This paper is focus on the applications of EM sensor in stress measurement for steel cables used in bridges. The
calibration of the EM sensors has been investigated. A new methodology makes this calibration process done either in
the laboratory or in the field. Application of EM sensors on QianJiang No.4 Bridge to monitor the stresses of key hanger
cables and post-tensioned cables is presented. Furthermore, a multi-EM sensor configuration has been developed to
monitor the stress in a multi-strand-cable system.
In this paper, the calibration and workability of magnetoelastic (ME or EM) stress sensors for large steel cables used in Qiangjiang 4th bridge in China are discussed. As an engineering application of magnetoelasticity, EM sensors make non-contact stress monitoring possible for steel hangers and post-tensioned cables on suspension and cable-stayed bridges, and other ferromagnetic structures. By quantifying the correlation of tension with magnetic properties represented by the relative permeability of the steel structure itself, the EM sensor inspects the loading level of the steel structure. The tension dependence of the relative permeability and the temperature influence was calibrated. The results revealed that the magnetoelasticity of the multi-wire hangers is consistent with one another, while the post-tensioned cables are similar to single wires. Cable stress monitoring on Qiangjiang (Qj) 4th bridge demonstrated the reliability of the EM sensors in safety evaluation of bridge.
A modern bridge is such a complicated system that is difficult to analyze by conventional mathematic tools. A rational bridge monitoring requires a good knowledge of the actual condition of various structural components. Fatigue analysis of concrete bridges is one of the most important problems. Concrete bridges are often undergoing a fatigue deterioration, starting with cracking and ending with large holes through the web. There is a need for the development of efficient health assessment system for fatigue evaluation and prediction of the remaining life. This information has clear economical consequences, as deficient bridges must be repaired or closed. The goal of this research is to provide a practical expert system in bridge health evaluation and improve the understanding of bridge behavior during their service. Efforts to develop a functional bridge monitoring system have mainly been concentrated upon successful implementation of experienced-based machine learning. The reliability of the techniques adopted for damage assessment is also important for bridge monitoring systems. By applying the system to an in-service PC bridge, it has been verified that this fuzzy logic expert system is effective and reliable for the bridge health evaluation.
Smart Health Monitoring System (SHMS) is a set of integrated system of hardware and software designed to automatically collect and analyze the data from a faraway bridge. The real-time data can be preprocessed in the sub-workstation on the bridge then transferred to the main server with a wired or wireless internet access. SHMS is based on the statistical analysis of the static and dynamic characteristics of structures. In order to automate the procedure of processing and analyzing all the raw data, a rule-based expert system was developed for the monitoring system with Bootstrap Method. In general, the estimation of parameters from measurement always contains systematic perturbations and random fluctuations. The systematic perturbations mainly come from periodic environmental factors, especially temperature. Random fluctuations result from irregular disturbance including instrumentation sources and numerical processing algorithms. The former can be identified and characterized. Based on the historical data, a set of correction models have been built to remove the influence from systematic perturbations. Random fluctuations are difficult to be characterized by traditional statistical methods. But with Bootstrap Method, we can minimize the random error.
Polyvinylidene fluoride (PVDF) is a piezoelectric polymer material. One of its most attractive applications is being used as a sensor for structure monitoring. A suitable circuit interface plays an important role in sensor design. PVDF sensor can be used in a large variety of situations according to different design of circuit. The approach to a special circuit interface, which enables PVDF sensor to be utilized as a wireless “dynamic strain gage”, is presented in this paper. The wireless PVDF sensor was then tested and all the results have been compared with strain gage output for strain and displacement measurements.
Investigation of surface profiling method for large aspheres becomes more and more important and imperative with the great development of the synchrotron radiaiton facility (SRF), since the latter puts greater demands on surface quality, shape and figure parameters of the optical elements used in itself. MEanwhile, things became more difficult because of the unique characteristics of the optics used in SRF. As a result, novel surface measurement methods and systems have to be developed to cope with such problems.
In this paper is introduced a new concept -- image point displacement (IPD), on the basis of which the principles and key tasks of light pattern projection profilometry are re- interpreted and evaluated. Two error sources that have not been addressed adequately are analyzed in detail. One is coordinate deviation, which comes from the different magnification at different depth. The other is the nonlinearity of the phase-IPD and phase-height relationship. These error sources will be more and more non-negligible as researchers make their ways to improve accuracy and increase measuring range. Some compensation approaches are also proposed, which have been verified by numerical simulations and experiments.
Phase unwrapping is an inevitable process in modern phase- evaluation based fringe analysis because the arctangent functions used in phase extraction only return phase modulo 2(pi) . In recent years, several temporal phase unwrapping methods have been proposed, which use a couple of phase maps with different sensitivities for the unique determination of the unwrapped phase on a pixel-by-pixel basis. We have found that, however, some of them are not effectively designed, probably due to the lack of proper philosophy for guidance. In this paper, we present a generalized mathematical excess fraction method, which is more compete and general in nature and can be used to re-interpret those temporal phase unwrapping algorithms in a unifying perspective. It is also demonstrated that more efficient and flexible algorithms can be designed based on the principle of the generalized excess fraction method.
Grating projection 3D profilometry has three major problems that have to be handled with great care. They are local shadows, phase discontinuities and surface isolations. Carrying no information, shadow areas give us no clue about the profile there. Phase discontinuities often baffle phase unwrappers because they may be generated for several reasons difficult to distinguish. Spatial phase unwrapping will inevitably fail if the object under teste have surface isolations. In this paper, a complementary grating projection profilometry is reported, which attempts to tackle the three aforementioned problems simultaneously. This technique involves projecting two grating patterns form both sides of the CCD camera. Phase unwrapping is carried out pixel by pixel using the two phase maps based on the excess fraction method, which is immune to phase discontinuities or surface isolations. Complementary projection makes sure that no area in the visible volume of CCD is devoid of fringe information, although in some cases a small area of the reconstructed profile is of low accuracy compared with others. The system calibration procedures and measurement results are presented in detail, and possible improvement is discussed.
Phase unwrapping, an unavoidable process in grating projection profilometry based on phase measurement, often manifests itself as the major obstacle that compromises measuring reliability and prevents automated profiling. In this paper, a robust temporal phase unwrapping algorithm, namely, non- linear excess fraction method (NLEFM) is proposed, which may enlarge the measuring range by dozens of times at no cost of accuracy. The algorithm presented in a general form demonstrates that the conventional linear excess fraction method can be readily extended into non-linear domain once certain condition is met. This novel method has been successfully applied to profiling complex objects with abrupt discontinuities and surface isolations. The multi-frequency grating projection profilometer based on NLEFM is promised to be developed into a robust, high accuracy and automated shape measurement system.
A frequency shift interferometer for absolute distance measurement using LD-pumped Nd:YVO<SUB>4</SUB>, microchip laser is proposed in this paper. The LD pumped Nd:YVO<SUB>4</SUB> crystal microchip laser here is an external cavity laser. By modulating the voltage supplied to PZT, the frequency of the laser beam is modulated. A frequency shift interferometer using the technique of frequency-modulated continuous-wave is established. In this experiment setup a reference interferometer is used to compensate for the drift of the central frequency of the laser. The experimental results show that drift of central frequency of the laser affects the accuracy of the measurement a lot and can be compensated effectively. But influence from the drift of frequency modulation ratio tot eh accuracy can not be compensated.
The paper proposes an absolute distance interferometry (ADI) system using the technique of heterodyne and superheterodyne detecting. A two wavelengths HeNe 633nm laser is employed to generate a long synthetic wavelength for measuring absolute distance. The characteristics of the heterodyne ADI system using AOMs are analyzed. The optical wave cross talk and poor wave front quality will decrease the performance of the heterodyne ADI system. By using the technique of superheterodyne and optical fiber, the performance of the new ADI system is improved. The common optical path and reference signal picking configuration in the interferometer eliminate the affection of air turbulence and problem form optical fiber effectively.
In this paper a novel method of measuring coaxiality and perpendicularity using laser alignment, diffraction imaging and CCD image detecting is proposed. In this method, we use a laser diode fiber alignment beam as the datum reference of straightness, deviation of the center of the hole from this datum, coaxiality, can be detected by a CCD camera. By rotating a penta-prism that makes this alignment beam bend a right angle a scanning datum plane is generated which can be used to detect the perpendicularity of the axis to an end plane. In the paper the factors that affect the accuracy of the system are analyzed. The experiment result shows that the measurement accuracy of the system is 3 + 2.3 X 10<SUP>+6</SUP>.
A research on using linear array CCD detecting technique in measuring form and position errors of large scale dimension is described in this paper. The basic idea of this kind of measurement is that using a stable aligned laser beam incident on a 1D asymmetrical phase plate to form a black line as the reference of the beam, form and position datum lines and planes can be established through a scanner. A linear array CCD is used to measure deviation between the datum and the quantity be measured directly and absolutely. In the paper, we also discuss the reading error of the CCD probe caused by various factors in detail and introduce the CCD probe and its signal processing circuit. Some of experiment results and its application are shown.
An improved signal processing method for laser interferometer is investigated to measure the radius of large precision centrifuge. Based on phase and integer periodic phase measurement, it not only makes the resolution up to 1 in 2000 of a wavelength, but also enlarges the range of the interferometer. The absolute radius measurement of the 5m centrifuge with the accuracy of +/- 3.4micrometers is achieved.
A new type of non-contact heterodyne profilometer with annular lens is described in this paper. The surface can be measured without the separated reference surface. The theoretical lateral resolution of this system becomes much higher of 1 m. The optical structure is easy to set up.
On the basis of the combination of phase measurement and integer periodic phase measurement, a signal processing
scheme, which is used for optical heterodyne interferometer with large range, is described. It not only maintains the
high measuring resolution , but also enlarges the measuring range, with solving the problem of large range and
measurement of dynamic displacement.
This paper proposes a heterodyne interferometer using a dual longitudinal mode HeNe 633 nm laser for absolute distance interferometry. It can measure the fractional fringe of the synthetic wavelength by an electrical phasemeter with high acccuracy. The measuring period is very short. The common optical path configuration in the interferometer eliminates the affection of air turbulence effectively.
The paper discusses some optical methods of measuring the profile of disk non-contacting. Grazing incident interferometer, Color schieren, and Moire deflectometer. Especially in the paper a new measuring method and instrument is introduced which is a pointwise instrument with an optical probe. It can measure a global and local profile of a disk in tangential and radial direction at the same time, without sacrificing the measurement accuracy. The resolution of the probe is 0.3 nm, the instrument accuracy better than 0.04 micrometers , the dynamic range larger than 5 mm.
A frequency-modulation absolute distance measurement techniques using a external-cavity semiconductor laser is proposed. A reference interferometer was adopted in order to reduce the effect of the central frequency drift of the laser on the measurement accuracy. The principle of the measurement is described and the experimental setup is introduced. The measurable range is as large as ten meters.
The paper introduces a new method to measure the flatness of the disk
with the laser heterodyne interferometer. Its most important advantage is
that it can measure the large and intermediate scale disk profile at the
same time, without sacrificing the measurement accuracy. The resolution
of the interferometer is 0.3 nm, the accuracy better than 0.04 jim, the
dynamic range larger than 5 mm. Otherwise the interferometer can nieasure
the disk global and local profile in tangential and radial direction.
A fiber displacement sensor with a Linear Frequency Modulation (LFM)
laser diode and its optical electric signal processing method are
described in the paper. It operates on the principle of a non-balance
optical fiber Michelson interferometer to realize heterodyne detecting.
During processing the heterodyne signal, phase locked loops (PLL) are
used to extend the resolution. The resolution of the sensor can reach to
π/40. In the paper the experiment result is being showed also.
Many Methods have been developed to .easure displace.ent with high accuracy, for
exap1e, with a dual frequency laser interferometer (AC interferometer) and an classic
interferoseter (DC interferoeter) which use a stabilized laser and fringe counter, and
an AC interfero.eter has ore advantage over the DC one.
An AC interfero.eter with a Zee.an laser can get a high resolution, in the order of
nanoMeters, but its resolution extension liRited by nonlinear relation between phase and
displace.ent which caused by the two-frequency coRponents in interferoaeter. Because
the fundaaental length scale of the interferometer is the wavelength of the light source
in the air. The accuracy of an interferoeter is li.ited by the operating envireaent,
teRperature, husidity, pressure, etc. because the aiRs of interferoseters expose in the
air. A high resolution optical fiber heterodyne interfermeter is described in the paper.