Predictive maintenance & condition monitoring


Figure 4. Three-axis MEMS CbM module with


integrated ADC, processor, FFT, and statistics, as well as a mechanical package with resonant frequency over 50 kHz.


for prolonged periods.7 While this is not a distinct weakness, it is worth considering if your PdM application exists in a harsh


environment with high humidity. In such cases, electret condenser microphones (ECMs) have shown advantages over MEMS microphones. Other environmental


does not correlate to adjacent columns. The CbM industry is due to have significant


growth over the next five years with wireless installations accounting for a significant amount of this growth. Piezo accelerometers are less suitable for wireless CbM systems due to a combination of size, lack of integrated features, and power consumption, but solutions do exist with typical consumption in the range of 0.2 mA to 0.5 mA. MEMS accelerometers and microphones are highly suited to battery- powered PdM systems due to their small size, low power, and high performance capabilities. All sensors have suitable bandwidths and low


noise, but MEMS accelerometers are the only sensors than can offer a response down to dc, useful for detection of imbalance at very low rotational speeds and tilt sensing. MEMS accelerometers also have a self-test feature where the sensor can be verified to be 100 per cent functional. This could prove useful in safety- critical installations where meeting system standards is made easier by being able to verify if a sensor is still functional. It is possible to completely hermetically seal MEMS accelerometers in ceramic packages and piezo accelerometers in mechanical packages for use in harsh, dirty environments. Table 4 focuses on physical, mechanical, and environmental performance of the sensors. This is where the key differences can be seen between each sensor such as integration, tolerance to harsh environments, mechanical performance, and attachment to a rotating machine or mount. Detecting vibration data in three axes offers more diagnostic insights and can lead to better fault detection. While this is not necessary in every PdM installation, it is a distinct advantage offered by piezo and MEMS accelerometers in terms of data quality, wiring, and space savings. MEMS microphones have shown distortion of up to –8 dB when exposed to increased humidity


Instrumentation Monthly October 2021


conditions that can affect microphones are wind, atmospheric pressure, electromagnetic fields, and mechanical shock. In benign environments, MEMS microphones


offer excellent performance in PdM applications. Currently, there is a lack of information available on mounting MEMS microphones in harsh operating environments with excessive vibrations, dirt, or humidity. Vibration can affect the performance of MEMS microphones, and this is an area that needs consideration; however, they do have lower vibration sensitivity than ECMs. If a wireless PdM solution were to use a MEMS microphone, the mounting box would need to have a hole or port to allow the acoustic signal to reach the sensor, adding further design complexity and potentially making other electronics susceptible to dirt or humidity. Recent advancements in capacitive MEMS accelerometer technology have allowed small, low cost, low power, wireless CbM solutions to be implemented on lower priority assets, allowing further diagnostic insights into facilities management and maintaining critical system uptime. These advancements also moved MEMS accelerometers closer to piezo performance for use in more traditional, wired CbM systems. Having such low noise and wide bandwidth, coupled with industry-standard connections (ICP and IEPE), piezo accelerometers have been the gold standard sensor used in vibration measurement for decades. MEMS accelerometers have been adapted to interface with IEPE standard modules, as shown in Figure 3. The conversion circuit is based on a Circuits from the Lab reference design. The circuit was designed on a special PCB that has been characterised to perform over wide bandwidths and is ready to be designed into a mechanical module at a later stage. The device shown in Figure 4 contains three single-axis MEMS accelerometers, three ADCs, a


processor, memory, and algorithms, all in a mechanical module with a resonance over 50 kHz. This highlights the capability of MEMS accelerometers to integrate intelligence at the sensor node, ensuring the sensor is paired with the best signal chain and processing to achieve the best performance possible. This module can perform FFTs, trigger various time domain or frequency domain alarms, and generate time domain statics vital for algorithms or machine learning tools to predict failures.


When it comes to choosing the most suitable


vibration sensor for your PdM solution, the real challenge lies in pairing sensors to meet the most likely potential failure modes of your assets. MEMS microphones are not yet proven to be robust enough to reliably detect all vibration- based failure modes in the harshest of environments, whereas the industry standard for vibration sensing, accelerometers, have been successfully implemented and performed reliably for decades. MEMS ultrasonic microphones have shown promising performance in detecting bearing faults earlier than accelerometers, and this potential symbiotic relationship could deliver the best PdM solution for your asset’s vibration analysis needs in the future. While it is difficult to recommend a single


vibration sensor for use in a PdM system, accelerometers have a successful history and continue to evolve and improve. Analog Devices offers a range of MEMS accelerometers from general purpose, low power, low noise, high stability, and high g, as well as intelligent edge-node modules shown in Figure 4. The ADcmXL3021 is just one example of a dedicated PdM module solution. Analog Devices was first to market with a family of PdM-capable MEMS accelerometers (20 kHz+ bandwidth, 25 µg/√Hz noise density) and remains the only MEMS accelerometer supplier with these performance levels. Analog Devices is continuing to lead the way in providing sensors, signal chain solutions, mechanical modules, platforms, machine learning algorithms, artificial intelligence software platforms, and total system solutions to enable predictive maintenance of industrial rotating machines in the most challenging environments.


Analog Devices analog.com/CbM 43


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