外文翻译--对振动侦查和测量的一种实用方法

河南理工大学万方科技学院本科毕业论文 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 河南理工大学万方科技学院本科毕业论文 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 河南理工大学万方科技学院本科毕业论文 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 河南理工大学万方科技学院本科毕业论文 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 河南理工大学万方科技学院本科毕业论文 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 河南理工大学万方科技学院本科毕业论文 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 河南理工大学万方科技学院本科毕业论文 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 . 河南理工大学万方科技学院本科毕业论文 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 河南理工大学万方科技学院本科毕业论文 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 河南理工大学万方科技学院本科毕业论文 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. 河南理工大学万方科技学院本科毕业论文 28 对振动侦查和测量的一种实用方法 物理原则和侦查技术 作者： John Wilson, 动态顾问 , LLC 这篇 论文 论述 振动物理 、 弹簧质量系统 的动力学 ， 阻止 、 位移、速度和加速度 ， 并且查出和测量这些物产传感器的操作原理。 振动摆动由振 动或作用在机构的力的变化引起振动的摆动。 振动行动反向。由于我们将看到，这振荡可能是在经过若干时间有价值的周期连续不断的或者可能间断的。 它可能是周期性或非周期性 , 那就是说，它可能或者可能不呈现一规则的周期的重复。 动摆的本质取决于力量的本质驾驶它和结构被驾驶。 运动是一个矢量，呈现一个方向和一个量。振动的方向通常被描述依据一些独立的坐标系（典型地笛卡尔的或者直角的）其运动的方向被称作坐标轴。这些坐标轴的正交座标系的原点是被任意地被定义在一些适当的的位置。 机构的多数振动的响应可以用当做单自由度弹簧质量系统模型 ，并且许多振动传感器使用他们的一个弹簧质量系统当做转导机构的机械部分。除外形尺寸之外，一个弹簧质量系统可以用弹簧的刚度 K，和质量 M，或者质量的重量 W等性能参数来阿描述。这些特征不仅决定来这机构的静态特性（静变位 d），而且决定来它的动态特性。 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 一个弹簧质量系统 的 动力学 河南理工大学万方科技学院本科毕业论文 29 一个弹簧质量系统的动力学的可以被体系的特性在自由振动及有效 的 振动表示。 自由振动 自由振动被那情况 情形哪里那弹簧是偏斜于是释放以及允许到自由地摇摆。例子包括一个跳板、一个跳簧跨接管，以及一个摆或摇摆偏斜以及留某事给自由地振动处理。 两个 特征特性应该注意。 第一、阻尼在那体系表示原因的那振幅的那振荡到减少将来。 那包括市区及郊区的那阻尼、那更快的那振幅随时间减小。（只要弹性极限不是超过），那频率或时期的那振荡无关原始的大小原始的偏转的的。 那自然地发生频率的那自由振动被呼叫那自然频率 fn: 受迫振动 受迫振动当能量是连续地被加到那弹簧质量系统由申请振动的力在一些受迫振动频率时的情形 ff. 两个二例子连续地推一个孩子上去一个摇摆和一失衡旋转电机元件。如果提供充足的能量到克服那阻尼是，那动作就会延续长达那激励延续之久。受迫振动可以取自励的或外部地激发振动的形式。自激振动发生在激发力是产生在或上去那悬挂质量的时候；外部地激发振动发生在激发力 作用于弹簧的时候。这是那情形、例如：、当那基础对此那弹簧附属于是移动时。 传导能力 当基础正在振动，而且力整个弹簧被传输到中止的质量时候 ,质量的动作将会是来自基础的动作差积。 我们将会认为基础的动作是输入， I, 和质量的动作响应 , R. 比率半径 /我被定义为传输度 ,Tr: Tr = R/I 共振 在力频率好低于体系的固有频率， R I, 和 Tr 1。 由于作用力的频率接近那固有频率，由于共振，所以传递率增加。共振是在机械系统中的量的存储。在力频率接近那固有频率、能量是存储和积聚、导致增加响应 振幅。河南理工大学万方科技学院本科毕业论文 30 阻尼也增加由于增加响应振幅、然而，并且最后那能量为阻尼所吸收、每一周期、等于能量增加由激振力，并且平衡状态到达。我们发现当 ff fn.时最大传递率发生，这个情况被称作共振。 隔振 如果激振力频率超过 fn， R 降低。当 ff = 1.414 fn, R = I 或 Tr = 1 时，在比较高的频率 R I 或 Tr 1。 在频率当 0.1 英寸，到使他们成为现实的。 一束对准在一个反射面上光束在强度或者角度的的变化能被使用当做一距离指示从震源的角度之上方面。如果该探测仪器是足够快的，变化的距离也可以被测定。最灵敏的、准确的和精密的测定距离或位移的光学装置是激光干扰仪。利用这个仪器，一束反射激光束间杂有原来的入射光束。这由相位差形 成的干涉图样可以测量位移下至 1 MHz 震动加速度仪。 最现代的 PR传感器是用单个碎片硅制造的。一般说来，造型整体传感器的优点从一个单一的材料块是更好的稳定性，较少热量的失配在部分之间，并且较高的可靠性。欠阻尼的 PR 加速度仪容易不比 PE 装置高低不平。 单一晶体矽能有特别的降伏强度 ,特别地以高的应变率，但是它是然而一个脆的事物。 矽的内磨擦非常低，因此，谐振扩大可能是比较高的超过对于 PE 传动器 。 两者河南理工大学万方科技学院本科毕业论文 42 的这些功能成为它的比较易脆性的因素 , 虽然如果适当地设计而且安装他们被规律性用测量震动很好上述的 100,000 g 。他们通常有较宽的频带宽度胜于 PE 传动器 (比较相似实物大小范围的模型 ), 连同较小的非线性，零的移位和磁滞特性。 因为他们有直流电反应，他们在将要产生长期计量时才使用。 在 PR 加速度仪的一个典型独石矽可察元件中 ,1 毫米角尺矽芯片合并整个的弹簧，质量和四个臂的 PR 应变计桥总成。 感知器经由各向异性的浸蚀和显微机械加工技术是利用一个单一晶体矽做成的。 应变计被本来平的矽一 个杂物的图案造形。 沟流的后来浸蚀释放规并且同时地定义如只是最初厚度的矽区域的质量。 桥路可以由放置并联补偿电阻或者级数用任何这木头支架平衡了，做相配的或者这阻抗值及价值的变化用温度的修正。补偿是一种艺术 ； 因为 PR 传动器能有非线性特性 , 用激发来自它被制作或校正的条件差积操作它是不受劝告的。 举例来说， PR 灵敏度只有大约成比例激发 , 通常是一个固定的电压或 , 在一些外壳 , 定流中有一些性能利益。因为热的性能将会大体上和激发电压的变化 ,在灵敏度和激发之间没有一个精密的比例。 另外的预防在处理电压驱动的桥方面 , 特别地有低的电阻那些 , 是确认桥拿适当的激发。 输入熔断丝的级数电阻担任一个分压器。注意这输入导线有低电阻，或者那一六线的大小是制成的（用读出线在这桥梁趋于允许这激励被校准）所以这桥梁获得这特有的激励。 恒定电流激励工作没有这些用串联电阻的问题。然而， PR 传动器通常被补整傲慢的固定电压激发并且不可能用定流给被需要的性能。 PR 桥的平衡是它的健康最敏感衡量 , 而且通常是传动器的总不确定度的占优势的功能。 平衡 ,有时叫做了偏向 , 零偏位 , 或 ZMO( 零可测量产量 ,和 0 g 的产量 ),能 被通常是热的特性或在内部或外面地诱导了感知器的应变变化的一些效应改变。传动河南理工大学万方科技学院本科毕业论文 43 器外壳设计尝试隔离来自外面的应变 , 像是热的暂态，基本的应变或固