Predictive maintenance & condition monitoring
Elusive tones M
In this article, Anthony DeSimone, Analog Devices, and Sid Tallur, Indian Institute of Technology Bombay, discuss aliasing effects in digital MEMS accelerometers in condition monitoring When using an accelerometer with digital
EMS accelerometers are attractive candidates for vibration-based condition monitoring of industrial
machinery, on account of their low cost, weight, power, and ease of use, as compared to incumbent piezoelectric accelerometers. These features allow equipment manufacturers to embed multiple MEMS accelerometers to detect vibration signals in various parts of the machinery, subsequently diagnose abnormalities and incipient failures by identifying vibration patterns that deviate from normal. While the patterns are unique to different machines and mounting locations for the sensors, the technique broadly relies upon gleaning information from the frequency spectrum of vibrations sensed by the accelerometers, and monitor for shift in frequencies of vibration tones, onset of harmonics, and change in broad-band vibration amplitude, in particular, frequency bands.
output, designers must take into account the bandwidth limitations imposed by the digital filters in the signal chain, and also be wary of aliasing of high frequency signals into the bandwidth of the sensor for a given output data (sampling) rate. For a sampling rate S, a vibration signal at frequency f (where f < S/2) is indistinguishable from a signal at higher frequency S/2 + f, due to folding. In condition monitoring applications, this could lead to misinterpretation of the vibration profile of the machinery. As an example, consider the ADXL355 and
ADXL354, accelerometers that are targeted for high resolution vibration measurement with very low noise to enable the early detection of structural defects via wireless sensor networks. The internal digital filters in the ADXL355 will cause aliasing of out of band frequency, content to frequencies within the bandwidth of the accelerometer. For operation at 4kHz ODR, for example, a
bandwidth of 1kHz frequencies above the Nyquist rate (2kHz) will experience aliasing. This is par ticularly impor tant for frequencies above 3kHz, which will be aliased to frequencies within the bandwidth of the accelerometer (1kHz), thereby causing an erroneous interpretation of the output. A combination of the filter and the sensor resonance results in an attenuation of ~25dB for the 3kHz signal aliased to 1kHz, as seen in Figure 1. The dynamic range of the accelerometer with noise density N = 25μg/√Hz and bandwidth BW = 1kHz, considering R = ±8g measurement range is:
The attenuation for all frequencies between
3kHz and 3.9kHz is less than the dynamic range and thus the aliased signal will be larger than the accelerometer noise floor. For applications where smaller bandwidth is acceptable, the ADXL355 could be used by digitally filtering the output to limit the bandwidth to lower frequencies, such that the attenuation of the aliased signal is higher than the dynamic range required in the application. If the application requires monitoring of vibration tones of amplitude above a cer tain threshold, such that the dynamic range required is 30dB (±8g to ±8mg), the bandwidth can be limited to 750Hz by using a digital filter (for example, four th order Butterwor th filter) at the output. If the attenuation requirement in the application for aliased signals is larger than the actual response in the ADXL355, it is recommended that the ADXL354 be used instead. Since the ADXL354 does not possess any internal digital filters, it does not exhibit aliasing.
Figure 1: ADXL355 output aliasing vs. frequency due to digital filtering
Instrumentation Monthly October 2018 Analog Devices
analog.com 17
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