外文翻譯--對振動偵查和測量的一種實用方法

河南理工大學萬方科技學院本科畢業(yè)論文 1 A Practical Approach to Vibration Detection and Measurement Physical Principles and Detection Techniques By: John Wilson, the Dynamic Consultant, LLC This tutorial addresses the physics of vibration； dynamics of a spring mass system； damping； displacement, velocity, and acceleration； and the operating principles of the sensors that detect and measure these properties. Vibration is oscillatory motion resulting from the application of oscillatory or varying forces to a structure. Oscillatory motion reverses direction. 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. The nature of the oscillation depends on the nature of the force driving it and on the structure being driven. Motion is a vector quantity, exhibiting a direction as well as a magnitude. The direction 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 arbitrarily defined at some convenient location. 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 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 河南理工大學萬方科技學院本科畢業(yè)論文 2 characteristics determine not only the static behavior (static deflection, d) of the structure, but also its dynamic characteristics. If g is the acceleration of gravity: F = MA W = Mg K = F/d = W/d d = F/K = W/K = Mg/K Dynamics of a Spring Mass System The dynamics of a spring mass system can be expressed by the systems behavior in free vibration and/or in forced vibration. 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 pendulum or swing deflected and left to freely oscillate. Two characteristic behaviors should be noted. First, damping in the system causes the amplitude of the oscillations to decrease over time. The 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 naturally occurring frequency of the free oscillations is called the natural frequency, fn: (1) Forced Vibration. Forced vibration is the case when energy is continuously added to the spring mass system by applying oscillatory force at some forcing 河南理工大學萬方科技學院本科畢業(yè)論文 3 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 motion 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 when the excitation force is generated in or on the suspended mass； externally excited vibration occurs when the excitation force is applied to the spring. This is the case, for example, when the foundation to which the spring is attached is moving. Transmissibility. When the 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 input, I, and the motion of the mass the response, R. The ratio R/I is defined as the transmissibility, Tr: Tr = R/I Resonance. At forcing frequencies well below the systems natural frequency, R I, and Tr 1. As the forcing frequency approaches the natural frequency, transmissibility increases due to resonance. Resonance is the storage of energy in the mechanical system. At 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 absorbed by damping, per cycle, equals the energy added by the exciting force, and 河南理工大學萬方科技學院本科畢業(yè)論文 4 equilibrium is reached. We find the peak transmissibility occurring when ff fn. This condition is called resonance. 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 R 0.1 in., to make them practical. The change in intensity or angle of a light beam directed onto a reflective surface can be used as an indication of its distance from the source. If the detection apparatus is fast enough, changes of distance can be detected as well. The most sensitive, accurate, and precise optical device for measuring distance 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 displacement down to 1 MHz in some PR shock accelerometers. Most contemporary PR sensors are manufactured from a single piece of silicon. In general, the advantages of sculpting the whole sensor from one homogeneous block of material are better stability, less thermal mismatch between parts, and higher reliability. Underdamped PR accelerometers tend to be less rugged than PE devices. Single-crystal silicon can have extraordinary yield strength, particularly with high strain rates, but it is a brittle material nonetheless. Internal friction in silicon is very low, so resonance amplification can be higher than for PE transducers. Both these features contribute to its comparative fragility, although if properly designed and installed they are used with regularity to measure shocks well above 100,000 g. They generally have wider bandwidths than PE transducers (comparing models of 河南理工大學萬方科技學院本科畢業(yè)論文 22 similar full-scale range), as well as smaller nonlinearities, zero shifting, and hysteresis characteristics. Because they have DC response, they are used when long-duration measurements are to be made. In a typical monolithic silicon sensing element of a PR accelerometer, the 1 mm square silicon chip incorporates the entire spring, mass, and four-arm PR strain gauge bridge assembly. The sensor is made from a single-crystal silicon by means of anisotropic etching and micromachining techniques. Strain gauges are formed by a pattern of dopant in the originally flat silicon. Subsequent etching of channels frees the gauges and simultaneously defines the masses as simply regions of silicon of original thickness. The bridge circuit can be balanced by placing compensation resistor(s) in parallel or series with any of the legs, correcting for the matching of either the resistance values and/or the change of the values with temperature. Compensation is an art； because the PR transducer can have nonlinear characteristics, it is inadvisable to operate it with excitation different from the conditions under which it was manufactured or calibrated. For example, PR sensitivity is only approximately proportional to excitation, which is usually a constant voltage or, in some cases, constant current, which has some performance advantages. Because thermal performance will in general change with excitation voltage, there is not a precise proportionality between sensitivity and excitation. Another precaution in dealing with voltage-driven bridges, particularly those with low resistance, is to verify that the bridge gets the proper excitation. The series resistance of the input lead wires acts as a voltage divider. Take care that the input lead wires have low resistance, or that a 河南理工大學萬方科技學院本科畢業(yè)論文 23 six-wire measurement be made (with sense lines at the bridge to allow the excitation to be adjusted) so the bridge gets the proper excitation. Constant current excitation does not have this problem with series resistance. However, PR transducers are generally compensated assuming constant voltage excitation and might not give the desired performance with constant current. The balance of the PR bridge is its most sensitive measure of health, and is usually the dominant feature in the total uncertainty of the transducer. The balance, sometimes called bias, zero offset, or ZMO (zero measurand output, the output with 0 g), can be changed by several effects that are usually thermal characteristics or internally or externally induced shifts in strains in the sensors. Transducer case designs attempt to isolate the sensors from external strains such as thermal transients, base strain, or mounting torque. Internal strain changes, e.g., epoxy creep, tend to contribute to long-term instabilities. All these generally low-frequency effects are more important for DC transducers than for AC-coupled devices because they occur more often in the wider frequency band of the DC-coupled transducer. Some PR designs, particularly high-sensitivity transducers, are designed with damping to extend frequency range and overrange capability. Damping coefficients of 0.7 are considered ideal. Such designs often use oil or some other viscous fluid. Two characteristics dictate that the technique is useful only at relatively low frequencies: damping forces are proportional to flow velocity, and adequate flow velocity is attained by pumping the fluid with large displacements. This is a happy coincidence for sensitive transducers in that they operate at the low acceleration frequencies where displacements are adequately large. Viscous damping can 河南理工大學萬方科技學院本科畢業(yè)論文 24 effectively eliminate resonance amplification, extend the overrange capability, and more than double the useful bandwidth. However, because the viscosity of the damping fluid is a strong function of temperature, the useful temperature range of the transducer is substantially limited. Variable Capacitance. VC transducers are usually designed as parallel-plate air gap capacitors in which motion is perpendicular to the plates. In some designs the plate is cantilevered from one edge, so motion is actually rotation； other plates are supported around the periphery, as in a trampoline. Changes in capacitance of the VC elements due to acceleration are sensed by a pair of current detectors that convert the changes into voltage output. Many VC sensors are micromachined as a sandwich of anisotropically etched silicon wafers with a gap only a few microns thick to allow air damping. The fact that air viscosity changes by just a few percent over a wide operating temperature range provides a frequency response more stable than is achievable with oil-damped PR designs. In a VC accelerometer, a high-frequency oscillator provides the necessary excitation for the VC elements. Changes in capacitance are sensed by the current detector. Output voltage is proportional to capacitance changes, and, therefore, to acceleration. The incorporation of overtravel stops in the gap can enhance ruggedness in the sensitive direction, although resistance to overrange in transverse directions must rely solely on the strength of the suspension, as is true of all other transducer designs without overtravel stops. Some designs can survive extremely high acceleration overrange conditions-as much as 1000 full-scale range . 河南理工大學萬方科技學院本科畢業(yè)論文 25 The sensor of a typical micromachined VC accelerometer is constructed of three silicon elements bonded together to form a hermetically sealed assembly. Two of the elements are the electrodes of an air dielectric, parallel-plate capacitor. The middle element is chemically etched to form a rigid central mass suspended by thin, flexible fingers. Damping characteristics are controlled by gas flow in the orifices located on the mass. VC sensors can provide many of the best features of the transducer types discussed earlier: large overrange, DC response, low-impedance output, and simple external signal conditioning. Disadvantages are the cost and size associated with the increased complexity of the onboard conditioning. Also, high-frequency capacitance detection circuits are used, and some of the high-frequency carrier usually appears on the output signal. It is generally not even noticed, being up to three orders of magnitude (i.e., 1000 ) higher in frequency than the output signals. Servo (Force Balance). Although servo accelerometers are used predominantly in inertial guidance systems, some of their performance characteristics make them desirable in certain vibration applications. All the accelerometer types described previously are open-loop devices in which the output due to deflection of the sensing element is read directly. In servo-controlled, or closed-loop, accelerometers, the deflection signal is used as feedback in a circuit that physically drives or rebalances the mass back to the equilibrium position. Servo accelerometer manufacturers suggest that open-loop instruments that rely on displacement (i.e., straining of crystals and piezoresistive elements) to produce an output signal often cause nonlinearity errors. In closed-loop designs, internal displacements are kept extremely 河南理工大學萬方科技學院本科畢業(yè)論文 26 small by electrical rebalancing of the proof mass, minimizing nonlinearity. In addition, closed-loop designs are said to have higher accuracy than open-loop types. However, definition of the term accuracy varies. Check with the sensor manufacturer. Servo accelerometers can take either of two basic geometries: linear (e.g., loudspeaker) and pendulous (meter movement). Pendulous geometry is most widely used in commercial designs. Until recently, the servo mechanism was primarily based on electromagnetic principles. Force is usually provided by driving current through coils on the mass in the presence of a magnetic field. In the pendulous servo accelerometer with an electromagnetic rebalancing mechanism, the pendulous mass develops a torque proportional to the product of the proof mass and the applied acceleration. Motion of the mass is detected by the position sensors (typically capacitive sensors), which send an error signal to the servo system. The error signal triggers the servo amplifier to output a feedback current to the torque motor, which develops an opposing torque equal in magnitude to the acceleration-generated torque from the pendulous mass. Output is the applied drive current itself (or across an output resistor), which, analogous to the deflection in the open-loop transducers, is proportional to the applied force and therefore to the acceleration. In contrast to the rugged spring elements of the open-loop transducers, the rebalancing force in the case of the closed-loop accelerometer is primarily electrical and exists only when power is provided. The springs are as flimsy in the sensitive 河南理工大學萬方科技學院本科畢業(yè)論文 27 direction as feasible and most damping is provided through the electronics. Unlike other DC-response accelerometers whose bias stability depends solely on the characteristics of the sensing element(s), it is the feedback electronics in the closed-loop design that controls bias stability. Servo accelerometers therefore tend to offer less zero drifting, which is the major reason for their uses in vibration measurements. In general, they have a useful bandwidth of 1000 Hz and are designed for use in applications with comparatively low acceleration levels and extremely low frequency components. 河南理工大學萬方科技學院本科畢業(yè)論文 28 對振動偵查和測量的一種實用方法 物理原則和偵查技術(shù) 作者： John Wilson, 動態(tài)顧問 , LLC 這篇 論文 論述 振動物理 、 彈簧質(zhì)量系統(tǒng) 的動力學 ， 阻止 、 位移、速度和加速度 ， 并且查出和測量這些物產(chǎn)傳感器的操作原理。 振動擺動由振 動或作用在機構(gòu)的力的變化引起振動的擺動。 振動行動反向。由于我們將看到，這振蕩可能是在經(jīng)過若干時間有價值的周期連續(xù)不斷的或者可能間斷的。 它可能是周期性或非周期性 , 那就是說，它可能或者可能不呈現(xiàn)一規(guī)則的周期的重復。 動擺的本質(zhì)取決于力量的本質(zhì)駕駛它和結(jié)構(gòu)被駕駛。 運動是一個矢量，呈現(xiàn)一個方向和一個量。振動的方向通常被描述依據(jù)一些獨立的坐標系（典型地笛卡爾的或者直角的）其運動的方向被稱作坐標軸。這些坐標軸的正交座標系的原點是被任意地被定義在一些適當?shù)牡奈恢谩?機構(gòu)的多數(shù)振動的響應可以用當做單自由度彈簧質(zhì)量系統(tǒng)模型 ，并且許多振動傳感器使用他們的一個彈簧質(zhì)量系統(tǒng)當做轉(zhuǎn)導機構(gòu)的機械部分。除外形尺寸之外，一個彈簧質(zhì)量系統(tǒng)可以用彈簧的剛度 K，和質(zhì)量 M，或者質(zhì)量的重量 W等性能參數(shù)來阿描述。這些特征不僅決定來這機構(gòu)的靜態(tài)特性（靜變位 d），而且決定來它的動態(tài)特性。 如果 g 是重力的加速度 : F = MA W = Mg K = F/d = W/d d = F/K = W/K = Mg/K 一個彈簧質(zhì)量系統(tǒng) 的 動力學 河南理工大學萬方科技學院本科畢業(yè)論文 29 一個彈簧質(zhì)量系統(tǒng)的動力學的可以被體系的特性在自由振動及有效 的 振動表示。 自由振動 自由振動被那情況 情形哪里那彈簧是偏斜于是釋放以及允許到自由地搖擺。例子包括一個跳板、一個跳簧跨接管，以及一個擺或搖擺偏斜以及留某事給自由地振動處理。 兩個 特征特性應該注意。 第一、阻尼在那體系表示原因的那振幅的那振蕩到減少將來。 那包括市區(qū)及郊區(qū)的那阻尼、那更快的那振幅隨時間減小。（只要彈性極限不是超過），那頻率或時期的那振蕩無關(guān)原始的大小原始的偏轉(zhuǎn)的的。 那自然地發(fā)生頻率的那自由振動被呼叫那自然頻率 fn: 受迫振動 受迫振動當能量是連續(xù)地被加到那彈簧質(zhì)量系統(tǒng)由申請振動的力在一些受迫振動頻率時的情形 ff. 兩個二例子連續(xù)地推一個孩子上去一個搖擺和一失衡旋轉(zhuǎn)電機元件。如果提供充足的能量到克服那阻尼是，那動作就會延續(xù)長達那激勵延續(xù)之久。受迫振動可以取自勵的或外部地激發(fā)振動的形式。自激振動發(fā)生在激發(fā)力是產(chǎn)生在或上去那懸掛質(zhì)量的時候；外部地激發(fā)振動發(fā)生在激發(fā)力 作用于彈簧的時候。這是那情形、例如：、當那基礎對此那彈簧附屬于是移動時。 傳導能力 當基礎正在振動，而且力整個彈簧被傳輸?shù)街兄沟馁|(zhì)量時候 ,質(zhì)量的動作將會是來自基礎的動作差積。 我們將會認為基礎的動作是輸入， I, 和質(zhì)量的動作響應 , R. 比率半徑 /我被定義為傳輸度 ,Tr: Tr = R/I 共振 在力頻率好低于體系的固有頻率， R I, 和 Tr 1。 由于作用力的頻率接近那固有頻率，由于共振，所以傳遞率增加。共振是在機械系統(tǒng)中的量的存儲。在力頻率接近那固有頻率、能量是存儲和積聚、導致增加響應 振幅。河南理工大學萬方科技學院本科畢業(yè)論文 30 阻尼也增加由于增加響應振幅、然而，并且最后那能量為阻尼所吸收、每一周期、等于能量增加由激振力，并且平衡狀態(tài)到達。我們發(fā)現(xiàn)當 ff fn.時最大傳遞率發(fā)生，這個情況被稱作共振。 隔振 如果激振力頻率超過 fn， R 降低。當 ff = 1.414 fn, R = I 或 Tr = 1 時，在比較高的頻率 R I 或 Tr 1。 在頻率當 0.1 英寸，到使他們成為現(xiàn)實的。 一束對準在一個反射面上光束在強度或者角度的的變化能被使用當做一距離指示從震源的角度之上方面。如果該探測儀器是足夠快的，變化的距離也可以被測定。最靈敏的、準確的和精密的測定距離或位移的光學裝置是激光干擾儀。利用這個儀器，一束反射激光束間雜有原來的入射光束。這由相位差形 成的干涉圖樣可以測量位移下至 1 MHz 震動加速度儀。 最現(xiàn)代的 PR傳感器是用單個碎片硅制造的。一般說來，造型整體傳感器的優(yōu)點從一個單一的材料塊是更好的穩(wěn)定性，較少熱量的失配在部分之間，并且較高的可靠性。欠阻尼的 PR 加速度儀容易不比 PE 裝置高低不平。 單一晶體矽能有特別的降伏強度 ,特別地以高的應變率，但是它是然而一個脆的事物。 矽的內(nèi)磨擦非常低，因此，諧振擴大可能是比較高的超過對于 PE 傳動器 。 兩者河南理工大學萬方科技學院本科畢業(yè)論文 42 的這些功能成為它的比較易脆性的因素 , 雖然如果適當?shù)卦O計而且安裝他們被規(guī)律性用測量震動很好上述的 100,000 g 。他們通常有較寬的頻帶寬度勝于 PE 傳動器 (比較相似實物大小范圍的模型 ), 連同較小的非線性，零的移位和磁滯特性。 因為他們有直流電反應，他們在將要產(chǎn)生長期計量時才使用。 在 PR 加速度儀的一個典型獨石矽可察元件中 ,1 毫米角尺矽芯片合并整個的彈簧，質(zhì)量和四個臂的 PR 應變計橋總成。 感知器經(jīng)由各向異性的浸蝕和顯微機械加工技術(shù)是利用一個單一晶體矽做成的。 應變計被本來平的矽一 個雜物的圖案造形。 溝流的后來浸蝕釋放規(guī)并且同時地定義如只是最初厚度的矽區(qū)域的質(zhì)量。 橋路可以由放置并聯(lián)補償電阻或者級數(shù)用任何這木頭支架平衡了，做相配的或者這阻抗值及價值的變化用溫度的修正。補償是一種藝術(shù) ； 因為 PR 傳動器能有非線性特性 , 用激發(fā)來自它被制作或校正的條件差積操作它是不受勸告的。 舉例來說， PR 靈敏度只有大約成比例激發(fā) , 通常是一個固定的電壓或 , 在一些外殼 , 定流中有一些性能利益。因為熱的性能將會大體上和激發(fā)電壓的變化 ,在靈敏度和激發(fā)之間沒有一個精密的比例。 另外的預防在處理電壓驅(qū)動的橋方面 , 特別地有低的電阻那些 , 是確認橋拿適當?shù)募ぐl(fā)。 輸入熔斷絲的級數(shù)電阻擔任一個分壓器。注意這輸入導線有低電阻，或者那一六線的大小是制成的（用讀出線在這橋梁趨于允許這激勵被校準）所以這橋梁獲得這特有的激勵。 恒定電流激勵工作沒有這些用串聯(lián)電阻的問題。然而， PR 傳動器通常被補整傲慢的固定電壓激發(fā)并且不可能用定流給被需要的性能。 PR 橋的平衡是它的健康最敏感衡量 , 而且通常是傳動器的總不確定度的占優(yōu)勢的功能。 平衡 ,有時叫做了偏向 , 零偏位 , 或 ZMO( 零可測量產(chǎn)量 ,和 0 g 的產(chǎn)量 ),能 被通常是熱的特性或在內(nèi)部或外面地誘導了感知器的應變變化的一些效應改變。傳動河南理工大學萬方科技學院本科畢業(yè)論文 43 器外殼設計嘗試隔離來自外面的應變 , 像是熱的暫態(tài)，基本的應變或固