Otopolariskop sistemi ve ölçüm düzeni

Abstract

Bu teze konu olan otomatik polariskop sistemleri mühendislik alanında uygulama bulan ve güncel olarak üzerinde araştırmalar yapılan aktif bir konudur. Veri toplama sistemlerinde otomasyon ihtiyacı, belirli bir ortamda birim alana etkiyen kuvveti deneysel olarak analiz eden yöntemlere bağlı olarak artmıştır. Fotoel as t i k ver i 1 er i n otomat i k ol ar ak t opl anması na dayanan met od, bilginin görsel ve geniş bir zaman di ününde toplandığı standart deneylere önemli bir boyut getirmiştir. Tezin ortaya koyduğu sistem fotodedeksiyon, konum belirleyici düzenek, faz dedeksiyonu ve görüntüleme yapı 1 ar ı ndan ol uş mak t adı r. Bu çalışmanın birinci bölümünde fotoel as tisi teye genel bir giriş yapılarak sistemi oluşturan bloklara ait temel fonksiyonlar, deneyin amaçları ve bu amaca ilişkin teori özet halde sunulmuştur. İkinci bölümden başlamak üzere sistem blokları detaylı olarak ele alınmış, teri ve fiziksel tasarımları elemanter yapıda şekil ve katalog verileri ile desteklenmiştir. Her bölümde alternatif bir veya birkaç metoda yer verilmiştir. Altıncı bölümde ölçme alanı ve duyarlılığı sistem kalibrasyonu ve uyarılar ele alınacaktır.

Photoelasticity is one of the enqineering tools. Due to the expanded use of experimental stress analysis, the need of automation of the data collection system in creased. The desirability of such a system is the greatest in photoelasticity, where the information was collected visually, using time- consuming compensation methods. The me tod of measuring photoelastic birefringen ce, based on the face - angle difference at two different wavelenghts, allows a complete automation of photoelastic col lection. The principal of polarization of scattered light is applied to determine the principal stresses in the interior of a model. On the basis of the teorem which states that " a series of birefringes is equivalent to an unique biref ringent, followed by a medium endowed with rotational power ". It can be assumed that, if the charecteristics. of an interior section. The measurements of the charecteristics of a biref ringent ( eventually following a medium endowed with rotational power ) can be accomplished by means of the new methods, making use of phototransistors and rotational analyzer.In this research, the basic aim is to find out the phase difference which includes the information of the photoelasticity parameters, and also to display this information to inform the specimen while testing. The interested point is located manually by the tester and also the information of the location would be displayed. This thesis can be seperated into three group by means of measuring the charecteristics of a brifringent: The first part is to dedect both the modulated monochromatic light which passed the material and the reference light which is only modulated rotational angle of the pol ar iscope. The dedection is achieved by suitable phototransistors The outputs of these phototransistors are input values of the second part of the design. The second part comprises the dedection of the phase angles which exists between the sinuisodal outputs of the phototransistors. In this part several methods are compared with each others and the best selection is made relatively. In third part, the output of phase dedectoris recovered as displaying it on a 3 1/2 liquid display. A general ADC integrated circuit whichserves as a mili voltmeter. The calibration for whole circuits is examined in the sections of the parts. Dedection Of The Light: xrThe inputs of the dedection circuit assembler are the lights which are spreaded from a monocromatic light source (laser beam ). The light is separated into two lights by a lens in front of the polarizer. One of them is the reference light and the other which passes the metarial and crosses the analyzer leads or lags the reference light with a specific phase angle. The selection of the phototransistor is made according to the their spectral responses at specific wavelenghts of laser beam. In addition to response, the radiation sensitivity, bias values forthe intensity of light, geometrical focus of lights are very importand while dedection. The concept of spinning of polarizers is easily adaptable to the measurements of direction of principal stresses. Indeed, the light intensity emergingfrom the system of crossed polarizer (P) and analyzer (A) is : I = 1^/2 [1- cos4 ( oc - ft )3 sinere - X where, esc is the position of the polarizer assembly and is the direction of one principle stresses to a selected reference. With the polarizer and analyzer crossed rotating at a constant speed : « = ft t The above expression becomes : 2 ° I = I./2 [1 - cos 4( ft t - ft )1 sin re - >:iiShowing that the light intensity is modulated with a frequency four times larger than the frequencyof polarizers rotation and becomes minimum then the polarizer is parallel to one of the principal stresses. This equation contains a constant and an alternating term. The phototransistor qeneratesan electric current, proportional to the light intensity. The alternating term produces across a load resistor an alternating voltage: V. = Vlmax cos 4 ( ft t - fi ) sin ît - X A reference voltage is generated, using an electromagnetic or photoelectric pickup, sensingthe passage of the polarizer axis by the references v,-, = v\* cos4ftt To measure the direction of principal stress, it is sufficient to compare the phase of Vi and VO signal, a function that is accomplished by a phase dedector. The frequency of polarizers rotation mostly importand to provide a sinuisodal output of phototransis tor. If this frequency is in the 1-10 Hz range, there will be no sufficient dedection and theresulting output waveform cannot be translated sufficiently to a square wave of TTL level or the larger time is consumed to provide a result of phase dedection. That's why that should be the Increased to kHz levels. In the first part of this thesis, several methods are informed to recover the sinuisodal output of phototransistor: firstly signal is amplified by a >tiiipre-amplif icator in case of low light. Then the signal are translated to TTL sguare wave forms. formsof Dedection Of The Phase Difference: This dedection can be made by several loqic circuits which is based on the pulse modulation methods. Several types of commonly used discrete phase dedectors are discussed in this part: for example exor, edge_trigger flip-flop, D- type flip-flop are used in these circuits. However these dedectors are sensitive to harmonic multiples of the incoming signal. Therefore, the phase_locked loop tends to lock onto these harmonics. In addition, these &re sensitive to changing duty cycles of the phase dedector's two inputs. If the duty cycle of either input of the e>;clusive_oR dedector is not 0,5, an e>;traneus error results. For the edge-triggered dedector, if the R input is at logic 1 when the S input i 5 also logic 1, the dedector will not function properly. The above facts shows that another assembly should be founds MC4044 which is a special integrated circuit avoids both the harmonic sensitivity and edge_triggered dedectors. This circuit consists of two digital phase dedectors, a charge pump, and an amplifier. The output dc voltage varies approximately from 0.75 to +2.25 volts as the phase difference between two inputs varies from - 2fi to +2rc radians. Displaying The Phase Difference XiVFor displaying the phase difference, a mili-vol tmeter is needed in order to display of the output volts generated by dedector. The 7176 is a 3 1/2 digit monolitic CMOS low power A/Dconverter. The single CMOS IC contains all the necessary active devices including seven segment decoders, display drivers, references and a clock. This integrated circuit interfaces with a liquid crystal display and includes a backplane drive. The ofset voltage is aplied for input range in order to obtain zero phase at a certain output voltage of the dedector. The scale factor is selected 2000mv. Additionaly the negative phase differences canbe displayed by this assembly.