機(jī)械專業(yè)外文文獻(xiàn)翻譯--對振動偵查和測量的一種實(shí)用方法

1、<p><b>　　英文原文</b></p><p>　　A Practical Approach to Vibration Detection and Measurement</p><p>　　——Physical Principles and Detection Techniques</p><p>　　By: John Wil

2、son, the Dynamic Consultant, LLC</p><p>　　This tutorial addresses the physics of vibration; dynamics of a spring mass system; damping; displacement, velocity, and acceleration; and the operating principles o

3、f the sensors that detect and measure these properties.</p><p>　　Vibration is oscillatory motion resulting from the application of oscillatory or varying forces to a structure. Oscillatory motion reverses di

4、rection. As we shall see, the oscillation may be continuous during some time period of interest or it may be intermittent. It may be periodic or nonperiodic, i.e., it may or may not exhibit a regular period of repetition

5、. The nature of the oscillation depends on the nature of the force driving it and on the structure being driven. </p><p>　　Motion is a vector quantity, exhibiting a direction as well as a magnitude. The dire

6、ction of vibration is usually described in terms of some arbitrary coordinate system (typically Cartesian or orthogonal) whose directions are called axes. The origin for the orthogonal coordinate system of axes is arbitr

7、arily defined at some convenient location.</p><p>　　Most vibratory responses of structures can be modeled as single-degree-of-freedom spring mass systems, and many vibration sensors use a spring mass system

8、as the mechanical part of their transduction mechanism. In addition to physical dimensions, a spring mass system can be characterized by the stiffness of the spring, K, and the mass, M, or weight, W, of the mass. These c

9、haracteristics determine not only the static behavior (static deflection, d) of the structure, but also its dynamic character</p><p>　　F = MA   W = Mg   K = F/d = W/d &#

10、160; d = F/K = W/K = Mg/K</p><p>　　Dynamics of a Spring Mass System</p><p>　　The dynamics of a spring mass system can be expressed by the system's behavior in free vibration and/or in

11、forced vibration. </p><p>　　Free Vibration. Free vibration is the case where the spring is deflected and then released and allowed to vibrate freely. Examples include a diving board, a bungee jumper, and a p

12、endulum or swing deflected and left to freely oscillate.</p><p>　　Two characteristic behaviors should be noted. First, damping in the system causes the amplitude of the oscillations to decrease over time. Th

13、e greater the damping, the faster the amplitude decreases. Second, the frequency or period of the oscillation is independent of the magnitude of the original deflection (as long as elastic limits are not exceeded). The n

14、aturally occurring frequency of the free oscillations is called the natural frequency, fn:</p><p>　　Forced Vibration. Forced vibration is the case when energy is continuously added to the spring mass system

15、by applying oscillatory force at some forcing frequency, ff. Two examples are continuously pushing a child on a swing and an unbalanced rotating machine element. If enough energy to overcome the damping is applid, the mo

16、tion will continue as long as the excitation continues. Forced vibration may take the form of self-excited or externally excited vibration. Self-excited vibration occurs whe</p><p>　　Transmissibility. When t

17、he foundation is oscillating, and force is transmitted through the spring to the suspended mass, the motion of the mass will be different from the motion of the foundation. We will call the motion of the foundation the i

18、nput, I, and the motion of the mass the response, R. The ratio R/I is defined as the transmissibility, Tr:</p><p><b>　　Tr = R/I </b></p><p>　　Resonance. At forcing frequencies well b

19、elow the system's natural frequency, RI, and Tr1. As the forcing frequency approaches the natural frequency, transmissibility increases due to resonance. Resonance is the storage of energy in the mechanical system. A

20、t forcing frequencies near the natural frequency, energy is stored and builds up, resulting in increasing response amplitude. Damping also increases with increasing response amplitude, however, and eventually the energy

21、absorbed by damping, per</p><p>　　Isolation. If the forcing frequency is increased above fn, R decreases. When ff = 1.414 fn, R = I and Tr = 1; at higher frequencies R <I and Tr <1. At frequencies when

22、 R <I, the system is said to be in isolation. That is, some of the vibratory motion input is isolated from the suspended mass.</p><p>　　Effects of Mass and Stiffness Variations. From Equation (1) it can b

23、e seen that natural frequency is proportional to the square root of stiffness, K, and inversely proportional to the square root of weight, W, or mass, M. Therefore, increasing the stiffness of the spring or decreasing th

24、e weight of the mass increases natural frequency.</p><p>　　DampingDamping is any effect that removes kinetic and/or potential energy from the spring mass system. It is usually the result of viscous (fluid)

25、or frictional effects. All materials and structures have some degree of internal damping. In addition, movement through air, water, or other fluids absorbs energy and converts it to heat. Internal intermolecular or inter

26、crystalline friction also converts material strain to heat. And, of course, external friction provides damping. </p><p>　　Damping causes the amplitude of free vibration to decrease over time, and also limits

27、 the peak transmissibility in forced vibration. It is normally characterized by the Greek letter zeta () , or by the ratio C/Cc, where c is the amount of damping in the structure or material and Cc is "critical damp

28、ing." Mathematically, critical damping is expressed as Cc = 2(KM)1/2. Conceptually, critical damping is that amount of damping which allows the deflected spring mass system to just return to its equil</p><

29、;p>　　Displacement, Velocity, and Acceleration</p><p>　　Since vibration is defined as oscillatory motion, it involves a change of position, or displacement (see Figure 1).</p><p>　　Figure 1. P

30、hase relationships among displacement, velocity, and acceleration are shown on these time history plots.</p><p>　　Velocity is defined as the time rate of change of displacement; acceleration is the time rate

31、 of change of velocity. Some technical disciplines use the term jerk to denote the time rate of change of acceleration.</p><p>　　Sinusoidal Motion Equation. The single-degree-of-freedom spring mass system, i

32、n forced vibration, maintained at a constant displacement amplitude, exhibits simple harmonic motion, or sinusoidal motion. That is, its displacement amplitude vs. time traces out a sinusoidal curve. Given a peak displac

33、ement of X, frequency f, and instantaneous displacement x:</p><p>　　at any time, t.</p><p>　　Velocity Equation. Velocity is the time rate of change of displacement, which is the derivative of th

34、e time function of displacement. For instantaneous velocity, v:</p><p>　　Since vibratory displacement is most often measured in terms of peak-to-peak, double amplitude, displacement D = 2X:</p><p&

35、gt;　　If we limit our interest to the peak amplitudes and ignore the time variation and phase relationships:</p><p><b>　　where: </b></p><p>　　V = peak velocity </p><p>　　

36、Acceleration Equation. Similarly, acceleration is the time rate of change of velocity, the derivative of the velocity expression:</p><p><b>　　and</b></p><p><b>　　where: </b&

37、gt;</p><p>　　A = peak acceleration </p><p>　　It thus can be shown that:</p><p>　　V = fDA = 22 f2 DD = V/fD = A/22 f2</p><p>　　From these equations, it can be seen th

38、at low-frequency motion is likely to exhibit low-amplitude accelerations even though displacement may be large. It can also be seen that high-frequency motion is likely to exhibit low-amplitude displacements, even though

39、 acceleration is large. Consider two examples:</p><p>　　? At 1 Hz, 1 in. pk-pk displacement is only ~0.05 g acceleration; 10 in. is ~0.5 g ? At 1000 Hz, 1g acceleration is only ~0.00002 in. displacement; 100

40、 g is ~0.002 in. </p><p>　　Measuring Vibratory Displacement   Optical Techniques. If displacement is large enough, as at low frequencies, it can be measured with a scale, calipers, or a measuring

41、 microscope. At higher frequencies, displacement measurement requires more sophisticated optical techniques. </p><p>　　High-speed movies and video can often be used to measure displacements and are especiall

42、y valuable for visualizing the motion of complex structures and mechanisms. The two methods are limited by resolution to fairly large displacements and low frequencies. Strobe lights and stroboscopic photography are also

43、 useful when displacements are large enough, usually >0.1 in., to make them practical.</p><p>　　The change in intensity or angle of a light beam directed onto a reflective surface can be used as an indica

44、tion of its distance from the source. If the detection apparatus is fast enough, changes of distance can be detected as well.</p><p>　　The most sensitive, accurate, and precise optical device for measuring d

45、istance or displacement is the laser interferometer. With this apparatus, a reflected laser beam is mixed with the original incident beam. The interference patterns formed by the phase differences can measure displacemen

46、t down to <100 nm. NIST and other national primary calibration agencies use laser interferometers for primary calibration of vibration measurement instruments at frequencies up to 25 kHz.</p><p>　　Electro

47、magnetic and Capacitive Sensors. Another important class of noncontact, special-purpose displacement sensors is the general category of proximity sensors. These are probes that are typically built into machinery to detec

48、t the motion of shafts inside journal bearings or the relative motion of other machine elements. The sensors measure relative distance or proximity as a function of either electromagnetic or capacitive (electrostatic) co

49、upling between the probe and the target. Because thes</p><p>　　Electromagnetic proximity sensors are often called eddy current probes because one of the most popular types uses eddy currents generated in the

50、 target as its measurement mechanism. More accurately, this type of sensor uses the energy dissipated by the eddy currents. The greater the distance from probe to target, the less electromagnetic coupling, the lower the

51、magnitude of the eddy currents, and the less energy they drain from the probe. Other electromagnetic probes sense the distortion of an e</p><p>　　Capacitive proximity sensor systems measure the capacitance b

52、etween the probe and the target and are calibrated to convert the capacitance to distance. Capacitance is affected by the dielectric properties of the material in the gap as well as by distance, so calibration can be aff

53、ected by a change of lubricant or contamination of the lubricant in a machine environment.</p><p>　　Contact Techniques. A variety of relative motion sensors use direct contact with two objects to measure rel

54、ative motion or distance between them. These include LVDTs, cable position transducers (stringpots), and linear potentiometers. All of these devices depend on mechanical linkages and electromechanical transducers.</p&

55、gt;<p>　　Seismic Displacement Transducers. These devices, discussed in detail later, were once popular but now are seldom used. They tend to be large, heavy, and short lived.</p><p>　　Double Integrati

56、on of Acceleration. With the increasing availability and decreasing cost of digital signal processing, more applications are using the more rugged and more versatile accelerometers as sensors, then double integrating the

57、 acceleration signal to derive displacements. While older analog integration techniques tended to be noisy and inaccurate, digital processing can provide quite high-quality, high-accuracy results.</p><p>　　M

58、easuring Vibratory Velocity   Transducers. Some of the earliest "high-frequency" vibration measurements were made with electrodynamic velocity sensors. These are a type of seismic transducer that in

59、corporates a magnet supported on a soft spring suspension system to form the seismic (spring mass) system. The magnetic member is suspended in a housing that contains one or more multiturn coils of wire. When the housing

60、 is vibrated at frequencies well above the natural frequency of the spring mass</p><p>　　Laser Vibrometers. Laser vibrometers or laser velocimeters are relatively new instruments capable of providing high se

61、nsitivity and accuracy. They use a frequency-modulated (typically around 44 MHz) laser beam reflected from a vibrating surface. The reflected beam is compared with the original beam and the Doppler frequency shift is use

62、d to calculate the velocity of the vibrating surface. Alignment and standoff distance are critical. Because of the geometric constraints on location, alignment, a</p><p>　　Integration of Acceleration. As wit

63、h displacement measurements, low-cost digital signal processing makes it practical to use rugged, reliable, versatile accelerometers as sensors and integrate their output to derive a velocity signal.</p><p>

64、　　Measuring Vibratory Acceleration</p><p>　　Most modern vibration measurements are made by measuring acceleration. If velocity or displacement data are required, the acceleration data can be integrated (velo

65、city) or double integrated (displacement). Some accelerometer signal conditioners have built-in integrators for that purpose. Accelerometers (acceleration sensors, pickups, or transducers) are available in a wide variety

66、 of sizes, shapes, performance characteristics, and prices. The five basic transducer types are servo force balance; </p><p>　　Seismic Accelerometer Principle. All seismic accelerometers use some variation o

67、f a seismic or proof mass suspended by a spring structure in a case (see Figure 3). When the case is accelerated, the proof mass is also accelerated by the force transmitted through the spring structure. Then the displac

68、ement of the spring, the displacement of the mass within the case, or the forcetransmitted by the spring is transduced into an electrical signal proportional to acceleration.</p><p>　　Accelerometers. Transdu

69、cers designed to measure vibratory acceleration are called accelerometers. There are many varieties including strain gauge, servo force balance, piezoresistive (silicon strain gauge), piezoelectric (crystal-type), variab

70、le capacitance, and integral electronic piezoelectric. Each basic type has many variations and trade names. Most manufacturers provide excellent applications engineering assistance to help the user choose the best type f

71、or the application, but because most </p><p>　　For most applications, my personal bias is toward piezoelectric accelerometers with internal electronics. The primary limitation of these devices is temperature

72、 range. Although they exhibit low-frequency roll-off, they are available with extremely low-frequency capabilities. They provide a preamplified low-impedance output, simple cabling, and simple signal conditioning, and ge

73、nerally have the lowest overall system cost.</p><p>　　Most important to the user are the performance and environmental specifications and the price. What's inside the box is irrelevant if the instrument

74、meets the requirements of the application, but when adding to existing instrumentation it is important to be sure that the accelerometer is compatible with the signal conditioning. Each type of accelerometer requires a d

75、ifferent type of signal conditioning.</p><p>　　Accelerometer Types. The most common seismic transducers for shock and vibration measurements are:</p><p>　　Piezoelectric (PE); high-impedance outp

76、ut </p><p>　　Integral electronics piezoelectric (IEPE); low-impedance output </p><p>　　Piezoresistive (PR); silicon strain gauge sensor </p><p>　　Variable capacitance (VC); low-leve

77、l, low-frequency </p><p>　　Servo force balance </p><p>　　Piezoelectric (PE) sensors use the piezoelectric effects of the sensing element(s) to produce a charge output. Because a PE sensor does n

78、ot require an external power source for operation, it is considered self-generating. The "spring" sensing elements provide a given number of electrons proportional to the amount of applied stress (piezein is a

79、Greek word meaning to squeeze). Many natural and man-made materials, mostly crystals or ceramics and a few polymers, display this characteristic. These m</p><p>　　Piezoelectric materials may also have a dipo

80、le (which is the net separation of positive and negative charge along a particular crystal direction) when unstressed. In these materials, fields can be generated by deformation from stress or temperature, causing piezoe

81、lectric or pyroelectric output, respectively. The pyroelectric outputs can be very large unwanted signals, generally occurring over the long time periods associated with most temperature changes. Polymer PE materials hav

82、e such high pyro</p><p>　　Charges are actually not "generated," but rather just displaced. (Like energy and momentum, charge is always conserved.) When an electric field is generated along the dire

83、ction of the dipole, metallic electrodes on faces at the opposite extremes of the gradient produce mobile electrons that move from one face, through the signal conditioning, to the other side of the sensor to cancel the

84、generated field. The quantity of electrons depends on the voltage created and the capacitance between the ele</p><p>　　Choosing among the many types of PE materials entails a tradeoff among charge sensitivit

85、y, dielectric coefficient (which, with geometry, determines the capacitance), thermal coefficients, maximum temperature, frequency characteristics, and stability. The best S/N ratios generally come from the highest piezo

86、electric coefficients.</p><p>　　Naturally occurring piezoelectric crystals such as tourmaline or quartz generally have low-charge sensitivity, about one-hundredth that of the more commonly used ferroelectric

87、 materials. (But these low-charge output materials are typically used in the voltage mode, which will be discussed later.) Allowing smaller size for a given sensitivity, ferroelectric materials are usually man-made ceram

88、ics in which the crystalline domains (i.e., regions in which dipoles are naturally aligned) are themselve</p><p>　　Polarization usually occurs at temperatures considerably higher than operating temperatures

89、to speed the process of alignment of the domains. Depolarization, or relaxation, can occur at lower temperatures, but at very much lower rates, and can also occur with applied voltages and preload pressures. Depolarizati

90、on always results in temporary or permanent loss of sensitivity. Tourmaline, a natural crystal that does not undergo depolarization, is particularly useful at very high temperatures.</p><p>　　Because they ar

91、e self-generating, PE transducers cannot be used to measure steady-state accelerations or force, which would put a fixed amount of energy into the crystal (a one-way squeeze) and therefore a fixed number of electrons at

92、the electrodes. Conventional voltage measurement would bleed electrons away, as does the sensor's internal resistance. (High temperature or humidity in the transducer would exacerbate the problem by reducing the resi

93、stance value.) Energy would be drained and the ou</p><p>　　External measurement of PE transducer voltage output requires special attention to the cable's dynamic behavior as well as the input characteris

94、tics of the preamplifier. Since cable capacitance directly affects the signal amplitude, excessive movement of the cable during measurement can cause changes in its capacitance and should be avoided. Close attention shou

95、ld also be paid to the preamp's input impedance; this should be on the order of 1000 M or higher to ensure sufficient low-frequency resp</p><p>　　In practice, a charge amplifier is normally used with a P

96、E transducer. </p><p>　　Instead of measuring voltage externally, a charge should be measured with a charge converter. It is a high-impedance op amp with a capacitor as its feedback. Its output is proportiona

97、l to the charge at the input and the feedback capacitor, and is nearly unaffected by the input capacitance of the transducer or attached cables. The high-pass corner frequency is set by the feedback capacitor and resisto

98、r in a charge converter, and not the transducer characteristics. (The transducer resistance chang</p><p>　　Perhaps the most important limitation of high-impedance output PE transducers is that they must be u

99、sed with "noise-treated" cables; otherwise, motion in the cable can displace triboelectric charge, which adds to the charge measured by the charge converter. Triboelectric noise is a common source of error foun