【正文】 this should be on the order of 1000 M or higher to ensure sufficient lowfrequency response. In practice, a charge amplifier is normally used with a PE transducer. Instead of measuring voltage externally, a charge should be measured with a charge converter. It is a highimpedance op amp with a capacitor as its feedback. Its output is proportional 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 highpass corner frequency is set by the feedback capacitor and resistor in a charge converter, and not the transducer characteristics. (The transducer resistance changes noise characteristics, not the frequency.) If time constants are long enough, the ACcoupled transducer will suffice for most vibration measurements. Perhaps the most important limitation of highimpedance output PE transducers is that they must be used with noisetreated cables。s inside the box is irrelevant if the instrument meets the requirements of the application, but when adding to existing instrumentation it is important to be sure that the accelerometer is patible with the signal conditioning. Each type of accelerometer requires a different type of signal conditioning. Accelerometer Types. The most mon seismic transducers for shock and vibration measurements are: ? Piezoelectric (PE)。 it approaches equilibrium asymptotically. Displacement, Velocity, and Acceleration Since vibration is defined as oscillatory motion, it involves a change of position, or displacement (see Figure 1). Figure 1. Phase relationships among displacement, velocity, and acceleration are shown on these time history plots. Velocity is defined as the time rate of change of displacement。 dynamics of a spring mass system。 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 system39。 piezoresistive or silicon strain gauge type。s internal resistance. (High temperature or humidity in the transducer would exacerbate the problem by reducing the resistance value.) Energy would be drained and the output would decay, despite the constant input acceleration/force. External measurement of PE transducer voltage output requires special attention to the cable39。 that is, the ratio of resistance change to the strain is small. Their response is dominated by the elastic response. They are effectively homogeneous blocks of material with resistivity of nearly constant value. As with any resistor, they have a value proportional to length and inversely proportional to crosssectional area. If a conventional material is stretched, its width reduces while the length increases. Both effects increase resistance. The Poisson ratio defines the amount a lateral dimension is narrowed pared to the amount the longitudinal dimension is stretched. Given a Poisson ratio of (a mon value), the gauge factor would be 。 silicon strain gauge sensor ? Variable capacitance (VC)。 100 g is ~ in. 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 microscope. At higher frequencies, displacement measurement requires more sophisticated optical techniques. Highspeed movies and video can often be used to measure displacements and are especially valuable for visualizing the motion of plex structures and mechanisms. The two methods are limited by resolution to fairly large displacements and low frequencies. Strobe lights and stroboscopic photography are also useful when displacements are large enough, usually 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 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. Electromagic and Capacitive Sensors. Another important class of noncontact, specialpurpose displacement sensors is the general category of proximity sensors. These are probes that are typically built into machinery to detect 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 electromagic or capacitive (electrostatic) coupling between the probe and the target. Because these devices rely on inductive or capacitive effects, they require an electrically conductive target. In most cases, they must be calibrated for a specific target and specific material characteristics in the gap between probe and target. Electromagic proximity sensors are often called eddy current probes because one of the most popular types uses eddy currents generated in the target as its measurement mechanism. More accurately, this type of sensor uses the energy dissipated by the eddy curr