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MEASUREMENT AND INSTRUMENTATION

withstand 200°C and higher, just like there are special-purpose such events because the dc content of the measured signal
MEMS accelerometers that can work up to 175°C, but this article is corresponds to tilt, assuming the resulting rocking of the asset
not focused on sensors for very specific applications. occurs in the sensitive axis.
Sensitivity defines the amount of output change per unit Vibration excitation is often directional, depending on the
of acceleration. Sensitivity change over temperature defines fault, such as a spall on an outer bearing race, mechanical
how the sensor’s sensitivity changes over temperature. It is not looseness, misalignment or a bad gear tooth. The direction of
uncommon to see piezo-electric accelerometers with scale factor the fault vibration is not always predictable, so one can’t know
error over temperature up to 20%, which could lead to significant which direction — axial, radial or tangential — the vibration will
drift, although 5% is more typical. Such errors require calibration travel. There can also be more than one fault causing abnormal
during production. MEMS accelerometer scale factor or sensitivity vibrations. One study focused on demonstrating the potential for
error over temperature is excellent due to being trimmed improved diagnostic capability when using triaxial piezo-electric
electrically during production, resulting in sensors that do not drift sensors vs. single axis radial and axial sensors. The study
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over temperature. As an example, if the ADXL1002 was exposed revealed that single-axis accelerometers can miss the diagnosis
to a temperature change from 25°C to 85°C, the sensitivity (40 of nearly 50% of the mechanical faults outlined previously if
mV/g) would change by 0.03%/°C × 60 = 1.8%, which means sensors are only placed radially or tangentially as shown in
the sensitivity change over 60°C is within 39.28 mV/g to 40.72 Figure 4. Seeing as the direction of the fault vibration is the issue,
mV/g. This shows that for MEMS accelerometers the sensitivity adding more sensors on the same axis will not solve this problem.
is quite stable vs. temperature change. For most applications, Adding an axial accelerometer improved fault detection to almost
temperature compensation for sensitivity is not required. 70%. Adding one more axial sensor increased the detection
Zero g offset is the output of the accelerometer when no to 80%. This shows that the extra diagnostic information from
acceleration is applied. Ideally this should be zero, but due to different axes can lead to better fault detection, but note that this
inherent imperfections within the MEMS sensor we see a dc must be done with a triaxial sensor. This study found that having
offset. In most cases, maintenance professionals are primarily data for all three axes was redundant in many cases, but still
concerned with dynamic data (ac output from the accelerometer) recommended measuring on three axes if possible.
such as deviations from baselines or trending away from an While having more data is always beneficial, it is not always
operating normal. For this reason, zero g offset is not a prime required, especially in wireless systems where measuring or
concern when using MEMS accelerometers for CbM. Zero g transmitting redundant data can shorten the life of the battery.
offset can be easily calibrated out of measurements, and most Proper placement of sensors whether they be single, dual or
high-performance digital sensors will provide registers to perform triaxial is critical but according to the above research, based on
this action with ease. Where dc or tilt detection is of interest, zero wired piezo-electric sensors, triaxial sensors should be used
g offset over temperature can also be calibrated out. The smaller whenever possible.
the operating temperature range, the easier this will be. For MEMS accelerometers, any existing triaxial sensors have
reduced performance compared to piezo-electric sensors, so the
Number of axes likelihood is that they will not be able to detect as many faults.
MEMS accelerometers are available in single, dual and triaxial Furthermore, the z-axis in most triaxial MEMS accelerometers
versions. Unlike piezo-electric accelerometers, there is no real has lower performance in noise, bandwidth or both, as shown
size difference between single and triaxial MEMS accelerometers. in Table 11, possibly weakening the potential added value of
Smaller size is one of the key advantages of MEMS over piezo- extra axes reported by studies based on triaxial piezo-electric
electric, along with much lower power consumption and higher accelerometers. In some cases, all axes will have different
levels of integration. With three-axis piezo-electric accelerometers performance in terms of noise and/or bandwidth, the two most
there are some clear disadvantages — such as cost, which can important specifications for CbM.
be up to three orders of magnitude higher compared to triaxial The implications of this mismatch in terms of noise and/
MEMS accelerometers, size, and accuracy; however one of the or bandwidth performance first appears to somewhat negate
main drivers for using triaxial piezo-electric accelerometers is
to allow easier collection of data for portable vibration readers.
Instead of having to prepare three sites (single-axis sensors)
then take three separate readings, one triaxial sensor can do
this alone. For assets with restricted access, this can be a major
advantage. Also, when measuring vibrations in multiple directions,
it can be important to maintain phase relations between axes and
a triaxial device will ensure this. For complex vibration analysis it
is important to see events on all axes with no phase mismatch, as
this could lead to misinterpretation of events.
With a triaxial piezo-electric sensor measuring vibration in
the x, y, and z directions, it is possible to measure the tangential
motion/vibration of the axis of rotation. Many mechanical forces
generated by rotating machines — soft footing, for example
— produce such tangential motions of the casing. This is not
possible to detect with a single-axis piezo-electric sensor. With
single-axis MEMS accelerometers, it may be possible to detect Figure 4: Motor vibration axes

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