Multi-degree-of-freedom measurement (MDFM) system is an effective way to measure multiple degrees of freedom errors simultaneously. In the MDFM system, the pitch angle and yaw angle are often measured based on the autocollimation measurement principle. Due to the lens fabrication and installation errors, the angle detector will deviate from the focal plane, causing defocus error. To address the problem, a defocus error model is established to analyze the influencing factors and solve the position of the focal plane. It is found that the straightness is the main factor of defocus error. When the defocus amount is present, the defocus error is larger with the increase of straightness. And the sensitivity of angle detector has a specific relationship with the intensity density of spot, which is related to the defocus amount. By using a precision displacement table, the position of the focal plane and the corresponding angle detector output can be accurately obtained. The experiment was carried out to verify the feasibility of this method. The angle detector and lens were installed precisely according to the angle detector output. When the vertical straightness is within ±400 μm, the influence of the defocus error on the pitch angle is less than 2 μrad. The results showed that the method can reduce the defocus error of auto-collimation measurement effectively, which can improve the accuracy of angle measurement in MDFM system.
Laser auto-collimation technology is an important method of precision micro-angle measurement. It has the advantages of high measurement accuracy and simple optical structure. However, the position-sensitive detector (PSD) has nonlinearity, and the uniformity of laser spot is affected by the working distance of the auto-collimation system, which seriously affects the accuracy of angle measurement based on laser auto-collimation technology. To improve the accuracy of laser auto-collimation technology in long-distance angle measurement, PSD is calibrated by linear interpolation to solve the error caused by its nonlinearity. In addition, due to the non-uniformity of the light spot, the PSD zero point is not collinear at different distances, and the measurement error can be corrected by a laser interferometer. Experimental results showed that the interpolation calibration method effectively improved the angle measurement accuracy of PSD. After the zero deviation was corrected, when the PSD moved within 5m, the maximum standard deviation (SD) of the pitch error was 0.12 ", and the SD of the yaw error was 0.09 ". The method proposed in this paper can achieve high precision angle measurement based on PSD at long working distances.
In our work, a rotor-stator axial gap non-contact real-time measurement system based on SWI with an auxiliary interferometer for nonlinearity correction is established. The measurement speed is 1kHz and the principle is given. The data of 6-10mm distance is collected, and the distance of each position is calculated by windowed FFT and Hilbert transform respectively. The simulation results indicate that the measurement error of Hilbert algorithm is on the order of a fraction of nanometers, while that of FFT is micrometer level. However, FFT takes 36μs in one measurement which is three times faster than Hilbert algorithm. Both of the two meet the precision requirement of axial gap measurement, it helps us to choose the more appropriate algorithm according to the actual situation.
Rotor-stator axial gap is a key design parameter that directly affects the efficiency and safety of large rotating machines. With optimum axial clearance value determined, the rotating machine can work with the highest efficiency. To realize active clearance control (ACC) of rotating machinery, high-precision non-contact online measurement of the axial gap must be carried out. However, traditional measurement methods are challenged by the extreme working environment, such as extreme temperature, high rotating speed, and narrow space conditions; no mature measurement method was reported to realize the high accuracy, non-contact, and online performance required by the axial gap measurement. In this paper, a rotor-stator axial gap measurement method is proposed based on frequency scanning interferometry, and the mathematical model of axial clearance measurement is established. The weak reflection signal of lens and time delay estimation algorithm is employed to undermine the impact of probe pigtail length drift caused by the temperature change under extreme conditions. The axial gap measurement prototype based on sweep frequency ranging was developed, and the measurement and system calibration was carried out. The experimental results verify the effectiveness and accuracy of the method. The drift of the system was less than 20μm in 30 minutes with millimeter level pigtail drift, and the measurement accuracy was better than 50μm.
The blade tip-timing (BTT) method has become the industry standard in blade vibration measurement. It monitors blade vibration based on the different times of arrival (ToAs) between blade vibration and not. The optical fiber bundle (OFB) probe is widely used to obtain ToA because of its small size, fast response, and high tip-timing accuracy. ToA is usually obtained by comparing the received signal with a fixed threshold level. However, the varying blade tip clearance (BTC) will affect the received signal, which directly causes the tip-timing error. We propose an optical fiber probe with GRIN lens, which eliminates the tip-timing error caused by varying BTC through the constant-fraction discrimination (CFD) method. The model of the OFB probe with GRIN lens is first established. Model analysis indicates that ToA obtained by the proposed method is constant no matter how BTC changes. Finally, the accuracy of the model and the effectiveness of the method are verified by two experiments. When BTC varied from 1.5 to 3.5 mm, the maximum tip-timing errors of OFB probe without and with GRIN lens were 181.12 μs and 10.44 μs, respectively.
Spectral domain OCT (SD-OCT) is a branch of Fourier domain OCT (FD-OCT) technology. Thanks to the characteristics of high sensitivity, high precision and non-invasion, SD-OCT can be widely used in clinical medical diagnosis, industrial nondestructive testing and distance measurement. However, one of the main drawbacks of the SD-OCT is the poor light collection efficiency of the fiber probe because the conventional single-mode fiber probe for light collection has very small diameter. In this paper, the multi-mode fiber probe was used for optical delivery and collection, the feasibility of this method has been verified both theoretically and experimentally. Furthermore, the higher efficiency of the light collection was obtained when the multi-mode fiber probe was used in the system. It is especially suitable for small distance and uncooperative target distance measurement.
In order to meet the demands of fabric defect detection under different lighting conditions, the multi-scale Retinex algorithm is proposed as preprocessing algorithm to limit the influence of lighting change on subsequent processing to a certain degree. Firstly, the fabric defect simulation database under complex lighting conditions is produced by rotating, flipping and transforming the data based on traditional TILDA fabric texture database. Aiming at the phenomenon of the obvious brightness changes between different images in the database and the more complicated illumination environment, the multi-scale Retinex algorithm as the preprocessing is used by logarithmically transforming the given input image and estimating the incident image in this paper. The input image and the estimated incident image are reflected images, which limits the influence of illumination changes on subsequent processing to a certain extent. The comparative experiments show that dynamic range compression, color constancy, edge enhancement and a balance between the three aspects can be achieved by the multi-scale Retinex algorithm at the same time. The experimental results show that the multi-scale Retinex algorithm is robust, and the local details of the processed image will be well maintained. The image information entropy and contrast is increased by 30%, and average gradient is increased by nearly 40%. Simultaneously, the change of light and noise will be limited to a certain degree, and high-quality fabric image under different illumination conditions can be obtained effectively.
KEYWORDS: Zemax, Distance measurement, Signal to noise ratio, Optical coherence tomography, Optical fibers, Light sources, Analytical research, Near field optics
Fiber probe is an important component in optical coherence tomography (OCT) and distance measurement system, its collection efficiency of the light reflected from target surface directly affects the signal-to-noise ratio (SNR) of the system, while the target distance will have direct influence on the collection efficiency. However, there are few researches on specific analysis of the collection efficiency variation of fiber probe when target distance change. In this paper, we proposed the theoretical model of fiber probe, and we deduce the collection efficiency of the light reflected from the target surface which is deviated from the ideal image plane of the fixed-focus fiber probe with Δl in the direction of the optical axis. The theoretical result is testified by ZEMAX simulation, and the difference between the simulation result and the theoretical calculation is 0.0023 when Δl = -5mm. In addition, ZEMAX is utilized to simulate the light collection efficiency when target distance change for reflective target surface. The results of simulation are basically consistent with theoretical calculation. It is concluded that the rate of descent for the collection efficiency is greater when target surface is placed in front of the ideal image plane, and the working range of the fiber probe increases with the increase of ideal image distance. The findings can guide us to make effective use of fiber probe and chose a detector with the right dynamic range to improve distance measurement precision.
This paper proposes a robust method based on a local intensity binary pattern for interest feature description. To avoid estimating reference orientation, local features were calculated on a rotation invariant system. Different from the local binary patterns (LBP) and center-symmetric-LBP operator, our proposed local circular contrast pattern (LCCP) operator calculates a local binary feature by comparing the relative intensity order information of each two adjacent elements in the sequence consisting of the sampling point and its neighboring points. To evaluate the performance of our proposed descriptor LCCP and other existing descriptors (e.g., scale-invariant feature transform, DAISY, HRI-CSLTP, multisupport region order-based gradient histogram-single, local intensity order pattern), image matching experiments were first conducted on the Oxford dataset, additional image pairs with complex light changes, image sequences with different noise, and three-dimensional objects dataset. To further evaluate the discriminative ability of local descriptors, a simple object recognition experiment was carried out on three public datasets. The experimental results show that our descriptor LCCP exhibits a better performance and robustness than other evaluated descriptors.
Blade vibration measurement based on the blade tip-timing method has become an industry-standard procedure. Fiber bundle sensors are widely used for tip-timing measurement. However, the variation of clearance between the sensor and the blade will bring a tip-timing error to fiber bundle sensors due to the change in signal amplitude. This article presents methods based on software and hardware to reduce the error caused by the tip clearance change. The software method utilizes both the rising and falling edges of the tip-timing signal to determine the blade arrival time, and a calibration process suitable for asymmetric tip-timing signals is presented. The hardware method uses an automatic gain control circuit to stabilize the signal amplitude. Experiments are conducted and the results prove that both methods can effectively reduce the impact of tip clearance variation on the blade tip-timing and improve the accuracy of measurements.
KEYWORDS: Sensors, Vibrometry, Signal detection, Neon, Analytical research, Calibration, Failure analysis, Time metrology, Precision measurement, Current controlled current source
In this article, a blade synchronous vibration measurement method based on tip-timing at constant rotating speed is presented. This method requires no once-per revolution sensor, which makes it more generally applicable, especially for high pressure compressors of the dual rotor engines. The vibration amplitude and engine order are identified with this method. The theoretical analysis is presented, and the least square method is utilized for vibration parameter identification. The method requires at least four tip-timing sensors if the Campbell diagram is previously known and five sensors if the Campbell diagram is unknown. The method has no strict requirement on the angles among sensors which facilitate the installation of the sensors in the measurement. In some special conditions the method will fail and these conditions are analyzed. Experiments are conducted on a high speed rotor with a fiber based tip-timing system, and the experimental results indicate that the theoretical analysis is correct and the method is feasible.
A fiber-optic sinusoidal phase modulating (SPM) interferometer was proposed for the acquisition and reconstruction of three-dimensional (3-D) surface profile. Sinusoidal phase modulation was induced by controlling the injection current of light source. The surface profile was constructed on the basis of fringe projection. Fringe patterns are vulnerable to external disturbances such as mechanical vibration and temperature fluctuation, which cause phase drift in the interference signal and decrease measuring accuracy. A closed-loop feedback phase compensation system was built. In the subsystem, the initial phase of the interference signal, which was caused by the initial optical path difference between interference arms, could be demodulated using phase generated carrier (PGC) method and counted out using coordinated rotation digital computer (CORDIC) , then a compensation voltage was generated for the PZT driver. The bias value of external disturbances superimposed on fringe patterns could be reduced to about 50 mrad, and the phase stability for interference fringes was less than 6 mrad. The feasibility for real-time profile measurement has been verified.
Blade tip-timing is the most effective method for online blade vibration measurement of large rotating machines like turbine engines. Fiber bundle sensors are utilized in tip-timing system to measure the arrival time of the blade. The model of the tip-timing signal of the fiber bundle sensor is established. Experiments are conducted and the results are in concordance with the model established. The rising speed of the tip-timing signal is analyzed. To minimize the tip-timing error, the effects of the clearance change between the sensor and the blade and the deflection of the tip surface are analyzed. Simulation results indicate that the variable gain amplifier, which amplifies the signals to a similar level, can eliminate the measurement error caused by the variation of the clearance between the sensor and blade. Increasing the clearance between the sensor and blade can reduce the measurement error introduced by deflection of the tip surface.
A novel fiber-optic interferometer fringe projector with the sinusoidal phase-modulating method is presented. The system utilizes the integrating bucket method to detect the desired phase or the displacement and a CMOS image sensor to detect four frames obtained by integration of the time-varying intensity in an interference image during the four quarters of the modulation period. Since this technique with the method modulating the injection current of the piezoelectric transducer (PZT), measurement accuracy is not affected by an intensity modulation that usually appears in the current modulation. The system also utilizes the Fresnel reflection signal to adjust the phase-modulation coefficient z to eliminate the disturbance of initial phase ψ0 . The experimental results for surface profiles of a convex hull show that the sinusoidal phase modulating interferometer proposed here confirms its applicability to practical application.
A fiber-optic sinusoidal phase-modulating (SPM) interferometer for fringe projection is presented. The system is based on the SPM technique and makes use of the Mach–Zehnder interferometer structure and Young’s double pinhole interference principle to achieve interference fringe projection. A Michelson interferometer, which contains the detection of Fresnel reflection on its fiber end face and interference at one input port of a 3 dB coupler, is utilized to achieve feedback precise control of the fringe phase, which is sensitive to phase drifting produced by the nature of the fiber. The phase diversity for the closed-loop SPM system can be real-time measured with a precision of 3 mrad. External disturbances mainly caused by temperature fluctuations can be reduced to 57 mrad for the fringe map. The experimental results have shown the usefulness of the system.
A novel all-metal fiber-optic accelerometer with Michelson interferometric configuration is proposed, using mass blocks
and flexible reed structure. The sensing arm is configured to directly transform the applied acceleration to fiber
deformation in axial direction, which increases the acceleration sensitivity of the sensor. The axis of the reference arm is
set orthogonal to that of the sensing arm in order to make it insensitive to applied acceleration, and thus decrease the
phase noise of the sensor. Using the flexible reed of 1mm thick and the mass block of 208g, an acceleration sensitivity of
556.7rad/g(54.9dB) is achieved by this structure. In the case of 10-4 rad/rt Hz(100Hz)noise floor, the minimum detectable
acceleration is 200ng/rt Hz.
The phase generated carrier (PGC) arctangent algorithm needs to be compensated when the carrier modulation depth C
deviates from 2.63 rad. Parameters measurement error can induce an incomplete compensation in the arctangent
algorithm. The demodulation distortion due to parameters estimation error is analyzed quantitatively in this paper. The
relative amplitude error (RAE) and harmonic suppression ratio (HSR) are considered, which indicate a linear distortion
and a nonlinear one separately. Theoretical analysis shows that the fluctuation range of RAE has a linear relation with an
absolute value of δC which is the estimation error of the modulation depth C. The fluctuation range of RAE also has a
square relation with δφ1, δφ2 which are the estimation error of the phase delay Δφ1Δφ2 of fundamental and second
harmonic carriers respectively. Similarly, minimum HSR decreases by 6dB along with a doubled absolute value of δC
and by 12dB with that of δφ1. Experimental results are presented to corroborate the theory.
Optimization and active control of the tip clearance of turbine blades has been identified as a key to improve fuel efficiency, reduce emission, and increase service life of the engine. However, reliable and real-time tip clearance measurement is difficult due to the adverse environmental conditions that are typically found in a turbine. We describe a dual-beam fiber optic measurement system that can measure the tip timing and tip clearance simultaneously. Because the tip timing information is used to calculate the tip clearance, the method is insensitive to the signal intensity variation caused by fluctuations in environmental conditions such as light source instability, contamination, and blade tip imperfection. The system was calibrated and tested using experimental rotors. The test results indicated a high resolution of 4.5 μm and measurement accuracy of ±20 μm over the rotation speed range of 2000 to 10,000 rpm.
Optimization and active control of the clearance between turbine blades and case of the engine is identified, especially in
aerospace community, as a key technology to increase engine efficiency, reduce fuel consumption and emissions and
increase service life .However, the tip clearance varies during different operating conditions. Thus a reliable non-contact
and online detection system is essential and ultimately used to close the tip clearance control loop. This paper described a
fiber optical clearance measuring system applying skewed dual optical beams to detect the traverse time of passing
blades. Two beams were specially designed with an outward angle of 18 degree and the beam spot diameters are less
than 100μm within 0-4mm working range to achieve high signal-to-noise and high sensitivity. It could be theoretically
analyzed that the measuring accuracy is not compromised by degradation of signal intensity caused by any number of
environmental conditions such as light source instability, contamination and blade tip imperfection. Experimental tests
were undertaken to achieve a high resolution of 10μm in the rotational speed range 2000-18000RPM and a measurement
accuracy of 15μm, indicating that the system is capable of providing accurate and reliable data for active clearance
control (ACC).
Particle image velocimetry (PIV) is the newest entrant to the field of fluid flow measurement and provides instantaneous velocity fields over global domains. A monocular stereoscopic sensor is designed according to the characteristics of a 3D-PIV system. It includes only one CCD camera and two groups of symmetry mirrors. A measured particle in the flow field is mapped to the CCD image surface from two angular directions and each directed-picture occupies half the image surface. Imaging procedure is decomposed to illustrate the principle of the monocular stereoscopic sensor. It is proved that folding optical path enables the possibility of miniaturized sensor design. A perspective transformation model and a geometric error model are presented for analysis and design of stereoscopic PIV system. The relationship between sensor performance and its geometric parameters is discussed in detail. These parameters contain the angle between two mirrors in the same group, the angle and distance between mirrors in each group, the distance between mirrors and camera and so on. Enough theoretical research is made for the optimum design of the stereovision system. In addition, measuring accuracy is also related to the position of the particle, especially in both x and z direction. Simulation results indicate that out-of-plane errors are higher than in-plane errors in most area.
A novel optical fiber displacement sensor is proposed and analyzed, which consists of a laser diode light source, an optical fiber probe, and three photodetectors. The bundling of optical fiber probe is sectioned into four parts: a centrally positioned fiber in the bundle for illumination, the first-neighbor circle of fibers for receiving (Group 1), the second circle of fibers for receiving (Group 2), and the remaining circle of fibers for receiving (Group 3). Then this paper describes the characteristic of the sensor compensation principle of three parts for receiving, mathematical model building and data acquisition system. The result confirms that the sensor can completely eliminate the influence of light source fluctuation, target surface reflectivity, and optical fiber loss. In addition, it can theoretically overcome the effect of inclined angle of the probe against target surface. The performance of the sensor using multi-grouped receiving fibers is improved and working distance is extended. Therefore, it can be more widely used, particularly in difficult measurement environments such as monitoring blade tip clearance and vibration of rotating turbo-machine.
KEYWORDS: Digital signal processing, Signal processing, Detection theory, Distortion, Light sources, Clocks, Sensors, Signal detection, Error analysis, Sun
A digital processing method for quasi-white light interference signal is proposed in the paper. A circuit based on EPP (Enhanced Parallel Port) is designed for high speed synchronous collecting. Envelope signals collecting is realized by means of interference fringe pulses synchronization, with the result that the space-domain interference envelope signals can be restored with high accuracy. A high accuracy algorithm for finding the acme of interference envelope signals, namely the point of zero optical path difference, is proposed in the paper. Compared with the traditional processing method based on analog circuit, the new method overcomes the time-domain wave distortion of quasi-white light signal caused by the uneven moving velocity, therefore, the errors caused by the distortion are eliminated. The method is applied in the interference signal processing of long coherent length. In the paper, the proposed method is emulated with computer and proved by experiments, and the results indicate that the locating accuracy of the points of zero optical path difference of fiber quasi-white interference signal is better than 0.5 interference fringes.
This paper presents a large range, high accuracy measuring method on base of quasi-light fiber interference technology. The system consists of quasi-light fiber locating interferometer and scanning interferometer. The light source of the locating interferometer adopts LD whose light spectrum has been widened by current injection. The locating interferometer has several fiber groups that have different light difference. Furthermore, several fibers are placed in the interference light path to multiply the measuring range two times. Therefore, the system can fulfill large range measurement (more than lOm) only using short guide (200mm). Moreover, the system sets up a zero position fiber group to fix an absolute zero position and fulfills the criterion transfer using light wavelength. The experimental results illustrate that the measuring accuracy is ±2?m and the repetition accuracy is better than 10-5. This paper demonstrates the basic principle of this method, introduces how to achieve the absolute zero position and the multiplication of the measuring range and at last, gives the corresponding experiment data.
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