Point load device for measuring rock deformation parameters based on digital image correlation technology

Abstract

The invention relates to the technical field of rock mechanics experiments, and discloses a point load device for measuring rock deformation parameters based on a digital image correlation technique, which comprises a support device, a loading mechanism, a control system and an image acquisition system. According to the point load device for measuring the rock deformation parameters based on the digital image correlation technology, on one hand, the rock point load strength is measured, on the other hand, measurement of the rock deformation parameters under the point load effect is synchronously achieved, and the device monitors the applied point load value through the pressure sensor; the deformation condition of the rock test piece during loading is measured by virtue of the CCD industrial camera, so that the purpose of measuring the rock point load strength and rock deformation parameters is achieved, compared with a traditional experiment process, the device generally realizes automatic control, the operation precision in the experiment loading process is improved, and the test efficiency is improved. Therefore, the experiment efficiency and accuracy are remarkably improved and optimized, and the rock deformation parameters are measured at the same time.

Description

Technical Field

[0001] The present invention relates to the technical field of rock mechanics experiments, and specifically to a point load device for measuring rock deformation parameters based on digital image correlation technology. Background Art

[0002] In the development and utilization of mineral resources, whether it is an underground mine or an open-pit mine, rock mechanics parameters (strength parameters and deformation parameters) are key factors affecting the construction safety and economic benefits during the mining process. The point load test is an experimental method for rock mechanics parameters. Due to the advantages of small instrument volume and simple operation, the point load test can be applied to laboratories or engineering sites. Therefore, the point load test is widely used in the determination of rock strength parameters quickly obtained on engineering sites.

[0003] However, in the actual operation process, the inventor found that there are still the following problems: At present, the existing rock point load test devices can be used to obtain rock strength parameters (rock point load strength), but do not have the function of measuring rock deformation parameters and the stress-strain curve during the loading process. At the same time, in the operation process, it relies too much on manual loading by experimental personnel, which not only poses certain safety hazards in the experimental process but also affects the experimental efficiency.

[0004] Based on this, the present invention provides a point load device for measuring rock deformation parameters based on digital image correlation technology. Summary of the Invention

[0005] Aiming at the deficiencies of the prior art, the present invention provides a point load device for measuring rock deformation parameters based on digital image correlation technology, which has the advantages of being able to measure point load deformation parameters and making the measurement process more automated, and solves the problems raised in the background art.

[0006] The present invention provides the following technical solutions: A point load device for measuring rock deformation parameters based on digital image correlation technology, comprising a support device, a loading mechanism, a control system, and an image acquisition system:

[0007] The support device is composed of a top horizontal bracket, a middle horizontal bracket, a base, an inclined bracket, and a vertical bracket;

[0008] The loading mechanism includes a pressure sensor, an upper point load head, a lower point load head, a ball screw, a flange, a worm gear, a worm, a bearing, a ductile iron housing, and a stepping motor. A stepping motor is fixedly installed at the top of the middle horizontal bracket. A driving assembly is arranged on the right side of the stepping motor. The driving assembly includes a worm gear, a worm, and a bearing. The output shaft of the stepping motor is fixedly connected to the worm. The worm is internally engaged with the worm gear. A bearing is fixedly installed inside the worm gear. The bearing is fixedly connected to the ball screw at the top. The surface of the ball screw is threadedly connected to the flange. The lower point load head is threadedly connected inside the flange. A calibration point is fixedly installed at the bottom end of the top horizontal bracket. A pressure sensor is fixedly installed at the bottom of the calibration point. The pressure sensor is fixedly connected to the upper point load head at the bottom. A rock specimen is arranged on the opposite sides of the upper point load head and the lower point load head. A ductile iron housing is arranged on the top of the base. The ball screw is arranged inside the ductile iron housing;

[0009] The control system includes a pressure sensor, an encoder, a driver, and a display screen. The pressure sensor is connected to the display screen by a circuit. An encoder is arranged on the left side of the stepping motor. The encoder is connected to a single-chip microcomputer by a circuit. The single-chip microcomputer is equipped with an external display screen and operation buttons. The signal of the single-chip microcomputer is transmitted to the signal of the driver;

[0010] The image acquisition system includes a calibration point, an L-shaped bracket, and a CCD industrial camera. An L-shaped bracket is fixedly installed at the bottom of the top horizontal bracket. A CCD industrial camera is arranged at the top end of the cavity of the L-shaped bracket. An LED fill light is arranged inside the L-shaped bracket. The CCD industrial camera is electrically connected to an external power supply.

[0011] Preferably, the ball screw penetrates through the middle horizontal bracket and extends to the lower part.

[0012] Preferably, a driver and a display screen are installed at the top of the middle horizontal bracket.

[0013] Preferably, an inclined bracket is fixedly connected to the bottom of the middle horizontal bracket, and the bottom of the inclined bracket is fixedly connected to the base.

[0014] Preferably, mounting holes are formed at both ends of the middle horizontal bracket, and vertical brackets are fixedly installed inside the mounting holes. The top horizontal bracket is fixedly installed at the top of the vertical brackets.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] The point load device for measuring rock deformation parameters based on digital image correlation technology integrates traditional point load experimental instruments, stepper motor systems, and digital image correlation technology. On the one hand, it measures the point load strength of rocks, and on the other hand, it simultaneously achieves the measurement of rock deformation parameters under point load. The device uses a pressure sensor to monitor the applied point load value and measures the deformation of the rock specimen during loading with a CCD industrial camera, thereby achieving the measurement objectives of rock point load strength and rock deformation parameters. Compared with the traditional experimental process, this device generally realizes automated control, improves the operation accuracy during the experimental loading process, significantly enhances and optimizes both the experimental efficiency and accuracy, and simultaneously achieves the measurement of rock deformation parameters. Brief Description of the Drawings

[0017] Figure 1 It is a schematic diagram of the overall structure of the device of the present invention;

[0018] Figure 2 It is a schematic top view structure diagram of the worm gear, worm, and bearing of the present invention;

[0019] Figure 3 It is a schematic structure diagram of the upper and lower point load heads (with calibration points) of the present invention;

[0020] Figure 4 It is a schematic structure diagram of the initial state of the present invention;

[0021] Figure 5 It is a schematic structure diagram of the preloading state of the present invention;

[0022] Figure 6 It is a schematic structure diagram of the loading state of the present invention;

[0023] Figure 7 It is a reference diagram of the calibration point photo of Lx and Ly between A and B, the speckle photos of the deformed sub-region and the undeformed sub-region of the present invention.

[0024] In the figure: 1. Support device; 2. Pressure sensor; 3. Upper point load head; 4. Rock specimen; 5. Lower point load head; 6. Calibration point; 7. Ball screw; 8. Flange; 9. Drive assembly; 10. Worm gear; 11. Worm; 12. Bearing; 13. Ductile iron housing; 14. Stepper motor; 15. Encoder; 16. Driver; 17. Display screen; 18. L-shaped bracket; 19. CCD industrial camera. Detailed Description of the Invention

[0025] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0026] Please refer to Figures 1-7 , a point load device for measuring rock deformation parameters based on digital image correlation technology, comprising a support device 1, a loading mechanism, a control system and an image acquisition system:

[0027] The support device 1 is composed of a top horizontal bracket, a middle horizontal bracket, a base, an inclined bracket and a vertical bracket;

[0028] The loading mechanism includes a pressure sensor 2, an upper point load head 3, a lower point load head 5, a ball screw 7, a flange 8, a worm gear 10, a worm 11, a bearing 12, a nodular cast iron housing 13 and a stepping motor 14. A stepping motor 14 is fixedly installed at the top of the middle horizontal bracket. A driving assembly 9 is arranged on the right side of the stepping motor 14. The driving assembly 9 includes a worm gear 10, a worm 11 and a bearing 12. The output shaft of the stepping motor 14 is fixedly connected to the worm 11. The worm 11 is internally engaged with the worm gear 10. The worm gear 10 is internally fixedly installed with a bearing 12. The top of the bearing 12 is fixedly connected to a ball screw 7. The surface of the ball screw 7 is threadedly connected to a flange 8. The inside of the flange 8 is threadedly connected to a lower point load head 5. A calibration point 6 is fixedly installed at the bottom end of the top horizontal bracket. A pressure sensor 2 is fixedly installed at the bottom of the calibration point 6. The bottom of the pressure sensor 2 is fixedly connected to an upper point load head 3. A rock specimen 4 is arranged on the opposite side of the upper point load head 3 and the lower point load head 5. A nodular cast iron housing 13 is arranged on the top of the base. The ball screw 7 is arranged inside the nodular cast iron housing 13;

[0029] The control system includes a pressure sensor 2, an encoder 15, a driver 16 and a display screen 17. The pressure sensor 2 is connected to the display screen 17 by a circuit. An encoder 15 is arranged on the left side of the stepping motor 14. The encoder 15 is connected to a single-chip microcomputer by a circuit. The single-chip microcomputer is equipped with an external display screen and operation buttons. The signal of the single-chip microcomputer is transmitted to the driver 16;

[0030] The image acquisition system includes a calibration point 6, an L-shaped bracket 18, and a CCD industrial camera 19. The bottom of the top horizontal bracket is fixedly installed with an L-shaped bracket 18. The top of the cavity of the L-shaped bracket 18 is provided with a CCD industrial camera 19. An LED fill light is arranged inside the L-shaped bracket 18. The CCD industrial camera 19 is electrically connected to an external power supply. This device combines traditional point load experimental instruments, a stepper motor 14 system, and digital image correlation technology into one. On the one hand, it measures the rock point load strength, and on the other hand, it simultaneously achieves the measurement of rock deformation parameters under point load. This device uses a pressure sensor 2 to monitor the applied point load value, and relies on the CCD industrial camera 19 to measure the deformation of the rock specimen during loading, thereby achieving the measurement objectives of rock point load strength and rock deformation parameters. Compared with the traditional experimental process, this device generally realizes automated control, improves the operation accuracy during the experimental loading process, significantly improves and optimizes the experimental efficiency and accuracy, and simultaneously achieves the measurement of rock deformation parameters.

[0031] Among them, the ball screw 7 is connected through the middle horizontal bracket and extends to the lower part. The top of the middle horizontal bracket is installed with a driver 16 and a display screen 17. The bottom of the middle horizontal bracket is fixedly connected with an inclined bracket, and the bottom of the inclined bracket is fixedly connected with a base. Installation holes are opened at both ends of the middle horizontal bracket, and vertical brackets are fixedly installed inside the installation holes. The top of the vertical bracket is fixedly installed with a top horizontal bracket.

[0032] Test steps of a point load experimental device for measuring rock deformation parameters based on digital image correlation technology:

[0033] ① Connect all parts of the device;

[0034] ② Connect the power supply, turn on the experiment start / end button, start the experimental device, and the initial state is as Figure 4 shown;

[0035] ③ Install the rock specimen sprayed with speckles, with the speckled side placed directly opposite the camera. Enter commands through the operation buttons on the display screen to make the motor operate and drive the screw to complete the preloading of the rock specimen. Then take pictures of the calibration point and the specimen;

[0036] ④ Turn on the experiment button, and then enter commands to load the specimen according to the experimental requirements until the rock specimen fails. During the loading process, the CCD industrial camera automatically collects the images of the specimen surface before and after loading and stores them in the memory. After the rock specimen fails, the maximum applied load of the rock specimen will be displayed;

[0037] ⑤ Control the device to return to the initial state as Figure 4 shown, and at the same time save the experimental data of the previous group;

[0038] ⑥ After cleaning the experimental device, the next set of experiments can be carried out. Turning off the power will shut down the entire device. If no more experiments are to be conducted, the motor can also be disassembled;

[0039] ⑦ Export the photos taken by the CCD industrial camera, analyze the calibrated photos taken, obtain the internal and external parameters of the camera, and establish the relationship between the camera coordinate system and the object position coordinate system. To prevent the influence brought by optical distortion and the change of the camera position, reduce the experimental error. Analyze and process the photos of the specimen during the loading process. Through the full-field average strain and pressure data, the stress-strain curve can be plotted, and the elastic modulus E (the ratio of stress to strain when reaching 50% of the peak stress) and Poisson's ratio μ (the ratio of the transverse strain to the axial strain when reaching 50% of the peak stress) of the rock under the point load of the rock can be obtained.

[0040] The measurement principle of a point load device for measuring rock deformation parameters based on digital image correlation technology:

[0041] In the initial state, the distance between the top horizontal bracket and the middle horizontal bracket is H, the height from the loading head on the point load to the top horizontal bracket is h1, and the height between the loading head under the point load and the middle horizontal bracket when reaching the preloading state is h2. Then the size h of the specimen in the vertical direction is h = H - h1 - h2. During the loading process, after the rock specimen is damaged, the maximum applied load P of the rock specimen will be shown. According to the maximum applied load P and the size h, the rock point load intensity can be calculated, and the point load intensity is Is (50) =(P / h 2 )*(h / 50) 0.45 Taking the single-chip microcomputer as the control core, it is responsible for processing input instructions, controlling the motor and reading encoder data. The single-chip microcomputer first sends a pulse signal to the driver to drive the stepping motor to drive the ball screw to move, clamp the rock specimen to reach the preloading state, and the encoder monitors the displacement of the motor in real time and feeds the monitored signal back to the single-chip microcomputer. The single-chip microcomputer compares the actual position with the preset target position. If there is a deviation, the single-chip microcomputer will adjust the pulse signal sent to the driver, thereby correcting the movement of the motor to make the movement of the ball screw more accurately reach the expected position and speed requirements.

[0042] Digital image-related technology is a non-contact measurement method based on image processing. By comparing images taken at different time points, feature points in the images are identified and tracked, so as to obtain the displacement and deformation information of an object under the action of external forces, with the aim of determining the deformation parameters of the rock corresponding to the point load. Before measurement, speckles are sprayed on the surface of the specimen. When a load is applied, the surface of the specimen deforms and undergoes displacement, and these speckles also move accordingly. By analyzing the speckle images, the displacements and deformations of each point on the surface of the specimen in different directions (such as the x and y directions) can be obtained. For example, first measure the actual distance between two calibration points in the horizontal direction as Lx and the actual distance in the vertical direction as Ly. Collect the images and use the digital image correlation method to calculate the number of pixel points mx and my between A and B in the horizontal and vertical directions. Then, the pixel equivalent in the x direction is Rx = Lx / mx, and the pixel equivalent in the y direction is Ry = Ly / my. The difference between the deformed sub-region u(x0′, y0′) and the undeformed sub-region u(x0, y0) is the pixel point displacement of the measurement point, that is, dx = x0′ - x0, dy = y0′ - y0 (refer to Figure 7 ), and the actual displacement of the specimen after conversion is X = Rx*dx, Y = Ry*dy. If it is necessary to calculate the displacement information of all pixel points within a region of interest, only by repeating the above method can it be achieved.

[0043] After obtaining the displacements of each point on the surface of the object, the measurement of the complete strain field of the sample can be realized by selecting different sub-regions on the surface of the sample. Suppose there is a series of adjacent sub-regions on the surface of the rock. By calculating the difference in displacements between adjacent sub-regions and then dividing by the distance between the sub-regions, the deformation can be obtained. For example, for a series of adjacent sub-regions on the surface of an object, if the displacement centers in the x direction are known as Dx1, Dx2, etc., and the spacing between the sub-regions in the x direction is Δx (the real physical spacing or the converted spacing), then the strain in the x direction is equal to εx = (Dx2 - Dx1) / Δx. Similarly, the strain εy in the y direction can be measured. Through the stress-strain values, the stress-strain curve can be plotted. When the stress reaches 50% of the peak stress, the elastic modulus of the rock E = σ / εy and the Poisson's ratio μ = εx / εy can be obtained.

[0044] The basic steps of digital image related technologies are as follows: (1) Apply speckles on the surface of the object to be measured manually or by other means; (2) Use a set of calibration plates with known sizes and shapes for photographing and analysis to obtain the internal and external parameters of the camera, establish the relationship between the camera coordinate system and the object position coordinate system, prevent the influence caused by optical distortion and the change of the camera position, and reduce experimental errors; (3) The image acquisition device takes and stores the images of the object before and after deformation; (4) Calculate the in-plane displacement field and strain field. The calculation of the displacement field and strain field mainly includes the following steps: (1) Whole-pixel displacement search: The first step in the calculation of the displacement field. In the digital image correlation technology method, the reference image is first divided into many small sub-regions. For each sub-region, find the most matching sub-region in the deformed image, and this matching process is based on the similarity of the pixel gray values within the sub-region. At the beginning, the search range is carried out at the whole-pixel level; (2) Sub-pixel displacement search: The result obtained from the whole-pixel displacement search has limited accuracy because the actual displacement may not be an integer number of pixels. The sub-pixel displacement search can further improve the accuracy of displacement measurement. It is a refined operation based on the whole-pixel displacement search; (3) Solution of the strain field: After obtaining the sub-pixel displacement, the analysis software will calculate the strain of the displacement in different directions. For example, for a series of adjacent sub-regions on the surface of the object, if the displacements in the x direction are known as Dx1, Dx2, etc., and the spacing between the centers of the sub-regions in the x direction is Δx (the actual physical spacing or the converted spacing), then the strain in the x direction is equal to εx = (Dx1 - Dx2) / Δx. Similarly, the strain εy in the y direction can be measured.

[0045] It should be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent to such process, method, article or device.

[0046] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A point load device for measuring rock deformation parameters based on digital image correlation technology, characterized in that: It includes a supporting device (1), a loading mechanism, a control system and an image acquisition system: The support device (1) is composed of a top transverse support, a middle transverse support, a base, an oblique support, and a vertical support; The loading mechanism comprises a pressure sensor (2), a point load upper loading head (3), a point load lower loading head (5), a ball screw (7), a flange (8), a worm wheel (10), a worm (11), a bearing (12), a spheroidal graphite housing (13) and a stepper motor (14); the stepper motor (14) is fixedly mounted on the top of the middle transverse bracket; a driving assembly (9) is arranged on the right side of the stepper motor (14); the driving assembly (9) comprises a worm wheel (10), a worm (11) and a bearing (12); the output shaft of the stepper motor (14) is fixedly connected to the worm (11); the worm wheel (10) is meshed inside the worm (11); the bearing is fixedly mounted inside the worm wheel (10); (12), the top of the bearing (12) is fixedly connected with a ball screw (7), the surface of the ball screw (7) is threadedly connected with a flange (8), the internal thread of the flange (8) is connected with a point load lower loading head (5), the bottom end of the top transverse bracket is fixedly installed with a calibration point (6), the bottom of the calibration point (6) is fixedly installed with a pressure sensor (2), the bottom of the pressure sensor (2) is fixedly connected with a point load upper loading head (3), the rock sample (4) is arranged on the opposite sides of the point load upper loading head (3) and the point load lower loading head (5), the top of the base is provided with a spherical graphite box (13), and the interior of the spherical graphite box (13) is provided with a ball screw (7); The control system comprises a pressure sensor (2), an encoder (15), a driver (16) and a display screen (17); the pressure sensor (2) is connected to the display screen (17); an encoder (15) is arranged on the left side of the stepping motor (14); the encoder (15) is connected to a single-chip microcomputer; the single-chip microcomputer is provided with an external display screen and operation buttons; the single-chip microcomputer signal is transmitted to the driver (16) signal; The image acquisition system comprises a calibration point (6), an L-shaped bracket (18) and a CCD industrial camera (19); the L-shaped bracket (18) is fixedly mounted on the bottom of the top transverse bracket; the CCD industrial camera (19) is arranged at the top of the cavity of the L-shaped bracket (18); an LED fill light is arranged on the inner side of the L-shaped bracket (18); and the CCD industrial camera (19) is electrically connected to an external power supply.

2. According to claim 1, a point load device for measuring rock deformation parameters based on digital image correlation technology is characterized in that: The ball screw (7) is connected to the middle transverse bracket through and extends to the lower part.

3. The point load device for measuring rock deformation parameters based on digital image correlation technology according to claim 1 is characterized in that: A driver (16) and a display screen (17) are installed on the top of the middle transverse support.

4. The point load device for measuring rock deformation parameters based on digital image correlation technology according to claim 1 is characterized in that: The bottom of the middle horizontal bracket is fixedly connected to an oblique bracket, and the bottom of the oblique bracket is fixedly connected to a base.

5. The point load device for measuring rock deformation parameters based on digital image correlation technology according to claim 1 is characterized in that: Both ends of the middle horizontal bracket are provided with mounting holes, and vertical brackets are fixedly mounted inside the mounting holes, and a top horizontal bracket is fixedly mounted on the top of the vertical bracket.

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