Accelerometers


Overview

Accelerometers

MEMS accelerometers consist of a damped mass system on a spring, etched in silicon. When the accelerometer is accelerating, the proof mass will move with respect to the casing. This movement is measured by electrodes. The analog measurement signal goes through a sequence of signal processing stages and gets converted to digital. Subsequently, the digital signal is converted to acceleration (SI unit, m/s2) by applying pre-defined calibration parameters. For inertial sensors the evaluation and thereby characterization of the MEMS-design and signal processing chain is typically done using Allan Variance graphs, which are typically used in the study of frequency stability of oscillators.

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How to choose your accelerometer?

Depending on the design, accelerometers behave differently. Especially 3D accelerometers that may have different designs for each axis have different characteristics depending on the orientation. The availability of 3D accelerometers makes it possible to have 3D IMUs and AHRSs on a SMD-chip, as shown in the MTi 1-series. Apart from the stochastic measures discussed previously other specifications that play an important role are the following:

Non-linearity is an important parameter to take into account, especially for applications where accelerations larger than 1g can be expected, e.g. aircraft.

Bias repeatability also called the (turn-on to turn-on) bias stability is often mistakenly used for referring the in-run bias stability discussed previously.

The bandwidth of an accelerometer is important for proper functionality in industrial applications. 

Accelerometers are available with different full-scale options parameters. The full range of accelerometers lies typically between 2g and 20g. The negative implications of choosing a non-optimal full range can be significant. As most industrial applications experience vibrations, the vibrations may exceed the full range of the accelerometer. As in many signal processing pipelines accelerometers are sampled at a high frequency (e.g. 10 kHz) and then converted to a lower frequency (e.g. 1 kHz or 2 kHz), short term clipping may not be noticed but does affect the output. Although the resolution of the accelerometer decreases and noise is higher, it is advised that that the full range is higher than the expected maximum vibrations or accelerations. Vibrations can easily be up to 10g in vehicles with engines. Military standards such as MIL-STD 202 prescribe immunity against vibrations of 6g RMS.

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