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MEASUREMENT AND INSTRUMENTATION

Table 8. Comparison of MEMS accelerometer g-range for use with Class I through Class IV motors

Peak Acceleration (g)

g-Range Class I Class II Class III Class IV
7.1 mm/s 6.4 g 11.2 mm/s 10.1 g 18 mm/s 16.3 g 28 mm/s 25.3 g

ADXL1002 50 g Pass Pass Pass Pass
ADXL317 16 g Pass Pass Fail Fail
MEMS B 16 g Pass Pass Fail Fail
MEMS C1 16 g Pass Pass Fail Fail
MEMS C2 64 g Pass Pass Pass Pass

When selecting a MEMS accelerometer for use in CbM Temperature
applications, you must refer to the asset manufacturer’s There are several specifications to consider when it comes
specifications to find potential fault vibration severity to temperature performance of MEMS accelerometers. Table
information, perform your own tests, and/or refer to standards 10 shows some very interesting comparisons between key
such as ISO 10816. By combining the information from Table temperature related data sheet specifications. Clearly there is
7 and Table 8, it is clear that the majority of CbM MEMS a significant range in terms of the numbers, but what does this
accelerometers on the market fail to meet the criteria outlined translate to in terms of performance?
in ISO 10816 in terms of noise performance to measure known A review of the most common applications for CbM (oil and
good vibration severity levels and g-range to detect potential gas, metal processing, food and beverage, and power generation)
faults per class of motor. The only sensor that has sufficient shows that potential temperatures on assets can easily exceed
noise performance and g-range is the ADXL1002, one of a 105°C due to factors such as over-driving the load capabilities,
family of sensors from Analog Devices designed specifically leading to excess current being drawn, contamination (dust, debris)
for CbM applications. It is very clear that the current state of raising the internal temperature of a motor and preventing it from
the art in MEMS accelerometers for CbM need to be classified cooling, and even creating vibrations that can generate excess
based on this evidence, and this is shown in Table 9. Noise and heat. External factors, such as potential gas or steam leaks, can
bandwidth are considered the highest importance, hence the also play a part in selecting a sensor. Piezo-electric manufacturers
weighting g-range is next, followed by temperature range and appear to favour a maximum temperature range of 120°C for
cross-axis sensitivity. most of their general-purpose vibration sensors, with some
ADXL1002 is a clear leader in terms of performance and so application specific sensors having 150°C maximum operating
is classified as the highest performance MEMS accelerometer in temperature. A survey of high frequency sensors (up to 10 kHz and
CbM applications. All other sensors, while still offering excellent greater) showed that 74% of sensors had a maximum operating
performance, are classified as medium performance CbM temperature range below 125°C, with 24% below or equal to 80°C.
accelerometers, given the gaps in performance. There are some special-purpose piezo-electric sensors that can

Table 9. Decision matrix for choosing the best MEMS accelerometer for PdM based on key criteria

Parameter [weight] ADXL1002 ADXL317 Other MEMS Vendors
±3 dB Bandwidth [5] 1 5 3 4 2
Noise Density [4] 1 2 3 4 5
g-Range [3] 2 3 3 3 1
Temperature Range [2] 1 1 2 2 2
Cross-Axis Sensitivity [1] 1 1 3 2 2
Total 18 45 43 51 42
Rank First Fourth Third Fifth Second

Table 10. Temperature performance comparison of MEMS accelerometers for CbM
ADXL1002 ADXL317 Other MEMS Vendors
Temperature Range –40°C to +125°C –40°C to +125°C –40°C to +105°C
Sensitivity Change ±5% ±2.5% (x, y) ±4.5% (z) ±1% to ±4.35%
0 g Bias Error ±10% ±9% ±0.1% to ±1%

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