Issue link: https://te.mouser.com/i/1382447
Predictive Maintenance with Vibration Sensors TE CONNECTIVITY SENSORS / WHITE PAPER PAGE 3 Long-Term Stability Long-term drift is a shift in the sensitivity and/or zero output measurement (zero output drift applies to MEMS sensors only). A shift in the sensitivity of the accelerometer could trigger a false alarm over time in the monitoring application. A shift in the zero output measurement will also have 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. MEMS VC sensor can have both zero and sensitivity drift over time. In the following section, we will review two different types of technologies that are offered for 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 piezo-electric. PZT crystals are ideal for condition monitoring applications since they offer wide temperature range, broad dynamic range, and wide frequency bandwidth (usable to >20kHz). There are basically two main types of PE accelerometer designs available: Compression Mode and Shear Mode (Flexure Mode is a rarely used alternative). Compression Mode designs are assembled by stressing the piezo-electric crystal in compression by loading a mass on top of the crystal and applying a preload force. These designs are outdated and becoming less popular due to performance limitations. The constructions are susceptible to mounting base strain and have higher thermal drifts. Shear Mode designs typically have an annular shear crystal and annular mass that is secured to a support post. This design has superior performance compared to compression mode designs in that it is base isolated and much less susceptible to thermal stresses ensuring improved stability. Most condition monitoring accelerometer designs offered today are shear mode and should be the choice of design for most condition monitoring installations. PE ACCELEROMETER DESIGNS