Standard referans ivmeölçerlerin enterferometrik yöntemle kalibrasyonu

Abstract

Bu çalışmamda titreşim ölçümlerinde yaygın olarak kullanılan piezoelektrik ivmeölçerler baklanda genel bilgiler verilmiş ve çalışmanın amacı olarak standard referans ivmeölçerin mutlak kalibrasyon düzeneğinin gerçekleştirilmesi esas alınmıştır. Birinci bölümde, titreşimin neden ölçülmesi gereken bir büyüklük olduğu ve bu ölçümler sırasında piezoelektrik ivmeölçerin seçilme nedenleri üzerinde durulmuştur. Bu bölümde ayrıca ivmeölçerin mevcut tiplerinden ve kalibrasyon metodlarından bahsedilmiştir. İkinci bolümde, ivmeölçer kalibrasyonunda dataların amaçlarımıza uygun olmasının gerekliliği üzerinde durulmuş ve bu amaçla titreşim ölçümünde kullanılan temel büyüklükler ve kullanım nedenlerinden bahsedilmiştir. Üçüncü bölümde, piezoelektrik ivmeölçerlerin yapılan, mevcut çeşitleri, dizayn özellikleri ve tüm yapısal özellikleri üzerinde durulmuş, sıkıştırma ve kayma dizaynları için gerekli teknik detaylar verilmiştir. Dördüncü bölümde, titreşim ölçümlerinde ivmeölçerin önemli bir parçası olan ve ivmeölçerden gelen düşük seviyeli sinyali güçlendiren önyükselteçler anlatılmıştır. Ayrıca yük ve voltaj önyükselteçleri sağladıkları avantaj ve dezavantajları ile ele alınmıştır. Beşinci bölümde, titreşim ölçümünde kullanılan ivmeölçerin içinde bulunduğu ortam şartlarından nasıl etkilendiği ve bu etkilerin ivmeölçer hassasiyetine etkisi anlatılmıştır. Altıncı bölümde, daha önceki bölümlerde bahsedilen sebeplerden dolayı ivmeölçerin neden bir kalibrasyona ihtiyaç duyduğu vurgulanmıştır. Dünyada mevcut olan kalibrasyon hiyerarşisi, metodları tanıtıldıktan sonra enterferometre tekniği ve sağladığı yararlar anlatılmıştır. Bölümde esas alman ise, tezin amacı olan standard referans ivmeölçerin enterferometrik yöntemle mutlak kalibrasyon metodu anlatılmıştır. Yedinci bölümde, UME' de gerçekleştirilmiş olan standard referans ivmeölçerlerin enterferometrik metodla mutlak kalibrasyon sistemi anlatıldı. Bir kalibrasyon sırasında gerçekleştirilmesi gereken kalibrasyon prosedürleri adım adım anlatılarak kalibrasyonla ilgili datalar ve kalibrasyon sonuçları verildi. Sekizinci bölümde, çalışmanın amacı olan ve gerçekleştirilen sistemle ilgili deneysel sonuçlar irdelenmiştir. Deney sırasında karşılaşılan zorluklar ve çözümleri verilerek sistemin nasıl geliştirilebileceği anlatılmıştır.

Since man began to build machines for industrial use, and especially since motors have been used to power them, problems of vibration reduction and isolation have engaged engineers. Gradually, as vibration isolation and reduction techniques have become an integral part of macine design, the need for accurate measurement and analysis of mechanical vibration has grown. This need was largely satisfied, for the slow and robust machines of yesteryear, by the experienced ear and touch of the plant engineer, or by simple optical instruments measuring vibratory displacement. Over there last decade a whole new technology of vibration measurement has been developed which is suitable for investigating modern highly stressed, high speed machinery. Using piezoelectric accelerometers to convert vibratory motion into an electrical signal, the process of measurement and analysis is ably performed by versatile abilities of electronics. Piezoelectric accelerometers have become very popular compared to traditional vibration transducers (velocity probes, eddy current probes, etc.), as mey permit full utilization of the wide dynamic and frequency range and high resolution of the advanced measurement analysis equipment of today. In order to make accurate measurement of vibration parameters such as displacement, velocity and acceleration the equipment used in the measurement must be calibrated. That is why precise calibration methods are needed. One of the precise calibration method used for piezoelectric accelerometer's calibration is interferometric method. This method relies on counting of interferometric fringes (Mich elson' s interferometer is used). The aim of this work is to realize the absolute calibration of Standard Reference Accelerometers by laser interferometry method. The present work consists of eight chapters and an appendix. PARTI. INTRODUCTION Vibration is measured for many different reasons general all uncontrolled vibration is an undesirable phenomenon which gives rise to noise, causes mechanical stress and is a possible cause of structural failure. There are several broad areas of vibration measurement. For instance, the human vibration measurement is one of them. This area concerns the measurement of the vibration transmited to human beings. These vibration can, for example, xm originate from passenger vehicles and hand-held power tools. The measured vibration levels are then related to human comfort and health by international standards. In present days piezoelectric accelerometer is considered to be the optimum choice of vibration transducer. The extensive range of high performance measuring equipment now avaible can fully utilize the very wide frequency range and dynamic range offered by this type of vibration transducer. In general two types of accelerometer's design exist : 1. Shear Type 2. Compression Type Piezoelectric accelerometers are rather stable toenviromental condition's changes, ijn spite of this the sensitivity of acceleremeter is change due to different factors such as temperature transients, acoustical noise, etc. However, during every measurements it is necessary to know exact value of accelerometer's sensitivity, which can be determined through calibration procedure. Two metods are used for accelerometer's calibration : 1. Absolute calibration metod 2. Comparison calibration metod In this work the experimantal set-up for absolute calibration of piezoelectric accelerometers and result of the calibration carried out at National Metrology Enstitute (NMI) of TÜBİTAK MAM have been presented. PART 2. VIBRATION AND SHOCK PARAMETERS In this part, the determination vibration and parameters characterizing a vibration are given. In general a body is said to be vibrate when it describes oscilation motion about referance position. The number of times a complete motion cycle takes place during of one second is called the frequency and measured in Hertz (Hz) The parameters characterizing the vibration process are following : 1. Amplitude 2. Velocity 3. Acceleration The vibration amplitude, which is the characteristic which describes the severity of vibration, can be quantified in several ways. 1. Peak to Peak Value : The peak to peak value is valuable in that it indicates maximum excursion of the wave, a useful quantity where, for example, the vibratory displacement of XIV a machine part is critical for maximum stress or mechanical clearance considerations. 2. Peak Value : The peak value is particulary valuable for indicating the level of short duration shock etc. But, peak values only indicate what maximum level occured, no account is taken of time history of the wave. The rectified average value, on the other hand, does take the time history of the way into account, but is considered of limited practical interest because it has no direct relationship with any useful physical quantity 3. RMS Value : The RMS value is most relevant measure of amplitude because it both takes the time history of the wave into account and gives an amplitude value which is directly related to the energy content, and therefore the destructive abilities of the vibration. PART 3. PIEZOELECTRIC ACCELEROMETER In this part the description of piezoelectric accelerometer is given. The transdusr which is more or less universally used for vibration measurements is the piezoelectric accelerometer. It exhibits better all round characteristics than any other type of vibration transduser. It has very wide frequency and dynamic ranges with good linearitythroughout the ranges. The piezoelectric accelerometer is self-generating, so it does not need a power supply, there are no moving parts to wear out, and finally, its acceleration propotional output can be integrated to give velocity and dispacement propotional signals. The heart of a piezoelectric accelerometer is the slice of piezoelectric material, usually an artificially polarized ferroelectric ceramic, which exhibits the unique piezoelectric effect. When it mechanically stressed, either in tension, compression or shear, it generates an electrical charge across its pole faces which is propotional to the applied force. The main characterizing apiezoelectric accelerometer are following : 1. Sensitivity 2. Mass 3. Frequency range PART 4. ACCELEROMETER PREAMPLIFIERS Direct loading of piezoelectric accelerometer's output, even by relatively high impedance loads, can greatly reduce the accelerometr's sensitivity as well aslimits its frequency response. To minimise these effects the accelerometer output signal is fed through xv apreamplifier which converts to a much lower impedanse, of measuring and analyzing instrumentation. With measuring amplifiers, analyzers, and voltmeters a separate accelerometer preamplifier is used while vibration meters intended for use with piezoelectric acceleremeters normally have the preamplifier built-in. In addition to the function of impedance conversion, most preamplifiers offer additional facilities for conditioning the signal. For example, a calibreted variable gain facility to amplify the signal to a suitable level for input to a tape recorder; A secondary gain adjustment to "normalize" transduser sensitivities; Integrators to convert the acceleration propotional output from accelerometers to either velocity or displacement signal; Varuis filter to limit the upper and lower frequency resonse to avoid interference from electrical noise or signal out side the linear portion of the accelerometer frequncy range. PART 5. INFLUENCE OF ENVIROMENTAL CONDITIONS ON ACCELEROMETER'S PERFORMANCE In this part main factors affecting on accelerometer's performance are described. These are : 1. Temperature transitions 2. Ground loop 3. Tribo-electric noise 4. Electromagnetic noise 5. Base strains 6. Nuclear radiation 7. Magnetic fields 8. Humidity 9. Acoustic noise 10. Transver vibrations In spite of that modern accelerometers and accelerometers cables are designed to have the minimum possible sensitivity to the many external influences, every factor from above mentioned is able to change an acceleometer's characteristics in certain degree. PART 6. CALIBRATION AND TEST OF AN ACCELEROMETERS In this part, accelerometer's calibration methods are discussed. xvi In general, two ways for accelerometers calibration are possible : 1. Absolute calibration 2. Comparasion calibration 1. Absolute Calibration : Absolute Calibration implies that the accelerometer sensivity is determined by measurements based on fundamental and derived units for the physical quantities involved. Today the most convenient method for absolute calibration is laser interferometry. Thus, since the advent of the laser, reciprocity calibration is becoming rare as this method is rather involved and requires very careful measurements to obtain good results. Comparasion Calibration : Comparison Calibration implies mat the sensivity of the transducer to be calibrated is measured relative to a Standard Reference Transducer with known sensivity. To avoid the necessity of carrying out absolute calibrations of all accelerometers for common use, a hierarchy of Standard Transducers, accelerometers used for calibration, is established. The Standard Transducers are ranked in three groups : Primary Standard Transducers are calibrated by absolute methods. They are kept at the International and National Standard Institutes or at the Calibration Laboratories where they have been calibrated. Transfer Standard Transducers are calibrated by comparison or absolute methods at the above-mentioned Institutes and Laboratories. By interchanging and recalibrating Transfer Standard Transducers among International and national Standard Institutes, consistency is established. By interchanging and calibrating Transfer Standard Transducers between a Standard Institute and various Calibration Laboratories the consistency of the calibration mey carry out is established. To keep track of which Standard Institute or Institutes have established the consistency of the calibration performed at a particular Calibration Laboratory, the transducers calibrated there are designated to be traceable to the Standard Institute/Institutes. Working Reference Standard Transducers are used for comparison calibration of accelerometers in common use, i.e. actual measurements. Working Reference Standard Transducers are calibrated by Standard Institutes or Calibration Laboratories by using absolute or comparison calibration methods. XVH The Working Reference Standard Transducers are checked by means of TransferStandard Transducers. In this way the consistency and hence the traceability of the calibration of Vibration Transducers in common use are ensured. PART 7. SET-UP FOR ACCELEROMETER'S CALIBRATION AND RESULT OF CALIBRATION In mis part, a description of the set-up for absolute calibration of an accelerometers establish at National Metrology Enstitute is given. Here it is shown the results of calibration. PART 8. CONCLUSION In this part, the technical result and other important observation for the experiment system are given. Also in this part, the technical result and other important observation for the experiment system are given. The details of measurement on which particular atention must be paid for obtaining correct results are also described.