![]() A bulk micromachined MEMS sensor will have the best long-term drift but will also be significantly more expensive and typically only used in inertial applications. Variable Capacitance MEMS accelerometers also have wide specification limits for long-term drift depending on the MEMS design structure. PZT (Lead-Zirconate-Titanate) crystals are the most common crystals used in PE accelerometers and are increasingly becoming the choice of crystal for most applications. Quartz has the best long-term stability of any PE accelerometer but it is rarely used in condition monitoring applications due to limited output and cost. The long-term drift parameters will also depend on the crystal formulation used so an actual value would be difficult to present. Piezo-electric crystals are inherently stable and have shown to have excellent stability over time. The long-term stability of PE sensors is well known with over 30 years field installations. Most condition monitoring accelerometer designs offered today are shear mode and should be the choice of design for most condition monitoring installations. 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. Shear Mode designs typically have an annular shear crystal and annular mass that is secured to a support post. The constructions are susceptible to mounting base strain and have higher thermal drifts. ![]() These designs are outdated and becoming less popular due to performance limitations. There are basically two main types of PE accelerometer designs available: Compression Mode and Shear Mode (Flexure Mode is a rarely used alternative).Ĭompression 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. PZT crystals are ideal for condition monitoring applications since they offer wide temperature range, broad dynamic range, and wide frequency bandwidth (usable to >20kHz). ![]() Most piezoelectric sensors are based on Lead Zirconate Titanate ceramics (PZT) which are poled to align the dipoles and make the crystals piezoelectric. Piezoelectric (PE) accelerometers incorporate piezoelectric crystals that are self-generating and provide a signal when stressed by external excitation such as vibrating machinery.
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