Axial clearance measurement method based on wavelength division multiplexing with all-fiber microwave photonic mixing.

Rotor-stator axial clearance plays a pivotal role in ensuring the safety and efficiency of major rotating machinery. This paper introduces an innovative clearance measurement method based on wavelength division multiplexing (WDM) combined with all-fiber microwave photonic mixing. The method is distinguished by large measurement range, high accuracy and low drift. The WDM-based common optical path structure is established. A comprehensive theoretical model of axial clearance drift determined by wavelength and temperature is developed based on the thermo-optic effect of optical fiber material. To efficiently separate measurement and reference light at the probe, the optical design for a compact optical bandpass filter (OBPF) fiber sensor probe is proposed. The performance of the method is substantiated by simulations and experiments. The results demonstrate an accuracy of better than 2.8µm over a 23.5 mm range, surpassing existing methods. The method's capability to mitigate temperature-induced drift is further confirmed through high-temperature drift and comparative experiments.

Parameters estimation-based calibration method for rotational axis of line-structured light 3D reconstruction.

Rotation is a critical component in 3D reconstruction systems, where accurate calibration of rotation axis parameters is essential for 3D stitching. In this study, what we believe to be a novel parameters estimation-based method for calibrating rotation axis parameters using 2D planar targets is proposed. Compared to traditional circle fitting methods, this method takes both orientation and position information into account, resulting in better precision performance. By leveraging the transmission of spatial pose relationships, the parameters estimation-based calibration method also effectively mitigates the impact of noise for more accurate calibration of rotation axis parameters. Error validation and 3D reconstruction experiments proved the superior performance of the proposed method. The experiment results demonstrate the effectiveness and applicability of the approach in enhancing the calibration of rotation axis parameters for 3D reconstruction systems.

Robust five-degree-of-freedom measurement system with self-compensation and air turbulence protection.

A robust five-degree-of-freedom (5-DOF) measurement system is proposed in this paper. The compact optical configuration with high resolution is designed based on lens combination and multiple reflections. Beam drift and dual-beam parallelism are monitored and compensated by autocollimator units and a polarizer unit respectively. In addition, a protection method is proposed to reduce the intensity of air turbulence by reducing the Reynolds number of the beam path. The performance of the system is verified by experiments. The experimental results show that the self-compensation methods and air turbulence protection can effectively improve the accuracy and stability of the system under the long-term interference of external environments. The proposed system has high precision, desirable robustness, and convenient pre-calibration, which can be used for error measurement of precision machines.

High-accuracy measurement system for rotor-stator axial clearance in narrow spaces based on all-fiber microwave photonic mixing.

In this paper, a high-accuracy measurement method for rotor-stator axial clearance in narrow spaces is proposed. The optical path structure based on all-fiber microwave photonic mixing is established. To improve the accuracy and expand the measurement range, the total coupling efficiency over the entire measurement range at different working distances of fiber probe was evaluated by Zemax analysis tool and theoretical model. The performance of the system was verified by experiments. The experimental results show that the measurement accuracy of axial clearance is better than 10.5 um within the range of 0.5-20.5 mm. The measurement accuracy has been effectively improved compared to previous methods. Additionally, the probe size is reduced to a mere diameter of 2.78 mm, which is more suitable for axial clearance measurement in narrow spaces inside rotating machines.

High precision dynamic measurement method for axial clearance based on time division multiplexing with dispersive interferometry.

Rotor-stator axial clearance is a crucial design parameter affecting rotating machines' efficiency and safety. To accurately measure the dynamic axial clearance in high-speed machinery, a precise method based on time division multiplexing with frequency domain interferometry has been proposed. This method has proven robust and accurate through simulations and experiments. The inclusion of an optical switch enables the utilization of dispersive interferometry(DPI) and time division multiplexing for multiple channels of the light source. It achieves a static accuracy of 1.5 µm for a 15 mm range and a dynamic accuracy of 9 µm at 3000 rpm.

High-speed nonlinear frequency sweeping signal distance extraction algorithm based on the table lookup method.

In the domain of frequency sweeping interferometry, the accurate extraction of distance information from nonlinear frequency scanning signals holds paramount significance in ensuring meticulous measurements of high precision. This paper presents a novel, to the best of our knowledge, high-speed distance extraction algorithm based on the table lookup method and validates its feasibility through theoretical models, simulations, and practical experiments. The proposed algorithm achieves comparable accuracy to traditional methods involving resampling and Hilbert transform. However, it outperforms them in robustness against noise and variations in sampling points. This method can accurately process signals sampled even below the Nyquist sampling rate. The simplicity and computational efficiency of the proposed approach make it suitable for various nonlinear sampling applications, promising broad applicability in scientific and engineering contexts.

A Stable, Efficient, and High-Precision Non-Coplanar Calibration Method: Applied for Multi-Camera-Based Stereo Vision Measurements.

Traditional non-coplanar calibration methods, represented by Tsai's method, are difficult to apply in multi-camera-based stereo vision measurements because of insufficient calibration accuracy, inconvenient operation, etc. Based on projective theory and matrix transformation theory, a novel mathematical model is established to characterize the transformation from targets' 3D affine coordinates to cameras' image coordinates. Then, novel non-coplanar calibration methods for both monocular and binocular camera systems are proposed in this paper. To further improve the stability and accuracy of calibration methods, a novel circular feature points extraction method based on region Otsu algorithm and radial section scanning method is proposed to precisely extract the circular feature points. Experiments verify that our novel calibration methods are easy to operate, and have better accuracy than several classical methods, including Tsai's and Zhang's methods. Intrinsic and extrinsic parameters of multi-camera-systems can be calibrated simultaneously by our methods. Our novel circular feature points extraction algorithm is stable, and with high precision can effectively improve calibration accuracy for coplanar and non-coplanar methods. Real stereo measurement experiments demonstrate that the proposed calibration method and feature extraction method have high accuracy and stability, and can further serve for complicated shape and deformation measurements, for instance, stereo-DIC measurements, etc.

Fast algorithm based on the Hilbert transform for high-speed absolute distance measurement using a frequency scanning interferometry method.

Frequency scanning interferometry using state-of-the-art high-speed frequency-swept laser source can be utilized to measure absolute distance on the order of micrometers to centimeters. Current distance demodulation methods based on fast Fourier transform (FFT) or fringe counting cannot achieve satisfactory accuracy when the number of sampling points within a frequency-sweeping period is small; the conventional Hilbert transform is more accurate, but it needs arctangent calculation and phase unwrapping, which is time consuming. So we propose a fast algorithm based on the conventional Hilbert transform to recover the distance from the interference signal. The algorithm is implemented by first performing a Hilbert transform and then solving the phase and the distance from the Hilbert signal with a novel, to the best of our knowledge, method that eliminates the need for arctangent calculation and phase unwrapping. The whole process took only 40 µs, and it is almost 2 times faster than the conventional Hilbert algorithm with little accuracy lost. Simulation results demonstrate that the proposed algorithm is more accurate than the FFT algorithm, and it achieved a standard deviation of 0.062 µm, which was less than that of the FFT, in our experiment at a distance of approximately 16 mm and measurement speed of 1 kHz.

An automated piecewise synthesis method for cetacean tonal sounds based on time-frequency spectrogram.

Bionic signal waveform design plays an important role in biological research, as well as bionic underwater acoustic detection and communication. Most conventional methods cannot construct high-similarity bionic waveforms to match complex cetacean sounds or easily modify the time-frequency structure of the synthesized bionic signals. In our previous work, we proposed a synthesis and modification method for cetacean tonal sounds, but it requires a lot of manpower to construct each bionic signal segment to match the tonal sound contour. To solve these problems, an automated piecewise synthesis method is proposed. First, based on the time-frequency spectrogram of each tonal sound, the fundamental contour and each harmonic contour of the tonal sound is automatically recognized and extracted. Then, based on the extracted contours, four sub power frequency modulation bionic signal models are combined to match cetacean sound contours. Finally, combining the envelopes of the fundamental frequency and each harmonic, the synthesized bionic signal is obtained. Experimental results show that the Pearson correlation coefficient (PCC) between all true cetacean sounds and their corresponding bionic signals are higher than 0.95, demonstrating that the proposed method can automatically imitate all kinds of simple and complex cetacean tonal sounds with high similarity.

Recognition method for the bionic camouflage click communication trains modulated by time delay difference.

Bionic camouflage covert underwater acoustic communication has recently attracted great attention. However, we have not found relevant methods or literature to recognize these bionic camouflage communication signals (BCCSs) in the area of anti-reconnaissance. Focused on recognizing the BCCSs, this article proposes a recognition method based on the statistics of inter-click intervals to recognize the camouflaged click communication train (CCCT), which is modulated by time delay difference (TDD). We first analyze the characteristics of TDD distributions of CCCT and real click train (RCT). According to the coding principle, the TDDs of CCCTs present a ladder-like distribution with a fixed time step, and the TDDs are equal to the integral multiple of the fixed time step. On the contrary, the TDDs of RCTs are approximately random distribution within a certain time range. Therefore, based on the different TDD distributions, this article classifies CCCTs and RCTs by utilizing the statistical property of TDD distributions. To measure the TDDs of diverse cetacean clicks accurately, a new click location scheme based on the dynamic window energy ratio is proposed. Next, based on the statistics of TDD distribution, the influences of the TDDs that are caused by multipath interferences are eliminated by iteration. Simulations demonstrate the accuracy of the recognition method under different conditions.

Deep Metallic Surface Defect Detection: The New Benchmark and Detection Network.

Metallic surface defect detection is an essential and necessary process to control the qualities of industrial products. However, due to the limited data scale and defect categories, existing defect datasets are generally unavailable for the deployment of the detection model. To address this problem, we contribute a new dataset called GC10-DET for large-scale metallic surface defect detection. The GC10-DET dataset has great challenges on defect categories, image number, and data scale. Besides, traditional detection approaches are poor in both efficiency and accuracy for the complex real-world environment. Thus, we also propose a novel end-to-end defect detection network (EDDN) based on the Single Shot MultiBox Detector. The EDDN model can deal with defects with different scales. Furthermore, a hard negative mining method is designed to alleviate the problem of data imbalance, while some data augmentation methods are adopted to enrich the training data for the expensive data collection problem. Finally, the extensive experiments on two datasets demonstrate that the proposed method is robust and can meet accuracy requirements for metallic defect detection.

Deep Active Learning for Surface Defect Detection.

Most of the current object detection approaches deliver competitive results with an assumption that a large number of labeled data are generally available and can be fed into a deep network at once. However, due to expensive labeling efforts, it is difficult to deploy the object detection systems into more complex and challenging real-world environments, especially for defect detection in real industries. In order to reduce the labeling efforts, this study proposes an active learning framework for defect detection. First, an Uncertainty Sampling is proposed to produce the candidate list for annotation. Uncertain images can provide more informative knowledge for the learning process. Then, an Average Margin method is designed to set the sampling scale for each defect category. In addition, an iterative pattern of training and selection is adopted to train an effective detection model. Extensive experiments demonstrate that the proposed method can render the required performance with fewer labeled data.

Identification of Vibration Events in Rotating Blades Using a Fiber Optical Tip Timing Sensor.

The blade tip timing (BTT) technique has been widely used in rotation machinery for non-contact blade vibration measurements. As BTT data is under-sampled, it requires complicated algorithms to reconstruct vibration parameters. Before reconstructing the vibration parameters, the right data segment should first be extracted from the massive volumes of BTT data that include noise from blade vibration events. This step requires manual intervention, is highly dependent on the skill of the operator, and has also made it difficult to automate BTT technique applications. This article proposes an included angle distribution (IAD) correlation method between adjacent revolutions to identify blade vibration events automatically in real time. All included angles of the rotor between any two adjacent blades were accurately detected by only one fiber optical tip timing sensor. Three formulas for calculating IAD correlation were then proposed to identify three types of blade vibration events: the blades' overall vibrations, vibration of the adjacent two blades, and vibration of a specific blade. Further, the IAD correlation method was optimized in the calculating process to reduce computation load when identifying every blade's vibration events. The presented IAD correlation method could be used for embedded, real-time, and automatic processing applications. Experimental results showed that the proposed method could identify all vibration events in rotating blades, even small events which may be wrongly identified by skillful operators.

Synchronous Vibration Measurements for Shrouded Blades Based on Fiber Optical Sensors with Lenses in a Steam Turbine.

It is important to obtain accurate dynamic vibrations of steam turbine blades for safe operation. Strain gauge (SG) measurements cannot fully obtain vibrations of all blades and nodal diameter (ND) details. The blade tip timing (BTT) technique could resolve this problem because it has the advantage of measuring all blades' vibrations. This article proposed an improved BTT technique for measuring synchronous vibrations of shrouded blades in a steam turbine based on fiber optical sensors with lenses. The newly developed sensor was equipped with a Plano-convex lens, and the optical path was specifically designed to collimate the beam within a large working distance from 4 to 19 mm and improve the measuring accuracy. A method to search the spectra of all peak vibration amplitudes of all the blades was proposed to obtain the ND details of synchronous vibration. Experimental results validated the efficiency and accuracy of the proposed methods and sensors. Comparison results of BTT measurements with SG measurements showed that the relative errors of normalized frequency and strain were small and acceptable.

Self-Contained High-SNR Underwater Acoustic Signal Acquisition Node and Synchronization Sampling Method for Multiple Distributed Nodes.

Aiming at the application demand in underwater noise monitoring, observation of marine animal, antisubmarine and underwater target localization, a high-SNR underwater acoustic signal acquisition (UASA) node that combines a self-contained acquisition system and floating platform is designed to improve the acquisition performance of a single UASA node, and a high-accuracy synchronization sampling method among multiple distributed UASA nodes based on master-slave dual phase-locked loops (MSDPLL) is proposed to improve the synchronization sampling accuracy. According to the equivalent model of hydrophone and application requirements, low noise signal conditioning circuit and large-capacity data storage modules are designed. Based on the long-term monitoring requirements for underwater acoustic signal and distributed positioning requirements for underwater targets, the structure of a single UASA node is designed and MSDPLL is developed for high-accuracy synchronization sampling among multiple UASA nodes. Related experimental results verified the performance of the UASA node and the synchronization sampling method.

Modeling and Experimental Testing of an Unmanned Surface Vehicle with Rudderless Double Thrusters.

Motion control of unmanned surface vehicles (USVs) is a crucial issue in sailing performance and navigation costs. The actuators of USVs currently available are mostly a combination of thrusters and rudders. The modeling for USVs with rudderless double thrusters is rarely studied. In this paper, the three degrees of freedom (DOFs) dynamic model and propeller thrust model of this kind of USV were derived and combined. The unknown parameters of the propeller thrust model were reduced from six to two. In the three-DOF model, the propulsion of the USV was completely provided by the resultant force generated by double thrusters and the rotational moment was related to the differential thrust. It combined the propeller thrust model to represent the thrust in more detail. We performed a series of tests for a 1.5 m long, 50 kg USV, in order to obtain the model parameters through system identification. Then, the accuracy of the modeling and identification results was verified by experimental testing. Finally, based on the established model and the proportional derivative+line of sight (PD+LOS) control algorithm, the path-following control of the USV was achieved through simulations and experiments. All these demonstrated the validity and practical value of the established model.

Forward fiber Fourier transform spectrometer modeling and design with PZT phase modulation real-time compensation.

Piezoelectric ceramic transducers (PZTs) are often applied in all-fiber Fourier transform spectrometers (FFTSs) to realize phase modulation. Combining a PZT and optical fiber features, a theoretical FFTS modeling is established. We systematically deduced the main causes of spectral errors and the factors of the instrumental resolution, then designed a new FFTS system and provided real-time compensation methods for spectral errors. We creatively employed two Mach-Zehnder interferometers by winding the sensing arms of the two on the same PZT cylinder to realize the dual optical path experiencing the same and simultaneous phase modulation. A processing circuit is designed to achieve selective sampling of the test interferogram, avoiding the spectral artifacts generated by the PZT phase discontinuities. The narrow line width of the reference spectrum is obtained at the same time for real-time calibration of spectral shift resulting from dispersion and other phase errors. By the comparison of the original interferogram and spectrum with those after compensation, the experiments validated that the system design could effectively compensate for the spectral errors caused by PZT and optical fiber physical limitations. Finally, the improved ideas are discussed if the FFTS prototype is to be applied to test a real-time gas spectrum.

Simple spectral reduction algorithm used for the echelle spectrometer.

The structure of the echelle spectrometer equipped with a prism cross-dispersion element is very complex because of the nonlinear interaction between the prism and the echelle grating, which makes it difficult to extract the wavelength information from the two-dimensional spectrogram. According to the dispersion equations of both the grating and the prism direction, a wavelength calibration model is derived, establishing a relationship between the pixel position and the wavelength. It is found that a calibration process is always needed in repeated experiments due to the slight change of the spectrometer parameters caused by the environment. To solve this problem and improve the accuracy of the reduction, a simple spectral reduction algorithm involving calibration and compensation is proposed, which combines the least-square method and the polynomial fitting method. The algorithm was implemented with LabVIEW software, and the result showed that the absolute error of each wavelength was less than 0.02 nm.

Bio-Inspired Covert Active Sonar Strategy.

The covertness of the active sonar is a very important issue and the sonar signal waveform design problem was studied to improve covertness of the system. Many marine mammals produce call pulses for communication and echolocation, and existing interception systems normally classify these biological signals as ocean noise and filter them out. Based on this, a bio-inspired covert active sonar strategy was proposed. The true, rather than man-made sperm whale, call pulses were used to serve as sonar waveforms so as to ensure the camouflage ability of sonar waveforms. A range and velocity measurement combination (RVMC) was designed by using two true sperm whale call pulses which had excellent range resolution (RR) and large Doppler tolerance (DT). The range and velocity estimation methods were developed based on the RVMC. In the sonar receiver, the correlation technology was used to confirm the start and end time of sonar signals and their echoes, and then based on the developed range and velocity estimation method, the range and velocity of the underwater target were obtained. Then, the RVMC was embedded into the true sperm whale call-train to improve the camouflage ability of the sonar signal-train. Finally, experiment results were provided to verify the performance of the proposed method.

Simple lock-in detection technique utilizing multiple harmonics for digital PGC demodulators.

A simple lock-in detection technique especially suited for digital phase-generated carrier (PGC) demodulators is proposed in this paper. It mixes the interference signal with rectangular waves whose Fourier expansions contain multiple odd or multiple even harmonics of the carrier to recover the quadrature components needed for interference phase demodulation. In this way, the use of a multiplier is avoided and the efficiency of the algorithm is improved. Noise performance with regard to light intensity variation and circuit noise is analyzed theoretically for both the proposed technique and the traditional lock-in technique, and results show that the former provides a better signal-to-noise ratio than the latter with proper modulation depth and average interference phase. Detailed simulations were conducted and the theoretical analysis was verified. A fiber-optic Michelson interferometer was constructed and the feasibility of the proposed technique is demonstrated.