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MEASUREMENT AND INSTRUMENTATION

Why MEMS accelerometers are becoming

the designer's best choice for condition-

based monitoring applications

By Chris Murphy, Applications Engineer

Introduction to condition-based monitoring frequencies is very useful for wind turbines and other types of
The global condition-based monitoring (CbM) market has slow rotating machinery used in metal processing, pulp/paper
experienced significant growth over the past few years, and processing, and food/beverage industries where slow rotating
1
this looks set to continue. This growth coincides with the rapid speeds of assets below 60 rpm (1 Hz) are common.
advancement of MEMS accelerometers for use in vibration Figure 2 shows that when piezo-electric sensors are
sensing applications, now rivalling the once dominant piezo- exposed to large shock events they can saturate, and due
electric or PZT accelerometer. There is an increased demand for to the large RC time constant they can take a long time to
CbM on less critical assets as well as a growing adoption rate of settle back to normal. MEMS, on the other hand, matches
wireless CbM systems, and MEMS accelerometers are the key the non-contact reference sensor by settling back to normal
to this. This article will compare MEMS accelerometers to piezo- almost instantly. The implications with a piezo-electric sensor
electric accelerometers to highlight just how far MEMS sensors undergoing a severe shock mean there is a risk that valuable
have come in their short lifetime. Key design considerations information or failures in the asset/process could go undetected,
for MEMS accelerometers in CbM applications will also be while MEMS sensors will detect impact events and subsequent
discussed with a comparison of five MEMS sensors from three events reliably.
different vendors. Table 1 highlights some more advantages of MEMS
accelerometers for CbM applications. Piezo-electric
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The current state of the art in vibration sensing — accelerometers are less suitable for wireless CbM systems
MEMS versus Piezo-electric due to a combination of size, power consumption and a
Vibration sensors have been used to detect machine health as lack of integrated features, but solutions do exist with typical
far back as the 1930s. Even now, vibration analysis is considered consumption in the range of 0.2 mA to 0.5 mA.
the most important modality for predictive maintenance (PdM). MEMS accelerometers also have a self-test feature where
Piezo-electric accelerometers have been long established as the sensor can be verified to be 100% functional. This could
the gold standard vibration sensor used on the most critical prove useful in safety critical installations where meeting system
assets to ensure they remain operational and perform efficiently. standards is made easier by the ability to verify if a deployed
Until recently, MEMS accelerometers’ limited bandwidth, noise sensor is still functional. In some applications this feature is one
performance and g-range capabilities prevented their use in CbM of the most important as it allows maintenance professionals to
of critical assets. While many high g-range accelerometers are
available (designed specifically for automotive impact detection),
they have very limited noise performance and bandwidth, making
them unsuitable for CbM. Likewise, some low noise MEMS
accelerometers (designed specifically to detect tilt) are available
but have insufficient bandwidth and g-range.
A small number of MEMS manufacturers have been striving
to overcome the noise, bandwidth, and g-range shortcomings
and have produced several medium and high performance
MEMS accelerometers with the latter being comparable to
piezo-electric accelerometers. MEMS sensors are based on
a completely different principle of operation to piezo-electric
sensors, and this is where the key differences arise. Figure
1 shows how MEMS can measure down to dc, allowing
measurements from very slow rotating machinery as well
as tilt detection. It is clearly understood that piezo-electric
sensors can offer better noise performance than MEMS at
higher frequencies, but at low frequencies MEMS sensors offer
lower noise all the way to dc. Being able to measure these low Figure 1: Noise density: MEMS vs. piezo-electric.

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