Research On The Strain Precision Measuring With Video Image

The material science is one of the most important technique courses of theworld development nowadays. It is basic condition of the material sciencetechnique development that the physics function of the material experiment. Strainis the material physics to experiment medium important and basic parameter,usually adopting the mechanical extensometer to clip and hold the specimen,measuring at the time of inflicting the load to specimen. For the rigid material,strain can use the traditional mechanical extensometer to clip and hold thespecimen or the thin extensometer to measure. However, these devices can't use forsuch as fiber, thin film, foam or soft material of experiment, because their weightand clipping method will affect to the result of experiment and split the point.Under the condition of many, It needs to measure to know the material functionthat super big strain scope keeps go to split. Limited to the route of travel,above-mentioned extensometer account to need to be dismantled before rupture.Under some particular environment condition of experiment, for example the heatcondition, the mechanical extensometer is restricted namely.For overcoming the above-mentioned restriction, adapting the new lately-develop demand of situation, this text study a kind of technique making use of thevideo image to measure object, That is to say the technical method of whole imageand high precision non-contact strain measurement. Develop a high performancenon-contact extensometer with measurement performance. The main work of thistext is to research following several contents.1. Measure principle and system constructionsThe hardware system with video image processing technology used for strainprecise measurement consists of the light system, CCD camera, PCI card, computerand output equipments. Figure 1 show the system concrete construction. Thesoftware system consists of three models that is getting image edge, markingsystem and real time calculating the geometry parameters. Figure 2 show thesystem concrete construction. Work principle is: first make use of CCD camera totake a specimen picture, then transport the picture into the computer by PCI card,finally get the specimen edge by digital image processing technology. Thespecimen edge in picture is proportional to the actual one, and the ratio is aconstant K determined by CCD camera magnified ratio. Calculating the straingeometry parameters and comparing with the actual size.2. Digital image processing and measuringThe basic characteristic of the image is the edge. In image processing, edge isimportant to both image division and measurement. Meanwhile, edge measurementis the key technology in non-contact measurement. Its results have directly impacton precision. This text adopts rough edge detection, together with precise edgedetection to process the image. First, search the rough edge by phasing inLight system Specimen CCD camera PCI card Output equipments Computer Fig1 Hardware of strain measuring systemCapture image Input image Detect edge Mark system Output parameters Measure image Fig2 Software of strain measuring systemdispersion, the precision of location attain pixel level. For attaining the higherprecision in searching the rough edge, we make use of subpixel arithmetic ---onedimension gray moment operators. It breaks the restrict of the CCD camera physicsresolution, and makes the edge location precision attain subpixel level.3. Characteristic measurementIf you want characteristic measurement result of strain, according toε =Δl/ l0, namely obtain the variety quantity of length, you must follow the edgeprocess. We obtain a frame of image first, according to subpixel edge location, youcan get picture elements every edge takes up, then use the system ratio K to get thegeometry value of measured specimen, this is the main measuring method.Initialize the speed of test-machine, according to the variety quantity of specimenlength and the speed, calculate the pixel dispersion of close frame of image.According to this method, make sure the searching area of measured specimen. Inthis way we complete real-time edge location.4. The factors of influencing the measuring precisionIn this text we find the factors of influencing the measuring precision byexperiment. Though measuring precision of the image measurement systemdepends mainly on resolution ratio of the digital camera, we must consider manyfactors which affect the precision in order to improve precision. The factors are asfollows: Error of oneself of CCD, Error of geometrical aberrance of imagingsystem, Noise of imaging system, Noise of light ken, calibration error, arithmeticerror and influence of vibrancy. By analyzing the factors of error, we choose thebest experiment condition. The error decreased to some extent by recursion averagefiltering and least square method.5. ConclusionThe key problem of the video image measuring system is the systematicprecision, Higher precision means better measuring result, higher accuracy. Inorder to improve precision of the system, we do many experiments. Throughcomparing different experiment condition and analyzing experiment result, wechoose the best experiment condition as the follow study condition. In order toimprove precision, we deal with measuring results of many pictures equally andfinalize the results. Order to test the correctness, we choose different lengthspecimen as target object. Experiment results show that the method on video imagemeasurement of strain precision measurement is correct and feasible.