A metal thermal expansion coefficient measuring device and a measuring method thereof
By combining optical imaging and image analysis, a metal thermal expansion coefficient measuring device has been developed, which solves the problems of high cost and low accuracy in existing technologies. It achieves non-contact, high-precision thermal expansion coefficient measurement, improving measurement efficiency and intuitiveness.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- TIANJIN UNIV
- Filing Date
- 2026-04-20
- Publication Date
- 2026-07-14
AI Technical Summary
Existing methods for measuring the coefficient of thermal expansion of metals suffer from problems such as high equipment costs, limited accuracy, complex operation, and stringent environmental requirements. There is a lack of high-precision, low-cost, and intuitive non-contact measurement devices.
A metal thermal expansion coefficient measuring device composed of a mechanical module, a vision module, a temperature control module, and an algorithm module is used. Combined with optical imaging and image analysis, it achieves non-contact high-precision measurement through image clarity evaluation, automatic search algorithm, and edge detection.
It enables non-contact, high-precision, and low-cost measurement of the thermal expansion coefficient of metal samples, improving measurement efficiency and intuitiveness. It can simultaneously measure the linear expansion coefficient and the volume expansion coefficient, reducing human reading errors.
Smart Images
Figure CN122385671A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of instruments for measuring the coefficient of thermal expansion, and in particular to a device and method for measuring the coefficient of thermal expansion of metals. Background Technology
[0002] Thermal expansion of materials is a fundamental and crucial property in physics and materials science. When the temperature of a material changes, the thermal motion of its internal atoms or molecules intensifies, leading to an increase in interatomic spacing, which manifests macroscopically as changes in size and volume. The key physical quantity describing this phenomenon is the coefficient of thermal expansion, including the coefficient of linear expansion and the coefficient of volumetric expansion. Accurate measurement and in-depth understanding of the coefficient of thermal expansion of materials are not only important means of verifying solid-state physics theories but also an indispensable foundation for the development of modern industrial technology.
[0003] With the development of technology, methods for measuring the coefficient of thermal expansion of metals have become increasingly diversified, including push-rod dilatometer method, extensometer method, optical interferometry, and optical lever amplification method. However, each method has certain problems in terms of principle, accuracy, cost, and applicability. The push-rod dilatometer method, as the most mature and widely used measurement technology in industry and scientific research, has a wide measurement range (from cryogenic to ultra-high temperature), high degree of automation, and stable data output. However, its equipment purchase cost is extremely expensive, reaching tens of thousands to hundreds of thousands of RMB. It also has high requirements for sample size and preparation, and the contact between the push rod and the sample may introduce frictional errors, compressive stress, or heat loss, affecting measurement accuracy. The extensometer method, although relatively low in cost and simple to operate, generally has lower measurement accuracy. Optical interferometry is currently the most accurate method for measuring thermal expansion, reaching the nanometer or even sub-nanometer level. However, the interferometer equipment is extremely precise and complex, expensive, and has stringent requirements for experimental environments such as vibration, airflow, and temperature fluctuations. The adjustment process is cumbersome, and it requires extremely high operator skills. The optical lever amplification method is a classic optical magnification measurement method. Its principle is intuitive and easy to understand, and the equipment is relatively simple, making it commonly used in teaching experiments. However, its measurement accuracy is limited by the optical path length, the accuracy of the scale reading, and the mechanical stability of the reflector, typically achieving only an accuracy of tens or even hundreds of micrometers, and it is easily affected by airflow disturbances. Therefore, developing a high-precision, non-contact, and intuitively visualized device for measuring the coefficient of thermal expansion of metals has significant practical application value. Summary of the Invention
[0004] The technical problem to be solved by the present invention is to provide a device for measuring the coefficient of thermal expansion of metals.
[0005] Another technical problem to be solved by the present invention is to provide a method for measuring thermal expansion using the above-mentioned metal thermal expansion coefficient measuring device.
[0006] The technical solution adopted in this invention is: A device for measuring the coefficient of thermal expansion of metals comprises a mechanical module, a vision module, a temperature control module, and an algorithm module, wherein... The mechanical module consists of an optical plate, a support rod, and an adjustment frame. The support rod is vertically and fixedly connected to the optical plate, and the adjustment frame is mounted on the support rod. The vision module consists of an imaging system and an illumination system. The illumination system is located at the bottom of the imaging system. The imaging system includes an industrial camera and a microscope lens. The illumination system is a ring light source. The industrial camera is fixedly connected to the ring light source through the microscope lens. The microscope lens is connected to a support rod through an adjustment bracket, thereby fixing the imaging system on the optical plate. The temperature control module is a metal bath heating device used to heat metal samples. The metal bath heating device is placed on an optical flat plate and located directly below the vision module. The algorithm module is a control system used for sharpness evaluation calculation, search algorithm and diameter calculation. The control system is electrically connected to the imaging system and the metal bath heating device.
[0007] Preferably, in the aforementioned metal thermal expansion coefficient measuring device, the sharpness evaluation function selected in the sharpness evaluation calculation is the improved Tenengrad gradient evaluation function. The Tenengrad gradient evaluation function is a typical spatial domain-based sharpness evaluation function. Since the edge is where the image brightness function changes drastically, it is a vector with amplitude and direction. The amplitude is the amplitude of the gradient, and the direction is perpendicular to the gradient direction. Visually, a sharp image has sharper edges, which mathematically means that the image has a larger gradient amplitude. The algorithm is as follows: , In the formula, g(x,y) represents the image grayscale function. Mask represents the convolution operation. h mask v Let M represent the horizontal and vertical convolution masks, respectively. M is the gradient magnitude, μ(M) is its average value, δ(M) is the standard deviation of M, T is the grayscale threshold, which is the critical value for distinguishing between "edge points" and "non-edge points," and F is the image sharpness result. Each pixel has a gradient value, but not all points are image edges. The proportion of actual edge points to the total number of pixels in the image is relatively low; therefore, an edge detection threshold T is defined. Referring to the properties of the normal distribution function, the probability that the value is greater than the sum of the mean and three times the standard deviation is only 0.135%, and points meeting this requirement must be actual edges.
[0008] Preferably, in the above-mentioned metal thermal expansion coefficient measuring device, the search algorithm adopts the Gaussian curve fitting method, and the Gaussian function expression is as follows: , The value of parameter b obtained by fitting is the position of sharpest image, i.e., the position of perfect focus.
[0009] When a lens or sample moves along the optical axis, a certain surface is sharpest near the optimal focusing position, with the largest edge gradient. The sharpness score S usually forms a peak with the displacement position x. In the above formula, e is the natural constant, a is the peak height, b is the peak center, which is the optimal focusing position, and c is the peak width.
[0010] Preferably, in the above-mentioned metal thermal expansion coefficient measuring device, the diameter calculation is a diameter measurement method based on image measurement and edge detection. After a high-resolution industrial camera acquires a clear two-dimensional image of the sample to be measured at the positive focus position, the sharpness of the upper surface sample image is calculated, and an edge binary map is returned. In the operable interface, the user clicks on the edge position close to the sample image. The software system searches for the edge near the clicked position based on the edge binary map, finds the nearest edge point for adsorption, and after completing the three points, the edge circle can be determined and the diameter data of the circle to be measured is returned.
[0011] A method for measuring thermal expansion using the aforementioned metal thermal expansion coefficient measuring device is based on a combination of optical focusing and image analysis. It utilizes an industrial camera imaging system combined with autofocus technology to determine the optimal focal point position on the sample surface through an image sharpness evaluation function and a search algorithm. The method then calculates the axial height change of the sample based on the focal point position change. Simultaneously, it obtains the sample cross-sectional diameter change through two-dimensional image edge detection and a circular fitting algorithm. This enables simultaneous measurement of the axial and radial thermal expansion of the metal sample, and the linear and volumetric expansion coefficients of the material are calculated accordingly. This measurement method effectively improves the accuracy and information dimension of thermal expansion measurement.
[0012] The beneficial effects of this invention are: The aforementioned metal thermal expansion coefficient measuring device, designed for cylindrical metal samples, can accurately measure the thermal expansion of their longitudinal height and transverse diameter, and then calculate the linear expansion coefficient and volumetric expansion coefficient. It is a non-contact, high-precision, low-cost, and highly intuitive measurement system. Applying the thermal expansion measurement method of this device, and utilizing a combination of hardware and software to achieve automated measurement, it enables non-contact, high-precision thermal expansion measurement. The measurement process is visualized, effectively improving experimental intuitiveness, and achieving simultaneous measurement of axial and radial expansion. Specifically: (1) By using image sharpness evaluation, automatic search algorithm and edge detection, the measurement data can be automatically acquired and calculated, reducing human reading error and improving experimental efficiency and data stability.
[0013] (2) The non-contact measurement principle based on optical focusing and image analysis is adopted, which avoids the measuring device from directly contacting the surface of the metal sample and the thermal expansion coefficient is more accurate.
[0014] (3) Using a camera to observe the surface of the metal sample in real time, compared with existing instruments, the entire process of sample expansion can be seen, which is highly intuitive.
[0015] (4) It can simultaneously measure the linear expansion coefficient and volume expansion coefficient of the sample, and can verify the approximate three-fold relationship between the two coefficients while measuring a higher dimension. Attached Figure Description
[0016] Figure 1 This is a schematic diagram of the metal thermal expansion coefficient measuring device of the present invention, wherein (a) is a front view and (b) is a side view; in the figure: 1-industrial camera, 2-microscope head, 3-ring light source, 4-metal bath heating device, 5-sample, 6-adjustment frame, 7-support rod, 8-optical plate, 9-control system.
[0017] Figure 2 This is a schematic diagram of the algorithm structure of the metal thermal expansion coefficient measuring device described in this invention.
[0018] Figure 3 This is a height measurement and sharpness evaluation chart.
[0019] Figure 4 This is a diagram for evaluating the clarity of diameter measurements.
[0020] Figure 5 This is the graph showing the Gaussian fitting process for height measurement.
[0021] Figure 6 This is the running graph of the Gaussian fitting part of the diameter measurement.
[0022] Figure 7 This is a screenshot of the code running for the diameter calculation part.
[0023] Figure 8 This is a flowchart of measuring the coefficient of linear expansion of aluminum alloy as described in Example 2.
[0024] Figure 9 This is a flowchart of the measurement of the linear expansion coefficient of aluminum alloy as described in Example 3. Detailed Implementation
[0025] To enable those skilled in the art to clearly understand the technical solution of the present invention, the technical solution will be described in detail below with reference to the accompanying drawings and specific embodiments.
[0026] Example 1 like Figure 1As shown, the metal thermal expansion coefficient measuring device consists of a mechanical module, a vision module, a temperature control module, and an algorithm module. The mechanical module consists of an optical plate 8, a support rod 7, and an adjustment frame 6. The support rod is vertically and fixedly connected to the optical plate, and the adjustment frame is mounted on the support rod. The vision module consists of an imaging system and an illumination system. The illumination system is located at the bottom of the imaging system. The imaging system includes an industrial camera 1 and a microscope head 2. The illumination system is a ring light source 3. The industrial camera is fixedly connected to the ring light source through the microscope head. The microscope head is connected to the support rod through an adjustment bracket, thereby fixing the imaging system on the optical plate. The temperature control module is a metal bath heating device 4, which is used to heat the metal sample 5. The metal bath heating device is placed on the optical flat plate and located directly below the vision module. During measurement, the metal sample to be measured is placed in the metal bath module used to carry the sample, the heat insulation cover is covered, and the metal bath module is placed on the metal bath heating device. The algorithm module is a control system 9, used for sharpness evaluation calculation, search algorithm, and diameter calculation. This control system is electrically connected to both the imaging system and the metal bath heating device. Figure 2 As shown, the algorithm consists of three parts: image acquisition, focal position search, and diameter calculation, and is performed in three stages.
[0027] First, after the code runs, it will display real-time footage captured by the camera in the program window. The experimenter can manually select an image within or outside the observation window and circle a region as the observation boundary. After selection, the program performs real-time sharpness evaluation calculations based on the Sobel operator for the observation area and displays the calculated sharpness value on the interface in real time, such as... Figure 3 , Figure 4 As shown. In the sharpness evaluation calculation, the improved Tenengrad gradient evaluation function is selected. The Tenengrad gradient evaluation function is a typical spatial domain-based sharpness evaluation function. Since edges are locations where the image brightness function changes drastically, they are vectors with amplitude and direction. The amplitude is the amplitude of the gradient, and the direction is perpendicular to the gradient direction. Visually, a sharp image has sharper edges, mathematically represented by a larger gradient amplitude. The algorithm is as follows:
[0028] In the formula, g(x,y) represents the image grayscale function. Mask represents the convolution operation. h mask vLet M represent the horizontal and vertical convolution masks, respectively. M is the gradient magnitude, μ(M) is its average value, δ(M) is the standard deviation of M, T is the grayscale threshold, which is the critical value for distinguishing between "edge points" and "non-edge points," and F is the image sharpness result. Each pixel has a gradient value, but not all points are image edges. The proportion of actual edge points to the total number of pixels in the image is relatively low; therefore, an edge detection threshold T is defined. Referring to the properties of the normal distribution function, the probability that the value is greater than the sum of the mean and three times the standard deviation is only 0.135%, and points meeting this requirement must be actual edges.
[0029] The search algorithm is then executed. A starting position is selected before the sharpest point. The distance between the camera and the sample is controlled by manually adjusting the linear adjustment frame. Photos are taken by pressing the spacebar according to the step size. After acquiring a sufficient number of sample points, the F key is pressed to search for the focal position. A Gaussian fitting function is used to obtain the height of the focal position based on the sharpness value and position of the photos. Figure 5 , Figure 6 As shown. The expression for the Gaussian fitting function is as follows:
[0030] The value of parameter b obtained by fitting is the position of sharpest image, i.e., the position of perfect focus.
[0031] When a lens or sample moves along the optical axis, a certain surface is sharpest near the optimal focusing position, with the largest edge gradient. The sharpness score S usually forms a peak with the displacement position x. In the above formula, e is the natural constant, a is the peak height, b is the peak center, which is the optimal focusing position, and c is the peak width.
[0032] After finding the focal point, select the closest photo to calculate the diameter.
[0033] Diameter calculation is a diameter measurement method based on image measurement and edge detection. After a high-resolution industrial camera acquires a clear 2D image of the sample at the focal position, the sharpness of the nearest image (i.e., the image of the upper surface sample) is calculated, and a binary edge map is returned. In the user interface, clicking on a nearby edge position in the sample image allows the software system to search for edges near the clicked position based on the binary edge map. The system then finds the nearest edge point and performs snap-on detection. After completing these three point detections, the edge circle is determined, and the diameter data of the measured circle is returned. Figure 7 As shown.
[0034] The models of each component in the above-mentioned metal thermal expansion coefficient measuring device are shown in Table 1.
[0035] Table 1
[0036] Example 2 The linear expansion coefficient of aluminum alloys is measured using a metal thermal expansion coefficient measuring device, such as... Figure 8 As shown, the specific steps are as follows: (1) Assemble the measuring instrument, adjust the lens to a high position, and place the metal bath heating stage directly below the lens. Place the metal sample (6061-T6 aluminum alloy cylinder) vertically into the metal bath module, cover it with the heat insulation cover, and wipe the heat insulation cover part above the sample clean; (2) Set the objective lens magnification to the highest level, connect the power supply, turn on the ring light source, run the height measurement program, and adjust the light source brightness, lens height and sample position until the sample surface texture can be clearly seen in the program window; (3) Record the current temperature as the initial temperature and begin the search for the focal position (see Figure 2 The coordinates of the focal position before heating are obtained as follows: ; (4) Keep the lens height unchanged, turn on the heating device, set the target temperature to 60°C, and start heating; (5) After the thermometer reading stabilizes at the target temperature, continue searching for the focal position. The second focusing is completed using a search algorithm and a sharpness evaluation function, and the coordinates of the heated focal position are obtained as follows: ; (6) Analyze the data and calculate the coefficient of linear expansion of the sample material at that temperature; (7) Set the target temperature of the heating platform to 80°C and 100°C and continue heating, then repeat steps (5) and (6). (8) Calculate the average linear expansion coefficient and compare it with the theoretical value.
[0037] Example 3 The linear expansion coefficient of aluminum alloys is measured using a metal thermal expansion coefficient measuring device, such as... Figure 9 As shown, the specific steps are as follows: (1) Assemble the measuring instrument, adjust the lens to a high position, and place the metal bath heating stage directly below the lens. Place the metal sample (6061-T6 aluminum alloy cylinder) vertically into the metal bath module, cover it with the heat insulation cover, and wipe the heat insulation cover part above the sample clean; (2) Set the objective lens magnification to the lowest level, connect the power supply, turn on the ring light source, run the diameter measurement program, and adjust the light source brightness, lens height and sample position until the sample surface texture can be clearly seen in the program window; (3) Record the current temperature as the initial temperature and begin the search for the focal position (see Figure 2 To obtain an image of the focal position before heating, the diameter of the sample's upper surface at this point is calculated and denoted as the initial diameter D0. (4) Keep the lens height unchanged, turn on the heating platform, set the target temperature to 60℃, and start heating; (5) After the thermometer reading stabilizes at the target temperature, run the diameter measurement program again to start the search for the positive focus position. Complete the second focusing through the search algorithm and sharpness evaluation function, obtain the positive focus position image, and calculate the diameter of the sample's upper surface at this time, which is recorded as the diameter after heating, D1. (6) Analyze the data and calculate the volume expansion coefficient of the sample material based on the height change measured in Experiment 1; (7) Set the target temperature of the heating platform to 80°C and 100°C and continue heating, then repeat steps 5 and 6; (8) Calculate the average volume expansion coefficient and compare it with the theoretical value to verify the relationship between the linear expansion coefficient and the volume expansion coefficient.
[0038] The initial size of the samples in Examples 2 and 3 above (in) (Measurement below) initial height
[0039] initial diameter
[0040] initial temperature
[0041] The original data records of the experiments measuring the coefficient of linear expansion and the coefficient of volume expansion of metals are shown in Table 2.
[0042] Table 2
[0043] The experimental data processing and results of the measurement of the linear expansion coefficient and volume expansion coefficient of metals are shown in Table 3.
[0044] Table 3
[0045] Among them: temperature change
[0046] Longitudinal length variation
[0047] Longitudinal linear expansion coefficient
[0048] Diameter change
[0049] Lateral linear expansion coefficient
[0050] The data is analyzed as follows: 1. Calculation of the average value of the linear expansion coefficient Mean longitudinal linear expansion coefficient:
[0051] Mean horizontal linear expansion coefficient:
[0052] Final average linear expansion coefficient:
[0053] 2. Calculate the volume expansion coefficient 1) The volume expansion coefficient measured experimentally
[0054] Initial volume:
[0055] exist At ℃,
[0056]
[0057]
[0058]
[0059]
[0060]
[0061] 2) According to Calculated volume expansion coefficient
[0062] Data Analysis Summary 1. Average linear expansion coefficient: The final average linear expansion coefficient measured by the experiment for This is consistent with the theoretical value of 6061-T6 aluminum alloy. The result is very close, indicating good accuracy. The result is slightly larger, possibly because there is a two-millimeter metal bath module below the sample, which also increases in height as it is heated.
[0063] 2. Verification of the relationship between volume expansion coefficients: The volume expansion coefficient calculated directly from volume change
[0064] according to The relationship is obtained by calculating the volume expansion coefficient using the average linear expansion coefficient.
[0065] The two are on the same order of magnitude, but there is a difference of about 7%, which is reasonable and common in actual experiments.
[0066] The above-described embodiments are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.
Claims
1. A device for measuring the coefficient of thermal expansion of metals, characterized in that: It consists of a mechanical module, a vision module, a temperature control module, and an algorithm module, among which, The mechanical module consists of an optical plate, a support rod, and an adjustment frame. The support rod is vertically and fixedly connected to the optical plate, and the adjustment frame is mounted on the support rod. The vision module consists of an imaging system and an illumination system. The illumination system is located at the bottom of the imaging system. The imaging system includes an industrial camera and a microscope lens. The illumination system is a ring light source. The industrial camera is fixedly connected to the ring light source through the microscope lens. The microscope lens is connected to a support rod through an adjustment bracket, thereby fixing the imaging system on the optical plate. The temperature control module is a metal bath heating device used to heat metal samples. The metal bath heating device is placed on an optical flat plate and located directly below the vision module. The algorithm module is a control system used for sharpness evaluation calculation, search algorithm and diameter calculation. The control system is electrically connected to the imaging system and the metal bath heating device.
2. The metal thermal expansion coefficient measuring device according to claim 1, characterized in that: The algorithm for calculating the sharpness evaluation is as follows: ， In the formula, g(x,y) represents the image grayscale function. Mask represents the convolution operation. h mask v Let M represent the horizontal and vertical convolutional masks, respectively. M is the gradient magnitude, μ(M) is its average value, δ(M) is the standard deviation of M, T is the grayscale threshold, and F is the image sharpness result.
3. The metal thermal expansion coefficient measuring device according to claim 1, characterized in that: The search algorithm employs Gaussian curve fitting, and the Gaussian function expression is as follows: ， In the formula, e is the natural constant, a is the peak height, b is the peak center, which is the optimal focal position, and c is the peak width.
4. The metal thermal expansion coefficient measuring device according to claim 1, characterized in that: The diameter calculation is a diameter measurement method based on image measurement for edge detection. After a high-resolution industrial camera acquires a clear two-dimensional image of the sample at the positive focus position, the sharpness of the upper surface sample image is calculated, and an edge binary map is returned. In the operable interface, the user clicks on the edge position close to the sample image. The software system searches for the edge near the clicked position based on the edge binary map, finds the nearest edge point and performs adsorption. After completing the three points, the edge circle can be determined and the diameter data of the circle to be measured is returned.
5. A method for measuring thermal expansion using the metal thermal expansion coefficient measuring device according to claims 1-4, characterized in that: Based on the combination of optical focusing and image analysis, this method utilizes an industrial camera imaging system combined with autofocus technology to determine the optimal focal position on the sample surface through an image sharpness evaluation function and search algorithm, and calculates the axial height change of the sample based on the change in focal position. At the same time, it obtains the change in the sample cross-sectional diameter through two-dimensional image edge detection and circular fitting algorithm, thereby realizing the synchronous measurement of the axial and radial thermal expansion of the metal sample, and calculating the linear expansion coefficient and volume expansion coefficient of the material accordingly.