation optical path unit, a measuring optical path unit, a reference optical path unit, a swept-frequency signal generating unit, and a signal acquisition and processing unit; the secondary modulation optical path unit is connected to the measuring optical path unit and the reference optical path unit, the secondary modulation optical path unit comprises a laser, a first collimator, an isolator, a polarization beam splitter prism, an electro-optical modulator, a quarter wave plate, a second collimator, and a coupler, the laser is connected to the first collimator via a single-mode optical fiber, and the coupler is connected to the second collimator via a polarization-maintaining optical fiber; an isolator, a polarization beam splitter prism, an electro-optical modulator, a quarter wave plate, and a second collimator are sequentially arranged after the first collimator; the swept-frequency signal generating unit is connected to the electro-optical modulator, and the signal acquisition and processing unit is arranged on one side of the polarization beam splitter prism.

[0009] Furthermore, a parallel optical path splitting structure or a common optical path splitting structure is adopted between the measuring optical path unit and the reference optical path unit.

[0010] Furthermore, when a parallel optical path splitting structure is adopted between the measuring optical path unit and the reference optical path unit, the measuring optical path unit is composed of a first optical fiber probe and an axial end face to be measured which are arranged in sequence; the reference optical path unit is composed of a second optical fiber probe and a reflective film, and the reflective film is plated on the output end face of the second optical fiber probe; the first optical fiber probe and the second optical fiber probe are respectively connected to the coupler through polarization-maintaining optical fibers.

[0011] Furthermore, when a common optical path splitting structure is adopted between the measuring optical path unit and the reference optical path unit, the measuring optical path unit and the reference optical path unit are jointly composed of a third optical fiber probe, and a semi-transparent and semi-reflective film is coated on the output end face of the third optical fiber probe. The semi-transparent and semi-reflective film can divide the laser into two parts: one part of the laser passes through the output end face of the third optical fiber probe, irradiates the axial end face to be measured, is reflected by the axial end face to be measured, and is then received by the third optical fiber probe. This part of the laser carries the axial gap information to be measured; the other part of the laser is directly reflected by the semi-transparent and semi-reflective film.

[0012] Furthermore, the frequency sweep signal generating unit is composed of a clock reference, a microcontroller, a phase-locked loop frequency synthesizer, and a power amplifier. The clock reference and the microcontroller are respectively connected to the phase-locked loop frequency synthesizer, the phase-locked loop frequency synthesizer is connected to the power amplifier, and the power amplifier is connected to the electro-optical phase modulator; the clock reference provides a reference clock for the phase-locked loop frequency synthesizer, the microcontroller controls the phase-locked loop frequency synthesizer to generate a frequency-variable modulation signal, and the modulation signal is amplified by the power amplifier and then loaded onto the electro-optical modulator.

[0013] Furthermore, the signal acquisition and processing unit is composed of a detector, an acquisition card, and a PC host computer which are connected in sequence; the detector is arranged on one side of the polarization beam splitter prism, and the polarization direction of the polarization beam splitter prism is at an angle of 45° with the x-axis.

[0014] The present invention also provides an online measurement method for the rotor-stator axial clearance based on swept frequency polarization modulation, comprising the following steps:

[0015] S1, the laser emits laser light, which enters the input end of the first collimator through single-mode optical fiber coupling; the collimated laser light output by the first collimator enters the free space and couples with the isolator, and the isolator only allows one-way laser light to pass through;

[0016] S2. The laser emitted from the isolator enters the polarization beam splitter prism. After passing through the polarization beam splitter prism, the laser forms linear polarized light with the same amplitude and perpendicular vibration directions, that is, P light vibrating along the x-axis direction and S light vibrating along the y-axis direction;

[0017] S3, by loading a sinusoidal modulation signal on the electro-optical modulator, performing a first polarization modulation on the P light and the S light entering the electro-optical modulator, and introducing a phase delay proportional to the modulation signal voltage to the P light and the S light;

[0018] S4, the P light and S light after the first polarization modulation pass through the 1 / 4 wave plate for the first time, and the angle between the fast axis of the 1 / 4 wave plate and the x-axis is 45°; the first modulated light emitted from the 1 / 4 wave plate enters the second collimator, and after passing through the second collimator, the first modulated light reaches the coupler through the polarization-maintaining fiber;

[0019] S5. The primary modulated light is divided into two paths of light through a coupler, namely, a measuring light and a reference light. The measuring light reaches the first optical fiber probe through a polarization-maintaining optical fiber, and the reference light reaches the second optical fiber probe through a polarization-maintaining optical fiber. The polarization-maintaining optical fibers of the measuring optical path and the reference optical path have the same length.

[0020] S6, the measuring light is emitted through the first optical fiber probe, passes through the gap space between the first optical fiber probe and the axial end face to be measured, reaches the axial end face to be measured, and then reflects, and part of the reflected light is received by the first optical fiber probe, and reaches the coupler again through the polarization-maintaining optical fiber; the reference light reaches the second optical fiber probe, and the reflective film realizes the mirror reflection of the reference light, and the reflected light reaches the coupler again through the polarization-maintaining optical fiber;

[0021] S7, the measuring light and the reference light are coupled into one light through a coupler and transmitted through a polarization-maintaining optical fiber to reach a second collimator; after being emitted through the second collimator, the measuring light and the reference light sequentially pass through a quarter wave plate and an electro-optic modulator to undergo a second polarization modulation; the secondary polarization modulated measuring light and the reference light emitted by the electro-optic modulator enter a polarization beam splitter prism, and polarization interference occurs at the polarization beam splitter prism between the P light and the S light of the measuring light and the reference light;

[0022] S8. After polarization interference occurs at the polarization beam splitter prism, the measuring light and the reference light are both received by the detector, and the light intensity received by the detector is the sum of the light intensities of the measuring light and the reference light;

[0023] S9, performing linear frequency sweep from frequency f1 to frequency f2 by means of a frequency sweep signal generating unit, that is, generating a plurality of modulation signals with equal frequency intervals within a frequency range (f1, f2), obtaining the magnitude of the total light intensity detected by the detector for each frequency modulation signal, and marking the magnitude of the total light intensity corresponding to each frequency modulation signal in the same coordinate system, wherein the abscissa of the coordinate system represents the modulation frequency, and the ordinate represents the magnitude of the total light intensity, thereby obtaining a curve of the total light intensity with respect to the modulation frequency;

[0024] S10, perform Fourier transform on the above curve of total light intensity with respect to modulation frequency with respect to the frequency of the RF modulation signal, and obtain a curve with two peaks, the horizontal coordinates corresponding to the peaks are F M and F R , F M and F R are the frequencies at which the light intensities of the reference signal and the measurement signal change with the frequency of the RF modulation signal;

[0025] S11. Obtain an expression for the axial clearance to be measured, perform data analysis and processing through a signal acquisition and processing unit, obtain a measurement result of the rotor-stator axial clearance, and realize online measurement of the rotor-stator axial clearance.

[0026] Furthermore, the diameters of the first optical fiber probe and the second optical fiber probe are within 5 mm.

[0027] Furthermore, the crystal main axis direction of the electro-optic modulator is parallel to the x-axis and y-axis of the coordinate system. After modulating the P light and the S light, the phase delay of the P light and the S light is proportional to the voltage of the modulation signal loaded on the electro-optic modulator.

[0028] Furthermore, the expression of the axial clearance to be measured in step S11 is:

[0029]

[0030] Where n1 represents the refractive index of light in air, d represents the axial gap to be measured, c represents the speed of light in vacuum, and F M and F R are the frequencies at which the light intensities of the reference signal and the measurement signal change as the frequency of the RF modulation signal changes.

[0031] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0032] 1. The technical solution of the present invention is an optical measurement method based on laser emission and optical fiber transmission, which is suitable for the measurement environment with complex signal extraction paths inside the aircraft engine. In addition, the laser is transmitted inside the aircraft engine and is not easily affected by electromagnetic interference compared to electrical signals.

[0033] 2. The present invention adopts a small-sized, high-temperature-resistant optical fiber probe, which is more suitable for the measurement environment of high temperature (above 600° C.) and narrow space inside an aircraft engine.

[0034] 3. The present invention proposes a measurement optical path based on dual parallel optical path frequency sweeping polarization secondary modulation, and a method of solving the axial gap to be measured using Fourier transform, which can compensate for the temperature drift caused by the large temperature change inside the aircraft engine, and is more suitable for measurement environments where the temperature inside the aircraft engine changes greatly.

[0035] 4. The present invention proposes to apply the absolute distance measurement method of swept-frequency polarization secondary modulation to the axial clearance measurement of the rotor and stator of aircraft engines, replacing the traditional phase-type laser absolute distance measurement method, and making up for the shortcomings of the traditional phase-type laser absolute distance measurement method. Compared with the traditional phase-type laser absolute distance measurement method, the absolute distance measurement method of swept-frequency polarization secondary modulation has improvements in both measurement range and measurement accuracy.

[0036] 5. The technical solution of the present invention breaks the current situation that there is no mature rotor-stator axial clearance measurement solution at home and abroad. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a schematic diagram of the structure of the measuring device of the present invention.

[0038] Figure 2 is the measured total light intensity I total Plot with respect to modulation frequency f.

[0039] Figure 3 It is the result of Fourier transform of the measured total light intensity signal with respect to the swept frequency modulation frequency f.

[0040] Figure 4 It is a schematic diagram of the structure of the overall optical path part of the measuring device of the present invention; in this figure, a common optical path splitting structure is adopted between the measuring optical path unit and the reference optical path unit.

[0041] Figure numerals: 1-secondary modulation optical path unit, 2-measurement optical path unit, 3-reference optical path unit, 4-sweep signal generating unit, 5-signal acquisition and processing unit, 101-laser, 102-first collimator, 103-isolator, 104-polarization beam splitter prism, 105-electro-optical modulator, 106-1 / 4 wave plate, 107-second collimator, 108-coupler, 109-first optical fiber probe, 110-axial end face to be measured, 111-second optical fiber probe, 112-reflection film, 117-detector, 118-acquisition card, 119-PC host computer, 201-third optical fiber probe. DETAILED DESCRIPTION

[0042] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0043] like Figure 1 As shown, the present invention provides an online measurement device for the axial clearance of a rotor and a stator based on swept-frequency polarization modulation, comprising a secondary modulation optical path unit 1, a measuring optical path unit 2, a reference optical path unit 3, a swept-frequency signal generating unit 4, and a signal acquisition and processing unit 5; the secondary modulation optical path unit 1 is connected to the measuring optical path unit 2 and the reference optical path unit 3, and the secondary modulation optical path unit 1 comprises a laser 101, a first collimator 102, an isolator 103, a polarization beam splitter prism 104, an electro-optical modulator 105, and a 1 / 4 wave plate 106. , a second collimator 107, and a coupler 108. The laser 101 is connected to the first collimator 102 via a single-mode optical fiber, and the coupler 108 is connected to the second collimator 107 via a polarization-maintaining optical fiber; an isolator 103, a polarization beam splitter prism 104, an electro-optical modulator 105, a quarter wave plate 106, and a second collimator 107 are sequentially arranged after the first collimator 102; a sweep signal generating unit 4 is connected to the electro-optical modulator 105, and a signal acquisition and processing unit 5 is arranged on one side of the polarization beam splitter prism 104.

[0044] In the present invention, a parallel optical path splitting structure or a common optical path splitting structure may be used between the measuring optical path unit 2 and the reference optical path unit 3 . Both optical path structures can achieve the measurement purpose.

[0045] When a light splitting structure of parallel light paths is adopted between the measuring light path unit 2 and the reference light path unit 3, the measuring light path unit 2 is composed of a first optical fiber probe 109 and an axial end face 110 to be measured, which are arranged in sequence; the reference light path unit 3 is composed of a second optical fiber probe 111 and a reflective film 112, and the reflective film 112 is plated on the emission end face of the second optical fiber probe 111; the first optical fiber probe 109 and the second optical fiber probe 111 are respectively connected to the coupler 108 through polarization-maintaining optical fibers. That is, the coupler 108 is used to divide the optical path into a measuring optical path and a reference optical path to perform the measurement process.

[0046] When the common optical path structure is adopted between the measurement optical path unit 2 and the reference optical path unit 3, Figure 4 The measuring optical path unit 2 and the reference optical path unit 3 are composed of a third optical fiber probe 201. The output end face of the third optical fiber probe 201 is coated with a high temperature resistant semi-transparent and semi-reflective film. The semi-transparent and semi-reflective film can divide the laser into two parts: one part of the laser passes through the output end face of the third optical fiber probe 201, irradiates the axial end face 110 to be measured, is reflected by the axial end face 110 to be measured, and is then received by the third optical fiber probe 201. This part of the laser carries the information of the axial gap to be measured; the other part of the laser is directly reflected by the semi-transparent and semi-reflective film. The two parts of light are used as the measuring light and the reference light for the measurement process.

[0047] The frequency sweep signal generating unit 4 is composed of a clock reference, a microcontroller, a phase-locked loop frequency synthesizer, and a power amplifier. The clock reference and the microcontroller are respectively connected to the phase-locked loop frequency synthesizer, the phase-locked loop frequency synthesizer is connected to the power amplifier, and the power amplifier is connected to the electro-optical phase modulator; the clock reference provides a reference clock for the phase-locked loop frequency synthesizer, the microcontroller controls the phase-locked loop frequency synthesizer to generate a frequency-variable modulation signal, and the modulation signal is amplified by the power amplifier and then loaded onto the electro-optical modulator.

[0048] Specifically, in order to explain the working principle and working process of the device, the following is established: Figure 1 The Cartesian coordinate system shown in the figure has a positive z-axis direction that is the same as the transmission direction of the forward laser. The on-line measurement method of the rotor-stator axial clearance based on frequency-sweep polarization modulation is as follows:

[0049] S1, laser 101 emits laser, which enters the input end of the first collimator 102 through single-mode optical fiber coupling; the collimated laser output by the first collimator 102 enters the free space and couples with the isolator 103. The isolator 103 only allows one-way laser to pass through, so as to prevent the reverse laser from entering the laser and affecting the laser.

[0050] S2. The laser emitted from the isolator 103 enters the polarization beam splitter prism 104. The polarization direction of the polarization beam splitter prism 104 is at an angle of 45° to the x-axis. After passing through the polarization beam splitter prism 104, the laser forms linear polarized light with the same amplitude and perpendicular vibration directions: P light (vibrating along the x-axis direction) and S light (vibrating along the y-axis direction), which can be expressed as the Jones matrix:

[0051]

[0052] In the formula, E x P represents the light amplitude, E y represents the amplitude of the S light, and E0 represents the amplitude of the synthetic linearly polarized light after passing through the polarization splitter prism.

[0053] S3, P light and S light enter the electro-optic phase modulator 105 (EOM) for the first polarization modulation. The crystal main axis direction of the electro-optic phase modulator 105 is parallel to the x-axis and y-axis of the coordinate system. After modulating the P light and S light, the phase delay of the P light and S light is proportional to the voltage of the modulation signal loaded on the electro-optic phase modulator 105.

[0054] S4, the frequency sweep signal generating unit 4 generates a modulation signal loaded on the electro-optical phase modulator 105. The frequency sweep signal generating unit is composed of a clock reference, a microcontroller (MCU), a phase-locked loop frequency synthesizer (PLL), and a power amplifier (PA). The clock reference provides a reference clock for the phase-locked loop frequency synthesizer. The microcontroller controls the phase-locked loop frequency synthesizer to generate a frequency-variable modulation signal. The modulation signal is amplified by the power amplifier and then loaded on the electro-optical phase modulator 105. The modulation signal loaded on the electro-optical phase modulator 105 is expressed as

[0055]

[0056] In the formula, A0 represents the amplitude of the modulation signal, ω represents the angular frequency of the modulation signal, Indicates the initial phase of the modulating signal.

[0057] The sinusoidal modulation signal loaded on the electro-optic phase modulator 105 performs the first polarization modulation on the P light and the S light entering the electro-optic phase modulator 105. The P light and the S light introduce a phase delay proportional to the modulation signal voltage. This phase delay can be expressed as

[0058]

[0059] make

[0060]

[0061] but

[0062]

[0063] Where V π represents the half-wave voltage of the electro-optical phase modulator 105.

[0064] The first polarization modulation process is expressed by the Jones matrix as

[0065]

[0066] i is the imaginary unit, e is the Euler number;

[0067] S5. The P light and S light after the first polarization modulation pass through the 1 / 4 wave plate 106 for the first time. The angle between the fast axis of the 1 / 4 wave plate 106 and the x-axis is 45°. This process is expressed by the Jones matrix as follows:

[0068]

[0069] S6. The primary modulated light is emitted from the 1 / 4 wave plate 106 and enters the second optical fiber collimator 107. After passing through the second optical fiber collimator 107, the primary modulated light reaches the coupler 108 through the polarization-maintaining optical fiber. The primary modulated light is divided into two paths of light through the coupler 108, namely the measurement light and the reference light. The measurement light passes through Figure 1 The measurement optical path unit 2 shown in the figure transmits the reference light through the reference optical path unit 3. The process of splitting the primary modulated light is the process of splitting the amplitude of the primary modulated light. This process can be expressed as follows using the Jones matrix:

[0070] Measuring light

[0071]

[0072] Reference light

[0073]

[0074] Where α1 and α2 are the splitting ratios of the measurement light and the reference light, respectively.

[0075] S7, the measurement light reaches the first fiber probe 109 through the polarization-maintaining fiber, and the reference light reaches the second fiber probe 111 through the polarization-maintaining fiber. The polarization-maintaining fibers of the measurement light path and the reference light path have the same length. The diameter of the first fiber probe 109 and the second fiber probe 111 can be within 5 mm to adapt to the narrow measurement environment inside the aircraft engine.

[0076] When measuring the axial clearance between the rotor and stator of an aircraft engine, the measuring optical path unit 2 and the reference optical path unit 3 are placed in a high-temperature measurement environment at the same time. In this way, when the temperature changes, the optical path of the measuring optical path and the reference optical path will change to the same extent, thereby compensating for the huge temperature drift caused by a large temperature change.

[0077] S8, the measuring light is emitted through the first optical fiber probe 109, passes through the gap space between the first optical fiber probe 109 and the axial end face 110 to be measured, reaches the axial end face 110 to be measured, and then reflects, and part of the reflected light is received by the first optical fiber probe 109, and reaches the coupler 108 again through the polarization-maintaining fiber. The reflection occurring at the axial end face 110 to be measured is not an ideal mirror reflection, so the received light of the first optical fiber probe 109 will have a loss compared to the emitted light, and this process is represented by the Jones matrix as

[0078]

[0079] Where α3 is the amplitude ratio of the received light to the emitted light.

[0080] S9, the reference light reaches the second optical fiber probe 111. The end face of the second optical fiber probe 111 is coated with a reflective film to achieve mirror reflection of the reference light. The reflected light reaches the coupler 108 again through the polarization-maintaining optical fiber. This process is represented by the Jones matrix:

[0081]

[0082] S10, the measuring light and the reference light are coupled into one optical transmission through the coupler 108, and are transmitted through the polarization-maintaining optical fiber to reach the second optical fiber collimator 107. The measuring light and the reference light are emitted through the second optical fiber collimator 107, and pass through the 1 / 4 wave plate 106 again. This process has the same effect on the measuring light and the reference light, which can be expressed as the Jones matrix:

[0083]

[0084] S11, the measurement light and the reference light that have passed through the 1 / 4 wave plate 106 enter the electro-optic modulator 105 again for a second polarization modulation. Compared with the reference light, the measurement light includes the optical path caused by the axial gap, so the optical path and time experienced by the measurement light and the reference light when they reach the electro-optic modulator 105 are different. When the measurement light and the reference light undergo the second polarization modulation, the modulation signal loaded on the electro-optic modulator 105 will change to varying degrees, thereby introducing different phase delays of the P light and the S light.

[0085] by and It indicates the phase delay of P light and S light introduced by the second polarization modulation of the measurement light and the reference measurement light. and It can be expressed as

[0086]

[0087] in, and The measurement light and the reference light experience different flight times t after the first polarization modulation of the electro-optic modulator 105. M and t R When the signal reaches the electro-optic modulator 105 for the second time, the phase of the modulated signal loaded on the electro-optic modulator 105 changes. and It can be expressed as

[0088]

[0089] Combining the above formula, we can see that It includes measuring the phase introduced by the light in the free space, optical fiber and axial gap space. and It only includes the phase introduced by the reference light during its flight in free space and in the optical fiber. and

[0090] Three-part phase and It can be expressed as

[0091]

[0092] Wherein, c represents the speed of light in vacuum, f represents the frequency of the modulation signal loaded on the electro-optic modulator 105, n1 and n2 represent the refractive index of light in air and the refractive index of light in the optical fiber, d1 and d2 represent the distances that light flies in free space and in the optical fiber, and d represents the axial gap to be measured.

[0093] but and It can be expressed as

[0094]

[0095] Then the second polarization modulation process of the electro-optic modulator 105 on the measuring light and the reference light can be represented by Jones matrices respectively.

[0096] The Jones matrix of the second polarization modulation of the measured light is:

[0097]

[0098] The Jones matrix of the second polarization modulation of the reference light is:

[0099]

[0100] The effect of temperature change on the refractive index of air is negligible, but the effect of temperature change on the refractive index and length of optical fiber is significant. Therefore, the optical path and flight time of light in the optical fiber will change due to temperature change. In this measurement method and device, part of the optical fiber is placed in the high temperature, large temperature change and narrow space measurement environment of the aircraft engine. The temperature change will cause the average refractive index of the polarization-maintaining optical fiber to change. Therefore, the phase difference introduced by the light in the optical fiber during its flight will be affected. It will change with temperature, introducing temperature drift during the measurement process. This is why the reference optical path is introduced to compensate for temperature drift. How to compensate for temperature drift will be explained in detail later.

[0101] S12, the secondary polarization modulated measuring light and reference light emitted from the electro-optical modulator 105 arrive at the polarization beam splitter prism 104, the polarization direction of the polarization beam splitter prism 104 is at an angle of 45° with the x-axis, and the P light and S light of the measuring light and the reference light undergo polarization interference at the polarization beam splitter prism 104. This process is represented by the Jones matrix as follows:

[0102]

[0103] Furthermore, the Jones matrix relationship is established for the entire propagation process of the measurement light as follows:

[0104]

[0105] The Jones matrix is ​​calculated as follows:

[0106]

[0107] The intensity of the measured light can be calculated from the amplitude:

[0108]

[0109] Simplifying further:

[0110]

[0111] Expanded by the Jacobi-Angel identity:

[0112]

[0113] By filtering out the high frequency terms, we can get:

[0114]

[0115] Upward Further expansion of the first kind of Bessel series yields:

[0116]

[0117] When M is less than 1 and M is not very large, the terms greater than 2nd order in the above formula can be filtered out. Substituting the filtered formula into the original formula, the expression of the measured light intensity can be obtained:

[0118]

[0119] Furthermore, the Jones matrix relationship for the entire propagation process of the reference light is established as follows:

[0120]

[0121] The Jones matrix is ​​calculated as follows:

[0122]

[0123] Similarly, according to the calculation process and steps in the previous step, the reference light intensity expression can be obtained:

[0124]

[0125] S13. After polarization interference occurs at the polarization beam splitter prism 104, the measurement light and the reference light are both received by the detector 117. The light intensity received by the detector 117 is the sum of the light intensities of the measurement light and the reference light. The detector 117 with an appropriate response speed can filter out the high-frequency components in the measurement light and the reference light. Then, the total light intensity received is expressed as:

[0126]

[0127] It can be seen from the above formula that the total light intensity received by the detector is related to the phase of the modulation signal of the measurement light and the reference light by the electro-optic modulator 105. and There is a definite relationship. The phase introduced by the measurement light in the free space, optical fiber and axial gap space and Decide, The phase introduced only by the reference light in the free space and optical fiber and Decide, and It is proportional to the product of the optical path length and modulation frequency in free space, optical fiber and axial gap respectively.

[0128] S14. Figure 1The microcontroller in the frequency sweep signal generating unit 4 controls the phase-locked loop frequency synthesizer to perform linear frequency sweep from frequency f1 to frequency f2, that is, to generate a series of modulation signals with equal frequency intervals within the frequency range (f1, f2). For each frequency modulation signal, the total light intensity detected by the detector 117 is obtained, and the total light intensity corresponding to the modulation signal of each frequency is marked in the same coordinate system, where the horizontal axis of the coordinate system represents the modulation frequency and the vertical axis represents the total light intensity. Thus, a total light intensity I total The curve of the modulation frequency f is as follows: Figure 2 shown.

[0129] Performing Fourier transform on the curve with respect to the sweep modulation frequency f, we can obtain a curve with two peaks, and the horizontal coordinates corresponding to the peaks are F M and F R , which are the frequencies at which the light intensity of the reference signal and the measured signal changes with the frequency of the RF modulation signal; the results are as follows Figure 3 shown.

[0130] From the above expression of total light intensity, we can know

[0131]

[0132] S15, from this we can get the expression of the axial clearance to be measured

[0133]

[0134] Finally, the PC host computer of the signal acquisition and processing unit performs data analysis and processing to obtain the measurement results of the rotor-stator axial clearance, thereby realizing the online measurement of the rotor-stator axial clearance.

[0135] It can be seen from the final expression of the axial clearance to be measured that the final expression does not contain the fiber refractive index n2 and the fiber length d2. Therefore, when the temperature changes significantly, the reference light measurement result compensates the measurement light measurement result, and the changes in the fiber refractive index and length caused by the significant temperature change will not affect the final axial clearance measurement result. This also shows that the aircraft engine axial clearance measurement method and device based on dual parallel optical path swept frequency secondary polarization modulation proposed in the present invention can compensate for the temperature drift caused by the significant change in the internal temperature of the aircraft engine, thereby realizing non-contact real-time online precise measurement of the rotor-stator axial clearance in the aircraft engine under high temperature, large temperature change and narrow space environment.

[0136] Finally, it should be pointed out that the above examples are only used to illustrate the calculation process of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above examples, a person skilled in the art should understand that the calculation process described in the above examples can still be modified, or some parameters can be replaced by equivalents, and these modifications or replacements do not make the essence of the corresponding calculation method deviate from the spirit and scope of the calculation method of the present invention.

[0137] The present invention is not limited to the embodiments described above. The above description of the specific embodiments is intended to describe and illustrate the technical solution of the present invention. The above specific embodiments are merely illustrative and not restrictive. Without departing from the scope of the present invention and the scope of protection of the claims, a person of ordinary skill in the art can also make many forms of specific changes under the guidance of the present invention, which all fall within the scope of protection of the present invention.

Claims

1. A rotor-stator axial clearance online measurement method based on swept frequency polarization modulation, and a rotor-stator axial clearance online measurement device, wherein the measurement device comprises a secondary modulation optical path unit (1), a measurement optical path unit (2), a reference optical path unit (3), a swept frequency signal generating unit (4), and a signal acquisition and processing unit (5); the secondary modulation optical path unit (1) is connected to the measurement optical path unit (2) and the reference optical path unit (3); the secondary modulation optical path unit (1) comprises a laser (101), a first collimator (102), an isolator (103), a polarization beam splitter prism (104), an electro-optical modulator (105), a quarter wave plate (106), and a second optical path unit (107). 06), a second collimator (107), and a coupler (108), wherein the laser (101) is connected to the first collimator (102) via a single-mode optical fiber, and the coupler (108) is connected to the second collimator (107) via a polarization-maintaining optical fiber; an isolator (103), a polarization beam splitter prism (104), an electro-optic modulator (105), a quarter wave plate (106), and a second collimator (107) are sequentially arranged after the first collimator (102); a sweep signal generating unit (4) is connected to the electro-optic modulator (105), and a signal acquisition and processing unit (5) is arranged on one side of the polarization beam splitter prism (104), characterized in that: The measurement method includes the following steps: S1, the laser (101) emits laser light, which is coupled to the input end of the first collimator (102) through a single-mode optical fiber; the collimated laser light output by the first collimator (102) enters the free space and is coupled to the isolator (103), and the isolator (103) only allows unidirectional laser light to pass through; S2, the laser light emitted from the isolator (103) enters the polarization beam splitter prism (104), and after passing through the polarization beam splitter prism (104), the laser light forms linear polarized light with the same amplitude and mutually perpendicular vibration directions, that is, P light vibrating along the x-axis direction and S light vibrating along the y-axis direction; S3, performing a first polarization modulation on the P light and the S light entering the electro-optical modulator (105) by loading a sinusoidal modulation signal on the electro-optical modulator (105), and introducing a phase delay proportional to the modulation signal voltage into the P light and the S light; S4, the P light and the S light that have undergone the first polarization modulation pass through the quarter wave plate (106) for the first time, and the angle between the fast axis of the quarter wave plate (106) and the x-axis is 45°; the first modulated light emitted from the quarter wave plate (106) enters the second collimator (107), and after passing through the second collimator (107), the first modulated light reaches the coupler (108) through the polarization-maintaining optical fiber; S5, the primary modulated light is divided into two paths of light through a coupler (108), namely, a measuring light and a reference light; the measuring light reaches the first optical fiber probe (109) through a polarization-maintaining optical fiber, and the reference light reaches the second optical fiber probe (111) through a polarization-maintaining optical fiber, and the polarization-maintaining optical fibers of the measuring optical path and the reference optical path have the same length; S6, the measuring light is emitted through the first optical fiber probe (109), passes through the gap space between the first optical fiber probe (109) and the axial end face (110) to be measured, reaches the axial end face (110) to be measured, and then is reflected, and part of the reflected light is received by the first optical fiber probe (109), and reaches the coupler (108) again through the polarization-maintaining optical fiber; the reference light reaches the second optical fiber probe (111), the reflective film (112) realizes the mirror reflection of the reference light, and the reflected light reaches the coupler (108) again through the polarization-maintaining optical fiber; S7, the measuring light and the reference light are coupled into one light path through a coupler (108) and transmitted through a polarization-maintaining optical fiber to reach a second collimator (107); after being emitted through the second collimator (107), the measuring light and the reference light sequentially pass through a quarter wave plate (106) and an electro-optic modulator (105) to undergo a second polarization modulation; the second polarization-modulated measuring light and the reference light emitted by the electro-optic modulator (105) enter a polarization beam splitter prism (104), and polarization interference occurs at the polarization beam splitter prism (104); S8. After polarization interference occurs at the polarization beam splitter prism (104), the measuring light and the reference light are both received by the detector (117), and the light intensity received by the detector (117) is the sum of the light intensities of the measuring light and the reference light; S9, performing a linear frequency sweep from frequency f1 to frequency f2 by means of a frequency sweep signal generating unit (4), that is, generating a plurality of modulation signals with equal frequency intervals within a frequency range (f1, f2), obtaining the magnitude of the total light intensity detected by the detector (117) for each modulation signal of each frequency, and marking the magnitude of the total light intensity corresponding to the modulation signal of each frequency in the same coordinate system, wherein the abscissa of the coordinate system represents the modulation frequency and the ordinate represents the magnitude of the total light intensity, thereby obtaining a curve of the total light intensity with respect to the modulation frequency; S10, perform Fourier transform on the above curve of total light intensity with respect to modulation frequency with respect to the frequency of the RF modulation signal, and obtain a curve with two peaks, the horizontal coordinates corresponding to the peaks are F M and F R , F M and F R are the frequencies at which the light intensities of the reference signal and the measurement signal change with the frequency of the RF modulation signal; S11, obtain the expression of the axial clearance to be measured, and perform data analysis and processing through the signal acquisition and processing unit (5), The measurement result of the rotor-stator axial clearance is obtained, and the online measurement of the rotor-stator axial clearance is realized.

2. According to claim 1, the method for online measurement of rotor-stator axial clearance based on swept frequency polarization modulation is characterized in that: A parallel optical path splitting structure or a common optical path splitting structure is adopted between the measuring optical path unit (2) and the reference optical path unit (3).

3. According to claim 2, the method for online measurement of rotor-stator axial clearance based on swept frequency polarization modulation is characterized in that: When a light splitting structure with parallel light paths is adopted between the measuring light path unit (2) and the reference light path unit (3), the measuring light path unit (2) is composed of a first optical fiber probe (109) and an axial end face (110) to be measured, which are arranged in sequence; the reference light path unit (3) is composed of a second optical fiber probe (111) and a reflective film (112), and the reflective film (112) is plated on the emission end face of the second optical fiber probe (111); and the first optical fiber probe (109) and the second optical fiber probe (111) are respectively connected to a coupler (108) via polarization-maintaining optical fibers.

4. According to claim 2, the method for online measurement of rotor-stator axial clearance based on swept frequency polarization modulation is characterized in that: When a common optical path light splitting structure is adopted between the measuring optical path unit (2) and the reference optical path unit (3), the measuring optical path unit (2) and the reference optical path unit (3) are jointly composed of a third optical fiber probe (201), and a semi-transparent and semi-reflective film is plated on the output end face of the third optical fiber probe (201), and the semi-transparent and semi-reflective film can divide the laser into two parts: one part of the laser light passes through the output end face of the third optical fiber probe (201), is irradiated to the axial end face (110) to be measured, is reflected by the axial end face (110) to be measured, and is then received by the third optical fiber probe (201), and this part of the laser light carries the axial gap information to be measured; and the other part of the laser light is directly reflected by the semi-transparent and semi-reflective film.

5. According to claim 1, the method for online measurement of rotor-stator axial clearance based on swept frequency polarization modulation is characterized in that: The frequency sweep signal generating unit (4) is composed of a clock reference, a microcontroller, a phase-locked loop frequency synthesizer, and a power amplifier. The clock reference and the microcontroller are respectively connected to the phase-locked loop frequency synthesizer, the phase-locked loop frequency synthesizer is connected to the power amplifier, and the power amplifier is connected to the electro-optical phase modulator. The clock reference provides a reference clock for the phase-locked loop frequency synthesizer. The microcontroller controls the phase-locked loop frequency synthesizer to generate a frequency-variable modulation signal. The modulation signal is amplified by the power amplifier and then loaded onto the electro-optic modulator.

6. According to claim 1, the method for online measurement of rotor-stator axial clearance based on swept frequency polarization modulation is characterized in that: The signal acquisition and processing unit (5) is composed of a detector (117), an acquisition card (118), and a PC host computer (119) which are connected in sequence; the detector (117) is arranged on one side of a polarization beam splitter prism (104), and the angle between the polarization direction of the polarization beam splitter prism (104) and the x-axis is 45 degrees.

7. The method for online measurement of rotor-stator axial clearance based on swept frequency polarization modulation according to claim 1, characterized in that: The diameters of the first optical fiber probe (109) and the second optical fiber probe (111) are within 5 mm.

8. The method for online measurement of rotor-stator axial clearance based on swept frequency polarization modulation according to claim 1, characterized in that: The crystal main axis direction of the electro-optic modulator (105) is parallel to the x-axis and y-axis of the coordinate system. After modulating the P light and the S light, the phase delay of the P light and the S light is proportional to the voltage of the modulation signal loaded on the electro-optic modulator (105).

9. The method for online measurement of rotor-stator axial clearance based on swept frequency polarization modulation according to claim 1, characterized in that: The expression of the axial clearance to be measured in step S11 is: Where n1 represents the refractive index of light in air, d represents the axial gap to be measured, c represents the speed of light in vacuum, and F M and F R are the frequencies at which the light intensities of the reference signal and the measurement signal change as the frequency of the RF modulation signal changes.

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