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MEASUREMENT AND INSTRUMENTATION

Table 1. Comparison of Piezo-electric and MEMS Accelerometers for CbM Applications

Sensor Cost 1k u.p. Noise 3 dB Bandwidth Potential Battery Life DC Response Self-Test Integrated Features

Piezo-electric Accelerometer $25 to $500+ <1 µg/√Hz to 50 µg/√Hz 2.5 kHz to 30 kHz+ Short to medium No No No

MEMS Accelerometer $10 to $30 25 µg/√Hz to 100 µg/√Hz 3 kHz to 20 kHz+ Medium to long Yes Yes Yes

know with absolute certainty the current state of the asset as well
as the accuracy and reliability of what they are measuring.
Design considerations for MEMS accelerometers in
CbM applications
MEMS accelerometers designed specifically for CbM applications
have some different characteristics compared to general-purpose
accelerometers. In this section we will discuss the key data sheet
parameters of MEMS accelerometers suitable for CbM and how
they relate to detecting machine faults. For example, how can we
select a sensor with the correct g-range or noise performance
to detect bearing faults on a 300 kW induction motor? Table 2
shows a list of the most important specifications for five MEMS
accelerometers targeted at CbM applications. Each specification
will be discussed in detail in the following sections.

Bandwidth
The bandwidth of a vibration sensor is typically linked to the
criticality of the asset it will be monitoring. A critical asset
or motor is crucial to keeping a process or larger machine
operational and online. If such an asset were to break down,
it would lead to unplanned downtime and potential loss of
revenue. In order to detect and diagnose faults as early as
possible, and avoid unplanned downtime, it is imperative to
have a wide bandwidth and low noise vibration sensor. Low
noise is required to detect faults or deviations at low magnitudes
and wide frequencies, as common faults with bearings, gear
meshing, and pump cavitation all occur — or at least can be
detected earliest — at higher frequencies greater than 5 kHz
and even up to 20 kHz and beyond. Therefore, it is incumbent
on MEMS sensors to be able to compete with the de facto
vibration sensors used for decades in industrial applications:
piezo-electric accelerometers. A noise level of less than 100
µg/√Hz and bandwidth of 5 kHz or greater is considered a high
performance MEMS accelerometer for CbM. Table 3 categorises
the two most important criteria for MEMS accelerometers used
in CbM and PdM applications. 3

Not all sensors are required to be ultra-low noise or wide
bandwidth; there are levels to vibration sensing that depend
on how critical it is to keep an asset running. Water cooling
pumps in a nuclear reactor could be considered extremely
critical and, in this case, detecting a fault early is required.
This means the criticality of the asset to be monitored will
generally dictate the level of vibration sensor required, which
relies on the following criteria:

Fault detection
To simply detect if vibrations have gone above a threshold or
Figure 2: Response to overload: MEMS vs. piezo-electric with warning level, a low performance MEMS accelerometer can be
laser reference. used. This method is typically employed on lower criticality assets.

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