Predictive maintenance & condition monitoring Sensor Piezo


Accelerometer MEMS


Accelerometer MEMS


Microphone MEMS


Ultrasonic Cost (1k unit price) $25 to $500+ $10 to $30 <$1 to $2 <$1 to $2 3 dB Bandwidth


2.5 kHz to 30 kHz+


3 kHz to 20 kHz+ 20 kHz 100 kHz Sensor Piezo


Accelerometer MEMS


Accelerometer MEMS


Microphone MEMS


Ultrasonic Size Medium Small/Medium Small Small


on an asset is correlated to the importance of assets being continuously able to operate reliably in the overall process and not at the cost of the asset itself.


The amount of energy in the vibration or


movement (peak, peak-to-peak, and rms) of a motor allows us to determine whether the machine is imbalanced or misaligned, among other things. Some faults, such as bearing or gear defects, are not as obvious, especially early on, and can’t be identified or predicted by an increase in vibration alone. These faults typically require a high performance predictive maintenance vibration sensor with low noise (<100 µg/√Hz) and wide bandwidth (>5 kHz) paired with a high performance signal chain, processing, transceivers, and postprocessing.


VibraTion, Sonic, and UlTraSonic SenSorS for PdM Microelectromechanical system (MEMS) microphones contain a MEMS element on a PCB, typically contained in a metal case with a bottom or top port to allow sound pressure waves inside. MEMS microphones offer low cost, small size, and effective means of sensing machine faults such as bearing condition, gear meshing, pump cavitation, misalignment, and imbalance. This makes MEMS microphones an ideal choice for battery-powered applications. They can be located at significant distances from the noise source and are noninvasive. When multiple assets are in operation, microphone- based performance may suffer due to the amount of audible noise from other machines or


42 Number of Axes 1 to 3 1 to 3 1 1


Vibration Mechanical Package


Yes Yes No No DC Response No Yes No No


Industry- Standard Interface


Yes Yes No No


environmental factors such as dirt or humidity, accessing the port hole in the microphone. Most MEMS microphone data sheets still list relatively benign applications such as mobile terminals, laptops, gaming devices, and cameras, etc. Some MEMS microphone data sheets list vibration


Noise/SNR


<1 µg/√Hz to 50 µg/√Hz


<25 µg/√Hz to 100 µg/√Hz


57 dB to 74 dB 65 dB


Potential Battery Life for Wireless CbM


Short to Medium Medium to Long Long Long Table 3. Predictive Maintenance Sensor Performance Specifications


Integrated Features


No Yes No No Table 4. Predictive Maintenance Sensor Mechanical Specifications


sensing or PdM as potential applications, but they also mention that sensors sensitive to mechanical shock and improper handling can cause permanent damage to the part. Other MEMS microphone data sheets state a mechanical shock survivability up to 10,000 g. There is still a lack of clarity on whether some of these sensors are suitable for operation in very harsh operating environments in the presence of potential shock events. MEMS ultrasonic microphone analysis enables the monitoring of motor health in complicated assets, in the presence of increased audible noise, because it listens to sounds in the nonaudible spectrum (20 kHz to 100 kHz) where there is far less noise. The wavelengths of low frequency audible signals typically range from approximately 1.7 cm to 17 m long. The wavelengths of high frequency signals range from approximately 0.3 cm to 1.6 cm long. When the frequency of the wavelength increases, the energy increases, making the ultrasound more directive. This is extremely useful when trying to pinpoint a failure in a bearing or housing. Accelerometers are the most commonly used


Figure 3. MEMS accelerometer, IEPE reference, PCB design allowing retrofit of the ADXL100x family of CbM accelerometers in IEPE mechanical modules. Note: Analog Devices does not produce IEPE mechanical modules.


vibration sensor and vibration analysis is the most commonly employed PdM technique, mainly used on large rotating equipment such as turbines, pumps, motors, and gearboxes. Table 3 and Table 4 show some of the key specifications for consideration when selecting high performance MEMS vibration and acoustic sensors vs. the gold standard piezo vibration sensor. Data in each column is representative of the min/max variation within that category and


October 2021 Instrumentation Monthly


Mechanical Attach


Yes Yes Noncontact Noncontact


Environmental Tolerance


Excellent Excellent Good Good Self-Test No Yes No No


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