Test & measurement

Measuring AC accelerations: To calibrate or not to calibrate?

In this article, Sid Tallur, product applications engineer, Analog Devices, discusses the ac response of accelerometers and factors that should be taken into account for deciding whether calibrating the output is necessary

interest, as the application requires sensing small changes in the static accelerometer output. In most applications, such as platform stabilization, downhole directional drilling, crane stability systems, and in the construction industry for leveling in road graders and surveying equipment, the inclination change can be assumed to be quasistatic, as the time scales involved in the change of inclination are typically much smaller than the bandwidth of the accelerometers. However, in applications such as vibration monitoring and structural health monitoring (SHM), the ac response of an accelerometer also becomes important, as the signals of interest may have a frequency spectrum with power spread over a wide range of frequencies. Since vibration is periodic, spectral


analysis offers a convenient way to characterise the vibration profile (the relationship between vibration, magnitude, and frequency). Every piece of moving equipment will have its own vibration profile, with spectral tones often representing the natural resonance frequencies of the equipment. Knowing the accelerometer’s frequency response of sensitivity, for example, how the sensitivity varies as a function of frequency of applied input vibration, is necessary to scale the frequency content of the accelerometer output from a voltage PSD (V/√Hz ) to acceleration PSD (g /√Hz). The ADXL354 and ADXL355 are part of a new family of low noise, low power MEMS accelerometers that enable low vibration level monitoring applications such as structural health monitoring. This article discusses the ac response of these accelerometers and factors that should be taken into account for deciding whether calibrating the accelerometer output is necessary in such applications.

FAcTorS conTrIbuTIng To Ac reSponSe Both the ADXL354 and ADXL355 accelerometers use an analogue low- pass, antialiasing filter to reduce out of

or applications such as inclinometers, the dc response of an accelerometer is the signal of

band noise. The analogue antialiasing filter is a sinc3 filter, and it provides a fixed bandwidth (3dB corner) of approximately 1.5kHz. This limits the bandwidth in the ADXL354 and the ADXL355, and, additionally, filters out

aliasing noise from the internal, 20-bit, Ʃ-Δ analogue-to-digital converter (ADC) in the ADXL355. The ADXL355 also incorporates an additional digital filter stage that consists of a low-pass decimation filter and a bypassable high- pass filter. A combination of all these filter stages defines the ac response of these devices. These filter stages effectively attenuate the sensitivity of the ADXL354 and the ADXL355 outside the 3dB corner. The MEMS sensor used in the ADXL354 and ADXL355 has a resonance frequency of approximately 2.5kHz on the x- and y- axis, and 2.1kHz on the z-axis, thereby causing a resonant enhancement of sensitivity around the sensor’s resonance frequency.

Instrumentation Monthly April 2019

MeASureMenT oF Ac SenSITIvITy A sinusoidal sweep vibration test is performed to evaluate the sensor’s frequency response. The accelerometer is bolted to a fixture and mounted on an Unholtz-Dickie model 20 shaker system. A reference accelerometer (PCB 320B14) is used for calibration of the shaker excitation, along with an Endevco model 133 signal conditioner. For the ADXL354, a vibration research VR9500 is used as a vibration controller and data acquisition system. For the ADXL355, an NI PCI 7850R is used as the data acquisition system. The frequency of the sinusoidal vibration signal is swept from 30Hz to 5kHz. The mounting of the accelerometer is changed after performing measurements to align a different axis of sensitivity to the axis of vibration of the shaker system.

ADXL354 The parts were operated in a ±8g range, with a 1g peak excitation on the sinusoidal

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Figure 1. ADXL354 sensitivity vs. vibration frequency

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