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MEASUREMENT AND INSTRUMENTATION

Table 7. Noise comparison of MEMS accelerometers for CbM as per ISO 10816 vibration severity standards

Minimum Noise Required
Noise Density (µg) Bandwidth (Hz) Sensor Noise (mg) Class I Class II Class III Class IV
0.71 mm/s 4.5 mg 1.12 mm/s 7.2 mg 1.8 mm/s 11.5 mg 2.8 mm/s 17.9 mg
ADXL1002 25 10,000 3.1 Pass Pass Pass Pass
ADXL317 [X, Y] 55 4000 4.4 Pass Pass Pass Pass
ADXL317 [Z] 120 2000 6.7 Fail Pass Pass Pass
MEMS B [X, Y] 75 6300 7.5 Fail Fail Pass Pass
MEMS B [Z] 110 6300 10.9 Fail Fail Pass Pass
MEMS C1 [X, Y] 130 4200 10.6 Fail Fail Pass Pass
MEMS C1 [Z] 130 2900 8.8 Fail Fail Pass Pass
MEMS C2 [X] 300 8200 34.0 Fail Fail Fail Fail
MEMS C2 [Y] 300 8500 34.7 Fail Fail Fail Fail
MEMS C2 [Z] 300 5600 28.1 Fail Fail Fail Fail

g-Range discussed in the “Noise Density” section.
This tells us the acceptable range of accelerations that a When selecting a MEMS accelerometer for use with a
sensor can reliably detect while guaranteeing the data sheet machine covered under ISO 10816, we can follow some easy
performance. Anyone who has ever tested a ±2 g sensor will have steps to determine if the g-range is acceptable for use. Equation
been able to generate more than 2 g while shaking the sensor 4 presents a specific example, which determines that measuring
in their hand. Most MEMS accelerometers, especially analogue unacceptable vibration severity on a Class IV asset, per ISO
output, have some headroom due to mechanical elements 10816-1 (VMAX= 28 mm/sec), at a frequency of 1000 Hz (fMAX),
and signal conditioning electronics. For CbM, typical g-range will require a measurement of good vibration severity levels and
requirements start at ±16 g for smaller assets (ISO 10816-7 g-range to detect potential faults per class of motor. The only
pumps), but some parts go all the way up to ±500 g for use on sensor that has sufficient noise performance and g-range is the
4
industrial gear boxes, compressors, medium and high voltage range of at least ±25.3 g.
induction motors, etc.
When measuring vibrations, it is important to understand the
relationship between acceleration, velocity and displacement. If a
vibration, measured on one axis, causes 250 nm of displacement
while vibrating at 1 kHz, the generated peak acceleration will be
A (250 nm, 1 kHz) = 1 g. For the same displacement at 10 kHz,
PK
the peak acceleration will now be A (250 nm, 10 kHz) = 100 g.
PK
It is vitally important to understand the potential vibrations that It should be noted that these fault classes do not consider a
can occur in your asset before selecting a vibration sensor. Some MEMS sensor’s ability to withstand base load vibration. Typically,
motor manufacturers will provide such information. There are also a sensor with a smaller g-range or full-scale range will be less
some standards such as ISO 10816 that can help with this, as resistant to wear and tear of its mechanical elements. Also, with a
smaller full-scale range it is easier for vibrations of interest to be
masked by baseline vibrations.
Table 8 shows ISO 10816 vibration severity charts both
in mm/s and g for each class of asset. A range of MEMS
accelerometers suitable for use in CbM applications are
compared. Approximately 16 g of g-range is not enough for use
on Class III and Class IV assets, but it is acceptable for Class
I and Class II. The only two sensors with sufficient g-range are
ADXL1002 and MEMS C2.
Low g-range MEMS accelerometers for CbM (<±16 g)
are limited to use on Class I and Class II machines, per ISO
10816, as the maximum vibration severity for Class III and
Class IV machines exceeds ±16 g. This means that noise
performance in low g-range MEMS accelerometers for CbM
becomes even more important to ensure they can be used
Figure 3: The relationship between acceleration, velocity, on Class I and Class II machines, as discussed in the “Noise
displacement and g-range. 5 Density” section.

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