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55 Engineering a More Sustainable Future | ADI Table 5. Different Accelerometer Types and Most Important Design Specifications How Do 3-Axis MEMS Sensors Compare to IEPE Vibration Sensors? Extensive testing has been carried out on the efficacy of triaxial MEMS accelerometers in identifying, with absolute confidence, specific faults that much higher performance single or even dual axis IEPE vibration sensors could not detect, as summarized in Table 6. 1 Faults like a bent shaft, eccentric rotor, bearing issues, and cocked rotor could not be detected by a single-axis vibration sensor with absolute certainty unless some efforts were taken to understand specific anomalies prior to mounting. When only a single-axis vibration sensor is available, other CbM sensors, such as motor current or magnetic field, would be required to identify certain faults with more confidence. A trade-off exists between a single-axis sensor with superior noise and bandwidth vs. triaxial sensing, and these extra axes can alleviate mounting position challenges as all the vertical, horizontal, and axial vibration will be detected, as well as offering deeper insights into the operation of an asset. According to the results presented in Table 6, most of the faults could not be confidently identified with a single axis sensor without reorienting and retesting, even if it has superior noise and bandwidth vs. a triaxial MEMS sensor. What Other Vibration Sensors Are Available and How Do They Compare? So, how do 3-axis MEMS sensors fit into the vibration sensor spectrum? Figure 9 shows an overview of the MEMS vibration sensors available today based on their noise vs. bandwidth. IEPE sensors are included for reference and to help highlight where exactly MEMS sensors fit onto the vibration sensor spectrum. It is clear that the different type of MEMS sensors naturally form clusters that we can use to assign potential use cases—for example, the lowest cost sensors ( MEMS triaxial ) would be used on lower criticality assets whereas the highest cost sensors ( IEPE ) will be used on the highest criticality assets. Single-axis IEPE sensors have been in use for decades and cover all areas from low to high criticality applications and are widespread in terms of cost and performance, as shown in Figure 9. It is clear to see that ADXL357 ADXL1002 Piezo Sensor (603C01) Size (mm) 6 × 5.6 × 2.2 5 × 5 × 1.8 18 × 42.2 Integrated ADC Yes No No No. Axes 3 1 1 Power Supply (V) 2.25 to 3.6 3.3 to 5.25 18 to 28 Interface SPI Analog Analog Weight (g) <0.2 <0.2 51 Noise (µg/√Hz) 80 25 350 Bandwidth (kHz) 1 11 10 Current Consumption 200 µA 1 mA 2 mA to 20 mA Spectrum Signatures Detectable with 1-Axis Notes Fault Detectable with z-axis (vertical) Detectable with y-axes (horizontal) Detectable with x-axes (axial) Confident detection with single axis Imbalanced Load Fundamental (1×) Fundamental (1×) 9×, 10× Yes Extra axes provide more confidence in measurement Eccentric Rotor Fundamental (1×) 3× No Axial signature clearly indicates rotor eccentricity Cocked Rotor Fundamental (1×) 3×, 4×, 5×, 6×, 7×, 8×, 9×, 10× No Axial signatures at high harmonics clearly identify cocked rotor Bent Shaft Fundamental (1×) Fundamental (1×) 3× Possible Axial signature clearly indicates bent shaft BPFO 3× (BPFO), 4× 3× (BPFO), 4× Yes Single axis measurement on the x-axis would miss this BPFI Fundamental (1×) 5× (BPFI) No Measuring on the x- or z-axis would miss this fault Table 6. Summary of Fault Frequency Signatures for Commonly Occurring Machine Faults Detected by a Triaxial MEMS Accelerometer Adobe Stock / WilliamJu – stock.adobe.com