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MEASUREMENT AND INSTRUMENTATION

Table 11. Variation in noise and bandwidth from axis to axis for MEMS accelerometers for CbM

ADXL1002 ADXL317 Other MEMS Vendors
Bandwidth X 11 kHz 4 kHz 4.2 kHz to 8.2 kHz
Bandwidth Y 4 kHz 4.2 kHz to 8.5 kHz
Bandwidth Z 2 kHz 2.9 kHz to 6.3 kHz
Noise X 25 µg/√Hz 55 µg/√Hz 75 µg/√Hz to 300 µg/√Hz
Noise Y 55 µg/√Hz 75 µg/√Hz to 300 µg/√Hz
Noise Z 120 µg/√Hz 110 µg/√Hz to 300 µg/√Hz

the advantages of having extra axes (y, z) in one place on an accelerometers, which are predominantly single axis, this
asset. This is well understood by designers familiar with MEMS will be given as transverse sensitivity, which describes the
sensors, but a few things need to be considered. MEMS triaxial sensitivity to any motion not on the same axis it was designed
accelerometers can be orders of magnitude lower in cost with to measure on. For a triaxial accelerometer experiencing
comparable performance to piezo-electric accelerometers and acceleration only on its y-axis, some acceleration will be
far smaller, so more sensors can be placed, even in wireless measured on the x and z axes due to CAS. Figure 5 shows
installations on less critical assets. This provides more diagnostic a CAS of 1% as the y (or z) axis experiences 1.5 g of
insights into the general operation of the asset. acceleration; this is also observed on the x-axis as 15 mg or
1% of 1.5 g. This phenomenon also affects single-axis MEMS
Cross-axis sensitivity accelerometers. The lower this percentage, the more accurate
Cross-axis sensitivity (CAS) refers to how much output is and reliable is the vibration data that can be measured and
seen on one axis when acceleration is imposed on a different used to detect faults, anomalies and drifting trend lines.
axis, typically expressed as a percentage. For piezo-electric Some MEMS manufacturers leave critical information like CAS
off their data sheets, as shown in Table 12, but for CbM and PdM
this is a vital specification that must be understood when trying to
detect failures early where they are likely to be close to the noise
floor of the sensor. The 1% listed for the ADXL1002 could be
considered conservative as tests have shown performance to be
slightly better.
Figure 6a shows a CAS test on a MEMS accelerometer.
Vibration was applied in the z-axis only. Figure 6a shows the
z-axis measured an acceleration of approximately 1.1 g peak,
whereas the x-axis in Figure 6b measured approximately 0.05 g
peak acceleration and the y-axis slightly less at 0.0425 g.
Table 13 shows the worst case CAS of 2.6% on the x-axis
and 2.2% on the y-axis. There could be misalignments in the test
setup, so the likelihood is the CAS is at least 2% but below 2.6%.
While it is possible to calibrate CAS, it is desirable to have this
value close to 1%, which is the industry-leading value for MEMS
CbM accelerometers. Piezo-electric CAS is typically around 5%,
but in some cases, up to 15% is reported.7 Transverse sensitivity
Figure 5: Cross-axis sensitivity observed on the x-axis of a 3-axis values below 5% are available on request from some piezo-
accelerometer accelerating in y or z axes. electric vendors at an extra cost.

Table 12. Cross-axis sensitivity comparison for MEMS accelerometers used in CbM

ADXL1002 ADXL317 Other MEMS Vendors

Cross-Axis Sensitivity 1% 1% Not listed, or up to 2%

Table 13. MEMS B sensor cross-axis sensitivity
Acceleration Peak (g) Acceleration RMS (g) CAS % (RMS)
Z-Axis 1.1 0.76
Y-Axis 0.0425 0.017 2.2
X-Axis 0.05 0.02 2.6

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