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Instrumentation, Measurement and Metrology|120 Article(s)
Wood Inertial Measurement Unit Based on Laser-induced Graphene
Chen LI, Hao LI, and Yanwei YANG
Inertial Measurement Unit (IMU) mainly measures and reports the specific force and angular velocity of an object. It usually consists of a three-axis accelerometer and a three-axis gyroscope. Traditional IMU have complex manufacturing processes and high costs. It is not easy to recycle, pollute the environment and cannot be biodegradable after being discarded. In this study, a kind of wood inertial measurement unit based on Laser-induced Graphene (LIG) is proposed.Camphor pine wood was selected as the sensor frame. The pine is placed in the vacuum chamber and adjust the vacuum degree in the vacuum chamber. Fiber laser and laser vibrators software was used to process LIG resistors at the pine frame resistor locations by adjusting laser frequency, laser power, scan speed, off-focus distance and other parameters, with a resistance value of about 100 Ω, and to connect these resistors into multiple Wheatstone bridges. Due to the sensor's design being symmetrical along the X- and Z-axes, only the Wheatstone bridge on the beam in the X- or Z-axis direction needs to be measured. Different inertial bodies are divided into balanced oscillator unit and unbalanced oscillator unit. The inertial body of the balanced oscillator unit is a spherical inertial body (Cr13), mainly measuring acceleration; The inertial body of the unbalanced dipole element is the inertial wheel (PLA), which mainly measures the angular acceleration. The theoretical formula shows that the acceleration range of IMU along X axis and Z axis is±10 g, the acceleration range along Y axis is -g~+3 g, the angular acceleration range around X axis and Z axis is±8 rad/s2, and the angular acceleration range around Y axis is±23 rad/s2.The product of this study is placed on a sports platform and uses a dynamic signal acquisition system to collect the output voltage of a Wheatstone bridge in the IMU. Through the joint test and comparison of the commercial acceleration sensor and unbalanced oscillator unit, it can be concluded that the designed sensor has the same change as the commercial sensor and has the ability to measure the angular rate. The results of this measurement of the Wheatstone bridge on the Z-axis beam are as follows: The sensitivity of the X-axis acceleration of the balanced oscillator unit is 0, and the sensitivity of the Y-axis acceleration is 8.695×10-4 mV/g, the Z-axis acceleration sensitivity is 0.200 mV/g, and the sensitivity to angular acceleration is 0. The sensitivity of Y-axis acceleration of the unbalanced oscillator unit is 1.110 mV/g, the sensitivity of the X-axis and Z-axis acceleration is 0, the sensitivity of the angular acceleration around the X-axis is 0.285 mV/(rad/s2), the sensitivity of the angular acceleration around the Y-axis is 0.305 mV/(rad/s2), and the sensitivity of the angular acceleration around the Z-axis is 0.765 mV/(rad/s2). By comparing the voltage output of Wheatstone in all directions of IMU under cyclic load, it is concluded that IMU of this design still has the ability to measure the angular rate and other parameters under multiple cyclic loads, so this design has a certain stability.We propose a wooden inertial measurement unit based on laser-induced graphene, which is made of biodegradable materials and has good sensitivity and certain stability. Compared to the IMU on the market, it is more environmentally friendly, cheaper, and more convenient to manufacture, with environmental advantages and market prospects. It can be used in the fields of wooden ships, vehicle engineering, or wooden buildings that require vibration measurement. In future research, the size of this wooden IMU can be further optimized, and element doping can be applied to LIG varistors to improve their conductivity and sensitivity. Inertial Measurement Unit (IMU) mainly measures and reports the specific force and angular velocity of an object. It usually consists of a three-axis accelerometer and a three-axis gyroscope. Traditional IMU have complex manufacturing processes and high costs. It is not easy to recycle, pollute the environment and cannot be biodegradable after being discarded. In this study, a kind of wood inertial measurement unit based on Laser-induced Graphene (LIG) is proposed.Camphor pine wood was selected as the sensor frame. The pine is placed in the vacuum chamber and adjust the vacuum degree in the vacuum chamber. Fiber laser and laser vibrators software was used to process LIG resistors at the pine frame resistor locations by adjusting laser frequency, laser power, scan speed, off-focus distance and other parameters, with a resistance value of about 100 Ω, and to connect these resistors into multiple Wheatstone bridges. Due to the sensor's design being symmetrical along the X- and Z-axes, only the Wheatstone bridge on the beam in the X- or Z-axis direction needs to be measured. Different inertial bodies are divided into balanced oscillator unit and unbalanced oscillator unit. The inertial body of the balanced oscillator unit is a spherical inertial body (Cr13), mainly measuring acceleration; The inertial body of the unbalanced dipole element is the inertial wheel (PLA), which mainly measures the angular acceleration. The theoretical formula shows that the acceleration range of IMU along X axis and Z axis is±10 g, the acceleration range along Y axis is -g~+3 g, the angular acceleration range around X axis and Z axis is±8 rad/s2, and the angular acceleration range around Y axis is±23 rad/s2.The product of this study is placed on a sports platform and uses a dynamic signal acquisition system to collect the output voltage of a Wheatstone bridge in the IMU. Through the joint test and comparison of the commercial acceleration sensor and unbalanced oscillator unit, it can be concluded that the designed sensor has the same change as the commercial sensor and has the ability to measure the angular rate. The results of this measurement of the Wheatstone bridge on the Z-axis beam are as follows: The sensitivity of the X-axis acceleration of the balanced oscillator unit is 0, and the sensitivity of the Y-axis acceleration is 8.695×10-4 mV/g, the Z-axis acceleration sensitivity is 0.200 mV/g, and the sensitivity to angular acceleration is 0. The sensitivity of Y-axis acceleration of the unbalanced oscillator unit is 1.110 mV/g, the sensitivity of the X-axis and Z-axis acceleration is 0, the sensitivity of the angular acceleration around the X-axis is 0.285 mV/(rad/s2), the sensitivity of the angular acceleration around the Y-axis is 0.305 mV/(rad/s2), and the sensitivity of the angular acceleration around the Z-axis is 0.765 mV/(rad/s2). By comparing the voltage output of Wheatstone in all directions of IMU under cyclic load, it is concluded that IMU of this design still has the ability to measure the angular rate and other parameters under multiple cyclic loads, so this design has a certain stability.We propose a wooden inertial measurement unit based on laser-induced graphene, which is made of biodegradable materials and has good sensitivity and certain stability. Compared to the IMU on the market, it is more environmentally friendly, cheaper, and more convenient to manufacture, with environmental advantages and market prospects. It can be used in the fields of wooden ships, vehicle engineering, or wooden buildings that require vibration measurement. In future research, the size of this wooden IMU can be further optimized, and element doping can be applied to LIG varistors to improve their conductivity and sensitivity.
Acta Photonica Sinica
- Publication Date: Feb. 25, 2024
- Vol. 53, Issue 2, 0212005 (2024)
Method of Calibrating Target Distance Using Dual Zone-block Light Screen Velocity Measuring Device
Jing LI, Zhonghui SUN, Jinping NI, and Hui TIAN
Light screen is a photoelectric test equipment that adopts the principle of measuring the flying time in a fixed distance, commonly used to measure the flying velocity of projectile which also known as the initial velocity of projectile near the muzzle. For the same diameter projectile, distance measurement error is the main factor affecting the velocity measurement error when timing error is determined. Direct measurement methods such as steel rules and laser range finders are used to measure long target distance greater than 1 m in light screen, and measurement error of distance is generally 0.5 mm, the relative error of distance is not more than 0.5‰, which has little impact on the relation error of velocity measurement. For light screen of short target distance with target distance less than 200 mm, the above method is still used to measure distance, and the distance relative error is greater than 2.5‰, then the relative error of velocity measurement exceeds 2.5‰, which does not meet the requirements of the actual velocity measurement. In this paper, a target distance calibration method using the dual zone-block velocity measuring device is proposed, which consists of two sets of calibration light screens with the same principle. The target distances of the two sets of light-screen are consistent and much larger than the distances of calibrated light screens. Based on the transmission formula of velocity measurement error, a calculation method of minimum calibration distance under the determination of that error is given. Two sets of light screen are utilized for calibration to measure the velocity of the flying projectile at two points on the ballistic separated by a fixed distance. Combined with the distance between two points, the resistance parameters of projectile and instantaneous velocity at any time between two points are developed based on the ballistic equation. Based on the fact that the average velocity within a certain distance is equal to the instantaneous velocity at the midpoint of projectile flying time within that distance, considering the time sequence of projectile passing through calibration light screen and the calibrated light screen, the distance calibration formula of calibrated light screen is given, and the calibration error calculation formula is derived. The influence of calibration distance and layout spacing in dual zone-block device on the calibration error is analyzed when projectile's flying velocity is determined. If projectile velocity is constant, the calibration error decreases with the increase of calibration distance and layout spacing. Furthermore, when the calibration distance and layout spacing are fixed, the larger the projectile flying velocity, the greater the calibration error. Taking the target calibration test of three light screen as an example, the single zone-block calibration test and the dual zone-block calibration test are carried out, in which the calibration distance is 1 237 mm and the layout spacing is 1 996 mm. When the projectile velocity is 150 m/s, the calibration relation error of single zone-block device and dual zone-block device are 1.6‰ and 0.8‰, respectively. The velocity measurement is carried out with calibration results of dual zone-block, and the relation error of velocity measurement is 0.37‰, which meets the requirements of velocity measurement. We proposes a calibration method for dual zone-block screen velocity measuring device, which indirectly measures the target distance of light screen with unknown distance by using the calibration light screen with known long target distance. Meanwhile, the calibration method proposed can be used to calibrate the target distance of sensors for zone-block velocity measurement on various principles and usage level. Light screen is a photoelectric test equipment that adopts the principle of measuring the flying time in a fixed distance, commonly used to measure the flying velocity of projectile which also known as the initial velocity of projectile near the muzzle. For the same diameter projectile, distance measurement error is the main factor affecting the velocity measurement error when timing error is determined. Direct measurement methods such as steel rules and laser range finders are used to measure long target distance greater than 1 m in light screen, and measurement error of distance is generally 0.5 mm, the relative error of distance is not more than 0.5‰, which has little impact on the relation error of velocity measurement. For light screen of short target distance with target distance less than 200 mm, the above method is still used to measure distance, and the distance relative error is greater than 2.5‰, then the relative error of velocity measurement exceeds 2.5‰, which does not meet the requirements of the actual velocity measurement. In this paper, a target distance calibration method using the dual zone-block velocity measuring device is proposed, which consists of two sets of calibration light screens with the same principle. The target distances of the two sets of light-screen are consistent and much larger than the distances of calibrated light screens. Based on the transmission formula of velocity measurement error, a calculation method of minimum calibration distance under the determination of that error is given. Two sets of light screen are utilized for calibration to measure the velocity of the flying projectile at two points on the ballistic separated by a fixed distance. Combined with the distance between two points, the resistance parameters of projectile and instantaneous velocity at any time between two points are developed based on the ballistic equation. Based on the fact that the average velocity within a certain distance is equal to the instantaneous velocity at the midpoint of projectile flying time within that distance, considering the time sequence of projectile passing through calibration light screen and the calibrated light screen, the distance calibration formula of calibrated light screen is given, and the calibration error calculation formula is derived. The influence of calibration distance and layout spacing in dual zone-block device on the calibration error is analyzed when projectile's flying velocity is determined. If projectile velocity is constant, the calibration error decreases with the increase of calibration distance and layout spacing. Furthermore, when the calibration distance and layout spacing are fixed, the larger the projectile flying velocity, the greater the calibration error. Taking the target calibration test of three light screen as an example, the single zone-block calibration test and the dual zone-block calibration test are carried out, in which the calibration distance is 1 237 mm and the layout spacing is 1 996 mm. When the projectile velocity is 150 m/s, the calibration relation error of single zone-block device and dual zone-block device are 1.6‰ and 0.8‰, respectively. The velocity measurement is carried out with calibration results of dual zone-block, and the relation error of velocity measurement is 0.37‰, which meets the requirements of velocity measurement. We proposes a calibration method for dual zone-block screen velocity measuring device, which indirectly measures the target distance of light screen with unknown distance by using the calibration light screen with known long target distance. Meanwhile, the calibration method proposed can be used to calibrate the target distance of sensors for zone-block velocity measurement on various principles and usage level.
Acta Photonica Sinica
- Publication Date: Feb. 25, 2024
- Vol. 53, Issue 2, 0212004 (2024)
Prepared of Ultrabroad Spectrum Calibration Blackbody Source for Deep Space Exploration
Huiwen DONG, Jiaping ZHANG, Minlong LIAN, Dazhou XIAO, Weigang WANG, and Yuehui LU
For small objects in deep space exploration, the radiation temperature is about 100 K to 420 K. According to Wien's displacement law, deep space exploration targets cover the ultra-wide spectral range of 5 μm to 50 μm, and high-precision on-board calibration is the key to the accurate detection of deep space exploration loads. Therefore, it is necessary to develop an on-board calibration source with high emissivity and high temperature stability of ultra-wide spectrum. Aiming at the technical problems of detecting ultra-wide spectrum, high reflectivity and high temperature stability of radiometric calibration blackbody source on ultra-wide spectrum stars in deep space, this paper carries out in-depth research and experimental verification from several aspects such as simulation, key component design and sample detection methods, so as to ensure the engineering realization and application of high-index precision radiometric calibration blackbody source on stars. COMSOL Multiphysics software simulation based on finite element method, microstructure design of blackbody source, emittance detection method of blackbody source and radiation calibration test in laboratory were studied.Firstly, starting from the parameter optimization design of the blackbody surface microstructure, the means of improving the emissivity is transformed from qualitative research to quantitative calculation. The optimal design parameters for wide-spectrum emissivity design are obtained by optimizing the microstructure of the radiating surface of the on-board calibration blackbody source, and then carbon-based ultra-black coating material is selected to further improve the spectral emissivity of the blackbody source. The microstructure optimization junction and the superblack coating of the blackbody source on the star are optimized to increase the emissivity of the blackbody source on the star from 5 μm to 50 μm in the spectral range to 0.986, in which the ultra-wide spectrum is realized. Depending on the superblack coating, each material has different intrinsic radiation ability in different bands, and the coating is a composite porous structure, which makes it have high emission ability in a wide band. Secondly, by careful selection of blackbody source temperature controller and optimal design of temperature control system, the temperature measurement accuracy and stability of on-board calibration source are improved. The MF61 series film NTC thermistor is embedded on the blackbody surface to monitor the temperature of the blackbody surface in real time. The Thermoelectric Cooler (TEC) is a heating and cooling device that has been developed utilizing the Seebeck effect; The high-precision temperature measuring device is capable of accurately and effectively measuring the real-time surface temperature of a blackbody. It utilizes the high-precision voltage reference REF6225 and precision metal resistance current limiting to provide constant current output for NTC as a constant current source, while employing the precision digital-to-analog conversion chip ADS124S08 and four-wire resistance temperature detector for temperature measurement. Using Ohm's law, the voltage value is converted and fed back to the high precision temperature measurement circuit. The temperature measuring circuit sends control instructions to TEC drive current through comparison and analysis with the actual required temperature. The power management chip TPS63020 is used as controller, which can pull and inject current to drive TEC, and then heats up or cool down TEC to control the blackbody surface temperature. The temperature stability of the on-board calibration blackbody source reaches 0.16 K. Finally, the design and process reliability of the blackbody source are tested by comprehensive parameter measurement and evaluation. The design results are verified by measuring the emissivity of the blackbody source and the temperature stability of the blackbody source in the vacuum environment. The test results show that the spectral range of ultra-wide spectral calibration blackbody source for deep space detection is 5 μm to 50 μm, the normal average emissivity is 0.986, and the temperature stability reaches 0.16 K. The blackbody source will greatly improve the spectral range of high-emissivity radiometric calibration sources, and provide basic support for on-orbit high-precision radiometric calibration of deep space exploration loads. For small objects in deep space exploration, the radiation temperature is about 100 K to 420 K. According to Wien's displacement law, deep space exploration targets cover the ultra-wide spectral range of 5 μm to 50 μm, and high-precision on-board calibration is the key to the accurate detection of deep space exploration loads. Therefore, it is necessary to develop an on-board calibration source with high emissivity and high temperature stability of ultra-wide spectrum. Aiming at the technical problems of detecting ultra-wide spectrum, high reflectivity and high temperature stability of radiometric calibration blackbody source on ultra-wide spectrum stars in deep space, this paper carries out in-depth research and experimental verification from several aspects such as simulation, key component design and sample detection methods, so as to ensure the engineering realization and application of high-index precision radiometric calibration blackbody source on stars. COMSOL Multiphysics software simulation based on finite element method, microstructure design of blackbody source, emittance detection method of blackbody source and radiation calibration test in laboratory were studied.Firstly, starting from the parameter optimization design of the blackbody surface microstructure, the means of improving the emissivity is transformed from qualitative research to quantitative calculation. The optimal design parameters for wide-spectrum emissivity design are obtained by optimizing the microstructure of the radiating surface of the on-board calibration blackbody source, and then carbon-based ultra-black coating material is selected to further improve the spectral emissivity of the blackbody source. The microstructure optimization junction and the superblack coating of the blackbody source on the star are optimized to increase the emissivity of the blackbody source on the star from 5 μm to 50 μm in the spectral range to 0.986, in which the ultra-wide spectrum is realized. Depending on the superblack coating, each material has different intrinsic radiation ability in different bands, and the coating is a composite porous structure, which makes it have high emission ability in a wide band. Secondly, by careful selection of blackbody source temperature controller and optimal design of temperature control system, the temperature measurement accuracy and stability of on-board calibration source are improved. The MF61 series film NTC thermistor is embedded on the blackbody surface to monitor the temperature of the blackbody surface in real time. The Thermoelectric Cooler (TEC) is a heating and cooling device that has been developed utilizing the Seebeck effect; The high-precision temperature measuring device is capable of accurately and effectively measuring the real-time surface temperature of a blackbody. It utilizes the high-precision voltage reference REF6225 and precision metal resistance current limiting to provide constant current output for NTC as a constant current source, while employing the precision digital-to-analog conversion chip ADS124S08 and four-wire resistance temperature detector for temperature measurement. Using Ohm's law, the voltage value is converted and fed back to the high precision temperature measurement circuit. The temperature measuring circuit sends control instructions to TEC drive current through comparison and analysis with the actual required temperature. The power management chip TPS63020 is used as controller, which can pull and inject current to drive TEC, and then heats up or cool down TEC to control the blackbody surface temperature. The temperature stability of the on-board calibration blackbody source reaches 0.16 K. Finally, the design and process reliability of the blackbody source are tested by comprehensive parameter measurement and evaluation. The design results are verified by measuring the emissivity of the blackbody source and the temperature stability of the blackbody source in the vacuum environment. The test results show that the spectral range of ultra-wide spectral calibration blackbody source for deep space detection is 5 μm to 50 μm, the normal average emissivity is 0.986, and the temperature stability reaches 0.16 K. The blackbody source will greatly improve the spectral range of high-emissivity radiometric calibration sources, and provide basic support for on-orbit high-precision radiometric calibration of deep space exploration loads.
Acta Photonica Sinica
- Publication Date: Feb. 25, 2024
- Vol. 53, Issue 2, 0212003 (2024)
Temperature Compensation Method for Optical Voltage Sensing Based on Temperature Field and D-Kalman Parameter Estimation
Shengshuo CHEN, Yansong LI, Dongxu CHEN, Shijia KANG, Zhiguang XU, and Jun LIU
Optical voltage sensors based on electro-optical materials have many advantages such as wide measurement band, fast response and small size, which can realize the non-contact measurement of grid voltage, and how to improve the measurement accuracy of optical voltage sensors has become an urgent problem. Temperature stability has become one of the important factors affecting the measurement accuracy of optical voltage sensors. When considering the effect of temperature on the sensing unit electro-optical crystal, two problems are faced: first, there is a temperature gradient in the electro-optical crystal when the temperature changes, resulting in unequal temperature between the crystal surface and the internal optical path; second, the physical parameters of the crystal are also affected by the temperature. Therefore, the bismuth germanate crystal is used as the research object in this paper, and the output response equation of the optical voltage sensor under multi-physics field is analyzed in combination with the previously derived one. In the output response equation, it is concluded that the temperature drift is a low-frequency component and the applied voltage is a high-frequency component, so that the output signal of the sensor is separated from the AC signals and DC signals. However, after adding the temperature variation parameter to the output response equation, the correction result shows that the temperature variation parameter and the refractive index parameter of the crystal affected by temperature also exist in the high-frequency component, so it is necessary to estimate the states of these two parameters. Considering that electro-optical crystals are optical materials, the internal temperature can not be measured directly by destroying the crystal, so it is necessary to establish a relationship between the surface temperature and the internal temperature to calculate the internal temperature indirectly. A semi-analytic method is first used to establish the crystal transient temperature field model, and the direct substitution of the measured surface temperature data to obtain the internal temperature will introduce a large amount of noise error, so the state estimation of the internal temperature of the crystal is realized by Kalman filtering. The refractive index parameters of the crystal queried through the literature are limited by light wavelength or temperature and can not be adapted to the time-varying environment. Therefore, the crystal refractive index at initial temperature is estimated by the central differential Kalman filter combined with the low frequency component of the sensor output signal. Finally, the compensation voltage is calculated by substituting the correction parameters into the high-frequency component of the sensor output signal. In summary, a temperature compensation method based on temperature field and dual Kalman filter parameter estimation is proposed. The experimental results show that the simulation accuracy of the transient temperature field resolution formula is within 0.02% and the experimental measurement accuracy is about 0.2% under the environment where the sensor is exposed to an external temperature of [20 ℃,40 ℃] at a heating rate of 0.5 ℃/min, which verifies the correctness of the transient temperature field model construction. The relative error of the refractive index parameters of the crystal obtained by the central differential Kalman filter is 0.017 6% compared with the calculated results in the literature. The accuracy of the output voltage measurement is better than 0.52% using two correction parameters to compensate. The method improves the sensor measurement accuracy compared with the temperature compensation effect of the Back Propagation Neural Network under the same platform and the temperature compensation effect in the relevant literature. Optical voltage sensors based on electro-optical materials have many advantages such as wide measurement band, fast response and small size, which can realize the non-contact measurement of grid voltage, and how to improve the measurement accuracy of optical voltage sensors has become an urgent problem. Temperature stability has become one of the important factors affecting the measurement accuracy of optical voltage sensors. When considering the effect of temperature on the sensing unit electro-optical crystal, two problems are faced: first, there is a temperature gradient in the electro-optical crystal when the temperature changes, resulting in unequal temperature between the crystal surface and the internal optical path; second, the physical parameters of the crystal are also affected by the temperature. Therefore, the bismuth germanate crystal is used as the research object in this paper, and the output response equation of the optical voltage sensor under multi-physics field is analyzed in combination with the previously derived one. In the output response equation, it is concluded that the temperature drift is a low-frequency component and the applied voltage is a high-frequency component, so that the output signal of the sensor is separated from the AC signals and DC signals. However, after adding the temperature variation parameter to the output response equation, the correction result shows that the temperature variation parameter and the refractive index parameter of the crystal affected by temperature also exist in the high-frequency component, so it is necessary to estimate the states of these two parameters. Considering that electro-optical crystals are optical materials, the internal temperature can not be measured directly by destroying the crystal, so it is necessary to establish a relationship between the surface temperature and the internal temperature to calculate the internal temperature indirectly. A semi-analytic method is first used to establish the crystal transient temperature field model, and the direct substitution of the measured surface temperature data to obtain the internal temperature will introduce a large amount of noise error, so the state estimation of the internal temperature of the crystal is realized by Kalman filtering. The refractive index parameters of the crystal queried through the literature are limited by light wavelength or temperature and can not be adapted to the time-varying environment. Therefore, the crystal refractive index at initial temperature is estimated by the central differential Kalman filter combined with the low frequency component of the sensor output signal. Finally, the compensation voltage is calculated by substituting the correction parameters into the high-frequency component of the sensor output signal. In summary, a temperature compensation method based on temperature field and dual Kalman filter parameter estimation is proposed. The experimental results show that the simulation accuracy of the transient temperature field resolution formula is within 0.02% and the experimental measurement accuracy is about 0.2% under the environment where the sensor is exposed to an external temperature of [20 ℃,40 ℃] at a heating rate of 0.5 ℃/min, which verifies the correctness of the transient temperature field model construction. The relative error of the refractive index parameters of the crystal obtained by the central differential Kalman filter is 0.017 6% compared with the calculated results in the literature. The accuracy of the output voltage measurement is better than 0.52% using two correction parameters to compensate. The method improves the sensor measurement accuracy compared with the temperature compensation effect of the Back Propagation Neural Network under the same platform and the temperature compensation effect in the relevant literature.
Acta Photonica Sinica
- Publication Date: Feb. 25, 2024
- Vol. 53, Issue 2, 0212002 (2024)
Method for Refractive Index Uniformity Measurement Based on Two-flat and Three-flat Test
Zhiyao MA, Donghui ZHENG, Lei CHEN, and Jun MA
Traditionally, the interferometric method has been employed to test the refractive index uniformity of flat optical materials. The method mainly involves two approaches: the overturning method and the transmission method. Both methods require a minimum of three flat samples, including the reference flat, the transmission flat, and the sample flat to be tested. This requirement for multiple flats increases the overall cost of testing.We have proposed a novel method for evaluating the refractive index uniformity of transmission flats, which only requires two flats, offering a significant improvement over existing method. The method is realized by using two surfaces of the transmission flat and one surface of the reference flat, and the sample flat is used as the transmission flat. Based on the even-and-odd functions, we can get the even-odd, odd-even and odd-odd parts of the refractive index uniformity directly. To the even-even part, we resolve it as the even-even-even (eee) and even-even-odd (eeo) part and we can calculate the eeo part by rotate 90° measurement result. Then we resolve the eee part into even-even-even-even (eeee) and even-even-even-odd (eeeo) part and get eeeo part through the rotate 45° measurement result. Finally, by neglecting high-order rotational symmetry terms, we can calculate the refractive index uniformity from the obtained components. This method with its step-by-step decomposition providing an effective and efficient approach to characterize the optical properties of the flat under test.Four random wavefronts are generated as the initial three surface error and the refractive index uniformity errors. The uniformity errors are recovered according to the proposed method. Compared the difference between the recovered results and the initial values, that only 0.6 ppm (10-6) in differ. Additionally, the residual error exhibits rotation invariance, which aligns with theoretical expectations. It is important to note that in the low and middle frequency ranges, the residual error is not affected by the surface error of the flat, but solely determined by the refractive index uniformity itself.The theory is verified the experiment on a 100 mm Zygo interferometer and the refractive index uniformity of the same flat is measured by the three flat transmission method. The test flat is made by quartz. The surface error of the two surfaces are better than λ/10. The flat has a certain wedge angle. The front and the back surface will not generate interference fringes. The reference flat is a microcrystalline flat, better than λ/10 as well.The result of the two flat method have the same shape with the three flat transmission method. The refractive index uniformity obtained by the two flats methods is only 0.2 ppm different from that obtained by the transmission method, and the peak-valley value is 3 nm different. To ensure accuracy in the evaluation process, error influence is analyzed considering factors such as rotation angle errors and pix offset errors. By evaluating the influence of error, the experimentally verified results illustrate the effectiveness of this method in accurately evaluating the refractive index uniformity of flat optical materials. The proposed method demonstrates promising potential in the evaluation of refractive index uniformity, reducing resource requirements, and improving cost-effectiveness. Traditionally, the interferometric method has been employed to test the refractive index uniformity of flat optical materials. The method mainly involves two approaches: the overturning method and the transmission method. Both methods require a minimum of three flat samples, including the reference flat, the transmission flat, and the sample flat to be tested. This requirement for multiple flats increases the overall cost of testing.We have proposed a novel method for evaluating the refractive index uniformity of transmission flats, which only requires two flats, offering a significant improvement over existing method. The method is realized by using two surfaces of the transmission flat and one surface of the reference flat, and the sample flat is used as the transmission flat. Based on the even-and-odd functions, we can get the even-odd, odd-even and odd-odd parts of the refractive index uniformity directly. To the even-even part, we resolve it as the even-even-even (eee) and even-even-odd (eeo) part and we can calculate the eeo part by rotate 90° measurement result. Then we resolve the eee part into even-even-even-even (eeee) and even-even-even-odd (eeeo) part and get eeeo part through the rotate 45° measurement result. Finally, by neglecting high-order rotational symmetry terms, we can calculate the refractive index uniformity from the obtained components. This method with its step-by-step decomposition providing an effective and efficient approach to characterize the optical properties of the flat under test.Four random wavefronts are generated as the initial three surface error and the refractive index uniformity errors. The uniformity errors are recovered according to the proposed method. Compared the difference between the recovered results and the initial values, that only 0.6 ppm (10-6) in differ. Additionally, the residual error exhibits rotation invariance, which aligns with theoretical expectations. It is important to note that in the low and middle frequency ranges, the residual error is not affected by the surface error of the flat, but solely determined by the refractive index uniformity itself.The theory is verified the experiment on a 100 mm Zygo interferometer and the refractive index uniformity of the same flat is measured by the three flat transmission method. The test flat is made by quartz. The surface error of the two surfaces are better than λ/10. The flat has a certain wedge angle. The front and the back surface will not generate interference fringes. The reference flat is a microcrystalline flat, better than λ/10 as well.The result of the two flat method have the same shape with the three flat transmission method. The refractive index uniformity obtained by the two flats methods is only 0.2 ppm different from that obtained by the transmission method, and the peak-valley value is 3 nm different. To ensure accuracy in the evaluation process, error influence is analyzed considering factors such as rotation angle errors and pix offset errors. By evaluating the influence of error, the experimentally verified results illustrate the effectiveness of this method in accurately evaluating the refractive index uniformity of flat optical materials. The proposed method demonstrates promising potential in the evaluation of refractive index uniformity, reducing resource requirements, and improving cost-effectiveness.
Acta Photonica Sinica
- Publication Date: Feb. 25, 2024
- Vol. 53, Issue 2, 0212001 (2024)
Research on Large Field-of-view White Light Interferometry Measurement System and Performance
Tao LIU, Zhibin WANG, Jiaqi HU, Yaonan HE, Weichang JING, Enjing CHEN, Wenlong ZHOU, Guoming YU, Ning YANG, Di ZHAO, Guofeng ZHANG, and Shuming YANG
White Light Interferometry (WLI) is a classic low-coherence interferometry. The surface height information can be obtained with ultra-high precision through the positioning of zero optical path difference position of white light interference. At the same time, compared with point-sectioning laser confocal microscope, WLI is a surface-sectioning tomography technology. In particular, its vertical resolution is comparable to that of an atomic force microscope. White light interferometry measurement technology has been widely used in the inspection of semiconductor wafer defects, Micro-electromechanical System (MEMS) sensing structures, ultra-precision optical components, and film thicknesses. Generally, this technology originated in the late 1980 s in the United States, and domestic researches in this field started late, with the overall technology lagging behind. In terms of the instrumentation, although there have been breakthroughs, core components still rely on imports, such as interference objectives, nano-positioning scanners. A large field-of-view white light interferometric measurement system was built and tested. In this system, a domestic white light interference objective with a lateral magnification of 2 was used and a 0.5× adapter lens was preferably configured in the imaging system in front of the image detector. For the white LED illumination source, a suitable bandpass filter was theoretically evaluated and experimentally confirmed. The center wavelength was determined through the experimental curve of white light interference along the axial direction. The actual magnification of WLI system and the distortion of the field of view were achieved through imaging of a two-dimensional microscopic grid sample. The actual maximum field-of-view at the object side has reached 14 mm. By selectively filtering the spectrum of the white light source, the white light interference signal can be effectively modulated. According to this, the axial resolution and horizontal resolution can be changed within a certain range. For example, by changing the center wavelength of the incident light, the horizontal resolution can be changed according to the Rayleigh criterion. Experimental results show that: a more ideal white light interference axial response curve is obtained through suitable spectral filtering; the maximum field-of-view at the object side is as large as 14 mm; the measurement results of standard step samples with heights of 2.04 μm and 20.43 μm are 2.05 μm and 20.47 μm, and the repeatability (standard deviation) of 10 measurements is 12 nm and 16 nm, respectively. The measurement result of the 2.04 μm height step was also compared with the result obtained by the atomic force microscope. Actual measurements were conducted on the roughness sample, MEMS sensing structure and semiconductor wafer film, demonstrating the feasibility of the developed system in the field of three-dimensional optical non-destructive precision inspection. For large field-of-view WLI systems, the horizontal or lateral resolution is on the order of a few micrometers, so it is difficult to apply WLI to the three-dimensional reconstruction of fine microstructures. This is an important shortcoming of large field-of-view WLI systems. Besides, through research, it was found that for ultra-smooth surfaces, such as polished wafers, it is difficult to measure using traditional vertical scanning interferometry technology, and the phase shifting method should be used. Measurement of film thickness may be used to monitor morphological changes in biological transparent film layers. Further research can focus on high-performance white-light interference objectives, automation of the white-light interferometry measurement process, and implementation of large field-of-view high-resolution white-light interferometry methods. White Light Interferometry (WLI) is a classic low-coherence interferometry. The surface height information can be obtained with ultra-high precision through the positioning of zero optical path difference position of white light interference. At the same time, compared with point-sectioning laser confocal microscope, WLI is a surface-sectioning tomography technology. In particular, its vertical resolution is comparable to that of an atomic force microscope. White light interferometry measurement technology has been widely used in the inspection of semiconductor wafer defects, Micro-electromechanical System (MEMS) sensing structures, ultra-precision optical components, and film thicknesses. Generally, this technology originated in the late 1980 s in the United States, and domestic researches in this field started late, with the overall technology lagging behind. In terms of the instrumentation, although there have been breakthroughs, core components still rely on imports, such as interference objectives, nano-positioning scanners. A large field-of-view white light interferometric measurement system was built and tested. In this system, a domestic white light interference objective with a lateral magnification of 2 was used and a 0.5× adapter lens was preferably configured in the imaging system in front of the image detector. For the white LED illumination source, a suitable bandpass filter was theoretically evaluated and experimentally confirmed. The center wavelength was determined through the experimental curve of white light interference along the axial direction. The actual magnification of WLI system and the distortion of the field of view were achieved through imaging of a two-dimensional microscopic grid sample. The actual maximum field-of-view at the object side has reached 14 mm. By selectively filtering the spectrum of the white light source, the white light interference signal can be effectively modulated. According to this, the axial resolution and horizontal resolution can be changed within a certain range. For example, by changing the center wavelength of the incident light, the horizontal resolution can be changed according to the Rayleigh criterion. Experimental results show that: a more ideal white light interference axial response curve is obtained through suitable spectral filtering; the maximum field-of-view at the object side is as large as 14 mm; the measurement results of standard step samples with heights of 2.04 μm and 20.43 μm are 2.05 μm and 20.47 μm, and the repeatability (standard deviation) of 10 measurements is 12 nm and 16 nm, respectively. The measurement result of the 2.04 μm height step was also compared with the result obtained by the atomic force microscope. Actual measurements were conducted on the roughness sample, MEMS sensing structure and semiconductor wafer film, demonstrating the feasibility of the developed system in the field of three-dimensional optical non-destructive precision inspection. For large field-of-view WLI systems, the horizontal or lateral resolution is on the order of a few micrometers, so it is difficult to apply WLI to the three-dimensional reconstruction of fine microstructures. This is an important shortcoming of large field-of-view WLI systems. Besides, through research, it was found that for ultra-smooth surfaces, such as polished wafers, it is difficult to measure using traditional vertical scanning interferometry technology, and the phase shifting method should be used. Measurement of film thickness may be used to monitor morphological changes in biological transparent film layers. Further research can focus on high-performance white-light interference objectives, automation of the white-light interferometry measurement process, and implementation of large field-of-view high-resolution white-light interferometry methods.
Acta Photonica Sinica
- Publication Date: Jan. 25, 2024
- Vol. 53, Issue 1, 0112003 (2024)
Gallium Fixed-point Blackbody Radiation Source for Calibration of Sea Surface Temperature Radiometer
Kailin ZHANG, Zhiyuan ZHAO, Chuanyun REN, Minglun YANG, and Liqin QU
A gallium fixed-point blackbody infrared radiation source is designed to calibrate the sea surface temperature radiometer. Due to the fact that the sea surface temperature radiometer requires additional regular calibration data before and after deployment to monitor the performance characteristics of the instrument, water bath blackbodies and ammonia heat pipe blackbodies are commonly used internationally for calibration. However, water bath blackbodies are greatly affected by atmospheric pressure, and ammonia heat pipe blackbodies are large in volume and difficult to transport. Therefore, a gallium blackbody is designed for the calibration of sea surface temperature radiometers.Firstly, the structure and working principle of a gallium fixed-point blackbody radiation source are introduced. The simulation software, which is based on the Monte Carlo algorithm, then simulate the emissivity of STEEP321 blackbodies and conducted experiments to measure the emissivity. The source of emissivity uncertainty is discussed. Experiments on phase transformation reproducibility of gallium under electric heating and water-bath heating are carried out and the effect of electric heating power on phase transformation reproducibility of gallium is discussed. Finally, the measurement experiment of ISAR thermometer radiometer is carried out to verify the blackbody.The results show that the emissivity is better than 0.998 8, and the emissivity measured by the radiation method is in good agreement with the theoretical simulation results, and the measurement repeatability (standard deviation) reaches 0.1%. The results show that the reproducibility of phase change temperature table is better than ±0.03 K. The difference between ISAR and FLUKE 1524 measurements of the temperature near the blackbody cavity is within ±0.15 K.The blackbody is developed successfully and can be used to calibrate the sea surface temperature radiometer after modification, and it can provide calibration sources for the development of sea surface temperature measurement equipment with independent intellectual property rights. The theoretical calculation of emissivity is high, but the actual measurement of emissivity is lower than the theory for the following reasons: the ratio of black body cavity length to opening diameter is not large enough, and the uncertainty in the measurement process of emissivity. The craft used for the fixed-point container is generally to spray polytetrafluoroethylene film on its surface. Thicker polytetrafluoroethylene cylinder and polytetrafluoroethylene sleeve are installed with gallium fixed point, which greatly avoids the pollution caused by direct contact of metal when the sprayed polytetrafluoroethylene film is relatively thin. A gallium fixed-point blackbody infrared radiation source is designed to calibrate the sea surface temperature radiometer. Due to the fact that the sea surface temperature radiometer requires additional regular calibration data before and after deployment to monitor the performance characteristics of the instrument, water bath blackbodies and ammonia heat pipe blackbodies are commonly used internationally for calibration. However, water bath blackbodies are greatly affected by atmospheric pressure, and ammonia heat pipe blackbodies are large in volume and difficult to transport. Therefore, a gallium blackbody is designed for the calibration of sea surface temperature radiometers.Firstly, the structure and working principle of a gallium fixed-point blackbody radiation source are introduced. The simulation software, which is based on the Monte Carlo algorithm, then simulate the emissivity of STEEP321 blackbodies and conducted experiments to measure the emissivity. The source of emissivity uncertainty is discussed. Experiments on phase transformation reproducibility of gallium under electric heating and water-bath heating are carried out and the effect of electric heating power on phase transformation reproducibility of gallium is discussed. Finally, the measurement experiment of ISAR thermometer radiometer is carried out to verify the blackbody.The results show that the emissivity is better than 0.998 8, and the emissivity measured by the radiation method is in good agreement with the theoretical simulation results, and the measurement repeatability (standard deviation) reaches 0.1%. The results show that the reproducibility of phase change temperature table is better than ±0.03 K. The difference between ISAR and FLUKE 1524 measurements of the temperature near the blackbody cavity is within ±0.15 K.The blackbody is developed successfully and can be used to calibrate the sea surface temperature radiometer after modification, and it can provide calibration sources for the development of sea surface temperature measurement equipment with independent intellectual property rights. The theoretical calculation of emissivity is high, but the actual measurement of emissivity is lower than the theory for the following reasons: the ratio of black body cavity length to opening diameter is not large enough, and the uncertainty in the measurement process of emissivity. The craft used for the fixed-point container is generally to spray polytetrafluoroethylene film on its surface. Thicker polytetrafluoroethylene cylinder and polytetrafluoroethylene sleeve are installed with gallium fixed point, which greatly avoids the pollution caused by direct contact of metal when the sprayed polytetrafluoroethylene film is relatively thin.
Acta Photonica Sinica
- Publication Date: Jan. 25, 2024
- Vol. 53, Issue 1, 0112002 (2024)
Oil Mist Concentration On-line Measurement by Optical Scintillation Method
Guangyao LIN, Fei HU, Zhongli JI, and Zhen LIU
In order to evaluate the efficiency and performance of the natural gas compressor outlet oil separator, a concentration detection method of micro-nano-scale oil mist droplets based on the light scintillation theory was proposed for the low-concentration lubricating oil mist downstream of the compressor outlet oil mist separator under the condition of 0~50 mg·m-3 low concentration oil mist concentration. Based on the particle phylolayer model, Lambert-Beer law and Poisson distribution, the theoretical model uses the total scattering theory to derive the particle concentration calculation model under the long-optical path parallel uniform beam by segmenting the measuring volume in the beam field and taking the transmittance of each monolayer measuring volume as the statistical random variable of Poisson distribution, and explains the causes and influencing factors of the optical scintillation method.For compressor lubricants in the natural gas industry, ISO viscosity grade 460 lubricants were selected, and spectral scanning and Principal Component Analysis (PCA) determined that the 400 nm wavelength beam had the best detection sensitivity and positive correlation in the visible light band. At the same time, the infrared spectrum and three-dimensional fluorescence spectrum of lubricating oil show that the infrared band has a strong absorption effect in hydrocarbon gas medium, and the fluorescence effect of lubricating oil in the ultraviolet band will aggravate the stray light in the optical path and affect the test accuracy. For the flicker frequency in the optical scintillation method, the selection of the flicker frequency is determined by the spectral change measurement, and the selection of the flicker frequency affects the detection accuracy by affecting the fitting variance of the scintillation signal, and the flicker frequency is determined according to the size of the measuring body, particle diameter and particle velocity.The results show that in the range of 0~50 mg·m-3 oil mist concentration, there is an obvious linear relationship between oil mist concentration and flicker signal variance under the appropriate flickering frequency by optical scintillation method, and the linear correlation coefficient R2=0.989. In the calibration test results, the linear correlation coefficient R2=0.989, and the test error is less than 10%. However, the error of the long-path light transmission method detection calibration results gradually decreases with the concentration, and the linear correlation coefficient R2=0.956. Compared with the long-path optical transmission method, the error of the optical flicker method is lower than that of the optical transmission method at very low concentration, and with the increase of the calibration concentration, the test error of the optical transmission method gradually decreases and is comparable to that of the optical flicker method. The above analysis shows that the optical flicker theory can realize the real-time online detection of oil mist concentration at low concentration, and the method is not affected by optical window pollution, and the optical structure is simple, which is expected to be used for real-time online monitoring of compressor outlet under high pressure in the future. In order to evaluate the efficiency and performance of the natural gas compressor outlet oil separator, a concentration detection method of micro-nano-scale oil mist droplets based on the light scintillation theory was proposed for the low-concentration lubricating oil mist downstream of the compressor outlet oil mist separator under the condition of 0~50 mg·m-3 low concentration oil mist concentration. Based on the particle phylolayer model, Lambert-Beer law and Poisson distribution, the theoretical model uses the total scattering theory to derive the particle concentration calculation model under the long-optical path parallel uniform beam by segmenting the measuring volume in the beam field and taking the transmittance of each monolayer measuring volume as the statistical random variable of Poisson distribution, and explains the causes and influencing factors of the optical scintillation method.For compressor lubricants in the natural gas industry, ISO viscosity grade 460 lubricants were selected, and spectral scanning and Principal Component Analysis (PCA) determined that the 400 nm wavelength beam had the best detection sensitivity and positive correlation in the visible light band. At the same time, the infrared spectrum and three-dimensional fluorescence spectrum of lubricating oil show that the infrared band has a strong absorption effect in hydrocarbon gas medium, and the fluorescence effect of lubricating oil in the ultraviolet band will aggravate the stray light in the optical path and affect the test accuracy. For the flicker frequency in the optical scintillation method, the selection of the flicker frequency is determined by the spectral change measurement, and the selection of the flicker frequency affects the detection accuracy by affecting the fitting variance of the scintillation signal, and the flicker frequency is determined according to the size of the measuring body, particle diameter and particle velocity.The results show that in the range of 0~50 mg·m-3 oil mist concentration, there is an obvious linear relationship between oil mist concentration and flicker signal variance under the appropriate flickering frequency by optical scintillation method, and the linear correlation coefficient R2=0.989. In the calibration test results, the linear correlation coefficient R2=0.989, and the test error is less than 10%. However, the error of the long-path light transmission method detection calibration results gradually decreases with the concentration, and the linear correlation coefficient R2=0.956. Compared with the long-path optical transmission method, the error of the optical flicker method is lower than that of the optical transmission method at very low concentration, and with the increase of the calibration concentration, the test error of the optical transmission method gradually decreases and is comparable to that of the optical flicker method. The above analysis shows that the optical flicker theory can realize the real-time online detection of oil mist concentration at low concentration, and the method is not affected by optical window pollution, and the optical structure is simple, which is expected to be used for real-time online monitoring of compressor outlet under high pressure in the future.
Acta Photonica Sinica
- Publication Date: Jan. 25, 2024
- Vol. 53, Issue 1, 0112001 (2024)
3D Object Detection with Fusion Point Attention Mechanism in LiDAR Point Cloud
Weili LIU, Deli ZHU, Huahao LUO, and Yi LI
With the rapid development of computer vision, object detection has made remarkable achievements in 2D vision tasks, but it still can not solve the problems such as light changes and lack of depth that occur in actual scenes. The 3D data acquired by LiDAR makes up for some defects existing in the 2D vision field, so 3D object detection is widely studied as an important field in 3D scene perception. 3D object detection in the field of autonomous driving is an important part of intelligent transportation, and the 3D object detection algorithm based on LiDAR point cloud provides an important perception means for it. Perception is a key component of autonomous driving, ensuring the intelligence and safety of driving. 3D object detection refers to the detection of physical objects from sensor data, predicting and estimating the category, bounding box, and spatial position of the target. However, due to the unstructured and non-fixed size characteristics of point clouds, they can not be directly processed by 3D object detectors and must be encoded into a more compact structure through some form of expression. There are currently two main types of expressions: point-based and voxel-based methods. Voxel-based methods have higher detection efficiency, but their detection accuracy is lower than that of methods based on raw point clouds. Therefore, how to improve the detection accuracy of voxel-based methods while ensuring detection efficiency has become a research hotspot in recent years.In view of the problems of loss of fine-grained information and insufficient ability to extract point cloud features in the 3D object detection algorithm for Pillar-encoded point clouds, this paper proposes a 3D object detection algorithm based on PointPillars that integrates point-wise spatial attention mechanism and CSPNet. Firstly, the point-wise spatial attention mechanism is integrated into the pillar feature network layer, which can enhance the network's ability to extract local geometric information and retain deep-level information, making the obtained key features more suitable for detection tasks. Point-wise spatial attention follows the basic structure of self-attention, which can effectively avoid the impact of redundant point clouds or noise points on features, strengthen the description of features with less coverage of point clouds, and to a certain extent solve the problem of information loss based on Pillar-encoded point clouds. Secondly, replacing the ordinary convolution in the downsampling module that extracts high-dimensional features from pseudo-images of point clouds with CSPNet can achieve gradient flow segmentation, further enhance the network's learning ability while reducing computational complexity, and improve model detection accuracy.Finally, the algorithm in this paper improves the 3D detection accuracy by 2.23%, 2.25%, and 2.30% in easy, medium, and hard cases, respectively, compared with the benchmark network under the application scenario of highway with car class in KITTI as the detection target. The experimental results show that the algorithm in this paper has significantly improved the detection performance, while the detection speed reaches the real-time detection level, which has some positive significance for the optimization and improvement of autonomous driving technology, and has great potential in the application scenario of highways. With the rapid development of computer vision, object detection has made remarkable achievements in 2D vision tasks, but it still can not solve the problems such as light changes and lack of depth that occur in actual scenes. The 3D data acquired by LiDAR makes up for some defects existing in the 2D vision field, so 3D object detection is widely studied as an important field in 3D scene perception. 3D object detection in the field of autonomous driving is an important part of intelligent transportation, and the 3D object detection algorithm based on LiDAR point cloud provides an important perception means for it. Perception is a key component of autonomous driving, ensuring the intelligence and safety of driving. 3D object detection refers to the detection of physical objects from sensor data, predicting and estimating the category, bounding box, and spatial position of the target. However, due to the unstructured and non-fixed size characteristics of point clouds, they can not be directly processed by 3D object detectors and must be encoded into a more compact structure through some form of expression. There are currently two main types of expressions: point-based and voxel-based methods. Voxel-based methods have higher detection efficiency, but their detection accuracy is lower than that of methods based on raw point clouds. Therefore, how to improve the detection accuracy of voxel-based methods while ensuring detection efficiency has become a research hotspot in recent years.In view of the problems of loss of fine-grained information and insufficient ability to extract point cloud features in the 3D object detection algorithm for Pillar-encoded point clouds, this paper proposes a 3D object detection algorithm based on PointPillars that integrates point-wise spatial attention mechanism and CSPNet. Firstly, the point-wise spatial attention mechanism is integrated into the pillar feature network layer, which can enhance the network's ability to extract local geometric information and retain deep-level information, making the obtained key features more suitable for detection tasks. Point-wise spatial attention follows the basic structure of self-attention, which can effectively avoid the impact of redundant point clouds or noise points on features, strengthen the description of features with less coverage of point clouds, and to a certain extent solve the problem of information loss based on Pillar-encoded point clouds. Secondly, replacing the ordinary convolution in the downsampling module that extracts high-dimensional features from pseudo-images of point clouds with CSPNet can achieve gradient flow segmentation, further enhance the network's learning ability while reducing computational complexity, and improve model detection accuracy.Finally, the algorithm in this paper improves the 3D detection accuracy by 2.23%, 2.25%, and 2.30% in easy, medium, and hard cases, respectively, compared with the benchmark network under the application scenario of highway with car class in KITTI as the detection target. The experimental results show that the algorithm in this paper has significantly improved the detection performance, while the detection speed reaches the real-time detection level, which has some positive significance for the optimization and improvement of autonomous driving technology, and has great potential in the application scenario of highways.
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0912002 (2023)
Highly Reflective Surface Shape Measurement Technology Based on Multiple Viewpoints and Fringes
Zhizhuo WANG, and Rongshen LU
Nowadays, three-dimensional measurement of highly reflective surface is an important part of industrial measurement. With the continuous development of automobile assembly, precision polishing, free-form surface processing and other industries, the demand for measurement of large size, large curvature and high reflective industrial parts such as aircraft tail, LCD panel, automobile body shell and windshield is increasingly strong. However, due to the large difference between the optical characteristics of the surface and the diffuse reflection object, it is difficult to achieve the ideal effect by the traditional three-dimensional measurement method.The phase deflection method is a non-contact high reflection surface 3D measurement method developed in recent years. Its high sensitivity and easy correction of system errors make it popular with many researchers. Generally, there are two main methods for the above measurement using deflection method: one is to improve the size of the screen and the field of view of the camera, but it is limited by accuracy and size. The other common method is to install a single-view system on the manipulator, and reconstruct a large size surface during the movement. However, the introduced manipulator system error reduces the accuracy, and the system mostly outputs the relative point cloud coordinates based on the gradient data, so the point cloud needs to be continuously spliced through the point cloud registration algorithm during the scanning process, so the calculation cost is high; however, this paper sets the camera array, and at the same time carries out the reconstruction based on absolute coordinates from multiple angles. It can effectively ensure the accuracy of local measurement, and realize accurate measurement of large size, large curvature and high reflection surface faster through the direct splicing of visual angles.In this paper, the LCD screen is used as the reference plane, and the camera array is used to realize the accurate reconstruction of the highly reflective surface shape based on multi-viewpoints. First, the mirror image of the reference plane target in the standard plane mirror is obtained using each camera, the external parameters of the reference plane and each camera through mirror calibration are obtained, and then the camera array is globally calibrated using the uniqueness of the reference plane. Then, the phase-shifting fringe are projected using the display screen to the measured surface, and the reflected modulated images are collected by each camera to obtain the absolute point cloud coordinates of the surface to be measured relative to each viewpoint. Finally, through global stitching, a larger size and higher curvature highly reflective surface shape measurement can be achieved compared to a single viewpoint system. At the same time, based on the planarity of the reference plane and the standard plane mirror, this paper proposes a coplanar constraint to optimize the geometric parameters and global parameters of the viewpoint and reduce the system error. The validity and accuracy of this method are verified by measuring the standard mirrors with different shapes. Nowadays, three-dimensional measurement of highly reflective surface is an important part of industrial measurement. With the continuous development of automobile assembly, precision polishing, free-form surface processing and other industries, the demand for measurement of large size, large curvature and high reflective industrial parts such as aircraft tail, LCD panel, automobile body shell and windshield is increasingly strong. However, due to the large difference between the optical characteristics of the surface and the diffuse reflection object, it is difficult to achieve the ideal effect by the traditional three-dimensional measurement method.The phase deflection method is a non-contact high reflection surface 3D measurement method developed in recent years. Its high sensitivity and easy correction of system errors make it popular with many researchers. Generally, there are two main methods for the above measurement using deflection method: one is to improve the size of the screen and the field of view of the camera, but it is limited by accuracy and size. The other common method is to install a single-view system on the manipulator, and reconstruct a large size surface during the movement. However, the introduced manipulator system error reduces the accuracy, and the system mostly outputs the relative point cloud coordinates based on the gradient data, so the point cloud needs to be continuously spliced through the point cloud registration algorithm during the scanning process, so the calculation cost is high; however, this paper sets the camera array, and at the same time carries out the reconstruction based on absolute coordinates from multiple angles. It can effectively ensure the accuracy of local measurement, and realize accurate measurement of large size, large curvature and high reflection surface faster through the direct splicing of visual angles.In this paper, the LCD screen is used as the reference plane, and the camera array is used to realize the accurate reconstruction of the highly reflective surface shape based on multi-viewpoints. First, the mirror image of the reference plane target in the standard plane mirror is obtained using each camera, the external parameters of the reference plane and each camera through mirror calibration are obtained, and then the camera array is globally calibrated using the uniqueness of the reference plane. Then, the phase-shifting fringe are projected using the display screen to the measured surface, and the reflected modulated images are collected by each camera to obtain the absolute point cloud coordinates of the surface to be measured relative to each viewpoint. Finally, through global stitching, a larger size and higher curvature highly reflective surface shape measurement can be achieved compared to a single viewpoint system. At the same time, based on the planarity of the reference plane and the standard plane mirror, this paper proposes a coplanar constraint to optimize the geometric parameters and global parameters of the viewpoint and reduce the system error. The validity and accuracy of this method are verified by measuring the standard mirrors with different shapes.
Acta Photonica Sinica
- Publication Date: Sep. 25, 2023
- Vol. 52, Issue 9, 0912001 (2023)
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