It can be observed that displacement is independent of frequency， velocityis proportional to frequency， and acceleration is proportional to the squareof the frequency. If the displacement and frequency are known， the velocity and acceleration can be calculated.
To measure any of the signals， a vibration transducer is used. A trans-ducer is a device that translates some aspect of machine vibration into a time-varying voltage output that can be analyzed. The frequency range to be analyzed should be carefully considered before selecting a transducer. Itshould be kept inmind， however， that there is no one best sensor， and severalkinds may be needed to analyze a given machine.Also， in many cases signal conditioning of the transducer signal may be required prior to analysis.
.isplacement Transducers
Eddy-current proximity probes are primarily used as displacement trans-ducers. Eddy probes generate an eddy-currentfield， which is absorbed by a conducting material at a rate proportional to the distance between the probe and the surface. They are often used to sense shaft motion relative to a bearing (by mounting them within the bearing itself) or to measure thrust motions. They are generally indifferent to hostile environments， includingtemperatures up to却5 oF(1却1 o.) and are not expensive. One drawback is that shaft surface conditions and electrical runout can result in false signals.Also， the smallest displacement that can be successfully measured is limitedby the SjN ratio of the system. Inpractice， it is difficult to measure values less than . 1 of an inch. If shaft displacement is being measured， the shaft runout (measured with the same pickup) should be less than the smallestmeasurable value. To achieve the proper shaft runout， it is necessary that theshaft be precision ground， polished， and demagnetized.
Velocity Transducers
Usual types of velocity transducers are made up of an armature mounted in a magnet. The motion of the armature in the magnet creates a voltage output proportional to the velocity of the armature.Usually， the forcesbeing measured must be relatively great to cause a signal output.However， the signal is quite strong when mounted onthe machine bearings， and amplification is usually not needed. They are very rugged but are also large and cost roughly 1 times as much as a proximity probe.
Becauseof damping， transfer function characteristics of the armature-magnet construction generally limit the low-frequency response to approxi-mately 1 Hz. At the high end of the frequencyrange， the resonant peak ofthe pickup itself is the limiting factor. Thus， the useful linear bandwidth is limited. The main advantage of the velocity pickup is that it is a high-outputjlow-impedancedevice， and hence， it provides an excellent SjN ratio-even under less than ideal conditions. The ma.or disadvantage of the velocity pickup is its sensitivity to placement. The probe is directional so that if thesame force is applied horizontally or vertically， the probe will give different readings.
Acceleration Transducers
Most accelerometers consist of some small mass mounted on a piezo-electric crystal. A voltage is produced when accelerations acting on the mass create a force acting on the crystal. Accelerometers have a wide frequency response and are not excessively costly. They also are temperature resistant. Accelerometers have two main limitations.First， they are extremely low-outputjhigh-impedance devices requiring loading impedances of at least 1MO. Such requirements rule out the use of long cables. One solution has been to have an amplifier built into the pickup to provide a low-impedancejamplified signal. A power supply is required， and the weight is increased.
Figure 16-.. .imitations on machinery vibrations analysis systems and transducers 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：燃气涡轮工程手册 Gas Turbine Engineering Handbook 3(9)
