Abstract:At present, the stability and uncertainty of optical atomic clocks have reached the level of 10 -18 , making them the most precise time-frequency measurement tools. Optical atomic clocks demonstrate potential for advanced scientific research in precision measurements and fundamental physics, potentially redefining the unit of time “second”. Among optical clocks, ytterbium (Yb) atomic clocks stand out due to their unique energy level advantages, making them one of the most mature and extensively researched optical clock candidates worldwide. The absolute frequency measurement of the ytterbium atomic clock transition and the precise measurement of the ytterbium atomic correlation transition spectrum are essential. This article provides an overview of the domestic and international progress in absolute frequency measurement of 6s 21 S0 -6s6p 3 P0 clock transition of cold ytterbium atomic clocks. The experiment measures the absolute frequency of Ytterbium atomic clock transition with an uncertainty of 7. 3×10 -16 is introduced whose measured value is 518 295 836 590 863. 30± 0. 38 Hz. Additionally, we present the results of precision absolute frequency measurements of relevant repumping transitions at 649, 770, and 1 389 nm, which use the absolute-frequency-measured ytterbium atomic clocks as a reference.

Abstract:Microwave atomic clocks based on trapped ions have advantages in miniaturization and mobility. Therefore, they have attracted widespread attention. Among them, the 113Cd + ion has become the main candidate ions for the new generation of practical ion microwave clocks due to its advantages of low laser requirements and large ground-state hyperfine splitting frequency. In recent decades, multiple experimental teams at home and abroad have conducted research on cadmium-ion microwave clocks. This article provides a detailed introduction in chronological order of the technical routes and the latest developments adopted by different experimental teams at home and abroad in the research of cadmium-ion microwave clocks.

Abstract:In recent years, with the rapid development of fiber manufacturing technology and femtosecond laser technology, the femtosecond optical frequency combs (FOFCs) technology represented by Er-FOFCs has gradually advanced the field of optical frequency measurement. It is playing an increasingly important role in length measurement, precision spectral analysis, ultra-low phase noise microwave frequency generation, precision time-frequency transfer, temperature measurement, and many other fields. It has become an important fundamental tool in high-end scientific research. However, the important problem to be solved by the FOFCs is to measure the optical frequency. This article mainly focuses on the demand for measuring optical frequency parameters. An optical frequency comb based on the Er-doped fiber femtosecond laser has been developed. With the stable operation of the Er-FOFC, the nonlinear optical frequency conversion is used to transfer the spectrum from the central wavelength of the Er-FOFC to other wavelengths. The beat note between the broadened spectrum in the Er-FOFC and the measured laser is detected. The Er-FOFC has a spectral range of 500 ~ 2 000 nm, frequency instability and accuracy of the level of 10 -16 , and a Hz-level comb tooth linewidth. These parameters meet the requirement for measuring laser frequency. The reported Er-FOFC provides a fundamental measurement tool for optical frequency, frequency drift, frequency linewidth, and other parameters.

Abstract:Time is the physical quantity with the best measurement uncertainty at present, representing the highest level of science and technology. After decades of development, fiber-optic time and frequency synchronization (FOTFS) technology has been widely used in many fields such as quantum metrology, radio interferometry, navigation, modern communication, power grid, high-energy physics, geodesy and so on. It has become a vital support for the efficient operation of human society. Thanks to the high reliability and stability of FOTFS technology, the long-standing contradiction between “distribution” and “real-time” of ensemble time scale has been solved. The paper introduces the development history of FOTFS technology and the research status of each technology path. At the same time, a real-time free-running time scale based on the Beijing FOTFS network is also introduced. Based on this foundation, it is pointed out that the development focus of FOTFS is shifting from technical research to large-scale networked applications. Conducting multi-functional and multi-application integrated research based on optical networks, to achieve integrated communication, sensing, computing, measurement, and control, will be an important development trend in the future.

Abstract:Narrow-linewidth external cavity diode lasers (ECDLs) have the advantages of compact structure, tunable wavelength, and low noise. They are widely used in quantum precision measurement, optical communication, laser radar, and other fields. In this article, four types of narrow linewidth ECDL using different frequency selection devices are introduced, including grating-ECDL, interference filtering ECDL, waveguide-ECDL, and Faraday laser. The article presents the basic structure and frequency selection mechanism, advantages and disadvantages of the ECDLs, as well as their international research progress. The first three types of ECDLs use non-quantum devices for frequency selection, while Faraday lasers utilize the resonant Faraday optical rotation effect for frequency selection. Therefore, the output wavelength corresponds to the atomic transition line directly and has good robustness against the fluctuations of the diode temperature and current as well. Then, the applications of these ECDLs are introduced, especially their typical applications in precision measurement. Finally, the future development of narrow linewidth ECDL is summarized and prospected.

Abstract:With the advancements in laser technology and improvements of the operation robustness, the fountain clocks can not only be used as frequency standards to calibrate atomic time scales, and improve the long-term stability and accuracy, but also be used to steer a hydrogen maser as a time-keeping clock. The ground state hyperfine transition of rubidium atoms, as a secondary representation of the second definition, has the advantages of small collision cross-sections and highly robust cooling lasers. It is a better candidate for commercial clocks. The prototype of a commercial rubidium fountain clock developed at the National Institute of Metrology (NIM) with a bi-metal microwave-vacuum integrated Ramsey cavity to increase the temperature adaptability of the system. A miniaturized optical system and a cage structure detection light system to improve operational reliability. A long-term quasi-continuous operation with an operation rate of 97. 5% in a year is realized. Though the one-year comparson tese, the long-term fractional frequency instability achieves 3. 7×10 -16 for the comparisons between the Rb fountain clock and NIM5 cesium fountain clock. The frequency instability of a single fountain clock is better than 2. 6×10 -16 .

Abstract:Strontium optical lattice clock plays a key role in fundamental physics research and precision measurement of time and frequency. Based on the research of its first strontium optical clock, NIM-Sr1, the National Institute of Metrology(NIM) has conducted in-depth study to enhance the performance of its optical clocks, designed and built the second strontium optical lattice clock, NIM-Sr2. Benefited from the experiences gained in the quantum reference preparation, clock transition interrogation and systematic frequency shift evaluation of NIM-Sr1, the physics apparatus of NIM-Sr2 is redesigned and improved. A vacuum differential pumping stage is added between the atomic oven and the magneto-optical trap (MOT) chamber, which reduces the MOT chamber pressure variation to 1×10 -8 Pa when the oven is switched between on and off. By replacing the coils in Zeeman slower with permanent magnets, optimizing the winding of the water-cooled anti-Helmholtz coils, and extending the viewport for Zeeman slowing laser beam, the inhomogeneity of the ambient temperature in the MOT region is reduced to 0. 166 K. The frequency evaluation revealed that these improvements significantly reduced the systematic shift uncertainty of NIM-Sr2 to 7. 2×10 -18 .

Abstract:The atomic clock, as the most precise timing instrument at present, is playing an important role in various fields of national economy and security. By operating atomic clocks on earth orbit satellites, many high-precision time synchronization and comparisonrelated applications can be implemented on a large scale on the earth. Unlike previous space cold atomic clocks that achieve microwave and atoms interaction by launching cold atoms cloud to traveling two microwave cavities, this article proposes a new space cold atomic microwave clock scheme based on in situ detection in the cavity. This scheme completes the laser cooling of rubidium 87 atoms, atoms and microwave interaction, cold atom detection, and other clock processes in the same microwave resonant cavity. It is possible to more fully utilize the microgravity environment to increase the duty cycle of the interaction time between atoms and microwaves in the clock cycle. Therefore, the impact of the Dick effect on the performance of atomic clocks is reduced. This article introduces the design and working principle of this kind of cold atoms microwave clock system, provides performance analysis and expected frequency stability in a microgravity environment, and finally shows that in the ground test results of the clock prototype. The tested 1. 35×10 -12 τ -1/ 2 frequency stability of the cold atom microwave clock demonstrates the potential performance of the clock operating in microgravity environments.

Abstract:With the rapid development of ion optical clock technology, the impact of the micro-motion effect caused by the radio frequency field used to trap ions in ion optical clock on its performance is becoming more and more negligible. In order to completely eliminate this effect, an experimental scheme of all-optical trapping ion optical clock was proposed. In view of this experimental scheme, an ion trapping device for all-optical trapping of calcium ions was designed and constructed. The device adopts a blade ion trap design, which can realize high-precision stray electric field compensation with a residual force of only on the order of 10 -20 N, and has a clearance aperture of 6 mm, which can well meet the passing requirements of dipole trapping laser in all-optical trapping experiments, and the radio frequency (RF) system based on the improved helical resonator design can achieve stable coupling at a low RF frequency of 9. 33(1) MHz, and can realize long-term ion trapping by combining vacuum preparation of up to 10 -9 Pa and vacuum chamber with conductive glass window. It provides an experimental basis for all-optical trapping of calcium ions, and is of great significance for the realization of optical trapping ion optical clocks.

Abstract:The article introduces a new compact cesium beam clock with a non-uniform magnetic field for state selection and a laser that resonates with D2 4-5 line for detection. The basic structure which contains the physical system, the optical system, and the servo system as well as the operation principle of the clock, is described. A 4U size prototype with stability of 4. 0×10 -13@ 100 s, 4. 5×10 -14@ 10 000 s and 2. 2×10 -14@ 1 d is realized, which is better than high performance type 5071A cesium beam clock. The clock is compared with the conventional cesium beam clock and the optically pumped cesium beam clock. The advantages and disadvantages of the clock are analyzed. Two feasible improved schemes are introduced, which are expected to further improve the stability to better than 3×10 -13@ 100 s. One scheme uses a laser that resonates with D2 3-2 line for detection. The other scheme uses optical pumping and non-uniform magnetic field together for state preparation.

Abstract:Coherent population trapping ( CPT) atomic clock has irreplaceable advantages in miniaturization. The mid-long-term frequency stability of vapor cell-based CPT clock is limited by buffer gas collision and broadening. To overcome these limitations, the laser-cooling atom technique is an alternative method to improve the mid-long-term performance. However, the conventional cold atom system remains relatively complicate. A MOT based on planar elements is proposed, which consists of grating chip, coils chip, and a compact vacuum chamber. The 10 6 cold atoms are trapped with a single beam. Moreover, a laser stabilized on the Rb D2 line combined with time-multiplexed frequency shifting is proposed to realize laser cooling and CPT interrogation with a single laser. This work evaluates the potential for developing a miniature / compact high-performance cold atom CPT clock.

Abstract:The independence of time and frequency standards is an important basis for the comprehensive PNT system construction in China. To further evaluate the timekeeping capability of the atomic time scale based on the national primary frequency standard, the improved FIR clock difference prediction and frequency control method is used to post-process the frequency difference of the cesium atomic fountain clock relative to the hydrogen maser. By post-processing, the autonomous time scale and traceable time scale are generated respectively. The two-time scales are compared with UTC by the international atomic cooperation link. The long-term experimental results show that both of the time differences between the two-time scales and UTC are less than ±4 ns and the frequency stability is less than 8×10 -16 / 5 d from December 2021 to December 2022.

Abstract:Optical clocks have great potential to generatie high-performance atomic time scales. This paper introduces the basic idea of generating a local time scale steered to NIM-Sr1 optical lattice clock of the National Institute of Metrology (NIM) , and the evaluation of the noise parameters of the HM57 hydrogen maser which is currently used as the flywheel oscillator. When NIM-Sr1 is operated as a reference clock, it runs consecutively with interrupts. A post-processing steering algorithm is designed to match this operation condition with the comparison data between NIM-Sr1 and HM57 in September and October 2022. The post-processing time scale TS(P) steered to NIM-Sr1 has a maximum time deviation of 0. 7 ns compared with TT(BIPM22) within 60 days. Moreover, a system with dedicated hardware and software is built to generate a real-time physical time scale by steering HM57 to NIM-Sr1. The generated real-time physical time scale TS(R) in April 2023 is evaluated with a maximum time deviation of 0. 89 ns compared with UTC within 30 days. Keyword

Abstract:Optical atomic clocks have been improved greatly in recent years. The Consultative Committee for Time and Frequency (CCTF) proposes the redefinition of the second in the International System of Units SI based on the optical clocks by 2030. The technology plans and practical steps are discussed. Therefore, the development of optical time scales with optical clocks and microwave clocks is the leading research topic at home and abroad. By an optical clock with a single 40Ca + ion (TOC-729) steering a microwave clock H, we study the optical time scale TS (Ca + ). The uptime of this TOC-729 is 87% over 7 months, and the frequency instability is 8E-17/ day expressed by the Allan deviation. There are two measurement methods of the optical clock TOC-729 relative to the microwave clock H, namely locking an optical frequency to a microwave clock frequency (the up-converting method) and conversion of an optical clock frequency into the RF range (the down-converting method). The up-converting method amplifies noises from the microwave clock H. The frequency instability of measurement data is inferior to the microwave clock H. For the down-converting method, the frequency instability of measurement data is almost the same as the microwave clock H, showing that the optical clock can accurately measure the microwave clock H with this method. The free-running H clock is compared with the signal down-converted from TOC-729 by recording the phase difference, and the frequency offset in H clock frequency is corrected by a phase micro stepper. The optical time scale (Ca + ) is independent, which steering to UTC is 0. 6ns in a month. The research is significant to establish and maintain the international time scale TAI with highly stable optical standards redefining the second, and the work can also improve and develop the national time scales in China.

Abstract:At present, microwave two-way time comparison technology is widely used in the field of inter-station time synchronization in the line-of-sight area, which can reach the nanosecond level. The application scenarios with higher time synchronization indicators need to be further improved. In this article, based on wideband signal modulation and demodulation technology, a microwave two-way carrier phase time comparison system is designed by taking advantage of the bandwidth spread spectrum signal and carrier phase measurement. The measurement uncertainty of the system is 0. 27 ps and the resolution of 4 ps is verified by using a hydrogen atomic clock and phase offset generator under laboratory conditions, which achieves the expected effectiveness. It provides a solid technical and experimental foundation for subsequent long-distance experiments and application promotion.

Abstract:The standard time transmission system developed by the National Time Service Center of the Chinese Academy of Sciences is capable of meeting the requirement of standard time-frequency signals for intercontinental users. This article takes the replication terminal located in Minsk, Belarus, and Europe as the analysis object. The impact of different comparison methods of common view and all-in-view on the performance of the replicated time-frequency signals is analyzed. For intercontinental applications, the number of satellites between the replication terminal and the reference station in common view decreases dramatically. The number of satellites in common view with GPS&BDS can reach about 9, which can benefit the accuracy of common view time comparison. All-in-view comparison has no requirements for the number of satellites common-in-view between the two sides. The experimental results show that the GPS all-in-view comparison can achieve better performance than that with GPS / BDS. Due to the inherent bias between BDS satellites, the reproduction results based on BDS all-in-view comparison are slightly worse than that with GPS. The performance of the reproduced time-frequency signal is optimal when the terminal runs in the GPS&BDS common view comparison fusion GPS all-in-view comparison mode. This article can provide some suggestions on the operation mode of standard time replication terminals which are as far as an intercontinental distance from the reference.

Abstract:The PPP time and frequency transfer method is a high-precision time and frequency transfer method for GNSS that has the characteristics of high precision, wide range, and low cost. The CV time and frequency transfer method can eliminate the satellite clock and reduce path errors, such as ionospheric and tropospheric errors. This study combines the advantages of the two methods and implements an improved PPP time and frequency transfer method. In the experiment, five IGS stations are connected to UTC( k) and one external high-precision hydrogen clock is selected to form zero, short, and long baseline links to evaluate the performance of the improved PPP. The results show that when IGS final clock products are used as time reference, the time transfer accuracy of the improved PPP is improved by 2. 55% ~17. 78% compared to PPP. The frequency stability can reach 2. 0×10 -17 / 600 000 s for the zero-baseline link. The short-term frequency stability is better than that of PPP when the common-view satellite does not change in the zero-baseline, and the long-term frequency stability is comparable to that of PPP. The frequency stability of the improved PPP of each link in the non-zero baseline link after the average time of 10 000 s is improved by about 10% compared with the IGS final products and the BIPM PPP.

Abstract:The time and frequency primary standard UTC (NIM) is maintained by the National Institute of Metrology (NIM) in China, and is traceable to the international time scale coordinated universal time ( UTC). With the continuous development of information technology, time stamp service is widely used in justice, health, finance, electronic commerce, intellectual property, etc. According to the Chinese government′s demand for legal metrology in areas such as trade settlement and medical, the time provided by time stamp authority (TSA) should be traced to UTC ( NIM) for domestic unification and international equivalence. This article proposes a calibrator to remotely and locally evaluate the traceability of TSA to UTC (NIM). When local calibration is implemented on the LAN, the uncertainties (k = 2) of the time offsets between UTC (NIM) and TSA are generally less than 2 ms. The uncertainties (k = 2) of the time offsets between UTC (NIM) and TSA located in Beijing are generally less than 713 ms on the WAN environment, even if the distance between the calibrator and TSA exceeds 1 000 km.

Abstract:To unify time on and out of the Earth, the viewpoint of relative time changes the time metrology standard from a unique standard time to two kinds of time: proper time and coordinate time, in which coordinate time can be traced to benchmark by pulsars. For the aim to build the ephemeris of orbit parameters simply locally, it is pointed out that the relationship of space inclusions is hierarchically nested,and the viewpoints of relative unification of time are formed. It deeply demonstrates the theory of coordinate time rebuilt by pulsars, summarizes the basic characteristics of a wide area coordinate system that unifies time must have, emphasizes the particularity of the original observer, and puts forward the definition method of pulsar′s sequence number and initial epoch based on the original observer, using the view that time and space are indivisible in the relative time view. First of all, to arrange a set of pulsar pulse signal as primary standard of the coordinate time, used to rebuild the coordinate time, convention a set of pulse period of pulsars for fixed constant array, no longer measuring after convention, the constant unit is still the international system of units[ seconds], constant can be understood as the pulse number coordinate time interval and conversion coefficient between[ seconds]. Then, based on the plane electromagnetic wave model, corresponding to the time when the pulse electromagnetic wave of the pulsar passes through the origin and the pulse sequence number one by one, so the coordinate time of original observer and continuous pulse number become linear function relationship. Finally, analyzes the pulsar stability characteristics, and given pulsar set stability criterion by the arguments of a set of sequence number.

Abstract:The transient step characteristic of the DC voltage transformer is very important to the control and protection device of the DC system. In this article, firstly, the factors affecting the transient transmission characteristics of DC electronic voltage transformers are studied. The technical parameters of step response and the performance requirements of the test device are analyzed. Based on this, a field test method for the step response characteristics of the DC voltage transformer is proposed. A step voltage source and test device based on a solid-state switch are developed. The synchronous measurement of steep rise, long pulse width square wave voltage output, and transient signal are realized. The test results show that the proposed field test method and device of step response characteristics of the DC electronic voltage transformer meet the requirements of the standard. The field test of step response is carried out in the actual ± 500 kV DC project converter station, which provides a technical basis for the transient performance test and evaluation of the DC voltage transformer.

Abstract:To meet the requirement of high-end ultra-precision equipment, such as photolithography machines and ultra-precision CNC machine tools for precise plane positioning, a planar two-dimensional displacement sensor based on multi-frequency magnetic field coupling is proposed. The sensor consists of a fixed length and a moving ruler. The fixed length is composed of a magnetically conductive substrate and an excitation coil in X- and Y-directions, and the moving ruler is composed of a magnetically conductive substrate and an induction coil in X- and Y-directions. By passing sine and cosine excitation signals to the excitation coils in X- and Y- direction, a two-dimensional uniform magnetic field array with multi-frequency magnetic field coupling is established on the fixed scale, and an electrical signal with displacement information is induced by the moving ruler. The feasibility of the multi-frequency magnetic field direct decoupling differential structure and amplitude modulation solution methods are evaluated through theoretical derivation and electromagnetic simulation. The simulation error is analyzed, and the sensor structure is optimized. Finally, the sensor prototype is manufactured by the PCB process and related experimental research is carried out. The experimental results show that the sensor can accurately measure the two-dimensional displacement in the measuring range of 150 mm × 150 mm. The measurement accuracy of the X-direction is ± 33. 08 μm and the measurement accuracy in the Y-direction is ± 36. 95 μm. The peak-to-peak values of the original internal displacement errors in the X and Y directions of the optimized sensor prototype are reduced by 49. 1% and 50. 7% on the original basis.

Abstract:The depth information of object surface topography is of great significance for intelligent robots to detect object surface features and human-computer interaction. Inspired by the mechanism of animal hair perception, a novel bionic magnetostrictive tactile sensor unit and array are designed. Based on the inverse magnetostrictive effect, Euler-Bernoulli beam theory, and Hooke′s law, the output voltage model of depth detection is derived. The simulation study determined the optimal bias magnetic field and array spacing. The output characteristics of the sensor unit under static and dynamic conditions were tested. In the depth detection range of 0. 05 ~ 4. 8 mm, the sensitivity is 185. 72 mV/ mm, and the response time and recovery time are 31 ms and 43 ms, respectively. It has good repeatability, and the coupling effect of the output voltage between the units in the sensor array does not exceed 2. 4% . The sensor unit and array are installed on the manipulator. When the appropriate sliding speed is selected to slide through the surface of different objects, various depths, and morphologies are accurately measured according to the output voltage waveform. The results show that the sensor unit and array can provide a reference for depth and morphology detection.

Abstract:This work fabricates and demonstrates a temperature sensor with high sensitivity and an expanded measuring range based on the microfiber knot resonator. Multiple-modes are generated in the microfiber knot resonator and participate in the resonance. The spectra of multiple mode resonance is superimposed on each other to form a vernier spectrum with envelope. By extracting the envelope of the vernier spectrum, high-sensitivity temperature measurement can be achieved with the sensitivity of up to - 10 nm/ ℃ . However, the temperature measurement range using the vernier spectrum is calculated to be only about 4℃ . To solve the problem of too small measurement range, the envelope spectrum and single frequency component spectrum are combined to expand the measurement range to about 20℃ . Compared with the resonance spectrum corresponding to the single frequency component, the vernier envelope spectrum achieves an amplification of sensitivity of about 1 600 times. This scheme uses the vernier effect to improve the sensitivity of temperature measurement. Meanwhile, the temperature measuring range of the vernier spectrum is expanded by using the resonant spectrum corresponding to a single frequency component, which improves the performance and practicability of the sensor.

Abstract:In response to the challenge of diminished accuracy in integrated navigation for smartphones due to satellite signal interference in complex environments, this article proposes a robust extended Kalman filter enhanced by fault recovery. Firstly, this method uses equivalent weight factors to adjust observation weights in real-time, effectively reducing the impact of gross errors on combination navigation accuracy. Considering the low redundancy of observations in smartphone-based loosely-coupled navigation, this algorithm divides the detection range into three segments for fault-free, bias, and anomalies. In the absence of faults, no further processing is undertaken. When a deviation occurs, the observation value is reduced in weight. For anomalies, the predicted innovation is used to repair the fault amplitude and correct the observation value. Practical experimental results show that when a satellite experiences a single-epoch fault, the robust filter method can effectively improve the positioning accuracy of the smartphone PDR/ GNSS combination navigation. The maximum error in the north direction is reduced from 7. 27 m to 3. 20 m, and the maximum position error in the east direction is reduced from 24. 01 m to 6. 60 m. In the case of multiple-epoch consecutive faults in GNSS positions, the proposed fault recovery-enhanced robust filter method shows an average reduction of over 50% in positioning error compared to the classical robust filter method.

Abstract:In order to solve the problems of poor stability, weak anti-interference ability and short transmission distance of existing passive wireless sensors, a new type of passive wireless strain sensor was proposed. The sensor consists of two coils and a volume acoustic sensor, which includes a force transmission structure and a quartz wafer, which converts the strain on the measured surface into a detectable quartz resonant frequency shift. Firstly, a semi-elliptical contact model was established to study the effects of contact force, excitation voltage, and contact element shape on quartz to improve sensor performance. The second, design a quartz loading device to realize the loading of small forces on the quartz surface. Last, build a passive wireless strain sensing system, the transmission distance can reach 5 cm, the resolution is about 4. 2 Hz/ με, the strain measurement range is 600 με, and the repeatability is 5. 47% . The experimental results show that the sensing system has good performance, and will be combined with UAVs in the future to provide an effective solution for large-scale structural strain measurement.

Abstract:To effectively overcome the interferences of GNSS signals and enhance the reliability of multi-sensor integration positioning in complex urban environments, a robust Mahalanobis distance statistic-based multi-sensor integration robust estimation method is proposed. With the basis of faulty measurements evaluation and typical model of variance inflation robust estimation, the robust Mahalanobis distance statistic is constructed based on the adjacent innovation sequences. The introduction of past innovation contributes to the observation redundancy. Meanwhile, the robustness of anomaly detection statistics can be improved by interacting between innovations from different measurements. According to the statistical property of this robust distance, the critical thresholds are ensured and then the measurement noise covariance can be adjusted adaptively with two traditional weighted strategies. Some experiments have been implemented on the INS / GNSS / LiDAR/ VINS vehicle positioning system in an urban canyon environment. It shows that compared with existing methods, the 3D positioning error root-mean-square of proposed method is limited within 3. 37 m. The superiority of our method is further validated by analyzing the positioning results with different significances.

Abstract:Aiming at the difficulty of power grid supraharmonic sampling and data processing, this paper proposes a supraharmonic signal detection method based on the windowed stochastic demodulation architecture. Firstly, the method utilizes analog information conversion technology combined with the idea of adding windows to obtain the compressed sampled data, which is realized by compressing and sampling the supraharmonic signal at the sampling end. Subsequently, the step-by-step bisection method is adopted to reconstruct the sampled data quickly, which is realized by setting the first-order differentiation of the residuals as the convergence threshold to improve the sparsity adaptive matching tracking (SAMP) algorithm. Simulation and test results show that the method can detect the frequency amplitude of supraharmonic signals accurately with only 10% of the data. The frequency amplitude detection error is less than 2% , and the frequency detection error is less than 3 Hz. The method provides a new way for the supraharmonic measurement.

Abstract:To address the issues of limited scene configuration and sparse traditional visual feature points in a ventilation duct environment, a method for modeling and localization of pipes based on point and line features is proposed by using two monocular cameras and two-line scan lidars as the primary sensing devices. Firstly, the LaneNet network and an improved random sample consensus algorithm are utilized to extract four wall-edge line features. Then, geometric and spatial constraints are employed to filter the detection results of the line segment detector algorithm, obtaining two vertical line features at the pipe joints. Next, the pipe width and robot yaw angle are calculated by using the line scan lidars. The depth information of the line features at the pipe joints is recovered and solved to obtain the height of the pipe. By combining the camera projection equations, the world coordinates of the line segment endpoints are obtained, and the pipe height is estimated. Finally, a pipe map coordinate system is established, and the two-dimensional robot position and pipe length are estimated. The experimental results show that the relative positioning error is within 9. 8 cm, and the relative modeling error is within 2. 9 cm, which could meet the requirements for pipe modeling and localization during robot inspection operations in the ducts.

Abstract:The rolling contact fatigue cracks on the rail surface and upper surface usually exist in the form of inclined cracks or multiangle complex cracks, which are difficult to detect and evaluate. Based on this, the wireless power transfer-eddy current testing (WPTECT) is adopted. A new probe structure is designed, and neural network algorithms are combined to detect and evaluate cracks. Firstly, different from the existing wireless power transfer-eddy current testing methods, the resonant circuit is constructed by increasing the excitation frequency instead of the series-parallel capacitance. Secondly, according to the characteristics of complex cracks, a directional probe structure consisting of two eight-figure excitation coils and two rectangular receiving coils is designed. Finally, the features of the detected signal are fully extracted, and the cracks are identified by the radial basis function neural network algorithm. Simulation and experimental results show that the proposed probe structure is sensitive to defects at any angle. Meanwhile, the recognition accuracy of the radial basis function algorithm for oblique crack, T crack, Y crack, and T crack with 1. 2 mm lift-off is 92. 00% , 95. 27% , 96. 64% , and 89. 50% , respectively.

Abstract:This paper proposes a solution for the autonomous obstacle avoidance of a hovering flapping-wing robot based on visual perception to meet the requirements of low-speed, safe, and long-endurance flight in obstacle-rich indoor environments. Firstly, inspired by birds′ visual information-based obstacle avoidance principle, a visual obstacle avoidance method based on optical flow detection is proposed to recognize both dynamic and static obstacles indoors. Secondly, a robot combining helium balloons with flapping wings is designed, utilizing helium buoyancy for primary lift and flapping wing motion for propulsion and secondary lift. A flight control method is designed to ensure stable flight and obstacle avoidance requirements. Finally, a robot prototype is fabricated, and its flight performance and autonomous obstacle avoidance capability are tested. Furthermore, a comparison is made between the proposed obstacle detection algorithm and another existing algorithm. The results indicate that the prototype exhibits a heading deviation of 5. 52° in straight-line flying and a turning speed of 23°/ s and achieves a 77% obstacle avoidance success rate with a single-frame detection time of 6. 1 ms. This demonstrates its capability to accomplish low-speed hovering flight and autonomous obstacle avoidance, laying the foundation for its execution of indoor exploration tasks.

Abstract:For the requirement of a high dexterity robot with high sensitivity of link parameters and precise positioning accuracy, there are the problems of low absolute positioning accuracy, poor parameter identification effectiveness, and calibration robustness in the random measurement configuration of robot kinematic calibration. To address these issues, a robot kinematic calibration measurement configuration stepwise optimization method based on ill-conditioned parameter separation and DETMAX and improved differential evolution (DETMAX-IDE) algorithm is proposed. Firstly, a robot position error model is formulated. Secondly, a comprehensive observability index is developed to evaluate the overall observability and sensitivity for different robot calibration measurement configurations. Finally, ill-conditioned parameters of the robot kinematic position error model are separated. The objective function and constraint conditions are established for optimizing the measurement configuration, the differential evolution algorithm is improved ( abbreviated as IDE algorithm), and a step-by-step iterative optimization algorithm based on the DETMAX algorithm and IDE algorithm is presented, which is referred to as DETMAX-improved differential evolution algorithm, and abbreviated as DETMAX-IDE algorithm. The step-by-step iterative optimization of robot kinematic calibration measurement configuration is achieved. Using numerical simulation and experimental robot kinematic calibration, the effectiveness of the proposed method is evaluated. Compared with the random measurement configuration, the experimental results show that the average and the mean square deviation of the robotic absolute positioning accuracy corresponding to the proposed method are improved, with an decrease of 62. 09% and 62. 45% , respectively.

Abstract:To solve the problem of global navigation satellite systems and inertial measurement unit fusion time asynchronous and improve the accuracy of pose estimation of plant protection UAV, this article proposes a delay pose compensation algorithm based on the improved error state Kalman filter by using the characteristics of large inertia and strong vibration of plant protection UAV. Firstly, the nominal state variables are linearly predicted, and a fading factor is introduced to improve the system stability in strong vibration environments. Then, complementary filtering is used to compensate for diagonal velocity and correct the attitude error state variables. Finally, combined with the delay time measured, complementary filtering is used to extrapolate the data and improve the velocity and position accuracy under high inertia characteristics. Experimental results show that, compared with the error state Kalman filter algorithm, the root mean square error of roll angle and pitch angle is reduced by 0. 266 9° and 0. 241 4°, and the root mean square error of yaw angle is reduced by 0. 076 4°. Under normal track plant protection operation, the root mean square error of velocity in the northeast sky direction values are decreased by 0. 210 5, 0. 184 9, and 0. 238 8 m/ s. The root mean square errors of the northeast celestial position are reduced by 0. 21, 0. 19, and 0. 23 m, respectively. The algorithm effectively improves the accuracy of pose estimation.