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MEASUREMENT AND INSTRUMENTATION

The output rms noise of a MEMS accelerometer can be Equation 3 offers this in a generic form, along with an example,
determined by the following formula: which estimates the total noise associated with an accelerometer
with a noise density of 80 µg/√Hz, when using it with a single-pole
low-pass filter that has a cut-off frequency of 1000 Hz (f = 1000
C
Hz). At 3.17 mg, the accelerometer appears to meet the boundary
Once the sensor noise is understood, it is important to match condition from Equation 2:
the most suitable sensor to the machine type, keeping in
mind some important questions such as: will the sensor’s
noise prohibit it from measuring important vibrations, and will
the g-range of the sensor be able to withstand potential fault
vibration levels? Luckily, there are standards that can help with
this, such as ISO 10816.
ISO 10816 establishes conditions and procedures for the Table 7 shows the prescribed vibration levels for each class
measurement and evaluation of vibrations from assets and of machine, from a known good state to dangerous fault level
machines. It defines a vibration severity standard where the rms vibrations and the corresponding minimum noise a MEMS
velocity (10 Hz to 1 kHz) of the installed machine’s housing is accelerometer requires to detect known good vibrations in region
used as a condition indicator, as shown in Table 6. The measured A (Class I at 4.5 mg, Class II at 7.2 mg, Class III at 11.5 mg, and
vibration from the machine is classified based on machine size, Class IV at 17.9 mg).
mounting strategy, and machine class (I = small, II = medium, III This data suggests that MEMS C2, MEMS C1, MEMS B,
= large with small foundation, and IV = large with rigid foundation). and ADXL317 (z-axis) are not suited for use on machines where
a noise level below 0.71 mm/s or 4.5 mg is required to detect
Please note that accelerometers typically output acceleration in a known good level of vibration (A). MEMS B, MEMS C2, and
g, whereas ISO 10816 uses velocity in mm/s or in/s. Equation MEMS C1 are not suited for use on machines requiring noise
2 can help us translate accelerations in g to velocity in mm/s. It below 1.12 mm/s or 7.2 mg. MEMS C2 do not have sufficient
determines that at a minimum vibration frequency of 10 Hz, the noise performance for use on any class of machine shown, to
noise in the acceleration measurement must be less than 7.18 detect known good vibration severity levels (A).
mg to detect vibration severity in the good range (A) for a Class Please note that all sensor noise values reported in Table 7
2 machine, per ISO 10816-1 (V MIN = 1.12 mm/s) as shown in are for full bandwidth measurements, even though ISO 10816
Table 6. 4 is only concerned with bandwidths up to 1 kHz. It is assumed
that if a vibration sensor has a wider bandwidth, this will typically
be used in order to not only detect vibration severity but also
to diagnose any potential faults at higher frequencies. With the
bandwidth limited to 1 kHz ,MEMS C1 fails Class I noise levels
while MEMS C2 only passes on Class IV.

Table 6. ISO 10816 vibration Severity Chart

RMS Vibration Velocity (mm/s) Class I Class II Class III Class IV
0.28
A A
0.45
0.71 A
1.12 A
B
1.8
B
2.8
C B
4.5
C B
7.1
C
11.2 C
18 D
D
28 D
D
45

■ A—Recently commissioned motor installation
■ B—Unlimited, satisfactory, long-time operation
■ C—Short-time operation
■ D—Vibration level that causes damage to the motor

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