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24 Advancements in Industrial Automation RPM for bearing monitoring. For fans and gearboxes, the minimum upper limit of the accelerometer should be four to five times the blade passing frequency. Measurement Resolution and Dynamic Range The measurement resolution of the vibration sensor is a function of the amplitude of the output signal to the broadband noise of the onboard electronics. An accelerometer with superior signal output will allow the measurement of smaller vibration levels in the machinery and predict earlier failure modes. Long-Term Stability Long-term drift is a shift in the sensitivity and/or zero output measurement. A shift in the sensitivity and/or zero output of the accelerometer could trigger a false alarm over time in the monitoring application. A shift in the zero-output measurement will have the same effect of potentially causing a false alarm indication. Since piezoelectric sensors do not have a DC response, they are not susceptible to zero drift, only sensitivity drift. Variable-capacitance (VC) MEMS sensors can have both zero and sensitivity drift over time. The following sections review two different types of technologies that offer condition-monitoring applications. Piezoelectric Vibration Sensors Piezoelectric (PE) accelerometers incorporate piezoelectric crystals that are self-generating and provide a signal when stressed by external excitation (such as vibrating machinery). Most piezoelectric sensors are based on lead zirconate titanate ceramics (PZT), which are poled to align the dipoles and make the crystals piezoelectric. PZT crystals are ideal for condition-monitoring applications since they offer a wide temperature range, broad dynamic range, and wide frequency bandwidth (usable to more than 20kHz). Variable-Capacitance Vibration Sensors VC sensors derive the acceleration measurement from a change in capacitance of a seismic mass moving between two parallel capacitor plates. The change in capacitance is directly proportional to the applied acceleration. VC accelerometers require an IC to be closely coupled to the sensing element to convert the very small capacitance changes into a voltage output. This conversion process often results in poor signal-to-noise ratio and limited dynamic range. Long-term drift is a shift in the sensitivity and/or zero output measurement. " "