Visual measurement method for structural vibration of laser transfer amplification

Abstract

The invention discloses a laser transfer and amplification structure vibration vision measurement method, which belongs to the technical field of structure vibration vision measurement methods, and is characterized in that structure vibration is converted into light spot vibration by means of a laser emission device, and vibration at multiple positions on a structure can also be converted into light spot vibration in a concentrated area; according to the conversion, the vision measurement problem of a large-size structure which can only be realized at a long distance is converted into a short-distance vision measurement problem, the advantage of image spatial resolution can be fully played without a telephoto lens, and the requirement on the stability of the camera is reduced.

Description

Technical Field

[0001] The invention relates to a structural vibration visual measurement method, in particular to a laser transfer amplified structural vibration visual measurement method, and belongs to the technical field of structural vibration visual measurement methods. Background Art

[0002] Structural health monitoring (SHM) technology is the key to ensuring the safe service of structures. It has been applied in structures such as bridges, wind turbine blades, dams, buildings, cables, etc. When a structure is damaged, its modal parameters (natural frequency, damping rate and vibration mode shape) will change accordingly. These changes can be obtained through the vibration process of the structure. Compared with ultrasonic, X-ray and infrared detection technologies, the vibration-based SHM method is easier to achieve global damage detection and has become one of the most popular methods in the SHM field.

[0003] With the rapid development of image sensors and machine vision technology, video-based vibration measurement technology has shown strong advantages in the SHM field. Video vibration measurement technology uses a camera to record the video of the structure during vibration and extracts the vibration displacement information of the structure through image processing algorithms. This method can achieve long-distance, multi-point and high-precision vibration detection without the need for complex sensor installation processes. Existing video vibration measurement technologies include point tracking, edge detection and digital image correlation. The point tracking method detects inherent feature points on the surface of the structure or artificially added marks, and uses a point tracking algorithm to extract the motion trajectory of the feature points. The edge detection rule uses the edge features of the structure to extract the structural vibration information. The digital image correlation rule obtains the structural vibration information by tracking a sub-area in the image. In order to improve the detection accuracy, the digital image correlation method requires a preset scattered point pattern on the surface of the structure.

[0004] The above-mentioned vision-based vibration detection technologies all follow a common rule, that is, directly photographing the structure through a camera. Such methods not only have to withstand the test of complex environmental lighting, but also their measurement accuracy is greatly affected by the shooting distance. Specifically, when the shooting distance is close, the spatial resolution of the image is high and the accuracy of the vibration expression is high; when the shooting distance is far, the spatial resolution of the image is reduced, and the accuracy of the vibration expression is reduced. This is also an important factor that makes it difficult for visual vibration measurement technology to play a role in the field of large-scale structure vibration detection.

[0005] In order to make up for this shortcoming, many technical solutions use telephoto lenses to achieve high-resolution imaging of local features of the structure at a longer shooting distance. Since telephoto lenses are expensive, such solutions increase the equipment cost, and the use of telephoto lenses also places higher requirements on the stability of the camera itself. Another technical solution is to use a drone equipped with a camera to take close-up photos of the structure. Although phase-stabilization technology has made great progress, the impact of the drone's own shaking on the vibration measurement results still restricts the use of such solutions. For this reason, a laser transfer amplified structural vibration visual measurement method is designed to solve the above problems. Summary of the invention

[0006] The main purpose of the invention is to provide a laser transfer amplified structural vibration visual measurement method.

[0007] The purpose of the present invention can be achieved by adopting the following technical solutions:

[0008] A laser transfer amplified structural vibration visual measurement method comprises the following steps:

[0009] S11: Install laser emitting device;

[0010] S12: measuring the height of the laser emitting device;

[0011] S13: measuring the distance between the laser spot and the structure to be measured;

[0012] S14: Install the camera;

[0013] S15: calibrate image spatial resolution;

[0014] S16: light spot vibration video acquisition;

[0015] S17: spot area segmentation;

[0016] S18: Calculate the center of gravity position of the light spot to obtain the light spot vibration in pixels;

[0017] S19: Obtain the light spot vibration in units of physical length according to the image resolution;

[0018] S110: Calculate the vibration of the structure.

[0019] Preferably, in S11, a laser emitting device is arranged on the structure to be measured so that the laser spot falls on the ground.

[0020] Preferably, in S12 and S13, the height of the laser emitting device is measured, and the height of the laser emitting device is represented by h; and the distance between each light spot and the measured structure is measured, and is represented by l.

[0021] Preferably, in S14, the camera is fixed at a position close to the light spot to ensure that the light spot is always located inside the image during the structural vibration;

[0022] In S15, the image is calibrated using a four-point calibration method to obtain the image trapezoidal correction transformation matrix H and the spatial resolution dmm / piexl of the corrected image.

[0023] Preferably, step S15 further includes the following steps:

[0024] S21: Place a rectangular calibration object with known dimensions, whose width is width and height is height (in millimeters), on the ground to ensure that the calibration object is within the camera shooting range and the laser spot does not exceed the calibration object range during the structural vibration process;

[0025] S22: Collect the calibration object image, and find the image coordinates (x, y) of the four vertices of the calibration object from the image, and represent them as (x1, y1), (x2, y2), (x3, y3), (x4, y4) in the order of upper left, upper right, lower left, and lower right respectively;

[0026] S23: Define the spatial resolution of the calibrated image pixels as d, in units of mm / pixel, and the coordinates (X, Y) of the four vertices of the calibration object in the calibrated image are respectively (1,1), (width / d+1,1), (1,height / d+1), (width / d+1,height / d+1);

[0027] S24: The coordinates of a point in the image before and after correction satisfy the following relationship:

[0028]

[0029] definition Then the transformation matrix H can be obtained by calibrating the four vertices of the object.

[0030] Preferably, in S16, a camera is used to record a video of the light spot vibration during the structural vibration process, and the camera frame rate is set according to the target frequency to be measured. If the upper limit of the structural vibration frequency to be measured is f, the frame rate should be no less than 2f;

[0031] In S17, the transformation matrix H obtained in S15 is used to perform trapezoidal correction on each frame in the video.

[0032] Preferably, in S18, the light spot area in each frame of image is segmented by image binarization, and the light spot area is represented by BW.

[0033] Preferably, in S19, the centroid position of each color area is calculated using the following formula:

[0034]

[0035] Where [x, y] is the center of gravity of the spot area, N is the number of points in the spot area, and the spot vibration signal Vib_spot_pixel in pixels is obtained.

[0036] Preferably, the vibration signal Vib_spot_pixel in S19 is converted into a vibration signal Vib_spot_mm in length units according to d defined in S23. Vib_spot_mm=Vib_spot_pixel×d.

[0037] Preferably, in S11, a vibration signal Vib_structure of the structure is obtained, wherein

[0038]

[0039] Beneficial technical effects of the present invention:

[0040] The present invention provides a laser transfer amplified structural vibration visual measurement method, which uses a laser emitting device to convert structural vibration into light spot vibration. This conversion transforms the visual measurement problem of large-scale structures that can only be achieved at a long distance into a close-range visual measurement problem. The advantage of image spatial resolution can be fully utilized without a telephoto lens, and the requirement for the stability of the camera itself is reduced.

[0041] It can be seen from formula (9) that by converting the structural vibration into the light spot vibration through the laser emitting device, under the premise of l>h, the amplified measurement of the structural vibration can be realized. This advantage broadens the recognition ability of visual vibration measurement for tiny vibrations.

[0042] Since the present invention belongs to close-range measurement, the imaging environment is controllable and the influence of ambient light on the measurement result is small.

[0043] In practical application, the present invention only needs to arrange the laser emission device on the structure and provide the necessary power supply. The image calibration and measurement process is simple and can be completed on site as needed. Compared with the traditional measurement method based on acceleration sensor, the present invention reduces the investment in data transmission lines and fixed acquisition equipment, and the system structure is simple and the cost is low. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 The schematic diagram is a principle diagram of a preferred embodiment of a laser transfer amplified structural vibration visual measurement method according to the present invention.

[0045] Figure 2 This is a diagram of an experimental scene according to a preferred embodiment of a laser transfer-amplified structural vibration visual measurement method of the present invention.

[0046] Figure 3 A laser displacement sensor position diagram according to a preferred embodiment of a laser transfer amplified structural vibration visual measurement method of the present invention.

[0047] Figure 4 This is a comparison diagram of the detection results of a preferred embodiment of a laser transfer amplified structural vibration visual measurement method according to the present invention and the detection results of a laser displacement sensor.

[0048] Figure 5 This is a diagram of the installation position of an acceleration sensor according to a preferred embodiment of a laser transfer amplified structural vibration visual measurement method of the present invention.

[0049] Figure 6 The figure is a comparison diagram between the detection results of a preferred embodiment of a laser transfer amplified structural vibration visual measurement method according to the present invention and the detection results of an acceleration sensor.

[0050] Figure 7 It is a schematic diagram of a preferred embodiment of a laser transfer amplified structural vibration visual measurement method according to the present invention. DETAILED DESCRIPTION

[0051] In order to make the technical solution of the present invention more clear and specific to those skilled in the art, the present invention is further described in detail below in conjunction with embodiments and drawings, but the implementation manner of the present invention is not limited thereto.

[0052] The principle of the present invention is described as follows:

[0053] Figure 1 The solid red line in the figure represents the initial state of the structure with a height of h. It can be assumed that the initial state of the structure is perpendicular to the ground. The bottom of the structure is fixed at point O, and the top is allowed to vibrate. The red dotted line represents the initial position of the laser light, and the light spot falls on point A on the ground. When the top of the structure is displaced ds, the structure is represented by a solid blue line (the angle between it and its initial position is θ), and the laser light is represented by a dotted blue line, and the light spot falls on point B on the ground; since the laser emitting device is fixed on the structure, the angle α between the laser emitter and the structure remains unchanged during the vibration of the structure. The distances from point A and point B to point O are respectively recorded as l A and l B , the displacement of the light spot position is recorded as dl.

[0054] Depend on Figure 1 The following formula can be obtained:

[0055]

[0056] l A -l B =dl (2)

[0057] Considering that ds is much smaller than h, after the displacement of ds occurs at the upper end of the structure, it can be considered that its height does not change, so the following formula can be obtained:

[0058]

[0059]

[0060] Formula (4) can be expanded as:

[0061]

[0062] Substituting formula (1) and formula (3) into formula (5) we can obtain the following formula:

[0063]

[0064] Since ds is much smaller than h, l A can be considered comparable to h, so l A ds is much smaller than h 2 , formula (6) can be simplified as:

[0065]

[0066] By removing the small square term of ds in formula (7), a more concise expression between dl and ds can be obtained:

[0067]

[0068] At this point, the relationship between the displacement of the upper end of the structure ds and the displacement of the laser spot dl is obtained. The displacement of the upper end of the structure can be inferred by the displacement of the laser spot.

[0069]

[0070] Preferred examples of the present invention:

[0071] The invention was simply verified under laboratory conditions. The experimental scenario is as follows: Figure 2 The laser transmitter is fixed at a height of about 1.71 meters on the wind turbine tower, and its spot falls 6.5 meters away from the wind turbine model. The coordinate system of the wind turbine tower is defined as shown in the figure, where the y direction reflects the vibration of the tower in the front and back directions, and the x direction reflects the vibration of the tower in the left and right directions.

[0072] The first experiment is to compare the proposed method with the laser displacement sensor. The two laser displacement sensors measure the vibration of the wind turbine tower in two directions respectively. Figure 3As shown in Figure 1, the wind turbine tower is stimulated by a force hammer to start vibrating, the sampling rate of the laser displacement sensor is 390.625Hz, and the sampling rate of the high-speed camera is 391fps.

[0073] The vibration time domain signals obtained by two different measurement methods are shown in Figure 2. Figure 4 As shown in the figure, it can be seen that the detection result of the invention in the x direction is well consistent with the detection result of the laser displacement sensor, and the detection result in the y direction is similar to the shape of the laser displacement sensor and is magnified by about 14.5 times, which is also consistent with the detection result of the invention. near.

[0074] The second experiment is to compare the measurement results of the method proposed by the present invention with those of the acceleration sensor. The installation position of the acceleration sensor is as follows: Figure 5 As shown, the y of the acceleration sensor acc Channel reaction tower vibration in x direction, z acc The channel reflects the vibration of the tower in the y direction. The wind turbine model is blown by a blower, and the tower vibrates under the excitation of wind and blade rotation. The sampling rate of the accelerometer is 195.3125Hz, and the frame rate of the high-speed camera is set to 195fps. The torque wrench is used to adjust the tightness of the bolts connecting the wind turbine tower and the base, with torques of 5Nm, 15Nm and 25Nm respectively, and the signal sampling time is 60 seconds.

[0075] The tower vibration spectra obtained by two different measurement methods are shown in Figure 2. Figure 6 As shown in the figure. It can be seen from the figure that, from the frequency point of view, the first two orders of the tower natural frequency obtained by the two measurement methods are highly consistent. The frequency information of about 1 Hz obtained by the acceleration sensor is close to the blade rotation frequency. This is due to the vibration in the up and down directions during the rotation of the blade. Since the method proposed in the present invention cannot obtain the vibration in the up and down directions of the tower, this part of the frequency component cannot be reflected in the spectrum. From the perspective of amplitude, the amplitude corresponding to the tower natural frequency in the y direction also maintains a good consistency. The amplitude of the detection result of the method proposed in the present invention is much larger than the amplitude of the acceleration sensor. This is because the detection method proposed in the present invention can realize the amplified detection of the vibration in the y direction. The natural frequencies of the tower obtained by the two detection methods in the x direction are different, which may be caused by the error in the sensor installation position. In short, this experiment once again verified the effectiveness of the detection method proposed in the present invention, especially the amplification effect on the vibration in the y direction.

[0076] The above description is only a further embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical solutions and concepts of the present invention within the scope disclosed by the present invention, which belong to the protection scope of the present invention.

Claims

1. A laser transfer amplified structural vibration visual measurement method, characterized in that: The steps include: S11: Install laser emitting device; S12: measuring the height of the laser emitting device; S13: measuring the distance between the laser spot and the structure to be measured; S14: Install the camera; S15: calibrate image spatial resolution; S16: light spot vibration video acquisition; S17: spot area segmentation; S18: Calculate the center of gravity position of the light spot to obtain the light spot vibration in pixels; S19: Obtain the light spot vibration in units of physical length according to the image resolution; S110: Calculate the vibration of the structure.

2. The laser transfer amplified structural vibration visual measurement method according to claim 1, characterized in that: In S11, a laser emitting device is arranged on the structure to be measured so that the laser spot falls on the ground.

3. The laser transfer amplified structural vibration visual measurement method according to claim 1, characterized in that: In S12 and S13, the height of the laser emitting device is measured, represented by h; and the distance between the light spot and the measured structure is measured, represented by l.

4. The laser transfer amplified structural vibration visual measurement method according to claim 1, characterized in that: In S14, the camera is fixed at a position close to the light spot to ensure that each light spot is always located inside the image during the structural vibration; In S15, the image is calibrated using a four-point calibration method to obtain the image trapezoidal correction transformation matrix H and the spatial resolution d of the corrected image, in units of mm / piexl.

5. The laser transfer amplified structural vibration visual measurement method according to claim 4, characterized in that: Step S15 also includes the following steps: S21: Place a rectangular calibration object with known dimensions, whose width is width and height is height (in millimeters), on the ground to ensure that the calibration object is within the camera shooting range and the laser spot does not exceed the calibration object range during the structural vibration process; S22: Collect the calibration object image, and find the image coordinates (x, y) of the four vertices of the calibration object from the image, and represent them as (x1, y1), (x2, y2), (x3, y3), (x4, y4) in the order of upper left, upper right, lower left, and lower right respectively; S23: Define the spatial resolution of the calibrated image pixels as d, in units of mm / pixel, and the coordinates (X, Y) of the four vertices of the calibration object in the calibrated image are respectively (1,1), (width / d+1,1), (1,height / d+1), (width / d+1,height / d+1); S24: The coordinates of a point in the image before and after correction satisfy the following relationship: definition Then the transformation matrix H can be obtained by calibrating the four vertices of the object.

6. The laser transfer amplified structural vibration visual measurement method according to claim 5, characterized in that: In S16, a camera is used to record a video of the light spot vibration during the structural vibration process. The camera frame rate is set according to the target frequency to be measured. If the upper limit of the structural vibration frequency to be measured is f, the frame rate should be no less than 2f; In S17, the transformation matrix H obtained in S15 is used to perform trapezoidal correction on each frame in the video.

7. The laser transfer amplified structural vibration visual measurement method according to claim 6, characterized in that: In S18, the light spot area in each frame of the image is segmented by image binarization, and the light spot area is represented by BW.

8. The laser transfer amplified structural vibration visual measurement method according to claim 7, characterized in that: In S19, the center of gravity position of the spot area is calculated using the following formula: Where [x, y] is the center of gravity of the spot area, N is the number of points in the spot area, and the spot vibration signal Vib_spot_pixel in pixels is obtained.

9. The laser transfer amplified structure vibration visual measurement method according to claim 8, characterized in that: The vibration signal Vib_spot_pixel in S19 is converted into a vibration signal Vib_spot_mm in length units according to d defined in S23. Vib_spot_mm=Vib_spot_pixel×d.

10. The laser transfer amplified structure vibration visual measurement method according to claim 9, characterized in that: In S11, a vibration signal Vib_structure of the structure is obtained, where

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