Demodulator working temperature drift correction method in aviation tool optical fiber strain measurement

Abstract

The invention belongs to the technical field of intelligent manufacturing, and discloses a demodulator working temperature drift correction method in aviation tooling optical fiber strain measurement, which aims at strain measurement temperature drift characteristics of different temperature intervals, completes curve segmentation on the basis of temperature change rate, and further applies a proper polynomial function to perform segmentation fitting to obtain a temperature drift correction result. And the temperature drift correction model is established, so that the fitting complexity is reduced, and the temperature drift correction accuracy is improved. And the measurement precision of the fiber bragg grating strain sensor under the condition that the working temperature of the demodulator changes is ensured. The method is suitable for the strain measurement of a tool in the assembly process of aircraft components, and is also suitable for the strain measurement process of any fiber bragg grating. The method has the advantages of wide application range, simple operation, easy popularization and the like.

Description

Technical Field

[0001] The invention belongs to the technical field of intelligent manufacturing, and in particular relates to a method for correcting working temperature drift of a demodulator in optical fiber strain measurement of aviation tooling. Background Art

[0002] During the assembly process of large aircraft components, assembly stress will inevitably be generated due to assembly process and assembly operation, resulting in deviation of key positions of tooling, which seriously affects the assembly quality of the aircraft. Therefore, accurately measuring the strain of tooling during assembly is an important prerequisite for accurately warning of forced assembly and ensuring the quality of aircraft assembly. Fiber Bragg grating strain sensors have great advantages in measuring tooling strain during aircraft assembly due to their high precision, high stability, small size, and networked measurement. However, the measurement results of this method will drift significantly with the change of the working temperature of the sensor demodulator. In addition, the temperature change of the measurement environment and the heating of the demodulator working circuit cannot be avoided, which seriously restricts the measurement accuracy of this method. Therefore, how to correct the drift of the measurement results caused by the change of the working temperature of the demodulator during the tooling strain measurement process has become a key issue that needs to be solved urgently.

[0003] In the article "Research on Temperature Strain Monitoring Technology of NBI Beam Deflector Components Based on Fiber Optic Sensing", Zhu Mengjiao designed a metal-packaged bonded fiber Bragg grating strain sensor, which has a temperature compensation function by changing the packaging material. The sensor can cover a range of 6000με, and there is no obvious drift in a 100-degree Celsius working environment for 2 months. The temperature drift correction caused by the temperature change of the sensor measurement environment is completed, and the strain measurement accuracy is improved. However, this method only corrects the strain temperature drift value at the measuring point, and does not consider the temperature drift at the fiber optic demodulator, resulting in the temperature drift still existing, which cannot meet the accuracy requirements of aviation tooling. Therefore, this method has certain deficiencies and needs to be improved.

[0004] In the article "Study on Fiber Bragg Grating Sensor for Dynamic Pressure Monitoring", Zhang Jingnan designed a fiber Bragg grating pressure sensor with a "T"-shaped metal ferrule structure, and optimized the temperature compensation for the "T"-shaped metal ferrule fiber Bragg grating pressure sensor. The temperature response of the strain measurement at the measuring point was calibrated to obtain the temperature response fitting curve of the sensor, and the strain measurement was corrected for temperature drift, thereby ensuring the force measurement accuracy. However, this method has too many restrictions on the use of fiber Bragg grating strain sensors, limited installation conditions, and only corrected the temperature drift for the temperature change at the measuring point. There is still a problem of incomplete temperature drift correction, and the temperature drift caused by the temperature change of the fiber demodulator cannot be ignored in actual monitoring. Therefore, this method is not enough to support the precision requirements of aviation tooling.

[0005] In summary, the existing research on temperature drift correction of fiber Bragg grating strain sensors mainly solves the problem of measurement result drift caused by the strain sensor's sensitivity to temperature, and cannot correct the strain measurement result drift caused by the working temperature change of the demodulator in the aviation tooling strain. To this end, the present invention proposes a method for correcting the working temperature drift of the demodulator in the aviation tooling optical fiber strain measurement to ensure the strain measurement accuracy when the working temperature of the demodulator changes. Summary of the invention

[0006] The technical problem to be solved by the present invention is to overcome the defects and inadequate correction of the prior art, and to invent a method for correcting the temperature drift produced by a fiber grating demodulator during the strain measurement of tooling during the assembly of aircraft components, thereby ensuring measurement accuracy, effectively warning of forced assembly, and providing feedback on assembly quality to ensure the product performance of the aircraft.

[0007] The technical solution of the present invention:

[0008] A method for correcting the working temperature drift of a demodulator in optical fiber strain measurement of aviation tooling is proposed. The direct measurement value of the fiber Bragg grating strain sensor and the working temperature of the demodulator are used as input quantities. The segmented correction model constructed by the method is used to calculate the temperature drift correction amount, that is, the drift error of the measurement result caused by the working temperature of the demodulator, so as to realize the working temperature drift compensation of the demodulator and improve the measurement accuracy of the tooling strain. The specific steps are as follows:

[0009] Step 1: Construction of temperature drift segment correction model

[0010] Since the fiber Bragg grating strain sensor has the characteristics of dual sensitivity to temperature and strain, when the fiber Bragg grating strain sensor is working, its measured value will fluctuate around the actual strain value with the change of ambient temperature and the working temperature of the fiber demodulator. The difference between the measured value and the actual value is the temperature drift value of the strain. The strain data measured by the fiber Bragg grating strain sensor when working is actually the superposition of the actual strain value and the temperature drift value of the strain. Therefore, in order to determine the actual strain value, the temperature drift value of the strain must be measured first, and then the actual strain value is calculated by combining the measured strain data. For the fiber Bragg grating strain sensor fixed on the workpiece, under the condition of no external force, at the initial working environment temperature, the strain measurement value is cleared to zero. As the experimental time increases, the temperature of the fiber demodulator continues to rise due to the heating of the equipment. The strain value measured by the fiber Bragg grating strain sensor is recorded, and the temperature of the fiber demodulator is measured with a thermocouple. After Gaussian filtering of the measurement results, the temperature-time curve of the strain measurement results after filtering and smoothing and the temperature-time curve of the fiber demodulator temperature over time are plotted. Assuming that the abstract function of the temperature-time curve is expressed as T = f(t), it should be f'(t) = 0 segmented curve; according to the segmented points of the temperature-time curve, the relationship between the temperature drift value of the strain and the temperature is fitted by a segmented polynomial to obtain the segmented curve of the temperature drift value of the strain with the temperature; when the temperature drift value of the strain is less than 1με at a certain temperature, it is considered that the temperature drift value of the strain does not affect the actual use, and only filtering and noise reduction processing is adopted without temperature fitting; when the temperature drift value of the strain is greater than 1με, filtering and noise reduction processing is adopted, and the segmented fitting curve of the temperature drift value and the temperature is obtained according to the segmented interval obtained by f'(t)=0, and finally the segmented correction function of the temperature drift is fitted:

[0011]

[0012] Among them, ε0 is the temperature drift value of strain, T is a certain temperature, and the other parameters are correction coefficients;

[0013] At this point, the construction of the temperature drift segmented correction model is realized;

[0014] Step 2: Set up the tooling strain measurement system

[0015] Build a distributed fiber optic strain monitoring system for the tooling locator, measure the strain signal of the finite discrete strain measurement point on the aircraft tooling locator in real time, and determine the installation range of the fiber grating strain sensor under the constraints. Mark the measurement point position on the locator, then arrange the fiber grating strain sensor at the strain measurement point position of the aircraft tooling locator, connect the fiber grating strain sensor to the corresponding channel of the demodulator, and collect the change of wavelength signal; the demodulator is connected to the computer to read the wavelength change and corresponding strain measurement value detected by each fiber grating strain sensor in the corresponding software system; measure the temperature of the grating demodulator through a thermocouple, and collect and record the temperature data through the temperature data acquisition module. The construction of the measurement system is now completed;

[0016] Step 3: Tooling strain measurement and demodulator working temperature drift correction

[0017] During the assembly process, the strain measurement values ​​sent back by the demodulator and the thermocouple temperature measurement values ​​sent back by the temperature data acquisition module are collected in real time, and the measured temperature values ​​are input into the constructed temperature drift correction model to obtain the drift correction value ε0, and then the correction of the working temperature drift of the demodulator is realized through the following formula:

[0018] ε r =ε-ε0 (T=T0) (2)

[0019] Among them, ε r is the strain measurement correction value at temperature T0, and ε is the original strain measurement value transmitted back by the demodulator at temperature T0;

[0020] At this point, accurate measurement of aviation tooling strain has been achieved.

[0021] Beneficial effects of the present invention: A method for correcting the working temperature drift of a demodulator in optical fiber strain measurement of aviation tooling is invented. The temperature drift characteristics of strain measurement in different temperature ranges are analyzed by segmenting the curve based on the temperature change rate, and then a suitable polynomial function is applied for segmented fitting to establish a temperature drift correction model, thereby reducing the complexity of fitting and improving the accuracy of temperature drift correction. The measurement accuracy of the fiber Bragg grating strain sensor under the condition of temperature change of the demodulator is guaranteed. This method is suitable for strain measurement of tooling during the assembly of aircraft components, and is also suitable for any fiber Bragg grating strain measurement process. It has the advantages of wide application range, simple operation, and easy promotion. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Schematic diagram of the fiber Bragg grating strain sensor measurement system for aircraft tooling locator

[0023] Figure 2 To demodulate the temperature drift

[0024] Figure 3 Temperature drift segment calibration diagram

[0025] Figure 4 The corrected strain measurement error diagram

[0026] Figure 5 The overall flow chart of this method

[0027] Among them, 1-fiber Bragg grating strain sensor, 2-strain measurement point position, 3-grating demodulator, 4-computer, 5-thermocouple, 6-temperature data acquisition module. DETAILED DESCRIPTION

[0028] The specific implementation of the present invention is described in detail below in conjunction with the technical scheme and the accompanying drawings.

[0029] In this embodiment, the demodulator used has a range of -15000-15000με, a demodulation accuracy of 1pm and a resolution of 0.5με; the fiber grating strain sensor used is a standard type; a thermocouple is used to measure the temperature; the temperature data acquisition module used is the Topre TP1608 data acquisition module. This measurement correction method is carried out on a certain model of aircraft horizontal tail elevator tooling in a global strain state perception experiment of a certain positioner.

[0030] Step 1: Construction of temperature drift segment correction model

[0031] For the optical fiber sensor fixed on the workpiece, without applying external force, at the initial working environment temperature, the strain measurement is reset to zero. As the experimental time increases, the temperature of the optical fiber demodulator gradually rises from 25°C to 42°C due to the continuous increase in the heat generated by the equipment. During this period, the strain value measured by the strain sensor is recorded at a sampling frequency of 5Hz, and the temperature of the optical fiber demodulator is measured with a thermocouple at the same sampling frequency. After Gaussian filtering of the measurement results, the strain measurement results after filtering and smoothing and the curve of the temperature change of the optical fiber demodulator over time are plotted, as shown in the figure below: Figure 2 As shown. Assuming that the abstract function of the temperature-time curve is expressed as T = f(t), the curve should be segmented at f'(t) = 0. According to the segmentation points of the temperature-time curve, the relationship between the strain drift value and the temperature is fitted with a piecewise polynomial, and a piecewise curve of the strain temperature drift value with temperature can be obtained. When the strain drift value is small, only filtering and noise reduction can be adopted without temperature fitting. The final fitted temperature drift piecewise correction function is as follows:

[0032]

[0033] Among them, ε0 is the strain temperature drift value, T is a certain temperature, and the remainder is the correction coefficient.

[0034] The fitting curve image is as follows Figure 3 shown.

[0035] Step 2: Set up the tooling strain measurement system

[0036] Paste the fiber Bragg grating strain sensor to the corresponding point according to the measuring point position. Arrange the fiber Bragg grating strain sensor 1 at the strain measuring point position of the aircraft tooling locator 2, connect the fiber Bragg grating strain sensor 1 to the corresponding channel of the demodulator 3, and collect the change of the wavelength signal; the demodulator 3 is connected to the computer 4, which is used to read the wavelength change and the corresponding strain measurement value detected by each fiber Bragg grating strain sensor 1 in the corresponding software system; measure the temperature of the grating demodulator through the thermocouple 5, and collect and record the temperature data through the temperature data acquisition module 6. Under no-load conditions, use thermocouples to measure the temperature, use the Topray TP1608 data acquisition module to collect temperature data from the grating demodulator, and record the strain value measured by the fiber optic sensor at the same time to obtain the temperature and strain curves that change with time, such as Figure 2 As shown, the temperature drift segmented calibration curve of strain is obtained by processing the data, as shown in Figure 3 shown.

[0037] Step 3: Tooling strain measurement and demodulator working temperature drift correction

[0038] Under the conditions of this project, actual measurements were carried out. Strain measurements were carried out based on the constructed fiber grating measurement system, and the measured values ​​of the grating demodulator working temperature, ambient temperature and strain were recorded simultaneously during the measurement. Based on the existing temperature calibration data and measured data, the correction value after strain temperature drift compensation can be obtained, and its expression is:

[0039] ε r =ε-ε0 (4)

[0040]

[0041] Where: r is the strain measurement correction value at T0 temperature; ε is the strain measurement value at T0 temperature; ε0 is the strain drift calibration value at T0 temperature.

[0042] In this way, the measured data after strain temperature correction can be obtained, and its error is significantly reduced. The temperature drift caused by the working temperature of the demodulator is reduced from about 30με to no more than 1.5με. Figure 4 As shown, it can be seen that this method can effectively improve the strain measurement accuracy.

[0043] Although the present invention has been disclosed as above in the form of a preferred embodiment, it is not intended to limit the present invention. Any person skilled in the art may make possible changes and modifications to the technical solution of the present invention by using the methods and technical contents disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention shall fall within the protection scope of the technical solution of the present invention.

Claims

1. A method for correcting the working temperature drift of a demodulator in optical fiber strain measurement of aviation tooling, characterized in that: The specific steps are as follows: Step 1: Construction of temperature drift segment correction model Based on the dual sensitivity of fiber Bragg grating strain sensor to temperature and strain, when the fiber Bragg grating strain sensor is working, its measured value will float on the strain measured value with the change of ambient temperature and the working temperature of the fiber demodulator. The difference between the measured value and the measured value is the temperature drift value of the strain. The strain data measured by the fiber Bragg grating strain sensor when working is actually the superposition of the strain measured value and the strain temperature drift value. Therefore, in order to determine the strain measured value, it is necessary to first measure the strain temperature drift value, and then calculate the strain measured value in combination with the measured strain data. For the fiber Bragg grating strain sensor fixed on the workpiece, the strain measurement value is reset to zero under the condition of no external force and at the initial working environment temperature; As the experimental time increases, the temperature of the fiber demodulator continues to rise due to the heating of the equipment. The strain value measured by the fiber Bragg grating strain sensor is recorded, and the temperature of the fiber demodulator is measured with a thermocouple. After the measurement results are Gaussian filtered, the strain measurement results after filtering and smoothing and the temperature-time curve of the fiber demodulator temperature over time are plotted. Assuming that the abstract function of the temperature-time curve is expressed as T = f(t), the curve should be segmented at f'(t) = 0. According to the segmentation points of the temperature-time curve, the relationship between the temperature drift value of the strain and the temperature is fitted with a piecewise polynomial to obtain the segmented curve of the temperature drift value of the strain with the temperature. When the temperature drift value of the strain is less than 1με at a certain temperature, it is considered that the temperature drift value of the strain does not affect the actual use, and only filtering and noise reduction processing is adopted without temperature fitting. When the temperature drift value of the strain is greater than 1με, filtering and noise reduction processing is adopted, and the segmented fitting curve of the temperature drift value and the temperature is obtained according to the segmented interval obtained by f'(t) = 0, and finally the temperature drift segmented correction function is fitted: Among them, ε0 is the temperature drift value of strain, T is a certain temperature, and the other parameters are correction coefficients; At this point, the construction of the temperature drift segmented correction model is realized; Step 2: Set up the tooling strain measurement system Build a distributed fiber optic strain monitoring system for the tooling locator, measure the strain signal of the finite discrete strain measurement point on the aircraft tooling locator in real time, and determine the installation range of the fiber grating strain sensor under the constraints. Mark the measurement point position on the locator, then arrange the fiber grating strain sensor at the strain measurement point position of the aircraft tooling locator, connect the fiber grating strain sensor to the corresponding channel of the demodulator, and collect the change of wavelength signal; the demodulator is connected to the computer to read the wavelength change and corresponding strain measurement value detected by each fiber grating strain sensor in the corresponding software system; measure the temperature of the grating demodulator through a thermocouple, and collect and record the temperature data through the temperature data acquisition module. The construction of the measurement system is now completed; Step 3: Tooling strain measurement and demodulator working temperature drift correction During the assembly process, the strain measurement values ​​sent back by the demodulator and the thermocouple temperature measurement values ​​sent back by the temperature data acquisition module are collected in real time, and the measured temperature values ​​are input into the constructed temperature drift correction model to obtain the drift correction value ε0, and then the correction of the working temperature drift of the demodulator is realized through the following formula: e r =ε-ε0 (T=T0) (2) Among them, ε r is the strain measurement correction value at temperature T0, and ε is the original strain measurement value transmitted back by the demodulator at temperature T0; At this point, accurate measurement of aviation tooling strain has been achieved.

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