Random(Gelişigüzel) Titreşimler Çalışan bir elektrik motorunda veya otomobilinizi çalıştırdığınızda hissettiğiniz titreşimler gelişigüzel titreşimlerdir. Gerçek hayatta, eğer özel olarak yaratılmıyorsa, düzgün salınındı titreşimlere rastlamak mümkün değildir. Gelişigüzel titreşimlerin harmonik salınımlar gibi belirli bir frekansı ve genliği yoktur. Dolayısıyla bu titreşimlere bakarak titreşime sebep olan kuvvet hakkında fikir yürütmek imkansızdır. Halbuki bizim titreşim analizi ile arızalan teşhis edebilmemiz için, bu titreşimlerin frekanslarını bilmemiz gerekir. İşte bu işlem için Fourier Dönüşümü'nü kullanıyoruz. Fourier dönüşümü vasıtası ile titreşimin sinüzoidal bileşenlerini bulabiliriz. Şekil- 11'de gelişigüzel bir titreşimin farklı frekans ve genliklere sahip sinüzoidal bileşenleri gösterilmiştir. Şekil-9'da gösterilen iki kütlenin yay sabitleri ve kütleleri farklıdır. Bu nedenle eğer her iki kütleyi de eşit miktarda çekip serbest olarak salınım yapmaya bırakırsak, her ikisi de farklı frekans ve genlikte titreşim yapacaktır. Bu iki farklı titreşimi topladığımız taktirde Şekil-10'da gösterilen grafiği elde ederiz. Olaya matematik yönünden bakacak olursak frekansları ve genlikleri farklı iki sinüzoidal eğriyi topladığımızda, sinüzoidal olmayan üçüncü bir eğri elde ederiz. Eğer bu işlemi iki değil de daha fazla sinüzoidal için yapacak olursak elde edeceğimiz grafik Şekil- 8'de gösterilen gibi bir eğri olacaktır. O halde eğer elimizde bu şekilde bir eğri varsa bu toplama işleminin tersini uygulayarak, bu düzensiz eğriyi düzgün sinüzoidaller halinde yazabiliriz. İşte bu İşleme Fourier Dönüşümü adı verilir. 1
Fourier Dönüşümü Fourier series are expansions of periodic functions f(x) interns of an infinite sum of sines and cosines of hte form. A simple statement of the Fourier Theorem is as follows: Any physical function that varies periodically with time with a frequency f can be expressed as a superposition of sinusoidal components of frequencies: f, 2f, 3f, 4f,... etc A quantitative statement of the same theorem is usually given in reverse form: If a periodic function of t, with period, can be expressed as the following summation Where, Fourier Dönüşümü Where, the coefficient Cn and the phase can be calculated from the above coefficients. By convention, the quantities appearing in these statements of the theorem are referred to by special names, as follows: The sinusoidal functions which are added in the equation are known as Fourier components. The Fourier component with the same period as the original function is known as the fundamental. Those with higher frequencies are called harmonics. The coefficients an and bn are known as Fourier coefficients. The complete list of the coefficients Cn, with the phases, is called the Fourier spectrum. In physical applications, it is occasionally useful to talk about the energy which is carried by each sinusoidal component, and is proportional to its square. The complete list of the squares of the coefficients,, appearing in in the above equation, is called the power spectrum. 2
Fast Fourier Dönüşümü The Fast Fourier Transform is a mathematical algorithm, which allows a computer to perform the Discrete Fourier Transform efficiently. The Fourier Transform produces the frequency domain signal from the time domain signal. The Fast Fourier Transform (commonly called an FFT ) computes the magnitude and phase of energy versus frequency for a given signal. An FFT does this by assuming the time domain signal is composed of a sum of sinusoids of various frequencies. The algorithm computes the amplitude of each of these sinusoids and the result is plotted as magnitude versus frequency. The FFT contains no information about the time evolution of a signal. If the frequency content of a signal changes within the time record, the FFT gives no indication of when or how that change occurred. It does, however, give a summary of all frequencies contained in the sampled data. Often we are interested only in the stable frequency content of a signal at a given time, though. There are techniques available to handle signals with changing frequency content (nonstationary signal), but here we will assume our signals here are stable in frequency(stationary signal). 3
Fourier Dönüşümü Fourier Dönüşümü Fast Fourier Dönüşümü 4
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Titreşim Ölçüm Sistemi How An "FFT" Plot Is Created? First, the vibration is "sampled" (collected) over a pre-determined period of time. The period of time used for the sample will be based on parameters programmed into either the database (for interval-based, route data collection) or the analyzer (for in-depth, or "spot", analysis). Although sometimes a relatively simple sine wave, it will far more often be a complex signal with a number of different frequency components. The "complex" signal shown below (still simplistic compared to data collected on most real machines) is made up of a 1x rpm component (e.g unbalance) and a 5x rpm component (e.g. number of vanes on the impeller - "vane pass" frequency) being generated by the machine. There can be (and usually are) far more influences - background (frictional) noise, misalignment, bearing problems, soft foot, looseness, frequency modulation, amplitude modulation, etc., etc., etc. What An "FFT" Is Actually Made Up Of?One of the "parameters" that must be programmed into the database or the analyzer is known as the "Number of Lines" (of resolution). This parameter determines how many individual amplitude values will make up the final FFT plot. That is what a spectrum is made up of - a certain number of amplitude values (e.g. 800) that each measure the vibration found in a relatively small frequency range. This parameter - number of lines - works in conjunction with your Maximum Frequency, or "Fmax", to establish your "Spectrum Resolution" - a critically important subject. The Fmax divided by the # of lines equals the spectrum resolution. The units are: "CPM per Line of Resolution" 7
Sinyaller Sürekli Sinyal(continuous time signal )(Analog sinyal) Everyday examples of analog signals include temperature, pressure, position, velocity, acceleration, force, torque, voltage and current, etc Ayrık sinyal(discrete time signal )(Dijital Sinyal) If the value of the signal is available only at certain discrete instants of time, it is called a discrete time signal. A common way to produce a discrete time signal x(k) is to take samples of an underlying analog signal xa(t) although some signals are inherently discrete time signals, economic data, for example. When finite precision is used to represent the value x(k), the sequence of quantized values is then called digital signal. If the number of bits of precision used to represent the value of x(k) is finite, then we say that x(k) is a quantized or discrete-amplitude signal. For example, if n bits are used to represent the value of x(k), then there are distinct values that x(k) can assume. Suppose the value of x(k) range over the interval. Then the quantization level, or spacing between adjacent discrete values of x(k), is 8
Sinyal Enerjisi Since we often think of signal as a function of varying amplitude through time, it seems to reason that a good measurement of the strength of a signal would be the area under the curve. However, this area may have a negative part. This negative part does not have less strength than a positive signal of the same size (reversing your grip on the paper clip in the socket is not going to make you any more lively). This suggests either squaring the signal or taking its absolute value, then finding the area under that curve. It turns out that what we call the energy of a signal is the area under the squared signal. 9
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Gürültü Experimental measurements are never perfect, even with sophisticated modern instruments. Two main types or measurement errors are recognized: systematic error, in which every measurement is either less than or greater than the "correct" value by a fixed percentage or amount, and random error, in which there are unpredictable variations in the measured signal from moment to moment or from measurement to measurement. This latter type of error is often called noise, by analogy to acoustic noise There are many sources of noise in physical measurements, such as building vibrations, air currents, electric power fluctuations, stray radiation from nearby electrical apparatus, interference from radio and TV transmissions, random thermal motion of molecules, and even the basic quantum nature of matter and energy itself. The quality of a signal is often expressed quantitatively as the signal-to-noise ratio, which is the ratio of the true signal amplitude (e.g. peak height) to the standard deviation of the noise. Signal-tonoise ratio is inversely proportional to the relative standard deviation of the signal amplitude. One of the fundamental problems in signal measurement is distinguishing the noise from the signal. The thing that really distinguishes signal from noise is that the noise is not reproducible, that is, it is not the same from one measurement of the signal to the next, whereas the genuine signal is at least partially reproducible. So if the signal can be measured more than once, use can be made of this fact by measuring the signal over and over again as fast as practical and adding up all the measurements point-by-point. This is called ensemble averaging, and it is one of the most powerful methods for improving signals, when it can be applied. 11
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Averaging There are three averaging mode available: RMS, Vector, and Peak Hold Averaging. RMS Averaging RMS averaging computes the average of the real part (X) and imaginary part (Y) of a measurement according to RMS averaging reduces fluctuations in the data but does not reduce the actual noise floor (squared values never cancel). With a sufficient number of averages, a very good approximation of the actual noise floor can be measured. Vector Averaging Vector averaging computes the average of the real part (X) and imaginary part (Y) of a measurement according to Since signed values are combined in the mean, random signals tend to average to zero. This reduces the noise floor since random signals are not phase coherent from measurement to measurement. Signals with a constant phase have real and imaginary parts which repeat from time record to time record and are preserved. Vector averaging can substantially improve the dynamic range of a measurement as long as the signals of interest have stable phases. Peak Hold Averaging Peak Hold is not really averaging, rather the magnitude of the new data is compared to the magnitude of the averaged data, and if the new magnitude is larger, then the new data becomes the averaged data. This is done on a frequency bin by bin basis. The result is averaged data with the largest magnitudes with occurred over a number measurement. 13
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LİN PEN ORTALAMA İLE GÜR. AZALIYOR The use of the FFT for frequency analysis implies two important relationships. The first relationship and one of the most fundamental rules of sampling is called the Nyquist Theorem. This theorem states that the maximum frequency which can be accurately analyzed in the frequency domain is one-half of the sampling rate used to capture the time domain signal. For example, if we want to represent the full 20 khz audio bandwidth that the human ear can hear, we must sample the audio signal at twice the maximum frequency, or at least 40 khz. In reality, samples are typically taken at 2.5 to 3 times the frequency of the highest component being measured. If the sample rate is not at least two times the rate of the highest frequency, these frequencies above one-half the sample rate will appear as a lower frequency component, or an incorrect measurement. This is called aliasing. A signal is aliased when higher frequency components appear in the lower frequency ranges due to incorrect sampling rates. Aliasing creates incorrect data and is impossible to filter. The second important relationship regards resolution. Just like the time domain, resolution determines how precisely you can examine data. However, in the frequency domain, frequency resolution is inversely proportional to the length of time of the waveform s acquisition, not the number of bits of the sample. Frequency resolution can also be expressed as the sample frequency divided by the number of sample points. One important note about frequency resolution is that you can get the same frequency resolution using various sample frequencies and values of N, or number of samples. Spectral leakage and aliasing are the biggest challenges in analyzing signals that are not ideally suited for spectral analysis. We can overcome these problems with the use of windows and anti-aliasing filters. 18
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Sensörler What Does The Transducer Actually Detect? Actual Bearing Movement: Elliptical A Transducer Mounted Vertically "Sees" Only Vertical Movement A Transducer Mounted Horizontally "Sees" Only Horizontal Movement Piezo-electric crystals are man-made or naturally occurring crystals that produce a charge output when they are compressed, flexed or subjected to shear forces. The word piezo is a corruption of the Greek word for squeeze. In a piezo-electric accelerometer a mass is attached to a piezoelectric crystal which is in turn mounted to the case of the accelerometer. When the body of the accelerometer is subjected to vibration the mass mounted on the crystal wants to stay still in space due to inertia and so compresses and stretches the piezo electric crystal. This force causes a charge to be generated and due to Newton law F=ma this force is in turn proportional to acceleration. The charge output is either is converted to a low impedance voltage output by the use of integral electronics (example: in an IEPE accelerometer) or made available as a charge output (Picocoulombs /g) in a charge output piezo-electric accelerometer. What are the different types of accelerometer? There are many different type of accelerometers and each has unique characteristics, advantages and disadvantages. The different types include: Different technologies Piezo-electric accelerometers Piezo-resistive accelerometers Strain gage based accelerometers Different output accelerometers Charge output IEPE output (2-wire voltage) Voltage output (3 wire) 4-20mA output Velocity output accelerometers Different designs of accelerometer Shear type design Single ended compression design Isolated compression 23
Çok geniş bir frekans aralığında kullanılabilir. Çok geniş bir dinamik ölçüm bölgesinde mükemmel lineer özelliğe sahiptir. ivme sinyali elektronik olarak kolayca integre edilerek hız ve deplasman bilgisine dönüştürülebilir. Çok farklı koşullarda mükemmel doğrulukta titreşim ölçümleri yapmak mümkündür. Kendi kendilerine sinyal ürettikleri için dış güç kaynağına gereksinim yoktur. Hareketli kısımları olmadığı için son derece dayanıldıdır. Titreşimlere karşı son derece duyarlıdır ve (duyarlılık / kütle) oranı yüksektir. Delta shear Tip Tasarım: Yay görevi gören üç adet piezoelektrik eleman ve üç kütle ortasındaki üçgen prizma üzerine yerleştirilmiştir. KütIeIer bulundukları yerlere yuksek öngerilimli bir tutucu kavrama halkası ile bastırılmıştır. Parçaları bir arada tutmak için yapıştırıcı veya cıvata kullanılmamıştır. Bu tasarım optimum perfamans ve ölçümde güvenilirlik sağlamaktadır. Tutucu kavrama halkasının öngerilmeli yapılmış olması piezoetektrik eiemanlara yüksek düzeyde lineeriik özelliği kazandırmaktadır. üretilen elektrik akımı kavrama halkası ile dış gövde arasında biriktirilir. DeIta kaymalı (shear) tasarımın hassasiyet i kütle oranı diğer transduserlere göre büyüktür ve oldukça yüksek rezonans frekansına sahiptir. Ayrıca bu tip transduserler, ölçüm yapılan yüzeydeki uzamalardan ve sıcaklıktan en az ölçüde etkilenir. -Delta Shear" genel amaçlı ve diğer özel ölçümler için ideal yapıdadır. 2. Düzlemsel Kaymalı Tasarım : Piezoelektrik eleman Delta tipinde oiduğu gibi kayma defamasyonuna uğrar. Transduserin merkezinde bulunan dikdörtgen kesitli gövdenin yüzüne yine dikdörtgen kesitli iki piezoelektrik eleman Şekildeki gibi yerleştirilmiştir. Bu tipte de öngerilmeli tutucu kavrama halkası piezoelektrik parçaları bağlı tutmaktadır. Transduserleri tabanı ve piezoelektrik elemanlar birbirlerinden izole edilmiştir, böylece transduser monte edildigi yüzeyin ezilmesi ve sıcaklık değişimi piezoelektrik elemanı etkilemez 3. Baskıda Çalısan Tip Tasarım :Bu geleneksel ve basit yapılı tasarım iyi sayılacak duyarlık kütle oranına sahiptir. Piezoelektrik. elemanın kütle-yay sistemi transdüserin tabanına ve tam ortada bulunan silindirik bir pim üzerine yerleştirilmiştir. Ancak transduserin tabanı ve ortadaki pim seri çalışan iki yay gibi davranır. Bu nedenle montaj yüzeyindeki dinamik değişimler eğilmeler ve sıcaklık etkileri piez0elektrik elemanda gerilmeler oluşturark sonuçların hatalı olmasına yolaçabilir. Çok kalın tabanların kullanılmasına rağmen eğilme ve gerilme kuvvetleri piezoeelektrik elemana iletilebilir. Bu durumda titreşim frekansında titreşim dışı hatalı sinyaller üretilecektir. İlaveten sıcaklık değişimleri de piezoelektrik elemanlarda elektrik şarjı üretebilir ve bu sinyaller titreşim değerlerinde hata oluşturabilir. Baskıda çalışan transduserler B&K tarafindan sadece ŞOK ölçümlerinde veya transduser kalibrasyon sistemlerinde kullanılır. Şok ölçümlerinde hata oranı, ölçülen titreşim sinyaıine göre oldukça küçüktür. Bu transdüserler laboratuar gibi kontrol edilebilen ortamlarda. standart referans transduser olarak kalibrasyon sistemlerinde kullanılır. 24
What is a single ended compression accelerometer? A single ended compression accelerometer is where the crystal is mounted to the base of the accelerometer andthemassis mountedtothecrystalby a setscrew, bolt or fastener. What is an isolated compression accelerometer? Single ended compression accelerometers can be susceptible to base strain and so to alleviate this problem the crystal is isolated from the base by mounting on an isolation washer or by reducing the mounting area bywhichthecrystalis mountedtothe base. Delta shear Tip Tasarım: Yay görevi gören üç adet piezoelektrik eleman ve üç kütle ortasındaki üçgen prizma üzerine yerleştirilmiştir. KütIeIer bulundukları yerlere yuksek öngerilimli bir tutucu kavrama halkası ile bastırılmıştır. Parçaları bir arada tutmak için yapıştırıcı veya cıvata kullanılmamıştır. Bu tasarım optimum perfamans ve ölçümde güvenilirlik sağlamaktadır. Tutucu kavrama halkasının öngerilmeli yapılmış olması piezoetektrik eiemanlara yüksek düzeyde lineeriik özelliği kazandırmaktadır. üretilen elektrik akımı kavrama halkası ile dış gövde arasında biriktirilir. DeIta kaymalı (shear) tasarımın hassasiyet i kütle oranı diğer transduserlere göre büyüktür ve oldukça yüksek rezonans frekansına sahiptir. Ayrıca bu tip transduserler, ölçüm yapılan yüzeydeki uzamalardan ve sıcaklıktan en az ölçüde etkilenir. -Delta Shear" genel amaçlı ve diğer özel ölçümler için ideal yapıdadır. 25
What is a piezo-resistive accelerometer? A piezo-resistive accelerometer is an accelerometer that uses a piezo-resistive substrate in place of the piezo electric crystal and the force exerted by the seismic mass changes the resistance of the etched bridge network and a whetstone bridge network detects this. Piezo-resistive accelerometers have the advantage over piezo-electric accelerometers in that they can measure accelerations down to zero Hertz. What is a strain gage based accelerometer? A strain gauged based accelerometer is based on detecting the deflection of a seismic mass by using a silicon or foil strain gauged element. A whetstone bridge network detects the deflection. The deflection is directly proportional to the acceleration applied to the sensor. Like the piezo-resistive accelerometer it has a frequency response down to zero Hz. What is the useable frequency range? For an accelerometer to be useful the output needs to be directly proportional to the acceleration that it is measuring. This fixed ratio of output to input is only true for a range of frequencies as described by the frequency response curve. What is an IEPE accelerometer? IEPE stands for Integrated Electronics Piezo Electric and defines a class of accelerometer that has built in electronics. Specifically it defines a class of accelerometer that has low impedance output electronics that works on a two wire constant current supply with an voltage output on a DC voltage bias. IEPE two wire accelerometers are easy to install, have a wide frequency response, can run over long cable lengths and are relatively cheap to purchase. The IEPE technology has generally replaced most 3 wire accelerometers and are broadly used for most applications except for specialist applications such as zero Hz accelerometers, high temperature applications or 4-20mA accelerometers used in the process industries. What is an ICP accelerometer? ICP is the trademarked PCB name for IEPE accelerometers. It stands for Integrated circuit-piezo electric'. The usable frequency response is the flat area of the frequency response curve and extends to approximately 1/3 to 1/2 of the natural frequency. The definition of flat also needs to be qualified and is done so by quoting the roll off of the curve in either percentage terms (typically 5% or 10%) or in db terms (typically +/- 3db). What is a charge output accelerometer? All piezo-electric accelerometers work by measuring the charge generated by a crystal that is being compressed or shear loaded by a mass influenced by acceleration. In most applications this high impedance charge output is converted to a low impedance voltage output by the use of integral electronics. However in some applications integral electronics are not appropriate such as high temperature or high radiation applications. Charge output accelerometers are self-generating and would typically have amplifying electronics mounted several feet away from the local heat or local radiation source. 26
What is dynamic range? The dynamic range of an accelerometer is the range between the smallest acceleration detectable by the accelerometer to the largest. A piezo-electric accelerometer produces a charge proportional the force applied to the crystal, which due to the seismic mass on the crystal is proportional to acceleration applied. The piezo electric effect can be detected for very small forces or accelerations all the way through to very large accelerations. In most cases the smallest acceleration is dictated by the amplifying electronics noise floor and for high g levels to the voltage rail used by the power supply. The design of the accelerometer will also play a part in what shock g levels an accelerometer can withstand before the crystal is irreparably damaged or the structure holding the crystal is distorted. Compression accelerometers are the most shock resistant design of accelerometer. Accelerometers with integral electronics have a maximum output voltage determined by the circuit design and the input voltage. The maximum output for an IEPE accelerometer is typically 4-8 volts. An accelerometer with a sensitivity of 100mV/g with electronics that has a maximum output of 5V will obviously have a dynamic range of +/- 50g while an accelerometer of sensitivity of 10mV/g will have a dynamic range of +/- 500g. What is amplitude linearity? The amplitude linearity of an accelerometer is the degree of accuracy that an accelerometer reports the output in voltage terms as it moves from being excited at the smallest detectable acceleration levels to the highest. This accuracy is qualified by the linearity. Typically the amplitude linearity is 1%. The dynamic range describes the minimum to maximum accelerations that can be detected. The output of an IEPE accelerometer can typically go from 100 micro g to 500g. This dynamic range is dependent on the electronics used with the accelerometer either internal or external, as is the output linearity over the dynamic range. When should I use a velocity output accelerometer? Velocity output accelerometers are usually used in condition monitoring applications where velocity is a much better parameter for judging the health of a machine. Doubling of velocity vibration equates to a doubling of the deterioration of the health of the machine. Velocity can also be used in lower frequency applications where the acceleration amplitude of vibration is too small to measure and the velocity vibration maybe of a higher and more meaningful value. Velocity vibration accelerometers are only really effective if the frequency of vibration is higher than 2Hz but more ideally 5 Hz. How do I choose the sensitivity of an accelerometer? Accelerometers with integral electronics have a maximum output voltage determined by the circuit design and the input voltage. The maximum output for an IEPE accelerometer is typically 4-8 volts. An accelerometer with a sensitivity of 100mV/g with electronics that has a maximum output of 5V will obviously have a dynamic range of +/- 50g while an accelerometer of sensitivity of 10mV/g will have a dynamic range of +/- 500g If the maximum g levels likely to be experienced is known then dividing this number by 5 volts will give the maximum sensitivity that should be used to get this dynamic range Example: Vibration expected to be seen is 300g. Sensitivity will be 5 divided by 300 which equals 16.6 mv/g. The nearest sensitivity would be a 10mV/g accelerometer. What is condition monitoring? Condition monitoring is where the health of a rotating machine is monitored using vibration levels. As the health of a machine (example becomes unbalanced, fan blades corrode, bearing surfaces degrade) deteriorates so the amplitude of the vibration the machine generates increases. By monitoring the vibration levels over a long period of time this gradual deterioration of the health of the machine can be assessed until the vibration levels get to a point where the machine needs to be taken out of service and overhauled. Analysis of the frequency content of the machine vibration signal will indicate not only that the health of the machine has deteriorated but also root causes can be attributed to the problem. Example: An 8 bladed pump running at 6000 rpm (100Hz) will produce a vibration signal with 100 Hz frequency if it becomes unbalanced, 200 Hz if it becomes mis-aligned 800 Hz if the blades become corroded and 43-47 Hz if the bearings start to go into oil whirl. 27
What is the natural frequency of an accelerometer? The natural frequency of an accelerometer is the frequency where the ratio of output is at it highest. The natural frequency of an accelerometer is defined by the equation: From a frequency roughly 1/3 to 1/2 of the natural frequency the ratio of output to input becomes non-linear and therefore makes measurements from this region difficult to interpret. Therefore the higher the natural frequency of an accelerometer the higher frequencies where the output to input is linear and the higher the frequencies that can be measured. It can be seen from the formula for natural frequency that to increase the natural frequency the mass needs to be as small as possible and the stiffness needs to be as high as possible. A small mass usually means a lower sensitivity and this is true of most high frequency accelerometers. It can be seen from the formula for natural frequency that to increase the natural frequency the mass needs to be as small as possible and the stiffness needs to be as high as possible. A small mass usually means a lower sensitivity and this is true of most high frequency accelerometers. What is the mounted natural frequency? What is base strain sensitivity? Base strain sensitivity is the erroneous signal that is generated by an accelerometer when the base is subjected to bending, torque or distortion either by mechanical movement or thermal stressing. The relative movement of the base of the accelerometer squeezes the crystal in an accelerometer and the seismic mass mounted on the crystal. Base strain is where the base distorts the mass while acceleration causes the seismic mass to distort the crystal. These two forces on the crystal are indistinguishable and so reduction of the base strain is vital for good signals only to be generated. The more indirectly that a crystal is mounted to the base under strain the less sensitive the accelerometer is to base strain. Single ended compression sensors are the most prone to base strain sensitivity and shear type accelerometers the least. Isolated compression accelerometers are a good compromise between have good base strain immunity and the disadvantages that shear type accelerometers bring in terms of sensitivity an robustness. 28
What is cross sensitivity or transverse sensitivity? An accelerometer produces a charge output when the crystal is compressed. That same crystal also produces a charge, albeit a much smaller one, when a shear load is exerted on the crystal. The accelerometer therefore produces a charge when it is vibrated in the axis 90 degrees to the main axis of measurement, which is indistinguishable from acceleration in the main axis. Conversely shear type accelerometers produce an erroneous signal when they experience cross axis acceleration only this time it loads the crystal in compressive mode. The sensitivity of the accelerometer to a transverse vibration is known as the transverse sensitivity and is typically less than 5% of the sensitivity to an "on axis" acceleration. The mounting of an accelerometer effects its frequency response. The mounted natural frequency is dependent directly on the stiffness of the mounting. The higher the stiffness the more the mounted natural frequency approaches its maximum. The least stiff mounting of an accelerometer is magnetic mounting and the highest stiffness is using a high tensile setscrew tightened to the correct torque mounted on a hard flat surface. Other mounting methods come in between these two extremes. İvmeölçer Tipleri 1E 1E 1D yada Doğrusal - Tek eksen boyunca ivme ölçmek 3D (üç( - eksenli) ivmeölçerler erler Tüm üç eksen boyunca ivme ölçmek Üç çıkışış sinyalini ayrı ayrı verir 29
İvmeölçer Parametreleri Ölçülebilir Sınır Maksimum ölçülebilir ivmeler - +g olarak verilir Hassasiyet -Çıkış voltajı ile ivmenin g oranı - mv/g olarak verilir Rezonans Frekansı - İvmeölçerin ikaz verdiği Frekans - khz olarak verilir Birçok İvmeölçerin 2 ila 20 miliamperlik çalışma Akımına ihtiyacı vardır Devre bileşenleri İvmeölçerleri tam randımanlı olarak sürebilmelidir 3 Genel Tip vardır Pil ve Direnç, Pil ve Diot ile Aktif Güç Kaynağı What are ground isolated accelerometers? Ground loops can be a significant problem to all type of sensors where the signal is un-amplified or the signal levels are low. Ground loops occur when different parts of the structure lab or building have different electrical grounds. These grounds may only differ by a few milli-volts or less. When areas with different grounds are connected by sensor cables then unless measures are taken to prevent it a ground loop are set up in the cable that can be significant when compared to low level voltage signals that come from the sensor. Ground loops are often very difficult to detect so it is prudent to take precautions to prevent their effects. There are a number of ways that ground loops can be prevented. The first is to hard wire different parts of the structure to ensure that each area has exactly the same ground. 30
Ensuring different parts of a plant have the same ground may not be so easy particularly when long distances are involved or structures carry noise generating machinery. In these cases it may be better not to eliminate ground loops but to prevent their effects influencing the sensor output. This can be achieved by mounting the accelerometer on an electrically isolated mounting stud. In this way the accelerometer sits on a locally constructed instrument ground and ensures that now ground loop exists between this and the measuring instrument. The same effect as mounting the accelerometer on an electrically isolated mounting base can be achieved by isolating the accelerometer internals from the outer case of the accelerometer. This is done by the manufacturer. Mounting the accelerometer on an isolating base or internally isolating the accelerometer does reduce the stiffness of the accelerometer and therefore reduces the mounted natural frequency. It is for this reason that not all accelerometers come automatically with internal isolation. Ground Loop Minimum gürültü sağlar Uygunsuz yapılırsa, hatalara neden olur Doğru Topraklamanın basitçe saptanması - Sensör hareketli ise, sistem çıkışı topraklanmalı - Sensör topraklanmışsa, sistem girişi topraklanmamalı What is an isolated stud? An accelerometer isolated stud is used in application where the possibility for ground loops exists which can corrupt the output of the sensor. Isolated studs do reduce the frequency response of the accelerometer somewhat so caution should be taken if high frequency data needs to be measured. How do I install a charge amplifier? Charge output accelerometers are used in applications where: High temperatures environments are encountered High radiation environments are encountered Very high frequency accelerometers are used where no room exists for internal electronics Charge output accelerometers are self-generating and so no excitation is required but a local charge amplifier is used to convert the charge output to a voltage. The charge output accelerometers do however have high output impedance. This high output impedance makes them susceptible to noise, cable movement (tribo-electric effect) and low insulation resistance. To minimize these effects it is important to have; a charge amplifier-impedance converter mounted as close to the accelerometer as possible, prevent cable movement, use low noise co-axial cable and ensure all surfaces are kept clean and dry. 31
What is the tribo-electric effect? Tribo-electric effect is when a spurious signal is generated by a charge output accelerometer by the movement of the co-axial cable. To prevent the tribo-electric effect the low noise cable needs to be clamped down as close to the accelerometer as possible. 32
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